Effect of oyster farms on the distribution of Rufa Red Knot foraging resources

Background/Question/Methods

Delaware Bay, New Jersey, USA hosts the world’s largest spawning population of horseshoe crabs (Limulus polyphemus, HSC) and is therefore the primary migratory stopover site for the federally threatened rufa red knot (Calidris canutus rufa). Red knots time their spring circumpolar migration to capitalize on the abundant HSC eggs along the beaches and tidal flats in Delaware Bay, fueling the last leg of their journey to the Arctic breeding grounds. The resurgent eastern oyster (Crassostrea virginica) aquaculture industry in this region may alter this trophic interaction by influencing access to foraging resources (i.e. farm avoidance, change in prey distribution). Research shows that foraging behavior is not impacted by the presence of farms, although the probability of shorebird presence is reduced by 2-7% while farms are tended. Oyster farms do not impact horseshoe crab access to spawning beaches; however, it is unknown how they may impact the distribution of eggs or other red knot prey. In Spring 2021, benthic sediment surveys were conducted in four paired farm-control plots (90 m x 180 m) across a 3.5-km stretch of tidal flats in Delaware Bay. Sediment cores determined the relative abundance of surficial eggs and other prey across each plot.

Results/Conclusions

Preliminary analysis indicated that HSC eggs and potential alternative prey resources vary across the intertidal zone (ANOVA, p-value < 0.05). Horseshoe crab eggs were the most abundant prey, but they were concentrated on the beach and rare across the tidal flats. Bivalves, gastropods, polychaete worms, and other potential prey were more evenly distributed but often more abundant on the tidal flats than the beach. Red knots foraging on tidal flats may be consuming these other prey types that were more abundant than horseshoe crab eggs on the flats. Paired t-tests showed that prey abundances were similar in farm and control plots, regardless of intertidal zone for each prey type. These findings suggest that oyster farms do not affect the distribution and abundance of rufa red knot prey resources. Results will be integrated with concurrent shorebird survey data to assess the overall impact of farms on red knot foraging through avoidance or changes in prey distribution. These findings inform the management of an important coastal industry (oyster farming) and the conservation of a threatened species (rufa red knots).