A study of the effects of shrimp trawling on benthic communities in two South Carolina sounds

Fisheries Research, 12 ( 1991 ) 139-156

139

Elsevier Science Publishers B.V., Amsterdam

A study of the effects of shrimp trawling on benthic communities in two South Carolina sounds* Robert F. Van Dolah, Priscilla H. Wendt and Martin V. Levisen Marine Resources Research Institute, P.O. Box 12559, South Carolina Wildlife and Marine Resources Department, Charleston, SC 29412-2559, USA (Accepted 19 March 1991 )

ABSTRACT Van Dolah, R.F., Wendt, P.H. and Levisen, M.V., 1991. A study of the effects of shrimp trawling on benthic communities in two South Carolina sounds. Fish. Res., 12: 139-156. Two estuarine sounds in South Carolina were studied to evaluate the effects of commercial shrimp trawling on the abundance, diversity and species composition of benthic infaunal assemblages. In each sound, two areas were sampled just prior to the opening of the shrimp trawling season and then again after 5 months of trawling activities. One area was located in a portion of the sound which was actively trawled and the other area was located in a nearby portion of the sound closed to trawling. Significant differences were observed between sampling periods in both sounds with respect to total faunal abundance, the relative abundance of dominant taxa, and the total number of species. Changes in species composition were also noted between sampling dates. Indices of species diversity and the relative proportion of species representing major taxonomic groups in each area were generally similar over time. The reduction in faunal abundance and number of species observed in all four areas during the second sampling period was more likely due to natural seasonal variability rather than trawling effects since there were no significant differences between trawled and non-trawled sites with respect to these parameters. There were also no obvious differences in species composition among the trawled vs. non-trawled areas based on cluster analysis. Although this study was not designed to address all of the potential impacts of trawling activities on benthic organisms, lack of any consistent differences among sites with respect to the community parameters assessed suggests that 5 months of trawling in the areas studied did not have a pronounced effect on the abundance, diversity or composition of the soft-bottom communities sampled.

INTRODUCTION

In 1986, the South Carolina Legislature passed a bill which temporarily closed the state's sounds to trawling for penaeid shrimp. This closure was prompted, in part, by public concern that trawlers were having a destructive effect on bottom habitats and communities in these areas. Infaunal benthic *Contribution No. 298 from the South Carolina Marine Resources Center.

0165-7836/91/$03.50

© 1991 Elsevier Science Publishers B.V. All fights reserved.

140

R.F. VAN DOLAH ET AL.

communities in the bays and sounds are of special concern because they are a primary food source for penaeid shrimp (McKenzie, 198 l; Hunter and Feller, 1987 ), and they are either directly or indirectly important in the diets of blue crab and finfish. Thus, detrimental effects on infaunal communities resulting from trawling could have far-reaching consequences. Relatively few studies have examined the effects of trawling in various marine habitats, and those studies have noted variable effects depending on the type of gear used and bottom conditions. For example, a study of fish trawls by Graham (1955) in the North Sea showed that trawl damage to benthic species preyed upon by fishes was not serious, although trawling with heavy tickler chains damaged Sabellaria worm tubes and other fragile projecting structures. Bridger (1970) noted that even small otter boards and nets equipped with lightweight chains caused considerable bottom disturbance, and damage to benthic marine life was enhanced when tickler chains or heavy leg chains were used. Gibbs et al. (1980), on the other hand, concluded that otter trawling in New South Wales for shrimp did not cause any detectable changes in the macrobenthic fauna of the trawl grounds. The net gear used in that study had no tickler chains, and the nets were dragged so that the footrope only lightly skimmed the bottom. Substantially greater damage has been noted when trawling was conducted over hard-bottom habitats where many sessile species, such as sponges and corals, are present (Tilmant, 1979; Wenner, 1983; Van Dolah et al., 1987). Hard-bottom habitats in South Carolina's bays and sounds are sparsely distributed and not considered to be prime habitat for white shrimp, Penaeus setiferus, or brown shrimp, Penaeus aztecus, the two most abundant species harvested in the state. While hard-bottom areas would probably be affected by shrimp trawling, the effects of trawling on soft-bottom benthos appear to be less certain. Two of the sounds temporarily closed by the State Legislature differ from the other embayments in South Carolina in that the established ocean sound trawl fishery boundary lines are well within the entrance to the sounds, resulting in only a partial closure of these estuaries. This provided an opportunity to evaluate changes in the abundance, diversity and species composition of benthic invertebrate communities related to commercial shrimp trawling in soft-bottom areas. METHODS

