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.     

Temporal variation in the utilization of an intertidal creek by the bay anchovy (Anchoa mitchilli)

A brackish-water, intertidal creek was sampled 19 times during the summer of 1976. Samples were analyzed to determine occurrence, abundance, and diversity of fish species present and to determine patterns of daily fish movement and utilization of intertidal creeks in an old rice field. Samples were collected from the primary station on the ebbing tide with a channel net. Three secondary stations were each sampled twice for comparative results and total biomass and average length per species calculated for each sample. We collected 125,579 individuals with a total biomass of 11.7 kg which represented 23 families and 37 species. Anchoa mitchilli was the dominant species and comprised 96.3% of the total numbers of individuals caught. The next two most abundant fish, Menidia menidia and Fundulus heteroclitus, represented 2.5% and 0.4% of the total numbers caught. The data from the nineteen sample collections were ordered by time of day and the resultant time series sequence of collections simulated a sample collection taken about once every 1.5 hours over 24 hours. A pattern of occurrence was noted for A. mitchilli as increased fish numbers occurred when the creek flooded during the daylight hours and there were significantly reduced numbers when the creek flooded during the early evening or at night. A relationship between light intensity, feeding, and predator avoidance is proposed to explain the creek loading pattern of A. mitchilli.     

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.     

Aging and growth of larval bay anchovy,Anchoa mitchilli,from the Newport River estuary,North Carolina

Bay anchovy (Anchoa mitchilli) larvae were hatched and reared in the laboratory from eggs collected near Beaufort, North Carolina. The first growth increment formed on otoliths on the fifth day after hatching when larvae were between 3.7 and 4.2 mm standard length. On the average, one otolith growth increment was formed per day thereafter in larvae up to 23 d posthatch. Age of wild larvae from the Newport River estuary in North Carolina was determined from otolith increment counts based on the assumption that increment deposition rates in nature are the same as in the laboratory. From their size and estimated age, it appears that the standard length (SL) of wild larvae in the estuary increases exponentially at about 4% d^?1 during their first 1.6 months, increasing from 0.24 mm d^?1 on day 12 to 1.11 mm d^?1 on day 49. Bay anchovy spawned early in the season (e.g., April–May) could grow to maturity and reach a size (>40 mm SL) that would enable them to spawn during their first summer.     

Spatial and temporal growth rate variation of Bay Anchovy (Anchoa mitchilli) larvae in the Mid Hudson River estuary

Estuaries are critical habitats for larvae and juveniles of many marine fishes, possibly because they promote high growth rates and survival rates. We investigated spatial and temporal changes in growth rate of larval bay anchovy (Anchoa mitchilli), in the middle Hudson River estuary where abundance of larvae is high. In two consecutive summer seasons, we sampled larvae at 4 sites evenly spaced over 45 km, at weekly intervals for up to a month. We examined otoliths to determine age in days and then used age-length regressions to estimate growth rate. In 1995, larval anchovy growth rates varied from 0.39 to 0.88 mm d⁻¹ (median=0.48 mm d⁻¹). In 1996, growth rates varied from 0.41 to 0.77 mm d⁻¹ (median=0.55 mm d⁻¹). In both years, we found significant spatial and temporal variation in growth rate. Larvae collected in the upper portion of Haverstraw Bay tended to grow more slowly than larvae collected in other sites. The dates on which the most rapidly growing larvae were collected varied from site to site. Neither temperature nor salinity variations explained growth rate differences. Growth rate variation, probably governed by patches of zooplankton, occurred on temporal scales of a week and spatial scales of 15 km.     

