EFFECT OF RATION AND SIZE HETEROGENEITY ON HARVEST TIME: TILAPIA CULTURE IN YUCATAN,MEXICO

Size heterogeneity is a common phenomenon in aquaculture systems and influences final production and economic yield. Among other factors, this variability is determined by ration. These variables directly affect potential returns and therefore influence the recommendations made for optimum aquaculture system management. As part of the search for more profitable culture strategies, a bioeconomic model was developed to analyze the effect of size heterogeneity and ration on optimum harvest time and size, thus creating a new methodological tool. Size dispersion was included using a continuous, size-structured population model incorporating the effect of ration on growth. Theoretical results were applied in a case study of tilapia culture in Yucatan State, Mexico, in which optimum ration levels and harvest times clearly differed between size heterogeneity and homogeneity models. Case study results indicated the use of different recommended ration and harvest management strategies depending on real and/or potential target market.     

Comparing size-limit strategies for exploitation of a self-thinned stream fish population

There is increasing evidence that territorial stream fish populations exhibit some degree of self-thinning. Four size-limit strategies were examined, under which a size-structured model population exhibiting self-thinning was exploited. The effects of: (1) increased minimum-size limits; (2) protection of spawners; (3) decreasing maximum-size limits; and (4) slot limits (prescribe lower and upper size limits of fish that must be released) were analysed in terms of population size and mean body size in the population after harvest. Biomass and numbers harvested, mean size of fish taken and proportions of different sizes in the population after harvesting were also analysed. Combinations of high exploitation rates and high minimum-size limits maximized both the number and biomass harvested while it favoured post-harvest abundance and the proportion of larger sizes in the population. When harvest rates and minimum-sizes were increased, the combinations of these that maintained or increased yield were successively narrowed. Protection of spawners and slot limits did not come close to reaching the levels of post-harvest abundance, yield, positive size structure and endpoints of harvest rates that were obtained with a high minimum size applied to the fishery. Maximum-size limits favoured the abundance of smaller size-classes. The results emphasize the advantages of setting the largest sizes of fish in the population as a minimum size that can legally be retained.     

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.     

Stock assessment and management for walleye pollock in Japan

We review the stock assessment strategies and management procedures for walleye pollock Theragra chalcogramma in Japan. In Japan, walleye pollock is classified into 4 stocks. Because biological data, fishing conditions, etc. are different for each stock, the stocks are assessed by different methods. Harvest strategies aiming at stock recovery are proposed for the Northern Japan Sea stock and the Nemuro Strait stock, which are currently in poor condition. For the Japanese Pacific stock and the Southern Okhotsk Sea stock, which are in good condition, harvest strategies for current fishery operations are proposed. In Japan, fisheries co-management has traditionally been carried out, and in recent years a total catch limitation system called the total allowable catch, a resource recovery plan, and a resource management plan have also been implemented. Although a plan is devised that accounts for the stock conditions of walleye pollock, it is also necessary to consider socioeconomic factors, ecosystem factors, and so on. However, we consider that the main focus of stock management for walleye pollock will still be maintaining fishing pressure at an appropriate level, which includes regulating fish size and price during the fishing season.     

A delay-differential model for representing small pelagic fish stock dynamics and its application for assessing alternative management strategies under environmental uncertainty

We present a novel adaptation of the classic discrete delay-difference model, a continuous delay-differential model (cDDM), which can adequately represent population dynamics of stocks that turn over rapidly and continuously over time (e.g., small pelagic fish, small tunas, and shrimps). We used the Northern-Central Peruvian anchoveta stock (Engraulis ringens, Engraulidae) as a case study for implementing the cDDM and conducted a management strategy evaluation (MSE) through stochastic optimization in policy space (SOPS). Our results showed that cDDM integrated with SOPS efficiently searches optimum and near-optimum harvest control rules (HCR) and is an alternative to pre-setting arbitrary HCRs as in traditional MSE. The cDDM showed comparable stock biomass and recruitment estimate reconstructions to more complex stock assessment models described for anchoveta. We concluded that the anchoveta stock is sustainably managed and is an example of adaptive fisheries management under high ocean-climate variability and uncertainty. Contrary to fishery textbooks, the anchoveta's collapse was not entirely due to the 1972 El Niño (EN) but a recruitment failure preceding EN. Our reconstructions revealed that low recruitment (or recruitment failure) could still occur at high stock biomass. Anchoveta's stock biomass is larger than pre-collapse, likely due to favourable environmental conditions (a cooling trend) and management, despite more frequent and stronger EN events. SOPS quickly revealed that harvest strategies with large base biomass (>5 mmt) lead to higher interannual stock variability and would not produce substantial increases in long-term yield. Alternative HCRs with lower base biomass, while adjusting for productivity regimes, have similar long-term yields without affecting the long-term average stock.     

