Use of otolith microchemistry and stable isotopes to investigate the ecology and anadromous migrations of Northern Dolly Varden from the Egegik River, Bristol Bay, Alaska

Abstract

Dolly Varden (Salvelinus malma) is a facultatively anadromous and iteroparous salmonid species, widely distributed around the Pacific Rim. Its distribution overlaps in northern areas with the circumpolar congeneric Arctic char (S. alpinus), which shares some of its life history traits. Despite the abundance of both species in many Alaskan watersheds and the diversity of their life histories, relatively little is known about their estuarine and marine ecology and migrations. Thirty char, taken as by-catch in the sockeye salmon (Oncorhynchus nerka) gill net fishery in the Egegik District, Bristol Bay, Alaska in June and July 2013, were identified using molecular genetic techniques as Dolly Varden. Otoliths indicated that the fish, ranging from 445 to 560 mm fork length, were from 4 to 9 years old, with a modal age of 6 (40 % of the specimens). Microchemical analysis indicated that all fish had first migrated to sea at ages 2 or 3 (16 and 14 individuals, respectively), migrated annually thereafter to a maximum of eight times, and did not over-winter in marine waters. Moreover, the fish were produced by a mix of anadromous mothers (14 fish) and non-anadromous mothers (11 fish); the maternal origin of the other five fish was uncertain. Despite the fact that the fish were captured in the riverine/estuarine environment of the Egegik River, stable isotope analysis of muscle tissue samples indicated that they had relied on marine prey, and this was supported by the high Sr levels in the otoliths, indicating prior occupancy of marine rather than fresh water for all individuals tested. Together, these analyses reveal extensive use of marine resources by Dolly Varden across a wide range of sizes and ages during regular but brief migrations, and provide new information on this facultatively anadromous, long-lived salmonid.