Recent declines in salmon body size impact ecosystems and fisheries

Abstract

Declines in animal body sizes are widely reported and likely impact ecological interactions and ecosystem services. For harvested species subject to multiple stressors, limited understanding of the causes and consequences of size declines impedes prediction, prevention, and mitigation. We highlight widespread declines in Pacific salmon size based on 60 years of measurements from 12.5 million fish across Alaska, the last largely pristine North American salmon-producing region. Declines in salmon size, primarily resulting from shifting age structure, are associated with climate and competition at sea. Compared to salmon maturing before 1990, the reduced size of adult salmon after 2010 has potentially resulted in substantial losses to ecosystems and people; for Chinook salmon we estimated average per-fish reductions in egg production (−16%), nutrient transport (−28%), fisheries value (−21%), and meals for rural people (−26%). Downsizing of organisms is a global concern, and current trends may pose substantial risks for nature and people.

Can late stage marine mortality explain observed shifts in age structure of Chinook salmon?

“…Some potential and non-mutually exclusive causes of marine mortality include changing environmental conditions that affect prey fields, size-selective harvest in directed fisheries, competition with hatchery-derived Pacific salmon, bycatch in marine fisheries and top down forcing by apex predators [ 30 ]. Recently emerging evidence suggests that predation is a stronger driver of late–stage marine mortality than selective human harvest and is a primary agent of changes in the age structure of Chinook salmon [ 31 ].…”

Redistribution of salmon populations in the northeast Pacific ocean in response to climate

“…An important caveat for our simulations is that we assume the abundance of the focal stocks is represented by the abundance in recent decades (1983 to present). While this assumption allows us to identify predicted shifts that stem solely from ocean climate, it is likely that future climate conditions will differentially affect the productivity of individual stocks through changes at various stages in the lifecycle (Crozier et al, 2008;Jones et al, 2020;Oke et al, 2020).…”

“…An important caveat for our simulations is that we assume the abundance of the focal stocks is represented by the abundance in recent decades (1983 to present). While this assumption allows us to identify predicted shifts that stem solely from ocean climate, it is likely that future climate conditions will differentially affect the productivity of individual stocks through changes at various stages in the lifecycle (Crozier et al., 2008; Jones et al., 2020; Oke et al., 2020). Currently, stock‐specific abundance projections are not available for all stocks, but a broad literature suggests climate change will affect the productivity and population dynamics for many salmon populations during freshwater life‐stages (Battin et al., 2007; Crozier et al., 2008; Kovach et al., 2015; Morita et al., 2014).…”

The Gill‐Oxygen Limitation Theory and size at maturity/maximum size relationships for salmonid populations occupying flowing waters

“…Two of these studies documented increased mean length in the youngest ocean age groups while mean sizes in older age groups declined across multidecadal scales (Ohlberger et al ., 2018; Siegel et al ., 2017). A similar “shrinking” phenomenon has also been reported in populations of Atlantic salmon Salmo salar (Todd et al ., 2008), steelhead Oncorhynchus mykiss (Bowersox et al ., 2019) and most recently for all five species of Alaska salmon (Oke et al ., 2020). Despite the fact that these temporal life history changes could be explained by ocean warming and the GOLT (Cheung et al ., 2013; Pauly & Cheung, 2018), it was only mentioned in a single one of the studies as a possible explanation.…”