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.     

山东近海口虾蛄单位补充量渔获量评估

为完善口虾蛄的基础生物学资料,并为口虾蛄资源的管理提供科学指导和理论依据,本研究根据2016至2017年山东近海渔业资源底拖网调查获得的口虾蛄体长、体质量数据,估算了口虾蛄的生长、死亡参数,构建了基于体长结构的单位补充量渔获量(YPR)模型,研究口虾蛄的资源动态和管理策略。采集调查口虾蛄样品共5028尾,体长—体质量关系的表达式为W=0.0145L2.88,为负异速生长;使用ELEFAN方法估算出口虾蛄的渐进体长L∞为19.87 cm,生长速率K为0.62 a−1。口虾蛄的生长表现出明显的季节性变化规律,生长参数的季节振幅C为0.76,10月份生长最快,4月份生长最慢。通过体长转换的渔获曲线估算出口虾蛄的总死亡系数Z为3.24 a−1,根据不同方法估算自然死亡系数M的范围为0.75~1.27 a−1,捕捞死亡系数F的估算范围为1.96~2.49 a−1,开发率的均值为0.67。YPR模型结果显示,随着F增大,YPR值呈现先上升后下降的趋势,生物学参考点F0.1和Fmax的值分别为0.92a−1和1.88a−1。口虾蛄资源处于过度开发的状态,应降低捕捞压力,同时调整开捕体长,以维持口虾蛄渔业资源量和渔获量。     

Catch length analysis, relation to minimum landing sizes and management implications from a Mediterranean small-scale fishery (Patraikos Gulf, Greece)

Fisheries management in the Mediterranean is planned on the basis of fishing effort allocation, gear technical characteristics and minimum landing sizes (MLSs), the latter being a measure recently updated in the EU Mediterranean fisheries policy. Very few elements on the evaluation of the MLS in regard to the catch and landings of the small-scale fisheries in the Mediterranean exist and this is due to both the great heterogeneity of the sector and the lack of systematic monitoring. The catch length composition of five species (Merluccius merluccius, Diplodus annularis, Mullus barbatus, Sepia officinalis and Squilla mantis) from a yearly record of small-scale fishing operations in the Patraikos Gulf (w. Greece, e. Mediterranean) was associated with temporal (season), spatial (depth of fishing, substrate type) and fishery (métier) characteristics by means of general linear models. Smaller individuals appeared in the catch in autumn for all five species indicating this season as suitable if a seasonal fishery closure was established. When length was found to be significantly different between different métiers this was either due to smaller mesh sizes (M. barbatus) or due to gear type (M. merluccius). For all species except S. officinalis, depth significantly affected length of individuals in the catch providing in the case of M. merluccius a clear association of bigger individuals with deeper fishing grounds. Substrate types also affected the length for the species D. annularis, M. barbatus. In relation to MLSs, the sizes of Mullus surmuletus and Scomber japonicus were fully legal, M. merluccius, M. barbatus, Solea vulgaris, Trachurus mediterraneus, Pagellus erythrinus had higher than 80% of the catch over MLS, while D. annularis, Diplodus sargus, Sparus aurata and Pagellus acarne had high fractions of the total catch below MLS. The métiers where undersized individuals appear were identified. Further management implications of the findings are discussed taking into account the current management scheme and the discarding practices in the area.     

基于单位补充量模型的西江赤眼鳟种群资源利用现状评价

赤眼鳟(Squaliobarbus curriculus)是珠江中下游最重要的经济鱼类之一,本研究利用2009—2014年西江肇庆江段渔业捕捞调查监测数据,分析了其生长和死亡参数历史变化,利用单位补充量渔获量(yieldperrecruitment,YPR)模型、单位补充量产卵群体生物量(spawning biomass per recruitment, SBR)和生物学参考点评估了赤眼鳟资源利用状况。结果表明,赤眼鳟体长(L)和体重(W)关系为W=2×10^-5L^2.9527(R^2=0.9595,n=2346),生长方程为L_t=725.802[1-e^{-0.110(t+0.613})]。目前西江的捕捞强度(F=0.96/a)和开发状况(E=0.86)远超种群可持续开发水平,赤眼鳟种群处于生长型捕捞过度状态。根据实际情况,建议将西江赤眼鳟开捕年龄提高至3龄(或开捕体长增大至238 mm),则预计单位补充量渔获量可增加175%,在珠江禁渔期制度的协同保护下,种群实际保护效果可能更好。     

