Age-structured Bioeconomic Model Research Articles

Bioeconomic analysis of stock rebuilding strategies for the green abalone fishery in Mexico under climate uncertainty

Globally, overexploitation of fishery stocks is a complex problem aggravated by climate change's effects. The green abalone in Mexico is an overexploited fishery, resulting in stakeholders committing to its recovery. Part of a comprehensive co-management strategy was to implement a fishing closure in some fishing areas. Now they are exploring reopening the fishery and gradually increasing fishing efforts. This research evaluates compatible management strategies with stock rebuilding under climate uncertainty. An age-structured bioeconomic model for the green abalone fishery was parameterized, which considered the impact of SST anomalies on population dynamics (i.e., recruitment, mortality, and growth). Three management strategies were evaluated under future climate scenarios (RCP 4.5, RCP 8.5, and a scenario without climate change). Results indicate that an effort level that aims to generate Maximum Sustainable Yield (MSY) based on historical fishery data may lead to overexploitation under warming scenarios. Instead, applying the minimum acceptable effort for the fishing community can support biomass recovery and is a strategy more resilient to climate change. This finding highlights the risk of overexploitation by assuming stock productivity stability under favorable climatic conditions when designing rebuilding plans. A stock rebuilding strategy will be more likely to succeed if it is cautious, recognizes uncertainties, and is flexible in considering stock productivity changes.

Estimating economic-based target reference points for key species in multi-species multi-métier fisheries

Consideration of economic outcomes is commonplace in most fisheries management systems globally, although only a few jurisdictions have adopted an economic objective as the primary target for fisheries management. Such an objective has been adopted for Australia's federally managed fisheries, with maximum economic yield (MEY) identified as the primary management objective. Correspondingly, target reference points defined in terms of biomass (i.e., BMEY) are used in harvest control rules. In the absence of explicit BMEY estimates, proxy estimates based on maximum sustainable yield (i.e., BMSY) are used. Identifying BMEY in multi-species fisheries is complicated as most stock assessments are undertaken at the individual species level, but economic activity occurs across species. This is further complicated when different fishing activities using different fishing gears and targeting practices (i.e., métiers) are present in a fishery. We employ an age-structured bioeconomic model to estimate BMEY for key species in a multi-species, multi-métier fishery. We find that optimal biomass levels are substantially higher than those assumed under the current proxy-based system, and that the economic targets are sensitive to prices and fishing costs, both of which change over time.

Fisheries Reference Points under Varying Stock Productivity and Discounting: European Anchovy as a Case Study

European anchovy (Engraulis encrasicolus) is the main commercially exploited fish stock in the Black Sea region, providing a vital source of livelihood and revenue for local communities and national economies. In recent decades, the Black Sea anchovy stock has faced many human-induced threats, including overfishing, eutrophication, invasive species, and climate change while these threats have raised concerns about the status and long-term productivity of the stock. To ensure sustainable levels of exploitation under potential future changes in stock productivity, we here estimate and compare a suite of biological and economic reference points under different levels of stock productivity and discount rates using an age-structured bioeconomic model setup. Our model simulations showed that optimal fishing mortalities achieving maximum sustainable yield (FMSY) and maximum economic yield (FMEY) increase at higher stock productivity but are always lower than the historically high mean levels of exploitation. Furthermore, we illustrate that the stock biomass at maximum economic yield (BMEY) is larger than the stock biomass at maximum sustainable yield (BMSY) at all stock productivities and discount rates, except at low stock productivity under high levels of discounting (i.e., 10%, 20%). By illustrating the ecological and economic benefits of reducing exploitation rates, we expect that our estimated reference points can add value to the decision-making process for the management of the European anchovy fishery and ensure long-term sustainable management even under future climate-driven changes in stock productivity.

Negative shocks in an age-structured bioeconomic model and how to deal with them

We consider an age-structured bioeconomic model of a fishery where periodic negative shocks affect recruitment and somatic growth. The model is relevant for fish stocks that are subject to environmentally driven events such as diseases, match-mismatch relationships between plankton blooms and spawning, or recruitment failure. The nature of the optimal fishing strategy is pulse fishing. We vary the period and severity of the shocks to investigate how the optimal management scheme adapts and what factors trigger fishing pulses. Two principles largely govern fishing. The first is to harvest when the number of small cohorts in the stock is minimal. Small cohorts emerge from negative shocks to recruitment. The second principle is to fish when shocks to growth have the least impact on the weight distribution in the stock. Furthermore, we consider how net present value and total catches depend on the period of shocks and the average impact (impact times frequency). In the case of average impacts, we observe close to linear relationships that make reasonable generalizations of our results, for example to different impact levels, viable. The structure of predictable negative shocks in the model is a significant simplification but allows us to crystallize drivers of adaptations in the fishing strategy.

