Modifying otter boards to reduce bottom contact: effects on catches and efficiencies of triple‐rigged penaeid trawls

Two otter‐board modifications designed to minimise bottom contact (and therefore habitat impacts) were compared against conventional otter boards for their relative effects on the engineering and catching efficiencies of triple‐rigged penaeid trawls. The first modification involved restricting the angle of attack (AOA) of a flat–rectangular otter board to <30^° by attaching a restraining line to an adjacent sled, while the second was a ‘batwing’ otter board comprising a sled‐and‐sail assembly operating at 20^° AOA. The modifications reduced otter‐board bottom contact by up to 8 ha over a night's fishing without affecting standardised catches (per ha) of the targeted eastern king prawns, Penaeus plebejus, nor the fuel required. Further, compared to all other trawls, the restrained trawls caught less discarded bycatch (by up to 37%), while the trawls spread by the modified flat–rectangular and batwing otter boards caught more individuals of three retained benthic teleost species per ha than the conventional configuration; results that were attributed to species‐specific herding responses. Both modifications represent simple alternatives where there are concerns over either benthic impacts or, for the restraining line, unwanted bycatches in penaeid‐trawl fisheries.     

Cumulative impacts of fisheries in the California Current

Ecosystem‐based fisheries management calls for the consideration of the indirect and cumulative effects of fishing, in addition to estimating direct fishing mortality. Here, we quantify such effects of fishing fleets, and their interactions, using a spatially explicit Atlantis simulation model of the food web and fisheries in the California Current. Simulations testing the effects of single fleets suggested that bottom trawl, fixed gear, and hake (Merluccius productus) trawl primarily have direct impacts on their target and bycatch species. Few indirect effects from these three fleets extended through predator–prey links to other parts of the food web. In contrast, effects of the purse seine fleet extended beyond the three groups it harvested, strongly altering the abundance of predators, planktonic prey, and benthos. In terms of nine ecosystem attributes, our experiments involving single fleets identified six fleets that caused the bulk of negative impacts. Specific fleets impacted different aspects of the ecosystem, for instance with groundfish gears causing reductions in piscivore abundance, and hake trawl and purse seine increasing krill through reducing abundance of planktivores. In terms of interactions among fleets' effects, the vast majority of effects were simply additive – the combined effect of two fleets was simply the sum of the individual fleets' effects. The analyses offer one way to sharpen the focus of ecosystem‐based fisheries management in the California Current, emphasizing impacts and interactions of particular stressors.     

浙江省单拖渔业现状分析与研究

利用2001~2006年浙江省单拖渔业典型性调查数据和2005~2008年的单拖作业监测资料,从浙江省单拖捕捞力量和产量分布、渔获对象和资源密度变化及经济效益、劳动力组成等几个方面进行了初步分析。结果表明,近几年浙江省单拖作业船数保持在2200艘左右,产量在40×104t左右,单船净利润存在年间波动,以2005年最高;单拖作业渔场分布较广,渔获物以头足类和中下层鱼类为主;近年来经济鱼类比例下降,而其它低值鱼类比例上升。文章最后提出了当前浙江单拖渔业存在的问题,并提出了相应管理建议以供参考。     

福建海区单拖渔业现状浅析

本文根据1998-2003年福建海区拖网渔业资源监测以及1995-1997年间闽南台湾浅渔场单拖渔业资源调查、1998年和2000-2001年福建海区拖网定点调查的资料,论述和分析福建海区单拖渔业的现状。结果表明:目前整个单拖渔业虽然船数削减,实际却加大渔船功率,作业时间延长,促使年产量、单位产量不断提高,导致渔获组成发生很大变化,由低质小型鱼类、生命周期短的头足类、虾蟹类支撑,渔获鱼类质量较差,经济效益下降;另一方面头足类、虾蟹类资源量年间波动性较大,易受捕捞过渡影响,很难长时间承担目前捕捞压力。为此,文中提出加强严格实行"双控制度"及控制网目规格等管理建议和意见,为有关渔业管理和生产部门提供参考依据。     

公海深海底层渔业国际管理进展

以底拖网为代表的深海底层渔业对深海脆弱海洋生态系统的危害受到国际社会的热切关注。2003年以来联合国大会多次通过决议,呼吁各国各自并通过RFMO/As采取行动,根据预防性原则,采用基于生态系统的管理方法,评估深海底层渔业对脆弱海洋生态系统的影响,若评估表明确有重大不利影响,则应采取有效措施限制深海底层渔业以降低这种影响;FAO主要从技术角度制定了《公海深海渔业管理国际指南》,为管理公海深海渔业和保护脆弱海洋生态系统提出了技术标准和管理框架;RFMO承担着具体执行深海底层渔业管理措施和监督管理的责任,在北大西洋、地中海、南太平洋的公海和南极水域,相关RFMO已采取了暂停部分区域底拖网渔业活动、收集数据、评估底拖网对脆弱海洋生态系统的影响等措施,在北太平洋,新成立的北太平洋渔业管理委员会将公海底层渔业管理作为首要目标。环保非政府组织和部分科学家呼吁禁止公海深海底层渔业,但各国对此的立场尚不一致,产业界大多持反对立场。近期来看,尚难以全面禁止公海的深海底层渔业。中国正在发展公海大洋渔业,需对此密切关注,加强跟踪研究以支撑决策,并应发展和使用选择性渔具和对生态环境无害的作业方式,防止对脆弱海洋生态系统产生损害性影响。     

Model of trawlable area using benthic terrain and oceanographic variables—Informing survey design and habitat maps in the Gulf of Alaska

