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.