Understanding the Impact of Sea Lettuce (Ulva spp.) Density on Pacific Oyster (Crassostrea gigas) Growth in Puget Sound, Washington

• Main Authors: , Audrey Lamb, Martin, Erin, Davis, Jonathan
• Format: Text
• Language: English
• Published: Western CEDAR 2017
• Subjects: Fresh Water Studies; Life Sciences; Marine Biology; Natural Resources and Conservation; Pacific; Crassostrea gigas; Pacific oyster
• Online Access: https://cedar.wwu.edu/ssec/2016ssec/food_and_food_security/20

Biofouling, or the growth of other species besides the cultured species on aquaculture gear, is a frequent challenge for shellfish farmers. In areas with dense macroalgae growth, shellfish farmers can frequently spend resources managing macroalgae through manual removal from aquaculture gear during various phases of oyster culture. However, there is little research examining the effects of macroalgae density on oyster growth and survival, and as such, it is unclear if these manual removal techniques are actually needed. This study investigated the relationship between sea lettuce (Ulva spp.) density and Pacific oyster (Crassostrea gigas) growth and survival on two commercial oyster farms in Puget Sound, WA. Juvenile C. gigas were grown in the North Hood Canal and South Puget Sound from April through October 2015 in grow bags with different added wet weights of Ulva spp. (0 kg, 1.5 kg, 3 kg). Presence of Ulva spp. resulted in significantly smaller shell heights of C. gigas at both sites (final mean shell height of 61.0 ± 3.2 mm for the 3 kg treatment and 83.8 ± 3.9 mm for the 0 kg treatment in North Hood Canal). The presence of Ulva spp., rather than the amount of Ulva spp. in the treatment, had significant negative impacts on oyster shell height. Ulva spp. presence did not have a significant effect on mortality. The wet weight of Ulva spp. in the treatments decreased in the late summer and fall due to diminished on-site macroalgae presence. However, the shell height of oysters that were exposed to Ulva spp. months earlier remained significantly smaller on average, suggesting that early exposure to Ulva spp. has a lasting effect on growth even when the oysters are no longer exposed to Ulva spp. These findings have significant implications for aquaculture practices in that managing Ulva spp. on aquaculture gear early in the growing season can lead to more rapid crop turnover and increased production in a growing area over time.