WENNER: SPONGE-CORAL HABITAT FISHES 



Table 7. — Summary of biomass estimates from South Atlantic Bight sponge-coral and open-shelf habi 



tats and selected tropical reefs. 



abundance data, observations from submersibles, 

 underwater television observations of fish and their 

 reactions to trawls, and comparative gear obser- 

 vations. Since we lack the necessary data to compile 

 these values for South Atlantic Bight reef species 

 and, hence, to judge trawl efficiency in sampling this 

 habitat, the estimates of the present study should 

 be considered minimum values. However, some 

 qualitative gear efficiency observations can be 

 made. 



Since the % Yankee otter trawl net has an average 

 vertical mouth opening of only 1.25 m at a towing 

 speed of 6.5 km/h, it is inefficient in sampling many of 

 the commercially important lutjanids and sparids. 

 Although Pagrus pagrus , the most commercially im- 

 portant sparid in the South Atlantic Bight (Ulrich et 

 al. 1976), is a generalized benthic feeder (Manooch 

 1977), it frequently forms aggregations 2 m off the 

 bottom on intermediate depth reefs. Vermilion, 

 Rhomboplites aurorubens, and red snapper, Lutjanus 

 campechanus, often show the same behavior. Al- 

 though these species are represented in trap collec- 

 tions from various reef sites and are taken with 

 hook-and-line gear, their low relative abundance 

 (with the exception of small R. aurorubens in day 

 trawl tows) can be explained by their unavailabil- 

 ity to the net due to their position in the water 

 column. 



Comparisons of the dominant species from sponge- 

 coral and open- shelf habitat studies in the South 

 Atlantic Bight show that, with one exception, the 

 sparid Stenotomus aculeatus is the most numerous 

 trawl-caught demersal teleost. Powles and Barans 

 (1980) cited tomteXe, Haemulon aurolineatum, as the 

 most abundant fish taken with the % Yankee trawl at 

 a sponge-coral site at a depth of 37 m. Stenotomus 

 aculeatus taken in stratified- random otter trawl sam- 



pling in the open-shelf habitat were most abundant in 

 depths <28 m (Wenner et al. 1979a, b, c, d). The 

 means of the natural logarithmic values of the num- 

 ber of S. aculeatus per tow for each of the sponge- 

 coral habitat sites are negatively correlated with the 

 depth of the site (r = —0.89, n = 7). The numerical 

 dominance of tomtate in the previous trawl study 

 (Powles and Barans 1980) is a reflection of the 

 greater site depth. 



The most apparent feature of the species diversity 

 and community analysis is the marked difference 

 between day and night tows. The increased species 

 diversity for night collections is attributed to a 

 greater number of species and a more even distribu- 

 tion of individuals among species. Hoese et al. ( 1 968) 

 reported more species taken in night than day trawl 

 tows in Texas coastal waters and attributed the dif- 

 ferences to changes in species availability (move- 

 ment from the sampling area; vertical movement 

 above the headrope; burrowing into the substrate) or 

 vulnerability (increased daytime net avoidance due 

 to trawl visibility). 



Most reef studies using visual census techniques 

 have shown the diversity of fish species to be higher 

 during the day than at night (Collette and Talbot 

 1972; Goldman and Talbot 1976; Helfman 1978). 

 The results of the present study indicate the opposite 

 to be true. Day trawl tows in the sponge-coral habitat 

 had a lower diversity because these areas lack many 

 of the diurnal species of labrids, scarids, pomacen- 

 trids, and acanthurids found in tropical systems. In 

 addition, where night trawl samples contained fish 

 that may be diurnal, the lack of hiding places 

 associated with the luxuriant coral growth in the 

 tropics increased the vulnerability of these fish to 

 night trawl tows, despite the fact they were not ac- 

 tively moving about and feeding. 



547 



