Is it hot in here or is it just me? Effects of warming temperatures on salmonid migratory ability

Suitability of aquatic habitats is characterized by a number of factors such as water temperature, streamflow, channel structure, cover, substrate, and food web relationships1. With climate change it is expected that there will be more frequent heat waves and intensified wet and dry season, thus impacting the normal hydrologic conditions in streams and rivers2. Water temperature is often considered one of the most problematic and widespread stressors for fishes2. Additionally, freshwater ecosystems are increasingly threatened due to additional anthropogenic stressors that can affect flow such as dam construction and removal of riparian vegetation2. As air temperature increases, water temperature is beginning to exceed the thermal tolerance of many fish species, therefore disrupting their normal geographic range and a number of physiological functions such as metabolism, stress response, reproduction, and locomotion2.

Unlike birds and mammals, who can metabolically control their temperature, fishes are something called obligate poikilothermic ectotherms; meaning that they rely heavily on their external environment to obtain and regulate their body temperature and perform vital biochemical reactions3. Slight increases in temperature (< 0.5oC) are likely to be beneficial to growth in the short term, however, over extended periods of time it has the potential to be deleterious as some individuals are unable to adapt3.

Shifts in the climatic and flow regime of aquatic habitats may be especially harmful to species that exhibit complex and energetically expensive life cycles that are often finely tuned with environmental cues2. Salmonids, in particular, may be especially susceptible to the changes in water temperature and flow2. Many of these species are anadromous meaning they migrate from between the ocean (feeding grounds) as juveniles to their natal river as adults (spawn grounds)2. Prior to migration, salmonids cease feeding and therefore, rely solely on energy reserves for their long journey. In order for individuals to successfully complete their migration and spawning, they must carefully allocate and expend their energetic stores2. This can be incredibly demanding, as salmon typically utilize 50% of their energetic stores just to make it to the spawning grounds4. Luckily, many are highly efficient swimmers and utilize a number of specialized techniques to minimize migratory stress2.

Figure 1: Anadromous fish life cycle

However, in the case of increasing temperatures and flow, these adaptations are challenged. As water temperature becomes warmer the organism’s metabolic and oxygen demand subsequently increase2. If this exceeds the rate at which the fish’s cardiovascular and respiratory system can keep up, they may shift toward limited anaerobic pathways2. Consequently, the fish will quickly become fatigued and begin to travel at slower swimming speeds, thus having the potential shift specialized migration timing2. In many cases, this leads to accumulated stress (i.e. increased levels of cortisol in plasma), poor body condition, immunosuppression, energetic trade-offs, and potentially pre-spawning mortality2. All of which could have immense detrimental effects on population structure and function.

Figure 2: Sockeye salmon (Oncorhynchus nerka)

Although quantifying the relationship between climate change and animal fitness is often difficult, some researches have accepted the challenge4. Over recent years, a number of studies have delved into this area of research to investigate potential responses and implications across various salmonid species. For example, Crossin et al. (2008) determined exposure to high temperature (18oC and above) had a great impact on the survival of Sockeye salmon (Oncorhynchus nerka), especially females. Less than one-third of the fish exposed to this temperature survived5. Similarly, Martins et al. (2012) further determined that survival decreased even further toward the last leg of the migration. Once again, especially in females6. This may be due to increased cortisol levels and immunosuppression, leaving the fish especially exhausted and susceptible to disease and mortality prior to spawning,6. These increased levels of female mortality could have the potential to limit the abundance and viability of salmon populations5,6. However, these studies are only the beginning and further studies will be necessary to conserve and manage for these species especially in light of projected future climate warming.


  1. Mohensi O, Stefan HG, Eaton JG (2003) Global warming and potential changes in fish habitat in U.S. streams. Climatic Change 59:389-409.
  2. Fenkes M, Shiels HA, Fitzpatrick JL, Nudds RL (2016) The potential impacts of migratory difficulty, including warmer waters and alter flow conditions, on the reproductive success of salmonid fishes. Comparative Biochemistry and Physiology 193:11-21.
  3. Elliott JM, Elliott JA (2010) Temperature requirements of Atlantic Salmon Salmo salar, brown trout Salmo trutta, and Arctic charr Salvelinus alpinus: predicting the effects of climate change. Journal of Fish Biology 77:1793-1817.
  4. Farrell AP, Hinch SG, Cooke SJ, Patterson DA, Crossin GT, Lapointe M, Mathes MT (2008) Pacific Salmon in Hot Water: Applying Aerobic Scope Models and Biotelemetry to Predict the Success of Spawning Migrations. Physiological and Biochemical Zoology 81:697-708.
  5. Crossin GT, Hinch SG, Cooke SJ, Welch DW, Patterson DA, Jones SRM, Lotto AG, Leggatt RA, Mathes MT, Shrimpton JM, Van Der Kraak G, Farrell AP (2008) Exposure to high temperature influences the behaviour, physiology, and survival of sockeye salmon during spawning migration. Canadian Journal of Zoology 86:127-140.
  6. Martins EG, Hinch SG, Patterson DA, Hague MJ, Cooke SJ, Miller KM, Robichaud D, English KK, Farrell AP (2012) High river temperature reduces survival of sockeye salmon (Oncorhynchus nerka) approaching spawning grounds and exacerbates female mortality. Canadian Journal of Fisheries and Aquatic Sciences 69:330-342.


Does something smell (pH)ishy?- Potential effects of ocean acidification on reef fishes

In aquatic environments, CO2 can naturally vary across the local and global scale due to biological activity, upwelling, winds, and currents1. However, with the onset of the industrial revolution and urbanization, the average CO2 levels in the oceans have dramatically increased1. In the last 200 years, it is expected that at least 30% of the CO2 from anthropogenic activities has been absorbed by our oceans2. When CO2 is hydrated in water it forms carbonic acid (H2CO3)1. Thus, causing a rapid decline of the pH in these ecosystems, a process often known as ocean acidification1. A lot of research has focused on its effect on plankton, corals, mollusks, and echinoderms due to their risk of calcification3. Yet, the ability for marine fishes to persist is widely unknown as many initially believed them to be safe under these conditions due to their specialized homeostatic mechanisms (e.g. Acid-Base balance) that allow for regulation of pH and CO2 in their blood and tissues1.

