Stable isotope analyses (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) of the trophic relationships of<Emphasis Type="Italic">Callinectes sapidus</Emphasis> in two North Carolina estuaries

The present study focused on detecting variations in trophic relationships among blue crab (Callinectes sapidus) consumers according to water quality along two estuaries in North Carolina. Stable isotope (δ¹⁵N and δ¹³C) analyses of particulate organic matter and bivalve(Rangia cuneata andCorbicula fluminea) food sources were examined in combination with an Isosource mixing model. Results suggest that blue crab δ¹³C values increased significantly with increasing salinity from upper to lower sites along the Neuse River estuary (NRE; R2 = 0.87, p < 0.01) and Alligator River estuary (R² = 0.92, p < 0.01). There was a positive relationship between blue crab δ¹⁵N values and nitrate concentrations for the NRE (R² = 0.48, p = 0.12). This study found that blue crab δ¹³C values increased with salinity from upper to lower regions along both estuaries. Results suggest that blue crab production may have used alternative food sources that were isotopically (δ¹³C) depleted, especially in the upper NRE, and enriched sources in the mid to lower regions of both estuaries. Consumers sampled from the upper NRE may be influenced by higher nitrogen input from urban land use and municipal wastewater.     

Distribution and abundance of early life history stages of the blue crab,Callinectes sapidus,in tidal marsh creeks near Charleston,South Carolina

A 16-month study of estuarine habitats in poly-, meso-, and oligohaline salinity regimes near Charleston Harbor assessed the distribution and abundance of megalopae and early crab stages of the blue crab,Callinectes sapidus. Blue crab were sampled with a plankton net and a cylindrical drop sampler. Blue crab were most abundant in plankton collections at night, accounting for 68% of the megalopae and over 88% of the juveniles collected in day and night tows combined. At night, densities of megalopae were greatest in surface samples, whereas densities in daylight collections were greater on the bottom. Juvenile densities were greatest on the bottom in both day and night collections, although catch rates at night were more variable than those of the megalopae. This suggests that megalopae, and possibly juvenile stages, experience a diel vertical migration. Results indicate that ingress to estuarine nursery areas occurs at the megalopal stage. Megalopal densities were highest at the polyhaline site, while juvenile blue crab were most abundant in the oligohaline area. Habitat utilization by juvenile blue crab was estimated using a cylindrical drop sampler and Venturi suction pump on three bottom types in the intertidal zone. Densities were greatest over the sandy-mud substrate, although catch rates were much lower than those reported for other geographical areas. These results suggest that juvenile blue crab do not occur in abundance on the marsh surface but remain on the creek bottom, possibly because creek physiography and large tidal amplitudes may restrict accessibility to the marsh surface.     

Salinity Variability and Water Exchange in Interconnected Estuaries

A high-resolution coastal ocean model is used to investigate salinity variability and water exchange in a complex coastal system off the southern U.S. characterized by three adjacent sounds that are interconnected by a network of channels, creeks, and intertidal areas. Model results are generally highly correlated with observations from the Georgia Coastal Ecosystem Long Term Ecological Research (GCE-LTER) program, revealing a high degree of salinity variability at the Altamaha River and Doboy Sound, decreasing sharply toward Sapelo Sound. A Lagrangian particle tracking method is used to investigate local residence time and connectivity in the system. Local residence time is highly variable, increasing with distance from the Altamaha River and decreasing with river flow, revealing that discharge plays a dominant role on transport processes and estuary-shelf exchange. The Altamaha River and Doboy Sound are connected to each other in all seasons, with exchange occurring both via coastal and estuarine pathways. While particles released at the Altamaha and Doboy rarely reach Sapelo Sound, particles released at Sapelo Sound and the creeks surrounding its main channel can reach the entire estuarine system.     