Optimal harvesting of farmed Atlantic salmon at two cohort management strategies and different harvest operation restrictions

Abstract

The main harvest planning problem in commercial fish farms is to determine the best time‐sequence of harvesting different fish cohorts in order to maximize the overall farm profit. Due to both annual and fish‐size variations in market prices, future forecasts of fish growth and size distribution are required to optimize harvest plans. Two management strategies for harvesting size‐structured fish cohorts are considered. The first strategy allows the fish farmer, at any time, to size‐grade, harvest and sell the most profitable fish sizes from the standing stock. The second strategy allows the fish farmer to harvest and sell a fish batch with similar size distribution as that of the standing stock. In this paper, two size‐structured fish growth models have been built to fit the two management strategies. The growth models are integrated in a multiperiod linear programming model that optimizes the harvest outputs for each of the two management strategies. Model outputs demonstrate clearly that it is more profitable to size‐grade fish prior to harvest compared to harvesting a batch of fish with similar size distribution to that of the standing stock. Five different harvest operations constraints have been identified for commercial salmon farming. The decrease in profitability of fish farming is shown for a variety of harvest operation constraints.     

A scientific alternative to moratoria for rebuilding depleted international tuna stocks

There is considerable international concern and scientific debate about the current state and future of tuna stocks worldwide and the capacity of Regional Fisheries Management Organisations to manage the associated fisheries effectively. In some cases, this concern has extended to predictions of imminent collapse with minimal chances of recovery, even under a commercial catch moratorium. As a viable alternative to a full fishery closure, the Commission for the Conservation of Southern Bluefin Tuna (CCSBT) has adopted a scientifically tested, adaptive rebuilding strategy for the depleted southern bluefin tuna (Thunnus maccoyii) stock. The management procedure (MP) adopted involves a harvest control rule that fully specifies the total allowable catch as a function of key indicators of stock status, adjusting future harvest levels every three years so as to meet the rebuilding targets agreed by CCSBT. It was chosen from a subset of candidate MPs selected following extensive simulation testing. This involved first selecting a wide range of plausible scenarios for stock status and input data, ranging from pessimistic to optimistic, against which the alternative candidate MPs were tested to ensure that they were robust to important uncertainties. This is the first time that a comprehensively evaluated MP has been adopted for an internationally managed tuna stock. Both the process and the outcomes have broad applicability to other internationally managed stocks.     

FISHY FISH? THE ECONOMIC IMPACTS OF ESCAPED FARMED FISH

The escape of cultured fish from a marine aquaculture facility is a type of biological invasion that may lead to a variety of potential ecological and economic effects on native fish. This article develops a general invasive species impact model to capture explicitly both the ecological and economic effects of invasive species, especially escaped farmed fish, on native populations and harvests. First, the possible effects of escaped farmed fish on the growth and stock size of a native fish are examined. Next, a bioeconomic model to analyze changes in yield, benefit distribution, and overall profitability is constructed. Different harvesting scenarios, such as commercial, recreational, and joint commercial and recreational fishing are explored. The model is illustrated by a case study of the interaction between native and farmed Atlantic salmon in Norway. The results suggest that both the harvest and profitability of a native fish stock may decline after an invasion, but the total profits from the harvest of both native and farmed stocks may increase or decrease, depending on the strength of the ecological and economic parameters.     

Temperature dependent growth and yield of pikeperch, Sander lucioperca, in Finnish lakes

Abstract  A non-linear growth model was used to evaluate the effects of temperature and age on annual length increments of pikeperch, Sander lucioperca (L.), in seven lakes in Finland. Length increments were derived by back-calculation using the Fraser–Lee method. Annual length increments increased from age 1 to age 3 and then decreased, while at the same time length increments and air temperature had positive correlation until age 12. Age- and size-structured yield per recruit models were used in two lakes to evaluate the effects of temperature and gillnet mesh size on pikeperch yield. In these two lakes maximum yield could be obtained with 60 and 70 mm (bar length) gill nets. In the second lake, as typically in Finland, 45–50 mm gill nets are the most frequently used. The use of larger mesh size gill nets would increase pikeperch yield from 685 to 1000 g per recruit based on the present mean temperature. In both lakes increase in temperatures would increase yield if mesh size is simultaneously increased. Higher pikeperch yield can be expected in the future because of climate warming.     

