Search

Assessment of marine protected areas (MPAs) requires the ability to quantify the effects of proposed MPA size and placement, habitat distribution, larval dispersal, and fishing on the persistence of protected populations. Here we describe a model-based approach to assessment of the contribution of a network of marine protected areas to the persistence of populations with a sedentary adult phase and a dispersing larval phase. The model integrates the effects of a patchy spatial distribution of habitat, the spatial scale of larval dispersal, and the level of fishing outside of reserves into a calculation of the spatial distribution of equilibrium settlement. We use the amount of coastline predicted to have equilibrium settlement rates that saturate post-settlement habitat as a response variable for the assessment and comparison of MPA network designs. We apply this model to a set of recently proposed MPA networks for the central coast of California, USA. Results show that the area of habitat set aside is a good predictor of the area over which population levels will be high for short-distance dispersers. However, persistence of longer distance dispersers depends critically on the spatial distribution of habitat and reserves, ranging from not persistent anywhere to persistent over a greater area than that set aside in reserves. These results depend on the mechanisms of persistence, with self-replacement supporting short-distance dispersers and network effects supporting long-distance dispersers. Persistence also depends critically on fishery status outside the MPAs, as well as how fishing effort is redistributed after MPA implementation. This assessment method provides important benchmarks, as well as a transparent modeling approach, for improving initial MPA configurations that may result from less-comprehensive rule- or habitat-based methods of designing MPAs.

Ecologists often point to excessive truncation of a population's size-structure as a deleterious effect of exploitation, yet the effect of this truncation on population persistence is seldom quantified. While persistence of marine populations requires maintenance of a sufficient level of lifetime reproduction, fishing reduces lifetime reproduction by increasing the total mortality rate, preventing individuals from growing old, large, and highly fecund. We employ a new method of estimating changes in lifetime egg production (LEP) using two samples of the size structure, one in the past and one current, to assess persistence of five species of nearshore rockfish (Sebastes spp.) in California and Oregon, U.S.A. Using length frequency data from catch in the recreational fishery, we estimate that since 1980, four of the five rockfish species considered have experienced declines in LEP to levels that suggest that persistence is impaired. When changes in LEP were estimated for subsets of the data corresponding to neighboring geographical regions, differences in LEP levels were apparent in the neighboring regions, implying that the effects of fishing mortality are not evenly distributed over space. We conclude by discussing the use of this estimation approach to assess the status of other species in data-poor situations.

Marine reserves are a spatial approach to marine management and conservation aimed at protecting and restoring multispecies assemblages and the structure and function of marine ecosystems. We used meta-analyses of published data to address the questions of how and over what time frames marine assemblages change within no-take marine reserves as they recover from fishing and other human uses. We used 20 studies of coastal fish assemblages from 31 temperate and tropical locations, reporting abundances, and in some cases biomass, of 10–134 species from reserve and reference conditions (i.e., conditions in nearby fished sites or before reserve establishment) spanning 1–25 years of protection. Synthesis of data from these diverse sets of assemblages showed that: (1) a species' level of exploitation, trophic level, and the duration of protection through the no-take reserve explain small but significant amounts of variation in individual species responses to protection, with only species that are targeted by fishing or by aquarium trade showing overall enhanced abundances in protected areas, and increasing positive effects of protection on abundances at top trophic levels through time; (2) up to a third of species in different studies (19% on average) appeared to be negatively affected by protection, indicating that indirect effects of protection through competitive or predatory interactions may be common; and (3) variation and lags in species responses to protection resulted in protected assemblages diverging from reference conditions, with greater proportions of total fish biomass at top trophic levels in protected compared to fished assemblages. These results indicate that marine reserves are effective in enhancing local abundances of exploited species and restoring the structure of whole communities, though these changes occur through a series of transient states and, for some communities, over long time frames (decades). In contrast with the more predictable increases of aggregate community variables such as total abundance and biomass, individual species and community structure exhibited broad variation in their responses to protection. Marine protected areas represent multiple human-exclusion “experiments,” replicated in a variety of ecosystem types and geographic locations, providing key insights on community-wide impacts of the removal of human extraction. Long-term monitoring of community trajectories in marine protected areas and modeling studies scaling up local effects to relevant spatial and temporal scales are needed to increase our ability to protect and restore whole marine systems and to set realistic targets for the conservation and restoration of specific assemblages.

