Climate warming and ocean competition alter Pacific Salmon life histories

Species with complex life cycles, such as Pacific salmon, may be particularly sensitive to global change because each life stage is influenced by a unique set of natural and anthropogenic stressors. Even in places where habitat is largely intact and salmon stocks are highly productive, populations are still vulnerable to a collection of large-scale stressors acting across life-history stages. Effective conservation of salmon populations and continued sustainable harvesting will require an understanding of how these stressors interact to affect population structure and productivity. Using age composition data from sockeye salmon returning to seven major rivers systems in Bristol Bay, we quantifed whether rapid climate warming, changing ocean conditions, competition in freshwater and the ocean, and fishery exploitation are influencing the duration of freshwater residency and age at maturity in the ocean—two critical life-history transitions for this species. We applied multivariate time-series analysis to identify changes in age among populations and quantify the effects of human and natural drivers.

Over the past five decades there have been substantial shifts in the age composition of Bristol Bay sockeye salmon, and these changes are coherent across the seven major river systems of this region. The duration of freshwater residency has decreased as lakes have warmed and become more productive. The influence of these changes on freshwater age, when combined with increased competition in the ocean, have delayed maturation and increased the proportion of sockeye spending 3 years in the ocean before returning to spawn in their natal rivers. These changes in life-history traits for sockeye salmon have the potential to influence age diversity, population productivity and fish size, all of which have implications for a highly valuable and reliable commercial fishery.

Cline et al. 2019.  Effects of warming climate and competition in the ocean for life-histories of Pacific salmon. Nature Ecology and Evolution. 3: 935-942. .

Black Lake: South Fork Alec River and Spit, with North Fork Alec in the background

Effects of simultaneous climate change and geomorphic evolution on the thermal characteristics of Black Lake, AK

Black Lake, in the upper Chignik watershed, has undergone rapid natural geomorphic evolution resulting in a 2m drop in lake level and a 40% decrease in lake volume since 1960.  In short, the primary inlet tributary to Black Lake, the Alec River, has migrated closer to the outlet of the lake and no longer feeds the main body of the lake while concurrent channel shifts in the West Fork River have resulted in the erosion of a natural sill that historically maintained Black Lake storage capacity. If these natural geomorphic changes continue, models predict that Black Lake will lose 80% of it’s 1960s storage capacity by 2100.

This landscape evolution, occurring simultaneously with rapid climate warming, has also coincided with, and perhaps driven, a shift in the life history of Black Lake juvenile sockeye salmon. Black Lake sockeye fry typically spend a full year rearing in Black Lake before migrating to Chignik Lake. Because of the loss of rearing capacity, many juveniles now migrate early and those fish have been shown to be in poor body condition relative to fish that remain in Black Lake. Concerns about diminishing harvests and lack of rearing capacity for sockeye salmon in Black Lake are the motivation for developing restoration strategies to reestablish historical lake water storage capacity, decrease winter oxygen stress, and reduce summer thermal stress. An outlet control structure (dam) has been proposed to stabilize the Black River sill and restore Black Lake storage capacity, but its effects on Black Lake habitat quality have not been evaluated (i.e. even with a dam, will Black Lake be too warm for juvenile sockeye under future warming scenarios?). 

We used a hydrodynamics model to assess the consequences of climate warming and contemporary geomorphic evolution for thermal conditions in this large, shallow Alaskan lake. Model results indicated that air temperature, not lake volume, was the driving factor in differences in lake temperatures between warm and cold climate scenarios. The direction and magnitude of future lake thermal responses will be driven largely by the extent of inlet stream migration when it occurs simultaneously with outlet erosion. Maintaining connectivity with inlet streams, like the Alec River and the West Fork, had substantial effects on buffering lake thermal responses to warming climate. What this ultimately means is that, under scenarios of future climate warming, simply increasing lake volume through restoration efforts (a dam) will not be enough to maintain the suitability of Black Lake as juvenile sockeye salmon rearing habitat if there is not also inflow from the Alec and West Fork rivers (as cold water inputs to counteract warmer air temperatures).  Failing to account for changing rates and types of geomorphic processes under continuing climate change may misidentify the primary drivers of lake thermal responses and reduce our ability to understand the consequences for aquatic organisms.

Griffiths et al. 2011. Effects of simultaneous climate change and geomorphic evolution on thermal
characteristics of a shallow Alaskan lake. Limnology and Oceanography. 56(1): 193-205. doi:10.4319/lo.2011.56.1.0193


Research summary