Influence of population density and length structure on angler catch rate in kokanee fisheries

Management agencies are often charged with providing fisheries that lead to angler participation. Catch rate is one of the primary drivers of angler participation but can be influenced by a suite of factors, including population structure (e.g., density and size structure). The complexity of understanding how population structure influences angler catch rate is typified in kokanee Oncorhynchus nerka fisheries. Previous research suggests that angler catch rates of kokanee are positively influenced by fish density and total length. However, that research was based on data collected using size-selective midwater trawls. Due to the potential limitation of previous research, we sought to (1) understand the relative bias of midwater trawls and gill nets for describing the size structure of kokanee available to anglers and (2) re-evaluate the influence of fish density and fish length on angler catch rates in kokanee fisheries. Midwater trawl, gill-net, and creel data were collected on five prominent kokanee fisheries throughout Idaho in 2016 and 2017. Catch composition and percent overlap of midwater trawls, gill nets, and angler-caught fish were compared to understand the efficacy of midwater trawls and gill nets for representing the size structure of kokanee available to anglers. In addition, the influence of kokanee density and length on angler catch rates was evaluated. Midwater trawls primarily sampled small kokanee (<330 mm) and exhibited little overlap with angler-caught fish, whereas gill nets sampled more large fish (>330 mm) and exhibited higher overlap with angler-caught fish when compared to midwater trawls. Fish length was not positively associated with angler catch rates. However, fish density exhibited a positive relationship with angler catch rates. Our results highlight the importance of gear choice for understanding how kokanee populations function and elucidate the tradeoffs associated with population density, fish length, and resulting kokanee fisheries.