Age and growth,movements and distribution of the cownose ray,Rhinoptera bonasus,in Chesapeake Bay

Ages were estimated for 115 of 899 cownose rays,Rhinoptera bonasus, collected primarily from commercial fishing gear, in lower Chesapeake Bay and vicinity from May through October, 1976–78. Age determinations were made using sectioned vertebral centra and estimates of von Bertalanffy parameters were for males DW_∞=119.2, K=0.126, and t₀=?3.699, and for females DW_∞=125.0, K=0.119, and t₀=?3.764. Females attained a larger adult size and the oldest specimen aged was a female 13 years old and 107 cm disc width. Both sexes mature after reaching about 70% of their maximum size and ages at maturity were estimated at 5 to 6 years for males and 7 to 8 years for females. In spring migrating rays schooled by size; they arrived along the North Carolina coast by April and entered Chesapeake Bay by early May. Rays were abundant in the major Virginia tributaries of Chesapeake Bay throughout summer and occurred in salinities as low as 8‰ and at water temperatures between 15–29 °C. Size segregation continued during summer and adults schooled by sex. Most rays left Chesapeake Bay by early October.     

Seasonal Occurrence of Cownose Rays (Rhinoptera bonasus) in North Carolina’s Estuarine and Coastal Waters

The seasonal occurrence of cownose rays (Rhinoptera bonasus) within North Carolina’s estuarine and coastal waters was examined from aerial surveys conducted during 2004–2006. Generalized linear models were used to assess the influence of several variables (month, year, habitat type, sea surface temperature, and turbidity) on predicted counts of cownose rays. The spatial distributions of rays were compared by season, and differences in group size were tested as a function of season and habitat. Cownose ray data associated with the North Carolina Division of Marine Fisheries (NCDMF) fishery independent gill net sampling program in Pamlico Sound was also examined as a function of season and year, and compared with aerial observations. Rays immigrated into the region in mid-spring (April), dispersed throughout the estuary in the summer (June–August), and emigrated by late autumn (November). Predicted counts were highest in the spring (April, May) and autumn (September–November) for coastal habitats and highest in the summer for estuarine habitats. Predicted counts were also higher in the coastal region than estuarine and higher when sea surface temperatures were above average. Comparison of group size by habitat type revealed substantially larger group sizes in the coastal habitat than the estuarine. In addition, for the estuary, spring surveys had larger group sizes than summer surveys; for the coastal habitat, autumn group sizes were significantly larger than spring or summer group sizes. The NCDMF gill net sampling surveys indicated similar trends in monthly migration patterns as well as increased ray abundance in 2008 and 2009 compared with 2003–2007. These results suggest that North Carolina’s waters serve as important habitat during the seasonal migration of cownose rays, as well as during the summer when the species may utilize the estuarine region as a nursery and/or for foraging.     

Eggs and early larvae of the bay anchovy,Anchoa mitchilli,and the weakfish,Cynoscion regalis,in lower Chesapeake Bay with notes on associated ichthyoplankton

The seasonal abundance and spatial distribution of eggs and early larvae of the bay anchovy,Anchoa mitchilli, and the weakfish,Cynoscion regalis, were determined from plankton collections taken during 1971–1976 in the lower Chesapeake Bay. Eggs and larvae of the bay anchovy,Anchoa mitchilli, dominated the ichthyoplankton, making up 96% of the total eggs and 88% of all larvae taken. A comparison of egg and larval densities from the lower Chesapeake Bay to existing data from other East Coast estuaries suggested that Chesapeake Bay is a major center of spawning activity for this species.Anchoa mitchilli spawning commenced in May when mean water column temperatures approached 17°C and abruptly ceased after August. Eggs and early larvae presented a continuous distribution throughout the study area during these months. Eggs and larvae of several sciaenid species, especiallyC. regalis, ranked second in numerical abundance. Larval weakfish were consistently taken in late summer of each sampling year but peak abundance and distribution was observed in August 1971. Sciaenid eggs exhibited a distinct polyhaline distribution with greatest concentrations observed at the Chesapeake Bay entrance or along the Bay eastern margin. Analysis of sciaenid egg morphometry and larval occurrence suggested spawning activity of at least four species. Additional important species represented by eggs and/or larvae in the lower Chesapeake Bay wereHypsoblennius hentzi, Gobiosoma ginsburgi, Trinectes maculatus, Symphurus plagiusa andParalichthys dentatus with the remaining species occurring infrequently.     

