Comparative resource utilization of eastern North Pacific skates with applications for fisheries management

Abstract

Skates (Rajiformes: Rajoidei) are an extremely diverse group of cartilaginous, marine fishes that are common mesopredators in benthic communities. Population declines, shifts in assemblage structure, and ecological interactions with exploited groundfishes have focused increased scientific and management attention on skates. The main purpose of this study was to address a knowledge gap in the ecology of eastern North Pacific (ENP) skates, and thereby inform the development of improved monitoring and management strategies for exploited skate stocks off the U.S. West Coast and in the Gulf of Alaska. Chapter 1 investigated spatial associations of ENP skates in continental shelf and upper continental slope waters of each study region. Chapter 2 focused on the other aspect of the traditional niche by examining trophic relationships within each skate assemblage and among distant populations of big and longnose skates. The goal of this chapter was to address the paradox of ecological redundancy in skates (i.e., skates occupy similar habitats and eat similar prey). Chapter 3 utilized an improved understanding of ENP skate ecology to establish baseline information for the development of fisheries management strategies. Space appears to be more important than food in structuring ENP skate assemblages. Spatial segregation among sympatric skate species and their life stages was demonstrated off central California and in the western Gulf of Alaska. In contrast, ENP skate assemblages displayed a high degree of trophic redundancy, with the timing and location of collection trawls explaining the great majority of dietary variation. Species-level and length-based differences were highly significant, but explained a relatively minor portion of the observed dietary variability, especially in the western Gulf of Alaska. Big and longnose skates had different predatory roles in each region, with a greater reliance on decapods in the Gulf and fishes off California. Distribution and abundance patterns of U.S. West Coast skate species and their life stages were modeled and can be used to characterize essential fish habitat. Updates to a Gulf of Alaska food web model indicated that skates (especially big skate) are major predators of Tanner crab and may considerably influence regional trophodynamics.