© 2006 International Council for the Exploration of the Sea
An investigation of the cumulative impacts of shrimp trawling on mud-bottom fishing grounds in the Gulf of Maine: effects on habitat and macrofaunal community structure
University of Maine, Darling Marine Center 193 Clark's Cove Road, Walpole, ME 04573, USA
*Correspondence to A. W. Simpson: tel: +1 2075633146, ext 254; fax: +1 2075638407. e-mail: anne.simpson{at}maine.edu.
The cumulative impacts (i.e. collective, multi-year effects) of seasonal commercial shrimp trawling on habitat and macrofaunal community structure were investigated for two mud-bottom fishing grounds and adjacent untrawled areas in the Gulf of Maine. Habitat structure on mud-bottom fishing grounds did not differ significantly from that in similar untrawled areas. Moreover, sediment resuspension associated with shrimp trawling did not appear to result in net loss of deposited material on fishing grounds, but there is evidence that trawling may alter sediment mixing regimes. Visual inspection of the sediment surface in trawled areas revealed minimal evidence of fishing gear disturbance (such as door, bobbin, or net marks), but biological disturbance features, including numerous large burrows, pits, and dense aggregations of the brittle star Ophiura sarsi, were present in both trawled and untrawled areas. Macrofaunal communities on the two fishing grounds exhibited different responses to shrimp trawling, which were attributed to disparities in levels of fishing activity during the 2000–2001 shrimp season. The results suggest that seasonal shrimp trawling produced at least short-term changes (<3 months) in macrofaunal community structure, but did not appear to result in long-term cumulative changes. Resilience to trawling disturbance may be due in part to high levels of biological disturbance generated by benthic megafauna, such as lobsters and fish. By burrowing, pit-digging, and possibly foraging, these animals rework sediments to a depth of 16–17 cm, creating a natural level of disturbance that appears to maintain macrofaunal communities in a perpetually low successional state, so potentially minimizing trawling impacts.
Keywords: fishing disturbance, fishing impacts, habitat structure, macrofaunal community structure, trawling impacts
Received 28 January 2005; accepted 27 July 2006.
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Introduction |
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 Top Introduction MethodsResultsDiscussionReferences |
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Bottom trawling is a pervasive agent of anthropogenic disturbance in many of the world's most productive marine environments (Messiah et al., 1991; Dayton et al., 1995; Auster et al., 1996; Kaiser, 1998; Rijnsdorp et al., 1998; Thrush et al., 1998; Watling and Norse, 1998; McConnaughey et al., 2000; National Research Council, 2002). Among the immediate and direct effects of trawling are the removal of target and non-target species (Jennings and Kaiser, 1998; Hall, 1999; Kaiser and de Groot, 2000), and physical disturbance of biogenic and abiotic habitat structures (Auster et al., 1996; Collie et al., 1997; Engel and Kvitek, 1998; Freese et al., 1999). The ecological consequences of trawling vary depending on both the substratum type and the degree of natural disturbance present in the environment. A growing body of scientific literature supports the idea of a continuum of trawling impacts in which benthic habitats with high-relief biogenic structures, such as sponges, corals, and seagrass beds, as well as areas with few large-scale natural disturbances, are more visibly impacted by trawling than shallow, low-relief areas of the seabed that experience frequent large-scale natural disturbances (see reviews in Collie, 1998; Jennings and Kaiser, 1998; Auster and Langton, 1999; Hall, 1999; Collie et al., 2000; National Research Council, 2002).
The effects of trawling on habitat structure are most visibly apparent in relatively high relief areas of the seabed such as boulder, gravel, and even rippled sand bottoms (reviews in Watling and Norse, 1998; Auster and Langton, 1999). Mud substrata that are characterized by overall low topographic structure, but a high degree of small-scale physical, chemical, and biological habitat complexity, may also be severely impacted. Thrush et al. (2001) showed that biodiversity in soft sediment environments is positively associated with habitat structure, and suggested that removal of structure by fishing disturbance could reduce diversity.
On mud bottoms, much of the biological community resides below the sediment surface, so most of the habitat structure is not visible from above the seabed. In these subsurface environments, bioturbating organisms (animals that disrupt the sediment matrix by their movements and/or feeding activities) create three-dimensional structure and habitat heterogeneity (Widdicombe et al., 2000). By increasing the depth of oxygen penetration into anoxic sediments, bioturbators may enhance biodiversity of infaunal communities (Widdicombe et al., 2004).
