Evaluating biodegradable alternatives to plastic mesh for small-scale oyster reef restoration

Abstract

Polyethylene plastic mesh is commonly used for containing oyster shells in small-scale oyster reef restoration, but environmental and public health concerns have prompted investigations of biodegradable alternatives. Shallow (<0.5 m) and deep (approximately 1 m) oyster reefs (approximately 6 m2) were constructed in the Mission-Aransas Estuary, Texas, U.S.A., in March 2020 using recycled oyster shells placed into four different replicated mesh bag types: polyethylene (plastic) and three biodegradable alternatives (cellulose, cotton, and jute). Biodegradable alternatives (cellulose, cotton, and jute) all completely degraded within 2 months of deployment, leaving piles of loose shell, while polyethylene bags remained intact. Despite rapid degradation, the biodegradable/loose shell successfully recruited and developed larger oysters (mean of 46 mm) than on the polyethylene-bagged shell (mean of 40 mm) after 7 months, although at less than half the density. Associated motile fauna density in the bagged shell was 2.4 times higher than in the loose shell after 7 months at both the deep and shallow locations. Faunal community composition and diversity varied more with reef depth than by bag type. The total cost of using polyethylene bags was lower than for biodegradable alternatives (22–45% the cost of cellulose, 35–72% the cost of jute, 49–99% the cost of cotton). However, because our estimate of the environmental cost of polyethylene plastic mesh only included impacts on marine natural capital, the true cost is likely much higher. Despite higher costs, biodegradable alternatives can still be successful for use in small-scale oyster restoration events without introducing plastics into the marine environment.