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Effects on Roller Trawling on A Hard Bottom Sponge and Coral Community
Prepared by: 
Robert F. Van Dolah
Priscilla Hinde
Nick Nicholson
INTRODUCTION
The continental shelf off Georgia and South Carolina is characterized by broad expanses of smooth sand bottom interspersed with areas of low relief hard ground and rocky outcrops. These habitats provide hard substrate for a diverse assemblage of sessile invertebrates which, in turn, attract a variety of motile species including many demersal fishes. Several species, such as snapper, grouper, porgy, and black sea bass, are important to offshore commercial and recreational bottom fisheries.
Traditionally, these fish have been caught with hook and line, fish traps and, to a lesser extent, with spear guns by divers. In recent years, the use of fish trawls in hard bottom areas has increased. This can be attributed to (l) the development of new gears designed to avoid hanging on rocks and large sessile invertebrates; (2) the depletion of fish stocks in other areas; (3) the recent-discovery of extensive hard bottom areas off the coasts of South Carolina and Georgia; and (4) an increased need for the shrimp industry to broaden its source of income.
In addition, several research programs have used or are currently using trawls to study groundfish communities in hard bottom habitats (Struhsaker, 1969; Ulrich et al., 1976; George and Staiger, 1978; Miller and Richards, 1980; Powles and Barans,1980; Grimes et al., 1982; South Carolina Wildlife and Marine Resources Department [SCWMRD], 1982; Wenner, 1983; Sedberry and Van Dolah, in press). Research trawls may have an even greater impact on benthic communities than commercial trawls since the latter may be towed more rapidly to avoid catching large sessile invertebrates.
Concern that trawl gear may cause long-term or irreversible damage to hard bottom habitats has been expressed by commercial and recreational fishing groups as well as management agencies. Previous studies on the effects of otter trawls on benthic communities generally have been restricted to sand bottom sites (Graham, 1955; Bridger, 1970; Anonymous, 1971; Caddy, 1973; Gibbs et al., 1980). Thus far, only one study has documented damage to hard bottom communities caused by roller-rigged trawls (Tilmant, 1979); however, the low faunal abundance's reported in that study suggest that the results may not have been statistically significant.
The threat of habitat destruction or alteration is of
particular concern to the Marine Sanctuaries Program, whose
sanctuaries coincide with areas where trawls may be used for
commercial fishing or scientific collecting. In light of
this concern, this study was undertaken with the following
three objectives: l) determine the effects of trawling on
large benthic invertebrates associated with an inshore hard
bottom habitat; 2) determine the rate at which these large
sessile invertebrates recover, in terms of growth and
recolonization, after trawling in this area; and 3) examine
the invertebrate communities associated with these large
sessile invertebrates.
DESCRIPTION OF STUDY AREA AND DOMINANT FAUNA
The site selected for study was located approximately 32.5 km east of St. Catherines Island, Georgia and 25 km north-northeast of the Gray's Reef National Marine Sanctuary. Water depth at the study site was approximately 20 m. This hard bottom area was chosen for study because it was typical of low relief hard bottom habitat in the South Atlantic Bight and because it was easily relocated due to its proximity to an artificial reef (Georgia "Reef J"). Additionally, macrofaunal assemblages in the area were similar to those present at Gray's Reef, permitting the study of trawling effects without risk to Sanctuary resources.
Preliminary diver surveys characterized the study area as having a smooth rock bottom covered by a thin layer of sand. Although rock ledges were noted nearby, no emergent rock outcrops were observed in the area to be trawled. Sessile invertebrate growth was extensive throughout the area with sponges and corals being the largest growth forms. Ascidians, hydroids, bryozoans and algae were also common, but these taxa were generally represented by low or encrusting growth forms which should not be as seriously affected by roller trawling as the larger sponges and corals.
Three of the most abundant large sponges, Haliclona oculata , Ircinia campana , and Cliona spp., were selected for assessment (Figs. 2-4). All of these species have been collected in roller trawl samples taken at inner shelf hard bottom stations (SCWMRD, 1982). The finger sponge, H. oculata , is an upright ramous sponge which usually occurs as a single stalk with multiple branches. It has a spongy texture and the branches are quite flexible. The purple vase sponge, I. campana , is a keratose sponge which persistently occurs in a cup or vase shape. The texture of this sponge is firmer than H. oculata , but it is still quite pliable. Finally, the boring sponges, Cliona spp., are upright and cake or barrel shaped in the gamma stage. These sponges have a distinct dermis covered with tubercles and are stiff in consistency. Cliona spp. are similar in shape and sometimes confused with the massive loggerhead sponge Spheciospongia vesparium , which is also common in hard bottom areas of the South Atlantic Bight (SCWMRD, 1982; Wenner et al., 1983).
The dominant octocorals in the area were the whip coral Leptogorgia virgulata , the false sea fan Lophogorgia hebes , and the stick coral Titanideum frauenfeldii . All of these species were more common than the sponges and generally grew tall enough to be collected in roller trawl sampling at inner shelf hard bottom areas (SCWMRD, 1982, Wenner et al., 1983).
The only hard coral found in the area was the branching
tree coral Oculina varicosa. This species occurred
only rarely in the study area, and the colonies were usually
small. Even though 0. varicosa was uncommon, it was
assessed in this study due to its fragile composition and,
consequently, to its susceptibility to damage from
trawling.
