Tracking the marine migrations of coho salmon through otolith microchemistry

Abstract

Coho salmon (Oncorhynchus kisutch) are an important species to Southern British Columbia both culturally, ecologically, and economically; unfortunately, since the 1990’s significant declines in abundance and survival have occurred in this region for several populations. While marine survival rates have been declining, where these fish migrate to in the ocean within these populations has also been changing. These shifts can vary considerable from year to year, depending on the population. While tracking marine migrations with traditional tagging approach is no longer feasible given the few returns, otolith microchemistry provides an alternative approach, by comparing otoliths of adult coho that returned to the freshwater to spawn to coho caught in different marine regions: in freshwater, adults (n = 147) were collected from various hatcheries in the fall of 2017 and 2018, and in marine waters, individuals (n = 202) were collected from the Strait of Georgia (SoG) and the West Coast of Vancouver Island (WCVI) in May through July of 2017 and 2018. Otolith samples were analyzed for both trace/minor elements and stable isotopes (δ¹³C, δ¹⁸O). Hatchery samples were classified as either SoG residents, WCVI migrants, or unknowns using random forest classification models. Otolith chemistry of ocean caught samples from the SoG and WCVI differed from one another (F=4.754, df= 1, p < 0.001), with ¹³⁸Ba and δ¹⁸O being the main drivers of these differences. Random forest models were accurate (∼95 %) with low out-of-bag error rates (<10 %) across both 2017 and 2018. Freshwater caught adults were consistently assigned (≥80 % of the random forest models across set seeds) to the SoG or WCVI 83.7 % of the time. 70.6 % of 2017 samples and 38.9 % of 2018 samples were predicted to be SoG residents. Using random forest classification models, and 41.2 % of 2017 samples and 42.5 % of 2018 samples were predicted to be Strait of Georgia residents. This study is the first to demonstrate the utility of both otolith trace/minor elements and stable isotopes to track the marine migrations of Pacific salmon. The use of the otolith as an intrinsic tag provides fisheries managers with an alternative, non-fisheries dependent approach to assess the marine distributions of these fish.