Effects of flood and drought events on multi‐species,multi‐method estuarine and coastal fisheries in eastern Australia

Abstract Multivariate patterns in commercial fisheries landings, effort and revenue from three adjacent estuarine and coastal systems were examined in eastern Australia between 9‐month periods of flood (September 2000–May 2001) and drought (September 2002–May 2003). Patterns in species landings, methods of fishing effort and revenue per species were significantly different between flood and drought. Spearman’s rank correlations between Bray–Curtis similarity matrices for landings, effort and revenue indicated that patterns in fisheries metrics represented a mixed signal of ecological response and fishers’ harvesting behaviour. Flood and drought events were associated with shifts in the species composition of landings that were reciprocated between estuarine and coastal systems. Estuarine migrant species (e.g. school prawn Metapenaeus macleayi Haswell) primarily contributed to landings during flood, whilst marine estuarine‐opportunist species (e.g. yellowfin bream Acanthopagrus australis Owen) primarily contributed to landings during drought. Flood and drought events redistributed fisheries resources between estuarine and coastal systems, modifying the bioeconomic productivity of commercial fisheries. Results indicated that flood and drought events influence commercial fisheries by modifying landings composition, fishers’ harvesting behaviour and revenue generation.     

Catch rates, size structure and sex ratio of Melicertus kerathurus (Decapoda: Penaeidae) from an Aegean Sea trawl fishery

Otter-trawl catch rates and population structure of the prawn Melicertus kerathurus were studied at the fishing ground of Thermaikos Gulf (North Aegean Sea, Eastern Mediterranean), where the stock is exploited both by otter-trawlers and the artisanal local fleet. Sampling was conducted on board commercial trawlers on a monthly basis in two consequent open fishing seasons (October to May 2000–2001 and 2001–2002) resulting to a total of 70 hauls. Samples were also obtained through experimental trawling (24 hauls) during the closed season. The main trawling ground is located near the border between the outer and the inner gulf, and along the western and northeastern coasts of the outer gulf in depths ranging from 35 to 70 m (most frequently at 40–50 m). Catch per unit effort (CPUE) varied significantly with month but not with fishing season (mean CPUE 3.18 and 1.78 kg/h for the 2 successive years) or depth. Catches were at maximum (up to 7.5 kg/h) at the beginning of the open season in autumn and decreased sharply as the season proceeded (<2.5 kg/h), which is likely to be due to induced fishing mortality in the preceding months, to prawn dispersal and/or to its tendency for longer burrowing. Commercial fishing trips consisted of multiple, short efforts when catches were high, whereas low yields during winter–spring led to less hauls of longer duration: ln CPUE = 2.974 − 2.047 ln HD, r = −0.57. Prawn size ranged from 17 to 60 mm CL, showing a consistent difference in favor of females. A weak recruitment to the fishery was observed from autumn through winter, while, as shown by experimental trawling, the main recruitment pulse to the fishery occurs just prior to the season's opening. The delayed arrival of the prawns on the fishing ground during the second fishing season, accompanied by smaller prawn size and shallower exploitation depth, was possibly related to decreased autumn rainfall. In both years, sex ratio showed a temporal pattern, which seems to be the result of sex-related differences in fishing vulnerability, in timing of inshore migration and in mating mortality.     

Prawn (Macrobrachium rosenbergii)–plant (Hydrilla verticillata) co‐culture system improves water quality,prawn production and economic benefit through stocking density and feeding regime manage