Evaluation of the advantage of marine protected areas over the reduction of the fishing mortality coefficient

ABSTRACT:   In the present study, we compared the advantage of marine protected areas (MPA) to the reduction of the fishing mortality coefficient ( F ). We accomplished this by comparing the yield per recruitment (YPR) and spawner biomass per recruitment (SPR) under two controls. This was accomplished using a mathematical model. We used the following five measures as comparisons: (i) the condition of the fishery where the increase in YPR is possible; (ii) the increase in YPR or SPR obtained by allocating half the fishing ground as an MPA and by reducing F by half; (iii) the maximum YPR obtainable when the fishing mortality coefficient or the proportion of MPA in the fishing ground is completely controlled; (iv) the proportion of the MPA and the reduction ratio of F required for attaining 30% SPR; and (v) the YPR obtainable while maintaining 30% SPR. Our results show that the MPA has a minor disadvantage in terms of the first measure. The MPA is advantageous in increasing SPR in the second measure, with a low migration rate of fish. The MPA is also advantageous in increasing YPR in the second measure, with some intermediate migration. The MPA is disadvantageous in the third measure. The MPA is advantageous in the fourth measure, with a low migration rate. The MPA is disadvantageous in the fifth measure.     

Saving large fish through harvest slots outperforms the classical minimum‐length limit when the aim is to achieve multiple harvest and catch‐related fisheries objectives

We address the problem of optimal size‐selective exploitation in an age‐structured fish population model by systematically examining how density and size dependency in growth, mortality and fecundity affect optimal harvesting patterns when judged against a set of fisheries objectives. The study offers five key insights. First, while minimum‐length limits often maximize the biomass yield, exploitation using harvest slots (i.e. regulations that protect both immature and very large individuals) can generate within 95% of maximum yield; harvest slots also generally maximize the number of fish that are harvested. Second, density dependence in growth and size‐dependent mortality predict more liberal optimal size limits than those derived under assumptions of no density and size dependence. Third, strong density dependence in growth maximizes the catch of trophy fish only when modest harvest is introduced; the same holds for numbers harvested, when the stock–recruitment function follows the Ricker type. Fourth, the inclusion of size‐dependent maternal effects on fecundity or egg viability has only limited effects on optimal size limits, unless the increase in fecundity with mass (“hyperallometry”) is very large. However, large hyperallometry in fecundity shifts the optimal size limit for biomass yield from the traditional minimum‐length limit to a harvest slot. Fifth, harvest slots generally provide the best compromises among multiple objectives. We conclude that harvest slots, or more generally dome‐shaped selectivity to harvest, can outperform the standard minimum‐length selectivity. The exact configuration of optimal size limits crucially depends on objectives, local fishing pressure, the stock–recruitment function, and the density and size dependency of growth, mortality and fecundity.     

