Effects Of Trawling Research Articles

Monitoring benthic biodiversity restoration in Lyme Bay marine protected area: Design, sampling and analysis

Long-standing concerns about the effects of scallop dredging and demersal trawling on high diversity mudstone reef and cobble habitats in Lyme Bay, southwest England, were addressed by the exclusion of bottom towed fishing gear from a 206km2 area in July 2008. A consortium led by Plymouth University Marine Institute was funded by the UK Department of Environment, Food and Rural Affairs to design and implement a study (initially funded for 3 years) to examine the effects of the closure on both nekton and epibenthos. This paper provides a detailed account of the methodology employed from survey design to data analysis to provide a protocol for future MPA monitoring programmes. Information on historical fishing effort, substrate distributions and current and previous closure boundaries was overlaid using GIS to locate suitable monitoring sites. Non-destructive and cost-effective techniques, including a towed high-definition video array and static baited video, were used to quantify changes in relative abundances of epibenthos and nekton over three years at sites previously fished but now closed to bottom towed fishing compared to both fished and un-fished reference sites. The monitoring programme as described provides a model for robust, cost-effective evaluation of the efficacy of policy instruments for feedback into the adaptive management cycle.

Modelling ecosystem effects of reduced prawn recruitment on the Thukela Bank trawling grounds, South Africa, following nursery loss

Penaeid prawns are targeted in the Thukela Bank shallow-water trawling grounds in south-eastern Africa. Based on previous studies, these targeted prawns are assumed to primarily use 2 large estuaries (St. Lucia and Richards Bay/Mhlathuze) as nurseries on the KwaZulu-Natal (KZN) coast, which comprise over 80% of the estuarine habitat in the region. However, St. Lucia has been closed to the Indian Ocean since 2002 due to continued separation from the Mfolozi River, cessation of dredging required to open the inlet, reduced freshwater flow and drought con- ditions. To study the effects of prawn trawling and prawn nursery loss on the ecosystem, Ecopath with Ecosim models of the Thukela Bank were constructed. Models were fitted to total catch (t) for 1990 to 2009, and biomass trends were estimated by scaling Ecosim feeding interaction parame- ters to prey trophic level. Prawn trawling was modelled using fishing effort data. Prawn recruit- ment was modelled via the 'hatchery stocking' function in Ecosim simulating nursery loss along the KZN coast. Various prawn recruitment and trawling effort levels were simulated from 2010 to 2040. Decreasing prawn recruitment positively affected benthic fish and carnivorous benthos bio- mass and negatively affected prawn, commercial crustacean and benthopelagic fish biomass. Negative effects were exacerbated and positive effects were decreased by high versus low trawl- ing effort. The results suggest that indirect anthropogenic and environmental factors, such as recruitment from estuarine nursery areas, can impact offshore fisheries catches. Therefore, when modelling and managing marine ecosystems, it is important to couple processes between adjacent systems, particularly for critical life-history stages.

Origins of the bottom trawling controversy in the British Isles: 19th century witness testimonies reveal evidence of early fishery declines

AbstractBottom trawling (nets towed along the seabed) spread around the British Isles from the 1820s, yet the collection of national fisheries statistics did not begin until 1886. Consequently, analysis of the impacts of trawling on fish stocks and habitats during this early period is difficult, yet without this information, we risk underestimating the extent of changes that have occurred as a result of trawling activities. We examined witness testimonies recorded during two Royal Commissions of Enquiry (1863–66 and 1883–85). These enquiries interviewed hundreds of fishers about the early effects of sail trawling and the changes they were witnessing to fish stocks, habitats and fishing practises during this time. We converted all quantitative statements of perceived change in fish stocks and fishing practices to relative change. Witnesses from the north‐east of England interviewed during 1863 revealed an average perceived decline in whitefish of 64% during their careers, which many blamed upon trawling. Between 1867 and 1892, trawl‐landing records from the same location suggest that this trajectory continued, with fish availability declining by 66% during the period. Fishers adapted to these declines by increasing distances travelled to fishing grounds and increasing gear size and quantity. However, inshore declines continued and by the early 1880s even trawl owners were calling for closures of territorial waters to trawling in order to protect fish nursery and spawning grounds. Until now, these testimonies have been largely forgotten, yet they reveal that alterations to near‐shore habitats as a result of trawling began long before official data collection was initiated.