Four areas were selected for sampling in Port Royal and St. Helena Sounds (Fig. l ). The "trawl" area in each sound was located in a portion of the sound which was actively trawled during the shrimping season. These areas are known to be favored "trawl alleys" by the shrimpers and they are bordered by shoals which helped to confine trawling through the sampling areas. The

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"control" area in each sound was located in a portion of the sound which remained closed to trawling during the 1987 shrimping season. In Port Royal Sound, the control site was located approximately 6 km from the trawl site in an area with similar hydrographic and bottom sediment conditions. This control area was also more than 3 km from the trawl boundary (Fig. 1 ) to minimize risk of the area being trawled illegally. In St. Helena Sound, the control site was located less than 1 km from the trawl site, but the two sites were separated by a draw bridge which precluded trawling past the trawl boundary line. Within each of the four study areas, 12 sampling stations were established in a fixed-grid design using Loran-C positioning referenced against stationary landmarks (Fig. 1 ). Each study area encompassed approximately 0.5 km 2 of bottom habitat in Port Royal Sound and approximately 0.2 km 2 in St. Helena Sound. Bottom sediments and infaunal communities were quantitatively sampled twice during 1987 at each of the 48 stations using a 0.05 m: Ponar grab. The first set of samples was collected in April, just prior to the opening of the shrimping season. The second set was collected in October, following approximately 5 months of trawling in the sounds. Each grab sample was processed in the field by collecting a subsample for sediment analyses, and sieving the remainder through a 0.5 m m mesh sieve. Organisms and sediment retained on the sieve were preserved in a 10% formalin/seawater solution having rose bengal stain. Hydrographic measurements of surface and bottom water temperature, salinity, dissolved oxygen, and pH were collected from the four study areas on each sampling date. In the laboratory, benthic organisms were sorted, identified to species level or the lowest taxonomic level possible, and counted. The sediment samples were analyzed to determine per cent weight of sand, shell and silt/clay fractions using procedures described by Folk ( 1965 ) and Pequenat et al. ( 1981 ). Sand fractions were dry sieved using a Ro-tap mechanical shaker fitted with 14 1/2 phi-interval screens for grain size determinations. Measurements of total organic matter were obtained by burning a portion of each sample at 550°C for 2 h as described by Plumb ( 1981 ). Statistical analyses of sediment and biological variables involved separate two-way analyses of variance to compare data collected in the trawl and control areas before and after 5 months of trawling in Port Royal and St. Helena Sounds. Numbers of organisms and species were first log-transformed to correct for non-normality and heterogeneity of variances. Power analyses performed on the transformed data using the power test described by Cohen ( 1988 ) showed that 12 samples per treatment area generally provided a greater than 80% probability of detecting at least a 25% change in the mean number of species and total abundance of organisms per sample. Sediment data were transformed using the angular transformation (arcsin of the square root o f p