Dynamic angling effort influences the value of minimum‐length limits to prevent recruitment overfishing

Recruitment overfishing occurs when stocks are fished to a level where recruitment declines proportionally with adult abundance. Although typically considered a commercial fishery problem, recruitment overfishing can also occur in freshwater recreational fisheries. This study developed an age‐structured model to determine if minimum‐length limits can prevent recruitment overfishing in black crappie, Pomoxis nigromaculatus (LeSueur), and walleye, Sander vitreus (Mitchill) fisheries considering angling effort response to changes in fish abundance. Simulations showed that minimum‐length limits prevented recruitment overfishing of black crappie and walleye, but larger minimum‐length limits were required if angler effort showed only weak responses to changes in fish abundance. Low angler‐effort responsiveness caused fishing mortality rates to remain high when stock abundance declined. By contrast, at high effort responsiveness, anglers left the fishery in response to stock declines and allowed stocks to recover. Angler effort for black crappie and walleye fisheries suggested that angler effort could be highly responsive for some fisheries and relatively stable for others, thereby increasing the risk of recruitment overfishing in real fisheries. Recruitment overfishing should be considered seriously in freshwater recreational fisheries, and more studies are needed to evaluate the responsiveness of angler effort to changes in fish abundance.     

Complexity of applying minimum legal sizes (MLS) of retention in an indigenous coral reef fishery

Abstract Retaining coral reef fish for subsistence during commercial fishing is a common practice for indigenous fishers in the Torres Strait, Australia, despite being inconsistent with legislation. Fisher access point surveys were completed between 2004 and 2006 on three islands in Torres Strait to characterise this subsistence practice and assess the level to which it undermines current minimum legal fish sizes. Approximately 15% of the annual total catch was retained for subsistence during commercial fishing. Notably, subsistence catch of the most commercially valuable species almost entirely comprised individuals smaller than the minimum legal sizes. The higher proportions of undersized individuals of valuable species retained during commercial fishing on some of the islands were most likely associated with an increase in professionalism of the fishers. These results demonstrate how the intended outcomes from a management strategy can be undermined when the specific operational conditions of the fishery are not considered. Successful implementation of management arrangements in indigenous communities ultimately depends on the sociocultural conditions of the communities and their understanding and adherence to the rules. A productive way forward for the management of this fishery is greater engagement of indigenous communities and managers in co‐management arrangements.     

The enhancement of abalone stocks: lessons from Japanese case studies

The dramatic declines in abalone Haliotis spp. fishery production have been documented all over the world. Release of hatchery‐reared juveniles into natural habitats has been considered as one measure to sustain and/or augment the current fishery production of abalone, as well as to restore collapsed abalone stocks. However, attempts at abalone release programmes have only been undertaken at experimental scales, except for Japan, where large‐scale stock enhancement programmes for abalone have been undertaken since late 1960s. To evaluate the potential of stock enhancement for abalone, we analysed the release surveys of 13 case studies in Japan in terms of the overall recapture rate (number of recaptures through a lifetime/number of juveniles released), yield per release (YPR, yields from released individuals), the economic efficiency of releases (ratio of income from recaptured abalone to release cost) for each release year, and the contribution of hatchery releases to total catches for each fishing year. The average estimates for overall recapture rates (0.014–0.238) and YPR (3.1–60.3 g/individual) varied between locations and release years. The economic efficiency was estimated at 0.4–6.2. The released abalone contributed 6.9–83.5% to total catches. Hatchery releases could augment total production at some locations, but the success of release programmes would be limited by the carrying capacity at release areas, because density‐dependent mortality occurred following releases in some cases. Throughout Japan, the annual catch of abalone has continuously declined from ～6500 t in 1970 to ～2000 t in the mid‐1990s, despite the increase in the number of hatchery releases. Based on the estimates for YPR, the magnitude of the abalone releases on a national scale has not been sufficiently large to sustain the total production of Japanese abalone, which has primarily fluctuated according to the abundances of wild populations. Our results suggest that releases should be targeted at local populations in regions where stock enhancement is predicted to have the greatest chance of success, and the magnitude of releases should be considered carefully and determined for each region by taking the local carrying capacity into account. We also address the future prospects of abalone stock enhancement.     

