Comparisons of isotopic niche between females and male alternative reproductive tactics of Chinook salmon (Oncorhynchus tshawytscha) from Lake Ontario

Chinook salmon (Oncorhynchus tshawytscha) have complex life histories with two distinct male alternative reproductive tactics (ARTs). The larger and older “hooknose” males battle for position to spawn with nesting females whereas the precociously maturing “jack” males use a sneaking tactic to gain access to females. Although dietary niche is known to differ between ARTs of other fish taxa, no research to date has examined these differences in a salmonid. Here, we used δ¹⁵N and δ¹³C stable isotope analyses to compare the dietary niches of female, hooknose male, and jack male Chinook salmon sampled from a Lake Ontario tributary. We found that all three life histories shared similar δ¹⁵N values, whereas jack males had greater δ¹³C values compared to hooknose males, with females having intermediate values. Jack males also had notably little overlap in their isotopic niche space with hooknose males and females, indicating that jack males occupy a distinct isotopic niche in Lake Ontario. We discuss habitat partitioning and differences in dietary niche as explanatory factors. Altogether, these data unveil novel distinctions in isotopic niche within the complex life history strategies of Chinook salmon in Lake Ontario that can inform management decisions and provide avenues for future research.