Medium-term impacts of hydraulic clam dredgers on a macrobenthic community of the Adriatic Sea (Italy) (original) (raw)
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Chronic fishing disturbance has changed shelf sea benthic community structure
Journal of Animal Ecology, 2000
1. Bottom ®shing using towed nets and dredges is one of the most widespread sources of physical disturbance to the continental shelf seas throughout the world. Previous studies suggest that degradation and ecosystem changes have occurred in intensively ®shed areas. Nevertheless, to date it has been dicult to attribute habitat and benthic community changes to ®shing eort at a spatial scale that is truly representative of commercial ®shing activities. 2. In this study we present convincing evidence that chronic bottom-®shing disturbance has caused signi®cant and widespread changes in the structure of two distinct soft-sediment benthic assemblages and habitats. 3. Our study compared the benthic fauna found in areas that have been exposed to either high or low levels of bottom-®shing disturbance over the past 10 years. We were able to validate the ®shing eort data in some areas using scars in the shells of a long-lived bivalve mollusc (Glycymeris glycymeris) which result from ®shing disturbance. Shell scars occurred most frequently in bivalves collected from the area of highest ®shing eort. 4. Multivariate analyses and the response of abundance/biomass curves indicated that chronic ®shing has caused a shift from communities dominated by relatively sessile, emergent, high biomass species to communities dominated by infaunal, smaller-bodied fauna. Removal of emergent fauna has thus degraded the topographic complexity of seabed habitats in areas of high ®shing eort. The communities within these areas currently may be in an alternative stable state.
Marine Ecology Progress Series, 2009
Demersal fishing alters seabed habitats and affects the structure and functioning of benthic invertebrate communities. At a critical level of disturbance, such communities may approach an equilibrium disturbed state in which a further increase in disturbance has little additional impact. Such arguments have been used to suggest that an ecosystem approach to fisheries management (EAFM) should protect lightly fished areas and deflect fishing activity into areas that are already intensively fished. In this study, the effects of variation in fishing disturbance on the secondary production, species diversity, abundance, biomass, and community structure of benthic infauna were examined in a region of the German Bight (North Sea) that has been intensively trawled for decades. Variation in fishing disturbance across the study area was determined using automated position registration and vessel monitoring through satellite. Even in such a heavily fished area, linear regression analyses revealed that biomass, species richness, and production decreased significantly with increasing fishing intensity. Although redundancy analyses (RDA) showed that sediment characteristics were influential in determining the structure of the infauna community, partial RDA revealed that fishing continued to have an impact on community structure in terms of biomass. These results suggest that, in implementing an EAFM, managers will need to consider the possibility that, even in areas with high chronic fishing disturbance, further increases in fishing activity may still cause additional damage to benthic invertebrate communities.
Disturbance of the Marine Benthic Habitat by Commercial Fishing: Impacts at the Scale of the Fishery
Ecological Applications, 1998
Commercial fishing is one of the most important human impacts on the marine benthic environment. One such impact is through disturbance to benthic habitats as fishing gear (trawls and dredges) are dragged across the seafloor. While the direct effects of such an impact on benthic communities appear obvious, the magnitude of the effects has been very difficult to evaluate. Experimental fishing-disturbance studies have demonstrated changes in small areas; however, the broader scale implications attributing these changes to fishing impacts are based on long-term data and have been considered equivocal. By testing a series of a priori predictions derived from the literature (mainly results of small-scale experiments), we attempted to identify changes in benthic communities at the regional scale that could be attributed to commercial fishing. Samples along a putative gradient of fishing pressure were collected from 18 sites in the Hauraki Gulf, New Zealand. These sites varied in water depth from ϳ17 to 35 m and in sediment characteristics from ϳ1 to 48% mud and from 3 to 8.5 g chlorophyll a/cm 3. Video transects were used for counting large epifauna and grab/suction dredge and core sampling were used for collecting macrofauna. After accounting for the effects of location and sediment characteristics, 15-20% of the variability in the macrofauna community composition sampled in the cores and grab/suction dredge samples was attributed to fishing. With decreasing fishing pressure we observed increases in the density of echinoderms, longlived surface dwellers, total number of species and individuals, and the Shannon-Weiner diversity index. In addition, there were decreases in the density of deposit feeders, small opportunists, and the ratio of small to large individuals of the infaunal heart urchin, Echinocardium australe. The effects of fishing on the larger macrofauna collected from the grab/ suction dredge samples were not as clear. However, changes in the predicted direction in epifaunal density and the total number of individuals were demonstrated. As predicted, decreased fishing pressure significantly increased the density of large epifauna observed in video transects. Our data provide evidence of broad-scale changes in benthic communities that can be directly related to fishing. As these changes were identifiable over broad spatial scales they are likely to have important ramifications for ecosystem management and the development of sustainable fisheries.
