A critique of models of the American lobster fishery

A number of studies have used the American lobster fishery to raise theoretical and empirical issues in the economic application of Schaefer yield-effort models. The present research shows that both published variants of the Schaefer yield-effort model are poor predictors of landings in the lobster fishery. The analysis suggests that minimum size rules in the lobster fishery make the Beverton-Holt dynamic pool model more appropriate to the lobster fishery than the Schaefer yield-effort model.     

Collaborative assessment of California spiny lobster population and fishery responses to a marine reserve network

Assessments of the conservation and fisheries effects of marine reserves typically focus on single reserves where sampling occurs over narrow spatiotemporal scales. A strategy for broadening the collection and interpretation of data is collaborative fisheries research (CFR). Here we report results of a CFR program formed in part to test whether reserves at the Santa Barbara Channel Islands, USA, influenced lobster size and trap yield, and whether abundance changes in reserves led to spillover that influenced trap yield and effort distribution near reserve borders. Industry training of scientists allowed us to sample reserves with fishery relevant metrics that we compared with pre-reserve fishing records, a concurrent port sampling program, fishery effort patterns, the local ecological knowledge (LEK) of fishermen, and fishery-independent visual surveys of lobster abundance. After six years of reserve protection, there was a four- to eightfold increase in trap yield, a 5-10% increase in the mean size (carapace length) of legal sized lobsters, and larger size structure of lobsters trapped inside vs. outside of three replicate reserves. Patterns in trap data were corroborated by visual scuba surveys that indicated a four- to sixfold increase in lobster density inside reserves. Population increases within reserves did not lead to increased trap yields or effort concentrations (fishing the line) immediately outside reserve borders. The absence of these catch and effort trends, which are indicative of spillover, may be due to moderate total mortality (Z = 0.59 for legal sized lobsters outside reserves), which was estimated from analysis of growth and length frequency data collected as part of our CFR program. Spillover at the Channel Islands reserves may be occurring but at levels that are insufficient to influence the fishery dynamics that we measured. Future increases in fishing effort (outside reserves) and lobster biomass (inside reserves) are likely and may lead to increased spillover, and CFR provides an ideal platform for continued assessment of fishery-reserve interactions.     

Functional response of sport divers to lobsters with application to fisheries management.

Fishery managers must understand the dynamics of fishers and their prey to successfully predict the outcome of management actions. We measured the impact of a two-day exclusively recreational fishery on Caribbean spiny lobster in the Florida Keys, USA, over large spatial scales (>100 km) and multiple years and used a theoretical, predator-prey functional response approach to identify whether or not sport diver catch rates were density-independent (type I) or density-dependent (type II or III functional response), and if catch rates were saturated (i.e., reached an asymptote) at relatively high lobster densities. We then describe how this predator-prey framework can be applied to fisheries management for spiny lobster and other species. In the lower Keys, divers exhibited a type-I functional response, whereby they removed a constant and relatively high proportion of lobsters (0.74-0.84) across all pre-fishing-season lobster densities. Diver fishing effort increased in a linear manner with lobster prey densities, as would be expected with a type-I functional response, and was an order of magnitude lower in the upper Keys than lower Keys. There were numerous instances in the upper Keys where the density of lobsters actually increased from before to after the fishing season, suggesting some type of "spill-in effect" from surrounding diver-disturbed areas. With the exception of isolated reefs in the upper Keys, the proportion of lobsters removed by divers was density independent (type-I functional response) and never reached saturation at natural lobster densities. Thus, recreational divers have a relatively simple predatory response to spiny lobster, whereby catch rates increase linearly with lobster density such that catch is a reliable indicator of abundance. Although diver predation is extremely high (approximately 80%), diver predation pressure is not expected to increase proportionally with a decline in lobster density (i.e., a depensatory response), which could exacerbate local extinction. Furthermore, management actions that reduce diver effort should have a concomitant and desired reduction in catch. The recreational diver-lobster predator-prey construct in this study provides a useful predictive framework to apply to both recreational and commercial fisheries, and on which to build as management actions are implemented.     

