Louis W. Botsford

PRINCIPLES FOR THE DESIGN OF MARINE RESERVES

The theory underlying the design of marine reserves, whether the goal is to preserve biodiversity or to manage fisheries, is still in its infancy. For both of these goals, there is a need for general principles on which to base marine reserve design, and because of the paucity of empirical experience, these principles must be based on models. However, most of the theoretical studies to date have been specific to a single situation, with few attempts to deduce general principles. Here we attempt to distill existing results into general principles useful to designers of marine reserves. To answer the question of how fishery management using reserves compares to conventional management, we provide two prin- ciples: (1) the effect of reserves on yield per recruit is similar to increasing the age of first capture, and (2) the effect of reserves on yield is similar to reducing effort. Another two principles answer the question of how to design reserve configurations so that species with movement in various stages will be sustainable: (3) higher juvenile and adult movement lowers sustainability of reserves for biodiversity, but an intermediate level of adult move- ment is required for reserves for fishery management, and (4) longer larval dispersal distance requires larger reserves for sustainability. These principles provide general guidelines for design, and attention to them will allow more rapid progress in future modeling studies. Whether populations or communities will persist under any specific reserve design is un- certain, and we suggest ways of dealing with that uncertainty.

POPULATION MODELS FOR MARINE RESERVE DESIGN: A RETROSPECTIVE AND PROSPECTIVE SYNTHESIS

We synthesize results from existing models of marine reserves to identify key theoretical issues that appear to be well understood, as well as issues in need of further exploration. Models of marine reserves are relatively new in the scientific literature; 32 of the 34 theoretical papers we reviewed were published after 1990. These models have focused primarily on questions concerning fishery management at the expense of other objectives such as conservation, scientific understanding, recreation, education, and tourism. Roughly one-third of the models analyze effects on cohorts while the remaining models have some form of complete population dynamics. Few models explicitly include larval dispersal. In a fisheries context, the primary conclusion drawn by many of the complete population models is that reserves increase yield when populations would otherwise be overfished. A second conclusion, resulting primarily from single-cohort models, is that reserves will provide fewer benefits for species with greater adult rates of movement. Although some models are beginning to yield information on the spatial configurations of reserves required for populations with specific dispersal distances to persist, it remains an aspect of reserve design in need of further analysis. Other outstanding issues include the effects of (1) particular forms of density dependence, (2) multispecies interactions, (3) fisher behavior, and (4) effects of concentrated fishing on habitat. Model results indicate that marine reserves could play a beneficial role in the protection of marine systems against overfishing. Additional modeling and analysis will greatly improve prospects for a better understanding of the potential of marine reserves for conserving biodiversity.

No-take Reserve Networks: Sustaining Fishery Populations and Marine Ecosystems

Abstract Improved management approaches are needed to reduce the rate at which humans are depleting exploited marine populations and degrading marine ecosystems. Networks of no-take marine reserves are promising management tools because of their potential to (1) protect coastal ecosystem structure and functioning, (2) benefit exploited populations and fisheries, (3) improve scientific understanding of marine ecosystems, and (4) provide enriched opportunities for non-extractive human activities. By protecting marine ecosystems and their populations, no-take reserve networks can reduce risk by providing important insurance for fishery managers against overexploitation of individual populations. Replicated reserves also foster strong scientific testing of fishery and conservation management strategies. Reserve networks will require social acceptance, adequate enforcement, and effective scientific evaluation to be successful. Processes for reserve establishment should accommodate adaptive management so bounda...

COMPARING DESIGNS OF MARINE RESERVES FOR FISHERIES AND FOR BIODIVERSITY

We compare and contrast the design of networks of marine reserves for two different, commonly stated goals: (1) maintaining high yield in fisheries and (2) conserving biodiversity, in an idealized setting using simple models. The models describe larval dispersal over a system of evenly spaced reserves of equal size, assuming sedentary adults. We initially demonstrate that, since populations in reserve systems can be sustained either by covering a minimal fraction of the coast with small reserves or by covering a smaller fraction of the coast with few large reserves, cost considerations dictate that the conservation goal would be best met by reserves as large as practically possible. In contrast, the fisheries goal of maximizing yield requires maximizing larval export outside of reserves, which we show means that reserves should be as small as practically possible. Meeting the fisheries goal is ultimately more costly because it suggests a larger area of the coastline should be in reserves, but it also improves on conservation goals by enhancing sustainability for species dispersing longer distances.

