Effects of cryptic mortality and the hidden costs of using length limits in fishery management

Fishery collapses cause substantial economic and ecological harm, but common management actions often fail to prevent overfishing. Minimum length limits are perhaps the most common fishing regulation used in both commercial and recreational fisheries, but their conservation benefits can be influenced by discard mortality of fish caught and released below the legal length. We constructed a computer model to evaluate how discard mortality could influence the conservation utility of minimum length regulations. We evaluated policy performance across two disparate fish life‐history types: short‐lived high‐productivity (SLHP) and long‐lived low‐productivity (LLLP) species. For the life‐history types, fishing mortality rates and minimum length limits that we examined, length limits alone generally failed to achieve sustainability when discard mortality rate exceeded about 0.2 for SLHP species and 0.05 for LLLP species. At these levels of discard mortality, reductions in overall fishing mortality (e.g. lower fishing effort) were required to prevent recruitment overfishing if fishing mortality was high. Similarly, relatively low discard mortality rates (>0.05) rendered maximum yield unobtainable and caused a substantial shift in the shape of the yield response surfaces. An analysis of fishery efficiency showed that length limits caused the simulated fisheries to be much less efficient, potentially exposing the target species and ecosystem to increased negative effects of the fishing process. Our findings suggest that for overexploited fisheries with moderate‐to‐high discard mortality rates, reductions in fishing mortality will be required to meet management goals. Resource managers should carefully consider impacts of cryptic mortality sources (e.g. discard mortality) on fishery sustainability, especially in recreational fisheries where release rates are high and effort is increasing in many areas of the world.     

Quantitative ecological risk assessment for fishing effects on diverse data-poor non-target species in a multi-sector and multi-gear fishery

Assessment of ecological sustainability for all species impacted by fishing is one of the most important and practical steps towards an Ecosystem Approach to Fisheries. We extend methods for Sustainability Assessment for Fishing Effects (SAFE) to assess diverse bycatch species in a multi-sector and multi-gear fishery. We develop methods for estimating fishing mortality rate, based on limited data, for demersal trawl, Danish seine, gillnet, and longline. The general approach involves estimating spatial overlap between species distribution and fishing effort distribution, catchability resulting from probability of encountering the gear and size-dependent selectivity, and post-capture mortality. We define three reference points (F_msm, F_lim, and F_crash) and use six methods to derive these reference points. As an example, we apply this method to nearly 500 fish species caught in the Southern and Eastern Scalefish and Shark Fishery, a multi-sector and multi-gear fishery in Australia. We assess sustainability risk for all captured fish species in each sub-fishery and the cumulative impact across all the sub-fisheries. The results indicate that chondrichthyans are more vulnerable to fishing impact than teleosts, and that impact differs among sectors of the fishery. This method could be easily applied to other fisheries. However, the results may require fine tuning by other means such as expert judgment.     

An assessment of recreational fishery harvest policies for Murray cod in southeast Australia

Murray cod Maccullochella peelii peelii is one of the world’s largest freshwater fish and supports popular fisheries in southeast Australia, but no previous modelling efforts have evaluated the effects of fisheries regulations or attempted to develop sustainable harvest policies. We compiled existing population metrics and constructed an age-structured model to evaluate the effects of minimum length limits (MLLs) and fishing mortality rates on Murray cod fisheries. The model incorporated a Beverton and Holt stock recruit curve, age-specific survivorship and vulnerability schedules, and discard (catch and release) mortality for fish caught and released. Output metrics included yield (kg), spawning potential ratio (SPR), total angler catch, total harvest, and the proportion of angler trips that would be influenced by each regulation based on recent creel survey data. The model suggested that annual exploitation (U) should be held to less than 0.15 under the current MLL of 500 mm total length to achieve an SPR > 0.3, a target usually considered to prevent recruitment overfishing. Exploitation rates at or exceeding 0.3 would cause SPR values to drop below typical management targets unless the MLL was set at or above 700 mm. Regulations that protected Murray cod from overfishing created higher angler catches and higher catch of trophy fish, but at a cost of reducing the proportion of angler trips resulting in a harvested fish. Expressing model output on a per-angler trip basis may help fishery managers explain regulation trade offs to anglers.     

