Seasonal variation in the lipid content of Fraser River Chinook Salmon (Oncorhynchus tshawytscha) and its implications for Southern Resident Killer Whale (Orcinus orca) prey quality

Abstract

In Southern British Columbia (BC), Canada, declines in southern resident killer whale (SRKW—Orcinus orca) populations have been linked to declines in numbers and average size of their preferred prey, Chinook salmon (Oncorhynchus tshawytscha). However, the life history diversity of Chinook suggests that there is a need to assess stock-specific differences in energy density to evaluate prey quality as a factor in SRKW declines. In this study, we calibrated a Distell fat meter to estimate Chinook whole-body lipid content, a proxy for energy density. The fat meter was deployed at the Fraser River, BC, Chinook test fishery during 2020, collecting lipid, weight, and length measurements from 1566 genetically stock identified individuals encompassing all major Fraser River Chinook population units (management units, MUs) at river entry. We found that MU-specific lipid content increased with distance and elevation to spawning grounds and was highest in the Spring-52 (12.8%) and Summer-52 (12.7%) MUs, intermediate in the Summer-41 MU (10.8%), and lowest in the Fall-41 MU (7.3%). Lipid content also decreased by up to 6 percentage points within MUs from the beginning to end of their migration period. Our data revealed SRKWs’ most endangered prey sources, the Spring-52 and Summer-52 MUs, are also its most energy rich. It also indicated SRKWs have access to progressively lower energy density Chinook through the year, requiring up to ~ 30% more fish to meet energy demands in the fall than in the spring.

Identifying drivers of demographic rates in an at‐risk population of marine mammals using integrated population models

“…Future analyses using our model could account for such variation by incorporating interactions between SRKW pod and salmon abundance in the survival and fecundity regressions. An alternative explanation, not explored here, is that the relationship between salmon abundance and SRKW vital rates is changing over time because the whales' physiology and demography may be responding to changes in Chinook salmon growth, which have decreased in size and energy density over time (Buckner et al 2022; Lerner & Hunt 2023; Malick et al 2023; Ohlberger et al 2019; O'Neill et al 2014). Similarly, it is possible that these relationships could change because of bias in fishery indices resulting from redistribution of Chinook populations in the eastern‐Pacific (Shelton et al 2021).…”

The combined effects of predation, fishing, and ocean productivity on salmon species targeted by marine mammals in the northeast Pacific

“…The intensifying predation by those marine mammals on mature fish can affect the average body size, maturation timing, and life-history characteristics of the Chinook salmon populations through the selective removal of larger fish [ 99 ]. In addition, leaving smaller Chinook salmon implies a reduction in caloric value, which could further affect other marine mammals feeding on similar preys, including SRKW [ 100 , 101 ]. Although one could argue that SRKW might have access to larger fish the same way NRKW do, recent evidence suggests that SRKW might exhibit different foraging strategies than NRWK [ 102 ].…”