The Effects of Small Dispersal Rates on Extinction Times in Structured Metapopulation Models

Habitat destruction is a critical factor that affects persistence in several taxa, including Pacific salmon. Salmon are noted for their ability to home to their natal streams for reproduction. Since straying (i.e., spawners reproducing in nonnatal streams) is typically low in salmon, its effects have not been appreciated. In this article, we develop both a general analytical model and a simple simulation model describing structured metapopulations to study how weak connections between subpopulations affect the ability of a species to tolerate habitat destruction and/or declines in habitat quality. Our goals are to develop general principles and to relate these principles to salmon population dynamics. The analytical model describes the dynamics of two density‐dependent subpopulations, connected by dispersal, whose growth rates fluctuate in response to environmental and demographic stochasticity. We find that, for moderate levels of environmental variability, small dispersal rates can significantly increase mean extinction times. This effect declines with increasing habitat quality, increasing temporal correlation, and increasing spatial correlation, but it is still significant for realistic parameter values. The simulation model shows there is a threshold rate of dispersal that minimizes extinction probabilities. These results cannot be seen in classical metapopulation models and provide new insights into the rescue effect.

THE EFFECTS OF INCREASED INDIVIDUAL GROWTH RATES ON DEPRESSED POPULATION SIZE

Several exploited aquatic populations have declined to low levels and remained low even after exploitation was reduced. In many of these, individual growth rate increased just prior to or during the decline and has remained at a high level. I propose herein that this increase in growth rate may contribute to the observed depressed equilibrium levels. Recruitment rate in the multiple age-class model used depends on a compensatory effect by older, larger individuals (e.g., cannibalism) and fecundity of mature individuals. The effect of density-dependent growth rate on equilibrium and stability of this population is analyzed for the case in which both the magnitude of compensatory effect and fecundity depend on individual size rather than age. The possible existence of multiple-equilibria corresponding to different individual growth rates is shown. Simulation results using life history parameters corresponding to the central California Dungeness crab demonstrate possible global behavior of this model. Fishing can increase the susceptibility of decline to a lower, stable equilibrium, or actually lead to a degenerative decline. The population remains at the lower equilibrium level even after exploitation is decreased. Characteristics of some real populations that suggest that increased individual growth rate may contribute to their depressed levels are presented. These characteristics and the results derived herein warrant investigation of increased growth rate as a possible contributor to depressed populations. The proposed new mechanism is shown to be different from those in existing population models with multiple equilibria.

A first estimate of white shark, Carcharodon carcharias, abundance off Central California

The decline of sharks in the global oceans underscores the need for careful assessment and monitoring of remaining populations. The northeastern Pacific is the home range for a genetically distinct clade of white sharks (Carcharodon carcharias). Little is known about the conservation status of this demographically isolated population, concentrated seasonally at two discrete aggregation sites: Central California (CCA) and Guadalupe Island, Mexico. We used photo-identification of dorsal fins in a sequential Bayesian mark–recapture algorithm to estimate white shark abundance off CCA. We collected 321 photographs identifying 130 unique individuals, and estimated the abundance off CCA to be 219 mature and sub-adult individuals ((130, 275) 95% credible intervals), substantially smaller than populations of other large marine predators. Our methods can be readily expanded to estimate shark population abundance at other locations, and over time, to monitor the status, population trends and protection needs of these globally distributed predators.

Density Dependence and Age Structure: Nonlinear Dynamics and Population Behavior

We characterize the dynamics of age-structured density-dependent populations with yearly reproduction. In contrast to prior studies focusing primarily on behavior near the stability boundary, we describe the dynamics over the full range of linear and nonlinear behavior. We describe model dynamics in terms that have direct biological interpretations. We illustrate the use of our approach by examining the dynamics of Dungeness crab (Cancer magister) in detail. Model dynamics are found to be very sensitive to changes in life-history parameters. Small changes in vital rates can cause population density to suddenly jump from low to high variability or vice versa. Nonmonotonic switching between chaotic and nonchaotic dynamics is also observed. The period (or dominant timescale) of cyclic behavior is loosely related to values of vital rates, typically increasing with adult survivorship, but can remain constant while vital rates change. Model dynamics are also found to be sensitive to environmental perturbations. For example, model dynamics may be chaotic or nonchaotic for fixed parameter values with environmental perturbations switching model dynamics between these distinct behaviors (i.e., the dynamics are nonstationary). These findings illustrate one possible explanation for the variety of dynamic behavior in Dungeness crab populations (and other natural populations) and temporal (or spatial) shifts in behavior.