Estuarine production of juvenile dungeness crab (cancer magister) and contribution to the Oregon-Washington coastal fishery

Estuaries provide nursery habitat for juvenile stages of several commercial decapod crustaceans worldwide, and those in the Northeastern Pacific are viewed as providing this function for Dungeness crab,Cancer magister. It is difficult to ascertain the degree to which such estuarine production of juveniles eventually contributes to coastal adult populations and fisheries since there are no direct surveys of adult abundance. As other authors have done, we used fishery landings data to compute the long-term average contribution of 1 + juvenile crab populations reared in estuaries to future coastal fisheries. We focused on Oregon and Washington states, but grouped landings in two large geographic zones by combining fishery ports as adjacent to Large Estuarine Zones (LEZ; Grays Harbor and Willapa Bay, Washington, and both sides of the Columbia River) and Small Estuarine Zones (SEZ; all other ports in Oregon). Mortality estimates were used to reduce 1 + crab abundance to surviving legal males, and portrayed as percent of the fisheries. Trends in the SEZ indicate that an average of only about 5–7% of estuarine production adds to the coastal adult population and contributes about $0.7 million to the fishery. The contribution is 25–30% in the LEZ (but may be higher since interannual density varies up to 5 times) and is worth about $3.9 million based on present ex-vessel value. Analyses of crab distribution and density indicate that the majority of an estuarine population (50–80%) is located in lower side channels (LSC) in spring and summer where temperature is higher and prey within and on adjacent intertidal flats is high. The potential average dollar value of equivalent legal male crab produced from the juvenile population is about $180 ha^?1 in LSC (but $280 ha^?1 in Grays Harbor where long-term density is highest), and lower in other estuarine habitats ($50–100 ha^?1). Estuarine juvenile production provides a relatively stable source of recruits to coastal adult populations, and large systems in the LEZ are important nurseries. Since direct coastal settlement of larvae does occur but is highly variable, the estuarine contribution may be especially important when physical forcing or unusual events lead to low survival of the coastal 0+ cohort. An unusually long period of very low landings in the LEZ from 1981–1987 is interpreted in light of the Mount St. Helens eruption (1980) and subsequent transport and deposition of very fine silt fractions over much of the LEZ nearshore shelf that may have adversely affected several year classes of small, early benthic phase juveniles at that time.     

Temperature-, Salinity-, and Size-Dependent Winter Mortality of Juvenile Blue Crabs (<Emphasis Type="BoldItalic">Callinectes sapidus</Emphasis>)

The blue crab, Callinectes sapidus, is an ecologically and economically valuable species in Chesapeake Bay. Field surveys and laboratory experiments indicate that blue crab mortality is significant during severe winters. We applied a temperature and salinity-dependent survival model to empirical temperature and salinity data to explore spatial and interannual patterns in overwintering mortality. Harmonic regression analysis and geostatistical techniques were used to create spatially explicit maps of estimated winter duration, average temperature, average salinity, and resulting crab survival probability for the winters of 1990–2004. Predicted survival was highest in the warmer, saline waters of the lower Bay and decreased with increasing latitude up bay. There was also significant interannual variation with survival being lowest after the severe winters of 1996 and 2003. We combine the survival probability maps with maps of blue crab abundance to show how winter mortality may reduce blue crab abundance prior to the start of the harvesting season.     

Analysis of tidal marsh vegetation patterns in two Georgia estuaries using aerial photography and GIS

Aerial photographs and GIS analysis were used to map the distribution of tidal marsh vegetation along the salinity gradients of the estuaries of the Altamaha and Satilla Rivers in coastal Georgia. Vegetation maps were constructed from 1993 U.S. Geological Survey Digital Orthophoto Quarter Quads, 1∶77,000-scale color infrared photographs taken in 1974 and 1∶24,000-scale black and white photographs taken in 1953, Changes between years were identified using a GIS overlay analysis. Four vegetation classifications were identified and groundtruthed with field surveys: salt marsh (areas containing primarilySpartina alterniflora), brackish marsh (Spartina cynosuroides andS. alterniflora), Juncus (Juncus roemerianus), and fresh marsh (Zizania aquatica, Zizaniopsis miliacae, and others). There was no evidence for an upstream shift in marsh vegetation along the longitudinal axis of either estuary over the time frame of this analysis, which implies there has not been a long-term increase in salinity. Although the inland extent of each marsh zone was further upstream in the Satilla than the Altamaha, they corresponded to similar average high tide salinities in each estuary: areas classified as salt marsh occurred from the mouth up to where average high tide salinity in the water was approximately 15 psu;Juncus ranged from 21 to 1 psu; brackish marsh ranged from 15 to 1 psu; and fresh marsh was upstream of 1 psu. Approximately 63% of the 6,786 ha of tidal marsh vegetation mapped in the Altamaha and 75% of the 10,220 ha mapped in the Satilla remained the same in all 3 yr.Juncus was the dominant classification in the intermediate regions of both estuaries, and shifts between areas classified asJuncus and either brackish or salt marsh constituted the primary vegetation change between 1953 and 1993 (87% of the changes observed in the Altamaha and 95% of those in the Satilla). This analysis suggests that the broad distribution of tidal marsh vegetation along these two estuaries is driven by salinity, but that at the local scale these are dynamic systems with a larger number of factors affecting the frequently changing borders of vegetation patches.     