EFFECTS OF SHRIMP TRAWLING ON BENTHIC COMMUNITIES

143

where p is the proportion of sand, silt/clay or CaCO3 in a particular sample). The Duncan's multiple range test was used for all a posteriori comparisons among sample groups. Normal cluster analyses were used to describe patterns of similarity in species composition and abundance among all grab samples collected from each of the two sounds. The Bray-Curtis similarity coefficient and flexible sorting strategy ( f l = - 0 . 2 5 ) were applied to log-transformed abundances of those species that occurred in at least two collections. The study sites in both sounds were observed on several occasions to confirm that commercial shrimpers were dragging nets over the established sampling grids. During those occasions, one to four boats were observed to be making repeated drags through the trawl areas established for sampling. Several shrimpers were also interviewed to confirm that the areas encompassed by the sampling grid were actively trawled during the shrimping season. Shrimpers working in the St. Helena Sound area commented that they had made at least 30-50 drags through the test area and that they had observed at least 20 other trawlers working the same area during the study period. Shrimpers working in the Port Royal Sound area commented that they had made at least 15-30 drags through the test area during the study period and that they had observed several other boats working in the area as well. Although the trawl areas sampled were actively dragged during this study, it should be noted that the 12 sampling sites within each trawl area were probably subjected to variable trawling pressure. RESULTS

Comparison of physical environments Hydrographic conditions were similar between the trawl and control areas of each sound (Table 1 ). Salinities varied between the trawl and control areas by less than 2 ppt during each season and temperature differences in both sounds were within 1 ° C. Dissolved oxygen levels were near saturation with differences of less than 1 ppm between the trawl and control areas of each sound. All pH readings were also similar between areas (differences < 0.2 ). Sediments in the Port Royal trawl and control areas were very similar (Table 1 ), with the predominant component being quartz sand. The proportion of silt and clay was slightly higher in samples collected during October in both areas, but the differences were not significant ( P > 0.05, ANOVA). The sand fractions of most samples collected in April and October were moderately to poorly sorted ( S D ~ = 0 . 7 - 1 . 4 ) . Mean grain sizes of the sand fractions were similar in the two Port Royal areas during both sampling periods, with medium-textured sand being the predominant size class. Mean percentages of shell hash, as measured by a CaCO3 content, remained spatially and tempo-

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rally consistent, contributing only about 4% by weight to each sediment sample from the trawl area and less than 2% to those from the control areas. Organic matter generally comprised less than 5% by weight in both areas. Sediment characteristics exhibited greater differences between sites and sampling periods in St. Helena Sound than in Port Royal Sound (Table 1 ). Sediments collected during April and October in the St. Helena trawl area were primarily sandy. Sediments in the control area, however, were more variable among sampling dates. Comparisons of the percent silt/clay fraction by area and time (ANOVA, Duncan test) indicated no significant difference between the trawl and control areas in April, but in October there was a significantly greater percentage of silt/clay in the control area than in the trawl area ( P < 0.05 ). Within each area, however, there was no significant difference in the percentage of silt/clay between seasons. Fine sands were predominant in both areas during each sampling period. Standard deviations associated with the sand fractions from the trawl and control areas indicated a wide range of sorting from well to poorly sorted sands (SDq~=0.4-1.9) in both April and October. Calcium carbonate ranged from trace amounts to over 15% at the trawl site and 24% at the control site in April. This variability was also evident in October although there was an overall decrease in the mean percent CaCO3 component to less than 12% by weight in the trawl and control areas. Organic matter represented, on average, less than 10% of total sample weight in both areas during each season.

Comparison of bottom communities Benthic communities in the two areas of Port Royal Sound were similar with respect to the number of taxa collected. During the April pre-trawl season sampling period, 107 taxa were collected at the 12 trawl stations as compared with 104 taxa collected at the 12 control stations (Table 2 ). Fewer taxa were collected in each area during October, but the relative number of taxa in the trawl versus control area remained nearly constant. An analysis of variance performed on the number of taxa per grab in each area showed a significant time effect, but no significant area effect or area-time interaction effects (Table 3 ). The total numbers of macrobenthic taxa collected in the two areas of St. Helena Sound were also nearly equivalent. During April, the numbers oftaxa collected from the trawl and control areas were 93 and 94, respectively (Table 2 ). The number of taxa declined significantly in both areas by the October sampling period (Table 3 ), but no significant area effects or area-time interaction effects were noted in St. Helena Sound. Indices of species diversity ( H ' ) , evenness ( J ' ) and richness (SR) (Margalef, 1958; Pielou, 1975 ) were generally similar at the two Port Royal sites sampled, both before the trawl season had opened and after 5 months of

146

R.F. VANDOLAHET AL.