Optimising harvest strategies in a multi‐species bivalve fishery

Abstract Optimising harvests from fishery resources requires appropriate data. In this study, fishery‐independent survey and size‐at‐first maturity (L₅₀) data were used to assess the suitability of current minimum legal sizes (MLS) and total allowable commercial catches (TACC) in the South Australian, mixed‐species, mud‐cockle (Katelysia spp.) fishery. Estimates of L₅₀ suggested the MLS was conservative in one fishing zone (Coffin Bay), but appropriate elsewhere. Harvestable‐biomass estimates (HB) demonstrated that TACCs were excessive in the Port River (41% of HB), suitable in Coffin Bay (10% of HB) and precautionary in the West Coast (1.5% of HB) fishing zones. Consequently, the MLS was decreased by 5 mm shell length in Coffin Bay and the TACCs for the Port River (reduced by 80%) and West Coast (increased by 40%) amended. This study demonstrated that harvest strategies in mixed‐species fisheries can be optimised by explicitly considering data on species composition, abundance and population biology.     

浙江南部海域银姑鱼的生活史参数估算及资源评价

为了评估银姑鱼资源开发状态,实验根据2016年在浙江南部海域底拖网的调查数据,研究了银姑鱼的生活史参数,并基于单位补充量模型对其资源状态进行评价,进而探讨不同自然死亡系数和捕捞选择性对资源评价结果的影响。结果显示,银姑鱼渐近体长估计值为25.36 cm,生长速率为0.32/年,当前开捕体长(13.52 cm)远小于其初次性成熟体长(17.79 cm);自然死亡系数估计值为0.74,总死亡系数为2.62,当前捕捞死亡系数为1.88。基于以上参数,构建了单位补充量渔获量YPR模型和单位补充量亲体生物量SSBR模型,随着F的增加,YPR先增大后减小,而SSBR则减少。银姑鱼生物学参考点F_0.1为0.78,F_max为3.43,F_20%为0.66,F_40%为0.33,可知当前捕捞强度远大于防止补充型过度捕捞警戒线F_20%。敏感性分析结果显示,自然死亡系数的不确定性将明显影响单位补充量模型的研究结果和相关生物学参考点的估算值,而不同选择性系数,尤其是开捕体长,也直接影响单位补充量模型的结果。研究表明,当前浙江南部近海银姑鱼种群已处于补充型过度捕捞状态,为维持渔业资源的可持续发展,建议适当减小开发力度,增大开捕体长;为提高资源状态评价的准确性,建议减小自然死亡系数的不确定性。本研究可为银姑鱼资源的养护和管理提供科学建议。     

Global fishery dynamics are poorly predicted by classical models

Fisheries dynamics can be thought of as the reciprocal relationship between an exploited population and the fishers and/or managers determining the exploitation patterns. Sustainable production of protein of these coupled human‐natural systems requires an understanding of their dynamics. Here, we characterized the fishery dynamics for 173 fisheries from around the globe by applying general additive models to estimated fishing mortality and spawning biomass from the RAM Legacy Database. GAMs specified to mimic production models and more flexible GAMs were applied. We show observed dynamics do not always match assumptions made in management using “classical” fisheries models, and the suitability of these assumptions varies significantly according to large marine ecosystem, habitat, variability in recruitment, maximum weight of a species and minimum observed stock biomass. These results identify circumstances in which simple models may be useful for management. However, adding flexibility to classical models often did not substantially improve performance, which suggests in many cases considering only biomass and removals will not be sufficient to model fishery dynamics. Knowledge of the suitability of common assumptions in management should be used in selecting modelling frameworks, setting management targets, testing management strategies and developing tools to manage data‐limited fisheries. Effectively balancing expectations of future protein production from capture fisheries and risk of undesirable outcomes (e.g., “fisheries collapse”) depends on understanding how well we can expect to predict future dynamics of a fishery using current management paradigms.     