Time series of the abundance of the post-larvae of the crabsCancer magister andcancer spp. on the Southern Oregon coast and their cross-shelf transport

Using stationary zooplankton nets that fished the tidal current we measured the daily abundance ofCancer crab megalopae near the mouth of Coos Bay, Oregon, during the 1997 spring settlement season. During the spring of 1997, the coastal waters were dominated by a significant El Niño event. Sea surface temperatures (SST) were higher than normal, upwelling indices were an order of magnitude smaller than during the two previous springs, and upwelling favorable winds were weak. Daily catches ofCancer magister megalopae ranged from 0 to 78 with 61% of the total catch occurring during four pulses. Peak catches tended to occur every 13.6 d close to 13.8 d average period between spring tides. Significant cross correlations were found between the maximum daily tidal range and the catch ofC. magister megalopae; large catches tended to occur 4 to 7 d after the spring tide. Daily catches ofCancer oregonensis andCancer productus ranged from 0 to 307 with catch significantly positively cross correlated to the maximum daily tidal range at a lag of ?5 days suggesting that the largest catches tended to occur after the spring tides. We hypothesize that a tidally-generated phenomenon internal waves, transportedCancer megalopae shoreward and caused the observed variation in their abundance in Coos Bay.     

Foraging by Two Estuarine Fishes, <Emphasis Type="Italic">Fundulus heteroclitus</Emphasis> and <Emphasis Type="Italic">Fundulus majalis</Emphasis>, on Juvenile Asian Shore Crabs (<Emphasis Type="Italic">Hemigrapsus sanguineus</Emphasis>) in Western Long Island Sound

The Asian shore crab, Hemigrapsus sanguineus, is a recent and particularly successful introduction to the east coast of the USA. Little research has been done on the utilization of Asian shore crabs for food by native species, a potential form of biocontrol. Over a 4-year period, we examined the gut contents of cogeners, Fundulus heteroclitus and Fundulus majalis, collected from two embayments in western Long Island Sound for the presence of juvenile H. sanguineus. Frequency (percent) of occurrence of food items in the guts of both species varied over year and study site. Asian shore crabs were consumed more often by F. heteroclitus than by F. majalis, but predation pressure by both species was low. Only 13% of F. heteroclitus and 7.7% of F. majalis found with food in their guts had ingested Asian shore crab remains. Of those, 1/3 had consumed whole crabs; the rest had only autotomized appendages in their guts. The mean carapace width of juvenile Asian shore crabs ingested by F. heteroclitus was 3.59 ± 2.22 mm (N = 33). Results of our study on killifish predation support the hypothesis that H. sanguienus abundance is partly explained by reduced impact of native predators (i.e., the “enemy release hypothesis”). Predation pressure of other potential enemies on both planktonic and benthic stages of the Asian shore crab must be investigated, however, to understand the full impact of predation on H. sanguineus population dynamics.     

Dispersion and recruitment of crab larvae in the Chesapeake Bay plume: Physical and biological controls

As part of the Microbial Exchanges and Coupling in Coastal Atlantic Systems (MECCAS) Project, crab larvae were collected in the shelf waters off Chesapeake Bay in June and August 1985 and April 1986. We conducted hydrographic (temperature, salinity, nutrients) and biological (chlorophyll, copepods) mapping in conjunction with Eulerian and Lagrangian time studies of the vertical distribution of crab larvae in the Chesapeake Bay plume. These abundance estimates are used with current meter records and drifter trajectories to infer mechanisms of larval crab dispersion to the shelf waters and recruitment back into Chesapeake Bay. The highest numbers of crab larvae were usually associated with the Chesapeake Bay plume, suggesting that it was the dominant source of crab larvae to shelf waters. Patches of crab larvae also were found in the higher salinity shelf waters, and possibly were remnants of previous plume discharge events. The distribution of crab larvae in the shelf waters changed on 1–2 d time scales as a consequence of both variations in the discharge rate of the Chesapeake Bay plume and local wind-driven currents. Downwelling-favorable winds (NW) intensified the coastal jet and confined the plume and crab larvae along the coast. In April during a downwelling event (when northwesterly winds predominated), crab zoeae were transported southward along the coast at speeds that at times exceeded 168 km d⁻¹. During June and August the upwelling-favorable winds (S, SW) opposed the anticyclonic turn of the plume and, via Ekman circulation, forced the plume and crab larvae to spread seaward. Plume velocities during these conditions generally were less than 48 km d⁻¹. The recruitment of crab larvae to Chesapeake Bay is facilitated in late summer by the dominance of southerly winds, which can reverse the southward flow of shelf waters. Periodic downwelling-favorable winds can result in surface waters and crab larvae moving toward the entrance of Chesapeake Bay. Approximately 27% of the larval crabs spend at least part of the day in bottom waters, which have a residual drift toward the bay mouth. There appears to be a variety of physical transport mechanisms that can enhance the recruitment of crab larvae into Chesapeake Bay.     