Food habits and feeding behavior of the cownose ray,Rhinoptera bonasus,in lower Chesapeake Bay

The most important food item of the cownose ray,Rhinoptera bonasus, in the Virginian tributaries of lower Chesapeake Bay is the soft shell clam,Mya arenaria. The Baltic macoma,Macoma balthica, ranks a distant second. Adult rays feed on deep burrowing mollusks, juveniles on shallow- or non-burrowing bivalves. Foraging schools of rays invade tidal flats during the flood tide. Stirring motions of the pectoral fins combined with suction from the expansive orobranchial chamber are probably used to excavate deep burrowing bivalves.     

Food habits and distribution of wintering canvasbacks,Aythya valisineria,on Chesapeake Bay

Baltic clams (Macoma balthica) were the predominant food items of 323 canvasbacks (Aythya valisineria) collected throughout Chesapeake Bay during 1970–1979. Natural vegetation constituted 4% of the food volume. Widgeongrass (Ruppia maritima) and redhead grass (Potamogeton perfoliatus) constituted the greatest percent volume and frequency of occurrence among the plant species, whereas wild celery (Vallisneria americana) constituted only a trace of the food volume. These results contrast with historical records of food habits of canvasbacks in Chesapeake Bay. Canvasback population estimates during the 1970’s were examined to detect annual and seasonal changes in distribution. Linear regression analyses of winter canvasback populations in the bay showed a significant decline in the upper-bay and middle-bay populations, but no significant changes in the lower-bay and Potomac River populations. The changes in winter distribution and abundance of the canvasback appear related to changes in natural food availability, which is the result of altered environmental conditions.     

Water quality associated with survival of submersed aquatic vegetation along an estuarine gradient

The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen <10 μM; mean dissolved phosphate <0.35 μM; mean suspended sediment <20 mg l^?1; mean chlorophylla in the water column <15 μg l^?1; and mean light attenuation coefficient (Kd) <2 m^?1. These values correspond well with those derived in other parts of the Chesapeake, particularly in the lower bay, and may provide managers with values that can be used as target concentrations for nutrient reduction strategies where SAV is an issue.     

Spatial and temporal variation in recent growth, overall growth, and mortality of juvenile weakfish (Cynoscion regalis) in Delaware Bay

We examined relative abundance of juvenile weakfish,Cynoscion regalis, collected during 1986 and 1987 and tested for spatial differences in growth and survival within Delaware Bay. Juvenile weakfish recruit to all areas of Delaware Bay, and two cohorts were present during each year of the study. Although catch per unit effort (CPUE) varied among areas within the bay, there was a general trend of higher CPUE at lower salinities; abundance quickly declined near the end of September in all areas of the bay. Estimated growth rates from otolith increment analysis of juvenile weakfish ranged from 0.69 mm d⁻¹ to 0.97 mm d⁻¹. Spatial and temporal patterns in recent growth rate followed a general pattern: highest in the middle bay, lowest in the upper bay, and intermediate in the lower bay. Mortality rates were usually lowest in the low salinity region of the middle and upper bay during both years. There was no difference in mortality between cohorts in the middle bay, while in the upper bay the later-spawned fish had lower mortality and in the lower bay the early-spawned fish had lower mortality. Analysis of spatial and temporal patterns in growth and mortality suggests that there is a seasonal trade-off between habitat usage and resource availability for juvenile weakfish. The function of oligohaline and mesohaline waters as optimal nursery areas (in terms of growth and survival) changes due to the seasonally dynamic physicochemical characteristics in Delaware Bay.     

