Nursery habitats

In marine environments, a nursery habitat is a subset of all habitats where juveniles of a species occur, having a greater level of productivity per unit area than other juvenile habitats (Beck et al. 2001). Mangroves, salt marshes and seagrass are typical nursery habitats for a range of marine species. Some species will use nonvegetated sites, such as Yellow Eyed Mullet, Blue Sprat and Flounder.

The nursery habitat hypothesis states that the contribution per unit area of a nursery habitat is greater than for other habitats used by juveniles for the species. Productivity may be measured by density, survival, growth and movement to adult habitat (Beck et al. 2001).

There are two general models for the location of juvenile habitats within the total range for a species which reflect life history strategies of the species. These are the Classic Concept: Juveniles and Adults in separate habitats. Juveniles migrate to adult habitat. General Concept: overlap of juvenile and adult habitats.

Some marine species do not have juvenile habitats, e.g. arthropods and scallops. Commonly fish, ells, some lobsters, blue crabs (and so forth) do have distinct juvenile habitats, whether with or without overlap with adult habitats.

In terms of management, use of the nursery role hypothesis may be limiting as it excludes some potentially important nursery sites. In these cases the Effective Juvenile Habitat concept may be more useful. This defines a nursery as that which supplies a higher percentage of individuals to adult populations.

Identification and subsequent management of nursery habitats may be important in supporting off shore fisheries and ensuring species survival into the future. If we are unable to preserve nursery habitats, recruitment of juveniles into adult populations may decline, reducing population numbers and compromising the survival of species for biodiversity and human harvesting.

In order to determine the nursery habitat for a species, all habitats used by juveniles must be surveyed. This may include kelp forest, seagrass, mangroves, tidal flat, mudflat, wetland, salt marsh and oyster reef. While density may be an indicator of productivity, it is suggested that alone, density does not adequately provide evidence of the role of a habitat as a nursery. Recruitment biomass from juvenile to adult population is the best measure of movement between the two habitats.

Consider also biotic, abiotic and landscape variability in the value of nursery habitats. This may be an important consideration when looking at which sites to manage and protect. Biotic factors include: structural complexity, food availability, larval settlement cues, competition, and predation. Abiotic: temperature, salinity, depth, dissolved oxygen, freshwater inflow, retention zone and disturbance. Landscape factors involve: proximity of juvenile and adult habitats, access to larvae, number of adjacent habitats, patch shape, area and fragmentation. The effects of these factors may be positive or negative depending on species and broader environmental conditions at any given time.

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