Effects of Geoduck Aquaculture Practices on Habitat and Trophic Dynamics of Nekton and Macroinvertebrates in Puget Sound

Habitat complexity is known to enhance diversity and abundance by ameliorating interactions among competitors, by sustaining predator and prey populations, and by enhancing settlement processes and food deposition. Epibenthic structure may also provide refuge and foraging habitat for mobile macrofauna. In estuaries and nearshore areas, organisms undertake intertidal migrations to access littoral habitats at high tide. Previous studies have found higher abundances of fish and mobile invertebrates in littoral habitats characterized by structure relative to unstructured habitats. However, the trophic implications of these patterns have rarely been addressed.

In Washington State, commercial culture of geoduck clams (Panopea generosa) involves large-scale out-planting of juveniles to littoral habitats and installation of PVC tubes and netting to exclude predators and increase early survival. Nets typically consist of either small plastic mesh caps stretched over the opening of individual tubes or large continuous covers over entire plots. Placement of predator exclusion structures may affect a number of ecological processes that result in altered diversity and abundance of associated flora and fauna. Such disturbances can modify predation pressure and alter trophic relationships with consequences cascading through local food webs. We examined whether structures associated with this nascent aquaculture method affect patterns of use by mobile macofauna and modify trophic dynamics.

We summarized observations of mobile macofauna made during regular SCUBA surveys of aquaculture areas and reference beaches at three sites. These data indicate that structures attract mobile predators that feed on associated biota but exclude others that rely on soft-bottom benthic prey. Additionally, we synthesized several small studies of the food habits of Pacific staghorn sculpin (Leptocottus armatus) collected from geoduck aquaculture areas and adjacent reference beaches. Seines were used to capture fish on flooding and ebbing tides within structured and unstructured areas, and individuals from each of two size categories were retained and preserved. In the laboratory we extracted and identified gut content; prey were separated into broad taxonomic categories and common items were identified to the lowest taxonomic level. Comparisons were done using MANOVA for major prey types, followed by rank-transformed ANOVA for individual prey categories. Multidimensional scaling (MDS) was used to reveal shifts in diet among habitats. We found evidence that introduced structures alter predator-prey relationships from those found in unstructured littoral habitats and affect energetic tradeoffs for foraging predators. Our results highlight linkages within communities modified by the addition of epibenthic structure that should be considered in tideland management and conservation.