A general bioeconomic simulation model for annual-crop marine fisheries

A generalized bioeconomic simulation model of annual-crop marine fisheries is described and its use in marine fisheries management is demonstrated. The biological submodel represents the recruitment of new organisms into the fishery, the movement of organisms from one fishing area to another and from one depth to another, the growth of organisms and the mortality of organisms resulting both from natural causes and from fishing. The economic submodel represents the fishing effort exerted on each resource species, the monetary costs of fishing, the value of the harvest and the rent (or excess profits) to the fishery.Basic dynamics of the model results from changes in the number of organisms in the fishery over time, which can be summarized as a set of difference equations of the general form ΔN/Δt = R + I ? E ? M ? F where ΔN/Δt is the net change in number of organisms in the fishery over time, R is recruitment, I is immigration, E is emigration, M is natural mortality and F is fishing mortality. R is a driving variable, whereas I, E, M and F are functions of the state of the system at any given point in time. The model can be run in a deterministic or stochastic mode. Values for parameters affecting rates of recruitment, movement, growth, natural mortality and fishing mortality can be selected from uniform, triangular or normal distributions.Use of the model within a fisheries-management framework is demonstrated by evaluating several management alternatives for the pink shrimp (Penaeus duorarum) fishery on the Tortugas grounds in the Gulf of Mexico. Steps involved in use of the model, including parameterization, validation, sensitivity analysis and stochastic simulations of management policies, are explained.     

Exploitation of the lobster fishery: Some empirical results

This paper analyzes the optimal and free market utilization of the lobster fishery and applies the results to two fishing areas in Canada. Biomass relationships and a production function are estimated and the empirical results are used to calculate hypothetical optimal fishing solutions. The welfare losses from overutilization of the fishing areas are examined.     

Aggregation and fishery dynamics: Multiple time scales,times to extinction,and random environments

This paper is a mathematical study of the models introduced by C.W. Clark and M. Mangel to describe the tropical tuna purse seine fishery. By casting the equations into dimensionless form, it is easy to study the bifurcations of the system models and to study the dynamical behavior of the models. The dynamics are analyzed by using singular perturbation theory. A number of stochastic problems related to the dynamics of the models are formulated and solutions are obtained by using the diffusion approximation and asymptotic analysis. Finally, the question of model identification is addressed, and two methods for model identification are sketched.     

Estimating the elasticity of substitution between restricted and unrestricted inputs in a regulated fishery: A probit approach

Catch, effort, vessel, and skipper characteristic data for a sample of boats involved in the Tasmanian rock lobster fishery in the 1983/1984 season are used to estimate a fishing effort production function nested in a Cobb-Douglas fishery production function. The initial effort function is translog but tests suggest that the CES form cannot be rejected. The vessel's choice of fishing ground is modeled by a probit equation, the output from which is included in the OLS estimation of the production function. The elasticity of substitution between the inputs which are restricted by regulation and the other inputs is of particular interest as it determines whether input restriction is an efficient form of regulation for the fishery. The results suggest a value significantly less than unity.     

A bioeconomic model of an international fishery

The conventional analyses of exploited fisheries is extended to situations where several national fleets harvest the resource. Using a simple biological model, the dynamics of such a fishery are explored and the open-access equilibria described. The model highlights the mutual interdependencies of the several fishing fleets, which are of importance when considering management policies in a regime of extended national jurisdiction. Several alternatives for managing such fisheries are also discussed.     

Management and exploitation dynamics of small-scale fisheries in the Bay of Biafra: An integrative analysis of purse seine fishing activity

For many decades, fisheries research has focused on stock assessment and the impact of the fishery effort on resources. Although this knowledge remains necessary, a more integrated analysis of the joint dynamics of resource and operational activities is needed to provide more useful advice for the management of fishery systems. Since 1994 a new approach to fishery science has been carried out for Cameroonian small-scale fisheries, the aim being the incorporation into fishery science of research on fishery management, fishing processes and fishermen's behaviour. This paper presents a more systemic data approach which combines biological parameters and operational factor analyses with the goal of sustainable development. From two years of data collected on the purse seine fishing units operating in the Bay of Biafra (2002 and 2003), a set of three correspondence analyses is applied: (1) to the length frequency distribution of Ethmalosa fimbriata, (2) to the number of visits per fishing ground, and (3) to the species appearance frequency in the landings. These three analyses were plotted per month-year period. The results are presented in the form of an annual exploitation cycle, linking fishing grounds, the main species caught, and corresponding fishing period.     