Causes of interestuarine variability in bay anchovy (<Emphasis Type="Italic">Anchoa mitchilli</Emphasis>) salinity at capture

Salinities occupied by different life stages of bay anchovy (Anchoa mitchilli) were compared over annual cycles at 128 stations in 12 Florida estuaries. The comparison included eight stations in an oligotrophic, groundwater-based estuary in which all life stages were rare or absent. At other stations, adults, eggs, and early larvae occurred in intermediate to high salinities (10-30 psu) with no apparent central salinity tendency. The larva-juvenile transition was marked by an upstream shift to lower salinities (0-15 psu), also with no central salinity tendency. Mean salinities of the juvenile catch were strongly dependent on the salinities of the sampling effort. This dependence was strongest in estuaries that had weak horizontal salinity gradients. Weak salinity gradients were either natural or resulted from estuarine dams. After using nonlinear regression to account for the interaction between effort salinity and catch salinity, catch salinities were found to be similar from year to year within estuaries, but widely different among estuaries, with interestuarine differences ranging as high as 10–13 psu. Lower salinities were occupied by juveniles in estuaries that had long freshwater turnover times. Inherent geomorphic and inflow-related effects on the distribution of prey resources, coupled with an ontogenetic diet shift, are proposed as the explanation for both the habitat shift and the strong interestuarine variability in salinity at capture.     

Delaware Bay and Chesapeake Bay populations of the horseshoe crabLimulus polyphemus are genetically distinct

The Delaware Bay contains the world’s largest population of horseshoe crabs, which constitute an ecologically significant component of this estuarine ecosystem. The North Atlantic speciesLimulus polyphemus has an extensive geographical distribution, ranging from New England to the Gulf of Mexico. Recent assessments of the Delaware Bay population based on beach spawning and trawling data have suggested a considerable decrease in the number of adult animals since 1990. Considerable debate has centered on the accuracy of these estimates and their impact on marine fisheries management planning. Compounding this problem is the lack of information concerning the genetic structure of Atlantic horseshoe crab populations. This study assessed patterns of genetic variation within and between the horseshoe crab populations of Delaware Bay and Chesapeake Bay, using both Random Amplification of Polymorphic DNA (RAPD) and DNA sequence analysis of the mitochondrial cytochrome oxidase I gene (COI). We examined 41 animals from Delaware Bay and 14 animals from the eastern shore of Chesapeake Bay. To provide high quality, uncontaminated genomic DNA for RAPD analysis, DNA was isolated from hemocytes by direct cardiac puncture, purified by spin column chromatography, and quantified by agarose gel electrophoresis. RAPD fingerprints revealed a relative paucity of polymorphic fragments, with generally homogeneous banding patterns both within and between populations. DNA sequence analysis of 515 bases of the 5′ portion of the mitochondrial COI gene showed haplotype diversity in the Chesapeake Bay sample to be significantly higher than in the Delaware Bay sample, despite the larger size of the latter. Haplotype analysis indicates minimal contemporary gene flow between Delaware Bay and Chesapeake Bay crab populations, and further suggests that the Delaware Bay population is recovering from a recent population decline.     

Genetic diversity and structure of natural and transplanted eelgrass populations in the Chesapeake and Chincoteague Bays

The objective of this study was to gain baseline population data on the genetic diversity and differentiation of eelgrass (Zostera marïna L.) populations in the Chesapeake and Chincoteague bays. Natural and transplanted eelgrass beds were compared using starch gel electrophoresis of allozymes. Transplanted eelgrass beds were not reduced in genetic diversity compared with natural beds. Inbreeding coefficients (F_IS) indicated that transplanted eelgrass beds had theoretically higher levels of outcrossing than natural beds, suggesting the significance of use of seeds as donor material and of seedling recruitment following transplantation diebacks. Natural populations exhibited very great genetic structure (F_ST=0.335), but transplanted beds were genetically similar to the donor bed and each other. Genetic diversity was lowest in Chincoteague Bay, reflecting recent restoration history since the 1930s wasting disease and geographical isolation from other east coast populations. These data provide a basis for developing a management plan for conserving eelgrass genetic diversity in the Chesapeake Bay and for guiding estuary-wide restoration efforts. It will be important to recognize that the natural genetic diversity of eelgrass in the estuary is distributed among various populations and is not well represented by single populations.     