Fishing impacts and the degradation or loss of habitat structure

The wider effects of fishing on marine ecosystems have become the focus of growing concern among scientists, fisheries managers and the fishing industry. The present review examines the role of habitat structure and habitat heterogeneity in marine ecosystems, and the effects of fishing (i.e. trawling and dredging) on these two components of habitat complexity. Three examples from New Zealand and Australia are considered, where available evidence suggests that fishing has been associated with the degradation or loss of habitat structure through the removal of large epibenthic organisms, with concomitant effects on fish species which occupy these habitats. With ever-increasing demands on fish-stocks and the need for sustainable use of fisheries resources, new approaches to fisheries management are needed. Fisheries management needs to address the sustainability of fish-stocks while minimizing the direct and indirect impacts of fishing on other components of the ecosystem. Two long-term management tools for mitigating degradation or loss of habitat structure while maintaining healthy sustainable fisheries which are increasingly considered by fisheries scientists and managers are: (1) protective habitat management, which involves the designation of protected marine and coastal areas which are afforded some level of protection from fishing; and (2) habitat restoration, whereby important habitat and ecological functions are restored following the loss of habitat and/or resources. Nevertheless, the protection of marine and coastal areas, and habitat restoration should not be seen as solutions replacing conventional management approaches, but need to be components of an integrated programme of coastal zone and fisheries management. A number of recent international fisheries agreements have specifically identified the need to provide for habitat protection and restoration to ensure long-term sustainability of fisheries. The protection and restoration of habitat are also common components of fisheries management programs under national fisheries law and policy.     

Rebuilding Mediterranean fisheries: a new paradigm for ecological sustainability

In Mediterranean European countries, 85% of the assessed stocks are currently overfished compared to a maximum sustainable yield reference value (MSY) while populations of many commercial species are characterized by truncated size‐ and age‐structures. Rebuilding the size‐ and age‐structure of exploited populations is a management objective that combines single species targets such as MSY with specific goals of the ecosystem approach to fisheries management (EAF), preserving community size‐structure and the ecological role of different species. Here, we show that under the current fishing regime, stock productivity and fleet profitability are generally impaired by a combination of high fishing mortality and inadequate selectivity patterns. For most of the stocks analysed, a simple reduction in the current fishing mortality (F_cur) towards an MSY reference value (F_MSY), without any change in the fishing selectivity, will allow neither stock biomass nor fisheries yield and revenue to be maximized. On the contrary, management targets can be achieved only through a radical change in fisheries selectivity. Shifting the size of first capture towards the size at which fish cohorts achieve their maximum biomass, the so‐called optimal length, would produce on average between two and three times higher economic yields and much higher biomass at sea for the exploited stocks. Moreover, it would contribute to restore marine ecosystem structure and resilience to enhance ecosystem services such as reservoirs of biodiversity and functioning food webs.     

Catching the future: Applying Bayesian belief networks to exploratory scenario storylines to assess long‐term changes in Baltic herring (Clupea harengus membras,Clupeidae) and salmon (Salmo salar,Salmonidae) fisheries

