Macroalgal growth on bivalve aquaculture netting enhances nursery habitat for mobile invertebrates and juvenile fishes

Abstract

Destruction and degradation of biogenic nursery habitats, such as seagrass and coral/ oyster/polychaete reefs, threaten coastal fisheries worldwide. We tested the hypothesis that macroal- gae and epifauna growing upwards from protective plastic mesh used in bottom clam culture substi- tutes for seagrass as a nursery habitat for mobile invertebrates and juvenile fish. By quantifying bio- mass of epibiota in each season and by seining both day and night on 11 occasions from August 1997 to April 1999, biogenic habitat structure and habitat use by mobile invertebrates and juvenile fishes were quantified on hard clam Mercenaria mercenaria aquaculture leases that were using 2 alter- native grow-out methods and on 2 natural habitats, a seagrass bed (Zostera marina and Halodule wrightii) and an unstructured sandflat. The macroalgal/epifaunal biomass per unit bottom area was significantly greater on aquaculture mesh than on the sandflat but did not differ significantly from the natural seagrass biomass density on most sampling dates. Community structure of mobile inverte- brates and juvenile fishes utilizing clam leases was more similar to that of seagrass than sandflat habitats. Community similarity among the structured habitats, the seagrass and the 2 lease types, was greater than the similarity between night and day within any given habitat. The total numbers of mobile invertebrates summed over all 11 dates were 75 times greater in the seagrass than in the sandflat habitat, whereas the structural habitat provided by epibiota attached to bottom mesh on leases provided a 44-fold enhancement of invertebrates over the corresponding counts on unstruc- tured sandflat. Utilization by juvenile fishes was 3 times greater in seagrass and 3 to 7 times greater in epibiota on mesh in clam leases than on sandflat habitat. For example, juvenile reef fishes exhib- ited almost equal utilization of epibiota on leases and seagrass while remaining absent over the sand- flat, which lacked emergent structure. Thus, the biogenic habitat provided largely by macroalgal growth on protective bottom mesh of clam leases supports elevated densities of mobile invertebrates and juvenile fishes similar to that of natural seagrass habitat, thereby representing a previously un- documented ecosystem benefit of bivalve aquaculture. This ecological role for structural habitat rising above clam aquaculture leases is consistent with a broader recognition that artificial reefs, plastic seagrass, oyster shell mounds, and other emergent bottom structures provide habitat services.