Inorganic carbon requirements of natural populations and laboratory cultures of some Chesapeake Bay phytoplankton

The rates of photosynthetic carbon fixation for some natural populations of Chesapeake Bay phytoplankton and for unialgal cultures of species isolated from those populations follow hyperbolic saturation kinetics with respect to total inorganic carbon concentrations. The apparent half-saturation constants, K_c, measured by ¹⁴C uptake are close to and in some cases larger than ambient concentrations of total inorganic carbon, indicating that inorganic carbon can be a limiting nutrient in eutrophic systems. Addition of bicarbonate ion to incubation bottles increased the measured ¹⁴C uptake rates by factors as high as 2.5 for natural samples and 4.5 for laboratory cultures.     

Distribution and movement of shortnose sturgeon (Acipenser brevirostrum) in the Chesapeake Bay

During a reward program for Atlantic sturgeon (Acipenser oxyrinchus), 40 federally endangered shortnose sturgeon (Acipenser brevirostrum) were captured and reported by commercial fishers between January 1996 and January 2000 from the Chesapeake Bay. Since this is more than double the number of published records of shortnose sturgeon in the Chesapeake Bay between 1876 and 1995, little information has been available on distributions and movement. We used fishery dependent data collected during the reward program to determine the distribution of shortnose sturgeon in the Chesapeake Bay. Sonically-tagged shortnose sturgeon in the Chesapeake Bay and Delaware River were tracked to determine if individuals swim through the Chesapeake and Delaware Canal. Shortnose sturgeon were primarily distributed within the upper Chesapeake Bay. The movements of one individual, tagged within the Chesapeake Bay and later relocated in the canal and Delaware River, indicated that individuals traverse the Chesapeake and Delaware Canal.     

Subtidal distribution of barnacles (Cirripedia: Balanidae) in Chesapeake Bay,Maryland

From 1977 to 1980, samples of barnacles were collected (as opportunities arose) from 61 subtidal locations (mostly oyster beds) around Chesapeake Bay, Maryland. Three species were identified from the area.Balanus improvisus dominated, comprising 83% of the 8,231 barnacles identified, and was collected at all locations but one. It occurred over a collection salinity range of 0.8‰ to 17.9‰.Balanus subalbidus (14% of the barnacles identified) was collected over the same salinity range, but mainly in lower salinity waters.Balanus eburneus was scarce (2% of the barnacles identified) and was collected at higher salinities (8.5‰ to 17.1‰).     

Seasonal oxygen depletion in Chesapeake Bay

The spring freshet increases density stratification in Chesapeake Bay and minimizes oxygen transfer from the surface to the deep layer so that waters below 10 m depth experiece oxygen depletion which may lead to anoxia during June to September. Respiration in the water of the deep layer is the major factor contributing to oxygen depletion. Benthic respiration seems secondary. Organic matter from the previous year which has settled into the deep layer during winter provides most of the oxygen demand but some new production in the surface layer may sink and thus supplement the organic matter accumulated in the deep layer.     

An estuarine benthic index of biotic integrity (B-IBI) for Chesapeake Bay

A multimetric benthic index of biotic integrity (B-IBI) was developed using data from five Chesapeake Bay sampling programs conducted between 1972 and 1991. Attributes of the index were selected by comparing the response of 17 candidate measures of benthic condition (metrics) between a set of minimally affected reference sites and at all other sites for which data were available. This procedure was conducted independently for each of seven habitats defined by salinity and substrate. Fifteen of the 17 candidate metrics differed significantly between reference sites and other sites for at least one habitat. No metric differed significantly in all seven habitats; however, four metrics, species diversity, abundance, biomass, and percent of abundance as pollution-indicative taxa, differed in six habitats. The index was calculated by scoring each selected metric as 5, 3, or 1 depending on whether its value at a site approximated, deviated slightly from, or deviated greatly from conditions at the best reference sites. Validation based on independent data collected between 1992 and 1994 indicated that the index correctly distinguished stressed sites from reference sites 93% of the time, with the highest validation rates occurring in high salinity habitats.     