Decision analysis for designing marine protected areas for multiple species with uncertain fishery status

Marine protected areas (MPAs) are growing in popularity as a conservation tool, and there are increasing calls for additional MPAs. Meta-analyses indicate that most MPAs successfully meet the minimal goal of increasing biomass inside the MPA, while some do not, leaving open the important question of what makes MPAs successful. An often-overlooked aspect of this problem is that the success of fishery management outside MPA boundaries (i.e., whether a population is overfished) affects how well MPAs meet both conservation goals (e.g., increased biomass) and economic goals (e.g., minimal negative effects on fishery yield). Using a simple example of a system with homogeneous habitat and periodically spaced MPAs, we show that, as area in MPAs increases, (1) conservation value (biomass) may initially be zero, implying no benefit, then at some point increases monotonically; and (2) fishery yield may be zero, then increases monotonically to a maximum beyond which further increase in MPA area causes yield to decline. Importantly, the points at which these changes in slope occur vary among species and depend on management outside MPAs. Decision makers considering the effects of a potential system of MPAs on multiple species are confronted by a number of such cost-benefit curves, and it is usually impossible to maximize benefits and minimize costs for all species. Moreover, the precise shape of each curve is unknown due to uncertainty regarding the fishery status of each species. Here we describe a decision-analytic approach that incorporates existing information on fishery stock status to present decision makers with the range of likely outcomes of MPA implementation. To summarize results from many species whose overfishing status is uncertain, our decision-analysis approach involves weighted averages over both overfishing uncertainty and species. In an example from an MPA decision process in California, USA, an optimistic projection of future fishery management success led to recommendation of fewer and smaller MPAs than that derived from a more pessimistic projection of future management success. This example illustrates how information on fishery status can be used to project potential outcomes of MPA implementation within a decision analysis framework and highlights the need for better population information.

Inter-annual variability in larval supply to populations of three invertebrate taxa in the northern California Current

Abstract We investigated sources of inter-annual variability in larval supply to crab and sea urchin populations at Bodega Head and Point Reyes in northern California. During the spring and summer upwelling seasons of the years 1992 through 1997 we monitored the weekly settlement rates of nine species of crabs and two species of sea urchins. As observed in previous studies, daily values of alongshore windstress, temperature and salinity provided evidence for the poleward flow of relatively warm, low salinity water from south of Point Reyes, an apparent retention zone, during upwelling relaxation events. In years dominated by these events (1992, 1993, 1995 and 1996) we observed that alongshore windstress, temperature and salinity were coherent and temperature was significantly correlated with cancrid crab settlement. During these years the magnitude of cancrid crab settlement and the fraction of cancrid crabs relative to other crab species settling were high. Over four years of concurrent sampling there was consistently greater cancrid crab settlement at the Point Reyes site, within the retention zone, than at Bodega Head. Settlement of non-cancrid crabs (porcellanids, grapsids, pagurids and majids) was not as closely linked to intra-annual patterns of upwelling and relaxation, possibly due to the shorter seasonal availability of larvae allowing for the influence of fewer relaxation events. Settlement of this group among years was positively correlated with environmental indicators of strong seasonal upwelling; high salinity, Bakun upwelling index and low temperature. Sea urchin settlement events were observed in June and July of 1992, 1994 and 1997 during warming periods when salinity and temperature were increasing and alongshore windstress was low. Across the six years of the study, we found that cancrid crab larvae had a more even seasonal availability than larvae of non-cancrid species, which settled in greatest numbers during the early portion of the upwelling season. Sea urchins settled in greatest numbers during the later part of the upwelling season. Together these patterns demonstrate the taxon-specific way that inter-annual variability in larval supply is forced by the coincidence of larval availability with favorable physical transport mechanisms.