Bottom trawl surveys provide fishery‐independent data on relative abundance and life history parameters for a wide range of marine taxa. Survey data are used to assess species distribution, biological interactions, and ecosystem structure and to manage marine resources. Not all bottom types or oceanographic conditions accommodate this survey method. We applied National Ocean Service hydrographic smooth sheets to evaluate physical attributes associated with habitat available to surveys. Random forests were used to evaluate the relative influence of benthic terrain and oceanographic predictors in determining accessibility to bottom trawl gear. We examined the marginal importance of each predictor, quantified the response gradient, and applied piecewise regression to determine threshold breakpoint values. Thresholds were used to develop predictive maps and distinguish untrawlable habitat at the scale of discrete towpaths and survey stations. Untrawlable habitat was associated with increased complexity in terrain, roughness, slope, surface curvature, substrate coarseness, current, and aspect. Maps of critical thresholds suggest different variables constrain the probability of a successful trawl in the nearshore, shelf, and continental slope. Overlay analysis of the model projection demonstrates the utility of archived smooth sheet data and identifies areas where higher resolution data might improve results. The model and maps produced in this analysis might be used to identify habitats available to and impacted by commercial trawl fisheries, inform the relative availability of various species and habitat types to bottom trawl surveys, evaluate bias in assessment indices and ecosystem metrics derived from survey data, and advance habitat‐specific biomass estimates.     

闽中地区单拖作业捕捞能力的因子分析

单拖作业是福建省闽中地区重要的作业渔具之一。在捕捞生产过程中渔具受到诸多因素的影响,各种因素对渔具捕捞能力的影响程度不同,给捕捞力量的量化计算和管理带来困难。为了了解各因子对单拖作业捕捞能力发挥的影响程度,本文选择影响单拖捕捞能力发挥的12个因子指标,采用因子分析法对其进行分析。结果表明,可将影响单拖捕捞能力发挥的诸多因素归并为网具结构、渔船作业能力、船龄、渔民捕捞技能4个公共因子,它们在单拖捕捞过程中所做的贡献率依次为网具结构52.87%,渔船作业能力24.46%,船龄7.05%,捕捞技能4.73%。本文还计算和讨论了各样品船的因子得分,通过各因子的得分和综合得分,可对各样品船的捕捞能力发挥情况进行综合评价,为渔业生产和管理提供参考。     

选择性虾拖网渔具渔法研究

根据虾拖网存在网目偏小，渔获物中经济鱼、虾类幼体和杂物所占比例较大等问题，于2002～2003年对在福建海区生产的网板虾拖网作业的网具进行改革，主要是在网具的后部采用上、下两个囊网。下囊网入口处装配刚性栅隔装置，起鱼、虾分离作用，上囊网采用放大网目尺寸，以达到释放经济鱼、虾类幼体为目的。通过改革试验取得了良好的效果。本文着重论述改革后的渔具特点与渔法。     

Response of benthic fauna to experimental bottom fishing: A global meta‐analysis

Bottom‐contact fishing gears are globally the most widespread anthropogenic sources of direct disturbance to the seabed and associated biota. Managing these fishing disturbances requires quantification of gear impacts on biota and the rate of recovery following disturbance. We undertook a systematic review and meta‐analysis of 122 experiments on the effects‐of‐bottom fishing to quantify the removal of benthos in the path of the fishing gear and to estimate rates of recovery following disturbance. A gear pass reduced benthic invertebrate abundance by 26% and species richness by 19%. The effect was strongly gear‐specific, with gears that penetrate deeper into the sediment having a significantly larger impact than those that penetrate less. Sediment composition (% mud and presence of biogenic habitat) and the history of fishing disturbance prior to an experimental fishing event were also important predictors of depletion, with communities in areas that were not previously fished, predominantly muddy or biogenic habitats being more strongly affected by fishing. Sessile and low mobility biota with longer life‐spans such as sponges, soft corals and bivalves took much longer to recover after fishing (>3 year) than mobile biota with shorter life‐spans such as polychaetes and malacostracans (<1 year). This meta‐analysis provides insights into the dynamics of recovery. Our estimates of depletion along with estimates of recovery rates and large‐scale, high‐resolution maps of fishing frequency and habitat will support more rigorous assessment of the environmental impacts of bottom‐contact gears, thus supporting better informed choices in trade‐offs between environmental impacts and fish production.     

Relationships between macro‐epibenthic communities and fish on the shelf grounds of the western Mediterranean

• 1. The present study characterizes the macro‐epibenthic assemblages, and the relationships between demersal species and benthic habitats on the shelf trawl fishing grounds off the Balearic Islands (western Mediterranean).
• 2. The data used were collected during experimental bottom trawl surveys from 2002 to 2005. A total of 157 samples from 38–255 m depth were analysed.
• 3. Three macro‐epibenthic assemblages were identified in both the shallow (at 38–91 m) and deep shelf (90–255 m). Macroalgae bathymetric distribution was identified as the main factor explaining the segregation between shallow and deep shelf assemblages.
• 4. Two especially sensitive habitats were identified: maërl and crinoid beds; a third habitat was identified as Peyssonnelia beds, which represented the highest biomass on the whole shelf, with a similar species richness to the maërl beds. On the deep shelf, crinoid beds represented the highest biomass.
• 5. Habitat type had a significant effect on the distribution of demersal commercial species, most of them being more abundant in the two sensitive habitats mentioned and in the Peyssonnelia beds. Some species showed size‐specific habitat preferences.
• 6. Fisheries management in the area should take into account the resilience of these benthic habitats, and their importance from both ecological and sustainable fisheries management perspectives.

Copyright © 2008 John Wiley & Sons, Ltd.