The pink prawn Farfantepenaeus paulensis is one of the most valuable fishery resources in the southwest Atlantic. It is exploited along the inner shelf and inside estuarine nursery grounds. Inside the nursery grounds, mainly artisanal fisheries are responsible for the capture of pink prawn juveniles. In this study, we experimentally assessed the impact of a light mini trawl, a gear extensively used for the capture of estuarine prawn along the southeast and south coast of Brazil. We simulated a single impact (represented by three consecutive trawling runs) on vegetated and non-vegetated bottom. Then we monitored changes in sediment properties and the response of the benthic fauna at the trawling and control sites at a logarithmic temporal scale in order to determine the initial impact and the recovery rates. Meiofauna and macrofauna, both numerically dominated by the ostracod Cyprideis multidentata, were significantly more diverse and abundant inside the submerged algal beds than in non-vegetated areas. In the vegetated areas, the only change caused by the mini trawl was a significant decrease in the percentages of fine sediments (silt?clay). By contrast, in the non-vegetated areas, the mini trawl caused a significant decrease of the total meiofauna, mainly due to lower nematode and C. multidentata abundances. Our results show that the mini trawl does not produce a loss of benthic diversity and causes minimal damage to the estuarine benthos. The absence of a general faunal response, as well as the absence of significant changes in sediment properties of the Communicated by Margaret Miller.
Frontiers in Marine Science, 2020
Bivalve molluscs fishery is of great importance along the Italian coasts, both in economic and landing terms, and different edible bivalve species are harvested both in Adriatic and Tyrrhenian Seas. A medium-term assessment of the impact of the hydraulic dredges targeting razor clam Ensis minor on macro-benthic community was made during two surveys carried out in winter 2017 and late spring-summer 2018 in central Tyrrhenian Sea, which represents the main fishing ground for this species. The study area was located between 1 and 4 m depth, within 0.3 nautical miles from the coast. A net sampler (40 cm width, 18 cm height, and 14 mm mesh size) was mounted on a commercial dredge (3 m width) and enabled to collect specimens of the smallest sizes for the entire community present in the areas. A control area was identified where fishing does not occur, in order to compare exploited and not exploited sea bottoms. The results show that benthic assemblages found in dredged areas are characterized by species living in high-energy habitat, due to the closeness to the shore, and thus showing a high resilience at medium-term disturbs. Differences in species richness were not clearly evident both for the entire community and for the mollusc assemblages evaluated over the two surveys, among the control and the impacted areas, with few exceptions mainly depending on local conditions and anthropic pressure. Thus, even if the benthic community is typical of a moderately disturbed environment, the effects of fishing on the community structure are still discernible over and above the natural variation.
Effect of fishing on community structure of demersal fish assemblages
Belgian Journal of Zoology
Seasonal experimental trawl surveys (fall 1991-winter 1993) carried out in the Thracian Sea and Thermaikos Gulf (N. E. Mediterranean, Greece). In this area, fishing pressure is very high, since approximately 50% of the Greek otter trawl fleet operates here, producing more than 57 % of the total demersal landings. From a total of 285 bottom trawls sampled at depths between 16-420 m, 157 fish species were caught. Indices of diversity, richness, evenness, dominance and ABC plots were used to assess spatial structure, seasonal changes and diversity of the demersal fish assemblages. In general, species diversity, richness and evenness decreased with water depth, with the highest values at depths < 100 m. Dominance increased with depth, getting its maximum at depths > 200 m. The effect of depth on the diversity patterns observed was always significant, while no seasonal trends were detected. Commercially important species were dominant in the shallowest zone, while non-commercial species predominated at depths below 200 m. At intermediate depths (30-200 m) almost 50% of the total catches comprised of non-commercially important fish species. The abundance/biomass comparison method proved a useful tool for assessing the impact of stresses on fish populations since it revealed moderate disturbance on fish communities at those depths where fishing pressure is the highest.
Clam dredging effects and subsequent recovery of benthic communities at different depth ranges
Marine Environmental Research, 2009
This study aimed to assess the potential effects of clam dredging and the subsequent recovery of the benthic environment. Two experimental areas located at 6 and 18 m depth were established in order to analyse whether impacts and recovery of benthic environment are depth-related. Study areas were located within an area closed to dredging and two different plots were established at both depths. One of the plots was subjected to intense clam dredge-fishing, while the other was undisturbed and therefore used as control. Sampling followed a BACI design, with samples for macrobenthic, meiobenthic and sediment particle size analysis being taken by SCUBA divers from both areas before and after fishing stress. For both depths, impacts on the benthic environment were very low resulting in high recovery rates. Nevertheless, at shallower depths communities demonstrated a faster recovery. It was shown that depending on the faunal component used as a bioindicator, different results can be observed. Generally deposit-feeding organisms with scales or chitinous plates and vermiform shape (mainly crustaceans, polychaetes and ophiuroids), without external protection, were the most affected by dredging, while some polychaetes without external protection and with a carnivorous feeding mode seemed to be enhanced by fishing. The low level of perturbations induced by the dredging activities was comparable to the impact of surface waves on the bottom, as experiments were undertaken in wave-dominated environments. The coexistence of storm events during the study period proved to have similar or even more deleterious effects on the benthic environment. It appears that communities from hydrodynamic fishing grounds that are well adapted to natural physical stress are not highly affected by dredging.