Figure 1: Ocean acidification process

However, over recent years there has been an increasing body of evidence that suggests that increased CO2 and changes in pH levels have a number of sublethal effects on fishes such as otolith overgrowth, alterations to metabolic rate, shifts in behavior, detection/interpretation of critical sensory cues, and impaired learning3,4. Olfaction, in particular, is especially important as it may affect the fish’s homing abilities, mate preference, and predator detection2,3,5. Therefore, posing a great threat to an individual’s fitness (i.e. survival and reproduction).

The potential underlying cause of behavioral changes and sensory disruption may be due to alterations in neurotransmitter function. In acidified waters, the concentration of CO2 in the blood will increase, known as hypercapnia1. In order to restore homeostasis, the fish will utilize their Acid-Base balance mechanisms by upregulating their excretion of hydrogen (H+) and accumulating HCO31. This also causes a decrease in plasma chloride (Cl). The shift in normal exchange of these ions has been found to reverse the normal function of GABA-A receptors (i.e. widespread neurotransmitter in the central nervous system)1. GABA-A receptors become excitatory instead of inhibitory, therefore altering behavior, stress response, and sensory system function1,4.

Many marine fish species exist in a pelagic larval phase as they transition into benthic adulthood. During this time, they use their learned and innate olfactory senses for predator recognition and avoidance3. Olfaction is vital in finding suitable habitat and avoid predation during settlement, as they typically settle at night when vision is limited3. However, various studies have demonstrated that ocean acidification impairs the olfactory function thus making it difficult for fish to detect, discriminate, and avoid predation risk.

Dixson et al. (2010) tested this theory in a population of newly hatched orange clownfish (Amphiprion percula). Clownfish has an innate ability to differentiate between the chemical cues of predators vs. non-predators. However, by simulating present and predicted CO2/pH levels Dixson et al. were able to determine that the fish’s ability to do so was lost in acidified seawater. In fact, the larvae shifted from complete avoidance to attraction to the predator cues. This fatal attraction may have a number of implications on the overall population structure of marine fish by limiting population replenishment and sustainability.

Figure 2: Orange Clownfish (Amphiprion percula)

Munday et al. (2010) performed a similar experiment to determine if damselfish larvae (Pomacentrus wardi) are also affected. Larvae exposed to acidified water engaged in riskier behavior as they spent more time active, were less responsive to predator cues, and moved farther away from places of refuge. Additionally, at high levels of CO2, the larvae demonstrated a preference for the predator cue. Therefore, indicating that they lose their ability to detect or differentiate the risk of the cues. These shifts in behavior have the potential to expose the larvae to increased rates of predation and alter energy allocation/metabolic rates.

Ward’s Damselfish (Pomacentrus wardi)

Based on these studies, it is suggested that 700 ppm CO2 may be the highest level at which reef fishes are able to somewhat adapt. Although, mortality is still likely to increase at that concentration. If emissions continue on their projected trajectory it is likely that atmospheric CO2 could reach 850 ppm by 21005. Further research is needed to clearly determine the various physiological processes and potential effects across taxa5. However, it is evident based on these studies that there is potential that ocean acidification will have implications for various populations of marine fish.



  1. Tresguerres M, Hamilton TJ (2017) Acid-base physiology, neurobiology, and behaviour in relation to CO2-induced ocean acidification. Journal of Experimental Biology 220:2136-2148.
  2. Munday PL, Dixson DL, Donelson JM, Jones GP, Pratchett MS, Devitsina GV, Døving KB (2008) Ocean acidification impairs olfactory discrimination and homing ability of marine fish. PNAS 106:1848-1852
  3. Dixson DL, Munday PL, Jones GP (2012) Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology Letters 13:68-75.
  4. Hamilton TJ, Holcombe A, Tresguerres M (2013) CO2-induced ocean acidification increases anxiety in Rockfish via alteration of GABAA receptor functioning
  5. Munday PL, Dixson DL, McCormick MI, Meekan M, Ferrari MCO, Chivers DP (2010) Replenishment of fish populations is threatened by ocean acidification. PNAS 107:12930-12934.

Microplastics and marine life: physiological implications of an emerging human-induced threat

One of the fastest growing areas of concern among scientists is the ever increasing quantity of plastic litter finding its way into aquatic environments. In 2014 alone, approximately 311 million metric tons of plastic were produced, with that number steadily increasing each year (Jonanovic, 2017). Of the plastic produced on a global scale, up to 10% of it ends up in aquatic environments. Specifically, by 2025 the mass ratio of plastic to fish in the oceans is predicted to be 1 to 3 and by 2050 it is expected that plastic will likely surpass fish stocks in the ocean by weight (Jonanovic, 2017). Microplastics, or plastic particles smaller than 5 mm derived from the break down of larger debris or entering the environment directly as microscopic fragments, has been estimated at over 5 trillion floating particles at a weight of 250,000 tons (Sussarellu et al., 2016). While much of the attention of scientists and the public eye have focused on the entanglement in and ingestion of macroplastics by vertebrates, the biological impacts of microplastics on marine organisms is only just now emerging (Wright et al., 2013). The aim of this post is to provide a summary of some of the current research completed looking at the deleterious impacts of microplastics on marine life.

Figure 1: Microplastics collected from the Chesapeake Bay Watershed .

Given the abundant nature and small dimensions of microplastics, ingestion by marine filter-feeders is of growing cause for concern. Filter-feeding species ingest large volumes of water, and subsequently, large quantities of particles potentially leaving them more susceptible to microplastic pollution. Sussarellu et al. (2016) experimentally exposed reproductively active Pacific oysters (Crassotstrea gigas) to microplastics for two months to access physiological impacts on both adults and offspring. The results of this study demonstrated that adult oysters altered their energy allocation from reproduction to structural growth and maintenance when exposed to microplastics. These alterations lead to reproductive impairment and decreased survival and growth of offspring (figure 2).

Figure 2: Comparison of larval growth of Pacific oysters between control group and those exposed to microplastics. Suppressed growth of larval oysters exposed to microplastics evident (Sussarellu et al., 2016).