The effects and commercial implications of aquatic plant addition, and variable prawn stocking density and supply of commercial aquafeed on water quality, prawn production and economic benefit, are investigated in a prawn (Macrobrachium rosenbergii) + plant (Hydrilla verticillata) co‐culture system. Our experimental design includes a control (PM, prawn monoculture without H. verticillata, with 30 prawns/m²) and four treatments with 15% plant cover of total pond area in each replicate. Dissolved oxygen, pH, N, P, total ammoniacal nitrogen, nitrite and nitrate in this co‐culture system were significantly lower than those of PM. Prawn survival (96.2 ± 14.9%), average final weight (68.5 ± 4.9 g) and yield (879.1 ± 102.4 kg/ha) in treatment 20W80%A (20 prawns/m², 80% of control aquafeed) were obviously greater than in PM and other treatments. Despite small males (SM) and immature females (IF) being predominant in prawn + plant cultures, more than 77.2% of prawns reached or exceeded 40 g on completion of trials, and by six months were of appropriate size for market. We report culture of M. rosenbergii with H. verticillata to be both feasible and profitable. The optimal treatment, a stocking density of 20 prawns/m² with these plants, enabled reduction of commercial aquafeed to 20% conventional culture levels. For this optimal treatment, we estimate total gross revenue, profit and internal rate of return to be US$ 6,593.3 ± 103.3 ha^?1, US$ 3,095.5 ± 42.6 ha^?1 and 127.5 ± 20.7% for 20W80%A respectively; we estimate U.S. Dollar (US$) invested generates 3.87 times conventional culture revenue. Co‐culture of M. rosenbergii and plants renders prawn production ecologically and economically feasible on larger farms.     

中东大西洋中部海域中上层鱼类资源结构与渔场分布

为了解中东大西洋中部海域中上层鱼类资源结构与渔场分布,对2007-2014年入渔FAO34渔区3.11和1.32小区(毛里塔尼亚专属经济区)从事中上层鱼类捕捞的7艘国内渔船的生产数据进行了分析。结果表明,我国入渔船只数量、入渔渔船总产量和作业渔船平均日产量均呈现先增加、后减少的趋势,且在2011年入渔渔船总产量和作业渔船平均日产量到达最大值,2012年入渔船只数量到达最大值。渔获种类主要包括沙丁鱼(Sardina pilchardus)、短体小沙丁鱼(Sardinella maderensis)、金色小沙丁鱼(Sardinella aurita)、日本鲭(Scomber japonicus)和竹筴鱼(Trachurus trachurus),各渔获种类的渔获量年际间波动较为明显。月均产量在17.2~23.6 t/haul,最高值和最低值分别出现在12月和8月;月均总产量在1 279.0~2 414.2 t,最高值和最低值分别出现在1月和8月,12月至次年3月为渔汛旺期。渔场季节变化现象明显,夏秋季向北移动,冬春季向南移动,20.0°~20.8°N、17.4°~18.0°W区域为高产量海域。该海域中上层鱼类洄游现象明显,这可能主要与该海域的不同洋流在不同月份的强弱变化有关,该海域整体渔业资源呈现一定衰退现象。建议渔船入渔该海域从事中上层鱼类捕捞持谨慎态度,加强该海域渔业资源的研究与保护。     

Distribution and intensity of bottom trawl fisheries in the Patagonian Shelf Large Marine Ecosystem and its relationship with marine fronts

Trawling is a major concern worldwide and there is considerable debate about its impact on marine ecosystems. The Patagonian Shelf Large Marine Ecosystem (PSLME) is an important fishing area in the Southwest Atlantic where bottom trawling is the dominant fishing method. We investigated the distribution of bottom trawl fishing within this region, defining the areas of highest trawling intensity (hotspots) and evaluating their relationship with marine fronts. We focused on the three main oceanographic fronts, the shelf‐break front, the southern Patagonia front and the mid‐shelf front. To estimate fishing effort and trawled areas, we used VMS data from 2006 to 2012. Despite being almost a fully trawlable shelf, we found that the spatial distribution of trawling activity is patchy and trawling hotspots were small, comprising annually <5% of the shelf extension or <7% of the total trawlable area. Contrary to what is believed worldwide, our findings suggest that over the PSLME the magnitude of habitat effects as a result of bottom trawling is relatively small. Regarding the three frontal systems studied, only the shelf‐break front showed a positive relationship with trawl fishing activity. Although trawling hotspots did not overlap with marine fronts, the shelf‐break front receives more trawling effort than expected. We hypothesize that this pattern is due to aggregation of species near or at the front taking advantage of the opportunities provided by this area.     

