Reef fish dynamics on Louisiana artificial reefs: the effects of reef size on biomass distribution and foraging dynamics

Abstract

Understanding the role that habitat plays in the life history of reef-associated fishes is particularly significant given the dramatic increase in the number of artificial reefs deployed in coastal ecosystems over the past 50 years. In the Gulf of Mexico, the oil and gas industry has added a significant amount of structure to the Louisiana continental shelf, creating the largest de facto artificial reef deployment area in the world. Noting their usefulness as fish habitat, the Louisiana Artificial Reef Program was established to convert decommissioned platforms into artificial reefs. However, very little quantitative information exists on how these habitats affect the associated fish assemblage. The two objectives of this study were to examine high-resolution spatial and temporal distribution around two standing and two toppled platforms, and to examine the trophic ecology of common reef-associated fishes, such as red snapper (Lutjanus campechanus). Spatial distribution of fish biomass was examined using a multifrequency hydroacoustic approach to examine the extent of the area of influence around the two habitats and to examine diel changes in distribution. Standing platforms supported roughly two times higher biomass than toppled platforms, particularly in the upper water column at close ranges to the structures. Diel periodicity was evident, with higher biomass in the upper water column during the night and higher biomass in the lower water column during daylight hours. Diel periodicity was dependent on habitat and distance from the reef, breaking down at close range to standing platforms, likely a result of the light field emitted by working platforms. Trophic ecology was assessed with a combination of gut content and stable isotope analyses to examine both prey and sources of basal resources to the reef habitats. Results indicate that red snapper are opportunistic feeders, and that artificial reef structures do not provide a unique set of prey items, indicating that prey and basal resources are consistently sourced from the surrounding water column and soft bottom sediments. Additionally, no evidence of an area of prey depletion (feeding halo) was found around the two habitats, further indicating that prey is derived opportunistically from areas surrounding the reef structures.