人工增殖放流技术的探讨

渔业资源增殖放流是指对野生鱼、虾、蟹和贝类等进行人工繁殖、养殖或捕捞天然苗种在人工条件下培育后,释放到渔业资源出现衰退的天然水域中,使其自然种群得以恢复的一种方法。增殖放流的目的在于保护濒危物种、增加渔业资源产量和修复生态环境,还具有培养渔民环保意识和增加渔民收入等多方面的重要意义。本文从苗种来源、标记方法、放流策略和效果评价四个方面总结了国内外增殖放流的技术,以期为今后的放流工作提供借鉴和参考。     

Proposed harvest control rule for managing the snow crab stock in the southwestern Gulf of St. Lawrence,Canada

We developed limit and target harvest control rules based on an age-, sex- and stage-structured model for managing the southwestern Gulf of St. Lawrence snow crab (Chionoecetes opilio) stock. We determined an F_x% (F resulting in a spawning biomass-per-recruit equivalent to x% of virgin spawning biomass-per-recruit) as a proxy for F_MSY and a minimum spawning stock biomass to open the fishery to incorporate them into the control rules. We evaluated the selected limit and target control rule parameters under stochastic simulations by considering various performance statistics. Because of the complexity in determining an effective female spawning biomass that involves a mating ratio, we choose the total mature male biomass (MMB) regardless of crab size as the spawning index to develop the stock–recruitment relationship for stochastic simulations. The MMB based F_37% appears a reasonable proxy for limit control rule while F_45% appears a reasonable target control rule. The corresponding limit and target harvest rates of legal size crab when the standing stock MMB exceeds the proxy MSY level are approximately 36% and 25%, respectively. The difficulty in establishing an appropriate stock–recruitment relationship for this stock was recognized hence a precautionary F_45% target level was chosen. Scenarios under biomass independent random recruitment were also investigated and performances of F_37% and F_45% control rules under this hypothesis behaved similar to those observed under S–R model based simulations.     

A strategic planning model for fisheries development

This paper presents a dynamic decision support model to aid coastal nations in the long-range planning of fishery development. The bioeconomic framework presented here provides a simple means for analysing complex interactions between fishery resource management, domestic fleet development, and the role of foreign fleets. The model includes explicit dynamics of the fish stock and the domestic fleet, and allows for harvest rate and investment rate controls. Several realistic behavioral components are also incorporated: (i) multi-objective optimization, balancing the direct benefits of rent generation with the indirect benefits of domestic fleet development; (ii) socio-political constraints, which lead the coastal state to accord its domestic fleet absolute priority in harvesting within coastal waters; (iii) limitations on access to capital, which restrict growth of the domestic fleet to that which can be financed through fishery revenues; (iv) interactions between the coastal state and the foreign fleets, in which the former is able to set a royalty rate, applied per unit of foreign harvest, while the foreign fleet responds to any specified royalty rate on a profit-maximizing basis. Analysis of the model indicates that optimal fishery development policies may depend particularly on the relative profitability of the domestic fleet, as well as the extent to which an explicit value is placed on the employment and secondary benefits of domestic fleet development.     

Efficient and economical way of operating a recirculation aquaculture system in an aquaponics farm

ABSTRACT

In this article, optimal control methods based on a metabolite-constrained fish growth model are applied to the operation of fish production in an aquaponic system. The system is formulated for the twin objective of fish growth and plant fertilization to maximize the benefits by optimal and efficient use of resources from aquaculture. The state equations, basically mass balances, required by the optimization algorithms are given in the form of differential equations for the number of fish in the stock, their average weight as mediated through metabolism and appetite, the water recirculation and waste treatment, hydroponic nutrient requirements and their loss functions. Six parameters, that is, water temperature, flow rate, stock density, feed ration size per fish, energy consumption rate and the quality of food (percentage of digestible proteins) are used to control the system under dynamic conditions. The time to harvest is treated as a static decision variable that is repeatedly adjusted to find the profit-maximizing solution. By modeling the complex interactions between the economic and biological systems, it is possible to obtain the most efficient decisions with respect to diet composition, feeding rates, harvesting time and nutrient releases. Some sample numerical results using data from a tilapia-tomato farm are presented and discussed.     