A Transactional and Collaborative Approach to Reducing Effects of Bottom Trawling

Private-sector financial and legal transactions have long been used to protect terrestrial habitats and working landscapes, but less commonly to address critical threats in marine environments. Transferrable and marketable fishing privileges, including permits and quotas, make it possible to use private-sector transactions as conservation strategies to address some fishery management issues. Abating the effects of bottom trawling on the seafloor and bycatch and discard associated with the practice has proven challenging. On the Central Coast of California, The Nature Conservancy (TNC), Environmental Defense Fund, local fishers and local, state, and federal authorities worked collaboratively to protect large areas of the seafloor from bottom trawling for groundfish while addressing economic impacts of trawl closures. Contingent on the adoption of trawl-closure areas by a federal regulatory agency, TNC used private funds to purchase federal groundfish trawl permits and vessels from willing sellers. Trawl-closure areas were designed collaboratively by combining regional biological diversity and fisheries data with local fishers' knowledge. The private transactional strategy was designed to remedy some deficiencies in previous federal buyouts, to mitigate economic impacts from trawl closures, and to carefully align with a public regulatory process to protect "essential fish habitat" under the Magnuson-Stevens Fishery Conservation and Management Act. This collaborative effort protected 1.5 million ha (3.8 million acres) of seafloor, reduced trawl effort in the area by 50%, and set a precedent for collaborative partnerships between conservation and fishing interests. This is the first time a large conservation organization has taken an ownership position in a fishery and demonstrates how nongovernmental organizations can invest in fisheries to improve environmental and economic performance.

Effects of trawling on the diets of common demersal fish by‐catch of a tropical prawn trawl fishery

The ecological effect of prawn trawling on the benthos of the Gulf of Carpentaria, northern Australia, was investigated by examining stomach contents of common demersal fishes incidentally caught as by-catch in the fishery. Fishes were collected from high and low fishing intensity sites in three regions based on vessel monitoring system data. The diets of eight species of benthic fish predators were compared between regions and fishing intensities. A regional effect on diet was evident for seven species. Only one generalist species had no significant difference in diet among the three regions. For the comparisons within each region, five predator species had significantly different diet between high and low fishing intensities in at least one region. Across the three regions, high fishing intensity sites had predators that consumed a greater biomass of crustaceans, molluscs and echinoderms. At low fishing intensity sites, predators had diets comprising a greater biomass of cnidarians and teleosts, and a different assemblage of molluscs, crustaceans and fishes. These changes in diet suggest that there may have been a shift in the structure of the benthic community following intensive fishing. Analysis of predator diets is a useful tool to help identify changes in the benthic community composition after exposure to fishing. This study also provided valuable diet information on a range of abundant generalist benthic predators to improve the ecosystem modelling tools needed to support ecosystem-based fisheries management.

Impact and performance of Mediterranean fishing gear by side-scan sonar technology

An innovative approach with side-scan sonar was implemented to observe directly during towing operations the behaviour and the performance of hydraulic dredge, Rapido trawl, and two types of Mediterranean bottom trawls. The innovative aspect of the current paper is that a side-scan sonar was used alongside fishing operations when towing was in progress. Based on available literature, the type and level of the physical impact assessed were used to infer biological and ecological consequences caused by the fishing gear. The sonar used in real time can be applied to a wide range of research and management issues, such as the fishing gear impact on benthic ecosystems and the in situ fishing gear performance. We demonstrated that gear type and rigging, together with environmental conditions, are the main factors affecting the disturbance caused by fishing. Hydraulic dredges scrape the surface of the substratum and dig into it by resuspending large amounts of sediment. Rapido trawls used in muddy areas for common sole (Solea solea) dig deeper into the sediment, making furrows up to 10–13 cm deep. The most evident physical effects of trawling were the furrows produced by the otterboards, while other parts of the trawl created only faint marks.