Along with filter feeders, indiscriminate deposit-feeders (aquatic organisms that feed on organic matter that has settled on the sea floor), could be negatively impacted by microplastic pollution. Wright et al. (2013) found that lugworms (Arenicola marina) exposed to natural sediments infused with 5% microplastics depleted energy reserves by up to 50%.  The time taken to egest (discharge undigested material) ingested material was 1.5 times longer in lugworms exposed to microplastics compared to control worms. Unfortunately, these two studies represent some of the only research looking at the ecological impacts of microplastic ingestion by filter- and deposit-feeders.

While often considered wholefully insufficiently investigated, recent discoveries regarding the potential negative impacts of microplastic ingestion by fish are emerging throughout the scientific realm (Jovanovic, 2017). Fish could ingest microplastics both intentionally and unintentionally during all stages of development. Laboratory studies focused on on fish ingestion of microplastics have demonstrated varying physiological impacts. Reduced food intake resulting from microplastic gut blockage could lead to decreased energy availability in fish (Mazurais et al., 2015). Rochman et al. (2013) found that marine fish ingesting microplastics sorbed with environmental contaminants bioaccumulate the pollutants and suffer liver toxicity. In the study, fish exposed to microplastics and sorbed contaminants expressed high levels of stress and hepatic (liver) inflammation. These studies indicate the potential for negative physiological impacts on fish populations, but further research is needed to validate these results in the field (Steer et al., 2017).

With potentially devastating physiological implications of microplastic ingestion evident, concentrated efforts on disentangling the ecological impacts of microplastic ingestion on our aquatic ecosystems remains vital. While the elimination of plastic use by humans may be unachievable and the removal of existing microplastics all but impossible, we have a moral obligation to reduce our plastic usage to help alleviate future negative impacts on all aquatic organisms.

Figure 3:


Jovanovic B (2017) Ingestion of microplastics by fish and its potential consequences from a physical perspective. Integrated Environmental Assessment and management 13(3): 510-515.

Mazurais D, Ernande B, Quazuguel P, Severe A, Huelvan C, Madec L, Mouchel O, Soudant P, Robbens J, Huvet A, Zambonino-Infante J (2015) Evaluation of the impact of polyethylene microbeads ingestion in Eurpoean sea bass (Dicentrarchus labrax) larvae. Marine Environmental Research 112: 78-85.

Rochman CM, Hoh E, Kurobe T, Teh SJ (2013) Ingested plastic transfers hazardous chemicals to fish and induces heptic stress. Scientific Reports 3(3263): doi.10.1038/srep03263.

Steer M, Cole M, Thompson RC, Lindeque PK (2017) Microplastic ingestion in fish larvae in the western European Channel. Environmental Pollution 226: 250-259.

Sussarellu R, Suquet M, Thomas Y, Lambert C, Fabioux C, Pernet MEJ, Goic NL, Quillien V, Mingant C, Epelboin, Corporeau C, Guyomarch J, Robbens J, Paul-Pont I, Soudant P, Huvet A (2016). Oyster reproduction is affected by exposure to polystyrene microplastics. PNAS. U.S.A. 113, 2430–2435.

Wright SL, Rowe D, Thompson RC, Galloway TS (2013) Microplastic ingestion decreases energy reserves in marine worms. Current Biology 23(23): 1031–1033.




Native Trout in North America: Impacts of Multiple Stressors

Cutthroat Trout (Oncorhynchus clarkii spp.) were once widespread across western North America and consisted of 14 subspecies.1 However, populations have since been declining due to a variety of threats; leaving only 9 subspecies left in the wild, all of which are under state or Federal protection.2

Cutthroat Trout (Oncorhynchus clarkii)

One of the primary threats to Cutthroat Trout populations is the introduction of nonnative trout. Beginning in the 1800s, nonnative trout introductions became common practice for improving recreational sport fishing in the western United States.3 The ecological ramifications caused by these introductions were not recognized until much later. Brown Trout (Salmo trutta), a native of Europe, have been introduced extensively throughout the Cutthroat Trout’s historical range. Negative effects have been observed in streams with sympatric (i.e. occurring within the same geographical area) populations of Brown and Cutthroat Trout. Due to an overlap in diet and a more aggressive predatory nature, nonnative Brown Trout outcompete native Cutthroat Trout.4 This disadvantage in foraging for food has resulted in a reduction of growth rates and reproductive function.5 Inability to compete in the presence of nonnative fish has also contributed to an upstream shift in the distribution of native trout. In many mountain streams today, the headwaters (i.e. the upstream portion of a river nearest its original source) are occupied by native Cutthroat Trout and the lower portions are dominated by nonnative Brown Trout. This zoning pattern has become useful in conservation practices that implement an “isolation management” strategy, whereby physical barriers are installed to prevent the movement of nonnative trout upstream into the last remaining habitat of native populations.6 Isolating populations in headwater streams away from the negative effects of nonnative Brown Trout is vital in preserving native trout populations. Likewise, preventing contact with nonnative Rainbow Trout (Oncorhynchus mykiss) is also important because of the potential for hybridization and loss of genetic diversity in Cutthroat Trout populations.7

Another growing concern for native Cutthroat Trout populations in western North America is climate change. The amount of snow in the Rocky Mountains is expected to decrease in coming years and with warmer temperatures also forecasted, snowmelt will occur earlier in the springtime thereby altering hydrological patterns in streams and rivers.8 This is problematic for fish that depend on a continuous flow of water throughout the year. Drought has been shown to cause a decrease in the overall abundance and size of Cutthroat Trout populations.9 It is likely that these conditions are a result of a scarcity of food. Trout often feed on aquatic insects that are swept downstream in the moving waters. The number of insects drifting in the water is greatest when flow rates are high, thus under conditions of drought when less water is flowing through a stream the abundance of aquatic insects is rather low.10 Also, when the water level is lower, fish become more concentrated in narrow streams and the effects of competition increases which can have negative impacts on abundance and size.11 Additionally, in response to drought and increased temperatures, Cutthroat Trout have also been shown to move upstream in search of deeper pools and colder water where they can best endure the challenging conditions created by climate change.9 However, this is near impossible when native trout are already living in the upper limits of their range.

