38 DECEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG invertebrates that were within the gear, and we also attached un-baited minnow traps for 24-hour periods to sample the animals that were around but not within the gear. We compared the influence of gear type on fishes or invertebrates by looking at abundance, the number of organisms present, and diversity, quantified as species richness, the number of unique species present, and Shannon diversity index, a metric that considers both the number of unique species present and how evenly represented those species are. Influence of Gear Type on Oysters We found that oysters in bottom cages were larger at the end of the study period in terms of both shell length and shell depth (Figure 6). We noticed that, despite random assignment, the oysters in the bottom cages were initially slightly longer than the oysters in the floating cages, so it is possible that the oysters in the bottom cages were intrinsically better growers. However, there was no significant difference in shell depth between the cage types to start, and shell depth was significantly greater in the bottom cages at the conclusion of the experiment suggesting that something other than intrinsic growth rates affected how oysters grew in the two cage types. Beyond intrinsic growth rates, size differences in oysters between cage types may have occurred in response to waveassociated tumbling or differences in fouling. The floating oyster cages were attached to anchored lines in such a way that prevented them from drifting around or entirely flipping over during extreme wave action, but slack in the anchor lines to account for changes in water height over a tidal cycle did allow for near constant light rocking of the cages in the waves. During times of elevated wave these oyster farming activities create population-level impacts for fishes is largely unknown. There are two general categories of oyster aquaculture gear used to grow oysters in commercial settings – cages that rest on the substrate, “bottom cages,” and cages that float at the water’s surface, “floating cages.” Bottom cages have been used traditionally, while floating cage designs are relatively new and thus less studied than bottom cages on both the oyster and ecosystem service fronts. To further scientific understanding of the influence of these two general gear types on oysters and other estuarine biota, we compared oyster size metrics, oyster death rates, and abundance and diversity of fishes and invertebrates between bottom cages and floating cages. We hypothesized that oysters would grow at dissimilar rates between cage types, and that diversity and abundance of other estuarine biota would vary similarly between the two gears. Our Methods We placed four oyster aquaculture cages – two bottom cages and two floating cages – at a nearshore brackish site in southern Delaware to examine oyster growth and the response of the aquatic community (Figures 4 and 5). We put 60 live oysters in each cage, and monitored the oysters, fishes, and invertebrates at each cage on a weekly basis from June through September 2022. We measured oyster shell length, the end-to-end distance from the hinge, and oyster shell depth, the thickness/cup-shape of the oyster, which are important factors influencing marketability. Thus, oyster farmers are interested in understanding how both shell length and shell depth may be influenced by gear type. To assess the use of the aquaculture cages by fishes and invertebrates, we removed portions of the gear from the water thereby removing the fishes and FIGURE 3. Juvenile black sea bass (Centropristis striata) captured in one of the oyster aquaculture cages. Photo by Timothy Smoot. FIGURE 4. A map of the study site. Four oyster aquaculture cages were situated in an estuarine boat basin connected to the Delaware Bay, USA. FIGURE 5. Oyster aquaculture gear types used in this study include (A) on-bottom cages that sit on the substrate and (B) floating cages which elevate the oysters to just below the water’s surface.
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