日本对虾育苗,养成,加工技术研究

对日本对虾亲虾运输，消毒暂养，催熟，幼体培育，养成，捕捞和活虾运输等全程技术进行了开发。１９９６年５００ｍ＾３育苗水体育出对虾苗４５００万尾，获得利８０余万元；８００亩精养池产虾１万ｋｇ，获利５０余万元，经济效益可观。     

南极磷虾中层拖网捕捞参数对CPUE变动的影响

根据中国在南极海洋生物资源养护委员会(CCAMLR)48.1亚区和48.2亚区内2010—2014年南极磷虾拖网渔船的捕捞数据,采用广义可加模型(generalized additive model, GAM)分析各拖网捕捞参数(因子)对南极磷虾汛期单位捕捞努力量渔获量(catch per unit effort, CPUE )的影响。结果表明:2010—2014年我国南极磷虾作业渔场主要位于南极半岛等岛屿的周边海域,CPUE 值月变化明显,2—5月为捕捞高峰期。 CPUE 最高值为2014年,最低值为2012年,年平均CPUE呈略上升趋势。从月变化来看,除6月外,标准化 CPUE 均大于名义 CPUE;且最大值出现在4月或5月,最小值均出现在10月。GAM分析表明各捕捞参数对 CPUE 的影响由大到小依次为:拖网速度、网口高度、网口水平扩张、拖曳水深、曳纲长度;各因子的偏差解释率分别为22.8%、14.5%、12.7%、6.0%、1.6%,所有因子对 CPUE 的总体偏差解释率为57.6%。研究认为,我国南极磷虾捕捞作业中最适拖网速度范围2.6~3.1 kn;最适网口高度范围22~30 m;最适网口水平扩张范围20~25 m;最适曳纲长度范围100~200 m;最适拖曳水深范围0~40 m。     

Yield characteristics of prawns, Macrobrachium rosenbergii (de Man), with fish polycultures at low feeding rates

Abstract. As currently practiced, freshwater prawn, Macrobrachium rosenbergii (de Man), aquaculture in Hawaii requires large inputs of supplementary feed, water and labour. The high cost of these inputs greatly restricts profit potential. An experiment was conducted to compare an alternative prawn production system which used lower feeding rates and fish polycultures to reduce inputs. Three treatments were run in quadruplicate: (1) control—prawns stocked at standard densities (11.5 per n2) in monoculture and fed standard prawn pellets at the commercial ration recommendations by New & Singholka (1982); (2) low feed—prawns at standard densities and fish [silver carp, Hypophthalmichthys molitrix (Valenciennes), grass carp, Ctenopharyngodon idella (Valenciennes), and grey mullet, Mugil cephalus (L.)] fed one-half commercial ration; and (3) maintenance feed—prawns at standard densities and fish fed one-tenth commercial ration. Although prawn yield characteristics (mean weights, harvestable sizes, survival, net yields) were not significantly different (P > 0.05) based on an analysis of variance in the three treatments over the approximately 9.5-month culture period, this lack of significance most likely resulted from interpond variability and the lack of a large number of replicates. The relatively poor survival (60–63%) of gray mullet and an almost twofold difference in prawn yields in the control and maintenance feed treatments suggest that there was in fact competition for resources in the nominally polyculture systems. Nevertheless, the wet weight yield of biomass from all species was twice as high in the low feed ponds as in the control ponds.     