Discriminating alternative stock–recruitment models and evaluating uncertainty in model structure

Fish stock–recruitment (S–R) assessment is one of the most essential keystones for fisheries management. Yet the analysis involves a variety of uncertainties. Amidst these difficulties, uncertainty in model structure is perhaps the most problematical to investigate because no rigorous statistical techniques can be used to explore the fundamental biological processes in S–R relationships. In this paper, I used computer simulations to investigate: (1) the differences between the estimated parameters of alternative S–R models as a function of stock characteristics: population growth rate, data range, fishing mortality, and process noise; and (2) the probability of selecting a correct model using information criteria. Two popular S–R functions, the Ricker and the Beverton–Holt models, were used as examples. Time series data were generated from a known S–R model and fitted by alternative models. The results show that when the two models fit the data similarly well, significant differences in parameters existed between the alternative models. The Ricker model tended to underestimate the population growth rate (initial slope) and the carrying capacity parameter, whereas the Beverton–Holt model overestimated these parameters. The management quantities (e.g., optimal virgin stock size) produced by one model were more conservative (i.e., larger optimal stock size or lower optimal harvest rate) under some conditions but became less conservative under other conditions. The differences between the alternative models were functions of the population growth rate, long-term fishing mortality, and data range of the stock size. The correct and incorrect models were statistically indistinguishable. For typical fishery data the probability of selecting the correct model based on information criteria was approximately 0.70 for the Ricker model and 0.61 for the Beverton–Holt model.     

The consequences of density-dependent individual growth for sustainable harvesting and management of fish stocks

Density dependence is likely to act as a regulatory mechanism in fish stocks that are recovering from overfishing. In general, density dependence in fish stocks is assumed to only occur in reproduction and early life stages and is therefore usually modelled as a stock-recruitment relationship. Recent research shows that density dependence can also reduce individual growth in body size later in life. In this study, we show how optimal fishing effort changes with the strength of density dependence in individual growth for four stocks of North Sea flatfish species. Using size-structured population models we show that density dependence arises due to a mechanistic link between the resource availability and life history processes at the individual level. We furthermore show that the stock response to harvesting is either driven by changes in individual reproduction when density dependence in individual growth is weak or by changes in individual growth rate when individual growth is strongly affected by density dependence. These two types or regimes are separated by a sudden shift in dynamics. It is therefore of great importance to account for density dependence in growth when managing fish stocks.     

海洋食物网拓扑学方法研究进展

基于生态系统的渔业管理(EBFM)对海洋食物网研究提出了新的理论和方法需求.作者针对食物网动态这一生态系统的核心问题,对其近期发展起来的研究领域——食物网拓扑学理论和方法进行综述.食物网拓扑学通过建立一系列拓扑学指标,定量描述食物网各个成员之间的联系紧密程度,以及各个成员对整个食物网的作用.评价渔业对食物网的影响或食物网对外界的响应,开发可持续的捕捞方案,需要建立并运用食物网的动态模型.鱼类生物的重要特点是其捕食关系在很大程度上由个体大小(体长、体重)决定.而体长结构的渔业种群动力学模型发展已经成熟.因此,建议将食物网拓扑学的动态建模、体长结构的渔业种群生物量模型、鱼类个体大小相关的捕捞过程研究结合起来,作为基于生态系统渔业管理的一个重要的、潜在解决方案.对此,下一步的研究重点是,选择食物网资料采集较为容易、渔业统计资料较为全面的海洋食物网,开展试点研究,从而逐步构建起完整的、具有实际应用能力的EBFM方法体系.     

Fisheries resource management of the daggertooth pike conger,Muraenesox cinereus,using existing limited datasets in the western Seto Inland Sea,Japan

The daggertooth pike conger, Muraenesox cinereus (Forsskål), has become an important fish resource in the western Seto Inland Sea, Japan, since the 1990s. However, introducing sustainable fisheries resource management for this species is difficult in this region because stock assessments have not been performed, and official fisheries statistics for this stock were discontinued after 2007. This study used existing limited data sets to compile the first report for fisheries resource management for this M. cinereus stock. Yield‐per‐recruit analyses showed that increasing fishing pressure above current levels would provide only a minimal increase in expected catch levels. Hence, the current harvest level is considered to represent the upper limit of fishing pressure. Age composition in a given year could potentially be used to forecast landing abundance for the following 2 years. This study provides a basis for establishing effective fisheries resource management strategies for M. cinereus.     

