Freshwater fishes are becoming increasingly vulnerable to human activities. Of the many threats they face (climate change, pollution, invasive species, over-fishing, etc.), habitat loss is thought to be one of the most significant factors in fish biodiversity loss and extinction. Understanding how fish respond to habitat loss is an important step in protecting those species that are most vulnerable. Scientists are currently studying different attributes that are thought to impact how fish respond to habitat loss. Adam S. van der Lee and Martin A. Koops, scientists working for Fisheries and Oceans Canada in the Great Lakes Laboratory for Fisheries and Aquatic Sciences, have used fish population models to predict how body size influences the ability of a fish to cope with habitat loss. van der Lee and Koops used estimates of fish growth, survival, and reproductive success to predict fish population sizes at different life stages. A simplified life cycle consisting of 3 stages (1 – eggs to age one; 2 – age 1 to age of maturity; and 3 – adults) was used. They were able to forecast how loss of fish habitat might affect the productivity (the number of surviving offspring per reproducing female) and growth of fish populations based on body length at each life stage.
According to their model predictions, it is likely that small-bodied fishes will be more susceptible to the effects of habitat loss. Small fishes are expected to show declines in population growth as habitat space decreases. Small fishes were also projected to exhibit higher losses in productivity with habitat loss. Larger fishes, while more equipped to withstand the impacts of habitat loss on population growth and productivity, were much more susceptible to losses from fishing mortality and are expected to display decreased catch rates at higher levels of habitat loss.
This work brings to light important considerations for future fisheries management and monitoring. Since large fishes are more likely to exhibit declining catch rates, at first glance it may seem as though large fishes are more susceptible to habitat loss. However, large fishes may actually be more resilient than small fishes to population declines and lowered reproductive success associated with habitat loss. Catch rates of small fishes on the other hand, are more likely to remain stable even as their habitat, and consequently population size and reproductive success, declines. The take home message: when designing monitoring programs for species at risk, if habitat loss is of concern, it might be prudent to assess small fishes first, even when catch rates would tell us to do the opposite.
van der Lee, A.S. and M.A. Koops. 2015. Are small fishes more sensitive to habitat loss? A generic size-based model. Canadian Journal of Fisheries and Aquatic Sciences, 72:1-11. dx.doi.org/10.1139/cjfas-2015-0026.