Uncovering Patterns and Mechanisms of Paralytic Shellfish Toxicity in Alaska’s Geoduck Clam Fishery

Abstract

This dissertation explores the patterns and mechanisms of paralytic shellfish toxicity in commercially harvested geoduck clams caused by the toxin-producing dinoflagellate Alexandrium catenella in Southeast Alaska. Alaska’s commercial fishery for geoduck clams (Panopea generosa) is a small but lucrative fishery, with annual ex-vessel values averaging US $3.9 million (2010-2022). In recent years, the presence of paralytic shellfish toxins (PSTs) in clam tissue resulted in declines in fishery openings and harvest. PSTs can bioaccumulate in the tissues of filter feeders when A. catenella blooms in the spring and summer seasons. However, high levels of toxicity in geoduck clams occur sporadically during the fishery in the fall and winter months long after toxic blooms have subsided. Levels of PSTs in geoduck clams vary substantially from week-to-week, and elevated PSTs are increasingly causing economic loss to the fishery through sampling costs from repeated testing and by delaying or closing harvests. In the past decade (2011-2021), about 60% of geoduck clams tested for PST failed regulatory screenings, up from a 36% failure rate in the decade prior (2001-2010). Knowledge about patterns and distributions of this harmful algal species and its toxins will help improve management of geoduck dive fisheries and provide information to reduce impacts of PSTs on this fishery. In Chapter 1, Patterns in geoduck clam paralytic shellfish toxicity from two decades of shellfish testing in southeast Alaska, we show that geoduck clams are increasingly failing biotoxin screening tests in some areas and these patterns are most closely correlated with regional air temperatures. In Chapter 2, Alexandrium catenella benthic cyst distribution, sediment characteristics, and geoduck clam (Panopea generosa) toxicity in Southeast Alaska, we provide the first A. catenella cyst distribution map for this region and found that cyst counts declined over the three-year study, but patterns were not related to geoduck PST levels or sediment characteristics. Lastly in Chapter 3, Geoduck clam (Panopea generosa) toxicity dynamics across harvest areas, time, age, and cyst gut content in Alaska’s commercial fishery, we confirmed the ability for geoduck clams to ingest dormant A. catenella cysts but revealed that neither this process, nor the age of a clam, is directly related to patterns of paralytic shellfish toxicity in this clam species. All together, these findings lead to a better understanding of the variability of PSTs in geoduck clams and are informative to future fishery management and the protection of human health.