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Blog - Bioenergetics for management and conservation
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    • Avoid predation or starvation: which strategy maximize rainbow trout juvenile survival?by Léa Bulon and Mylène Jury

      Published by Charlotte Recapet the September 9, 2019 on 7:12 AM

      Natural populations are increasingly exposed to a range of biotic stressors, such as predators, and abiotic environmental stressors such as environmental variations (seasonality -  Wingfield, 2013) or chemical pollution (Fisher et al., 2013). (We could think to grandma Margaret who throws away her bleach bucket directly into the river or grandpa George who loses all his plastic lures into the lack). The first year of life is complicated for all organisms because they are more sensitive to those kinds of stressors and their survival is highly impacted.

      In temperate zones, fry are subjected to high predation during the growing season and a nutritive resource deficit during winter. This is why juveniles need to find the best way to maximize their survival and make population viability durable through the time. Predators prefers a fry-up of little fish, it is why predation mortality is higher in small fish than large (Parkinson et al., 2004). However, growth itself may impose a significant energy mobilization which can drive trade-offs between growth and other metabolic processes. If you are really interested by the topic but not by fish, we recommend you to look at the article written by Mcleod et al., 2008, about birds.

      During winter, the metabolism needs some fuels like lipids and proteins to work because resources are often limited. Production of energy storage is energetically expensive, and energy contributes less to increasing their body-size.

      Is it better to allocate their energy into the growing season to avoid predators or into the lipid storage to survive during winter?

      Stephanie Morgensen and John R. Post, scientists from Canada, are been interested in this process. They led an experiment with juvenile rainbow trout (Oncorhynchus mykiss). They developed a mathematical model to determine the energy allocation strategy maximizing the first-year survival of rainbow trout.

      Rainbow trout juveniles are sampled from two sets of lakes in British Columbia in Canada. The first site is located on the Bonaparte Plateau and it corresponds to highly and cold lakes. The second site is located near the town of Merritt and it corresponds to low altitude and warm lakes. In warm lakes, the winter season is shorter than in cold lakes and there are more resources for juveniles.

      They found that juvenile growth is different between the two kinds of lakes: fish from the cold lakes growth more than fish from warm lakes. As we said before, production of lipid storage consumes more energy than growth and resources are more abundant in warm lakes. It is why, fish from warm lakes are more able to stock and fish from cold lakes, to growth. However, fish do not follow only one strategy. Indeed, they grow during the first part of the non-winter season and then they put their energy into the lipid storage to survive during winter. This switch between the strategies is controlled by environmental conditions and determined trout survival during winter.

      The juvenile survival trade-off influencing by environmental conditions such as temperature and resource availability would be important to understand population viability with the evolution of environmental conditions. Rainbow trout has been introduced into many streams and water bodies for recreational fishing because they are easy to catch and quite combative (fishing federation). They constitute an important economic interest it is why, it is one of the most studied species by biologists (INRA). This may lead to management measures to improve pisciculture conditions or to instore fishing quotas and a minimum size of capture. It could be also interesting to know if energy allocation strategies affect physiological processes like growth or reproduction.

      And you what would you choose to survive during winter?

      Cited study: Mogensen, S., & Post, J. R. (2011). Energy allocation strategy modifies growth–survival trade-offs in juvenile fish across ecological and environmental gradients. Oecologia, 168(4), 923–933. doi:10.1007/s00442-011-2164-0

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