Regional and temporal variability in distribution and abundance of bay anchovy (Anchoa mitchilli) eggs, larvae, and adult biomass in the Chesapeake Bay

Patterns and variability in reproductive output of pelagic fish are seldom determined at the ecosystem scale. We examined temporal and spatial variability in spawning by bay anchovy (Anchoa mitchilli), and in distribution and abundances of its pelagic early-life stages, throughout Chesapeake Bay. On two cruises in June and July 1993, ichthyoplankton and zooplankton were collected on 15 transects at 18.5-km (10 nautical mile) intervals over the 260-km length of the bay. Finer-scale sampling was carried out in a grid of stations between two transects on each cruise. Regional abundance patterns of bay anchovy eggs and larvae in the lower, mid, and upper Bay were compared with zooplankton abundances, environmental variables, and biovolumes of two gelatinous predators—the scyphomedusa Chrysaora quinquecirrha and the lobate ctenophore Mnemiopsis leidyi. Abundances of anchovy eggs, and, especially, larvae were higher in July than in June. Baywide daily egg production increased from 4.25×10¹² in June to 8.43×10¹² in July. Concentrations of zooplankton that are potential anchovy prey nearly doubled on a baywide basis between June and July, while biovolumes of the ctenophore declined. Except for scyphomedusan biovolumes, all analyzed organisms differed regionally in abundance and were patchily distributed at 1-km to 10-km sampling scales. Negative correlations between larval anchovy abundances and gelatinous predator biovolumes suggested that predation may have controlled abundances of bay anchovy early-life stages. Biomasses of adult anchovy, estimated from daily egg productions, were higher in the lower Bay and remarkably similar—23,433 tons in June and 23,194 tons in July. Most spawning by bay anchovy occurred during July in the seaward third of Chesapeake Bay, emphasizing the importance of this region for recruitment potential of the Bay's most abundant fish.     

Population structure, growth rates, and seasonal abundance of twoSyngnathus pipefish species

Northern pipefish,Syngnathus fuscus, and dusky pipefish,Syngnathus floridae, are among the most abundant ichthyofauna components of the Chesapeake Bay, USA, eelgrass beds,Zostera marina, but population structure and many life history traits remain uncharacterized. We conducted monthly collections from May through September 2003–2005 in Chincoteague Bay, Virginia, to investigate seasonal migration and spawning, sex ratios, size at maturity, sexual dimorphism in length, and growth rates. BothS. fuscus andS. floridae spawned from May through September. Water temperature was significantly correlated withS. fuscus catches, whereasS. floridae abundance peaked after maximum water temperatures. Sex ratio data indicatedS. floridae populations are balanced, whileS. fuscus populations are strongly female-biased. Both species can quickly reach reproductive maturity, potentially within one season, becauseS. fuscus andS. floridae population growth rates average 1.0 mm d⁻¹ and minimum standard length at maturity measures 125 and 103 mm, respectively, for females and 99 and 91 mm, respectively, for males. ForS. fuscus, females were significantly longer than conspecific males during time periods when juveniles were not rapidly maturing. Size sexual dimorphism in this species coincides with reports of extensive paternal care and supports the hypothesis that the strength of sexual selection differs in these species.     

Spatial and Interannual Variability in Winter Mortality of the Blue Crab (<Emphasis Type="Italic">Callinectes sapidus</Emphasis>) in the Chesapeake Bay

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.     

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.     

Habitat Affects Survival of Translocated Bay Scallops, Argopecten irradians concentricus (Say 1822), in Lower Chesapeake Bay

Bay scallop (Argopecten irradians) populations existed in Chesapeake Bay until 1933, when they declined dramatically due to a loss of seagrass habitat. Since then, there have been no documented populations within the Bay. However, some anecdotal observations of live bay scallops within the lower Bay suggest that restoration of the bay scallop is feasible. We therefore tested whether translocated adults of the southern bay scallop, Argopecten irradians concentricus, could survive during the reproductive season in vegetated and unvegetated habitats of the Lynnhaven River sub-estuary of lower Chesapeake Bay in the absence of predation. Manipulative field experiments evaluated survival of translocated, caged adult scallops in eelgrass Zostera marina, macroalgae Gracilaria spp., oyster shell, and rubble plots at three locations. After a 3-week experimental period, scallop survival was high in vegetated habitats, ranging from 98% in their preferred habitat, Z. marina, to 90% in Gracilaria spp. Survival in Z. marina was significantly higher than that in rubble (76%) and oyster shell (78%). These findings indicate that reproductive individuals can survive in vegetated habitats of lower Chesapeake Bay when protected from predators and that establishment of bay scallop populations within Chesapeake Bay may be viable.     