Reducing effects of trawl fishing on seabirds by limiting foraging opportunities provided by fishery waste

By-catch of seabirds on trawl-fishing gear has been reported worldwide, and is exacerbated by the discharge of fisheries waste. We compared the attraction of seabirds to three forms of fishery waste—unprocessed discharge (offal, fish discards), hashed discharge (smaller chunks passed through a hasher pump) and cutter pump discharge (waste passed through the hasher and a cutter pump to further reduce particle size)—to identify the discharge form that most effectively reduced the risk of seabird by-catch. Seabird responses measured within specified areas astern of the vessel were the abundance of: large albatrosses (Diomedea spp.), small albatrosses and giant-petrels (Thalassarche spp.; Southern Giant-Petrels, Macronectes giganteus; and Northern Giant-Petrels, M. halli), Cape Petrels (Daption capense) and all other procellariid species. Seabirds on the water were less numerous during cutter pump and hashed discharge relative to unprocessed discharge (except small albatrosses—cutter treatment). Also, in some cases, the total number of birds decreased, relative to unprocessed discharge treatments (but not small and large albatrosses—cutter treatment). Particle size may be less important for reducing abundances than temporal discharge patterns, which affected how birds tracked the discharge stream. Manipulating discharge characteristics can reduce seabird attraction to fishing vessels. However, the risk of by-catch remained lowest when no discharging occurred.

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.

Abundance analyses of mega‐epibenthic species on the Dogger Bank (North Sea): Diurnal rhythms and short‐term effects caused by repeated trawling, observed at a permanent station

Within our long term monitoring programme at the Dogger Bank (North Sea), a permanent station at the north-eastern tail end of the Dogger Bank was sampled yearly with a 2m beam trawl since the year 1994. The sampling procedure was repeated every 3h within a timeframe of 48h. All species above a size of 1cm were recorded quantitatively. The analysis of the dataset, consisting of thirteen sampling years, revealed rhythmic abundance fluctuations of one crustacean and two fish species, depending on the time of day. In order to check the accuracy of the results obtained, we further analysed the dataset for short-term effects of continuous trawling at the same track on the abundance of individual species. No direct effect on the abundance of particular species was detected, but the analysis revealed a periodic fluctuation of the mean number of individuals and the mean catch volume.

Evaluation of the modern submarine landscape off southwestern Turkey through the documentation of ancient shipwreck sites

The issue of damage to shipwreck sites caused by the operation of mobile fishing gear has only recently begun to be addressed by the archaeological community. However, the nature, extent, and intensity of this damage has yet to be quantified. Acoustic and video surveys conducted between 2008 and 2010 located and imaged sixteen ancient shipwrecks around the Bodrum and Datça Peninsulas, Turkey, many of which were heavily damaged by trawling activity. The results of this research illustrate the unfortunate reality that many wreck sites in the Aegean Sea are heavily damaged by modern fishing activities. Quantifying the extent and intensity of trawl scars on the seabed further reveals the geographic spread of damage in these areas. The results of these mapping projects call attention to the dismantling of cultural sites by the use of mobile fishing gear on the seabed. By comparing the number of broken artifacts on these wreck sites to other sites that have escaped the effects of trawling, such as those in the Black Sea, we see that shipwrecks that are or were at one time in areas of trawling activity show a considerable amount of damage. The location and condition of these wreck sites helps map and quantify past and recent trawling activity, and pinpoint areas on the shallow coastal shelf where additional trawling restrictions or protected zones may be able to help the preservation of archaeological material.

