Demographic structure of Chinook salmon populations is shifting

November 4, 2018

Ohlberger J, Ward EJ, Schindler DE, and Lewis B. 2018. Demographic changes in Chinook salmon across the Northeast Pacific Ocean. Fish and Fisheries 19:533-546.

In a nutshell
  • The demographic structure of anadromous salmon populations is controlled by complex interactions among numerous biotic and abiotic factors, including ocean and climatic conditions, predator–prey dynamics, interspecific competition, hatchery production, and fisheries practices
  • Analysis of several long-term time-series datasets indicates that the age–size class structure of many Chinook salmon populations has changed over the past several decades, with salmon becoming on average both smaller and younger
  • These changes in salmon demography trend along a north-to-south gradient, being most prominent in Alaskan populations and less pronounced in more southerly populations
  • Identifying the specific drivers of shifts in the relative age and size classes of Chinook populations is challenging, however, and are most likely the result of changes in multiple biotic and abiotic factors
  • Reductions in the relative abundance of older and larger Chinook have potentially negative ecological and economic implications
The average size and age of salmon populations are shaped by complicated interactions between multiple biotic and abiotic influences. Changes in fishery harvest, climate change, evolving ocean conditions, and many other natural and anthropogenic factors may account for evidence that salmon are getting smaller and younger throughout their range. Both scientific observations and local knowledge from fishers have suggested that large Chinook salmon have become increasingly rare in recent decades. However, the magnitude of these changes and the geographic area over which they have occurred is only now becoming clear. Such trends are worrisome for any fish population, but given the high commercial value of Chinook salmon – due in part to their large size – reductions in the relative abundance of the biggest and oldest individuals are not only of ecological and traditional subsistence concern but are of economic significance as well. Although the effects of changes in size and age structure are still not well understood, the loss of older and larger salmon is likely to diminish future Chinook population productivity and stability. In this study, Ohlberger et al. analyzed long-term time-series data drawn from several state and regional databases to quantify how Chinook populations along the west coast of North America have changed over the past ~40 years. In total, approximately 1.5 million individual measurements derived from 85 Chinook salmon populations were included in the analysis. The data were used to estimate the contributions of different ages of fish to the adult population and the size-at-age of fish, the latter of which is an indicator of the growth rate of individuals surviving to adulthood. The authors also sought to correlate trends to Chinook salmon age and size to time-series data relating to the most likely biotic and abiotic drivers of change, specifically overall fishing pressure, hatchery salmon releases, climate variables, and marine mammal predator abundance. The results of their analysis indicate that most Chinook populations along the west coast are now both younger and smaller than they used to be; that is, the relative proportion of the younger and smaller age–size classes has increased over the past four decades, whereas that of the older and larger age–size classes has decreased. Additionally, the size-at-age of Chinook salmon has also changed during this time period, whereby the oldest age classes (fish that spent 4–5 years in the ocean) were found to be about 20% smaller by weight in 2010 than they were in 1980. Surprisingly, the youngest age classes (fish that spent 1–2 years in the ocean) were larger in recent decades than they were in the 1980s. These trends appeared to vary along a north-to-south geographic gradient, being most prominent in Alaska and moderate to weak in Washington, Oregon, and California (an exception were Chinook populations in British Columbia, which seemed to buck the trend altogether, though very little data were available for these stocks). The authors note that similar age–size reductions have also been observed in Chinook populations in northeastern Asia. Harvest rates, environmental changes, and hatchery effects were not well correlated with the demographic trends in Chinook salmon populations, according to the authors. In particular, while fishery harvests are size-selective on the oldest and largest Chinook salmon, fewer fish are caught in fisheries now compared to the 1980s; thus, fisheries do not appear to be the main driver of these changes. Interestingly, the authors found that increasing predation on big Chinook salmon by growing populations of resident killer whales could account for the observed shifts, reflecting the findings of several other recent studies, but caution that more focused quantitative studies are needed to confirm this hypothesis. In summary, Chinook salmon are getting younger and smaller in most populations in the northeastern Pacific. The authors conclude that these shifts are most likely the result of changes in multiple biotic and abiotic factors that are playing out at a scale encompassing most of the coastline of the region. The consequences of these changes for the ecosystems, people, and economies that rely on these salmon are not clear. Science Spotlight by Ken Ferguson