北太平洋公海日本鲭资源分布及其渔场环境特征

根据2014~2015年两年收集的北太平洋公海围拖网作业的日本鲭(Scomber japonicas,又称鲐鱼)生产月度数据,结合同期卫星遥感反演技术获取的海表温度(SST)、海水叶绿素a(Chl-a)浓度、海流等环境数据,运用渔获量重心法,地统计插值等方法,分析了北太平洋公海鲐鱼的资源分布情况与渔获量重心的时空变化及其与主要环境因子之间的关系。研究表明,鲐鱼渔场季节性差异明显,渔场重心集中分布在39°N~43°N、147°E~154°E范围内。两年渔场重心均呈现先向东北方向移动,自9月开始再向西南方向移动的趋势。GAM模型显示,北太平洋鲐鱼渔场的最适海表温度范围是16~18℃,最适叶绿素a浓度范围是0.3~0.8 mg·m~(-3),空间上集中分布在40°N~41°N、148°E~151°E,海流对鲐鱼渔场形成尤为重要。     

Estuarine fishes in two large eastern Australian coastal rivers—does prawn trawling influence fish community structure?

Fish communities were sampled quarterly over a 2-year period at sites along the estuarine gradient of two large coastal rivers, the Richmond and Clarence, located in eastern Australian (NSW). A total of 175 582 fish from 73 species was captured, with two species making up 65% of this total. Many fish species of economic importance in local coastal waters were captured, predominantly as juveniles. Multivariate analysis revealed three significantly different groupings of sites corresponding to the marine, brackish and tidal-freshwater zones of the two rivers. The two rivers were remarkably similar in terms of overall abundance and diversity of fish, and structuring of fish communities, despite a large-scale and intensive prawn trawling industry operating on the Clarence River. Dependency of fishes on estuarine habitats and the distribution of some of the dominant fish species in relation to estuarine gradients are discussed.     

Strategies for improving fuel efficiency in the Portuguese trawl fishery

The recent rise in oil prices has brought renewed attention to energy savings in the fishing industry, and particularly in trawling. Coastal trawlers spend most of their time on fishing grounds near the coast. In such cases, the most successful energy-saving modifications ought to result from changes in the fishing gear and towing conditions. The purpose of this study was to identify the energy-economy potential for Portuguese fish trawlers after altering a vessel's operating conditions and improving its trawl gear performance. Two trawlers, named Tricana de Aveiro and João Macedo were selected as subjects in this project. Both vessels work with gear of similar design and size. Experimental sea trials were carried out to elucidate the actual vessel and gear performance. A model trawl was then built and tested in a flume tank, which provided the basis for improving the gear design. Full-scale trials were then carried out with both vessels using the modified trawls in order to assess changes in gear performance. The new trawls maintained their previous ability to catch species of different ecological groups and consumed less fuel at the same commercial trawling speed. An economic study showed potential increases in the net cash flow (NCF) of up to 27% over the range of operational navigation and trawling speeds. Having demonstrated the performance of the new trawls, the skippers of both vessels subsequently adopted the new design for commercial fishing.     

渤海对虾流网作业中掉虾数量及其成活率的研究

根据对虾流网结构特点设计兼有捕虾和接获网上掉虾功能的专用网具,研究了对虾流网作业全过程的掉虾数量;同时还对暂养模拟掉虾和对虾在网上直接死亡率作了调查;并研究了流网掉下对虾的成活率。所得结果为调整渤海区秋季对虾汛的捕捞结构,实行以流网代替拖网作业的管理决策提供了科学依据。     

Genetic evidence for a single stock of giant tiger prawns Penaeus monodon in demarcated prawn fishing zones of Tanzania

Tanzania demarcated three prawn fishing zones along its coast to spread fishing pressure and help the prawn fishery to recover. However, it is unknown whether the demarcated zones correspond to the genetic stock structure of the world's most commercially important prawn, the giant tiger prawn (Penaeus monodon). Thus, this study used partial mitochondrial control region sequences (534 base pairs) to test the hypotheses that (1) giant tiger prawns in Tanzania's demarcated prawn fishing zones are a single stock and (2) the giant tiger prawns on the Tanzanian coast experienced a recent demographic expansion. The sequences showed high haplotype diversity (h = 0.998–1.0) and low nucleotide diversity (θ_π = 1.89%–2.24%). The neutrality test and mismatch analysis showed that the hypothesis of recent demographic expansion could not be rejected. The analysis of molecular variance revealed low and insignificant fixation indices between the zones (F_ST = 0.00025, p > 0.05; Φ_ST = −0.00027, p > 0.05), suggesting that the three demarcated fishing zones constitute a single stock and that fishers may be targeting the same stock. Furthermore, it was discovered that Zone 2 has the potential to replenish depleted areas; thus, it should be prioritised in future conservation planning.     