Fisheries management aims to ensure that the fishing activities are environmentally sustainable in the long term, while also achieving the economic, social and food security related management objectives. To facilitate this, both the ecological and human dimensions of sustainability need to be included in fisheries assessment. In addition, assessing long‐term sustainability calls for taking into account plausible changes in the surrounding societal conditions that shape the characteristics of the fisheries governance system, as well as the ecological conditions. The paper uses a combination of qualitative exploratory scenario storylines (ESS) and Bayesian belief networks (BBN) to integrate the environmental, economic, social and food security dimensions in an interdisciplinary assessment of the future sustainability of Baltic herring (Clupea harengus membras, Clupeidae) and salmon (Salmo salar, Salmonidae) fisheries. First, four alternative ESS were created based on plausible changes in societal drivers. The ESS were then formulated into a BBN to (a) visualize the assumed causalities, and (b) examine quantitatively how changes in the societal drivers affect the social‐ecological fisheries system and ultimately the fisheries management objectives. This type of probabilistic scenario synthesis can help in thinking qualitative scenarios in a quantitative way. Moreover, it can increase understanding on the causal links between societal driving forces and the complex fisheries system and on how the management objectives can be achieved, thereby providing valuable information for strategic decision‐making under uncertainty.     

Evolutionary impact assessment: accounting for evolutionary consequences of fishing in an ecosystem approach to fisheries management

Managing fisheries resources to maintain healthy ecosystems is one of the main goals of the ecosystem approach to fisheries (EAF). While a number of international treaties call for the implementation of EAF, there are still gaps in the underlying methodology. One aspect that has received substantial scientific attention recently is fisheries‐induced evolution (FIE). Increasing evidence indicates that intensive fishing has the potential to exert strong directional selection on life‐history traits, behaviour, physiology, and morphology of exploited fish. Of particular concern is that reversing evolutionary responses to fishing can be much more difficult than reversing demographic or phenotypically plastic responses. Furthermore, like climate change, multiple agents cause FIE, with effects accumulating over time. Consequently, FIE may alter the utility derived from fish stocks, which in turn can modify the monetary value living aquatic resources provide to society. Quantifying and predicting the evolutionary effects of fishing is therefore important for both ecological and economic reasons. An important reason this is not happening is the lack of an appropriate assessment framework. We therefore describe the evolutionary impact assessment (EvoIA) as a structured approach for assessing the evolutionary consequences of fishing and evaluating the predicted evolutionary outcomes of alternative management options. EvoIA can contribute to EAF by clarifying how evolution may alter stock properties and ecological relations, support the precautionary approach to fisheries management by addressing a previously overlooked source of uncertainty and risk, and thus contribute to sustainable fisheries.     

Keeping watch on the unwatchable: technological solutions for the problems generated by ecosystem-based management

Ecosystem‐based management is an emerging paradigm influencing the management of commercial fisheries. Increasingly, developed nations are adopting explicit legislation and policy governing the assessment and management of their fisheries against criteria of ecological sustainability. Yet the ability to evaluate ecosystem impacts of fisheries is compromised by a general lack of understanding of marine ecosystem function (beyond the population level) and a lack of robust and practical indicators for ecosystem health and management. Recent technological advances can assist in developing criteria, including structural analyses of seafloor communities potentially impacted by fishing gears (e.g. demersal trawling). Similarly, advances in fishing gear technology, including improved selectivity and the development of gears which have a more benign environmental impact, can mitigate some of the ecological impacts of fishing. Such technological advances are summarized in the context of contemporary fisheries management.     

Key attributes related to fishery improvement project (FIP) effectiveness in promoting improvements towards sustainability

Fishery improvement projects (FIPs) are emerging as a popular market‐based means to improve fisheries sustainability and have been employed in scores of fisheries around the world; however, project ability to realize improvements has been highly variable, and little is known about how fishery and project conditions affect improvement efforts. In order to evaluate the effectiveness of the FIP model as a tool for improving diverse fisheries around the world, we compile a unique dataset of social, ecological and economic characteristics for over 60 FIPs globally, which we use to identify key attributes correlated with improvements in fishing practices, management and/or on‐the‐water outcomes. Using a random forest classifier, we identify three important attributes related to FIP effectiveness in demonstrating improvements. Specifically, FIPs are more likely to have achieved improvements with increased cumulative project time, when regional‐level management arrangements are present and when the target species has a moderate inherent vulnerability to fishing. Interestingly, improvements were not correlated with a number of expected features, including a fishery's socio‐economic setting or baseline performance against the desired sustainability standard (e.g. the Marine Stewardship Council fisheries standard). This study improves our understanding of factors related to FIP effectiveness in improving fisheries practices and management and provides key insights for practitioners into important attributes to consider when implementing the FIP model to promote fisheries sustainability.     