Modeling of wind-induced destratification in Chesapeake Bay

It has been observed that storms in early fall can result in top-to-bottom mixing of Chesapeake Bay. A three-dimensional, time-dependent circulation model is used to examine this destratification process for September 1983, when extensive current and hydrographic data were available. The model bay is forced at the surface by observed hourly winds, at the ocean boundary by observed hourly surface and bottom salinities and sea level fluctuations, and at the head by observed daily discharges for a 28-d period. A second-moment, turbulence-closure submodel, with no adjustments from previous applications to its requisite coefficients, is used to calculate the vertical turbulence mixing coefficients. Comparisons with data inside the model domain indicate relative errors of 7% to 14% for sea level, 7% to 35% for current, and 11% to 21% for salinity. The tidal portion of the spectrum is modeled better than the subtidal portion. The model is used to examine both the mechanisms of wind mixing and the temporal and spatial distribution of vertical mixing within the estuary. Wind-driven internal shear is shown to be a more effective mechanism of inducing destratification than turbulence generated at the surface. The model is also used to show that the vertical temperature inversion which occurs in the fall does not affect the timing of the destratification as much as its completeness. The distribution of mid-depth vertical mixing shows highly variable values in the mid-bay region, where wind-induced mixing is dominant. This suggests that the source of oxygen to mid-bay bottom waters is similarly variable. Vertical turbulence mixing coefficients of 10^?2 cm² s^?1 (background) to 10³ cm² s^?1 were needed to simulate the September period, indicating the need for time-variable mixing in models of dissolved and suspended estuarine constituents.     

Changes in submerged aquatic macrophyte populations at the head of Chesapeake Bay, 1958–1975

Submerged aquatic plant populations in the Susquehanna Flats of the Chesapeake Bay were followed for 18 years. An exotic species, eurasian water milfoil,Myriophyllum spicatum, increased dramatically from 1958 to 1962; at the same time the dominant native species declined. After 1962, milfoil populations declined and the native rooted aquatics gradually began to return to their former levels. In the late 1960's all species declined and in 1972 almost disappeared from the Susquehanna Flats. These fluctuations may have been related to several interrelated environmental factors in the Chesapeake Bay, including tropical storms, turbidity, salinity and disease. The utilization of the Susquehanna Flats by waterfowl appears to be related to the abundance and species composition of the submerged macrophytes present.     

A pollution history of Chesapeake Bay

Present day anthropogenic fluxes of some heavy metals to central Chesapeake Bay appear to be intermediate to those of the southern California coastal region and those of Narragansett Bay. The natural fluxes, however, are in general higher. On the bases of Pb-210 and Pu-239 + 240 geochronologies and of the time changes in interstitial water compositions, there is a mixing of the upper 30 or so centimeters of the sediments in the mid-Chesapeake Bay area through bioturbation by burrowing mollusks and polychaetes. Coal, coke and charcoal levels reach one percent or more by dry weight in the deposits, primarily as a consequence of coal mining operations.     