Effects of trawling on fish and invertebrates from deep-sea coral facies of Isidella elongata in the western Mediterranean

Deep-water corals are known to provide essential habitat for fish and invertebrates along the continental slope in different areas of the world, offering prey to coral predators, protection or attachment substrate to other suspension feeders. In the Mediterranean Sea the coral Isidella elongata (Esper, 1788) characterizes a facies of bathyal compact mud substrates between 500 and 1200 m depth. Based on 4 experimental surveys carried out between 1985 and 2008, we obtained quantitative data on the fauna associated with live coral stands. We show that species richness of invertebrates and crustaceans, as well as abundance and biomass of crustaceans, is higher in areas with large stands of the coral. Some commercial fishery species are also more abundant or reach larger sizes in areas with high density of the coral, particularly the red shrimps Aristeus antennatus (Risso, 1816) and Plesionika martia (A. Milne Edwards, 1883). Trawling over I. elongata facies causes direct impacts on the biological assemblages by removing the habitat-forming corals, decreasing invertebrate species diversity and negatively affecting fisheries production in the long term.

Could our fisheries be more productive? Indirect negative effects of bottom trawl fisheries on fish condition

Summary1. The nets used in bottom trawl fisheries cause mortality of benthic invertebrates and this can decrease the long‐term availability of prey to exploited fish species by reducing the abundance of benthic invertebrates. This may have consequences for the sustainability of fisheries.2. We assessed the impact of bottom trawling on the food availability of fish by comparing the condition of fish (as weight‐at‐length) in an area that had a steep commercial bottom‐trawling gradient in the Irish Sea but otherwise homogeneous environmental conditions.3. We found that the condition of the important commercial flatfish plaice Pleuronectes platessa was negatively related to trawling frequency, and this could be explained by a reduced production of the infaunal invertebrates they feed on. Density‐dependent changes in competition over food could not explain this difference. No effect of trawling on the condition of the flatfish dab Limanda limanda was detected. Whiting Merlangius merlangus feeds primarily on fish, and therefore, no effect of bottom trawling on its condition was expected or detected.4. This study therefore indicates that bottom trawl fisheries may have a negative effect on the condition of some of their target species, but not others, by reducing the abundance of their benthic prey.5. Synthesis and application. Bottom trawls may indirectly affect the population size and growth rate of the target fish species and result in lower fishing yields. Such reductions in the yield and sustainability of fisheries are highly undesirable. The effects of bottom trawls may be mitigated by the modification of fishing gears or by minimizing the area of the seabed fished by bottom trawls.

Macrobenthic associations in a South Atlantic Brazilian enclosed bay: The historical influence of shrimp trawling

This study characterized the macrobenthos of Tijucas Bay and related the distribution and dominance patterns with historical trawling activities. Fifteen sampling sites were established in the bay and were sampled in summer and winter for the characterization of the macrobenthos. To evaluate the trawling effects, three passages were established at each site with control and impacted areas analyzed before and after trawl-net action in impacted area. Seasonal differences occurred in macrobenthos, possibly influenced by rainfall where the relative water level of the Tijucas River was directly associated with the discharge of freshwater and particulate matter. The associations identified in the bay were distinct in comparison with the Zimbros Embayment. The trawling assessment showed no significant variation in the macrobenthos after the trawl, possibly due to decades of shrimp exploitation in the bay. Trawling and instable environmental conditions inside the bay make the establishment of a stable macrobenthic community difficult.

Effects of demersal trawling along the west coast of southern Africa: multivariate analysis of benthic assemblages