TABLE 2 Total number of species, percent of total by taxa, and species diversity estimates of infaunal assemblages sampled in Port Royal and St. Helena Sounds immediately before and during the shrimp trawling season Category

Port Royal Sound Control

Total numberofspecies Percent of total by taxa Polychaetes Amphipods Molluscs Decapods Echinoderms Isopods Cumaceans Nermerteans Other Species diversity H' J' SR

St. Helena Sound Trawl

Control

Trawl

Before season

During season

Before season

During season

Before season

During season

Before season

During season

104

78

107

83

94

63

93

80

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40 19 15 5 4 1 0 4 12

43 18 15 4 5 2 3 2 9

42 13 14 10 5 1 2 1 11

44 12 16 10 2 1 2 4 9

44 11 17 3 3 0 3 3 14

42 17 17 4 3 1 3 4 8

40 10 15 11 6 0 3 6 9

4.79 0.71 14.05

4.45 0.71 11.18

4.33 0.64 13.42

4.42 0.69 12.04

2.06 0.31 10.51

4.53 0.69 12.08

4.32 0.68 11.05

3.56 0.60 9.02

TABLE3 Coefficients of determination (r 2), g ratios and levels of significance associated with two-way ANOVA tests comparing the abundance of fauna and the number of species between trawl and control areas before and after 5 months of commercial shrimp trawling in Port Royal and St. Helena Sounds Port Royal

St. Helena

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EFFECTS OF SHRIMP TRAWLING ON BENTHIC COMMUNITIES

147

TABLE 4 Total faunal abundance, and percent o f total by taxa, for infaunal assemblages sampled in Port Royal and St. Helena Sounds before and during the s h r i m p trawling season Category

Port Royal Sound Control Before season

Meanabundance/0.05 m z Percent o f total by taxa Polychaetes Mollusks Amphipods Oligochaetes Nematodes Decapods Echinoderms Cumaceans Nemerteans Other

St. Helena Sound Trawl

During season

Before season

Control During season

Before season

Trawl During season

Before season

During season

127

82

224

76

581

80

169

106

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24 22 5 20 20 2 3 1 1 2

17 77 1 1 < 1 < 1 < 1 2 1 1

73 5 5 5 4 1 < 1 4 1 1

38 36 7 5 4 < 1 < 1 5 3 1

39 22 3 15 0 3 3 6 8 1

trawling had occurred at one of the sites (Table 2). Greater differences in diversity were noted between the trawl and control areas of St. Helena Sound both before and during the trawl season. Species diversity increased in the control area and decreased slightly in the trawl area from April to October, but diversity values remained higher at the trawl site compared to the control site even after 5 months of commercial shrimp trawling. Taxonomic groups with the greatest number of species in both sounds were polychaetes, amphipods and molluscs (Table 2). Other taxa represented by several species included decapods, echinoderms, cumaceans and nemerteans. Within each sound, the relative proportions of species representing the dominant major taxa were generally very similar between the sites before the trawl season commenced, as well as 5 months after the trawl season had opened. The total number of organisms comprising the major taxonomic groups, on the other hand, differed considerably between areas within each sound, and between sounds (Table 4). In Port Royal Sound, the most abundant taxa in the control area during April were amphipods, molluscs and polychaetes, whereas the most abundant taxa in the trawl area were polychaetes, oligochaetes, nematodes and molluscs. Six months later, the relative abundance of these and other taxa remained generally similar in the trawl area, but the most abundant taxa had changed in the control area to a predominance of polychaetes, nematodes and molluscs. In St. Helena Sound, polychaetes and molluscs comprised almost three-