Fish biomass size spectra in Chesapeake Bay

Biomass size spectra of pelagic fish were modeled to describe community structure, estimate potential fish production, and delineate trophic relationships in Chesapeake Bay. Spectra were constructed from midwater trawl collections each year in April, June–August, and October 1995–2000. The size spectra were bimodal: the first spectral dome corresponded to small zooplanktivorous fish, primarily bay anchovyAnchoa mitchilli; the second dome consisted of larger fish from several feeding guilds that are supported by multiple prey-predator linkages. Annual production estimates of pelagic fish, derived from a mean production to biomass ratio, varied nearly three-fold, ranging from 162 × 109 kcal (125 × 103 tons) in 1996 to 457 × 109 kcal (352 × 103 tons) in 2000. Seasonally, the biomass level and mean individual sizes of fish in the first dome increased from April to October, while the biomass level of the second dome was relatively stable. Regionally, biomass levels in the second dome were higher than biomasses in the first dome for the upper and lower Bay, but were minimal in the middle Bay where seasonal and episodic hypoxia occurs. To test a benthic-pelagic coupling hypothesis that could explain the higher biomass in the second domes for the lower and upper Bay, a cyclic size-spectrum model was fit that included only species in the zooplanktivorous-piscivorous fish guilds. The mean, normalized slope equaled ?1, indicating that zooplanktivorous fish may support piscivore production, but that a benthic-pelagic linkage is required to fully support fish production in the second dome. Interannual variability in slopes and intercepts of modeled size spectra was related to salinity, recruitment level of bay anchovy, and the primary axis of a correspondence analysis (salinity effect) on fish community structure. The spectral slope and intercept of normalized spectra were lowest in 1996, a near-record wet year. Results suggest that fish size spectra can be developed as useful indicators of ecosystem state and response to perturbations, especially if prey-predator relationships are explicitly represented.     

Hydrogeochemistry and transport of organic contaminants in an urban watershed of Chesapeake Bay (USA)

Stream water samples were collected in the two main free-flowing branches of the Anacostia River watershed above the head of tide over a one year time period. Both the Northeast and Northwest Branches drain large suburban and urban land areas that flow into the more urbanized tidal portion of the Anacostia River within Washington, DC. Large volume (40–75 l) water samples were filtered, and the suspended particulate matter and filtrate were analyzed for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and organochlorine pesticides (OCPs) at sub-nanogram per liter concentrations using ultra-trace analytical methods. Higher amounts of PCBs, PAH, and OCPs in the tidal Anacostia River occurred primarily in the particulate phase during high flow events. Polycyclic aromatic hydrocarbons in the particulate phase within fluvial transport consisted primarily of pyrogenic homologues characteristic of weathered or combusted petroleum products. Fluxes were exceptionally high for PAHs which showed annual fluxes to the tidal Anacostia River comparable to those determined for the much larger mainstem Potomac River. Aromatic hydrocarbons in runoff from urban regions may serve as an important source of PAH fluxes to the tidal waters of Chesapeake Bay.     

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

The structure of the fish community associated with eelgrass beds in the lower Chesapeake Bay was studied over a 14 month period. A total of 24,182 individuals in 48 species was collected by otter trawl with Leiostomus xanthurus (spot) comprising 63% of the collection, Syngnathus fuscus (northern pipefish) 14%, Anchoa mitchilli (bay anchovy) 9%, and Bairdiella chrysoura (silver perch) 5%. The density and diversity of fishes were higher in vegetated areas compared to unvegetated areas; fishes were more abundant in night collections Fish abundance and species number increased in the spring and early summer as both water temperature and eelgrass biomass increased and decreased in the fall and winter as temperature and eelgrass biomass decreased. Gill netting revealed some of the top predators in the system, especially the sandbar shark, Carcharhinus milberti. The fish community in the Chesapeake Bay was quite different from North Carolina eelgrass fish communities. Most notable was the rarity of the pinfish, Lagodon rhomboides, which may be a very important predator in the structuring of the epifaunal communities.     

Wind Modulation of Dissolved Oxygen in Chesapeake Bay

A numerical circulation model with a simplified dissolved oxygen module is used to examine the importance of wind-driven ventilation of hypoxic waters in Chesapeake Bay. The model demonstrates that the interaction between wind-driven lateral circulation and enhanced vertical mixing over shoal regions is the dominant mechanism for providing oxygen to hypoxic sub-pycnocline waters. The effectiveness of this mechanism is strongly influenced by the direction of the wind forcing. Winds from the south are most effective at supplying oxygen to hypoxic regions, and winds from the west are shown to be least effective. Simple numerical simulations demonstrate that the volume of hypoxia in the bay is nearly 2.5 times bigger when the mean wind is from the southwest as compared to the southeast. These results provide support for a recent analysis that suggests much of the long-term variability of hypoxia in Chesapeake Bay can be explained by variations in the summertime wind direction.