Habitat use,temporal abundance variability,and diet of blue crabs from a New Jersey estuarine system

In a long-term, spatially comprehensive beam trawl survey of the Navesink River-Sandy Hook Bay estuary, the blue crabCallinectes sapidus was one of the most abundant species. Seasonal changes in abundance were evident, with low abundances in summer followed by peak abundances in the fall, after juveniles recruited to the estuary. We saw no long-term trends in abundance during the 5 yr study. Location in the navesink River or Sandy Hook Bay explained most of the variance in abundance within any one survey. In diet analyses, we found evidence of cannibalism in all seasons, but in the size range of crabs caught in this study (10–180 mm), we did not find a relationship between cannibalism and juvenile crab abundance. Within surveys, crabs divided into 20 mm size categories showed no sizerelated differences in location within the estuary or among 7 habitat types examined (algae bed, amphipod bed, beach, channel, marsh edge, mid-depth, and sandbar). Channels and sandbars tended to exhibit lower crab abundance than other habitats. Shallow habitats with and without cover were equally preferred by juvenile blue crabs, implying that the presence of structure was not critical. Spatial models of crab abundance (<- 80 mm carapace width) to environmental data were fit from several seasons of intensive sampling in the Navesink River-Sandy Hook Bay estuary between summer 1996 and spring 1998. These models indicated that fine-grained sediments, tmmperature, depth, and salinity were good indicators of crab abundance in spring, summer, and fall. Using these spatial models and environmental data collected in subsequent seasons (summer 1998−fall 1999), we were able to predict blue crab abundance in the river as evidenced by significant correlations between predicted and observed abundances. For the size range of crabs examined here, physical conditions may be as important as structural habitat types or cannibalism in determining habitat use in northerly estuaries.     

Distribution of Diatoms and Development of Diatom-Based Models for Inferring Salinity and Nutrient Concentrations in Florida Bay and Adjacent Coastal Wetlands of South Florida (USA)

The composition and distribution of diatom algae inhabiting estuaries and coasts of the subtropical Americas are poorly documented, especially relative to the central role diatoms play in coastal food webs and to their potential utility as sentinels of environmental change in these threatened ecosystems. Here, we document the distribution of diatoms among the diverse habitat types and long environmental gradients represented by the shallow topographic relief of the South Florida, USA, coastline. A total of 592 species were encountered from 38 freshwater, mangrove, and marine locations in the Everglades wetland and Florida Bay during two seasonal collections, with the highest diversity occurring at sites of high salinity and low water column organic carbon concentration (WTOC). Freshwater, mangrove, and estuarine assemblages were compositionally distinct, but seasonal differences were only detected in mangrove and estuarine sites where solute concentration differed greatly between wet and dry seasons. Epiphytic, planktonic, and sediment assemblages were compositionally similar, implying a high degree of mixing along the shallow, tidal, and storm-prone coast. The relationships between diatom taxa and salinity, water total phosphorus (WTP), water total nitrogen (WTN), and WTOC concentrations were determined and incorporated into weighted averaging partial least squares regression models. Salinity was the most influential variable, resulting in a highly predictive model (r _apparent² = 0.97, r _jackknife² = 0.95) that can be used in the future to infer changes in coastal freshwater delivery or sea-level rise in South Florida and compositionally similar environments. Models predicting WTN (r _apparent² = 0.75, r _jackknife² = 0.46), WTP (r _apparent² = 0.75, r _jackknife² = 0.49), and WTOC (r _apparent² = 0.79, r _jackknife² = 0.57) were also strong, suggesting that diatoms can provide reliable inferences of changes in solute delivery to the coastal ecosystem.     