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.     

当代海洋捕捞的发展现状和展望

海洋捕捞事关国际粮食安全和海洋生态系统健康。得益于科学技术的进步, 当代海洋捕捞发展迅速, 但依然面临着渔业资源衰退、海洋底栖环境破坏、兼捕与抛弃、废弃渔具引起的幽灵捕捞和塑料污染、气候变化、区域发展不平横等系列问题。本文综述了国内外为缓解上述问题在海洋捕捞管理、技术创新、最佳实践探索等方面的工作进展, 并提出了如下发展建议: (1) 应进一步加强生态友好型渔具渔法、高效节能技术、废弃渔具回收利用技术、生物可降解材料等方面的基础和应用研究; (2) 开展减少兼捕、降低底拖网对底栖环境的影响、海洋捕捞高效节能、减少废弃渔具流向海洋等方面最佳实践的探索; (3) 加强激励机制的研究和应用, 加强对废弃渔具回收产业的政策扶持, 完善海洋捕捞管理体系和废弃渔具管理体系建设, 增强治理能力; (4) 加强国家或地区间渔业合作, 分享管理知识, 促进区域间平衡发展。(5) 加强气候变化应对策略的探索研究。立足长远、多措并举, 促进海洋捕捞可持续发展。     

四片式高口双拖网

本文反映黄海区群众渔业双拖网改进研究取得的一项新成果,报导了设计的依据、改进的特点、网模水池试验及海上实测的验证数据。经三年捕捞试验,取得了比原型网增产40％以上的显著效果。     

长天井、大网目囊网的机帆渔船双拖网增产对虾和释放幼鱼的试验研究

为合理利用我国北方海区对虾资源，每年秋季在渤海使用机动渔船双拖网捕捞生产。由于渤海既是主要的对虾渔场，又是多种经济鱼类产卵繁殖和幼鱼生长的良好场所。在拖网捕虾的同时，大量捕到当年的幼鱼，损害渔业资源。因此，在1974—1977年研究一种能够多捕对虾、大量释放幼鱼的新型双拖网。新型网的主要特点如下：它的天井网比原型网长一倍左右，并增加侧网使网口的覆盖面积和网高相应扩大，防止对虾接触网具后弹跳逃逸；将囊网网目放大到50毫米，以释放网内的幼鱼。试验效果较为满意。在模型网水槽试验中，新型网的主要优点是阻力较小、网口较高和网口前方覆盖面积较大。捕捞生产试验证明可以增产对虾40％以上，少捕幼鱼不少于60％。     

Formulation of management strategies for two exploited West African prawn populations

The life histories of the estuarine prawn, Nematopalaemon hastatus Aurivillius, and the brackish river prawn, Macrobrachium macrobrachion Herklots, were compared in relation to their seasonal exploitation patterns in the Cross River estuary, Nigeria. The period of intensive exploitation of N. hastatus (March to June) was outside the main spawning season (July to October), because these prawns migrate from the main fishing area in the estuary and near shore coastal waters into the deeper continental shelf waters to spawn. This probably contributes to their reproductive success and conservation of the stock. With M. macrobrachion , the peak spawning period and the major fishing season converge between July and September. These prawns also migrate from the fresh water into the major fishing zone within the estuary during the spawning season. This exposes the spawning population to intense human exploitation. On the basis of their spawning dynamics, M. macrobrachion requires more urgent management attention. An option would be a closed season for M. macrobrachion within the estuary during the spawning period.     

