WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2023 39 (CONTINUED ON PAGE 40) height and increased wave frequency, the unattached oysters likely rolled around within their bags, and these small collisions with both the bags and other oysters may have resulted in minor chips to new growth that cumulatively resulted in smaller oysters over a monthslong timescale. The submerged bottom cages appeared to experience less surface wave activity related to a decline in wave energy with depth and the weight of the bottom cages anchoring them to the substrate. Additionally, we noted, but did not directly measure, a relative increase in the presence of barnacle settlement, algal growth, and sediment coverage of the oysters within floating cages compared to oysters in the bottom cages despite identical weekly cleaning routines. We did not detect any significant differences in oyster mortality between the cage types, but the presence of other filterfeeding organisms, such as barnacles and materials covering the oysters in the floating cages may have prohibited maximum oyster growth through local competition or other inhibitory means. Influence of Gear Type on Fishes and Invertebrates When examining trends in the fishes around the oyster aquaculture cages, we noticed that there were some stark differences between cages of the same type or between the two floating cages. Abundance and diversity of fishes was lower at one of the floating cages than the other three cages (Figure 7), and we believe that those findings may be influenced by characteristics of the surrounding environment. The substrate directly underneath the floating cage with lower fish abundance and diversity was an open, sandy area with little to no submerged aquatic vegetation or other cover. The second floating cage was initially placed in similar conditions to the first, but it became surrounded by dense growths of sea lettuce (Ulva lactuca) over the progression of the summer months. The two bottom cages were in sandy areas with low to moderate vegetation cover. In comparison to the benthic-oriented bottom cages and the vegetation-surrounded floating cage, the sandy-area floating cage may have represented a predation risk for young fishes which would have had to cross an open water column to reach the floating structure. Many fishes prefer to avoid open water to reduce risks associated with predation from larger fish or birds, so it is likely that fewer fish chose to spend time around this sandy-area floating cage. Interestingly, while the sea lettuce growth around the vegetation-surrounded floating cages led to more total fishes present, the diversity of fishes there was intermediate and not quite as high as at the bottom cages. Vegetation cover appears to provide a pathway that encourages some, but not all, fish species to travel higher in the water column to reach floating cages. When looking at trends in invertebrates between the oyster aquaculture cages, we saw that abundance of total invertebrates present in the floating cages was higher than in the bottom cages (Figure 8). This was attributed to high numbers of grass shrimp (Palaemon sp.) at the floating cages (Figure 9), which is in contrast to lower fish abundance at floating cages unless additional vegetative cover was present. One potential explanation for high FIGURE 6. Oyster length over time in the bottom and floating oyster aquaculture cages. FIGURE 7. Cumulative counts of individual fishes captured from within and around each oyster aquaculture cage from June 14 – September 25, 2022 in a Mid-Atlantic estuary. FIGURE 8. Cumulative counts of individual motile invertebrates captured from within and around each oyster aquaculture cage from June 14 – September 25, 2022 in a Mid-Atlantic estuary.
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