Induction of Metamorphosis in the Asian Shore Crab <Emphasis Type="Italic">Hemigrapsus sanguineus</Emphasis>: Effects of Biofilms and Substratum Texture

Since its initial invasion in the late 1980s, the Asian shore crab Hemigrapsus sanguineus has spread over 650 km along the Atlantic Coast of the United States. Larvae of this species can live in the water column for over 3 weeks, which provides ample time for transport to new locations. Previous research on the settlement and metamorphosis of H. sanguineus showed that postlarvae (megalopae) accelerate metamorphosis in response to very specific water-soluble cues. However, the response to substratum cues appears to be much broader. The experiments presented here were designed to test the metamorphic response to biofilms and substratum texture. We found that nylon mesh with pore size of 100 and 1,000 μm had a greater effect on mean time to metamorphosis than 10 and 2,000 μm mesh. Results also showed a positive response to natural rock substrata, and this response was most pronounced when megalopae were exposed to substrata with rough texture. In addition, we found that a combination of a textured surface and a natural biofilm accelerated metamorphosis more than either an abiotic substratum or a smooth surface covered with natural biofilm. A final experiment compared the effect of textural cues to that of a water-soluble cue found in exudate from adult H. sanguineus. Results indicated that both types of cue-induced metamorphosis to the same extent.     

Settlement patterns of brachyuran megalopae in a Louisiana estuary

Artificial substrate collectors were used to sample settled brachyuran megalopae in Terrebonne Bay, Louisiana, northern Gulf of Mexico. Three taxa,Callinectes sapidus, Uca spp., andRhithropanopeus harrisii, were the most common settlers. Settlement occurred almost year-round, although settlement densities were highest in late summer and early fall. In 1990, all three taxa had several simultaneous settlement peaks; in 1991, one simultaneous settlement peak occurred. FewerC. sapidus andUca spp. settled in 1991 than in 1990, but settlement abundance ofR. harrissii was similar between years. Local weather events, such as heavy precipitation, may have influenced the lower abundances ofC. sapidus andUca spp. in 1991. The megalopae ofC. sapidus andUca spp., which re-invade the estuary from higher salinity offshore waters where larval development occurs, may have been more affected by the lower 1991 estuarine salinity values than the larvae ofR. harrissii, which are retained within the estuary throughout larval development. Settlement ofC. sapidus andUca spp. was correlated with maximum tidal height, tidal amplitude, salinity, temperature, and lunar phase. In 1990, settlement ofC. sapidus was correlated with lunar declination cycles; settlement peaks occurred during equatorial minimum amplitude tides. The settlement ofR. harrisii was associated with tidal amplitude and lunar phase. Associations between environmental variables and settlement of megalopae were not consistent between the two years sampled. The opposing or reinforcing effects of various environmental variables on settlement, e.g., tidal height and rainfall, are superimposed on long-term hydrologic cycles so that dramatically altered cycles of settlement occur among years.     

The impact of yellow shore crabs,<Emphasis Type="Italic">Hemigrapsus oregonensis</Emphasis>, on Early Benthic Phase dungeness crabs,<Emphasis Type="Italic">Cancer magister</Emphasis>, in intertidal oyster shell mitigation habitat

Dungeness crab (Cancer magister) megalopae recruit to northeastern Pacific coastal estuaries, and settle into intertidal and subtidal habitats where they molt into Early Benthic Phase (EBP) crabs, and are dependent on epibenthic structure for shelter from predation. Given the importance of shell refuge to their post-settlement ontogeny, the U.S. Army Corps of Engineers began constructing intertidal plots of oyster shell in the Grays Harbor estuary, Washington, to enhance recruitment and mitigate losses of subtidalC. magister entrained and killed during extensive dredging efforts. When shell habitat was newly constructed, settlement and survival ofC. magister were high, and expectations for the mitigation project were met. During the first several years (1992-1997), plots greater than 1 yr postconstruction were colonized by yellow shore crabs,Hemigrapsus oregonensis (often≥75 crabs m⁻², and abundance of EBPC. magister was reduced to nearly zero. While some predation of settling megalopae byH. oregonensis does occur, the relationship between these species is characterized by density-dependent competitive interactions. Laboratory observations of competition for shell habitat indicate thatH. oregonensis are dominant over EBPC. magister and can evictC. magister from refuge spaces. Field experiments show that high densities of the former cause, the latter to emigrate from shell, and suggest detection and avoidance of areas with high densities ofH. oregonensis by settlingC. magister megalopae. More recently (1998-2001), abundance ofH. oregonensis has declined dramatically within plots of oyster shell, apparently due to recruitment failure, and patterns ofC. magister abundance and production have returned to levels consistent with original expectations of the mitigation project. Both intraspecific and interspecific competition for space are significant factors effecting population, regulation ofC. magister when they are strongly dependent on refuge from predation. Efficacy of constructed oyster shell plots as a mitigation tool may hinge on the population dynamics of a species previously considered of little consequence to the target species.     