Combined effects of nonnative trout and climate change are predicted to extirpate (i.e. the extinction of a population at a local level) roughly 40% of the Colorado River Cutthroat Trout subspecies and leave another 40% vulnerable to extirpation.12 Loss at this scale would be devastating to the species as a whole. Appropriate management is needed to prevent this catastrophe from happening. New introductions of nonnative trout need to be prevented across all regions of North America. Fish barriers need to be repaired or improved to prevent any further migration of nonnative trout upstream. Where possible, nonnatives need to be removed and habitat restored for expanding the current range of native trout. Furthermore, protection of the genetic diversity of native trout is important for allowing populations to have the capacity for adapting to the growing threat of climate change.


  1. Behnke RJ (1988) Phylogeny and classification of Cutthroat Trout. In: Gresswell RE, eds. Status and management of interior stocks of Cutthroat Trout. American Fisheries Society, Symposium 4, Bethesda, Maryland, pp 1-7.
  2. Wilson WD, Turner TF (2009) Phylogenetic analysis of the Pacific Cutthroat Trout (Oncorhynchus clarkii : Salmonidae) based on partial mtDNA ND4 sequences: a closer look at the highly fragmented inland species. Molecular Phylogenetics and Evolution 52: 406-415.
  3. Pister EP (2001) Wilderness Fish Stocking: History and Perspective. Ecosystems 4: 279-286. doi: 10.1007/s10021-001-0010-7
  4. Meredith CS, Budy P, Thiede GP (2015) Predation on native sculpin by exotic brown trout exceeds that by native cutthroat trout within a mountain watershed (Logan, UT, USA). Ecology of Freshwater Fish 24: 133-147. doi: 10.1111/eff.12134
  5. Al-Chokhachy R, Sepulveda AJ (2019) Impacts of Nonnative Brown Trout on Yellowstone Cutthroat Trout in a Tributary Stream. North American Journal of Fisheries Management 39: 17-28. doi: 10.1002/nafm.10244
  6. Kirk MA, Rosswog AN, Ressel KN, Wissinger SA (2018) Evaluating the Trade-Offs between Invasion and Isolation for Native Brook Trout in Pennsylvania Streams. Transactions of the American Fisheries Society 147: 806-817. doi: 10.1002/tafs.10078
  7. McKelvey KS, Young MK, Wilcox TM, Bingham DM, Pilgrim KL, Schwartz MK (2016) Patterns of hybridization among Cutthroat Trout and Rainbow Trout in northern Rocky Mountain streams. Ecology and Evolution 6:688–706.
  8. Stewart IT, Cayan DR, Dettinger MD (2005) Changes toward earlier streamflow timing across western North America. Journal of Climate 18: 1136–1155.
  9. VerWey BJ, Kaylor MJ, Garcia TS, Warren DR (2018) Effects of Severe Drought on Summer Abundance, Growth, and Movement of Cutthroat Trout in a Western Oregon Headwater Stream. Northwestern Naturalist 99(3): 209-221.
  10. Harvey BC, Nakamoto RJ, White JL (2006) Reduced streamflow lowers dry-season growth of Rainbow Trout in a small stream. Transactions of the American Fisheries Society 135: 998–1005.
  11. Uthe P, Al-Chokhachy R, Shepard BB, Zale AV, Kersher JL (2019) Effects of Climate-Related Stream Factors on Patterns of Individual Summer Growth of Cutthroat Trout. Transactions of the American Fisheries Society 148: 21-34. doi: 10.1002/tafs.10106
  12. Roberts JJ, Fausch KD, Hooten MB, Peterson DP (2017) Nonnative Trout Invasions Combined with Climate Change Threaten Persistence of Isolated Cutthroat Trout Populations in the Southern Rocky Mountains. North American Journal of Fisheries Management 37: 314-325. doi: 10.1080/02755947.2016.1264507

*All images are the property of J.Evans

Blinded by the Light

Credit: Christian Giese

Light pollution is a commonly occurring issue in urban areas and other areas of high human activity. An excess of artificial lighting, while it may be useful to us, has the potentially to negatively impact nocturnal or other light sensitive species. Bats make up the second largest order of mammals on earth but roughly a quarter of these species are threatened which may in no small part be due to the disruptive effects of light pollution (Stone et al 2015). However, some bat species have readily colonized city habitats so it is important to understand how artificial light may negatively impact these species

Exposure to artificial lights can significantly alter foraging and travel patterns in many bat species. Bats have been shown to alter their travel routes in order to avoid sources of artificial light (Kuijper et al 2008, Stone et al 2008). The avoidance of light sources by many bats effectively fragments their habitat. Additionally, alternate routes that bats may take in response to light pollution may be longer or impose a greater risk, such as predation, on bat species. Bats will also avoid foraging areas that are too well lit which can limit food availability and force bats to travel to farther or lower quality foraging sites (Polak et al 2011). However, not all bats avoid artificial light sources. Some species of insectivorous bat are actually drawn to artificial light sources as these sites often contain a higher density of insects (Schoeman 2015). These illuminated foraging sites are not without risk though as attraction to light sources put bats at a greater risk of being struck by vehicles and light levels have also been shown to interfere with a bat’s ability to avoid obstacles (McGuire and Fenton 2010, Stone et al 2015).

Bats can also experience light pollution at the roost. The presence of artificial light near bat roosts can artificially extend daylight conditions delaying the emergence of bats in the evening and reducing the time available for them to forage (Stone et al 2009). This may explain in part why light pollution around the roost sites has been linked to reduced growth rates in young bats (Stone et al 2015). Light pollution around roosts may also deter returning bats from re-entering the roost and in extreme cases may result in abandonment of the roost (Stone et al 2015).

While light pollution can have a number of impacts on bats the negative effects do not just stop there. Many bats provide essential ecosystem services which may be disrupted by artificial lighting. Fruit-eating bats in the tropics are seed dispersers and play an important role in the recolonization of abandoned farmland. However, the presence of artificial light near these abandon fields can cause fruit-eating bats to avoid the area which can delay the introduction of native plant life back into the area (Lewanzik and Voigt 2014). This is just one example of the cascading effects that light pollution can cause by disturbing bat species, but bats provide a range of important ecological services such as pollinating plants and regulating insect populations. In order to prevent the loss of these services and species that provide them measures need to be taken in order to reduce the impact of light pollution on bats. Some recent work has found that tree cover can help to mitigate the negative effects of light pollution so the expansion of green spaces within urban centers may be one viable method of conservation (Straka et al 2019). Other strategies such as limiting the number of streetlights near bat habitats or using lower intensity bulbs may also be viable, but more work still needs to be done to fully understand how light pollution impacts bat species and how it can be managed.



