Publications

Many nearshore fish and invertebrate populations are overexploited even when apparently coherent management structures are in place. One potential cause of mismanagement may be a poor understanding and accounting of stochasticity, particularly for stock recruitment. Many of the fishes and invertebrates that comprise nearshore fisheries are relatively sedentary as adults but have an obligate larval pelagic stage that is dispersed by ocean currents. Here, we demonstrate that larval connectivity is inherently an intermittent and heterogeneous process on annual time scales. This stochasticity arises from the advection of pelagic larvae by chaotic coastal circulations. This result departs from typical assumptions where larvae simply diffuse from one site to another or where complex connectivity patterns are created by transport within spatially complicated environments. We derive a statistical model for the expected variability in larval settlement patterns and demonstrate how larval connectivity varies as a function of different biological and physical processes. The stochastic nature of larval connectivity creates an unavoidable uncertainty in the assessment of fish recruitment and the resulting forecasts of sustainable yields.

David Festa, Diane Regas, and Judson Boomhower's compelling account "Sharing the Catch, Conserving the Fish" ( Issues , Winter 2008) echoes mounting theoretical and empirical evidence on the powerful ability of economic incentives to provide resource users with strong motivation for good stewardship. The question for policymakers should no longer be if or when to implement these innovations, but how to design them to achieve biological, economic, and social objectives.
Most current fisheries management approaches generate two inherent conflicts: between a fisherman's short- and long-term interests and between the short-term incentives of a fisherman and the longer-term public good. These conflicts exist because there are strong incentives for fishermen to overfish and spend too much money doing it. These mismatches can be reversed with appropriate design of secure-access programs that align short- and long-term interests. Providing fishermen with economic incentives to conserve is smart
management. 

We characterize the optimal harvest of a renewable resource in a generalized stochastic spatially explicit model. Despite the complexity of the model, we are able to obtain sharp analytical results. We find that the optimal harvest rule in general depends upon dispersal patterns of the resource across space, and only in special circumstances do we find a modified golden rule of growth that is independent of dispersal patterns. We also find that the optimal harvest rule may include closure of some areas to harvest, either on a temporary or permanent basis (biological reserves). Reserves alone cannot correct open access, but may, under sufficient spatial heterogeneity and connectivity, increase profits if appropriate harvest controls are in place outside of reserves.

Spillover of fish from marine reserves to adjacent harvested waters may be mediated by density-independent movement, density-dependent movement, or both. If dispersal is by random movement, populations within the reserve must be regulated by density-dependent population growth (DDG). Density-dependent movement (DDM) can also regulate the population if accelerated emigration from a reserve to the surrounding fishing grounds leads to substantially increased mortality. Using spatially explicit models, we show that stock per unit area is bounded for DDG and increases with size for DDM. With DDG, spillover rate per unit area of reserve is maximized with reserves around 50% larger in linear dimension than the minimum size for population persistence. With DDM, spillover per unit area of reserve increases with reserve size. The results highlight the need for the mechanism of population regulation to be incorporated into theoretical and empirical investigations of marine reserve ecology.

Some studies suggest that fishery yields can be higher with reserves than under conventional management. However, the economic performance of fisheries depends on economic profit, not fish yield. The predictions of higher yields with reserves rely on intensive fishing pressures between reserves; the exorbitant costs of harvesting low-density populations erode profits. We incorporated this effect into a bioeconomic model to evaluate the economic performance of reserve-based management. Our results indicate that reserves can still benefit fisheries, even those targeting species that are expensive to harvest. However, in contrast to studies focused on yield, only a moderate proportion of the coast in reserves (with moderate harvest pressures outside reserves) is required to maximize profit. Furthermore, reserve area and harvest intensity can be traded off with little impact on profits, allowing for management flexibility while still providing higher profit than attainable under conventional management.

We study the dynamic harvest incentives faced by a renewable resource harvester with insecure property rights. A resource “concession” is granted for a fixed duration, after which it is renewed (with a known probability) only if a target stock is achieved. Despite the insecurity of this property right, simple concessions contracts can be designed to induce first best harvest trajectories. We examine how those contracts will depend on economic, ecological, and institutional variables, and apply theoretical insights to two concessions-managed fisheries in Baja California, Mexico.

Throughout the world “fishing the line” is a frequent harvesting tactic in communities where no-take marine reserves are designated. This practice of concentrating fishing effort at the boundary of a marine reserve is predicated upon the principle of spillover, the net export of stock from the marine reserve to the surrounding unprotected waters. We explore the consequences and optimality of fishing the line using a spatially explicit theoretical model. We show that fishing the line: (1) is part of the optimal effort distribution near no-take marine reserves with mobile species regardless of the cooperation level among harvesters; (2) has a significant impact on the spatial patterns of catch per unit effort (CPUE) and fish density both within and outside of the reserve; and (3) can enhance total population size and catch simultaneously under a limited set of conditions for overexploited populations. Additionally, we explore the consequences of basing the spatial distribution of fishing effort for a multispecies fishery upon the optimality of the most mobile species that exhibits the greatest spillover. Our results show that the intensity of effort allocated to fishing the line should instead be based upon more intermediate rates of mobility within the targeted community. We conclude with a comparison between model predictions and empirical findings from a density gradient study of two important game fish in the vicinity of a no-take marine-life refuge on Santa Catalina Island, California (USA). These results reveal the need for empirical studies to account for harvester behavior and suggest that the implications of spatial discontinuities such as fishing the line should be incorporated into marine-reserve design.

Temperature controls the rate of fundamental biochemical processes and thereby regulates organismal attributes including development rate and survival. The increase in metabolic rate with temperature explains substantial among-species variation in life-history traits, population dynamics, and ecosystem processes. Temperature can also cause variability in metabolic rate within species. Here, we compare the effect of temperature on a key component of marine life cycles among a geographically and taxonomically diverse group of marine fish and invertebrates. Although innumerable lab studies document the negative effect of temperature on larval development time, little is known about the generality versus taxon-dependence of this relationship. We present a unified, parameterized model for the temperature dependence of larval development in marine animals. Because the duration of the larval period is known to influence larval dispersal distance and survival, changes in ocean temperature could have a direct and predictable influence on population connectivity, community structure, and regional-to-global scale patterns of biodiversity.

Individual transferable quota (ITQ) regulation relies on a decentralized market mechanism and a single price to allocate access to a stock of fish. The resulting allocation will not be fully efficient if the stock being allocated is heterogeneous or if there are potential gains from centralized coordination of harvesting effort. If stocks are heterogeneous in their density, location, or unit value during the season, harvesters governed by an ITQ policy will not be indifferent to when or where they exercise their quotas. Stocks that are relatively dense and/or close to port will be preferred to those less dense or more remote. Because an ITQ policy assigns the same opportunity cost for each unit harvested, individual harvesters have an incentive to compete for higher-valued units, and such competition may dissipate part of the fishery's potential rent. A similar phenomenon arises when stock densities vary in an unknown way over space or time, so harvesters must engage in costly search. Individual harvesters governed by an ITQ policy still face a collective action problem which limits the incentive to share information on stock locations. This can lead to redundant search effort. We demonstrate that both sources of inefficiency can be eliminated either by defining ITQ rights more precisely or by an agreement among harvesters to coordinate their effort. We develop models that illustrate these effects and identify the factors that determine their likely size. Anecdotal evidence on practices adopted by fishery cooperatives is presented to illustrate the practical relevance of the issues we raise.