There has been widespread concern that reduction in density of broadcast spawners by a fishery can have severe effects on reproduction. As fishing decreases density, fertilization efficiency declines, so that reproduction declines more rapidly than indicated by density, producing an Allee effect. However, this potentially precipitous decline in reproduction with declining density or stock size is not well understood at the population level. We used a simple deterministic model of the reproductive dynamics of a broadcast spawning invertebrate to show that the abrupt threshold often proposed as the form of this Allee effect is possible only when individuals are evenly spaced, and when sperm dispersal distribution is a constant out to a specified distance. When the spatial distribution is random and the sperm dispersal distribution is either Gaussian or exponential, fertilization success is linear up to 100%; thus, zygote production is quadratic up to this density. As the width of the dispersal function decreases, fertilization reaches 100% at higher densities, so that it may be beyond the highest density experienced by the population. In that case, zygote production is functionally linear. We then used a simulation model to assess the effects of aggregative behavior and stochastic variability in spatial patterns. We compared narrow and broad sperm dispersal distributions that were based on empirically determined fertilization of the green sea urchin, Strongylocentrotus droebachiensis, under two flow regimes. Zygote production exhibited a gradual nonlinear Allee effect as density decreased for the broader gamete dispersal distribution, but a functionally linear relationship with a much lower slope for the narrower sperm dispersal distributions. Increasing aggregation increased the slope of both fertilization and zygote production vs. density at the origin, but they remained linear for the narrower distribution. High interannual variability in zygote production due to different spatial configurations suggested that the geometric mean of recruitment is a better indicator of population consequences than the arithmetic mean, and it displays a sharper threshold than the arithmetic mean. Our results imply that potential Allee effects at low density in broadcast spawning populations rarely exhibit threshold behavior, and that these Allee effects may be too weak to be detected in stock–recruitment relationships.

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.

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 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.

Recovery plans for endangered and threatened species will not benefit conservation efforts unless prescribed tasks are actually implemented. We analyzed data collected on task implementation in early 1999 and found that an average of 70.3% of recovery tasks were either partially or completely implemented, although they ranged from 0% to 100% implemented. Plans approved in 1990 or earlier had relatively uniform task implementation, whereas implementation of more recent plans varied with plan and species attributes. Among plans approved after 1990, multi-species plans had lower task implementation than single-species plans. Recovery plans for plants, terrestrial species, and insular species had a lower degree of task implemention than those for animals, aquatic species, and continental species, respectively. Analyses further indicated that species with: (1) critical habitat designation, (2) conflict designation, (3) revised recovery plans, (4) a recovery coordinator, and (5) a dedicated database had greater task implementation than species lacking these management features. These results suggest that multi-species plans are implemented more slowly than single-species plans, and that recovery plans for species with greater public or agency profiles (as evidenced by critical habitat designation, conflict designation, and plan revision) are implemented at a higher rate. The effect of administrative strategies on higher rates of task implementation indicate that recovery efforts should include a recovery coordinator and database whenever possible. When developing recovery plans, responsible agencies should explicitly consider attributes of species and plans that influence task implementation.

For the endangered fish totoaba (Totoaba macdonaldi), as for other endangered species, the relative amount of environmental variability in demographic rates at various life stages is uncertain. Because of the ontogenetic migratory pattern of this species, four life history stages, (a) prerecruits, (b) juveniles, (c) pre-adults, and (d) adults, experience different environmental conditions. We used a diffusion equation approximation of random Leslie matrices to explore the effects on extinction risk of environmental variability in each of these life stages and correlation between stages. Variability in pre-adults and adults had a greater effect on the probability of extinction than did variability in prerecruits and juveniles. In general, correlation between stages increases the rate of increase of the variance of the logarithm of abundance and, hence, the probability of extinction. For example, when all life stages vary coherently, the rate of increase of the variance is the square of a weighted sum of elasticities, while when all stages vary independently, the rate of increase is the same weighted sum of the squared elasticities. The maximum extinction rate occurs when all life history rates in a Leslie matrix vary coherently. Specifically, when the coefficient of variation is the same in each parameter, the rate of increase of the standard deviation of the logarithm of abundance equals the coefficient of variation of the environmental variability. We also evaluated the accuracy of the diffusion equation approximation by comparing predicted extinction rates with those from Monte Carlo simulations of totoaba with variability in each of the four stages. The diffusion equation approximation accurately predicted probabilities of extinction in all cases except one, random variability in recruitment, where it underestimated simulation results substantially. This is apparently due to the large random jumps in abundance in this case. The diffusion equation approximation accurately predicted quasi-extinction of the adult portion of the population.