Kuijper DPJ, Schut J, van Dullemen D, Toorman H, Goossens N, Ouwehand J, Limpens HJGA (2008) Experimental evidence of light disturbance along the commuting routes of ponds bats (Myotis dasycneme). Lutra 51:37-49.

Lewanzik D, Voigt CC (2014) Artificial light puts ecosystem services of frugivorous bats at risk. Journal of Applied Ecology 51:388-394.

McGuire LP, Fenton MB (2010) Hitting the Wall: Light Affects the Obstacle Avoidance Ability of Free-Flying Little Brown Bats (Myotis lucifugus). Acta Chiropterologica 12:247-250.

Polak T, Korine C, Yair S, Holderied MW (2011) Differential effects of artificial lighting on flight and foraging behaviour of sympatric bat species. Journal of Zoology 285:21-27.

Schoeman MC (2015) Light pollution at stadiums favors urban exploiter bats. Animal Conservation 19:120-130.

Stone EL, Jones G, Harris S (2009) Street Lighting Disturbs Commuting Bats. Current Biology 19:1123-1127.

Stone EL, Harris S, Jones G (2015) Impacts of artificial lighting on bats: a review of challenges and solutions. Mammalian Biology 80:213-219.

Straka TM, Wolf M, Gras P, Buchholz S, Voigt CC (2019) Tree Cover Mediates the Effect of Artificial Light on Urban Bats. Frontiers in Ecology and Evolution

Where does all that waste go?! The effect of estrogens and endocrine-disrupting compounds on fish reproduction

Have you ever wondered where things go after they’re flushed down the toilet? If you’re a kindred spirit, maybe you’ve written several term papers on waste water treatment. More likely, you’ve probably truly never given the process much thought or aren’t exactly dying to admit that you’ve spent time pondering the transport and fate of toilet waste. One of the most common misconceptions about biological waste is that it goes to a wastewater treatment plant and every single contaminant in the waste is filtered out. Unfortunately, this isn’t always the case. In humans, chemicals that contain estrogen (e.g., birth control) are metabolized in the body, however, a small amount of un-metabolized estrogens are excreted in urine or feces (Racz & Goel 2010). While it is true that wastewater treatment plants are able to remove almost one hundred percent of the estrogen in urine and feces, some cities still have combined sewers (i.e., underground pipes that store excess waste and precipitation until it is treated) in which it is possible for untreated wastewater to overflow into nearby waterways during periods of high precipitation. This means that untreated waste containing estrogen compounds enters streams directly (EPA 2017; Racz & Goel 2010). Unfortunately, these estrogens can have detrimental physiological impacts on fish reproduction. Additionally, naturally-occurring chemicals and man-made chemicals such as pesticides enter streams as runoff from agricultural fields and contain endocrine-disrupting compounds (EDCs) that mimic sex hormones, having similar negative physiological impacts on fish reproduction (Arcand-Hoy & Benson 1998).

Untreated wastewater being deposited into a stream from a combined sewer overflow (CSO) after a period of heavy precipitation (Image drawn by Krystal Pocock).

A fish being affected by the untreated wastewater from the combined sewer overflow (Image drawn by Krystal Pocock).


Endocrine-disrupting compounds can have detrimental effects on reproductive development during early life stages in fishes (e.g., larval, juvenile) (Arcand-Hoy & Benson 1998). Exposure to EDCs at the larval or juvenile stage can interfere with the determination (i.e., development) of sex organs and can even cause hermaphroditism, which occurs when an individual organism has both female and male sex organs (Arcand-Hoy & Benson 1998; Mill et al., 2011). While chemically-induced hermaphroditism does not always cause reproductive decline in fishes, there is evidence that hermaphroditism reduces the hatchability of eggs, the number of successful eggs, and swimming success (Hill Jr. and Janz 2003). Additionally, exposure to EDCs during the larval or juvenile stages can cause fishes to reach reproductive maturity earlier or later than expected, which can result in reduced lifetime reproductive output. Furthermore, male fishes exposed to estrogens or endocrine disrupting compounds later in life can experience feminization of the testes, which can negatively interfere with reproductive efforts. The processes by which these transformations occur in the body are complex, however, they have serious implications for fish reproduction (Arcand-Hoy & Benson 1998). In the future, proper management of wastewater and the phasing-out of CSO systems can help minimize the amount of estrogen-laden wastewater entering streams. Applying fertilizers and pesticides when there is little to no chance of precipitation will also help minimize the contamination of streams with endocrine-disrupting compounds.

Arcand-Hoy LD, Benson WH (1998) Fish reproduction: an ecologically relevant indicator of endocrine disruption. Environmental Toxicology and Chemistry 17(1):49-57.

Hill Jr. RL, Janz DM (2003) Developmental estrogenic exposure in zebrafish (Danio rerio): I. Effects on sex ratio and breeding success. Aquatic Toxcology 63(4):417-429.

Racz L, Goel K (2010) Fate and removal of estrogens in municipal wastewater. Journal of Environmental Monitoring 12:58-70.

EPA (2017) What are combined sewer overflows (CSOs)? Webpage. Retrieved from on 22 April 2019.

The physiological effects of angling stress on fishes

Bobbers in a pond (Image taken by Krystal Pocock)

Have you ever been catch-and-release fishing and found yourself wondering if the fishes you catch go on to live healthy, productive lives after you toss them back? Maybe you’ve contemplated how long it takes for the puncture mark left by your hook to heal or how removing fishes from the water for short periods of time affects them. If these thoughts have ever crossed your mind, you’re not alone. In fact, there has been a lot of research completed on the subject.  Studies have shown that catch-and-release fishing is one of the biggest sources of stress to game fishes (Meka & Mc Cormick 2004; Twardel et al., 2018). When a fish is exposed to extremely high amounts of stress, hormones that are produced by the stress response can be fatal. Luckily, complex processes in the body work to regulate hormone levels and are typically able to quickly return the body to its normal state before stress hormones are fatal. More often, angling-induced stress tends to have what is called sub-lethal effects, which cause other important functions in the body to slow down or stop altogether. As such, recovery from elevated stress levels in captured fishes requires a lot of energy and often causes energy to be taken away from other important functions like reproduction and survival to compensate (Meka & McCormick 2004).

