The Role of Nearshore Ecosystems as Fish and Shellfish Nurseries

Summary

Coastal ecosystems provide many vital ecological and economic services, including shoreline protection, productive commercial and sport fisheries, and nutrient cycling. Key nearshore ecosystems such as seagrass meadows, marshes, and mangrove forests are particularly valued for their extremely high productivity, which supports a great abundance and diversity of fish as well as shrimp, oysters, crabs, and other invertebrates. Because of the abundance of juvenile fish and shellfish they contain, nearshore ecosystems are widely considered "nurseries." The nursery role of coastal estuaries and marine ecosystems is well accepted by scientists, conservation organizations, fisheries managers, and the public, and it is often cited to support protection and conservation of these areas.

Nonetheless, comparatively little money and effort is being directed at protecting and managing these ecosystems. Until recently, even fisheries managers have largely ignored the issue of identification and conservation of juvenile habitat. This neglect, combined with intense pressures from human activities, is causing continued decline in vital nearshore habitats. We believe a better understanding of habitats that serve as nurseries for marine species is needed to help prioritize the limited funding and effort available for their protection and management.

Based on the scientific evidence, we conclude that:

• The concept of nursery habitat has been poorly defined.
• Lack of a clear definition has hindered identification of valuable nursery habitats.
• There is variation between and within ecosystems in their value as nurseries, and the nursery value of seagrass meadows, wetlands, and other ecosystems varies geographically.
• Many ecosystems such as oyster reefs and kelp forests have been relatively unexamined as nurseries.
• A better understanding of the factors that create site-specific variability in nursery quality will help prioritize efforts to halt their decline.

We suggest as a testable hypothesis that a nearshore habitat serves as a nursery for juveniles of a particular fish or invertebrate species if it contributes disproportionately to the size and numbers of adults relative to other juvenile habitats. The disproportionate contribution to the production of adults can come from any combination of four factors: density, growth, and survival of juvenile animals, and their movement to adult habitats. We further suggest that in future research on putative nurseries:

• It is not sufficient to measure a single factor such as density of juveniles.
• Researchers must compare multiple habitats, and an area should be considered important nursery habitat only if it produces relatively more adults per unit of area than other juvenile habitats the species uses.
• Despite the difficulties, researchers must track the number of individuals that move from juvenile to adult habitats; this number is the best measure of nursery value.
• Researchers should examine the factors that contribute to local variations in the value of nursery habitat. For example, not all marshes function equally as nurseries. An understanding of local variations could also help to explain regional changes in the nursery value of some habitats.

Conservation and management organizations now commonly consider all seagrass meadows and wetlands as nurseries, an assumption that may hinder the protection of other ecosystems vital to the protection of marine biodiversity as well as commercial fishery stocks. In the past, management effort has often focused on the restoration of these ecosystems. Future research needs to be devoted to measuring whether restoration reinstates the functional value of ecosystems as nurseries. Currently, results of restoration efforts are equivocal at best. Where restoration and mitigation cannot be shown to return nursery value, more effort should be focused on conservation. Better research and a clearer understanding of nursery habitats will allow more efficient use of limited money, time, and effort in conservation and management and contribute to the development of true ecosystem-based management of coastal resources.