Mitigating unaccounted fishing mortality from gillnets and traps

Gillnets and traps often are considered to have fewer holistic environmental impacts than active fishing gears. However, in addition to the targeted catches, gillnets and traps still cause unwanted mortalities due to (i) discarding, (ii) ghost fishing of derelict gear, (iii) depredation, (iv) escaping or dropping out of gear, (v) habitat damage, and potentially (vi) avoiding gear and predation and (vii) infection of injuries sustained from most of the above. Population‐level concerns associated with such ‘unaccounted fishing mortalities’ from gillnets and traps have been sufficient to warrant numerous attempts at mitigation. In this article, we reviewed relevant research efforts, locating 130 studies in the primary literature that concomitantly quantified mortalities and their resolution through technical modifications, with the division of effort indicating ongoing concerns. Most studies (85) have focused on discard mortality, followed by ghost‐fishing (24), depredation (10) and escape (8) mortalities. The remaining components have been poorly studied (3). All problematic mortality components are affected by key biological (e.g. species), technical (e.g. fishing mechanisms) and/or environmental (e.g. temperature) factors. We propose that these key factors should be considered as part of a strategy to reduce impacts of these gears by first assessing modifications within and then beyond conventional configurations, followed by changes to operational and handling practices. Justification for this three‐tiered approach is based not only on the potential for cumulative reduction benefits, but also on the likely ease of adoption, legislation and compliance.     

捕捞网具逃逸死亡研究进展

幼鱼及非目标种类兼捕已成为许多网具作业共同存在的问题,提高网具选择性能是降低这种兼捕的重要途径。但是选择性能研究主要是基于从网具中逃逸后的鱼类较轻遭受到网具损害并最终存活。如果逃逸死亡率较高,即使选择性提高也是无宜的,因此应当将鱼类逃逸后生存能力作为网具改进效果的评价指标。本文主要由以下三个部分组成:(1)逃逸后死亡的主要影响因素概述,主要包括网具特征、作业时间、作业深度、渔获种类及体长、渔获量及环境因子等;(2)逃逸死亡对资源量和渔业管理决策的影响;(3)总结了近年来通过网具革新措施提高存活率的最新研究概况,如逃逸网片、分离栅及新型材料的应用等。综合分析,认为采取保护幼鱼和非目标种类的一系列措施尽管能够提高鱼类从网具的逃逸几率,但是并不能降低遭遇网具的概率,实施禁渔区、禁渔期制度管理,避免在幼鱼资源高密度区和产卵季节作业是降低这种未报告死亡的最重要的途径。     

Changes in a coral reef fishery along a gradient of fishing pressure in an Indonesian marine protected area

1. Human population growth, rising incomes, and increased commercialization of marine resources promote demand for reef fish, yet few studies in Indonesia have examined how artisanal fisheries are influenced by the socio‐cultural conditions that contribute to their exploitation. This study examined artisanal fisheries of Karimunjawa National Park, Java, to understand how the condition of an artisanal fishery was related to socio‐cultural factors, along a gradient in fishing pressure.
2. A total of 8674 fishes landed in Karimunjawa by fishers using four artisanal fishing gears were examined to understand how the condition of the artisanal fishery (standard and infinite fish length, trophic level and weight) related to fishing gear use, village fishing grounds, management, human population size, human population density and estimated fishing pressure.
3. Depletion in fish lengths and trophic structure were found at or above 46 fishing trips day^‐1 km^‐2, suggesting that fishing pressure is a key factor driving fishery catch structure. When catch characteristics were examined in relation to the fishing pressure estimates from each village, negative correlations were found between inshore fishing pressure (no. trips day^‐1 km^‐2) and all four fish catch characteristics, but owing to small sample sizes (n = 5), only the effects on trophic level were significant.
4. Fishery closures had limited impact on fish characteristics, and lack of any effect of spatial controls on fishing also supports the notion that fishing pressure and the types of fishing gears used, most likely driven by human population densities, are the greatest drivers of reef fish catch characteristics in the Karimunjawa fishery.
5. In the absence of support for fishery closures from local fishing communities or adequate enforcement of fishery closures, targeted gear or species management strategies that limit impacts on large‐bodied fish and aim to conserve key species may be more effective in improving the size and trophic structure of fish populations.

Copyright © 2013 John Wiley & Sons, Ltd.     