Trawling causes resuspension of the upper layers of fine sediments and may also mix or bury remaining substratum (Caddy, 1973; Churchill, 1989; Mayer et al., 1991). This intense disturbance can alter physical and chemical sediment profiles (Mayer et al., 1991; Pilskaln et al., 1998; Smith et al., 2000; Watling et al., 2001) and surficial habitat complexity by creating deep gouges or ruts on the sediment surface (Caddy, 1973; Friedlander et al., 1999; Roberts et al., 2000). However, the impact of chronic trawling disturbance on infaunal habitat structure has not been well studied. Trawling may secondarily reduce mud-bottom habitat structure by removal or mortality of the sediment-dwelling organisms (especially bioturbators) that increase physical and chemical heterogeneity in soft sediments through burrowing, feeding, and related activities (Aller, 1982, 1988; Wheatcroft et al., 1990; Widdicombe et al., 2004).
Relative to sand, gravel, and mixed sediment types, comparatively few studies on trawling impact have been conducted on mud (silt/clay) bottoms, despite the existence of commercial trawl fisheries for groundfish and shellfish in fine-sediment habitats. Most studies conducted on mud bottoms have employed manipulative field experiments to examine short-term effects of known levels of trawling effort (Mayer et al., 1991; Riemann and Hoffmann, 1991; Tuck et al., 1998; Hansson et al., 2000; Lindegarth et al., 2000; Sanchez et al., 2000; Sparks-McConkey and Watling, 2001). The results of short-term experiments provide insight into specific mechanisms of trawling disturbance, but these experiments cannot approximate either the spatial extent or the chronic levels of disturbance present in commercial trawl fisheries (Thrush et al., 1998; Jennings et al., 2002).
The only study to date to examine the effects of a commercial trawl fishery on a mud-bottom habitat was conducted by Smith et al. (2000) in the eastern Mediterranean Sea. Results from that study show strongly negative impacts of trawling on benthic macrofauna and megafauna, as well as significant differences in sedimentary organic carbon, chlorophyll, and phaeopigments between lightly and heavily fished areas (Smith et al., 2000). Other studies on fishing grounds with mixed sand and mud (silt/clay) substrata have shown that chronic levels of trawling disturbance reduce the biomass of infauna and epifauna (Jennings et al., 2001), decrease production of large infauna (Jennings et al., 2002), and alter the size structure of benthic infaunal communities (Duplisea et al., 2002).
One of the most important trawl fisheries on mud-bottom habitats in the Gulf of Maine is an annual winter fishery for the northern or pink shrimp (Pandalus borealis). In an area that extends from approximately the southwestern portion of the Gulf of Maine through western Penobscot Bay to Cape Cod, the species is harvested by trawls and traps when ovigerous female shrimp migrate from deeper waters to shallower, nearshore areas where eggs are released. The shrimp-trawling fleet is made up mainly of vessels <25 m long that tow otter trawls. Nets generally have 21 m sweeps, 5 cm mesh, and are equipped with rockhopper gear. The fishing season is characterized by intense fishing activity on mud, sandy mud, and where these unconsolidated sediments intersect with rockier seabed. The length of the fishing season is highly variable from year to year (0–180 days) owing to dramatic fluctuations in annual shrimp populations.
We examined the cumulative impacts of the seasonal shrimp-trawling fishery on (i) the physical structure of sedimentary habitats, and (ii) the biological structure of the resident benthic macrofaunal community of relatively deep (>80 m) nearshore mud-bottom fishing grounds in the mid-coast region of Maine. Based on the assumption that these mud-bottom habitats possessed a high degree of small-scale structural complexity and were subject to few large-scale natural disturbances, we anticipated that habitat and macrofaunal community structure would be significantly affected by trawling.
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Methods |
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The study consisted of an 18-month field sampling programme conducted on two shrimp fishing grounds and adjacent unfished areas. Common difficulties associated with locating appropriate untrawled areas for control sampling sites (Hall, 1999) were dealt with by situating control treatment sampling stations in areas that are the current and historical territory of fixed gear fishers, and therefore not subject to trawling. In addition to the presence of fixed gear as an obstacle to trawling, one of the two control sites was located in a lobster conservation zone, whereas the other control was in an area that is difficult to access with mobile gear.