Ingress of Brachyuran Crab Post-larvae to the Newport River Estuary

Planktonic larvae of estuarine species often develop in the coastal ocean and return to estuaries using favorable currents. This study investigated spatial distributions of brachyuran crab post-larvae during ingress to the Newport River estuary, North Carolina, USA (34°41′ N, 76°40′ W). Nearshore plankton tows were conducted across the inlet to the estuary. Settlement on passive ‘hog’s hair’ collectors was simultaneously monitored in each of four estuarine channels. Callinectes sapidus density was highest east of the inlet, whereas relative estuarine abundance was higher in western channels. In separate sampling with collectors at coastal and estuarine locations, spatial distributions of post-larvae were consistent through time but differed for C. sapidus, Uca spp., and Pachygrapsus transversus. The diel timing of C. sapidus settlement on collectors was determined at the coast and compared to previous studies of settlement in the estuary. Behavioral responses to environmental cues may alter transport pathways from those predicted by hydrodynamic models.     

The influence of introduced European green crabs (<Emphasis Type="Italic">Carcinus maenas</Emphasis>) on habitat selection by juvenile native blue crabs (<Emphasis Type="Italic">Portunus pelagicus</Emphasis>)

The European green crab (Carcinus maenas) is a highly successful marine invader, having established populations in a number of areas outside its natural range in the last 100 years. In South Australia,C. maenas can be abundant on intertidal mud flats, which are used by juveniles of the native blue swimmer crab (Portunus pelagicus) and could have the potential to cause substantial negative effects on this species. The influence of adult blue and green crabs on habitat selection by juvenile blue crabs was tested to determine if they responded to both predators in a similar fashion. The presence of predators did not influence habitat selection by juvenile blue crabs in either laboratory or field experiments, but juvenile behavior in the selected habitat did differ between the two adult species. Many more crabs buried themselves beneath the substrate when adult conspecifics were present than when adult green crabs were present. Burying in the presence of adult green crabs was no more frequent than when predators were absent. It remains to be determined if this makes juvenile blue crabs more vulnerable to predation by green crabs than by adults of their own species, or if the difference in response is because green crabs pose a different or lesser threat.     

Effects of environmental stress on blue crab (<Emphasis Type="Italic">Callinectes sapidus</Emphasis>) harvests in Chesapeake Bay tributaries

Trotline blue crab (Callinectes sapidus) fishermen in the Patuxent, Chester, and Choptank tributaries of Chesapeake Bay set their gear in areas where the water quality, characterized by average mid-channel bottom dissolved oxygen, varies across the river systems and over the fishing season. Two harvest production models are developed to capture the potential effects of this source of environmental stress. One model treats the impact of water quality as influencing the availability of crabs to the gear, while the second treats the impact as another source of mortality. Both models are estimated assuming that dissolved oxygen has no effect on crab harvests above an upper threshold of 5 mg I⁻¹. Contrary to the mortality model, the availability model produces estimates that are consistent with our prior beliefs that productivity will be negatively impacted by poor water quality. To determine the percentage of the available crab stock in an area that will be harvested by a given amount of gear under different water quality conditions a simulation of the availability model is developed. The simulations show that a decline in average mid-channel bottom dissolved oxygen in the Patuxent River to 4 mg I⁻¹ may lead to a 48% decline in the percentage of the blue crab population in the area that will be harvested with the same amount of fishing effort.     