Population dynamics and management of culture-based fisheries

Abstract The population dynamics of culture-based fisheries are studied by means of a mathematical model, which incorporates explicit sub-models for density-dependent individual growth and size-dependent mortality. The model applies to populations of carps, i.e. common carp, Chinese carps, and Indian major carps, and coregonids.
The effects on production of stocking density and size of seed fish, fishing mortality, and size at harvesting are studied in a model population of carp. Management implications of the modelling results are emphasized. An adaptive approach to management, involving judicious experimentation with stocking and harvesting regimes, is proposed to gain information on the dynamics of actual fisheries, and to optimize their production.     

基于分期的实际种群分析法研究

实际种群分析法(virtual population analysis,VPA)是开展渔业资源评估最有效的技术之一,一般以世代为基础开展评估.基于实际渔业存在渔汛期、休渔期等特点,本研究运用分期评估的概念对传统实际种群分析进行了扩展,即分期种群分析法,并根据不同时期的捕捞死亡特征,评估与分析了4种不同分期情景对评估结果的影响.模拟研究表明,由于分期不当造成评估结果的误差为6％～33％.文中一并给出了开展分期实际种群分析法对资料收集和参数评估的要求.该方法克服了传统实际种群分析法中没有全面分期产生的误差,使其扩展至适合于评估全年捕捞死亡率不稳定或非连续性渔业种群,评估结果也更接近于评估种群的真实值.     

Multicentury trends and the sustainability of coral reef fisheries in Hawai‘i and Florida

Global overfishing indicates a need to define fisheries sustainability thresholds and identify social factors promoting successful management, but rates of fishing and factors mediating sustainability over long timescales are largely unknown. Here, we reconstruct fisheries yield for the entire period of human habitation (five to seven centuries) for two coral reef ecosystems with substantially different fisheries histories (Florida Keys and the Hawaiian Islands) and evaluate the management strategies associated with periods of sustainable fishing. This involved a mixed methods approach, in which we estimated yield by fishery sector (commercial, subsistence, recreational and aquaculture) and characterized management strategies associated with periods of sustained high yields. We found differences between the two locations, with Hawai‘i sustaining yields of more than 12 mt km^?2 for four centuries prior to the arrival of Europeans. This period was characterized by adaptive management whose design and enforcement exhibited characteristics of common property resource governance systems, and which effectively protected reef habitat, vulnerable life‐history stages for fish, and species with high susceptibilities to overfishing. Reefs in both Florida and Hawai‘i were exploited intensively after European contact, with sequential export‐driven depletion evident in Florida over the past century. Today, both exhibit strikingly similar modern catch levels, with landings exceeding 10 mt km^?2 and evidence of overfishing. Our results demonstrate that management strategies and social institutions that support strict enforcement by a local rule‐making authority have had substantial impacts on fisheries yields in the past and suggest that long‐term sustainability of fisheries is possible, although rare today.     

Experimental harvesting of Mytilus galloprovincialis: Can an alien mussel support a small-scale fishery?