This study is the first to examine the benthic impacts of the otter-trawl fishery on hake in the southern Benguela upwelling region. Infauna were sampled at 4 sites, from southern Namibia to Cape Town by means of 5 replicate grab samples at each of 2 trawling treatments (heavily and lightly trawled areas), paired at each site. The large invertebrate epifauna was also sampled at 2 of these sites using a fine-meshed otter trawl. Sites ranged in depth from 350 to 450 m. Environmental attributes (sediment particle size, total organic carbon, depth, salinity, temperature and dissolved O2 concentra- tion) were examined along with faunal assemblage composition. Vertical profiles of water mass char- acteristics showed little long-shore variation, apart from slightly lower O2 concentrations in the north. Difficulties of pseudo-replication in benthic impact studies are discussed, and methods for circumvent- ing these suggested. There were significant differences in sediment characteristics among the 4 sites, but only 2 sites showed different sediment characteristics between trawling treatments. Studies of spe- cies richness, evenness and numbers of infaunal individuals showed little difference between trawling treatments at 3 sites and species diversity was similar between treatments at all 4 sites. Multivariate analyses show marked differences in both infaunal and epifaunal assemblages among the sites and between trawling treatments at all sites. The analyses suggest that differences in trawling intensity are at least partially responsible for significant variation in benthic assemblage composition between heavily and lightly trawled areas. These findings contrast to those in shallower waters in the northern hemisphere, where infauna are more sensitive to trawling than epifauna. This study shows that epifau- nal abundances, number of species and species diversity decrease with increasing trawling intensity, and that there are also considerable changes in epifaunal assemblages in more heavily trawled sites.

Developing seamount fishery produces localized reductions in abundance and changes in species composition of bycatch

AbstractEvidence of localized and cumulative impacts of bottom‐trawling upon bycatch abundance and diversity was found during the 8‐year period the Chilean orange roughy (Hoplostethus atlanticus) fishery was open. We analyzed acoustic, orange roughy catch and fish bycatch data available from a voluntary monitoring program implemented by the fishing‐rights holders. Acoustic data, recorded from the third year of the fishery (2002) until its closure in 2006, permitted us to extract detailed bathymetry and precise information about trawl tows. From these data we reconstructed the yearly and cumulative extent of the bottom trawling footprint and compared biodiversity indices, bycatch abundance and assemblage structure between nominally pristine areas and others that exhibited different levels of previous trawling impacts (as measured by within‐ and across‐years impact indices). Although the footprint area expanded as the cumulated number of tows increased, degrees of overlapping between tows were found in all seamounts. The trawled footprint was 22 km2 for the 2002–2006 period, equivalent to 2.7% of the study area. An extrapolation to include unassigned tows and years without data (1999–2001) increased this estimate to 81 km2 (9.8% of the study area) for the entire duration of the fishery. The relative abundance of bycatch species decreased at more heavily impacted areas, suggesting a local depletion response. Evidence of bycatch species richness decline and significant changes in diversity and species assemblage composition were also found at these areas. Seamount management and conservation efforts must consider the cumulative and localized nature of bottom trawling effects we found to occur upon seamount fish communities.

Can bottom trawling indirectly diminish carrying capacity in a marine ecosystem?

Bottom trawling is associated with reduced biomass and production in the marine benthic community. Abundance of hard-bodied organisms such as bivalves, crustaceans and echinoderms typically declines in favour of soft-bodied opportunists such as polychaetes. Trawling effects vary with habitat; impact and recovery time are typically greater for more complex substrates/communities and those with lower rates of natural disturbance. Benthic organisms represent the prey base of a large component of the demersal fish assemblage. Hence, trawling-induced change in benthic community structure and function may exert an indirect effect on feeding success and growth of important commercially exploited fishes such as plaice Pleuronectes platessa. We present habitat-specific mixed effects models of plaice length as a function of age, bottom-trawling effort, population density and near-bottom temperature, with sampling year and area, and fish sex incorporated as random effects. Across an observed gradient of trawling effort in the Celtic Sea, plaice on gravel habitat showed significant declines in length at age while plaice on sand habitat showed significant increases in length at age. Contrasting trawling effects likely reflect dietary differences between habitats. Plaice on sand substrates are known to consume predominately polychaetes, which may proliferate at moderate trawling intensity in this habitat. Conversely, plaice on gravel substrates are reported to consume more of the fragile organisms such as echinoderms and bivalves that show marked declines with bottom trawling. An indirect effect of trawling on prey availability and growth of demersal fish has substantial implications for fisheries sustainability via reduced ecosystem carrying capacity and production of commercial fish.

