Value of coastal habitats for exploited species: introduction to a theme set of articles

Many exploited fish and invertebrate species use coastal habitats during one or more life-history stages as spawning, feeding, and nursery areas; yet, the value of these habitats has not been adequately characterized. As habitat availability can be a bottleneck for many populations, concerns about habitat effects on exploited species have been increasing. We have compiled nine articles presenting the state of knowledge and future research priorities regarding the importance of habitat for exploited species. Reviews from European habitats and several geographical locations throughout the United States demonstrate the influence of coastal habitats on survival, growth, and movement, especially during the early life-history stages, in a wide variety of species, spatial scales, and habitats. Moreover, many of these species contribute substantially to commercial landings, highlighting the importance of coastal habitats to population persistence and fishery yields. Management of fishery species can also be enhanced through modelling efforts incorporating habitat. Finally, there is a need for more effort in quantifying population demographics rates, creating comprehensive habitat maps, and developing better census techniques for complex hard bottom habitats; thus, future work is needed on the value of coastal habitats for exploited species

Habitat complexity and benthic predator-prey interactions in Chesapeake Bay

In Chesapeake Bay, the soft-shell clam Mya arenaria (thin-shelled, deep-burrowing) exhibits population declines when predators are active, and it persists at low densities. In contrast, the hard clam Mercenaria mercenaria (thick-shelled, shallow-burrowing) has a stable population and age distribution. We examined the potential for habitat and predators to control densities and distributions of bivalves in a field caging experiment (Mya only) and laboratory mesocosm experiments (both species). In the field, clams exposed to predators experienced 76.3% greater mortality as compared to caged individuals, and blue crabs were likely responsible for most of the mortality of juvenile Mya. In mesocosm experiments, Mya had lower survival in sand and seagrass than in shell hash or oyster shell habitats. However, crabs often missed one or more prey items in seagrass, shell, and oyster shell habitats. Predator search times and encounter rates declined when prey were at low densities, likely due to the added cost of inefficient foraging; however, this effect was more pronounced for Mya than for Mercenaria. Mercenaria had higher survival than Mya in mesocosm experiments, likely because predators feeding on Mercenaria spent less time foraging than those feeding on Mya. Mya may retain a low-density refuge from predation even with the loss of structurally complex habitats, though a loss of habitat refuge may result in clam densities that are not sustainable. A better understanding of density-dependent predator-prey interactions is necessary to prevent loss of food-web integrity and to conserve marine resources

Probing the Universe with Weak Lensing

Gravitational lenses can provide crucial information on the geometry of the Universe, on the cosmological scenario of formation of its structures as well as on the history of its components with look-back time. In this review, I focus on the most recent results obtained during the last five years from the analysis of the weak lensing regime. The interest of weak lensing as a probe of dark matter and the for study of the coupling between light and mass on scales of clusters of galaxies, large scale structures and galaxies is discussed first. Then I present the impact of weak lensing for the study of distant galaxies and of the population of lensed sources as function of redshift. Finally, I discuss the potential interest of weak lensing to constrain the cosmological parameters, either from pure geometrical effects observed in peculiar lenses, or from the coupling of weak lensing with the CMB.Comment: To appear Annual Review of Astronomy and Astrophysiscs Vol. 37. Latex and psfig.sty. Version without figure, 54 pages, 73Kb. Complete version including 13 figures (60 pages) available on ftp.iap.fr anonymous account in /pub/from_users/mellier/AnnualReview ; file ARAAmellier.ps.gz 1.6 M