福建省近海渔业管理目标的比较

应用Schaefer和Fox剩余产量模式及其由此衍生的生物经济模式和Gulland最适产量Y0．1模式，分别估算了福建省近海海洋捕捞业的最大持续产量，最大持续捕捞力量，最适产量，最适捕捞力量，最大经济产量，最大经济捕捞力量，最佳经济效益，并对各模式计算的诸项经济指标进行比较，建立了渔业管理3种模式，并根据福建省近海渔业实际情况和渔业发展趋势 ，提出实现3种管理目标的实施步骤。     

Efficacy of harvest and minimum size limit regulations for controlling short‐term harvest in recreational fisheries

It is important to consider the potential effectiveness of regulations for reducing total harvest levels when developing fishery management plans. A random forest (RF) modelling approach was used to examine how changing per‐angler harvest or minimum size limit regulations affected sport fishery harvest in US Atlantic coast recreational fisheries. Harvest limits per angler (i.e. bag limits) were typically high initially and subsequently reduced, whereas almost half of minimum length limits were initially below the length‐at‐maturity and subsequently increased. Across most fisheries examined, extreme reductions in harvest limits (e.g. from unlimited to catch‐and‐release) were largely ineffective at limiting total fishery harvest. Increasingly restrictive minimum length limits caused a greater average harvest reduction than per‐angler harvest limits. Some regulation changes were associated with higher angling effort and thus increased harvest, which suggests that when effort cannot be constrained, more direct harvest limitations should be considered.     

Mortality of Diplodus annularis and Lithognathus mormyrus released by recreational anglers: implications for recreational fisheries management

Abstract  The factors affecting mortality in Diplodus annularis (L.) and Lithognathus mormyrus (L.) caught and released in a recreational fishery in the Balearic Islands (NW Mediterranean) were examined. A total of 194 and 214 specimens of each species, respectively, were caught and individually monitored in containment tanks for 10 days. The circumstances of the catch, such as capture depth, water temperature, fish size, hook type, hook location, bleeding, unhooking time and cutting the hook line were tested with a logistic regression model. Diplodus annularis experienced moderate rates of mortality (15%), and only deep-hooking was a significant predictor of death. The incidence of undersized fish for this species was 48%. By contrast, L. mormyrus exhibited higher mortality rates (33%) with over 90% of the catch under the minimum legal size. Deep-hooking was the strongest predictor of mortality. When circle hooks were used, or if the line was cut when the fish were deep-hooked, mortality was considerably reduced. Strategies, such as promoting the use of more selective gears that reduce the capture of undersized fish and implementing gentler release techniques, should be considered in managing these species.     

Analysing extensive fish culture systems by transparent population modelling: bighead carp, Aristichthys nobilis (Richardson 1845), culture in a Chinese reservoir

A transparent population modelling approach is used to analyse a large-scale extensive fish culture system, the bighead carp, Aristichthys nobilis (Richardson 1845), fishery in a Chinese reservoir. The population model incorporates explicit submodels for density-dependent growth and size-dependent mortality, and allows the assessment of stocking density, seed fish size, fishing mortality (fishing effort), and gear selectivity. The process of model building and parameter estimation from stocking and catch data is described in detail. The full analysis is carried out in computer spreadsheets where all model components are visible and can easily be modified to take account of specific conditions, or to explore different assumptions. The practical use of the model in management decision making is discussed, together with data requirements and the possible need for experimental management.     

闽台近海渔业管理目标的研究

本文应用Ｓｃｈａｅｆｅｒ和Ｆｏｘ两种剩余产量模式,及其由此衍生的两种生物经济模式,分别估算了闽台近海最大持续产量,最适捕捞力量和最大经济产量,最适经济捕捞 力量,最佳经济效益等指标,建立了闽台渔烽管理模式,比较了以最大渔获量和以最佳经济效益为目标的各项经济指标,讨论了并确定了近期以最大持续产量为目标的渔业管理方案。     

Weight-based yield per recruitment and spawning-biomass per recruitment analyses of pacific cod <Emphasis Type="Italic">Gadus macrocephalus</Emphasis> off the Pacific coast of southern Hokkaido,Japan

