Using otolith strontium isotopes to elucidate population structure and movements of Bering cisco (Coregonus laurettae)

Abstract

Methods for stock discrimination and tracking the movements and distribution of fishes have often involved expensive field logistics, a problem compounded in remote regions such as Alaska. An alternative approach is to use the chemical signatures preserved in otoliths, or ear bones, of teleost fishes to discriminate stocks or to track the movement history of fish. Currently, a commercial fishery targeting the anadromous Bering cisco Coregonus laurettae is occurring in the Yukon River, Alaska. There are only three known Bering cisco spawning rivers worldwide, the Yukon, South Fork Kuskokwim (Kuskokwim), and Susitna rivers. Managers and researchers believed that two of the three spawning-river populations (Yukon and Kuskokwim rivers) were being harvested in the fishery, due to major coastal currents linking two of the spawning rivers' deltas. To determine the likelihood of a mixed-stock fishery, in Chapter 1, I used the strontium isotope signature (⁸⁷Sr/⁸⁶Sr) preserved in the freshwater portion of otoliths to establish a baseline for the three natal rivers. The baseline data set was composed of otoliths from spawning adult Bering cisco of known origin (n=82). Subsequently, the baseline was used to classify commercially harvested Bering cisco (n=139) and determine the stock composition of the fishery. Greater than 97% of the commercial samples were classified as Yukon River origin. However, 0.7%, and 1.4% of the commercial samples were classified as originating from Kuskokwim and Susitna rivers, respectively. In Chapter 2, I used the baseline data to classify Bering cisco from three coastal rearing areas (Alaska Arctic coast, n=49; Y-K Delta, n=70; and the Alaska Peninsula, n=8). More than 96% of the coastal rearing Bering cisco had ⁸⁷Sr/⁸⁶Sr signatures consistent with a Yukon River origin. These data demonstrate the wide-spread coastal distribution of Bering cisco, with some travelling >4,900 km between coastal rearing and spawning habitats. This approach illustrates that ⁸⁷Sr/⁸⁶Sr can determine the natal river of Bering cisco. Subsequently, this method can be used for stock discrimination and elucidating migration patterns for unknown origin Bering cisco.