Diet Selectivity of Juvenile Blue Crabs (Callinectes sapidus) in Chesapeake Bay

Shallow coves in Chesapeake Bay have abundant food and serve as nursery grounds for juvenile blue crabs. In this study, we examined the relationships between the diet of very small (4-40 mm CW) juvenile blue crabs and the benthic infauna in shallow, unvegetated nursery coves. We compared infauna in benthic samples with gut contents of juvenile blue crabs from six shallow coves in each of two sub-estuaries (Rappahannock and York Rivers) in Chesapeake Bay, Virginia, USA. Benthic communities differed depending on river and location, with abundant clams in upriver regions and abundant polychaetes in downriver regions. Juvenile crabs, like adults, appeared to be opportunistic feeders, with gut contents including clams, amphipods, polychaetes, small crustaceans, plant matter, and detritus. There was a positive relationship between polychaetes in the benthic samples and in crab guts, suggesting that juvenile crabs are opportunistic feeders on polychaetes in the benthos. Moreover, Ivlev\u27s electivity index and foraging ratio showed that clams and polychaetes were selectively eaten at all locations. Alternatively, crabs selectively rejected amphipods. Crab densities corresponded positively with polychaete densities, which suggests that there may be bottom-up control of crab distributions and that food resources are important in nursery habitats

Effects Of Estuarine Acidification On An Oyster-Associated Community In New South Wales, Australia

Many of the features that make estuaries among the most productive natural systems on earth also make them prone to acidification. Understanding the effects of estuarine acidification on different components of an ecological community is an important step in identifying indicators of ecosystem degradation. This study examined the impact of estuarine acidification, as a result of acid sulfate soil runoff, on wild Sydney rock oysters Saccostrea glomerata and their associated epifaunal communities in estuaries experiencing acid sulfate soil runoff in New South Wales, Australia. The responses of oysters and their invertebrate epifaunal communities to chronic acidification (greater than 6 mo; represented by oyster source site conditions) were assessed by examining the differences in oyster communities associated with moderate acidification (3.5 km from the source of acidification) or low acidification (8.2 km from the source). Oysters from moderate-or low-acidification sites were transplanted to a site with high exposure to acidification (less than 3 km from the source) or back to their original source sites (control) to mimic episodic acidification (2 wk). Epifaunal mussels Xenostrobus securis and limpets Patelloida mimula showed a negative association with oyster mortality, suggesting that these communities are closely tied to oyster survival. Oyster-associated epifaunal communities exposed to both chronic and episodic acidification were significantly different from communities with low exposure. Epifaunal communities exposed to episodic acidification were significantly less diverse than the control. Spionid and syllid worms were significantly less abundant and the mussel X. securis was significantly more abundant on oysters with moderate exposure or chronic exposure to acidification, as compared with communities from areas with low exposure to acidification. The mussel X. securis and the snail Bembicium auratum were significantly less abundant in oyster communities that were exposed to episodic acidification, as compared with the control. In systems where community composition depends on a single habitat-forming species, maintaining resistance may rely on the ability of that species to persist in the face of environmental stress

Living Shorelines Support Nearshore Benthic Communities in Upper and Lower Chesapeake Bay

Human population growth and sea-level rise are increasing the demand for protection of coastal property against shoreline erosion. Living shorelines are designed to provide shoreline protection and are constructed or reinforced using natural elements. While living shorelines are gaining popularity with homeowners, their ability to provide ecological services (e.g., habitat provision and trophic transfer) is not well understood, and information is needed to improve coastal and resource management decision-making. We examined benthic community responses to living shorelines in two case-study subestuaries of Chesapeake Bay using a before-after control-impact study design. At Windy Hill, a bulkhead was removed and replaced by three tombolos, sand fill, and native marsh vegetation. At Lynnhaven, 25 m of eroding marsh shoreline was stabilized with coir logs, sand fill, and native marsh vegetation. Communities of large (\u3e 3 mm) infauna adjacent to living shorelines at both locations tended to increase in biomass by the end of the study period. Community compositions changed significantly following living shoreline construction at Windy Hill, reflecting a trend toward higher density and biomass of large bivalves at living shorelines compared to pre-construction. Increasing trends in density and biomass of clams and simultaneously decreasing density and decreasing trends in biomass of polychaetes suggest a transition toward stable infaunal communities at living shorelines over time, though longer-term studies are warranted