Human disturbances altering sockeye life-history
Cline TJ, Ohlberger J, and Schindler DE. 2019. Effects of warming climate and competition in the ocean for life-histories of Pacific salmon. Nature Ecology & Evolution 3: 935–42.
In a nutshell
- Analysis of long-term datasets on the ages and sizes of sockeye salmon from Alaska’s Bristol Bay reveals that juveniles are developing at a faster rate and entering the North Pacific Ocean a full year earlier than in the past due to increased freshwater prey abundance triggered by climate warming
- Once in the Pacific, however, increased competition with hatchery-reared pink and chum salmon is forcing sockeye to remain in the ocean for a longer period of time prior to reaching maturity
- Bristol Bay sockeye are thus spending less time in freshwater and more time in the ocean before returning to rivers to spawn at roughly the same total age as they have over the past 50 years
- Changes in life-history due to the combined effects of these factors has potentially significant implications for future sockeye productivity given uncertainty in future environmental conditions
Human interference in the natural world affects many of the world’s fishes, but anadromous species are particularly vulnerable to anthropogenic disturbances due to their dependence on both freshwater and marine ecosystems. In the case of Pacific salmon, new research suggests that conditions are changing so rapidly in northern rivers, lakes, and seas as a result of human activity that the life-cycles of some salmon populations have already undergone striking transformations.
Analysis of a database of the age and size classes of sockeye salmon returning to rivers in Alaska’s Bristol Bay watershed extending back to the 1950s, along with modeling of biomass-based competition in the North Pacific Ocean, allowed Timothy Cline and colleagues to explore the impacts that human-caused stressors are having on the Bay’s sockeye populations. Historically, young sockeye remained in the lakes and rivers that empty into Bristol Bay for up to 2 years to feed and grow before migrating to the North Pacific, where most would spend a further 2–3 years prior to reaching adulthood and returning to their natal waters to spawn. The researchers found, however, that juvenile sockeye are now more likely to enter the North Pacific a full year earlier than in the past, migrating to the ocean after just a single year of freshwater development instead of two. They attribute this shift in migratory timing to the increasingly warmer waters of the sockeye’s natal rivers and lakes in this region. Previous research has shown that higher water temperatures promote the production of freshwater zooplankton, a dietary staple of juvenile sockeye. As zooplankton abundance increases, so too does sockeye growth rates; put simply, the amount of food available to juvenile salmon has increased as their natal rivers and lakes have warmed over the past several decades, enabling faster development and earlier migration to the ocean.
Although it would seem that progressing into the marine phase of their life-cycle at a younger age would confer competitive advantages to these salmon, life in the North Pacific Ocean has, in contrast, become considerably more difficult for sockeye. Commercial salmon hatcheries ringing the North Pacific release an estimated 6 billion pink and chum salmon into the ocean every year to boost commercial catches, a figure that has expanded greatly over the past five decades. Such an enormous influx of competitors has made it far more difficult for sockeye to consume enough food to grow to the size needed for adulthood, forcing them to spend an extra year in the Pacific. In effect, the combined impacts of human-driven climate change and hatchery releases have resulted in sockeye essentially exchanging a year of development in freshwater for an additional year of development in the ocean, due primarily to inverse trends in freshwater and marine resource availability.
The authors warn that such a marked change in life-history may have considerable negative implications for sockeye productivity in this region. Prior to significant warming of the freshwaters forming the Bristol Bay watershed, fish would enter the North Pacific Ocean across a diversity of ages. This variability in age-class migration helped to buffer the Bay’s sockeye population against the potential impacts of poor marine conditions. Now, however, juveniles from an individual spawning year are migrating to the ocean at more or less the same time, at about one year of age, putting an entire generation of young salmon at risk should they encounter adverse conditions in the marine environment.
Cline et al.’s analysis further demonstrates how human disturbances are affecting salmon populations in unexpected and unforeseen ways, particularly in northern latitudes. Global climate change, hatchery overproduction, and the interactions between these forces have already triggered a marked alteration in the life-history of Bristol Bay sockeye salmon, with potentially significant negative implications for the future productivity of this stock. In addition, the results of this study serve as a reminder that current changes in salmon life-histories almost certainly represent only the tip of the iceberg of future transformations as the planet continues to warm and hatchery production continues to expand.
Science Spotlight by Ken Ferguson