Demographic changes in Chinook salmon across the Northeast Pacific Ocean

Abstract

The demographic structure of populations is affected by life history strategies and how these interact with natural and anthropogenic factors such as exploitation, climate change, and biotic interactions. Previous work suggests that the mean size and age of some North American populations of Chinook salmon (Oncorhynchus tshawytscha, Salmonidae) are declining. These trends are of concern because Chinook salmon are highly valued commercially for their exceptional size and because the loss of the largest and oldest individuals may lead to reduced population productivity. Using long‐term data from wild and hatchery populations, we quantified changes in the demographic structure of Chinook salmon populations over the past four decades across the Northeast Pacific Ocean, from California through western Alaska. Our results show that wild and hatchery fish are becoming smaller and younger throughout most of the Pacific coast. Proportions of older age classes have decreased over time in most regions. Simultaneously, the length‐at‐age of older fish has declined while the length‐at‐age of younger fish has typically increased. However, negative size trends of older ages were weak or non‐existent at the southern end of the range. While it remains to be explored whether these trends are caused by changes in climate, fishing practices or species interactions such as predation, our qualitative review of the potential causes of demographic change suggests that selective removal of large fish has likely contributed to the apparent widespread declines in average body sizes.

Interactions between Marine Growth and Life History Diversity of Steelhead from the Situk River, Alaska

“…, ), we were not able to distinguish this “bottom‐up” environmental hypothesis from an alternative hypothesis that variation in mortality rates of older fish could drive temporal patterns in age at maturity of salmonids (e.g., Ohlberger et al. ). Allelic variation (e.g., Martyniuk et al.…”

Changing salmon: An analysis of body mass, abundance, survival, and productivity trends across 45 years in Puget Sound

“…While causes of contrasting patterns of salmon body size and abundance are not completely understood, the studies cited above and others discussed possible links to environmental conditions, management practices and density dependence (Claiborne et al, in press; Debertin, Irvine, Holt, Oka, & Trudel, ). Together, these and numerous other studies (Kendall, Marston, & Klungle, ; Kilduff, Botsford, & Teo, ; Ohlberger, Ward, Schindler, & Lewis, ; Zimmerman et al, ) highlight the importance of multidecadal time series documenting basic salmon demographic data across multiple populations for management and conservation purposes.…”

“…Reduced body size of Pacific salmon has been linked to a variety of factors including fisheries-induced evolution (Fukuwaka & Morita, 2008;Kendall, Dieckmann, Heino, Punt, & Quinn, 2014), hatchery practices (Hankin, Fitzgibbons, & Chen, 2009;Swain, Riddell, & Murray, 1991), environmental and climate conditions (Debertin et al, 2017;Jeffrey et al, 2017;Morita & Fukuwaka, 2007), density-dependent effects (Ruggerone & Irvine, 2018) and increased natural mortality (e.g. predation by killer whales; Ohlberger et al, 2018). Larger juvenile salmon body size, earlier maturation and thus smaller adult body sizes in coho and Chinook salmon have also been associated with larger juvenile salmon body size prior to seawater entry (due to hatchery practices or changing freshwater conditions; Vøllestad, Peterson, & Quinn, 2004).…”

The roles of extrinsic and intrinsic factors in the freshwater life‐history dynamics of a migratory salmonid

“…Our research adds to the growing appreciation for the ecological impacts of pink salmon, which are increasing in many locations (Malick and Cox 2016). For example, there is a growing body of evidence that pink salmon in the marine ecosystem are important competitors, and years with high abundances are associated with decreased survival and size in sockeye salmon and nesting seabirds (Ohlberger et al 2018, Springer et al 2018). Here, we provide rare evidence that the temporal dynamics of one species of salmon is linked with the life history of a co-occurring salmonid during the freshwater phase of their life and that it may directly translate into increased per capita productivity.…”

“…For example, there is a growing body of evidence that pink salmon in the marine ecosystem are important competitors, and years with high abundances are associated with decreased survival and size in sockeye salmon and nesting seabirds (Ohlberger et al. , Springer et al. ).…”