Distribution and abundance of the cownose ray,Rhinoptera bonasus,in lower Chesapeake Bay

Aerial surveys were conducted in the lower Chesapeake Bay during 1986–1989 to estimate abundance and examine the distribution of the cownose ray,Rhinoptera bonasus, during its seasonal residence, May–October. Most of the survey effort was concentrated in the lower and mid-bay regions. Cownose rays appeared uniformly distributed across the bay during mid-summer, but were more abundant in the eastern portion of the bay during migration. North-south distribution varied and reflected the general seasonal migration pattern. Mean abundance increased stepwise monthly from June through September and declined dramatically in October with their emigration from the bay. Abundance estimates from individual surveys varied. The greatest range of individual survey abundance estimates occurred in September (0–3.7×10⁷ cownose rays0 due to high variation in school size and abundance between surveys. Monthly mean cownose ray abundance ranged from 0 in May and November to an estimated maximum of 9.3×10⁶ individuals in September. The magnitude of the population suggests that the cownose ray plays an important role in the trophic dynamics of the Chesapeake Bay ecosystem. The historical data were insufficient to determine whether the population has increased, but these surveys provided the baseline data which would allow future investigation of cownose ray population dynamics in lower Chesapeake Bay.     

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.     

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 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.     

Population Structure of Adult Blue Crabs, <Emphasis Type="Italic">Callinectes sapidus</Emphasis>, in Relation to Physical Characteristics in Barnegat Bay,New Jersey

Blue crabs (Callinectes sapidus) are an important species in coastal or lagoonal estuaries where adult population characteristics may differ as compared to drowned-river estuaries. Barnegat Bay, in southern New Jersey, is composed of two large embayments: one without and one with a salinity gradient. We tested the influence of physical characteristics on the abundance, sex ratio, and size of adult blue crabs and examined variation in measures of reproductive potential (e.g., sperm stores) in both sexes in Barnegat Bay from June to September, 2008–2009. Population structure was distinct between the embayments due to sex-specific responses to salinity: male abundance was negatively correlated with salinity whereas adult females were more abundant in high salinity because of proximity to Barnegat Inlet. This produced high sex ratios in low salinity areas and low sex ratios in high salinity areas. Summer was a growing season for adult males while in late summer-early fall, juvenile males recruited to the adult size class. The spawning season lasted from May to August and ovigerous females were concentrated near the inlets. Information on female sperm stores and ovarian development identified two cohorts of adult females: females that will spawn in the current summer and females that will not spawn until the following summer. Thus, not all adult females near the spawning grounds were members of the current spawning stock. This suggests that annual estimates of spawning stock size which overlook the proximity of females to spawning are overestimating the current spawning stock in Barnegat Bay and other estuaries.     

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.     

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.     

Phytoplankton index of biotic integrity for Chesapeake Bay and its tidal tributaries

A Phytoplankton Index of Biotic Integrity (P-IBI) was developed from data collected during 18 yr 91985–2002) of the Chesapeake Bay Water Quality Monitoring Program. Dissolved inorganic nitrogen (DIN), orthophosphate (PO₄), and Secchi depth were used to characterize phytoplankton habitat conditions. Low DIN and PO₄ concentrations and high Secchi depths characterized least-impaire conditions. Thirty-eight phytoplankton metrics were tested for their ability to discriminate between impaired and least-impaired habitat conditions. Twelve discriminatory metrics were chosen, and different combinations of these twelve metrics were scored and used to create phytoplankton community indexes for spring and summer in the four salinity regimes in Chesapeake Bay. The scoring criteria for each metric were based on the distribution of the metric’s values in least-impaired conditions relative to the distribution in impaired conditions. An independent data set and jackknife validation procedure were used to examine P-IBI performance. The P-IBI correctly classified 70.0–84.4% of the impaired and least-impaired samples, grouped by season and salinity, in the calibration data set. The P-IBI is a management tool to assess phytoplankton community status relative to estuarine nutrient and light conditions.     