Population density, prey selection, and predator avoidance of the burrowing anemone (Ceriantheopsis americanus) in Narragansett Bay, Rhode Island

The maximum population density of the burrowing anemone (Ceriantheopsis americanus) was estimated at 17–28 animals m^?2 in soft-bottom sediments of mid Narragansett Bay. The gut contents of the anemone indicated primary prey of harpacticoid and calanoid copepods. The consumption of calanoid copepods was higher in October than April, which may be due to decreased density of hapacticoids in the fall. The anemones apparently avoid fish predation in late summer by withdrawing into the sediments. After the seasonal fall migration of fish out of the bay, anemones reappear.     

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.     

Aspects of the population dynamics of the polychaeteSabellaria vulgaris Verrill,in the Delaware Bay

Aspects of the population dynamics of the polychaeteSabellaria vulgaris Verrill 1874 were studied by observing the temporal occurrence of larvae in the plankton of Delaware Bay. Vertical plankton samples were collected monthly from July 1970 to October 1971. Four 25-hour plankton studies were conducted within this time period, and on one occasion samples were collected on a transect across the mouth of the bay.Sabellaria vulgaris larvae were present in the bay only from mid-April through October. July (1970) and August (1971) 25-hour plankton studies showed larvae present in the water column at virtually all times of the day and night. Horizontal dispersion of larvae in the plankton was clearly aggregated. However, correlation of larval presence, absence or abundance with the measured physical factors in the estuary was not apparent except on a seasonal scale. Six developmental stages were defined based upon observation of laboratory-reared larvae. Young larvae appeared in the plankton on numerous occasions, indicating that spawning occurred repeatedly during the April–October time period in Delaware Bay. Relative to other habitats within the geographic range ofS. vulgaris, Delaware Bay is a particularly well-suited environment for the construction of massive colonies by the species. Adults living in large aggregates would exhibit greater fitness because of the higher probability of eggs being fertilized. Indications are that a portion of the larvae produced in the Bay are retained in the estuary and undergo settlement there. Delaware Bay may be a population center for the species. A comparison of reproductive phenomena among sabellariid species is presented. It is apparent that the species,S. vulgaris, consists of several physiologically distinct populations, and that this is true of certain other sabellariid species as well.     

Abundance and distribution of ichthyoplankton in Narragansett Bay, Rhode Island, 1989–1990

An ichthyoplankton survey (18 stations in seven sampling sectors) was conducted in Narragansett Bay in 1990 to provide information on abundance, distribution, and seasonal occurrence of eggs and larvae of estuarine fishes, including seasonal migrants. An additional goal was to examine changes in species composition, abundance, and distribution occurring since the last baywide survey in 1972–73. The taxonomic composition of eggs and larvae in 1990 (41 species in 25 families from 684 plankton samples) and in 1972–73 (43 species in 28 families from 6900 samples) was similar. Maximum abundance of fish eggs occurred in June and larvae in July, minimum abundance in September to February. Species diversity was greatest in May–July and lowest during January in both surveys. However, egg and larval densities in 1990 were considerably lower than in 1972–73. Bay anchovy, tautog, and cunner accounted for 86% of the eggs and 87% of the larvae in the bay in 1990. These three species accounted for only 55% of the eggs and 51% of the larvae in 1972–73, with menhaden accounting for another 18% of the eggs and 34% of the larvae. Searobins, scup, and butterfish eggs were common in 1973 (19%) but rare in 1990 (2%). Ichthyoplankton abundance for several of the most abundant species was significantly lower (p<0.05) in the Providence River, upper bay, and Greenwich Bay in 1990 than in 1972–73. Density of fish eggs and larvae in the lower portions of the bay was lower in 1990 for some species but not others. Distribution data suggested a general down-bay shift in density in 1990. *** DIRECT SUPPORT *** A01BY085 00015     

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Decapod crustacea

Otter trawl collections of eelgrass habitats in the lower Chesapeake Bay during 1976–1977 produced 14 species of decapod crustaceans. These collections were dominated by palaemonid shrimp (Palaemonetes spp.), blue crabs (Callinectes sapidus), and sand shrimp (Crangon septemspinosa), each of which exhibited unimodal seasonal abundance curves with large summer peaks. Decapod abundance was positively correlated with plant biomass throughout the year. Decapod densities on vegetated bottoms were greater than on unvegetated bottoms, and nighttime abundance on each bottom type was greater than corresponding daytime abundance. Total decapod abundances in Chesapeake Bay eelgrass meadows appear to be much greater than those reported in North Carolina eelgrass or Gulf of Mexico turtlegrass habitats.     

