Mark H. Carr

PROPAGULE DISPERSAL DISTANCE AND THE SIZE AND SPACING OF MARINE RESERVES

This study compiled available information on the dispersal distance of the propagules of benthic marine organisms and used this information in the development of criteria for the design of marine reserves. Many benthic marine organisms release propagules that spend time in the water column before settlement. During this period, ocean currents transport or disperse the propagules. When considering the size of a marine reserve and the spacing between reserves, one must consider the distance which propagules disperse. We could find estimates of dispersal distance for 32 taxa; for 25 of these, we were also able to find data on the time the propagules spent dispersing. Dispersal distance ranged from meters to thousands of kilometers, and time in the plankton ranged from minutes to months. A significant positive correlation was found between the log-transformed duration in the plankton and the log-transformed dispersal distance ( r 5 0.7776, r 2 5 0.60, df 5 1, 25, P 5 0.000); the more time propagules spend in the water column the further they tend to be dispersed. The frequency distribution of the log-transformed dispersal distance is bimodal (kurtosis 52 1.29, t 52 4.062, P , 0.001) with a gap between 1 and 20 km. Propagules that dispersed ,1 km spent less time in the plankton (,100 h), or if they remained in the plankton for a longer period, they tended to remain in the waters near the bottom. Propagules that dispersed .20 km spent more than 300 h in the plankton. The bimodal nature of the distribution suggests that evolutionary constraints may reduce the likelihood of evolving mid-range dispersal strategies (i.e., dispersal between 1 and 20 km) resulting in two evolutionarily stable dispersal strategies: dispersal , 1k m or.;20 km. We suggest that reserves be designed large enough to contain the short-distance dispersing propagules and be spaced far enough apart that long-distance dispersing propagules released from one reserve can settle in adjacent reserves. A reserve 4-6 km in diameter should be large enough to contain the larvae of short-distance dispersers, and reserves spaced 10- 20 km apart should be close enough to capture propagules released from adjacent reserves.

MARINE RESERVES ARE NECESSARY BUT NOT SUFFICIENT FOR MARINE CONSERVATION

The intensity of human pressure on marine systems has led to a push for stronger marine conservation efforts. Recently, marine reserves have become one highly advocated form of marine conservation, and the number of newly designated reserves has increased dramatically. Reserves will be essential for conservation efforts because they can provide unique protection for critical areas, they can provide a spatial escape for intensely exploited species, and they can potentially act as buffers against some management miscalculations and unforeseen or unusual conditions. Reserve design and effectiveness can be dramatically improved by better use of existing scientific understanding. Reserves are insufficient protection alone, however, because they are not isolated from all critical impacts. Communities residing within marine reserves are strongly influenced by the highly variable conditions of the water masses that continuously flow through them. To a much greater degree than in terrestrial systems, the scales of fundamental processes, such as population replenishment, are often much larger than reserves can encompass. Further, they offer no protection from some important threats, such as contamination by chemicals. Therefore, without adequate protection of species and ecosystems outside reserves, effectiveness of reserves will be severely compromised. We outline conditions under which reserves are likely to be effective, provide some guidelines for increasing their conservation potential, and suggest some research priorities to fill critical information gaps. We strongly support vastly increasing the number and size of marine reserves; at the same time, strong conservation efforts outside reserves must complement this effort. To date, most reserve design and site selection have involved little scientific justification. They must begin to do so to increase the likelihood of attaining conservation objectives.

Designing marine reserve networks for both conservation and fisheries management

Marine protected areas (MPAs) that exclude fishing have been shown repeatedly to enhance the abundance, size, and diversity of species. These benefits, however, mean little to most marine species, because individual protected areas typically are small. To meet the larger-scale conservation challenges facing ocean ecosystems, several nations are expanding the benefits of individual protected areas by building networks of protected areas. Doing so successfully requires a detailed understanding of the ecological and physical characteristics of ocean ecosystems and the responses of humans to spatial closures. There has been enormous scientific interest in these topics, and frameworks for the design of MPA networks for meeting conservation and fishery management goals are emerging. Persistent in the literature is the perception of an inherent tradeoff between achieving conservation and fishery goals. Through a synthetic analysis across these conservation and bioeconomic studies, we construct guidelines for MPA network design that reduce or eliminate this tradeoff. We present size, spacing, location, and configuration guidelines for designing networks that simultaneously can enhance biological conservation and reduce fishery costs or even increase fishery yields and profits. Indeed, in some settings, a well-designed MPA network is critical to the optimal harvest strategy. When reserves benefit fisheries, the optimal area in reserves is moderately large (mode ≈30%). Assessing network design principals is limited currently by the absence of empirical data from large-scale networks. Emerging networks will soon rectify this constraint.

