Recent declines in salmon body size impact ecosystems and fisheries

K B Oke,M L Baskett,B Lewis,E P Palkovacs,A P Hendry,J Clark,P A H Westley,H K Kindsvater,G K Vick,J Kibele,C J Cunningham,N W Kendall,J D Reynolds,V A Karatayev,S M Carlson,K M Kobayashi,S Munch

doi:10.1038/s41467-020-17726-z

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.

Highlights

Declines in animal body sizes are widely reported and likely impact ecological interactions and ecosystem services
Chinook salmon populations in Westward Alaska and Arctic–Yukon–Kuskokwim declined by 10% on average, whereas conspecifics in Southeast Alaska declined by 4%
General additive models (GAMs) confirmed that average sizes declined through time in each species, the common pattern in average size across time differed between species

Summary

Introduction

Declines in animal body sizes are widely reported and likely impact ecological interactions and ecosystem services. Major selective forces such as climate change and harvest may be causing widespread declines in organismal body size[2,3,4,5]. Understanding the causes of body size declines is daunting given the influence of numerous, potentially interacting factors. Or in unison, these underlying factors can influence body size through shifting population age structure, changing growth rates, or a combination thereof. We examine changes in body size for four species of Pacific salmon (Oncorhynchus spp.), by assembling a 60-year (1957–2018) database of size and age measurements from 12.5 million individually-measured fish. Our overarching goals were to understand the magnitude and consistency of size declines across regions and species, evaluate potential causes, and quantify the consequences of these changes for ecosystems and people. Accumulating evidence from local and indigenous knowledge suggests that adult salmon body sizes are decreasing, including in Alaska where salmon provide critical support for ecosystems and people[17,18,19], cf. ref. 20

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