A framework for adaptive gear and ecosystem‐based management in the artisanal coral reef fishery of Papua New Guinea

• 1. Artisanal fishing on coral reefs in Papua New Guinea is an important livelihood activity that is managed primarily at the level of local communities. Pockets of overexploitation exist and are expected to increase with plans for increased commercialization.
• 2. This paper provides a current assessment of the artisanal multi‐species coral reef fishery by examining selectivity of the dominant gear, namely line fishing, spearguns, and gill nets. Each gear has its own strengths and weaknesses in terms of use and conservation of resources, with no clear problem gear.
• 3. The three gears utilize different resources but there was moderate overlap in the species caught, particularly between gill nets and line fishing and marginally between lines and spearguns. Gill nets have the disadvantage of being destructive to coral and the advantage of catching commercial species. Line fishing catches an intermediate number of species but mostly large‐bodied and predatory species that could potentially reduce predation and the mean trophic level of the fishery. Spearguns catch the highest numbers of species, including many non‐commercial and herbivorous fish and could reduce the diversity of fish and encourage algal growth.
• 4. This information could be used in combination with scientific monitoring and traditional ecological knowledge to develop an adaptive management framework that uses local restrictions on the various gears to restore or balance the fishery and ecosystem. Restrictions could be selectively imposed: on gill nets when coral cover is low, line fishing when large‐bodied predators are depleted, and spearguns when biodiversity is reduced and algal abundance high.

Copyright © 2008 John Wiley & Sons, Ltd.     

Short-term stress and mortality of juvenile school prawns, Metapenaeus macleayi, discarded from seines and trawls

Abstract  Two experiments were carried out to assess the short-term impacts to juvenile school prawns, Metapenaeus macleayi (Haswell) discarded from commercial seining and trawling in New South Wales, Australia, and the potential utility of modifications to operational and/or onboard handling practices designed to improve survival. For both gears, discards were handled according to two general categories of treatment (termed mild and extreme) chosen to represent the plausible limits of severity during existing commercial operations. A total of 600 trawled and 480 seined school prawns (from both treatments), along with appropriate numbers of controls (previously collected and housed in acclimation tanks), were placed into cages (in groups of 10). Subsets of each of these groups were then destructively sampled over periods of up to 5 days and examined for their mortality and physiological response (measured as levels of l -lactate in the haemolymph). In both experiments, the temporal mortalities in the treatment groups ranged between 0% and 15% and, for the most part, were not significantly different to controls. Prawns in the treatment groups had similar, significantly elevated levels of l -lactate approximately 40 min after being caught, but these returned to levels approaching baseline estimates within 48 h. Under the conditions examined, juvenile school prawns appear capable of withstanding a range of stressors associated with being discarded from active gears, and their fate is not greatly influenced by the duration of gear deployment and subsequent air exposure. However, further work is required to quantify other sources of unaccounted fishing mortality and, ultimately, the impacts that discarding has on stocks.     

The understudied and underappreciated role of predation in the mortality of fish released from fishing gears

The assumption that animals released from fishing gears survive has frequently been scrutinized by researchers in recent years. Mortality estimates from these research efforts can be incorporated into management models to ensure the sustainability of fisheries and the conservation of threatened species. Post‐release mortality estimates are typically made by holding the catch in a tank, pen or cage for short‐term monitoring (e.g. 48 h). These estimates may be inaccurate in some cases because they fail to integrate the challenges of the wild environment. Most obvious among these challenges is predator evasion. Stress and injury from a capture experience can temporarily impair physiological capacity and alter behaviour in released animals, a period during which predation risk is likely elevated. In large‐scale commercial fisheries, predators have adapted their behaviour to capitalize on impaired fishes being discarded, while in recreational catch‐and‐release fisheries, exercise and air exposure can similarly impede the capacity for released fish to evade opportunistic predators. Owing to the indirect and often cryptic nature of this source of mortality, very few studies have attempted to document it. A survey of the literature demonstrated that <2% of the papers in the combined realms of bycatch and catch‐and‐release have directly addressed or considered post‐release predation. Future research should combine field telemetry and laboratory studies using both natural and simulated predation encounters and incorporate physiological and behavioural endpoints. Quite simply, predation is an understudied and underappreciated contributor to the mortality of animals released from fishing gears.     

Fishing profiles of Danish seiners and bottom trawlers in relation to current EU management regulations

Danish seines and bottom trawls operate differently and have different catching processes. Both gears belong to the same legislative category in European fisheries, but different management strategies in other countries and criticism by fishers on grouping Danish seines and trawls together indicate disagreement on current gear classification. This study compared both gears in terms of their fishing characteristics and catches of commercial species based on 16 years of observer data. Danish seining is a specialised fishing method that targeted few species but with higher total catch rates than bottom trawlers. Bottom trawling is a more all‐purpose fishing method that targets a larger number of species, and bottom trawlers use larger engines than Danish seiners. A generalised additive mixed model indicated that catch rates of flatfish are generally higher for Danish seines, and catch rates of roundfish species are higher for trawlers. The results do not directly suggest a separation of the gears in terms of legislation as the quantities of fish below current minimum size were similar, but for example future survival studies may reach different conclusions. Additional factors were found to be important in determining catches of both gears.     