Bioeconomics of technological interdependencies in a sequential shrimp fishery: optimal size of the industrial fleet in southern Gulf of California

The bioeconomic sequential externalities in the white shrimp (Litopenaeus vannamei) fishery during 2014-2015 season in southern Sinaloa, Mexico, were analyzed by calculating the optimal size of the industrial fleet (trawlers) given the current oversized small-scale fleet (SSF: cayucos). For this purpose, we constructed a dynamic multi-fleet age-structured bioeconomic model with natural mortality-at-age and catchability-at-age. The distributed delay model was applied to simulate recruitment seasonality. The reproductive effect of seasonal sea surface temperature (SST) was also considered. The constant size of SSF was represented in status quo while reducing industrial fleet by -25%, -50% and -70%Maximizing net present value “NPV” of industrial fleet. The model showed the technological interdependencies of two fleets competing for one species and their respective externalities. The status quo fishery showed a deteriorated biomass (0.27, below MSY); overcapitalized fleet (especially for the SSF); fleets low NPV of resource rent, especially for the cayucos ($761 USD per season). The access limitation of trawlers presented a progressive improvement in all bioeconomic variables, mainly on biomass (+27%) and NPV resource rent for trawlers (+700%). The SSF presented positive marginal externalities (4%). However, biomass was still below MSY (0.34) and the fishery trade-off showed a 70% industrial fleet loss to overcome the negative externalities of the actual SSF status. Given the current conditions of the sequential shrimp fishery (i.e., overcapitalized, overexploited, difficulty to control SSF) the limitation of industrial effort to the area could be a viable strategy to improve the fishery bioeconomic performance. However, it is necessary to explore more management strategies that include the small-scale fishery to attained sustainable levels and higher profitability for users.

Incorporating economics into fishery policies: Developing integrated ecological-economics harvest control rules

Despite the efforts of management authorities, several fish stocks continue to be harvested at unsustainable levels. While integrating economics into catch advice has been identified as key in developing more effective management, few studies investigate how bioeconomic modelling can inform decision-making. At the same time, increasing evidence has been gathered on the occurrence of regime shifts in marine ecosystems, raising greater concerns regarding fish stocks' recovery ability. Policy evaluation methods, however, still resort to compensation population models. As a result, these may overestimate reproductive success, thus jeopardizing the design and evaluation of stock rebuilding policies. This paper addresses those challenges proposing a methodology that approximates a harvest control rule using an age-structured bioeconomic model. As a result, we deliver a policy rule that complies with current advice procedures and endogenously captures economics. Also, we consider concerns on the existence of regime shifts in marine populations and estimate a bioeconomic model with critical depensation. Results from our case study suggest that policies neglecting the existence of critical depensation may compromise stock rebuilding objectives and might even result in fishery collapses. Using bioeconomic modelling to define harvest control rules may enhance policy design aimed at reconciling fisheries' economic returns and stock recovery.

The Role of Food Web Interactions in Multispecies Fisheries Management: Bio-economic Analysis of Salmon, Herring and Grey Seal in the Northern Baltic Sea

Multispecies bio-economic models are useful tools to give insights into ecosystem thinking and ecosystem-based management. This paper developed an age-structured multispecies bio-economic model that includes the food web relations of the grey seal, salmon, and herring, along with salmon and herring fisheries in the Baltic Sea. The results show that the increasing seal population influences salmon fisheries and stock, but the impacts on the harvest are stronger than on the stock if the targeted management policies are obeyed. If seal population growth and a low herring stock occur simultaneously, the salmon harvest could face a serious threat. In addition, scenarios of the multispecies management approach in this paper reveal a benefit that our model can evaluate the performance of different fisheries with identical or different management strategies simultaneously. The results show the most profitable scenario is that both fisheries pursuit aggregated profits and reveal a trade-off between herring fisheries and salmon fisheries. Our model indicates that the herring harvest level and the approaches to managing herring fisheries can influence the performance of salmon fisheries. The study also demonstrates a way to develop a multispecies bio-economic model that includes both migratory fish and mammalian predators.