Imagine you’re out fishing on a beautiful day and you’ve finally gotten a bite. What happens next can directly contribute to the amount of physiological stress that a fish experiences during catch-and-release angling. There are three major sources of stress and potential contributors to mortality that fish are exposed to while being captured: exhaustion from the reeling and landing process (i.e., the amount of time that it takes an angler to successfully remove the fish from the water), injuries from hooks, and air exposure (Meka & McCormick 2004). Just like humans, fishes can become extremely tired from too much exercise and the exercise fish get while an angler attempts to reel them in can be physically exhausting. If a fish has not fully recovered from reeling and landing exercise once they are returned to the water or quickly thereafter, they can easily be eaten by a larger fish (Meka & McCormick 2004; Twardek et al., 2018). Additionally, injuries obtained from hooks during the angling process can be deep and cause a lot of bleeding, which can cause death or infection (Meka & McCormick 2004). Furthermore, the longer that a fish is out of water and exposed to air, the more stressed they become (Meka & McCormick 2004; Ferguson & Tufts 1992). When fish cannot pass water over their gills to obtain oxygen, they start breathing very quickly, which causes a lot of carbon dioxide to accumulate in their bodies. Once there is excess carbon dioxide in their blood, fishes have a hard time retaining oxygen and can die if they aren’t able to obtain enough oxygen (Ferguson & Tufts 1992). Thus, exhaustion, injuries, and air exposure all have the potential to cause mortality, however, this is rare and typically happens under extreme circumstances.

More often, angling stressors such as those discussed above have sublethal impacts on fishes due to stress causing disruption of other important functions to compensate for the energy needed to recover (Meka & McCormick 2004). For example, sublethal impacts of stress associated with catch-and-release fishing are reduced reproductive output, reduced growth and time spent foraging, reduced immune response, and altered migration behaviors (Meka & McCormick 2004; Twardek et al., 2018). Additionally, research has shown that fish become more stressed as landing time, air exposure, and the time it takes to remove a hook increases. Fish are also more stressed when water temperatures are high (Meka & McCormick 2004). Furthermore, the longer fish are exposed to air, the longer it takes them to move again once they’ve been tossed back into the water (Twardek et al., 2018). One study even suggested that when fish are stressed, it can take as long as a day for stress levels to return to normal (Meka & McCormick 2004)!

Minimizing the time a fish is held outside of water, exposed to air, can greatly minimize the amount of capture stress (Meka & McCormick 2004; Photograph taken by Krystal Pocock).

Luckily, there are some best practices that anglers can utilize to reduce the amount of physiological stress that fish experience from catch-and release fishing. For example, using barbless hooks reduces injury during capture and use of natural bait can reduce injuries obtained from fish swallowing large, plastic lures (Brownscombe et al., 2017). Taking care to reduce the amount of time it takes to reel the fish in and time spent handling the fish out of water can also reduce stress. Finally, using specialized tools to remove hooks or cutting and leaving the hook in if it is found in a sensitive spot can reduce stress associated with angling (Brownscombe et al., 2017). Using smart angling practices can reduce stress and mortality associated with catch-and-release fishing and help ensure that there are future populations of fishes for years to come.

Brownscombe JW, Danylchuk AJ, Chapman JM, Gutowsky LFG, Cooke SJ (2017) Best practices for catch-and-release recreational fisheries- angling tools and tactics. Fisheries Research 186:693-705.

Ferguson RA, Tufts BL (1992) Physiological effects of brief air exposure in exhaustively exercised rainbow trout (Oncorhynchus mykiss): Implications for “catch and release” fisheries. Canadian Journal of Fisheries and Aquatic Sciences 49:1157-1162.

Meka JM, McCormick SD (2005) Physiological response of wild rainbow trout to angling: impact of angling on duration, fish size, body condition, and temperature. Fisheries Research 72(2,3): 311-322.

Twardek WM, Gagne TO, Elmer LK, Cooke SJ, Beere MC, Danylchuk AJ (2018) Consequences of catch-and-release angling on the physiology, behavior, and survival of wild steelhead Oncorhynchusm mykiss in the Bulkley River, British Columbia. Fisheries Research 206:235-246.

Hey siri, can you dim the lights?: artificial lighting and aquatic ecosystems

The vast majority of the Earth’s ecosystem’s have been influenced and modified by human activities, especially freshwater ecosystems. As the preferred site for human activity and development, these ecosystems often accumulate the effects of activities within their catchments (Perkins et al., 2011) Research focused on the effects of human-induced chemical pollution, alteration to nutrient cycles and natural flows, invasive species, urbanization, and loss of riparian zones have dominated our understanding how we are impacting freshwater ecosystems for the past 40-50 years (Perkins et al., 2011). Conversely, the influence of artificial lighting on freshwater ecosystems has long been overlooked. An estimated 67% of Americans and 20% of people globally live in locations in which the Milky Way is no longer visible as a result of interference from artificial light sources (Perkins et al., 2011). While freshwater ecosystems only cover about 0.8% of the Earth’s surface, approximately 9.5% of all animal’s species and one-third of all vertebrates call these systems home (Perkins et al., 2011) Studies aimed at identifying how these ecosystems, and the organisms living in them, are influenced by artificial lighting is a growing priority among science. Improving our knowledge on how artificial light may modify community structure and ecosystem function within freshwater ecosystems could help guide our management and conservation strategies in the near future.

Figure 1: Artificial lighting in the United States as seen from space.