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quarters of the total fauna collected from the trawl area in April (Table 4). Amphipods, cumaceans and oligochaetes were also abundant ( 18% of total fauna). Six months later, polychaetes and molluscs were still the predominant taxa although proportionally more polychaetes were present than molluscs. Relative abundances of other taxa in the trawl area did not change markedly, except for oligochaetes, which nearly doubled in abundance. Polychaetes and molluscs were also the two most abundant taxa in the control area of St. Helena Sound in April, but molluscs (primarily Mulinia lateralis) were much more abundant than polychaetes (77% versus 17% of total fauna collected). Six months later, polychaetes comprised 73% of the total number of organisms collected, whereas the relative abundance of molluscs had declined to only 5% of the total fauna collected.

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Fig. 3. Dendrogram showingthe relative similarity of samples collectedat the trawl and control sites of St. Helena Sound using the Bray-Curtis similarity coefficient. Comparisons of species composition and the relative abundance of fauna in each sound using cluster analysis showed no obvious separation of station groups by area (i.e. trawl versus control stations). The dendrogram resulting from an analysis of the Port Royal data separated stations into eight groups with low to moderate similarity values among the stations within each group (Fig. 2 ). Seven of the eight groups contained both trawl and control samples collected during one or both seasons. In St. Helena Sound, the cluster analysis produced seven station groups (Fig. 3). Only two of the station groups contained samples from both seasons and those included only trawl sites. In general, faunal similarity was greater among pre-season and mid-season samples collected from the trawl area than among those from the control area. Comparisons of mean faunal abundance per grab sample showed little evidence for trawl effects on the density of benthic infauna in either sound. Prior

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TABLE 5

Species accounting for > 2.00% of the total fauna collected from each area of St. Helena Sound during a sampling period. Values represent mean abundance, + standard error, during April (before trawl season ) and October (during trawl season), 1987 Species

Mulinialateralis Paraprionospiopinnata Lumbrineristenuis Mediomastuscaliforniensis Oligochaeta

Leuconamericanus Nematoda

Sabellariavulgaris Galeommatacea

llyanassa obsoleta Leitoscoloplosfragilis Tellinatexana Bateacatherinensis Heteromastusfiliformis Nuculaproxima Spheniaantillensis

Trawl area

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Before

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45.1+11.8 8.7+4.2 15.8+ 4.5 13.4+ 5.2 8.4+ 3.4 5.9+ 3.4 6.8+ 3.2 1.1+ 0.7 1.6+ 1.0 -* 15.8+ 4.5 4.4+ 1.6 3.4+ 3.0 4.1+ 2.4 2.5+ 1.2 -

1.3+0.4 22.3+8.3 4.1+ 1.5 2.3+1.5 15.5+4.7 6.3+3.6 6.4+2.5 2.3+1.7 13.7+6.3 4.1 + 1.5 2.3+1.3 1.9+1.5 0.8+0.1 2.6+1.2 2.3+ 1.6

425.6+121.1 7.3+ 3.7 32.2+ I1.0 21.8+ 4.9 3.5+ 1.I 10.7+ 3.7 0.5+ 0.3 1.6+ 1.0 0.1 + 0.1 12.3 + 12.2 32.2+ 11.0 0.5+ 0.4 1.0+ 0.6 1.4+ 0.5 0.1+ 0.1 0.2+ 0.2

0.3+ 32.4+ 7.4+ 1.3+ 3.9+ 3.2+ 3.5+ 10.8+ 7.4+ 0.7+ 1.2+ 0.2+ 0.1+ 1.8+

0.2 10.0 3.2 0.6 0.9 1.5 1.6 8.3

3.2 1.0 0.7 0.1 0.1 1.3

*-, organism not collected.

