Synergistic serial depletion of marine invertebrates leads to the decline of a strongly interacting grazer

We investigated the relative roles of natural factors and shoreline harvest leading to recent declines of the black leather chiton (Katharina tunicata) on the outer Kenai Peninsula, Alaska. This intertidal mollusc is a strongly interacting grazer and a culturally important subsistence fisheries resource for Sugpiaq (Chugach Alutiiq) Natives. We took multiple approaches to determine causes of decline; field surveys examined the significant predictors of Katharina density and biomass across 11 sites varying in harvest pressure, and an integrated analysis of archeological faunal remains, historical records, traditional ecological knowledge, and contemporary subsistence invertebrate landings examined changes in subsistence practices through time. Strong evidence suggests that current spatial variation in Katharina density and biomass is driven by both human exploitation and sea otter (Enhydra lutris) predation. Traditional knowledge, calibrated by subsistence harvest data, further revealed that several benthic marine invertebrates (sea urchin, crab, clams, and cockles) have declined serially beginning in the 1960s, with reduced densities and sizes of Katharina being the most recent. The timing of these declines was coincident with changes in human behavior – from semi-nomadic to increasingly permanent settlements patterns, improved extractive technologies, regional commercial crustacean exploitation, the erosion of culturally-based season and size restrictions – and with the reestablishment of sea otters. We propose that a spatial concentration in shoreline collection pressure through time, increased harvest efficiency, and the serial depletion of alternative marine invertebrate prey have led to intensified per capita predator impacts on Katharina and thus its recent localized decline.