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 https://commons.wikimedia.org/wiki/File:Lifecycle_of_salmon.gif

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) https://commons.wikimedia.org/wiki/File:Sockeye_salmon_(31376019895).jpg

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.

REFERENCES:

  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.

 

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