Using life‐history information to assess potential effects of shrimp trawling on small fishes

The current study presents information on size distributions, size at recruitment to the fishery, size at maturity and patterns of reproduction for several small benthic fishes caught as by-catch in the southern Gulf of California (Mexico) shrimp trawl fishery: sand perch Diplectrum spp., lumptail searobin Prionotus stephanophrys, bigscale goatfish Pseudupeneus grandisquamis and silver stardrum Stellifer illecebrosus. Pseudupeneus grandisquamis, P. stephanophrys and S. illecebrosus populations were all sexually dimorphic in size. Total-length (L(T))-based analyses did not provide reliable information on survival and growth. The majority of sampled P. grandisquamis and S. illecebrosus were caught before reproductive maturity, whereas the majority of Diplectrum spp. and almost all P. stephanophrys were mature when caught. L(T) at 50% gear retention (L(Tc), mm) v. 50% maturity (L(Tm), mm): Diplectrum spp. 124.53 v. 131.43; P. grandisquamis 90.98 v. 135.20; S. illecebrosus 82.55 v. 137.30. L(Tc) for P. stephanophrys was 104.73, but L(Tm) could not be modelled for this species as almost all captured individuals were mature. Diplectrum spp., P. grandisquamis and S. illecebrosus were indeterminate spawners, whereas P. stephanophrys appeared to be a determinate spawner. Sex ratios were equal for each of the gonochoristic species. In general, the gonado-somatic index (I(G)) increased with increasing L(T) for all except P. stephanophrys, where I(G) decreased with increasing L(T) for both males and females. Mature individuals of all taxa were found throughout the sampling period (September to March), and I(G) increased with sample day for all except females of P. grandisquamis. The current data suggest the potential for fishery effects on sampled populations of P. grandisquamis and S. illecebrosus.

Trophodynamic effects of trawling on the feeding ecology of pandora, Pagellus erythrinus, off the northern Sicily coast (Mediterranean Sea)

Because trawling disturbs benthic organisms, it could affect the diet of benthic-feeding fish with implications for food-web dynamics. The present study assessed the effects of commercial trawling on the trophodynamics and diet of pandora, Pagellus erythrinus, by comparing its stomach contents and stable-isotope (δ15N and δ13C) composition in two trawled and two untrawled gulfs in northern Sicily (central Mediterranean). Fish were collected on muddy bottoms at 50–100-m depth. Higher abundance and biomass and a slightly larger mean body length were found in the untrawled gulfs. The feeding habits were similar although more selective in the untrawled gulfs. The diet was mainly composed of decapod crustaceans (especially the brachyuran crab Goneplax rhomboides) and of polychaetes. The trophic level of pandora, estimated by its δ15N values, was higher in the untrawled gulfs. No clear trend between trawled and untrawled gulfs was found for the source of carbon in the diet (δ13C). The diet of a benthic feeder such as pandora may be used as an indirect indicator of trawling disturbance, as long as stomach contents and stable-isotope analysis are used jointly to assess the diet and trophodynamics of a species.