Guiding ecological principles for marine spatial planning

The declining health of marine ecosystems around the world is evidence that current piecemeal governance is inadequate to successfully support healthy coastal and ocean ecosystems and sustain human uses of the ocean. One proposed solution to this problem is ecosystem-based marine spatial planning (MSP), which is a process that informs the spatial distribution of activities in the ocean so that existing and emerging uses can be maintained, use conflicts reduced, and ecosystem health and services protected and sustained for future generations. Because a key goal of ecosystem-based MSP is to maintain the delivery of ecosystem services that humans want and need, it must be based on ecological principles that articulate the scientifically recognized attributes of healthy, functioning ecosystems. These principles should be incorporated into a decision-making framework with clearly defined targets for these ecological attributes. This paper identifies ecological principles for MSP based on a synthesis of previously suggested and/or operationalized principles, along with recommendations generated by a group of twenty ecologists and marine scientists with diverse backgrounds and perspectives on MSP. The proposed four main ecological principles to guide MSP--maintaining or restoring: native species diversity, habitat diversity and heterogeneity, key species, and connectivity--and two additional guidelines, the need to account for context and uncertainty, must be explicitly taken into account in the planning process. When applied in concert with social, economic, and governance principles, these ecological principles can inform the designation and siting of ocean uses and the management of activities in the ocean to maintain or restore healthy ecosystems, allow delivery of marine ecosystem services, and ensure sustainable economic and social benefits.

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...

Predation effects on early post-settlement survivorship of coral-reef fishes

Little is known of the sources of mortality that affect local population dynam~cs of coralreef fishes. To examine the role of predat~on, resident p~scivorous fishes [moray eels (Muraenidae), large squirrelfishes (Holocentridae), groupers (Serranidae), and snappers (Lutjanidae)] were removed from 3 of 6 isolated patch reefs of living coral near Lee Stocking Island, Bahamas, in 1992. All 6 reefs were then seeded with natural densities of newly settled recruits of 3 species: Chromis cyanea (blue chromis, Pomacentridae), Halichoerespictus (rainbow wrasse, Labridae), and Thalassoma bifasciatum (bluehead wrasse, Labridae). Controls showed that any secondary effects of transplanting new recruits were negligible. Over the next month, survivorship of C. cyanea (mean 41.3%) and H. pictus (80.8%) on the predator-absent reefs was significantly greater than on the predator-present (control) reefs (9.4% for C. cyanea and 43.2% for H. pictus). No statistical difference was evident for T bitasciatum (48.5 vs 37.8% survival), perhaps because juveniles of this species are cleaner fish and/or because they were less conspicuous to predators than the other species. Although the size distributions of the wrasses did not differ between treatments, the size distribution of C. cyanea shifted significantly. At the end of the experiment, surviv.ing C. cyanea were slightly larger on the predator-present reefs (mode = 4.0 cm total length, TL) than on the predator-absent reefs (mode = 3.5 cm TL), despite no significant difference at the start of the experiment (mode for both treatments = 3.0 cm TL). We interpret this size shift as predators differentially consuming smaller recruits and/or surviving recruits growing faster after densities were reduced by predators. Preliminary remote video monitoring of the activity of transient p~scivores [mostly jacks (Carangidae)] over the experimental reefs suggested that such predators may regularly visit isolated reefs in search of prey. If so, transient predators may have accounted for the surprisingly low first-month survivorship (about 40 to 80%) of new recruits on reefs where resident predators were removed. In any case, we conclude that resident predators can substantially alter the local density and size structure of reef fishes shortly after they settle from the plankton. Because piscivores differentially affected the survivorship of different species, predation may also influence the structure of reef-fish communities by altering the relative abundances of prey species established at the time of settlement. K E Y WORDS: Coral-reef fishes . Predat~on . Remuitment . Size distribution . Survivorship