Changes in life history and ecological characteristics of coral reef fish catch composition with increasing fishery management

Abstract Length, life history and ecological characteristics of landed fish communities were studied over a 10‐year period to test theories of fishing disturbance during a time of increased gear and closure management in heavily utilised fisheries. It was predicted that with greater management restrictions: (1) the earliest and fastest responses in the fishery will be seen in those species with faster turnovers, or relatively lower vulnerabilities to fishing; (2) the fishery would transition to a landed catch with higher mean trophic levels, and greater mean body lengths. In addition, the removal of a non‐selective, small‐mesh seine nets should benefit the catch of gears that previously had the greatest species selectivity overlap with the seine net. Many predictions were supported, although maximum lengths and lengths at maturity responded more rapidly than anticipated. The response to eliminating the non‐selective seine net was a more rapid increase in sizes caught by gears with a larger overlap in size (hook and lines) than species selectivity (gill nets). The simultaneous comparison of management systems over time indicates that open‐access fishing grounds can benefit from restrictions imposed in adjacent fishing grounds. The study indicated that multi‐species coral reef fisheries management objectives of maximising yields, as well as maintaining the fish community’s life‐history diversity, require management trade‐offs that balance local socio‐economic and biodiversity needs.     

Modification of marine habitats by trawling activities: prognosis and solutions

Fishing affects the seabed habitat worldwide on the continental shelf. These impacts are patchily distributed according to the spatial and temporal variation in fishing effort that results from fishers' behaviour. As a consequence, the frequency and intensity of fishing disturbance varies among different habitat types. Different fishing methodologies vary in the degree to which they affect the seabed. Structurally complex habitats (e.g. seagrass meadows, biogenic reefs) and those that are relatively undisturbed by natural perturbations (e.g. deep‐water mud substrata) are more adversely affected by fishing than unconsolidated sediment habitats that occur in shallow coastal waters. These habitats also have the longest recovery trajectories in terms of the recolonization of the habitat by the associated fauna. Comparative studies of areas of the seabed that have experienced different levels of fishing activity demonstrate that chronic fishing disturbance leads to the removal of high‐biomass species that are composed mostly of emergent seabed organisms. Contrary to the belief of fishers that fishing enhances seabed production and generates food for target fish species, productivity is actually lowered as fishing intensity increases and high‐biomass species are removed from the benthic habitat. These organisms also increase the topographic complexity of the seabed which has been shown to provide shelter for juvenile fishes, reducing their vulnerability to predation. Conversely, scavengers and small‐bodied organisms, such as polychaete worms, dominate heavily fished areas. Major changes in habitat can lead to changes in the composition of the resident fish fauna. Fishing has indirect effects on habitat through the removal of predators that control bio‐engineering organisms such as algal‐grazing urchins. Fishing gear resuspend the upper layers of sedimentary seabed habitats and hence remobilize contaminants and fine particulate matter into the water column. The ecological significance of these fishing effects has not yet been determined but could have implications for eutrophication and biogeochemical cycling. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators. In contrast, high levels of trawling disturbance cause sediment systems to become unstable due to large carbon fluxes between oxic and anoxic carbon compartments. In low energy habitats, intensive trawling disturbance may destabilize benthic system chemical fluxes, which has the potential to propagate more widely through the marine ecosystem. Management regimes that aim to incorporate both fisheries and habitat conservation objectives can be achieved through the appropriate use of a number of approaches, including total and partial exclusion of towed bottom fishing gears, and seasonal and rotational closure techniques. However, the inappropriate use of closed areas may displace fishing activities into habitats that are more vulnerable to disturbance than those currently trawled by fishers. In many cases, the behaviour of fishers constrains the extent of the impact of their fishing activities. Management actions that force them to redistribute their effort may be more damaging in the longer term.     

涉海建设项目中深水海域渔业资源调查问题的研究

为解决涉海工程深水渔业资源评估的问题,于2014~2015年在南海北部局部海域开展底拖网与灯光罩网联合调查渔业资源活动。结果表明:在深水区底拖网渔获组成与灯光罩网有明显的不同,并受到季节与海域的影响(P0.05)。拖网渔获物中鱼类平均占渔获总尾数的93.36%,头足类占3.53%,甲壳类占3.11%;灯光罩网渔获物中鱼类平均占渔获总尾数的63.33%,头足类占36.65%,甲壳类占0.02%。两种调查的相同渔获物1~6种,主要为鱼类和头足类,在拖网中所占比值较低,平均占渔获总尾数的2.07%,而在灯光罩网中比值极高,平均占渔获总尾数的67.03%。灯光罩网与底拖网调查的渔业资源密度直接比值平均为0.06,灯光罩网与底拖网扣除相同渔获种类后的资源密度比值平均为0.06,相应的质量密度直接比值平均为0.62,扣除相同渔获种类后的比值平均为0.67。研究认为,深水海域的渔业资源量应该是两种渔具调查结果之和,对两种渔具调查中出现的相同渔获物,宜取其评估数据的平均值。由于各站点调查结果差异较大,采用所有站点的平均值来计算评估海域的损害赔偿较为合适。建议对SC/T9110-2007技术规程修订,规范中上层渔业资源调查方法。