Study sites
Based on information on the location of past and present shrimp-trawling effort, two study areas with similar habitat features and fishing history were selected. Each area encompassed both traditional fishing grounds and an adjacent area of similar habitat type where shrimp trawling does not take place. Detailed spatial records of fishing effort are not available for the Maine shrimp fishery, so it is not possible to know the precise trawling intensity for any given area. By consulting with both fishing industry partners and the State of Maine Department of Marine Resources (DMR), the state governmental agency that monitors the fishery, we were able to obtain general information on both historic and recent fishing effort for the two study areas. Site 1 was located off Monhegan Island and site 2 was south of Boothbay Harbor, near the Outer Pumpkin Ledges (Figure 1). Water depths in the study areas ranged from 86 m to 102 m and 84 m to 91 m at Monhegan and Pumpkin, respectively. Bottom water temperature and salinity were similar at both sites, with temperature ranging seasonally from 2.8°C to 9.4°C and salinity from 32.4 to 33.1. Sediment grain sizes (as measured by grain surface area; Mayer and Rossi, 1982) were characteristic of silt and clay, and were similar at both study sites.
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The Monhegan study area was located in one of the state of Maine's two Lobster Conservation Zones. Lobster fishers in this zone have the exclusive right to fish the waters in a 4.8 km radius around Monhegan Island with fixed gear during a 6-month fishing season, 1 December–31 May. Several Monhegan Island lobster fishers stated that no shrimp trawling had been observed, and no fixed gear had been lost to trawls in several deep mud basins off the east side of the island (S. Stanley and M. Thompson, pers. comm.). The area appears to be sheltered from shrimp-trawling activity by both historic lobster fishing territory boundaries and relatively high densities of fixed gear during the winter shrimp season. Based on the above information, the untrawled area at Monhegan was located in one large, and a second adjacent smaller, muddy basin (43°45'N 69°17'W) roughly 2 km east of Monhegan Island. The trawled area at Monhegan (43°46'N 69°17'W) was situated approximately 1 km northeast of the island on a relatively wide expanse of mud bottom. The area has been historically trawled for shrimp by boats from the neighbouring Port Clyde fishing fleet. Monhegan Island lobster fishers as well as fishers from the nearby fishing town of South Bristol reported seeing Port Clyde fishing vessels trawling for shrimp in this location in recent years (E. Gastaldo and M. Thompson, pers. comm.).
The untrawled area at the Outer Pumpkin Ledges study area was in a narrow, short, north–south running channel (43°43'N 69°35'W), which makes it a difficult and undesirable area to tow. Both mobile and fixed gear fishers noted that the area is dominated by fixed gear (lobster and shrimp traps) during the winter shrimp-fishing season (B. McLean, E. Gastaldo, and W. Autio, pers. comm.). Local fishers confirm, and fisheries monitoring data support, the notion that shrimp trawling has not occurred at this location in recent or past decades (DMR, unpublished data). The trawled area at Pumpkin was situated in a wide, easily towed, north–south running channel (43°44'N 69°33'W). Information supplied by fishers and independent fisheries monitoring data document historic and recent shrimp trawling there (E. Gastaldo, B. McLean, M. McLellan, and W. Autio, pers. comm.; DMR, unpublished data).
Study design
The field study was developed as a block design experiment where the treatment variable, the presence of shrimp trawling, was assigned with a priori knowledge of fishing activity. To avoid difficulties associated with pseudo-replication, data were collected at two separate study sites, each consisting of an area of trawled and adjacent untrawled bottom. At each study site, five replicate box cores were collected at initially haphazardly selected stations in each treatment area. Weather and equipment problems resulted in fewer samples being collected during the first two sampling periods, yielding an unbalanced design in June 2000 and October 2000. Sampling stations were re-located using a Global Positioning System (GPS), and cores were collected within approximately 25 m of the designated station location. In most cases, data from Monhegan and Pumpkin were analysed separately on the assumption that different sampling blocks were not equivalent (Krebs, 1999).
Sample collection
From June 2000 through December 2001, sediment samples were collected approximately every 60–120 days at both study sites using a 0.0625 m2 GOMEX style box core (Boland and Rowe, 1991). Cores were subsampled for sediment X-radiography (data not presented here), porosity, and macrofauna. In March 2001, one box core collected in the Monhegan trawled area and a second collected in the untrawled study area were subsampled for vertical profiles of excess 210Pb activity. In August 2001, all porosity cores were subsampled for grain surface area measurement. Video quadrat surveys were conducted in April 2002 and June 2002 to determine surface burrow densities in both study areas using a drop camera fitted with a 1.0 m2 frame in the field of view. Qualitative observations of surficial sediment structures and surface-dwelling macro- and megafauna were carried out in October 2001 and 2002 using a Phantom 300 remotely operated vehicle (ROV). Bottom temperature and salinity measurements were made throughout the study using a CTD (Sea-bird Electronics, Inc.) and temperature data logger (HOBOTemp, Onset Computer Corporation).
