Distribution of acoustic fish backscatter associated with natural and artificial reefs in the Northeastern Gulf of Mexico

Abstract

Artificial reefs have been deployed throughout the northern Gulf of Mexico in an effort to enhance fish production and mitigate losses from the depletion of natural habitats. Measuring the efficacy of artificial reefs in these capacities requires both fine- and large-scale direct comparisons between natural and artificial reef structures. We conducted mobile active acoustic surveys to examine intra-reef and regional spatial patterns of acoustic fish backscatter in ~2 km2 survey areas over 14 natural and 17 artificial reefs along the northwestern Florida continental shelf. Nested generalized additive models (GAMs) showed that intra-reef backscatter from fish at natural reefs was diffused throughout the survey area forming low density patches, whereas backscatter at artificial reefs was concentrated in high density hot-spots that were constricted horizontally within a mean of 116 m away from the structure. The exact distance from artificial structures at which backscatter was decreased to background levels, termed the area of influence, was found to be mediated by reef longitude, relief, area, and depth. On a regional scale, hierarchical GAMs (HGAMs) which incorporated altitude from the seafloor explained a larger proportion of deviance in backscatter than either latitude or longitude at both habitat types, indicating that the horizontal distribution of backscatter at these habitats varied throughout the water column. Trends in the horizontal distribution of backscatter were primarily driven by scatterers within 20 m of the seafloor at natural reefs and within 10 m of the surface at both natural and artificial reefs. HGAMs which examined only the horizontal distribution of backscatter showed a negative gradient of fish backscatter at natural reefs from west to east along the northwestern Florida continental shelf which was significantly influenced by reef depth. At both reef types, backscatter from fish was highest in the bottom 10 m of the water column and declined significantly with increasing altitude from the seafloor.