Creation of a Gilded Trap by the High Economic Value of the Maine Lobster Fishery

Abstract: Unsustainable fishing simplifies food chains and, as with aquaculture, can result in reliance on a few economically valuable species. This lack of diversity may increase risks of ecological and economic disruptions. Centuries of intense fishing have extirpated most apex predators in the Gulf of Maine (United States and Canada), effectively creating an American lobster (Homarus americanus) monoculture. Over the past 20 years, the economic diversity of marine resources harvested in Maine has declined by almost 70%. Today, over 80% of the value of Maine's fish and seafood landings is from highly abundant lobsters. Inflation‐corrected income from lobsters in Maine has steadily increased by nearly 400% since 1985. Fisheries managers, policy makers, and fishers view this as a success. However, such lucrative monocultures increase the social and ecological consequences of future declines in lobsters. In southern New England, disease and stresses related to increases in ocean temperature resulted in more than a 70% decline in lobster abundance, prompting managers to propose closing that fishery. A similar collapse in Maine could fundamentally disrupt the social and economic foundation of its coast. We suggest the current success of Maine's lobster fishery is a gilded trap. Gilded traps are a type of social trap in which collective actions resulting from economically attractive opportunities outweigh concerns over associated social and ecological risks or consequences. Large financial gain creates a strong reinforcing feedback that deepens the trap. Avoiding or escaping gilded traps requires managing for increased biological and economic diversity. This is difficult to do prior to a crisis while financial incentives for maintaining the status quo are large. The long‐term challenge is to shift fisheries management away from single species toward integrated social‐ecological approaches that diversify local ecosystems, societies, and economies.     

Optimal escapement levels in stochastic and deterministic harvesting models

A constant-escapement feedback policy is shown to be optimal in maximizing expected discounted net revenue from an animal resource whose dynamics are described by a stochastic stock-recruitment model, provided that unit harvesting costs satisfy certain conditions. The optimal escapement in this model is compared with that in the corresponding deterministic model and it is shown how the way in which unit harvesting costs vary with population abundance can be important in determining the relative sizes of the optimal escapements. In most cases, the optimal stochastic escapement is no less than the optimal deterministic escapement.     

The role of habitat disturbance and recovery in metapopulation persistence

Classical metapopulation theory assumes a static landscape. However, empirical evidence indicates many metapopulations are driven by habitat succession and disturbance. We develop a stochastic metapopulation model, incorporating habitat disturbance and recovery, coupled with patch colonization and extinction, to investigate the effect of habitat dynamics on persistence. We discover that habitat dynamics play a fundamental role in metapopulation dynamics. The mean number of suitable habitat patches is not adequate for characterizing the dynamics of the metapopulation. For a fixed mean number of suitable patches, we discover that the details of how disturbance affects patches and how patches recover influences metapopulation dynamics in a fundamental way. Moreover, metapopulation persistence is dependent not only on the average lifetime of a patch, but also on the variance in patch lifetime and the synchrony in patch dynamics that results from disturbance. Finally, there is an interaction between the habitat and metapopulation dynamics, for instance declining metapopulations react differently to habitat dynamics than expanding metapopulations. We close, emphasizing the importance of using performance measures appropriate to stochastic systems when evaluating their behavior, such as the probability distribution of the state of the metapopulation, conditional on it being extant (i.e., the quasistationary distribution).     

