Geoduck Clam (Panopea Abrupta) Demographics and Mortality Rates in the Presence of Sea Otters (Enhydra Lutris) and Commercial Harvesting

Abstract

In British Columbia, expanding sea otter ( Enhydra lutris ) populations are creating concerns among commercial harvesters about the potential predation impacts on exploitable geoduck clam ( Panopea abrupta ) stocks. We analysed fishery-independent surveys of exploited geoduck clam populations along a gradient of sea otter occupancy on the west coast of Vancouver Island, British Columbia, Canada to assess relationships between otter presence, commercial fishery removals of geoduck, and geoduck population demographics. Geoduck mean density, age composition, and estimated total mortality were influenced by a combination of variables, and therefore, we could not differentiate among geoduck populations according to sea otter presence or absence alone. As expected, we found a strong association between commercial fishery removals and geoduck clam total mortality rates. In contrast, the local numbers of sea otters were not an important factor affecting geoduck total mortality. A more balanced study design and greater sampling intensity would increase the power to detect whether sea otter predation affects harvestable geoduck stocks. Also, knowledge of the consumption rate by sea otters of geoduck throughout the year, in combination with survey data of unfished geoduck populations, would facilitate better prediction of how geoduck clam mortality rates might change as sea otters re-colonise new areas.