Genetic structure of bay anchovy (Anchoa mitchilli) populations in Chesapeake Bay

To determine the genetic structure of the bay anchovy (Anchoa mitchilli) within Chesapeake Bay, 16 isozyme systems encoding 21 loci for 20 population were examined using horizontal starch gel electrophoresis. Contingency Chisquare analysis revealed significant allelic frequency differences at nine loci (AAT-1, AAT-2, ALD-1, CPK-2, GAP-1, GLY-1, LDH-1, MDH-1, and MDH-2). Two loci, ALD-1 and MDH-1, were responsible for nine of 14 tests not conforming to Hardy-Weinberg expectations, with some of these deviations attributed to possible scoring and/or sampling error. Estimates for mean average heterozygosity were relatively high, ranging from 0.40 to 0.096, with 33–57% of the loci polymorphic. A low F_st value (0.041) along with high genetic identity estimates (I=0.997) indicated little substructuring of bay anchovy populations within Chesapeake Bay.     

Forecasting the abundance of the sea nettle,Chrysaora quinquecirrha,in the Chesapeake Bay

The sea nettle shows variable seasonal infestation in the Chesapeake Bay. Public interest in the medusal population prompted an examination of the effect of climatic, hydrographic, and biological variables on such changes. Visual medusal counts since 1960 were regressed in a stepwise fashion against the suite of variables, to produce an abundance model which allows a reasonable prediction of the forthcoming summer’s infestation. Streamflow in the entire Chesapeake watershed for the months of January through June and the water temperature for May were most important. Lower streamflow apparently provides a salinity regime which supports the sessile stages early in the year and allows the survival and rapid growth of ephyrae in early summer. The water temperatures in May furnishes the trigger for strobilation at a propitious time.     

Spatial and Temporal Patterns of Winter–Spring Oxygen Depletion in Chesapeake Bay Bottom Water

Although seasonal hypoxia is a well-studied phenomenon in many coastal systems, most previous studies have only focused on variability and controls on low-oxygen water masses during warm months when hypoxia is most extensive. Surprisingly, little attention has been given to investigations of what controls the development of hypoxic water in the months leading up to seasonal oxygen minima in temperate ecosystems. Thus, we investigated aspects of winter–spring oxygen depletion using a 25-year time series (1985–2009) by computing rates of water column O₂ depletion and the timing of hypoxia onset for bottom waters of Chesapeake Bay. On average, hypoxia (O₂ <62.5 μM) initiated in the northernmost region of the deep, central channel in early May and extended southward over ensuing months; however, the range of hypoxia onset dates spanned >50 days (April 6 to May 31 in the upper Bay). O₂ depletion rates were consistently highest in the upper Bay, and elevated Susquehanna River flow resulted in more rapid O₂ depletion and earlier hypoxia onset. Winter–spring chlorophyll a concentration in the bottom water was highly correlated with interannual variability in hypoxia onset dates and water column O₂ depletion rates in the upper and middle Bay, while stratification strength was a more significant driver in the timing of lower Bay hypoxia onset. Hypoxia started earlier in 2012 (April 6) than previously recorded, which may be related to unique climatic and biological conditions in the winter–spring of 2012, including the potential carryover of organic matter delivered to the system during a tropical storm in September 2011. In general, mid-to-late summer hypoxic volumes were not correlated to winter–spring O₂ depletion rates and onset, suggesting that the maintenance of summer hypoxia is controlled more by summer algal production and physical forcing than winter-spring processes. This study provides a novel synthesis of O₂ depletion rates and hypoxia onset dates for Chesapeake Bay, revealing controls on the phenology of hypoxia development in this estuary.