EFFECTS OF MACROALGAL DYNAMICS ON RECRUITMENT OF A TEMPERATE REEF FISH

Spatial and temporal variation in the recruitment of dispersive life stages can strongly affect the distribution, dynamics, structure, and gene flow of local populations. I investigated the causes of variable larval recruitment to populations of a temperate reef fish, the kelp bass (Paralabrax clathratus). Spatial and temporal variation in the density of settlement habitat on a reef, the giant kelp (Macrocystis pyrifera), explained much of the variation in recruitment of kelp bass. Although recruitment was positively related to Macrocystis density, the relationship was asymptotic at high plant densities, indicating that kelp bass recruitment saturates at intermediate densities of Macrocystis (a 100- 130 stipes/ 30 M2). Field experiments demonstrated causation between Macrocystis density and kelp bass recruitment and indicated that kelp bass recruitment was linearly related to the local abundance of kelp structure (i.e., the number and biomass of overlapping Macrocystis blades). This linear (nonasymptotic) relationship between Macrocystis blade biomass and density of fish recruits indicated that both the quantity and quality of the recruitment habitat limited larval recruitment. Structural complexity per unit length of plant (e.g., blade biomass per unit plant length) was inversely related to plant density. This relationship underlies the asymptotic relationship between structural complexity per unit reef area (the product of per-plant structural complexity and plant density) and recruit density. Thus, density-dependent constraints on blade biomass in this macroalga, rather than larval supply, better explained the general asymptotic nature of the relationship between Macrocystis and kelp bass recruitment. These results indicate that density-dependent effects on plant biomass and architecture (e.g., plant height and structural complexity), common among both mac- rophytes and terrestrial plants, can greatly influence the relationship between plant density and recruitment of other species. For organisms with dispersive life stages whose local recruitment is influenced by the occurrence of living habitat structure, spatial and temporal variability characteristic of such biogenic habitats can be a major source of variation in their recruitment.

Artificial Reefs: The Importance of Comparisons with Natural Reefs

Abstract Methods used to evaluate the performance of an artificial reef will vary according to the purpose for which the reef was built. To determine how well artificial reefs mitigate losses due to human activities on natural reefs, the performance of artificial reefs should be evaluated using contemporaneous comparisons with relatively undisturbed natural reefs. Unfortunately, comparisons between artificial and natural reefs are typically confounded by differences in reef size, age, and isolation. We compared colonization and subsequent assemblage structure of reef fishes on coral and artificial (concrete block) reefs in which reef size, age, and isolation were standardized. Species richness and fish abundance (all species combined) were greater on reefs of natural rather than artificial structure, but substantial differences in species composition were not detected. Our results suggest that artificial reefs with structural complexity and other abiotic and biotic features similar to those of natural ree...

Habitat selection and recruitment of an assemblage of temperate zone reef fishes

Abstract Spatial and temporal patterns of recruitment are described for young-of-year of nine species of rockfish (Scorpaenidae, genus Sebastes ) in a giant kelp Macrocystis pyrifera (L.) C.Ag. forest off central California, USA. Though all nine species recruited during or soon after the months of maximum coastal upwelling, variation in timing of peak recruitment among species corresponded to the sequential parturition of pelagic larvae. Recruits of each rockfish species exhibited strong and significantly distinct habitat selection based on substratum type and relief, algal type, and vertical position in the water column. To determine the effect of habitat structure on the distribution and abundance of recruits, observations of habitat selection were made within an unmanipulated M. pyrifera forest and compared to sites where the presence of M. pyrifera was altered. Whereas variation in substratum type can contribute to spatial pattens of rockfish recruitment, the temporal dynamics of algal abundance, especially M. pyrifera , may strongly influence temporal, as well as spatial variability of rockfish recruitment. These results indicate that the structural composition of a reef, particularly the occurrence of M. pyrifera , strongly influences the magnitude and species composition of local recruitment of this rockfish assemblage.

The ecology of substrate-associated juveniles of the genus Sebastes

This paper reviews current knowledge of substrate-associated juvenileSebastes. In general, juvenile rockfish recruit to shallower depths than those occupied by conspecific adults. Habitat use by newly recruited rockfish differs markedly among species. While a few species recruit to shallow-dwelling macrophytes, such asZostera, many others recruit to larger brown algae, such asNereocystis andMacrocystis. A few deeper dwelling species recruit to low relief or soft substrata. However, little is known on recruitment of these deeper dwellers. Crustaceans are the major component of the diets of newly recruitedSebastes. Species which continue to forage in the water column shift to larger crustaceans (e.g. euphausids) and fish as they grow. Species which shift to substrate-associated prey soon begin to feed on larger algal-associated gammarid amphipods, shrimps and isopods. Field studies indicate variation in intraspecific growth rates over large geographic distances, among nearby sites and among algal habitats on the same reef, with food availability and water temperature being major factors in the differences. In particular, laboratory studies have shown that temperature is one of the most important factors in growth rates. Many species of juvenile rockfish make ontogenetic movements, often moving into deeper water as they age. Most seasonal movements appear to be related to changes in water temperature and turbulence.