Sampling procedures and data analysis
Video quadrat surveys: surface burrow densities
Surface-penetrating burrow densities were measured using video quadrat surveys to determine whether shrimp trawling had an impact on surficial habitat complexity. Digital video tape from the quadrat survey was analysed by obtaining a single frame immediately before the quadrat came to rest on the bottom. At each study site 100 quadrats were obtained, 50 from each treatment area. From each image, the number and the size (diameter) of all visible burrows were systematically recorded. Burrows were grouped into seven 1-cm size categories, based on diameter. Variation in burrow density between untrawled and trawled areas was examined for each size category using a Mann–Whitney rank test.
Porosity
Porosity samples were collected from each box core using a clear acrylic tube with a 10 cm internal diameter. Each sample was sectioned at 1 cm intervals to a depth of 10 cm. Sections were placed into individual airtight containers for transport to shore. In the laboratory, each sample was homogenized, and an approximately 2.0 g (wet weight) subsample was transferred to an aluminium drying pan. Sediment wet weight was measured, and samples were subsequently dried to a constant weight in an oven at 60°C for approximately 24 h. Dry samples were allowed to cool in a moisture-free environment, then re-weighed to obtain total dry weight.
The relationship between sediment porosity and depth was examined by a one-way analysis of variance (ANOVA) for August and December 2001. Differences in sediment porosity between trawled and untrawled areas were analysed by individual 1-cm depth increments from 0 cm to 10 cm using a three-way, fixed factor ANOVA with sampling date (DATE), study site (SITE), and the presence/absence of trawling activity (TREATMENT) as the main factors. Data were tested for normality and homoscedasticity using the Kolmogorov–Smirnov and modified Levene tests, respectively, and met these criteria in all but a few cases.
Excess 210Pb activity
Physical re-working of upper sediment layers was examined using 210Pb, a naturally occurring particle-reactive radioisotope commonly used as a tracer for sediment mixing studies. Cores for excess 210Pb activity profiles were collected from both trawled and untrawled stations at Monhegan using a 10 cm internal diameter clear, acrylic tube. Sediment from each sample was sectioned at 1 cm intervals for the complete length of the core. The outermost layer (
5 mm) of sediment was carefully scraped away and discarded to avoid sample contamination resulting from down-core smearing during core extrusion. In the laboratory, each sediment sample was homogenized and a 2.0 g subsample was removed for porosity measurement. The remaining portion of each sample was lyophilized to remove water, then disaggregated, and 5 g of dried sediment was transferred to pre-weighed plastic vials where samples were packed to produce uniform geometry among samples. Uranium-series activities, including that of 210Pb, were measured by non-destructive gamma spectrometry using a single, closed-end, coaxial well germanium detector (Canberra model GCW3523/S). 210Pb activity was measured at the 46.5 keV energy peak. Supported levels of 210Pb were obtained by measuring the activity of 214Pb at the 352 energy peak. Detector counting efficiencies were determined by counting a U.S. Department of Energy certified "pitchblende ore" uranium standard (CRM-103A) of known activity, with all daughter isotopes in secular equilibrium. Sources of error associated with counting, background radioactivity, and detector counting efficiency were calculated and corrected for in final 210Pb activity measurements. Both error and activity measurements were corrected for radioactive decay, which occurred between sample collection and counting time. Excess 210Pb activity values were plotted by depth. Total excess 210Pb inventories were obtained by summing excess activity profiles of both the trawled and the untrawled cores.
Sediment macrofauna
One sample for quantitative community analysis was collected from each box core using a clear acrylic tube of 6.5 cm internal diameter. Each tube core was sectioned into 0–2 cm, 2–4 cm, 4–6 cm, and 6–10 cm sediment depth increments. In the laboratory, all samples were sieved using a 0.5 mm screen mesh and preserved with a 10% buffered formalin solution within 12 h of sample collection. To obtain larger and deeper (>10 cm) burrowing macrofauna for qualitative community description, sediment remaining in the box core after the removal of sub-cores was sieved over a 2.0 mm mesh. Material remaining on the sieve was placed in a sample container containing 10% buffered formalin.
In the laboratory, macrofauna samples were sorted from non-biological material under a dissecting microscope. As a quality control, 20% of samples were re-examined to ensure that all macrofauna had been removed. All individuals were counted and identified to family level (Somerfield and Clarke, 1995), or to the next lowest possible level above family.
Macrofaunal abundance data from both tube cores and the sediment remaining in the box core were combined to determine the relative proportions of phyletic groups in the study areas. Temporal patterns of total abundance, average taxa abundance (richness), diversity (Shannon–Wiener), and evenness (Pielou's) were examined using data from the macrofaunal tube cores and analysed using Mann–Whitney rank sum tests. Because of unequal sample sizes, data from the first two sampling periods, June 2000 and October 2000, were not included when examining temporal trends.
