Native fish community structure and Indo-Pacific lionfish Pterois volitans densities along a depth-temperature gradient in Onslow Bay, North Carolina, USA

Abstract

We quantified native fish densities along a depth-temperature gradient within a tem- perate-tropical marine transition zone (North Carolina continental shelf) to examine the role of depth and temperature in structuring these communities. We also examined the distribution of in- vasive lionfish (Pterois volitans, P. miles) to evaluate a laboratory-derived thermal minimum for lion- fish. We collected year-round continuous bottom water temperature data from 2000 to 2010 and sur- veyed lionfish (2004 to 2010), conspicuous fishes (2006 to 2010) and cryptic smaller-bodied fishes (2007 to 2010) at depths from 5 to 46 m using SCUBA. Bottom water temperatures were constant across the depth gradient during summer and increased from inshore to offshore during winter. The conspicuous fish community was structured by 3 depth zones, 5-14, 15-37 and 38-46 m, that corre- sponded with winter mean temperatures of 13.9, 17.9 and 20.9°C, respectively. The cryptic fish community was structured by 4 depth zones, 5-15, 18-24, 27-38.5 and 39.5-46 m, with correspon- ding winter mean bottom temperatures of 13.8, 15.6, 18.7 and 20.9°C. In contrast, summer tempera- tures were not important in structuring either the conspicuous or the cryptic fish community. Thus, fish communities in the spring/summer appear to be structured by the pattern of bottom water tem- perature experienced the previous winter, supporting previous studies that indicate winter mini- mum temperature is important for determining fish distribution and abundance in temperate marine ecosystems. In addition, the deeper fish communities were dominated by tropical species. Lionfish, a tropical species, was found in the highest densities from 38 to 46 m and present in loca- tions with a winter mean of 15.3°C and higher. Increasing temperatures could favor a potential ex- pansion of invasive lionfish and native tropical species into the nearshore waters on the North Car- olina shelf, resulting in unforeseen community structure and trophic disruptions.