One way in which aquatic ecosystems are impacted by artificial light is the disturbance in natural dispersal tendencies of both aquatic insects and certain species of fish. Aquatic insects move throughout aquatic ecosystems and the adjacent terrestrial environments as they transition from their larval to adult stages, providing essential nutrients and acting as a prey source in streams, rivers, and lakes (Meyer & Sullivan, 2013; Perkin et al., 2011). Perkin et al. (2011) identified three main ways in which artificial light may impact aquatic insect dispersal. The fist, know as the fixation of captivity effects, involves emergent adult insects located near lights flying directly to them. In this case the insects may be killed directly by the lights, or mortality may occur when these insects are unable to leave and die from exhaustion, predation, or heat. The second mechanism is termed the crash barrier effect, in which insect dispersal and migration are impeded by artificial light sources. In this case, nocturnal aquatic insects may actively avoid areas in which artificial light alters the visual environment, eliminating dispersal of these important aquatic subsidies. Lastly, insects from a large area may be attracted to a nearby light source, altering movement and predator-prey relationships in both aquatic and terrestrial systems found near freshwater bodies of water. However, these impacts are only hypothesized. Carefully designed research and experiments are needed to determine how these mechanisms may actually play out in disrupting aquatic insect dispersal. For example, studies have identified elevated artificial lighting as a means of diminishing invertebrate drift rates, while extending or improving fish foraging (Meyer & Sullivan, 2013). This gives a distinct advantage to invertivore fishes, potentially reducing population sizes of aquatic invertebrates through predation-induced mortality (Meyer & Sullivan, 2013). However, in some locations, attraction of terrestrial insects to the water as a result of increased reflection of light off the surface of water has been shown to increase terrestrial prey subsidies for stream fish and release predation pressure on benthic insects (Meyer & Sullivan, 2013). Incorporating these hypothesized impacts of artificial lighting on dispersal and movement of aquatic insects with studies of ecosystem functioning will allow us to tease apart just how how big of an impact enhanced lighting could have on our streams, rivers, and lakes.

Figure 2: Solid line shows natural light provided by the moon in a temperate region. Dashed line is light level measured in the Berlin on clear night. Dotted line is the light level in the center of Berlin on cloudy night. Figure depicts the ability of artificial lighting to eliminate natural variation in light levels (Perkin et al., 2011).

               Fish, specifically those in which rely upon lighting cycles to cue migration, dispersal, or feeding, could also be disrupted by artificial lighting (Perkin et al., 2011). Studies have shown altered migratory timing of Pacific and Atlantic salmon species in the presence of artificial lighting. While many of these species typically wait until sunset or dusk to move throughout their systems, when exposed to artificial light, migration started at random times, impacting arrival times to breeding grounds (Perkin et al., 2011; Tabor et al., 2012). Differing light conditions may also impact predation of fish species. Many species of fish often wait to forge until the cover of night in order to avoid predation (Perkin et al., 2011). In systems where artificial lighting may eliminate darkness altogether, this protective cover may vanish. Tabor et al. (2012) found that predation mortality of sockeye salmon increased by 40% when they were exposed to artificial lighting. Impacts of artificial light on migration patterns and predation could prove to be extremely detrimental to species that are already struggling to survive in their altered natural environments.

            Further enhancing our current knowledge beyond hypothesized impacts and single taxon studies by incorporating research addressing the influence of artificial lighting on food webs and ecosystem functioning will be crucial in conserving biodiversity in freshwater ecosystems. Of the many impacts that humans have on these ecosystems, artificial lighting lighting could modify dispersion and predation of both aquatic organisms and the terrestrial organisms that rely upon aquatic energy subsides to thrive in their natural environments (Meyer & Sullivan, 2013; Perkin et al., 2011). These influences could alter future population abundances and genetics within these ecosystems, especially when compounded with other human-induced stressors such flow modification or chemical pollution (Perkin et al., 2011). The need for management of these systems in the wake of human disturbance is essential and continued effort from all stakeholders involved in the use of freshwater ecosystems must be achieved in order to minimize our impacts on these environments.



Meyer LA, Sullivan SMP (2013) Bright light, big city: influences of ecological light pollution on reciprocal stream-riparian invertebrate fluxes. Ecological Applications 23(6): 1322-1330.

Perkin EK, Holker F, Richardson JS, Sadler JP, Wolter C, Tockner K (2011) The influence of artificial lighting on stream and riparian ecosystems: questions, challenges, and perspectives. Ecosphere 2(11): doi.10.1890/ES11-00241.11.

Tabor RA, Brown GS, Luiting VT (2004) The effect of light intensity on sockeye salmon fry migratory behavior and predation by cottids in the Cedar River, Washington. North American Journal of Fisheries Management 24: 128-145.

Nervous Laughter: The Mixed Effects of Human Activity on Spotted Hyenas

Credit: Heather Paul, CC by 2.0

Human activity in an area whether it be urban development, farming, or tourism can have significant impacts on wild species. Among the species most effected by human activity are large carnivores. Large carnivores struggle to coexist with humans because they often face persecution for the perceived threat they present to livestock and residence. Additionally, large carnivores can often occupy large territories and human activity in an area can limit the abundance of acceptable territory. However, not all carnivores suffer equally in the face of human encroachment some species like the spotted hyena (Crocuta crocuta) have been shown to adapt to and even thrive in areas of human activity.

The spotted hyena is often portrayed in popular media as a villainous scavenger while in actuality it is a top predator and keystone species in many of the environments it inhabits. Spotted hyenas are also highly social forming matriarchal clans of up to 80 individuals. So how then does a large predator living in such large social groups manage to persist in human disturbed areas? It may in part be due to the negative effects that human activity has on other carnivore species. The African lion (Panthera leo) is another top predator and the main competitor of the spotted hyena. Lions and hyenas will often steal each other’s kills as well as killing isolated or juvenile individuals that they come across. However, the African lion has shown striking declines in population size in response to human disturbance (Green et al 2017). The decline of their main competitor and predator has allowed hyena populations to grow rapidly in human disturbed areas (Green et al 2017). However, this fact alone is not sufficient to explain the success of the spotted hyena. Why does it not suffer the same fate as its rival? Well the answer may be in part due to the hyena’s iron stomach. Although hyenas will hunt and kill the majority of the food, they eat they have shown a remarkable ability to consume almost any organic material they can scavenge (Yirga et al 2012). In the wild hyenas consume even the bones, hair, and hooves of their prey, and will consume putrefied or diseased carcasses they encounter. While near human settlements hyenas will eat a wide variety of garbage from offal, to scraps, to even feces (Yirga et al 2012). The incredible flexibility of their diet may allow hyenas to utilize anthropogenic food sources that other carnivore species are unable to consume.