Ordination analysis with non-metric multi-dimensional scaling (MDS) of Bray–Curtis similarity measures was used to identify patterns in assemblage composition (PRIMER 5.0; Clarke and Warwick, 1994). A one-way Analysis of Similarities (ANOSIM; Clarke and Green, 1988) was performed to test for statistically significant differences in taxon abundance between treatments (untrawled and trawled areas). Taxonomic groups that contributed the most proportionally to average Bray–Curtis similarity between sample groups were identified using the similarity of percentages in the SIMPER (Clarke, 1993) routine in PRIMER 5.0.
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Results |
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Impacts of shrimp trawling on habitat structure
The substratum in both study areas was predominantly fine, with relatively high water content mud with grain surface areas (20–30 m2 g–1) characteristic of silt-sized particles (Mayer and Rossi, 1982). Occasional large, glacial relic boulders were present at Monhegan. Structures created by biological activity, including tubes, burrows, and pits, were plentiful at both sites. Commonly observed surface-dwelling animals were the brittle star (Ophiura sarsi), lobster (Homarus americanus), northern shrimp (Pandalus borealis), silver hake (Merluccius bilinearis), snake blenny (Lumpenus lumpraeteformis), and unidentified burrow-inhabiting hake (Urophycis sp.).
Analysis of surface-penetrating burrow densities showed no clear persistent effect of shrimp trawling on this biogenic habitat feature. Burrows were significantly more numerous in the trawled area (p < 0.05, Mann–Whitney rank sum test) for all but the smallest and largest burrow diameter classes examined at Monhegan (Figure 2). This was not the case for burrows of any diameter in the trawled area at Pumpkin. Rather, burrows were significantly more numerous (p < 0.05, Mann–Whitney rank sum test) in the 3- and 4-cm diameter classes in the untrawled area at Pumpkin.
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Because of a sampling processing error, only porosity data for the sampling periods August 2001 and December 2001 were analysed. Porosity decreased significantly with depth in sediment (p < 0.05) at both study sites (Figures 3 and 4). Analysis of individual 1-cm depth layers from 0 cm to 10 cm showed significant differences in porosity (three-factor ANOVA) between Monhegan and Pumpkin at all depths, but no significant differences between untrawled and trawled areas at either site were detected (Table 1).
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= 0.05.Excess 210Pb activity profiles from untrawled and trawled areas at Monhegan revealed a relatively deep mixed layer (
16–17 cm) in both areas (Figure 5). Sediment inventory of excess 210Pb activity was higher (3.311 Bq g–1) in the trawled area than in the untrawled area (2.909 Bq g–1). The elevated inventory in the trawled area was due primarily to higher excess 210Pb activity in a 6 cm thick layer from 11- to 17-cm depth in the sediment. Surface activity of this isotope was slightly higher in the untrawled area (0.233 Bq g–1), but decreased with depth in sediment to activity values similar to those observed in the trawled area. There was a single subsurface peak in 210Pb activity in the 5–6-cm depth layer in the untrawled area profile, but all other activity values in the upper 20 cm of the profile were lower than those in the trawled area. The higher activity values in the excess 210Pb profile from the trawled area are consistent with more recently deposited (i.e. higher activity) material being mixed throughout the profile. The reason(s) for this are not clear, but may be due to elevated levels of mixing in the trawled area.
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Impacts of shrimp trawling on macrofaunal community structure
During the course of the study, 14 224 individuals in 52 families and eight higher taxonomic levels were collected. Annelids, represented by 24 polychaete families, were the most numerous and diverse group, constituting 61–66% of the community assemblage at Monhegan and 75–81% of the community at Pumpkin. The second most numerous phylum was molluscs, which was represented by five families and accounted for 27–29% of the community at Monhegan and 8–19% at Pumpkin. Echinoderms made up <8% of the community assemblage at all sites, and were represented by just two families, Caudinidae and Ophiuridae. Arthropoda and Nemertea accounted for just 1–3% of the community assemblage at all sites.
The greatest abundance of macrofauna in the upper 10 cm of sediment was found in the 0–2-cm depth interval (Figure 6). At Monhegan, 56–62% and at Pumpkin 46–66% of the total macrofauna was recorded in the upper 2 cm of sediment (Figure 6). Temporal patterns in total macrofaunal abundance in untrawled and trawled areas at each study site showed a seasonal peak in recruitment during summer (Figure 7). Macrofauna abundance was greatest in the untrawled site at Monhegan. Seasonal abundance peaks were lower at Pumpkin, but roughly equal in both untrawled and trawled areas there.
