FISHERY BULLETIN: VOL. 81, NO. 3 



Comments on Use of Otter Trawl 



Nets in 



Sponge-Coral Habitat Surveys 



Demersal trawl surveys are conducted in the 

 northeast region of the United States to determine 

 the species composition, distribution, and relative 

 abundance of finfish, squids, and decapod crus- 

 taceans (Grosslein 1969). This survey has been 

 designed so that the investigators can use some as- 

 pect of the catch per unit effort from research vessels 

 to obtain an index of relative abundance for some 

 groundfish stocks. This index, with appropriate ad- 

 justments, can be used (see Clark and Brown 1977) 

 to derive regional stock-size estimates independent 

 of the commercial catch. These, in turn, can be 

 utilized for management decisions. In addition, some 

 indication of the abundance of prerecruits can be ob- 

 tained from the juveniles retained in the small mesh 

 cod end liner of most research survey nets. 



Although trawl gear can determine the size and rela- 

 tive abundance of many species that are available 

 and vulnerable to the gear, its use to assess commer- 

 cially harvested species of the sponge-coral habitat is 

 suspect for the following reasons. First, as previously 

 indicated, most commercially important lutjanids, 

 sparids, and serranids are adept at avoiding the sur- 

 vey gear. High rise nets (otter trawls with greater ver- 

 tical mouth openings) are more efficient in obtaining 

 representative samples of these fishes, but swept- 

 area estimates of the density of these species are un- 

 reliable. Most sponge- coral habitats have sections of 

 small rock outcroppings and ledges where epi- 

 nepheline serranids, lutjanids, and sparids ag- 

 gregate. Because of possible gear destruction due to 

 the high relief, these areas are untrawlable and 

 catches in the remainder of the habitat show these 

 species to be either absent or of minor importance. 

 Trap and handline collections in untrawlable areas of 

 this habitat document the localized abundance of 

 these animals and indicate the importance of these 

 species to the fish community. 



Second, even with cod end liners, the % Yankee and 

 high-rise roller trawl nets fail to give indications of 

 the abundance of prerecruits (Wenner unpubl. data). 

 Available data indicate that the juveniles of epi- 

 nepheline serranids are either spatially separated 

 from the adults (Mycteroperca microplepis) or are 

 cryptic forms which hide in cracks and crevices in this 

 habitat (Johnson 5 ). Juvenile Rhomboplites auro- 



5 G. David Johnson, South Carolina Wildlife and Marine Resources 

 Dept, P.O. Box 12559, Charleston, SC 29412, pers. commun., 

 1981. 



rubens are available and vulnerable to the gear during 

 late summer months on 30-40 m sampling sites. This 

 is the only species of the commercially important 

 complex that is taken with any regularity. 



Thus, otter trawl gear is neither effective in predict- 

 ing recruitment of most commercially important 

 species, nor reliable in providing accurate abundance 

 estimates of these species. In addition, the physical 

 habitat may sustain considerable damage caused by 

 the bottom trawl net. 



The 3 4 Yankee trawl net effectively covers a much 

 wider area of the bottom than the measured sweep 

 (8.7 m) due to the configuration of the otter doors, 

 ground cables, and bottom leg lines. Although this 

 arrangement cannot increase the actual spread of the 

 net beyond the headrope length, the passage of these 

 cables over the substrate creates a disturbance that 

 serves to herd fish in the path of the net (Baranov 

 1969). This net does, however, damage the sponge- 

 coral habitat by shearing off sponges, soft corals, 

 bryozoans, and other attached invertebrates. The 56 

 trawl tows made in the sponge- coral habitat for this 

 study collected 2,351 kg of attached invertebrates 

 (including sponges, soft corals, tunicates, bryozoans, 

 and hydroids) yielding an average 42 kg/tow. This is 

 only the amount of bottom material actually removed 

 from the habitat. An estimate of the total amount of 

 bottom destroyed by the doors, ground cables, and 

 leg lines cannot be ascertained from the current 

 study. 



Personal observations and interviews with commer- 

 cial fishermen attest to the productivity of the 

 sponge-coral habitat. Most studies indicate the im- 

 portance of habitat availability and space in deter- 

 mining the abundance and diversity of reef fishes 

 (Emery 1978). With this in mind, and given the 

 knowledge that 1) the use of the % Yankee trawl net 

 reduces the amount of attached invertebrate growth 

 (the amount damaged by doors and ground cables is 

 presently not quantifiable); 2) the places where the 

 invertebrates had been attached may be sanded over 

 and rendered unsuitable for recolonization; and 3) 

 the removal of these attached invertebrates reduces 

 refugees for decapods, polychaetes, etc., that are 

 food items for Centropristis striata and other benthic 

 feeders, one must conclude that the continued use of 

 this trawl net reduces the amount of productive fish 

 habitat. For these reasons, in addition to the ineffec- 

 tiveness of the gear in sampling commercially impor- 

 tant species, alternate nondestructive methods, such 

 as direct observations or the use of mark-recapture 

 techniques with trap catches, should be employed in 

 assessment surveys of the commercially important 

 species of this habitat. 



550 