Potential of mixed culture of freshwater prawn (Macrobrachium rosenbergii) and self-recruiting small species mola (Amblypharyngodon mola) in rotational rice–fish/prawn culture systems in Bangladesh

The effects of different stocking densities of freshwater prawn (Macrobrachium rosenbergii) on its growth and production in relation to the presence of small self‐recruiting species mola (Amblypharyngodon mola) were investigated in modified rice fields after rice harvest at Mymensingh, Bangladesh. Keeping the stocking density of mola fixed at 20 000 ha^?1 in each treatment, four densities of freshwater prawn (treatments) were maintained: 10 000, 15 000, 20 000 and 25 000 ha^?1 respectively. The rice plots were limed (CaCO₃) and fertilized with urea, triple super phosphate and cowdung regularly. The prawns were fed daily with commercial pellets. Water quality parameters such as temperature, dissolved oxygen, transparency, pH, total alkalinity, inorganic nitrogen (nitrate, nitrite and total ammonia), chlorophyll a and orthophosphate were determined fortnightly. Numerical analysis of plankton communities was performed monthly. All water quality parameters were found to be within the suitable range for freshwater prawn culture, except high temperature, in the peak summer months. There was a homogenous abundance of plankton communities in all treatments throughout the experimental period. Mola started to breed in the second month of the original stocking with partial harvesting after the second month and continued until the final harvest. The total production of mola ranged between 124 and 152 kg ha^?1 during the 4‐month culture period. The average prawn survival ranged from 49% to 57% without any significant difference among treatments. Freshwater prawn production ranged from 294 to 596 kg ha^?1 with significantly higher production in the treatment where 20 000 ha^?1 freshwater prawn were stocked. This treatment also resulted in a higher net profit margin (74%), indicating that stocking at a combination of 20 000 ha^?1 freshwater prawn and 20 000 ha^?1 mola could be the optimum proposition for prawn–mola culture in modified rain‐fed rice fields after rice cultivation.     

Measurement of the rate of depletion of benthic fauna by prawn (shrimp) otter trawls: an experiment in the Great Barrier Reef, Australia

We carried out a trawl-depletion experiment for benthic fauna in an area closed to commercial trawling. A 12-fathom prawn (shrimp) trawl net having a swept path of about 18 m was towed 13 times along each of six tracks. Each track was 2.7 km long and known from previous surveys to contain several relatively dense patches of megabenthos. The depletion rate was assumed to vary between trawls on a given track, and was modelled by a beta distribution defined by a mean rate and a dispersion parameter. Maximum likelihood analysis of the sequence of catches on each track suggests that a single trawl removes a small, but non-negligible, fraction of the benthos. The combined effect of 13 trawls on each track, assuming the trawls were aligned, was to remove a substantial proportion of the initial biomass. Gastropods suffered the greatest impact (an estimated 95% removed, on average). Ascidians, sponges, echinoids, crustaceans and gorgonians were depleted by an estimated 74–86%, and all other taxonomic groups except algae (27% removed) were reduced by at least 54%. Using differential global positioning systems and careful navigation, we aimed to achieve a high degree of overlap between successive trawls. Nonetheless, records show that the trawler deviated from this path and it is likely that the net did not sweep exactly the same path during the 13 trawls on each track. A simulation study was conducted to examine the effect on parameter estimates of ignoring the probable non-alignment of trawls. There was little bias in the estimate of the dispersion parameter. With the level of dispersion found on these tracks, the estimated mean depletion rates would, however, have been biased (upwards when the true mean depletion rate was below 15%, downwards when the true mean depletion rate was above 15%). This means that most mean depletion rates estimated from the catch data are probably biased by 2–3%. The depletion rates estimated from this study may be acceptable in areas that are trawled infrequently or sparsely. However, the cumulative effect of frequent trawls in an area of high intensity trawling is likely to be substantial both in terms of organisms directly affected by trawling and indirectly due to attracting scavengers and removing refuge habitat for fish and other mobile organisms.