to the shrimping season, the average abundance of infauna at stations in the trawled area of Port Royal Sound was 224/0.05 m 2 (Table 4). Six months later, infaunal densities in this area had declined to an average density of 7 6 / 0.05 m 2. Faunal abundance in the control area also decreased between April and October, although the decline was less marked and initial densities of infauna in the control area were lower than in the trawl area. Analysis of variance indicated significant time effects, but no significant area effects or areatime interaction effects, suggesting that the observed differences were the result of seasonal variation rather than an effect of trawling (Table 3 ). In St. Helena Sound, faunal density in the control area was initially much greater than that observed in the trawl area (Table 4). The density in both areas had declined by October. A two-way analysis of variance indicated significant main effects due to area and time, as well as a significant area × time interaction effect (Table 3 ). The a posteriori comparisons indicated no significant difference in faunal abundance at the trawl area over time. A significant difference was noted, however, in faunal abundance at the control area between seasons. Among the numerically dominant species collected in St. Helena Sound, four (the polychaetes Paraprionospio pinnata, Lumbrineris tenuis, Leitoscoloplos fragilis and Mediomastus californiensis) were significantly less abun-

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TABLE7

Species accounting for >- 2.00% of the total fauna collected from each area of Port Royal Sound during a sampling period. Values represent mean abundance, + standard error, during April (before trawl season ) and October (during trawl season), 1987 Species

Trawl area Before

Nematoda Oligochaeta Nuculaproxima Tellina texana

Protohaustorius deichmannae Clymenella torquata Mediomastus californiensis Batea catharinensis Nephtyspicta Oxyurostylis smithi Tharyxsp. Unciola serrata Sphenia antillensis Ampeliscaabdita

Actiniaria sp. A Polycirrus eximius

49.7 + 57.8 + 10.7+ 15.6 + 1.3 + 14.8 + 5.3 + 0.1 + 3.2+ 3.6 + 1.7+ 2.4 + 0.2+ 2.7+ -

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Before

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14.4 + 6.3 15.5 + 7.1 16.5+6.3 9.3 + 3.1 0.3 + 0.3 0.3 + 0.2 2.2 + 1.3 4.0 + 2.2 0.9+0.3 4.8 + 1.7 4.2+2.0 3.3 + 1.8 2 . 6 + 1.8 2.9 + 2.8

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dant 5 months after the initial sampling period (Tables 5 and 6). This was the case in both the trawl and control areas, however, indicating that the declines were a consequence of natural seasonal variability rather than an effect of commercial shrimp trawling. The lack of any significant interaction effect between the two factors tested (area and time) supports the contention that seasonal differences in abundance of those species analyzed were consistent between the two areas. The bivalve Mulinia lateralis also exhibited a pronounced seasonal decline in abundance in both the trawl and control areas, but its highly contagious distribution resulted in heteroscedastic variances which precluded any analysis of its abundance using parametric statistics. This was true for several other species as well. The pattern was essentially the same in Port Royal Sound. Among the numerically dominant species, two bivalves (Nucula proxima and Tellina texana ) and one amphipod (Ampelisca abdita) were significantly less abundant in October than they were in May in both the trawl and control areas (Tables 7 and 8 ). As in St. Helena Sound, these declines in abundance were probably the result of seasonal variations unrelated to the effects of commercial shrimp trawling. Other numerically dominant species present in both areas of Port Royal Sound had distributions that were not amenable to statistical

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R.F. VAN DOLAH ET AL.

comparisons and the effects of commercial shrimp trawling on their abundance remain largely unknown. DISCUSSION