Distribution ofVibrio parahaemolyticus in Chesapeake Bay during the summer season

The distribution ofVibrio parahaemolyticus in Chesapeake Bay during the warmer weather of the summer months was examined. This species was found throughout the Chesapeake Bay and its tributaries, even in areas of very low salinity. Counts of this species ranged from 0.04 per 100 ml to 46 per 100 ml in the water column and 2.03 to ≥2.4×10³ per 100 cc of sediment. A variety of physical, chemical and bacteriological properties associated with the incidence and distribution ofV. parahaemolyticus were examined and salinity was found to be the major influence among the factors examined. Correlation and regression analysis showed that the population size of this species increased with increasing salinity in the estuary.     

Seasonal distributions of suspended particulate material and dissolved nutrients in a coastal plain estuary

The temporal and spatial distributions of salinity, dissolved oxygen, suspended particulate material (SPM), and dissolved nutrients were determined during 1983 in the Choptank River, an estuarine tributary of Chesapeake Bay. During winter and spring freshets, the middle estuary was strongly stratified with changes in salinity of up to 5‰ occurring over 1 m depth intervals. Periodically, the lower estuary was stratified due to the intrusion of higher salinity water from the main channel of Chesapeake Bay. During summer this intrusion caused minimum oxygen and maximum NH₄ ⁺ concentrations at the mouth of the Choptank River estuary. Highest concentrations of SPM, particulate carbon (PC), particulate nitrogen (PN), total nitrogen (TN), total phosphorous (TP) and dissolved inorganic nitrogen (DIN) occurred in the upper estuary during the early spring freshet. In contrast, minimum soluble reactive phosphate (SRP) concentrations were highest in the upper estuary in summer when freshwater discharge was low. In spring, PC:PN ratios were >13, indicating a strong influence by allochthonous plant detritus on PC and PN concentrations. However, high concentrations of PC and PN in fall coincided with maximum chlorophyll a concentrations and PC:PN ratios were <8, indicating in situ productivity controlled PC and PN levels. During late spring and summer, DIN concentrations decreased from >100 to <10 μg-at l^?1, resulting mainly from the nonconservative behavior of NO₃ ^?, which dominated the DIN pool. Atomic ratios of both the inorganic and total forms of N and P exceeded 100 in spring, but by summer, ratios decreased to <5 and <15, respectively. The seasonal and spatial changes in both absolute concentrations and ratios of N and P reflect the strong influence of allochthonous inputs on nutrient distributions in spring, followed by the effects of internal processes in summer and fall.     

Seasonal phytoplankton composition in the lower Chesapeake Bay and old plantation Creek,Cape Charles,Virginia

During a 14-month phytoplankton study in the lower Chesapeake Bay, there was a bi-modal pattern of population peaks with fall and spring maxima. The phytoplankton was dominated bySkeletonema costatum and other diatoms similar to major dominants found on the continental shelf. The composition in an inlet adjacent to the Bay was similar throughout most of the period, but differed from Bay populations during the summer months when larger concentrations and diversity of phytoflagellates and small sized diatoms occurred. Seasonal phytoplankton assemblages characteristic for the lower and entire Chesapeake Bay are given with the seasonal appearances noted for 219 phytoplankters. The importance of nanophytoplankters, both diatoms and the phytoflagellates, to the total phytoplankton composition is also emphasized.     

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.     

Harmful algal blooms in the Chesapeake and Coastal Bays of Maryland, USA: Comparison of 1997, 1998, and 1999 events

Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA, are not a new phenomenon, but may be increasing in frequency and diversity. Outbreaks ofPfiesteria piscicida (Dinophyceae) were observed during 1997 in several Chesapeake Bay tributaries, while in 1998,Pfiesteria-related events were not found but massive blooms ofProrocentrum minimum (Dinophyceae) occurred. In 1999,Aureococcus anophagefferens (Pelagophyceae) developed in the coastal bays in early summer in sufficient densities to cause a brown tide. In 1997, toxicPfiesteria was responsible for fish kills at relatively low cell densities. In 1998 and 1999, the blooms ofP. minimum andA. anophagefferens were not toxic, but reached sufficiently high densities to have ecological consequences. These years differed in the amount and timing of rainfall events and resulting nutrient loading from the largely agricultural watershed. Nutrient loading to the eastern tributaries of Chesapeake Bay has been increasing over the past decade. Much of this nutrient delivery is in organic form. The sites of thePfiesteria outbreaks ranked among those with the highest organic loading of all sites monitored bay-wide. The availability of dissolved organic carbon and phosphorus were also higher at sites experiencingA. anophagefferens blooms than at those without blooms. The ability to supplement photosynthesis with grazing or organic substrates and to use a diversity of organic nutrients may play a role in the development and maintenance of these species. ForP. minimum andA. anophagefferens, urea is used preferentially over nitrate.Pfiesteria is a grazer, but also has the ability to take up nutrients directly. The timing of nutrient delivery may also be of critical importance in determining the success of certain species.     