SUMMARY AND CONCLUSIONS
1. A trawl alley was established in a hard bottom area located approximately 25 km NNE of the Gray's Reef National Marine Sanctuary to evaluate the effects of roller trawling on sponge and coral assemblages commonly found in hard bottom habitats of the South Atlantic Bight.
2. The species selected for study were the finger sponge Haliclona oculata , the vase sponge Ircinia campana , the barrel sponges Cliona spp., the whip coral Leptogorgia virgulata , the fan coral Lophogorgia hebes , the stick coral Titanideum frauenfeldii , and the stony tree coral Oculina varicosa . These were the most abundant large sessile organisms in the area.
3. Five transects were established across the alley: three in an area of high coral and sponge density, and two in an area of high coral but low sponge density. For the pre-trawl assessment divers counted all sponges and corals in 20 replicate quadrats located along each transect (total of 100 quadrats). Quadrats measured 25 m2 for all species except the stick coral ( T. frauenfeldii ), which was only counted in a l-m2 area within each larger quadrat. After the pre-trawl census, a 40/54 fly trawl was dragged through the alley once. This trawl is identical to the one used for research at Gray's Reef and is similar to those used by commercial trawlers. After trawling operations, transects were reestablished and eight of the 20 quadrats initially censused on each transect were randomly selected for reassessment: four in the trawl path and four in control (non-trawled) areas. These same quadrats were assessed again one year later. Changes in the density of undamaged sponges and corals were evaluated statistically, and qualitative observations were made on the type of damage to each species.
4. Results of the trawl damage assessment study documented damage to all species counted, but only the density of barrel sponges ( Cliona spp. ) was significantly decreased (P < 0.05, ANOVA) by trawling activities. The extent of damage or loss among the other large sponges ( I. campana, H. oculata ), octocorals ( L. virgulata, L. hebes, T. frauenfeldii ) and hard corals ( 0. varicosa ) varied depending on the species, but changes were not statistically significant (P > 0.05). Twelve months after trawling, no damaged sponges or corals were observed in the area and the abundance of specimens counted in the trawled quadrats had increased to pre-trawl densities or greater.
5. Trawl damage documented in this study was less severe than the damage observed in the only other similar study in Florida waters. The discrepancies between studies are attributed to (l) differences in the roller-rig design of the trawls, and (2) differences in the number of times the same bottom was trawled. Repeated trawling over the same bottom with the research trawl used in this study would probably have caused much greater damage to the sponge and coral populations than that caused by a single tow.
6. In the second phase of this investigation, growth rate studies were conducted on selected species ( Cliona spp., I. campana, H. oculata, L. hebes, 0. varicosa ) to further evaluate recovery rates. Both damaged and undamaged specimens of most species were tagged and then measured at different time intervals over a nine-month period. Measurements included maximum height for all species, as well .as various diameter and circumference measurements for selected sponges.
7. Measurements of sponge and coral specimens provided only limited information on species growth rates because of unexpected changes in sediment depths at the base of tagged specimens. Estimates of yearly increases in the size of sponges, based on changes in circumference and diameter, indicate that Cliona spp. and I. campana grow relatively slowly, suggesting that it may take several years for damaged specimens to reach pre-trawl sizes.
8. In the third and final phase of this investigation, three specimens of each sponge and coral species censused in the trawl damage study were collected in the vicinity of the trawling alley for study of their associated fauna.
9. Results of the associated fauna study indicated that all sponge and coral specimens harbored numerous commensal species, several of which have been shown to be important prey items for hard-bottom demersal fishes. Polychaetes and amphipods represented the most diverse and abundant taxa among all host specimens as a group. Polychaetes were numerically dominant on the finger sponge H. oculata , the vase sponge I. campana and the stony coral 0. varicosa , while amphipods were numerically dominant on the barrel sponge Cliona spp. and all three octocorals (L. virgulata, L. hebes , and T. frauenfeldii ). The stony coral 0. varicosa harbored the most species and had the highest species diversity (H') among all hosts studied. All the octocorals had higher diversity values than the sponges, despite having fewer commensal species. This was due to the fact that species associated with octocoral hosts were more evenly represented, with respect to abundance, than those associated with sponges.
10. Most of the species inhabiting the sponges and-corals examined in this study appear to be fairly ubiquitous with respect to their association with various host species. Some have also been shown to be free-living inhabitants of sediments in hard bottom areas. Nevertheless, several species were much more abundant on sponges and corals in this study than they were in bottom samples taken in other studies. This suggests that, while few of the species collected in this study are obligate commensals of sponges and corals, many are attracted to these large macroinvertebrates, possibly as a source of food or as refuge from predation.
11. Based on the results of this study, previous research
trawling in the Gray's Reef Marine Sanctuary (SCWMRD, 1982;
Sedberry and Van Dolah, in press) may have affected the
sponge and coral populations present in the area. This, in
turn, could have reduced the populations of invertebrate
fauna associated with these species. However, the use of our
trawl roller rig, in combination with a deployment strategy
which avoided trawling over the same bottom more than once,
probably prevented any serious damage to the hard bottom
habitat or its associated fauna.
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