A stochastic simulation model of brown shrimp (Penaeus aztecus ives) growth,movement, and survival in Galveston Bay,Texas

A stochastic simulation model of brown shrimp (Penaeus aztecus Ives) population dynamics in Galveston Bay, Texas, is described, validated, and used to evaluate the effects of management alternatives and changing environmental conditions on shrimp dynamics. The model is composed of submodels representing: (1) recruitment, (2) growth, (3) natural mortality, (4) fishing mortality, and (5) emigration of brown shrimp. The model predicts significant changes in total annual harvest from the food shrimp, bait, and recreational fisheries resulting from (1) closure of the bay system to all fishing except during the spring and fall open seasons, (2) two-week postponement of the opening and closing of the open seasons for the food shrimp fishery, (3) a 2.5°C increase and (4) a 2.5°C decrease in mean water temperature, (5) an 80% increase and (6) an 80% decrease in fishing effort. No significant change in the total annual harvest is predicted when the food shrimp fishing season is extended from May 15 through December 15. Sensitivity analysis suggests that field experimentation designed specifically to test the hypothesis of a 60-day time lag between brown shrimp recruitment into the bays and exposure to the fishery should receive high priority. Simulation results are discussed within a management framework.     

Long-term trends in invertebrate–habitat relationships under protected and fished conditions

Few studies examine the long-term effects of changing predator size and abundance on the habitat associations of resident organisms despite that this knowledge is critical to understand the ecosystem effects of fishing. Marine reserves offer the opportunity to determine ecosystem-level effects of manipulated predator densities, while parallel monitoring of adjacent fished areas allows separating these effects from regional-scale change. Relationships between two measures of benthic habitat structure (reef architecture and topographic complexity) and key invertebrate species were followed over 17 years at fished and protected subtidal rocky reefs associated with two southern Australian marine reserves. Two commercially harvested species, the southern rock lobster (Jasus edwardsii) and blacklip abalone (Haliotis rubra) were initially weakly associated with habitat structure across all fished and protected sites. The strength of association with habitat for both species increased markedly at protected sites 2 years after marine reserve declaration, and then gradually weakened over subsequent years. The increasing size of rock lobster within reserves apparently reduced their dependency on reef shelters as refuges from predation. Rising predation by fish and rock lobster in the reserves corresponded with weakening invertebrate–habitat relationships for H. rubra and sea urchins (Heliocidaris erythrogramma). These results emphasise that animal–habitat relationships are not necessarily stable through time and highlight the value of marine reserves as reference sites. Our work shows that fishery closures to enhance populations of commercially important and keystone species should be in areas with a range of habitat features to accommodate shifting ecological requirements with ontogenesis.     

围海造陆形成后对生态环境和渔业资源的影响--以天津临港工业区滩涂开发一期工程为例

研究的目的在于利用有关数学模型的研究成果,结合项目区域海洋生态环境的现状,分析围海造陆工程形成后由于项目本身侵占湿地以及周围海域水动力条件的改变对生态环境和渔业资源的影响。分析结果表明,滩涂开发工程对养殖区等生态敏感目标的影响不明显,但对滩涂湿地生态系统有一定程度的影响,表现为占用部分湿地,导致湿地总面积减少,但本地区海河口湿地面积缩小对渤海湾生态环境的影响有待进一步研究。文章从管理与规划角度出发提出滩涂开发过程中的有关建议和要求。     

How important are uncertainty and dynamics for environmental and climate policy? Some analytics

We introduce ten papers on sustainable resource dynamics. In addition, we provide analytical results on the effect of stochastic damages on optimal economic growth, the effects of habits and loss aversion on the cost-benefit discount rate, and the effect of a carbon budget and carbon capture and storage (CCS) on optimal investment in technical change.     