ABSTRACT:   The present study assessed the stock state of Pacific cod Gadus macrocephalus caught off the coast of southern Hokkaido, Japan. Weight-based yield per recruitment (YPR) and spawning-biomass per recruitment (SPR) analyses were used for this assessment. The current fishing mortality (average from 1998 to 2000) was 0.65 and weight at first capture was 0.5 kg bodyweight. Under these fishing pressures, the YPR of Pacific cod in southern Hokkaido was 1.06 kg/recruitment and percentage of SPR (%SPR) was 6.9%. The %SPR was lower than the critical limit at 20%SPR. The main reason that values of both YPR and %SPR were not optimum, would be that the weight at first capture was too small. Raising the weight at first capture was thought to be a better strategy from the biological viewpoint, and reducing fishing mortality to 0.3 would be the next alternative strategy from the fisheries management viewpoint.     

Management of the Peruvian bay scallop (Argopecten purpuratus) metapopulation with regard to environmental change

1. This paper sets out to: (1) review previous ecological studies and analyse recent trends of the Peruvian bay scallop fishery in order to better understand and to model the species’ temporal and spatial (meta) population dynamics along the South Pacific coast; (2) develop a fisheries model to protect the stock from overexploitation and optimize the annual yield of the pulse fishery in Independence Bay, the centre of the scallop diving fishery in Peru. 2. Natural stock fluctuations are very pronounced in this species and are positively correlated with the El Niño–Southern Oscillation (ENSO). During such an event, habitat conditions for the Peruvian bay scallop are improved either regionally or locally, such that populations proliferate and larval production and dispersal are greatly increased. Extinct beds and new habitats are recolonized during these periods (producing a strong pulse of metapopulation biomass), although most become extinct very shortly thereafter. 3. For management purposes, two considerations are fundamental: (1) heavy overfishing or extinction of the main scallop source populations would endanger the metapopulation as a whole; (2) rates of growth and survival greatly increase (and with them the potential yield of the scallop stock) locally over an El Niño cycle in a way that can be roughly estimated from past experience, including the most recent El Niño event (1997–1998). 4. We suggest a fisheries management regime capable of adapting to natural changes and propose a procedure for calculating both the optimal yield and the respective fishing effort under normal upwelling and El Niño scenarios, considering changes in the size at first capture (Lc) and fishery mortality (F). Copyright © 2000 John Wiley & Sons, Ltd.     

Fish life history,angler behaviour and optimal management of recreational fisheries

To predict recreational‐fishing impacts on freshwater fish species, it is important to understand the interplay between fish populations, anglers and management actions. We use an integrated bioeconomic model to study the importance of fish life‐history type (LHT) for determining (i) vulnerability to over‐exploitation by diverse angler types (generic, consumptive and trophy anglers), who respond dynamically to fishing‐quality changes; (ii) regulations [i.e., minimum‐size limits (MSLs) and licence densities] that maximize the social welfare of angler populations; and (iii) biological and social conditions resulting under such socially optimal regulations. We examine five prototypical freshwater species: European perch (Perca fluviatilis), brown trout (Salmo trutta), pikeperch (Sander lucioperca), pike (Esox lucius) and bull trout (Salvelinus confluentus). We find that LHT is important for determining the vulnerability of fish populations to overfishing, with pike, pikeperch, and bull trout being more vulnerable than perch and brown trout. Angler type influences the magnitude of fishing impacts, because of differences in fishing practices and angler‐type‐specific effects of LHT on angling effort. Our results indicate that angler types are systematically attracted to particular LHTs. Socially optimal minimum‐size limits generally increase with LHT vulnerability, whereas optimal licence densities are similar across LHTs. Yet, both regulations vary among angler types. Despite this variation, we find that biological sustainability occurs under socially optimal regulations, with one exception. Our results highlight the importance of jointly considering fish diversity, angler diversity and regulations when predicting sustainable management strategies for recreational fisheries. Failure to do so could result in socially suboptimal management and/or fishery collapse.