In an effort to stimulate new fisheries and address historic imbalances in access to fishing rights, there has been a recent focus on the development of small-scale fisheries in South Africa. To assess the biological viability of a fishery for the alien mussel Mytilus galloprovincialis, an experimental fishery operated by two impoverished coastal communities was initiated. Harvesting took place on a rotational basis at three sites, nested within four locations. At each of these 12 sites, 5 treatments were undertaken to span a spectrum of harvesting intensities (F = 0, 0.3, 0.6, 0.9 and a once-off total removal). A dynamic biomass-based fisheries model was developed to predict changes in exploited populations over time. Monthly maximum sustainable yield (MSY) estimates peaked at 1560 kg per 100 m of shore in March–April and September–October, but dropped by two orders of magnitude to 15 kg per 100 m of shore during the remainder of the year. The two peaks in MSY corresponded to the peak spawning periods of M. galloprovincialis along the South African west coast. Consequently, harvesting will only be viable if focused within two seasons spanning the peaks in MSY. Under these conditions, a range of harvesting intensities between F = 0.1 and 0.3 would permit long-term biological sustainability of a fishery. If implemented, this would represent the first instance of a marine invasive species being utilised to achieve socio-economic goals in South Africa.     

Stock assessment of the blue jack mackerel,Trachurus picturatus,in the North‐eastern Atlantic

A total of 49,151 blue jack mackerel, Trachurus picturatus, (Bowdich) was collected in Madeira Island (North‐eastern Atlantic) between 2002 and 2016 to evaluate possible influence of fishing on landings and reproductive parameters. A decreasing trend in the length composition was observed over the study period and length at first maturity decreased by 2.78 cm TL. Maximum yield per recruit decreased from 2002 to 2016 but the corresponding fishing mortality was constant (F_max = 0.4/year). Considering the fishing mortality level in 2016, it is evident that the stock may be exploited beyond its sustainability limit. Amendments of the purse‐seine fishing regulations and implementation of measures to reduce fishing effort are suggested.     

Beyond Individual Transferrable Quotas: methodologies for integrating ecosystem impacts of fishing into fisheries catch rights

While there has been a growing concern for the adverse ecological impacts of fishing, progress on incorporating these into operational fisheries management has been slow. Many fisheries management organizations have addressed the problem of overharvesting and over‐capitalization first. In this domain, the question of access regulation has gained growing recognition as a key dimension of fisheries sustainability, leading to recommendation and progressive implementation of rights‐based systems, in particular Individual Transferrable Quotas (ITQs). While adjustments in fishing capacity resulting from the implementation of these systems may entail a reduction in some unwanted ecosystem impacts of fishing, it is also recognized that they will not be sufficient to achieve the ecological outcomes increasingly demanded by the global community. There is thus a need to examine the possibilities for a common management framework for dealing with both over‐capitalization of fisheries and adverse ecological effects of fishing. In this paper, we examine the feasibility of incorporating greater ecosystem goods and services into ITQ policy instruments initially designed with a narrow focus on commercial target species. We consider the advantages and limitations of alternative approaches in this respect and identify some of the practical issues associated with the different alternatives, in particular the underpinning knowledge requirements. We argue that given the need for increasingly streamlined management processes, further investigation into practical ways forward in this domain is crucial if management of fisheries is to achieve economic efficiency while fully encompassing the ecologically sustainable development objectives of ecosystem‐based fisheries management.     

How commercial fishing effort is managed

Wild capture fisheries produce 90 million tonnes of food each year and have the potential to provide sustainable livelihoods for nearly 40 million people around the world ( http://www.fao.org/3/a-i5555e.pdf ). After decades of overfishing since industrialization, many global fish stocks have recovered, a change brought about through effective management. We provide a synthetic overview of three approaches that managers use to sustain stocks: regulating catch and fishing mortality, regulating effort and regulating spatial access. Within each of these approaches, we describe common restrictions, how they alter incentives to change fishing behaviour, and the resultant ecological, economic and community‐level outcomes. For each approach, we present prominent case‐studies that illustrate behaviour and the corresponding performance. These case‐studies show that sustaining target stocks requires a hard limit on fishing mortality under most conditions, but that additional measures are required to generate economic benefits. Different systems for allocation allow stakeholder communities to strike a locally acceptable balance between profitability and employment.     