However, not all aspects of human disturbance are so beneficial to spotted hyenas. Although hyenas can consume putrefied food with seemingly few consequences, they are still vulnerable to disease introduced as a result of human activity. The introduction of canine distemper virus to Africa led to an outbreak in the 90s that decimated lion and hyena populations in the Serengeti (Marescot et al 2018). However, while lion populations recovered in a matter of years spotted hyena populations took nearly five times as long to recover (Marescot et al 2018). This is due to the low reproductive rate of hyenas. Hyena reproduction is a tricky business as females are very masculinized and possess a pseudo-penis through which copulation and birthing must occur. This makes reproduction stressful and painful for hyenas and human disturbances have been shown to increase levels of stress hormones in hyenas which may make reproduction even more taxing (Van Meter et al 2009). Finally, hyenas only tend to have 1 or 2 cubs in a litter and mothers will nurse their cubs for up to a year and a half, much longer than other carnivore species. All these factors taken together mean that hyenas are slow to recover from populations declines.  In addition to their slow rate of recovery, human disturbance may also make hyenas more vulnerable to future threats. A study by Belton et al (2017) found that hyena clans inhabiting areas of higher human activity had lower connectivity within their social groups than clans in less disturbed areas. This lower connectivity may make clans more vulnerable to future disturbances (Belton et al 2017).

While hyenas may benefit from human activity they do not do so without risk. Although populations may be thriving locally the global population of spotted hyenas is on the decline due to habitat loss and persecution by local peoples. While spotted hyenas are currently listed as least concern by the IUCN their low reproductive rate could potentially make recovery difficult if their population were to decline to much. Therefore, it is important to understand how human activities may affect the long-term trends of this population and to change public perception of spotted hyenas in order to garner support for conservation efforts.

Credit: Oliver Honer


Belton LE, Cameron EZ, Dalerum F (2017) Social networks of spotted hyaenas in areas of contrasting human activity and infrastructure. Animal Behaviour 135:13-23.

Green DS, Johnson-Ulrich L, Couraud HE, Holekamp KE (2018) Anthropogenic disturbance induces opposing population trends in spotted hyenas and African lions. Biodiversity and Conservation 27:871-889.

Marescot L, Benhaiem S, Gimenez O, Hofer H, Lebreton JD, Olarte-Castillo XA, Kramer-Schadt S, East ML (2018) Social status mediates the fitness costs of infection with canine distemper virus in Serengeti spotted hyenas. Functional Ecology 32:1237.

Van Meter PE, French JA, Dloniak SM, Watts HE, Kolowski JM, Holekamp KE (2009) Fecal glucocorticoids reflect socio-ecological and anthropogenic stressors in the lives of wild spotted hyenas. Hormones and Behavior 55:329-337.

Yirga G, De Iongh HH, Leirs H, Gebrihiwot K, Deckers J, Bauer H (2012) Adaptability of large carnivores to changing anthropogenic food sources: diet change of spotted hyena (Crocuta crocuta) during Christian fasting period in northern Ethiopia. Journal of Animal Ecology 81:1052-1055.

Save the Butterflies

Over the last 20 years, the population of Monarch Butterflies (Danaus Plexippus) in North America has declined by approximately 90% (Moore, 2019). This reason for this decline has been identified to be the loss of their wintering grounds in Mexico and the increased used of herbicide, which is toxic weed control, in the Midwest. Monarch butterflies are herbivorous and are very fond of milkweeds, which are a type of toxic flowering plant which is native to North America. To be clear, the toxin has no effect on the butterfly’s but, can be harmful to humans and grazing animals. Monarch butterflies are partial migrants, meaning not all will migrate. Eastern populations migrate over to high elevation forests in Mexico, and western monarchs’ winter in the trees on the coast of California (Malcolm, 2018).

Migratory behavior in these butterflies is driven by one factor, the abundance of milkweed host plants. With North American agriculture growing and adopting GM crops the fields over which the butterflies migrate have become a toxic warzone. The soil around Genetically modified crops are host to a bacterium called Bacillus thuringiensis which is a natural form of pest control. The bacteria consist of a spore and a protein crystal within the spore that is toxic to insects and other organisms deemed “pests”. It was concluded that the risk this bacteria posed to monarchs was negligible because the exposure probability was too low in the event of pollen specific expression but, this assessment was never put into context of the spatial and temporal variations in the life history of these butterflies (Malcolm, 2018).

There has been no research on the bacterium and mortality of butterflies but, given constant exposure due to milkweed being very common in the Midwest region, there could be consequences in the future. Many other pesticides are also toxic to these organisms. Approved barrier treatments for mosquito control with synthetic pyrethroid permethrin that was sprayed on milkweed in Minnesota showed that 95% of monarch larvae were killed between 50% and 0.1% dilutions (Malcolm, 2018). With the temperature shifts due to global warming, it is expected to see milkweed shifts northward in North America. With this shift northward monarchs will now have to migrate across less suitable habitats in their searches.



Monarch Butterfly on swamp milkweed


In Mexico, forests cover, and the contiguous canopy is necessary to modify the temperature extreme where the monarchs overwinter. The closed canopy is thought to be important to minimize lipid usage during overwintering, so monarchs do not waste these lipids from unnecessary thermoregulation (Malcolm, 2018) The population of butterflies that winter in Mexico is protected within the “Monarch Butterfly Biosphere Reserve” located in Trans – Mexican Volcanic Belt. Despite logging bans in high elevation fir forests in Mexico, between the years 1986 and 2012, 4,300 hectares of protected forests have been altered from human activities such as logging both large and small scale leaving less than 10% of the canopy intact (Malcolm, 2018). In California, Monarchs winter in over 400 locations within the state and are seen to roost on introduced blue gum eucalyptus, Monterey pine, red gum eucalyptus, and Monterey cypress.

Monarch populations in both the east and west have declined and this only seems to be accelerating. The declines in milkweed resources across the Midwest and specifically around fields where herbicides are used are not helping the problem. Human activities in Mexico and California are also directly linked to the decline of these butterflies despite conservation efforts.  Conserving their migration is only possible if migratory conditions are fully understood and If something isn’t done these butterflies will disappear forever and other species will soon follow.



Malcolm, S. B. (2018). Anthropogenic Impacts on mortality and population viability of the monarch butterfly. annual review of entomology, 277-302.

Moore, A. (2019, april 9). Fall of the monarchs. Retrieved from