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The average taxa richness was significantly higher (p < 0.05, Mann–Whitney rank sum test) at Monhegan than at Pumpkin in all sampling periods from December 2000 to December 2001 (Table 2). No significant differences in mean taxa richness between untrawled and trawled areas were detected at either study site (Figure 8). Univariate measures of taxa diversity (Shannon–Wiener) and evenness (Pielou's) in untrawled and trawled areas were not significantly different at either site during the course of this study (Figure 9). Taxa diversity at Monhegan was significantly higher (p < 0.05, Mann–Whitney rank sum test) than at Pumpkin (Table 2). Evenness did not differ significantly between sites (Table 2).
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MDS ordination of pooled Monhegan and Pumpkin macrofaunal abundance data revealed apparent differences in assemblage structure between the study sites (Figure 10). Differences between sites were tested using a one-way ANOSIM and were statistically significant (p = 0.01; Global r = 0.528).
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There also appears to be a difference in the response of the macrofaunal community to trawling. At Monhegan, there were no clear differences in community composition between untrawled and trawled areas (Figure 10b). In contrast, there was a clear separation of community assemblages in untrawled and trawled areas at Pumpkin (Figure 10c). Statistically significant separation of treatments at Pumpkin was confirmed by the results of ANOSIM (p = 0.012; Global r = 0.355). No significant difference was detected between trawled and untrawled areas at Monhegan (ANOSIM; p = 0.579; Global r = –0.032).
SIMPER analysis was used to determine the contributions of macrofaunal taxon abundance to the average Bray–Curtis dissimilarity between untrawled and trawled sites at Monhegan and Pumpkin depicted in the MDS ordination diagrams. The average dissimilarity between areas at Monhegan was lower (average dissimilarity 29.70) than at Pumpkin (average dissimilarity 35.00). Families contributing cumulatively to 50% of the average dissimilarity (Cossuridae, Paraonidae, Sabellidae, Nuculidae, Spionidae, Ampharetidae, Capitellidae) were always on average more numerous in the untrawled area (Tables 3 and 4). Conversely, only two (Spionidae and Nuculidae) of the eight families contributing cumulatively to >50% of the dissimilarity between areas at Pumpkin were on average more abundant in the untrawled area; the remaining six families (Paraonidae, Cossuridae, Ampharetidae, Sabellidae, Phoxocephalidae, Cirratulidae) were all on average more numerous in the trawled area.
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Discussion |
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TopIntroductionMethodsResultsDiscussionReferences |
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The impact of mobile fishing gear on the seabed is related to both the intensity and the frequency of fishing (Watling and Norse, 1998; Auster and Langton, 1999). The intensity of disturbance created by otter trawls, such as those used in the Gulf of Maine shrimp fishery, is considered to be lower than for other types of mobile gear, such as beam trawls and dredges (Kaiser et al., 1996; Collie et al., 2000). During the shrimp-fishing season, from late winter through early spring, the frequency of disturbance in certain areas may be very high because the same well-known areas of unobstructed bottom are repeatedly towed. However, no shrimp-trawling activity is present in these areas during the rest of the year (the off-season). There is considerable interannual variability in fishing effort attributable to the large fluctuations in the annual populations of Pandalus borealis in the Gulf of Maine. For the duration of this study (2000–2001), very low shrimp abundance (Assessment Report for the Gulf of Maine Northern Shrimp 2000) resulted in extremely limited fishing activity, lasting just 25 days in the state waters of Maine in 2001 and 51 days in 2000, compared with a high of 180 days in 1994 (M. Hunter, DMR, pers. comm.).
Substantial differences in levels of shrimp-trawling activity between the two sites during this study had not been anticipated. Because the impacts of trawling disturbance may be highly temporally dependent (Watling and Norse, 1998; Auster and Langton, 1999; Kaiser et al., 2002), one can compare short-term vs. longer-term recovery dynamics by contrasting the responses of the more recently impacted community (Pumpkin) with those of a similar community that may show only persistent effects from long-term impacts (Monhegan). Results of MDS ordination analysis comparing communities in trawled and untrawled areas at both study sites show no apparent differences between treatment area (Figure 10). However, when community similarity between trawled and untrawled areas was analysed separately for each study site, it was clear that recent shrimp-trawling disturbance at Pumpkin produced changes in macrofaunal community structure, whereas the effects of older (at least one year previous) trawling activity could not be detected in the macrofaunal community (Figure 10) at Monhegan.