Testing hypotheses about temporary and persistent effects of otter trawling on infauna: changes in diversity rather than abundance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 385:51-64 (2009) - DOI: https://doi.org/10.3354/meps08056 Testing hypotheses about temporary and persistent effects of otter trawling on infauna: changes in diversity rather than abundance Stefán Áki Ragnarsson1,*, Mats Lindegarth2 1Marine Research Institute, Skúlagata 4, PO Box 1390, 121 Reykjavík, Iceland 2Department of Marine Ecology—Tjärnö, University of Gothenburg, Tjärnö, 452 96 Strömstad, Sweden *Email: steara@hafro.is ABSTRACT: A field experiment was conducted to examine the short- and long-term effects of otter trawling on a macrobenthic infaunal community in shallow subtidal waters of Faxaflói Bay (SW Iceland) that had never been trawled before. The experimental design consisted of 4 sites trawled 10 times and 4 areas left undisturbed (controls). Sampling of fauna and sediments was carried out in June 1997, immediately after trawling, and subsequently 2 and 7 mo later, in order to investigate longer term impacts of trawling. No significant treatment effects could be detected on total abundance or on multivariate structure, and tests for individual species revealed only a single short-term effect (for the bivalve Thyasira flexuosa). However, trawling affected several aspects of diversity with significant short-term reduction in species richness and persistent effects on the Shannon-Wiener index. Power analysis revealed that larger changes were needed to detect changes in abundance compared to measures of diversity. Analyses of qualitative changes revealed a decreasing trend in abundance of the majority of taxa (especially the rarer ones) immediately after trawling. This was the most likely cause for the reduced diversity and the augmented small-scale variability in species richness. The size of initial impacts and the persistence of effects were largely consistent with those of previous studies in similar environments. Similar to other types of environmental impacts in aquatic environments, our analyses reveal that impacts of trawling on measures of diversity can be detected at a high statistical power and that larger effects are necessary to detect impacts on abundances. KEY WORDS: Otter trawl · Experimental fishing · Infauna · Variability · Analysis of variance · Iceland · Diversity · Power analysis Full text in pdf format PreviousNextCite this article as: Ragnarsson SÁ, Lindegarth M (2009) Testing hypotheses about temporary and persistent effects of otter trawling on infauna: changes in diversity rather than abundance. Mar Ecol Prog Ser 385:51-64. https://doi.org/10.3354/meps08056 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 385. Online publication date: June 18, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

A large scale BACI experiment to test the effects of prawn trawling on seabed biota in a closed area of the Great Barrier Reef Marine Park, Australia

A multiple Before-After-Control-Impact experiment was conducted to estimate the large scale effects of a single coverage by prawn trawlers on an offshore inter-reef area in the northern Great Barrier Reef that was closed to fishing. Prospective power and cost-benefit analyses facilitated the design and optimization of the experiment, the effect size of which was guided by prior publications at the time that had indicated impacts of 10–100×. The optimal design, given the number of factors to be tested and the constraint of available ship time (100 days) and resources needed to process samples, was for 12 control and 12 impact plots each measuring ∼2.8 km × 1.2 km. The final design specification was capable of detecting an average −80% effect size (5×). The experiment was divided over two seasons, wet and dry. During the experimental trawl impact phase, a total of 32 t of benthic biota was removed from six shallow plots (15–25 m depth) and 6 t from six deep plots (30–50 m depth). Sampling before and 6 months after impact was conducted by an epibenthic dredge, a prawn trawl and a fish trawl. Analysis of means before and after impact, relative to controls, showed very few significant differences. This indicated that the impact of a single prawn trawl was less than the design specification. The lower than expected impact, compared to previous studies, may have been partly because this study included all benthic groups, not just the most sensitive as earlier studies had focussed on. Comparison of catch rates from the prawn trawl and the dredge indicated the overall impact on total biomass was around −3% but ranged from close to 0% for some species to around −20% for sensitive sessile species. A review of effect sizes in 30 other recent trawl experiments indicated that this result was not exceptional; i.e. the reported effects of single trawls generally were not large, and prawn trawls appeared to have smaller effect sizes than fish trawls, beam trawls and scallop dredges—also, several studies reported recovery within ca. 6 months. Analysis of fishery effort data indicated that this result was appropriate for the majority of trawl fishing grounds, where effort is sparse and infrequent. Many published experiments had confounded designs and most had not used a priori power analyses or pre-specified effect sizes. Nevertheless, recurrent trawling can be expected to have cumulative impacts on benthos, as has been demonstrated by repeat-trawl depletion experiments. On the other hand, the spatial extent of fishing grounds trawled as intensively as these depletion experiments is quite limited.