Catching industrial fishing incursions into inshore waters of Africa from space

Small‐scale fisheries contribute substantially to the sustainability of coastal communities by providing livelihood and economic opportunities and ensuring food security. However, their geographic range of operation overlaps with that of industrial fisheries, increasing the resource competition, risk of vessel collision and inter‐sector conflicts, while jeopardizing the sustainability of fish stocks. When industrial vessels venture into waters that are reserved to artisanal fisheries, their operations become illegal. In Africa, the extent of such operations, beyond their legal implications, has resulted in severe economic, food security and maritime safety issues. In this paper, we use automatic identification system data derived from satellite technology to predict fishing operations and find that industrial fleets spend 3%–6% of their time fishing within inshore areas reserved for small‐scale fisheries between 2012 and 2016, of the total 4.2 million industrial fishing hours within the Exclusive Economic Zones of African countries. We assessed the total fishing effort by this form of illegal fishing operations at 166 million kWhours at least out of 4.9 billion kWhours in total. We discuss this dangerous form of illegal fishing, which often results in deadly collisions with small‐scale sector operators, increases competition and conflicts over fisheries access, threatens the sustainability of fish stocks, and calls for better governance, and protection.     

Behavioural diversity in fishing—Towards a next generation of fishery models

Despite improved knowledge and stricter regulations, numerous fish stocks remain overharvested. Previous research has shown that fisheries management may fail when the models and assessments used to inform management are based on unrealistic assumptions regarding fishers' decision‐making and responses to policies. Improving the understanding of fisher behaviour requires addressing its diversity and complexity through the integration of social science knowledge into modelling. In our paper, we review and synthesize state‐of‐the‐art research on both social science's understanding of fisher behaviour and the representation of fisher decision‐making in scientific models. We then develop and experiment with an agent‐based social–ecological fisheries model that formalizes three different fishing styles. Thereby we reflect on the implications of our incorporation of behavioural diversity and contrast it with the predominant assumption in fishery models: fishing practices being driven by rational profit maximizing. We envision a next generation of fisheries models and management that account for social scientific knowledge on individual and collective human behaviours. Through our agent‐based model, we demonstrate how such an integration is possible and propose a scientific approach for reducing uncertainty based on human behavioural diversity in fisheries. This study serves to lay the foundations for a next generation of social–ecological fishery models that account for human behavioural diversity and social and ecological complexity that are relevant for a realistic assessment and management of fishery sustainability problems.     

肖四海湖五种渔具的鳜渔获结构特征及其对鳜资源的影响

为评估不同渔具对鳜资源的影响,于2007年5月和12月对长江中游浅水湖泊肖四海湖刺网、延绳钓、网簖、电拖网和电捕仪5种渔具捕获的鳜渔获物结构特征进行了调查分析.结果发现,共采集鳜样本359尾,全长分布范围为92 ～600 mm,优势全长集中于251～350 mm;体质量分布范围为10～3 380 g,优势体质量集中于300 ～500 g.种群由5个年龄组构成,2～3龄为优势龄组,占总数的74.6％.刺网、延绳钓和网簖对鳜有较强的捕捞选择性,网目大小为80 mm和100 mm刺网的鳜渔获物中2龄及以上成熟个体占总数的93.3％,个体平均体质量466 g,“标鳜”(0.4 ～0.75 kg)个体占总数量的57.0％;延绳钓捕获的鳜渔获物中2龄及以上成熟个体占总数的86.9％,“标鳜”个体占总数量的43.5％;网簖捕获的鳜渔获物中90％以上为1龄的未成熟个体.电拖网和电捕仪捕获的鳜全长范围明显较大,其渔获物以1龄和2龄个体为主.综合分析表明,刺网适于作为鳜捕捞的主要渔具,延绳钓可以作为一种鳜捕捞的辅助渔具.网簖对鳜补充群体有较大危害,不适于作为鳜的捕捞网具.电拖网和电捕仪均属于违法渔具,对鳜资源危害巨大,应该加大监管力度,严禁使用.