Age-structured bioeconomic model for strategic interaction: an application to pomfret stock in the Arabian/Persian Gulf

Abstract When fish stocks migrate across multiple exclusive economic zones (EEZs), they compel managers to examine management at both national and international levels. A strategic interaction emerges when the fishing activity of one country impacts fishing opportunities available for other countries sharing the stock. Left unaddressed, strategic interaction could lead to overexploitation and suboptimal payoffs. Here, we develop and apply a bioeconomic model to address the competitive fishing for silver pomfret in the Arabian/Persian Gulf—a highly commercial fish stock shared between Kuwait and Iran—and evaluate biological–economic trade-offs under competition, cooperation, and country-independent management using maximum sustainable yield (MSY) and fishing mortality that maintain MSY (Fmsy) policies. When cooperation involves an equal share of the overall Fmsy or a share based on the proportion of the stock available in each EEZ, countries are expected to cooperate given the substantially higher catch, biomass, and relative profits compared to other management regimes. However, other than these two arrangements, countries would favour different regimes. Besides providing policy insights to improve the perilous status of silver pomfret, our approach could be useful in exploring alternative fishing arrangements to sustainably harvest a transboundary fish stock while maximizing average yields.

Livestock and Carnivores: Economic and Ecological Interactions

Carnivores-livestock interactions cause human-wildlife conflicts worldwide. These interactions are present under a wide range of ecological and economic circumstances. This paper studies the relationship between predation mortality and natural mortality, when food availability affects natural mortality of the livestock. Semi-domestic Saami reindeer (Rangifer t. tarandus) herding in Norway is used as a case study. When predation affects reindeer density, food competition among reindeer changes, which changes weights and natural mortality in the reindeer population. An age-structured bio-economic model is presented, where this relationship is taken into account. While predation mortality may be additional to natural mortality in absence of food limitation, it can compensate for natural mortality in situations of food scarcity. Furthermore, due to density dependency in livestock weights, predation may increase the meat value of livestock. The paper analyzes how predation affects livestock production and economic performance under an optimized management scheme. One main result is that predation shifts the optimal harvesting composition towards calf harvesting and, therefore, the optimal stock composition among the different categories of animals. This contrasts findings in the existing bioeconomic literature. Furthermore, a changing harvesting pattern towards calf harvest is an important adjustment that highly limits the negative impact on profit of predation.

Optimization of Age-Structured Bioeconomic Model: Recruitment, Weight Gain and Environmental Effects

More and more fishery researchers begin to acknowledge that one-dimensional biomass models may omit key information when generating management guidelines. For the more complicated age-structured models, numerous parameters require a proper estimation or a reasonable assumption. In this paper, the effects of recruitment patterns and environmental impacts on the optimal exploitation of a fish population are investigated. Based on a discrete-time age-structured bioeconomic model of Northeast Atlantic mackerel, we introduce the mechanisms that generate 6 scenarios of the problem. Using the simplest scenario, optimizations are conducted under 8 different parameter combinations. Then, the problem is solved for each scenario and simulations are conducted with constant fishing mortalities. It is found that a higher environmental volatility leads to more net profits but with a lower probability of achieving the mean values. Any parameter combination that favours the older fish tends to lend itself to pulse fishing pattern. The simulations indicate that a constant fishing mortality around 0.06 performs the best. A comparison between the optimal and the historical harvest shows that for more than 70% of the time, the optimal exploitation precedes the historical one, leading to 43% higher net profit and 34% lower fishing cost.

Analytical and numerical investigation of optimal harvest with a continuously age-structured model

An age-structured bioeconomic model, which is completely continuous in age and time, is developed in order to compare with traditional discrete models. Both types have advantages and disadvantages. The continuous framework complements discrete models as it allows for deeper and more transparent analytical study and leads to analytical results that would be difficult to achieve within a discrete framework. To make the model realistic, a nonlinear recruitment function is introduced and steady state solutions and constant-effort optimal fishing are studied analytically. In addition, the framework has been used for numerical analysis. Simulations are used to investigate how optimal harvesting patterns vary with parameter values.

Preference for Landings’ Smoothing and Risk of Collapse in Optimal Fishery Policies: The Ibero-Atlantic Sardine Fishery

Several world fish stocks are being explored at unsustainable levels and require management plans to rebuild stock abundance. Defining a management plan is, however, a complex task that entails multidisciplinary work. In fact, while it requires solid scientific knowledge of fish stocks, the inclusion of economic and social objectives is crucial to a successful management implementation. In this paper we develop an age-structured bioeconomic model where the objective function is modified to accommodate preferences from different stakeholders. In particular, we consider important characteristics that a management plan should take into account: profit maximization, fishermen’s preference for reducing landings’ fluctuations and risk of fishery collapse. Modeling preferences for reducing landings’ fluctuations is accomplished by defining a utility function with aversion to intertemporal income fluctuations. Building upon biology literature, we model precautionary concerns by incorporating a probability of collapse that depends on current spawning biomass. We illustrate how this framework is able to assist in the analysis and design of harvest control rules applying it to the Ibero-Atlantic sardine stock.

