Life as a salmon is tough. Salmon travel long distances from stream to sea and back again, meeting obstacles every step of the way. As climate change alters their habitats, their journeys are expected to become even more challenging. Droughts are projected to intensify, which may make it harder for Northwest streams to rear salmon and other stream-dwelling fish. As natural resource managers contend with this reality in their work to protect at-risk species like salmon, there is increasing interest in species management interventions. New NW CASC research set out to evaluate a controversial strategy called “fish rescue”, which has potential to help fish cope with seasonal stream drying, but until now, has been largely unexplored as a climate adaptation strategy.
Drought may cause some seasonal streams in the Northwest to dry up
In the Northwest, many watersheds receive their water from rainfall (as opposed to groundwater), making them especially sensitive to drought. The projected intensification of drought may cause some seasonal streams that provide rearing habitat for at-risk species like the endangered coho salmon to experience longer and more severe periods of drying, resulting in stream fragmentation. This is prompting managers to consider intervention strategies such as “fish rescue” to help salmonid populations cope with increasing stream drying expected under climate change.
Fish rescue is a potential strategy for helping fish deal with drought
“Fish rescue” involves capturing fish during times of seasonal environmental stress and rearing them in captivity until habitat conditions improve and they can be released back to their stream. In contrast with other management interventions like fish salvage, intended to protect fish from immediate threats that may last only a few days, or assisted migration, a longer-term strategy designed to expand a species’ range, fish rescue occurs at an intermediate timescale (monthly or annual) that better matches the seasonality of environmental stressors that stream-dwelling fish are experiencing.
To evaluate the effectiveness of fish rescue, which has not been well studied until now, NW CASC researchers developed a coho life cycle model and a corresponding interactive web app that sheds light on the benefits and risks of fish rescue. This simulation model compares different scenarios of fish rescue to understand their potential effects on coho salmon population dynamics. Rescue scenarios vary based on the quantities of rescued fish, time in captivity, drought severity and other factors. NW CASC researchers used this model to evaluate an existing fish rescue program in southwestern Washington.
Considering context and tradeoffs is key
Research results show both benefits and risks of fish rescue, depending on the context. When young fish are held in captivity for a full year, the model suggests that fish rescue is capable of increasing the abundance of returning adult fish, thereby lowering extinction risk. However, if fish are only held for a summer and there is limited winter habitat, fish rescue has the potential to decrease the abundance of adults, thereby increasing extinction risk. The interactive R Shiny app (freely available online through the link below) produced through this research allows users to populate the life cycle model with data and parameters for specific fish populations of interest. This can help managers better understand how populations may respond to fish rescue programs.
Like any management decision, deciding whether to employ fish rescue requires understanding the context and weighing the tradeoffs. On the one hand, fish rescue may increase fish abundance under certain conditions and help save fish from temporary threats and poor conditions. On the other hand, fish rescue could negatively affect fish survival and may not help them adapt to an altered future.