Ontogenetic changes in habitat use by postlarvae and young juveniles of the blue crab

Changing habitat requirements are evident during the developmental cycles of many species. In this field investigation, we attempted to distinguish between depth (shallow vs deep), habitat structure (seagrass species), and study site as factors influencing the distribution and abundance of postlarvae and juvenile blue crabs Callinectes sapidus in the Chesapeake Bay. Deep (greater than or equal to 70 cm mean low water [MLW]) and shallow (less than or equal to 50 cm MLW) suction samples in monospecific Zostera marina and Ruppia maritima beds were taken in the York River, a tributary of the Chesapeake Bay. Our studies revealed ontogenetic changes in habitat use, which suggested that blue crabs are influenced differently by physical and biological factors even during the earliest life stages. Postlarvae through 3rd instar distributions were not related to seagrass species, but their densities increased with distance upriver (regression, p \u3c 0.004, n = 36 to 38, postlarvae: r(2) = 0.173, 1st instars: r(2) = 0.308, 2nd-3rd instars: r(2) = 0.231). This suggests that the smallest instar distributions are related to larval supply and physical forces, such as currents and winds, which determine water-column transport. In contrast, 4th and greater instars were significantly more abundant in Ruppia than in Zostera (ANOVA, df = 1, p \u3c 0.05), possibly because of the high shoot density of Ruppia beds. Habitat use by 4th and greater instars may be related to seasonal changes in seagrass shoot density. Water depth did not influence the distribution of any crab stage. We suggest that habitat selection and differential mortality among habitats influence larger instar distributions more strongly than they influence the distribution of postlarvae and the earliest instars of C. sapidus

The roles of seagrasses in structuring associated fish assemblages and fisheries

Seagrasses are known to provide important habitats for a diversity of fish and fisheries species. Continued research has allowed us to re-evaluate the generalisations, and identify the gaps in our knowledge regarding these habitats, particularly in an Australian context. Seagrasses generally form part of a mosaic with other habitats within a seascape that contributes to its overall biodiversity of fish. Patterns of abundance and diversity of fish between seagrass and other habitats, such as unvegetated flats and reef habitats, is inconsistent and depends on the region, fish and seagrass species, and sampling method. Edge effects, adjacent habitats, and fragmentation can strongly influence fish assemblages. Seagrass structural complexity can enhance survival and growth of juvenile fishes, but recent studies show that survival rates of individual prey do not vary greatly across seagrass densities when densities of both prey and predators increase with seagrass density. The concept of the nursery habitat has been built on data from studies in estuaries or highly seasonal seagrass habitats, whereas recent studies in marine systems or cool temperate seagrass meadows suggest that this role does not always hold. Direct grazing on seagrasses by fishes occurs mainly in tropical regions, although there is a paucity of data on this process along with several other processes, from tropical Australia. Grazing on seagrasses by fishes appears to be limited in temperate regions, with consumption of seagrass restricted mainly to omnivorous species. However, tropicalisation, that is, the immigration of tropical grazers to higher latitudes due to global ocean warming, is predicted to increase grazing rates on temperate seagrasses. Reductions in seagrass biomass caused by increased grazing will disrupt connectivity processes between seagrass meadows and surrounding habitats, and are likely to have significant ramifications for the biodiversity and ecosystem services those other coastal habitats provide. Although other habitats rely on inputs of seagrass detritus, and the immigration of fish and fisheries species from their juvenile seagrass habitats, quantitative data on this link are limited. Evidence that fisheries declines, either directly or indirectly, have resulted from seagrass loss is equivocal to date, and therefore, the quantification of this role is still needed. Managing seagrass for fisheries is complex, and many fisheries agencies embrace ecosystem-based management, but do not have direct responsibility for seagrass habitat. Significant progress has been made in our knowledge of fish and fisheries in seagrasses, but our review highlights significant knowledge gaps where further research is recommended