Optimal Fishing Mortalities with Age-Structured Bioeconomic Model - A Case of NEA Mackerel

The effects of random environmental impacts on optimal exploitation of a fish population are investigated using both optimization and simulation, based on a discrete-time age-structured bioeconomic model. The optimization problem is solved as a non-linear programming problem in GAMS. First, a basic model structure and 6 different scenarios, dealing with two interactions between fish and environment, are introduced. Based on the simplest scenario, eight different parameter combinations are tested. Then the optimization problem is solved for each of the 6 scenarios for a period of 100 years in order to gain long term insights. The main finding is that higher volatility from the environment leads to higher net profits but together with a lower probability of actually hitting the mean values. Simulations are conducted with different fixed fishing mortality levels under 6 scenarios. It seems that a constant fishing mortality around 0.06 is optimal. In the end, a comparison is made between historical and optimal harvest for a period of 40 years. It turns out that in more than 70% of the time, the optimal exploitation offered by our optimization model dominates the historical one, leading to 43% higher net profit and 34% lower fishing cost on average.

Quota implementation of the maximum sustainable yield for age-structured fisheries

One of the main goals stated in the proposals for the Common Fisheries Policy (CFP) reform was achieving maximum sustainable yield (MSY) for all European fisheries. In this paper, we propose a fishing rights allocation mechanism or management system, which specifies catch limits for individual fishing fleets to implement MSY harvesting conditions in an age-structured bioeconomic model. An age-structured model in a single species fishery with two fleets having perfect or imperfect fishing selectivity is studied. If fishing technology or gear selectivity depends on the relative age composition of the mature fish stock, fixed harvest proportions, derived from catchability and bycatch coefficients, is not valid anymore. As a result, not only the age-structure and fishing technology but also the estimated level of MSY is steering the allocation of quota shares. The results also show that allocation of quota shares based on historical catches or auctioning may not provide viable solutions to achieve MSY.

Bioeconomics of ocean acidification effects on fisheries targeting calcifier species: A decision theory approach

The impact of ocean acidification on fisheries is a relatively new issue facing decision-makers, and one for which very little empirical data is available to draw upon. This paper demonstrates how, despite the lack of knowledge, well-established methods of bioeconomic modelling and decision analysis can be applied to address the challenge. A decision support framework is developed, incorporating a dynamic age-structured bioeconomic model together with a set of decision tables applicable in the absence of known probabilities of future change. With such a model it is possible to trace ocean acidification as an additional stressor, specifically on fisheries targeting calcifier species, such as many high value mollusks. We do so by shifting growth and natural mortality parameters into time varying functions of ocean acidity (pH), as forecasted by climate scenarios reported by the Intergovernmental Panel on Climate Change (IPCC). Possible effects of ocean acidification on calcifier species with various life cycles were modeled beginning with initial biological parameters of the growth and mortality dynamic functions reflecting differences in individual growth, natural mortality and species longevity. The analysis illustrates how fishery outcomes depend on the extent of ocean acidification and the life cycle of calcifier species. Results also indicate that under uncertainty, there is value in taking precautionary management measures, such as reducing fishing intensity.

Bioeconomic analysis of child-targeted subsidies for artemisinin combination therapies: a cost-effectiveness analysis.

The Affordable Medicines Facility for malaria (AMFm) was conceived as a global market-based mechanism to increase access to effective malaria treatment and prolong effectiveness of artemisinin. Although results from a pilot implementation suggested that the subsidy was effective in increasing access to high-quality artemisinin combination therapies (ACTs), the Global Fund has converted AMFm into a country-driven mechanism whereby individual countries could choose to fund the subsidy from within their country envelopes. Because the initial costs of the subsidy in the pilot countries was higher than expected, countries are also exploring alternatives to a universal subsidy, such as subsidizing only child doses. We examined the incremental cost-effectiveness of a child-targeted policy using an age-structured bioeconomic model of malaria from the provider perspective. Because the vast majority of malaria deaths occur in children, targeting children could potentially improve the cost-effectiveness of the subsidy, though it would avert significantly fewer deaths. However, the benefits of a child-targeted subsidy (i.e. deaths averted) are eroded as leakage (i.e. older individuals taking young child-targeted doses) increases, with few of the benefits of a universal subsidy gained (i.e. reductions in overall prevalence). Although potentially more cost-effective, a child-targeted subsidy must contain measures to reduce the possibility of leakage.