SIMPER analysis showed that communities in trawled and untrawled areas at Pumpkin were more dissimilar (average dissimilarity 35.00) than at Monhegan (average dissimilarity 29.70), the less recently impacted site. Dissimilarity between trawled and untrawled areas from recent trawling impacts at Pumpkin was due in part to increased abundance of burrowing and/or disturbance-tolerant taxa such as Cirratulidae, Cossuridae, and Paraonidae on fishing grounds (Tables 3 and 4). Trawling-sensitive taxa, such as the bivalve families Nuculidae and Nuculanidae, were more abundant in the untrawled area. Similar patterns in abundance of common taxa were not recorded at Monhegan, where nearly all taxa (except Nuculanidae) were more abundant in the untrawled area.
Mortality rates of infaunal organisms attributable to trawling are related to their body size and depth distribution in the sediment (Duplisea et al., 2002). Nearly all the animals collected in this study were very small (<1.0 cm) and were surface or near-surface dwelling. The depth distribution of sediment-dwelling macrofauna showed that 50–65% of the total abundance of organisms was in the upper 2 cm of sediment, except in the Pumpkin trawled area, where just 45% of the total macrofauna inhabited this upper sediment layer (Figure 7). Macrofaunal depth distributions suggest that recent shrimp-trawling activity may have caused mortality at Pumpkin, reducing the abundance of organisms in the upper 2 cm of sediment, perhaps by resuspension, given the small size of the surface sediment dwellers and the high water content of the sediment.
The macrofaunal communities on these mud-bottom fishing grounds showed a rapid rate of recovery following shrimp trawling. These results are generally consistent with those reported in other studies of community recovery following trawling disturbance on mud bottoms, or at least those dominated by small macrofauna species with high reproductive output. Elsewhere in the Gulf of Maine, Sparks-McConkey and Watling (2001) found that most of the infaunal community recovered to ambient levels within 3.5 months of experimental trawling. After a period of 16 months of experimental trawling, Tuck et al. (1998) reported nearly complete recovery of the infaunal community in 12 months.
The disturbance of the seabed by shrimp trawling should also be assessed in the context of sediment mixing by resident megabenthic fauna. Previous studies have shown that bioturbation by megafaunal species is a factor that influences infaunal community diversity in soft sediment habitats (Widdicombe et al., 2000, 2004). Biological sediment disturbance on the mud-bottom fishing grounds we studied was associated with the activities of lobsters, shrimp, brittle stars, and fish. ROV observations of the sediment surface revealed many small and large burrows and pits, probably created by crustaceans (lobsters and shrimp) and demersal fish. Dense, mobile patches of the brittle star, Ophiura sarsi, were also common. All the large taxa were observed creating localized sediment disturbance while simply moving over or foraging on the mud bottom of the study areas.
The effects of trawl-induced sediment disturbance on habitat structure were assessed using burrow density and sediment porosity. Burrows increase the sediment surface area exposed to the overlying water column and are sites of chemical reactions involving oxygen, dissolved metals, and other elements in solution (Aller, 1982). In addition, burrows are also sites of enhanced diagenesis and nutrient cycling (Aller, 1982, 1988; Furukawa et al., 2001). Substantive reduction in burrow densities by trawling might be expected to impact localized sediment elemental cycling and organic matter diagenesis (Aller, 1994; Widdicombe et al., 2004). Sediment porosity, on the other hand, is an indicator of sediment grain size and arrangement, and more generally of the capability of the sediment to support (in the physical, load-bearing sense) animals and their tubes at the sediment–water interface. The species composition of benthic communities is determined in part by porosity, water content, and grain size (Harrison and Wass, 1965; Rhoads, 1974; Ellingsen, 2002). Trawling mediated changes in porosity and/or sediment grain size, and related habitat structural features would, consequently, result in changes in benthic community structure.
At Monhegan, burrows of diameter 1–5 cm were more abundant in the trawled area (Figure 3). However, at Pumpkin, burrow densities were never significantly greater in the trawled area, and there were significantly more burrows of diameter 3–4 cm in the untrawled area. The presence of burrows of small to intermediate size on the Monhegan trawling grounds suggests that some recolonization had commenced in the absence of recent trawling. (This area, although historically trawled, was not fished during the course of this study.) It is possible that small burrows were not as abundant in the untrawled area because those burrow-makers are gradually replaced by the larger burrow-making megafauna.
