Alaska Salmon are Shrinking
This summer we shared our latest paper on the importance of freshwater conditions for Cook Inlet Chinook Salmon and highlighted how important it is to understand stream-specific responses to climate […]
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This summer we shared our latest paper on the importance of freshwater conditions for Cook Inlet Chinook Salmon and highlighted how important it is to understand stream-specific responses to climate change for better management of our valuable fisheries. Now we’d like to put the spotlight on other important research going on that is helping us understand how our changing climate is affecting our salmon.

The article below is reprinted with permission from the authors: Krista Oke and Bert Lewis, and was first published in ONCORHYNCHUS, the newsletter of the Alaska Chapter of the American Fisheries Society (Fall 2020).

If you live in Alaska, you know that salmon play a critical role in Alaska’s ecosystems, economies, and communities. You may also have heard that there is a general trend towards smaller body size in salmon, or that the very large, old Chinook Salmon (Oncorhynchus tshawytscha) once common in many parts of the state are becoming rare. Through a State of Alaska Salmon and People (SASAP) project working group sponsored by the National Center for Ecological Synthesis and Analysis (NCEAS), we conducted a synthesis of salmon size data from across the state to better document and understand the trend for smaller size seen in Chinook, Chum (O. keta), Coho (O. kisutch), and Sockeye (O. nerka) salmon. Using data collected over the past 60 years by the Alaska Department of Fish and Game on over 12.5 million individual salmon, we show broadscale declines in the body size of all four salmon species. We found that younger age at maturity was primarily responsible for decline in size.

In each species, fish maturing after 2010 were significantly smaller on average than fish that matured prior to 1990. Variation in the extent of body size change existed among species, with Chinook Salmon showing the greatest magnitude decline. Within species, the magnitude of decline also varied among regions and populations, but the overall pattern of body size decline was significant in all four species. Importantly, our study shows that changes in body size resulted primarily from changing age structure and to a much lesser extent from changing growth rates.

On average, salmon body size was smaller post-2010 compared with pre-1990 across all areas and species examined. A) Study area with regions numbered and colored according to Alaska Department of Fish and Game management areas. B) Percent change in mean length between data collected before 1990 and after 2010. Points show change in mean length for individual populations. Red line indicates no change. Adapted from Oke et al. (2020).

We also investigated potential causes of the declines including climate, competition, and size selective harvest. Our analysis found that no single factor could explain body size declines across all four species, which instead appear to result from complex and species-specific effects of several factors. However, both climate and competition at sea with highly abundant salmon species are clearly contributing to the observed body size declines. Our ability to test for an impact of size-selective fishing was limited to certain regions and species, where there was not an effect of fishing, but a role of size-selective fishing in other species or regions cannot be ruled out.

Given the importance of salmon for Alaska’s ecosystems, economies, and culture, we investigated whether smaller body size could result in ecological or socioeconomic consequences. Our calculations suggest that today, each salmon provides fewer meals for subsistence users and less profit for commercial users, while also transporting less marine-derived nutrients into freshwater ecosystems and leaving fewer eggs in redds, compared to the average salmon prior to 1990. These impacts will be most acutely felt when reduced body size is compounded by reduced salmon abundances, as is the case for many Chinook Salmon populations. Body size declines have been observed in many taxa, but the consequences of these changes are rarely quantified. Our results suggest that these consequences could be substantial.

For more, the peer-reviewed paper is freely available at https://rdcu.be/b6mc2.