An age-structured bio-economic model of invasive species management: insights and strategies for optimal control

Controlling invasive species is a highly complex problem defined by the biological characteristics of the organisms, the landscape context, and a management objective of minimizing invasion damages given limited financial resources. While bio-economic optimization models provide a promising approach for invasive species control, current spatio-temporal optimization models omit key ecological details such as age structures—which could be essential to predict how populations grow and spread spatially over time and determine the most effective control strategies. We develop a novel age-structured optimization model as a spatial-dynamic decision framework for controlling invasive species. In particular, we propose a new carrying capacity sub-model, which allows us to take into account the biological competition among different age classes within the population. The potential use of the model is demonstrated on controlling the invasion of sericea (Lespedeza cuneata), a perennial legume threatening native grasslands in the Great Plains. The results show that incorporating age-structure into the model captures important biological characteristics of the species and leads to unexpected results such as multi-logistic population growth with multiple, sequential, and overlapping phases of logistic form. These new findings can contribute to understanding time-lags and invasion growth dynamics. Additionally, given budget constraints, utilizing control measures every 2–3 years is found to be more effective than yearly control because of the time to reproductive maturity. Results of the bio-economic optimization approach provide both ecological and economic insights into the control of invasive species. Furthermore, while the proposed model is specific enough to capture biological realism, it also has the potential to be generalized to a wide range of invasive plant and animal species under various management scenarios in order to identify the most efficient control strategies for managing invasive species.

Evaluation of selectivity and bycatch mitigation measures using bioeconomic modelling. The cases of Madagascar and French Guiana shrimp fisheries

Tropical shrimp fisheries are characterized by various interactions with their natural environment and with other fisheries. These latter interactions can be explained by the high quantity of bycatch taken by industrial trawler fleets, which has a significant impact on fish populations associated with shrimps and thus also on finfish fisheries. By- catch also includes emblematic species, which are subject to strict conservation measures decided by the international community. It seems important to identify and assess the biological and economic consequences of different mitiga- tion measures (increase of mesh size, turtle excluder devices and bycatch reducing devices). This communication is based on case studies undertaken on the Indian white prawn (Fenneropenaeus indicus) and speckled shrimp (Metape- naeus monoceros) fisheries in Madagascar and on the brown shrimp (Farfantepenaeus subtilis) and pink spotted shrimp (Farfantepenaeus brasiliensis) fishery in French Guiana. A review of the impacts of these fisheries on resources and ecosystems is made and some results of experiments on mitigation devices given. Finally, the results of simulations obtained using a multi-species, multi-fleet, age-structured bioeconomic model, including modifications of catchability and costs related to the adoption of these devices, is presented and discussed.

Effort allocation and marine protected areas: is the North Sea Plaice Box a management compromise?

Abstract Kjærsgaard, J., and Frost, H. 2008. Effort allocation and marine protected areas: is the North Sea Plaice Box a management compromise? – ICES Journal of Marine Science, 65: 1203–1215. A complex bioeconomic model is used to study the economic and biological consequences of establishing a marine protected area (MPA). The model is a multispecies age-structured bioeconomic model that treats days at sea and number of vessels, for different fleets fishing inside and outside the protected area, as endogenous variables. A simulation applies an adaptive investment rule that determines fleet size from year to year, and an optimization procedure provides a benchmark for a profit-maximizing solution over time. In contrast to most conventional studies on MPAs, fishing within the protected area is possible. Moreover, the stock is not divided between inside and outside the protected area, although the abundance of different age classes in each area differs. Therefore, the economic and biological impacts of fishing inside or outside are different. The North Sea flatfish fishery is used as a case study, so the analysis is particularly relevant because North Sea flatfish regulation is currently under revision.

Improving Utilization of the Atlantic Sea Scallop Resource: An Analysis of Rotational Management of Fishing Grounds

<i>An age-structured bioeconomic model was constructed to determine optimal patterns of exploitation for the U.S. Atlantic sea scallop fishery. Results indicated that economic rents are maximized by engaging in pulse fishing strategies, whereby fishing only occurs following a multi-year closure period. Closures allow biomass to accumulate undisturbed for several years in a row, leading to the harvest of premium-size scallops upon reopening of the fishing grounds. Closures also result in substantial reductions in operating fishing costs. The rotational harvesting strategy was found to be robust with respect to a number of assumptions in the model. Policy implications are discussed.</i> (JEL Q22)