Shrimp trawling did not result in significant changes in sediment porosity on the fishing grounds (Table 2). Although porosity profile data were analysed for only the last two of the seven sampling periods, these results support the conclusion drawn from analysis of sediment X-radiographs (Simpson, 2003) that shrimp-trawling disturbance was not associated with changes in subsurface habitat structure. For a similar mud-bottom habitat in the Gulf of Maine, Sparks-McConkey and Watling (2001) reported significant reductions only in surface porosity, and no significant change in subsurface porosity immediately following experimental shrimp trawling. Three months later, surface porosity values had rebounded to ambient levels, indicating rapid recovery following the disturbance. In both the Sparks-McConkey and Watling (2001) study area and this one, porosity values were very high. Mayer et al. (1991) suggested that the passage of an otter trawl injects water into the substratum, but it is also possible that very high water content surficial sediment layers are resuspended by the passage of the gear (Pilskaln et al., 1998; Palanques et al., 2001), leaving lower water content subsurface (2–4 cm) sediments behind (Watling et al., 2001).
The naturally occurring radioisotope, 210Pb, was also used as a tracer to determine the extent to which commercial trawling gear influences sediment mixing. Profiles of excess 210Pb activity from trawled and untrawled areas revealed sediment inventories suggestive of differences in sediment mixing regimes. However, shrimp trawling did not appear to cause net loss or burial of sediment on fishing grounds. Both profiles showed relatively deep mixing to a depth of 16–17 cm (Figure 6). Water depth (100 m) and the lack of visual evidence of storm wave, tidal current, or other abiotic disturbances, suggest that sediment mixing was not the result of large-scale natural physical disturbances. Likewise, the small body size and predominantly near-surface depth distribution of the macrofaunal community preclude their capability of producing the deeply mixed layer. Most likely, this deep mixed layer was produced by the burrowing and foraging activities of the resident megafauna (lobsters, fish, crabs, etc.)
Interestingly, the inventory of 210Pb was higher in the trawled area (Figure 6). Aller et al. (1980) proposed that elevated rates of physical or biological re-working of surficial sediments may produce higher inventories of 234Th. Although 210Pb has a much longer half-life (22.3 years) than 234Th (24.5 days), both radioisotopes are particle-reactive and are expected to be subject to similar mixing processes, even though mixing rates may differ (Smith et al., 1993). One possible explanation, which will need further evaluation, is that the elevated 210Pb inventory in the trawled area is a result of historically higher levels of gear-related mixing in that location.
The macrofaunal community in both the trawled and the untrawled areas exhibited features characteristic of a disturbed faunal assemblage (sensu Rhoads and Boyer, 1982). Small, mainly short-lived, polychaetes were the most abundant group at both study sites. In the Pumpkin trawled area, the fishing ground known to have experienced trawling activity during the course of this study, polychaetes displayed the overall highest proportional abundance (81%), suggesting that shrimp trawling may have had at least short-term impacts on the structure of an apparently disturbed community by creating conditions favourable for small, disturbance-tolerant taxa. The second most abundant group observed in this study was molluscs (8–29%), represented almost exclusively by small bivalves of the families Nuculidae and Nuculanidae. Bivalves are sensitive to trawling (Tuck et al., 1998; Ball et al., 1999) and may be useful as indicator species of fishing gear disturbance (Witbaard and Klein, 1994). The proportional abundance of molluscs was similar in the Monhegan untrawled and trawled areas (27% and 29%, respectively), the latter of which was not trawled during the course of this study. In contrast, the untrawled area at Pumpkin exhibited more than double the relative proportion of molluscs (19%) than the trawled area (8%). These results suggest that the effects of shrimp trawling on community structure may be related to relatively recent trawling impacts, and are therefore not necessarily cumulative in nature.
In summary, commercial shrimp trawling did not appear to have a cumulative or lasting impact on overall habitat or macrofaunal community structure, though significant short-term changes in macrofaunal communities were clearly apparent on fishing grounds within 3 months of trawling. Low intensity and frequency of trawling effort, especially over the course of the study, are likely factors that contributed to the rapid recovery rate. However, we suggest that there is evidence that high levels of biological sediment disturbance, probably caused by large, predatory megafauna, maintained macrofaunal communities in a disturbed, low successional state that may have minimized the impact of shrimp trawling on both habitat and community structure.
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Acknowledgements |
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This research was funded by a Pew Marine Conservation Fellowship to LW. We thank L. Mayer, L. Schick, and D. Shull for assistance with sediment analysis, and C. Wilson, R. Russell, D. Shull, L. Mayer, J. Stevens, B. Grannis, J. Vavrinec, J. Brewer, J. Higgins, R. Downs, and numerous interns for help with fieldwork. Special thanks are due to Ed Gastaldo, captain of the fishing vessel Avalon, for his help in the field, and to the radiology staff at Miles Memorial Hospital in Damariscotta, Maine, for use of their darkroom and X-ray film-developing equipment. P. J. Auster provided valuable comments on an earlier draft of the manuscript.
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