Straddling the line: cooperative and non-cooperative strategies for management of Bering Sea pollock

The eastern Bering Sea fishery for pollock, Theragra chalcogramma, yields a first wholesale value over $1 billion; it is the premier US fishery. While there is general agreement that this fishery is managed under principles that foster sustainability, the stock is not wholly contained within the US Exclusive Economic Zone. Management of straddling stocks can be highly contentious, particularly when, as is the case for pollock, the spatial distribution varies considerably. When the center of pollock abundance shifts to the northwest, an increased portion of the stock is exposed to harvest by vessels operating in the Russian Federation Exclusive Economic Zone. The lack of coordination in the management of this transboundary stock presents a risk that is not reflected in current management strategies. We use a multiple product/multiple market bioeconomic model to characterize optimal cooperative and non-cooperative harvest management strategies from the perspective of US and Russian pollock fisheries under environmentally induced changes in pollock abundance and the distribution of that abundance.     

Rethinking length‐based fisheries regulations: the value of protecting old and large fish with harvest slots

Managing fisheries using length‐based harvest regulations is common, but such policies often create trade‐offs among conservation (e.g. maintaining natural age‐structure or spawning stock biomass) and fishery objectives (e.g. maximizing yield or harvest numbers). By focusing harvest on the larger (older) fish, minimum‐length limits are thought to maximize biomass yield, but at the potential cost of severe age and size truncation at high fishing mortality. Harvest‐slot‐length limits (harvest slots) restrict harvest to intermediate lengths (ages), which may contribute to maintaining high harvest numbers and a more natural age‐structure. However, an evaluation of minimum‐length limits vs. harvest slots for jointly meeting fisheries and conservation objectives across a range of fish life‐history strategies is currently lacking. We present a general age‐ and size‐structured population model calibrated to several recreationally important fish species. Harvest slots and minimum‐length limits were both effective at compromising between yield, numbers harvested and catch of trophy fish while conserving reproductive biomass. However, harvest slots consistently produced greater numbers of fish harvested and greater catches of trophy fish while conserving reproductive biomass and a more natural population age‐structure. Additionally, harvest slots resulted in less waste in the presence of hooking mortality. Our results held across a range of exploitation rates, life‐history strategies and fisheries objectives. Overall, we found harvest slots to represent a valuable option to meet both conservation and recreational fisheries objectives. Given the ubiquitous benefits of harvest slots across all life histories modelled, rethinking the widespread use of minimum‐length limits is warranted.     

Coupling dynamic energy budget and population dynamic models to inform stock enhancement in fisheries management

Extensive applications of fishery stock enhancement worldwide bring up broad concerns about its negative effects, creating a pivotal need for science-based assessment and planning of enhancement strategies. However, the lack of mechanistic understanding of enhanced population dynamics, particularly the density-dependent processes, leads to compromise in model development and limits the capacity in predicting enhancement effects. Here, we developed an individual-based model based on dynamic energy budget theory and full life-history processes, to understand the mechanism of density dependence in population dynamics that emerge from individual-level processes. We demonstrated the utility of the model framework by applying it to an extensively enhanced species, Chinese prawn (Fenneropenaeus chinensis, Penaeidae). The model could yield projections reflecting the observed trajectory of population biomass and yields. The model also delineated the key effects of density dependence on the vital rates of growth, fecundity and starvation mortality. Regarding the manifold effects of stock enhancement, we demonstrated a dampened shape in population biomass and yields with increasing magnitude of enhancement, and trade-offs between the ecological and economic objectives, that is, pursuing high benefit might compromise the wild population without proper management. Furthermore, we illustrated the possibility of combining stock enhancement and harvest regulation in promoting population recovery while maintaining fisheries yields. We highlight the potential of the proposed model for understanding density dependence in enhancement programme, and for designing integrated management strategies. The approach developed herein may serve as a general approach to assess the population dynamics in stock enhancement and inform enhancement management.