A simple empirical optical model for simulating light attenuation variability in a partially mixed estuary

Representation of the subsurface light field is a crucial component of pelagic ecosystem and water quality models. Modeling the light field in estuaries is a particularly complicated problem due to the significant influence of high concentrations of dissolved and particulate matter that are derived from both terrestrial and estuarine sources. The goal of this study was to develop a relatively simple but effective way to model light attenuation variability in a turbuid estuary (Chesapeake Bay, United States) in a coupled physical-biological model. We adopted a simple, nonspectral empirical approach. Surface water quality data (salinity was used as a proxy of chromophoric dissolved organic matter [CDOM]) and light measurements from the Chesapeake Bay Program were used to determine the absorption coefficients in a linear attenuation model using regression methods. This model predicts K_c (specific attenuation due to phytoplankton/chlorophylla [chla]), K_t (specific attenuation due to total suspended solids), and K_s (a function of specific attenuation coefficients of CDOM in relation to salinity). The Bay-wide fitted relation between the light attenuation coefficient and water quality concentrations gives generally good estimates of total light attenuation, K_d. The direct inclusion of salinity in the relationship has one disadvantage: it can yield negative values for K_d at high salinities. We developed two separate models for two different salinity regimes. This approach, in addition to solving the negative K_d problem, also accounts for some changes in specific light absorption by chla, seston (nonphytoplankton particulate matter), and CDOM that apparently occur in different salinity regimes in Chesapeake Bay. The resulting model predicts the statistical characteristics (i.e., the mean and variance) of K_d quite accurately in most regions of Chesapeake Bay. We also discuss in this paper the feasibility and caveats of using K_d converted from Secchi depth in the empirical method.     

Temperature effects on the timing of striped bass egg production,larval viability,and recruitment potential in the Patuxent River (Chesapeake Bay)

The relationships between egg production (spawning behavior), larval growth and survival, and environmental conditions that larvae encounter were investigated in the Patuxent River tributary of Chesapeake Bay in 1991. Striped, bass (Morone saxatilis) eggs and larvae occurred predominantly above the salt front where conductivity was ≤800 μmhos cm^?1. There were three prominent peaks in egg production, each coinciding with increasing temperatures. Estimated growth rates of 6-d, otolith-aged cohorts, which ranged from 0.15 mm d^?1 to 0.22 mm d^?1 (mean=0.17 mm d^?1), were not demonstrated to differ significantly from each other. Observed zooplankton densities and temperature did not significantly affect growth rates. Stage-specific cumulative mortalities of combined cohorts were calculated for eggs (Z_stage=0.20=18.1%), yolk-sac larvae (Z_stage=5.80=99.7%), and first-feeding larvae (Z_stage=2.95=94.8%). The very high mortality of yolk-sac larvae suggests that dynamic during this stage may have had a major impact on subsquent recruitment. Cohort-specific mortality rates of larvae were variable, ranging from Z=0.045 d^?1 to 0.719 d^?1, and were strongly temperature-dependent. Cohorts that experiented average temperature <15°C or >20°C during the first 25 d after hatching had significantly higher mortality rates than those which experienced intermediate temperatures. Estimated hatch-date frequencies of larvae ≥8 mm SL indicated goo, very good, and very low potential recruitments for cohorst spawned during early-season (April 2–11), mid-season (April 12–24) and late-season (April 25–May 5), respectively. Because seasonal temperature trends and fluctuations are unpredictable, striped bass females cannot select a spawning time that guarantees their offspring will be exposed to optimum temperatures. Consequently, selection may have occured for spawning over a broad range of temperatures and dates, a behavior insuring that some larval cohorts will encounter favorable temperatures.     

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.     