Comparisons of various community parameters assessed in the trawled and non-trawled portions of Port Royal and St. Helena Sounds provided no clear evidence of trawl effects on the benthic infaunal communities related to commercial shrimping. Declines were observed in the density and number of species present in both the trawled and control areas of both sounds. While some of these declines may be attributable to trawl effects, others were probably a consequence of natural seasonal variability. Additionally, there were no statistically significant differences between areas with respect to the community parameters measured during the October sampling period; nor was there any obvious separation among sites based on the cluster analyses. It is apparent from the results of the two-way analyses of variance that the factors tested (area and time ) never accounted for more than 51% of the total variance in abundance of any numerically dominant species, as indicated by the coefficient of determination (Tables 6 and 8). This suggests that some factor (s) other than those tested (e.g. differential responses to sediment type or the natural tendency of some species toward contagion) was largely responsible for the observed differences in the distribution and abundance of species in both St. Helena and Port Royal Sounds. Although sediments did vary among sites within each study area, hydrographic and sediment conditions at the two sites in Port Royal Sound were generally similar. Hydrographic parameters were also similar between the two areas of St. Helena Sound, but the sediments were not. The generally finer sediments observed in the control area may have accounted for some of the differences between sites in species composition and faunal abundance during both sampling periods. However, 8 of the 10 numerically dominant taxa collected in the trawl area were also among the dominant species found in the control area (Table 5 ) during April, and many of the other general community parameters were similar between sites. The lack of any consistent differences between trawled and control areas with respect to the abundance, diversity or species composition of benthic communities in both of the sounds we studied supports the observation by Graham ( 1955 ) and Gibbs et al. (1980) that otter trawling on sandy/muddy bottoms causes little, if any, degradation of the infaunal communities. Gibbs et al. (1980) concluded that the lack of any effect on the benthos in their study of the prawn fishery in two estuaries of New South Wales was due to the fact that the trawl gear only lightly skimmed the bottom. This is not the case in South Carolina where shrimpers often use trawls equipped with tickler chains, which would be expected to cause greater disturbance in the upper

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few centimeters of sediments than trawls without chains. Trawler agitation has also been shown to increase the risk of predation for many infaunal species (Caddy, 1973), and may result in the injury or death of some benthic organisms, particularly filter feeders and surface deposit feeders (Graham, 1955; Bridger, 1970). However, our data provide no evidence that there was any significant difference in the abundance or diversity of benthic organisms between the trawled and non-trawled areas studied, even after 5 months of commercial trawling activity in the sounds. The degree of trawling pressure at each of the 24 sites sampled in both sounds is unknown but was probably quite variable. This may have affected the variability among stations with respect to many of the parameters analyzed; however, the study areas selected were within popular trawling alleys which are probably subjected to more intensive trawling than many other areas in the sounds. Therefore, the lack of any conclusive evidence of significant trawl effects on the abundance, diversity or community structure of benthic organisms in these areas suggests that benthic communities in other soft-bottom areas within the sounds may be affected similarly or to an even lesser extent. No hard-bottom habitat was detected in any of the study areas. Hard-bottom habitats support numerous species of hydroids, erect and encrusting bryozoans, ascidians, sponges and soft corals. These epifaunal communities, in turn, provides excellent habitat for many demersal fishes, including commercially and recreationally valuable species. Because previous studies of hardbottom areas have shown that trawling causes damage or loss of epifaunal organisms (Wenner, 1983; Van Dolah et al., 1987), these areas should be avoided by trawlers to minimize adverse impacts. In summary, the results from this study suggest that commercial shrimp trawling in two of South Carolina's sounds did not have a significant adverse effect on the benthic community parameters measured. However, it is important to note that the study was not designed to assess all ecological changes that might result from trawling. ACKNOWLEDGMENTS We wish to thank several people who assisted on this project. David Knott and Bob Martore assisted in the field sampling. Charlie Zemp helped by monitoring the shrimping activity in the sounds and interviewing several shrimpers. He and the staff at the Waddell Mariculture Center also provided space for processing our samples. Sarah Miller, Debbie Paquette, Terry Bates, and Bob Steffen assisted in sample sorting and/or data tabulation. Karen Swanson provided graphics assistance, Betty Wenner provided statistical advice and Margaret Lentz typed the report. We also wish to thank Betty Wenner

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and David Whitaker and the anonymous reviewers for their comments on previous drafts o f this manuscript.

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