Interactions of Salinity,Marsh Fragmentation and Submerged Aquatic Vegetation on Resident Nekton Assemblages of Coastal Marsh Ponds

Increases in relative sea level are fragmenting the emergent vegetation of Louisiana’s coastal marshes. Nekton abundance is likely impacted by salinity and whether emergent vegetation is replaced by submerged aquatic vegetation (SAV) or open water. To assess these effects, we sampled nekton densities along a salinity gradient (categorized as freshwater, intermediate, and brackish marsh) in fragmented and non-fragmented areas. Total nekton density increased strongly with SAV in brackish marsh but only weakly in freshwater marsh (F _2,238 = 10.03, p < 0.0001). Freshwater and intermediate marshes had higher nekton densities when fragmented than when non-fragmented; this relationship was reversed in brackish marsh (F _2,238 = 8.89, p = 0.0002). Fragmentation, SAV, and salinity interacted to affect the densities of Gambusia affinis, Poecilia latipinna, Cyprinodon variegates, and Lucania parva. Our results suggest that the presence of both emergent vegetation and SAV was necessary for maintaining high nekton densities, with this combination being especially important in brackish marshes.     

Ocean distribution of dungeness crab megalopae and recruitment patterns to estuaries in Southern Washington State

We investigated the distribution of meroplankton and water properties off southern Washington and simultaneously measured time series of larval abundance and water properties in two adjacent estuaries, Grays Harbor and Willapa Bay. The cruise period, in late May 1999, coincided with large variation in the alongshore wind stress that caused dynamic change in the position of the Columbia River plume, coastal upelling and downwelling, and offshore phytoplankton production. In the coastal ocean, meroplankton groups responded differently to this wind event and the associated advection of water masses. Dungeness crab (Cancer magister) megalopae were largely indifferent to the wide salinity variation, and were found throughout the surveyed area in both plume and recently upwelled waters. Megalopae of kelp crab (Pugettia producta) and hermit crab (Pagurus spp). were more abundant in upwelled water and low numbers were caught in the plume water. Barnacle cyprids appeared to track the advective transport suggesting that they may be more passively dispersed. Within the estuaries, hydrography responded rapidly and synchronously to variation in wind stress. Intrusions of both plume and newly upwelled waters were detected at estuarine sites, depending on the type of water present at the coast, indicating a tight link between the estuaries and the coastal ocean in this region. A 90-d record ofC. magister megalopae abundance was made at 3 estuarine sites using light traps. The bulk of theC. magister recruitment was limited to a relatively brief period in late May through June. Within this window, megalopae occurred in distinct pulses of 3–5 d interspaced with periods of low or zero abundance.C. magister megalopae recruited to the estuaries over a wide range of wind forcing, and were transported into the estuary within varied water types. There were no periodic patterns indicative of spring-neap tidal variations in the abundance time series. Abundance was only weakly cross-correlated between the adjacent Grays Harbor and Willapa Bay estuaries, which contrasts with the more synchronous estuarine-coastal linkages measured for water properties. These results suggest the interaction of larval aggregation size in the ocean with estuary-ocean exchange processes likely controls patterns of estuarine recruitment.     

Swimming velocities and behavior of blue crab (Callinectes sapidus rathbun) megalopae in still and flowing water

Habitat selection capabilities of the recruiting larval stages of marine invertebrates are limited, in part, by their ability to maneuver in flowing water. Distributional and experimental evidence suggest that blue crab (Callinectes sapidus) megalopae may preferentially settle into vegetated habitats. However, the behavior and swimming capabilities of megalopae in flowing water have not previously been investigated. Laboratory experiments were conducted in a small, recirculating seawater flume to determine the swimming response of megalopae to varying flow velocities. Nighttime trials were conducted at six flow velocities: 0, 1.9, 3.6, 4.8, 6.3, and 9.3 cm s^?1. Behavior and swimming velocities of field-collected C. sapidus megalopae were video recorded. Megalopae exhibited negative phototaxis and were found in the water column at all flows in the dark. The maximum sustained swimming speed observed was 12.6 cm s^?1 and the mean swimming speed in still water was 5.0 cm s^?1, with short bursts in excess of 20 cm s^?1. Megalopae frequently oriented into the current and were capable of swimming upstream against the current at flow speeds <4.8 cm s^?1; at greater velocities they were not able to do so. The results suggest that at low to moderate current velocities C. sapidus megalopae have the ability to actively move in search of settlement sites and to maintain their positions in desirable sites rather than relying strictly on passive movements by currents.