Kepone distribution in selected tissues of blue crabs,Callinectes sapidus,collected from the James River and lower Chesapeake Bay

Male, ovigerous female and nonovigerous female blue crabs were collected from seven stations in the lower Chesapeake Bay and lower James River, Virginia. The Kepone concentration in muscle, gonad and hepatopancreas was determined by GC analysis. All crabs from the lower Bay contained little Kepone, while many of those from the lower James River were contaminated. Male crabs contained more Kepone in the backfin muscle than females. Female crabs concentrated Kepone into the gonad, and, to a lesser extent, the hepatopancreas whereas the males did not. Ovigerous female crabs with spent ovaries, had less Kepone in the ovary than the corresponding egg masses. Further, ovigerous females, especially those with spent ovaries, had more Kepone in backfin muscle than nonovigerous females. It is concluded that the differences in Kepone partitioning among tissues of male and female crabs may explain the previously reported observation that males contain more Kepone in the backfin muscle than do females.     

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.     

Relationship between Hypoxia and Macrobenthic Production in Chesapeake Bay

Human development has degraded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O₂ l^-1). The Bay's hypoxic zones have an adverse effect on both community structure and secondary production of macrobenthos. From 1996 to 2004, the effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and its three main tributaries (Potomac, Rappahannock, and York Rivers). Each year, in the summer (late July???early September), 25 random samples of the benthic macrofauna were collected from each system, and macrobenthic production in the polyhaline and mesohaline regions was estimated using Edgar's allometric equation. Fluctuations in macrobenthic production were significantly correlated with dissolved oxygen. Macrobenthic production was 90 % lower during hypoxia relative to normoxia. As a result, there was a biomass loss of ~7,320–13,200 metric tons C over an area of 7,720 km², which is estimated to equate to a 20 % to 35 % displacement of the Bay's macrobenthic productivity during the summer. While higher consumers may benefit from easy access to stressed prey in some areas, the large spatial and temporal extent of seasonal hypoxia limits higher trophic level transfer, via the inhibition of macrobenthic production. Such a massive loss of macrobenthic production would be detrimental to the overall health of the Bay, as it comes at a time when epibenthic and demersal predators have high-energy demands.     

Spawning densities,egg densities,size structure,and movement patterns of spawning horseshoe crabs,<Emphasis Type="Italic">Limulus polyphemus</Emphasis>, within four coastal embayments on Cape Cod,Massachusetts

Spawning densities, spawning indices, egg densities, size distributions, and movement patterns of horseshoe crabs (Limulus polyphemus) were quantified for four coastal embayments (Monomoy National Wildlife Refuge, Pleasant Bay, Nauset Estuary, and Cape Cod Bay) on Cape Cod, Massachusetts from 2000 to 2002. Spawning activity was highest from mid May through mid June, but densities varied throughout the Cape Cod region. Average spawning densities (male and female crabs combined), measured using 25-m² quadrats, were lower than 1 crab 25 m^?2, although certain locations had consistently higher densities averaging 2 to 3 crabs 25 m^?2 with individual survey densities recorded as high as 17 crabs 25 m^?2. Spawning densities during night surveys were either similar or slightly higher than day surveys, except at a few sites within Pleasant Bay. Spawning indices were considerably lower ranging from 0 to 1.3 females 25 m^?2 throughout the Cape Cod region. Spawning sex ratios varied from 1∶1.6 to 1∶3.1 (females:males) throughout the region, except within Pleasant Bay where highly male skewed ratios were observed (e.g., 1∶5.8, 3-yr average). Egg densities were low overall (<1 egg cm^?2) throughout Cape Cod and egg densities tended to be higher in deeper sediments (5–20 cm deep) compared to shallow sediments (0–5 cm deep) at most locations. Over 7,800 horseshoe crabs were tagged on Cape Cod from 2000 to 2002. Average size and size frequency distributions of tagged crabs varied among regions. Larger individuals were observed at Monomoy National Wildlife Refuge while the smallest individuals were from Cape Cod Bay. We documented an overall recapture rate of 6.7% and our tag-recapture data indicated that 62% of crabs were recaptured at the original tagging location and 70% of recaptures traveled less than 2 km from the original tagging location, providing evidence for localized populations on Cape Cod. We have observed that horseshoe crabs differ among embayments within a regional area, suggesting the potential need for management plans specific to embayments or subregions depending on the characteristics of a population.