Otolith derived hatch dates, growth rates, and microchemistry of Arctic cod (Boreogadus saida) support the existence of several spawning populations in Alaskan waters

Abstract

The changing climate in the Arctic is resulting in increased air and water temperatures as well as a reduction in sea ice, affecting native species that evolved to live in the unique habitats of the Arctic Ocean. One species of significant importance to Arctic marine ecosystems is the Arctic Cod (Boreogadus saida), a keystone species that serves as vital prey for many marine mammals, seabirds, and fishes. Arctic Cod have a strong association with sea ice for spawning and for much of their early life history. In the Pacific Arctic, little is known about their early life history, especially with regards to hatch timing and locations. To address these gaps, I estimated the hatch timing and growth rates of Arctic Cod during their first year by examining incremental growth patterns in their otoliths. Specifically, I counted daily growth increments to estimate age, and used those estimates to describe the relationship between length and age. Using this relationship, length measurements of age-0 Arctic Cod were converted to estimated daily ages and subtracted from the day of capture to estimate hatch date distributions for multiple sampling regions. Results suggest that fish caught during spring in the northern Bering Sea and southern Chukchi Sea hatched near their capture location over a relatively short period that coincided with the timing of local sea ice recession. Hatch dates from summer samples over multiple sampling regions indicated a prolonged hatching event that lasted from early winter (December) through early summer (July). Summer aggregations in the northeast Chukchi Sea likely represented a mix of different hatching populations that had been transported from the south and retained in the northern regions. Within each sampling region, mean hatch dates differed between pelagic and demersal caught Arctic Cod, which supports the existence of multiple hatching populations mixing within each region during the summer. In general, hatching occurred earlier the further south they were captured for summer captured fish, whereas their growth rate declined as one moved northward, possibly due to the higher average temperatures during the larval stage in southern hatching locations. By analyzing the elemental composition of otoliths, I was able to infer environmental conditions such as salinity near the time of hatching of age-0 Arctic Cod. Regional differences in elemental concentrations at the time of hatching suggest a stronger freshwater influence in the eastern Beaufort Sea compared to the Chukchi Sea and western Beaufort Sea and support the existence of separate hatching populations. This study expands the understanding of the early life history of Arctic Cod and informs managers and policy makers to better protect critical life stages of this key species in a changing environment.