Strontium isotope analyses (87Sr/86Sr) of otoliths from anadromous Bering cisco (Coregonus laurettae) to determine stock composition

A commercial fishery targeting the anadromous Bering cisco (Coregonus laurettae) is occurring in the Yukon River, Alaska, USA. All three of the known global spawning populations occur in Alaska. Managers believed that two of the three populations were being harvested in the fishery. To determine the likelihood of a mixed-stock fishery, we used 87Sr/86Sr values from the freshwater region of otoliths, from spawning adult Bering cisco of known origin (n = 82), to create a baseline. A 10-fold cross-validated, quadratic discriminant function analysis (DFA) of the three baseline population 87Sr/86Sr values (Yukon River, n = 27; South Fork Kuskokwim River [Kuskokwim River], n = 25; and Susitna River, n = 30) correctly reclassified 98.8% of the fish analysed. The baseline DFA model was then used to classify the 87Sr/86Sr values from a set of otoliths removed from commercially harvested Bering cisco (n = 139). Using a posterior probability threshold of 90%, we found that >97% of the commercial samples were classified as originating in the Yukon River. The remainder of the commercial samples were classified as originating in the Kuskokwim River (0.7%) or from the Susitna River (1.5%). The presence of 87Sr/86Sr values consistent with the Susitna River discovered in the Yukon River baseline (n = 1) and commercial samples (n = 2) suggested either multiple isotope signatures within the Yukon River population or straying among populations. Strontium isotope data provide an effective tool to monitor the movements and stock composition of Bering cisco.

Contributions to the Biology of the Bering Cisco (Coregonus laurettae) in Alaska

Taxonomic and biological data are presented on Bering cisco, Coregonus laurettae Bean, from the South Fork of the Kuskokwim River; Hess Creek and the Porcupine River in the Yukon System; and the Koyuk River and Port Clarence–Grantley Harbor area of western Alaska. Gill raker counts ranged from 31–40 with means of 33.8–36.6, those on the lower arch 18–24 with means of 20.4–23.6.Potential spawners were found 1610 km up the Yukon River in mid-June and 262 were taken at the mouth of Hess Creek in the Yukon River 1270 km upstream from the mouth June 10–14, 1972.Bering cisco from Hess Creek grow faster than cisco from Port Clarence–Grantley Harbor, reaching a maximum length of 480 mm and 8 years of age. Growth rates for males and females are similar.