Spatial and temporal variation in shark communities of the lower Florida Keys and evidence for historical population declines

Abstract

Sharks are top predators in many marine ecosystems. Despite recent concerns over declines in shark populations, studies of shark communities in coastal habitats are limited. We used drumlines and longlines to determine shark community composition and habitat affinities in the Florida Keys, USA. Community composition varied among habitats. Catch rates of smaller sharks were highest in protected shallow waters, while large sharks were more abundant in deep channels. Overall probabilities of catching large sharks on drumlines did not vary with water temperature, while catches of small sharks on longlines increased with increasing water temperature. Individual species differed in their responsiveness to variation in water temperatures and habitat. Bait type affected catch rates of some species, suggesting that fishing methods should be considered explicitly in studies describing shark communities or temporal trends in abundance. Catch rates of large-bodied sharks were higher in a remote and protected location compared with similar habitats near inhabited Keys. Also, historical accounts of a shark fishery in the study area during the 1920s suggest substantial declines in large shark abundance and shifts in community composition. By implication, ecosystem impacts of changes in the large shark community may be dramatic and likely occurred before adequate baselines were established.