Generalised linear modelling of fecundity at length in southern rock lobsters, <Emphasis Type="Italic">Jasus edwardsii</Emphasis>

Accurate fecundity estimates are central to population modelling of reproductive status and egg production in a fishery. Southern rock lobster (Jasus edwardsii) fisheries are managed with the consideration of performance measures based on the level of current egg production relative to the unfished stock. Egg production is a function of the size at the onset of maturity, the size structure of the stock, and the fecundity at length of lobsters. Fecundity at length of the Tasmania stock of J. edwardsii has not been reported previously and was estimated from seven sites around Tasmania. Data were analysed by classical linear regression and also generalised linear models (GLM) with gamma and Poisson distributions. GLM with a gamma distribution produced the best fit and did not require a correction factor to generate a model on the linear scale. Individual fecundity ranged from 43,918 to 660,156 and could be predicted from carapace length according to the equation F = −1.707 + 2.969 (log length). The coefficient of length approximated three, implying the fecundity at length relationship is cuboidal. Despite the spatial heterogeneity in reproductive parameters in this fishery, the inclusion of spatial information and egg development stage into the analysis only explained an additional 1% of the variance in fecundity. Thus, it appears valid to apply the relationship across the fishery without spatial separation.     

Non-renewable resource prices: Deterministic or stochastic trends?

In this paper, we examine temporal properties of 11 natural resource real price series from 1870 to 1990. Recent studies by Ahrens and Sharma [Trends in natural resource commodity prices: deterministic or stochastic? J. Environ. Econom. Manage. 33(1997)59–74], Berck and Roberts [Natural resource prices: will they ever turn up? J. Environ. Econom. Manage. 31(1996)65–78], and Slade [Grade selection under uncertainty: least cost last and other anomalies, J. Environ. Econom. Manage. 15(1988)189–205], among others, find that many non-renewable resource prices have a stochastic trend. We revisit this issue by employing a Lagrangian multiplier unit root test that allows for two endogenously determined structural breaks with and without a quadratic trend. Contrary to previous research, we find evidence against the unit root hypothesis for all price series. Our findings support characterizing natural resource prices as stationary around deterministic trends with structural breaks. We additionally show that both pre-testing for unit roots with breaks and allowing for breaks in the forecast model can improve forecast accuracy. Overall, the results in this paper are important in both a positive and normative sense; without an appropriate understanding of the dynamics of a time series, empirical verification of theories, forecasting, and proper inference are potentially fruitless.     

Genetic differentiation of the California spiny lobster Panulirus interruptus (Randall, 1840) along the west coast of the Baja California Peninsula, Mexico

Identifying reproductive stocks in commercial species is relevant to fishery management strategies. We obtained muscle samples of California spiny lobster (Panulirus interruptus) from six localities along the west coast of the Baja California Peninsula and analyzed the genetic structure using mtDNA RFLPs. Our results indicated that all localities shared the same major haplotypes and showed a spatial homogeneity in the distribution of haplotype frequencies. An analysis of molecular variance indicated that only 0.84% of the genetic variability was explained by differences among localities and was not significantly different from zero. Weak divergences were found between Bahía Magdalena, the most southerly locality, in relation to other populations. Major oceanographic processes along this coast, combined with a long larval period that supports passive transport among localities, are suggested to explain the results.     

The Robert H. MacArthur Award Lecture. Timescales, dynamics, and ecological understanding

Explicit consideration of timescales and dynamics is required for an understanding of fundamental issues in ecology. Endogenous dynamics can lead to transient states where asymptotic behavior is very different from dynamics on short timescales. The causes of these kinds of transients can be placed in one of three classes: linear systems with different timescales embedded or exhibiting reactive behavior, the potentially long times to reach synchrony across space for oscillating systems, and the complex dynamics of systems with strong density-dependent (nonlinear) interactions. It is also important to include the potentially disparate timescales inherent in ecological systems when determining the endogenous dynamics. I argue that the dynamics of ecological systems can best be understood as the response, which may include transient dynamics, to exogenous influences leading to the observed dynamics on ecologically relevant timescales. This view of ecosystem behavior as responses of ecological systems governed by endogenous dynamics to exogenous influences provides a synthetic way to unify different approaches to population dynamics, to understand mechanisms that determine the distribution and abundance of species, and to manage ecosystems on appropriate timescales. There are implications for theoretical approaches, empirical approaches, and the statistical approaches that bridge theory and observation.