Michael J. Millard

Spatial and temporal distribution of horseshoe crab (Limulus polyphemus) spawning in Delaware Bay: Implications for monitoring

Concern for the status of horseshoe crab (Limulus polyphemus) has increased as harvest for conch and eel bait has increased and spawning habitat has decreased. In early 1999 a workshop was held at the behest of the Atlantic States Marine Fisheries Commission to design a statistically valid survey of horseshoe crab spawning in Delaware Bay. The survey that resulted was a redesign of a volunteer-based spawning survey that began in 1990, and its network of volunteers was relied on to implement the three-stage sampling design in 1999. During May and June of 1999, 163 participants surveyed during the highest of the daily high tides on 16 beaches (8 on each site of Delaware Bay). During the first half of the spawning season, spawning was associated with lunar phases, but moderated by wave height. Disproportionately more spawning occurred within 3 d of the first new and full moons, and spawning activity (measured by an index of female density) was correlated inversely to the percent of beaches with waves ≥0.3 m. Spawning was heaviest on the Delaware shore around the full moon in May in spite of low waves in New Jersey during the new and full moons in May. Number of beaches sampled was the most important factor in determining the precision of the spawning index and power to detect a decline. Explicit consideration of statistical power has been absent from the current debate on horseshoe crab status and harvest. Those who argue against harvest restrictions because of a lack of statistically significant declines take on a burden to show that the surveys they cite have high statistical power. We show the Delaware Bay spawning survey will achieve high statistical power with sufficient sampling intensity and duration. We recommend that future Delaware Bay spawning surveys sample on 3 d around each new and full moon in May and June and increase the number of beaches to ensure high statistical power to detect trends in baywide spawning activity.

Comparative Status and Assessment of Limulus polyphemus with Emphasis on the New England and Delaware Bay Populations

Increases in harvest of the American horseshoe crab (Limulus polyphemus) during the 1990s, particularly for whelk bait, coupled with decreases in species that depend on their eggs has reduced horseshoe crab abundance, threatened their ecological relationships, and dictated precautionary management of the horseshoe crab resource. Accordingly, population assessments and monitoring programs have been developed throughout much of the horseshoe crab’s range. We review and discuss implications for several recent assessments of Delaware Bay and New England populations and a meta-analysis of region-specific trends. These assessments show that the western Atlantic distribution of the horseshoe crab is comprised of regional or estuarine-specific meta-populations, which exhibit distinct population dynamics and require management as separate units. Modeling of Delaware Bay and Cape Cod populations confirmed that overharvest caused declines, but indicated that some harvest levels are sustainable and consistent with population growth. Coast-wide harvest was reduced by 70% from 1998 to 2006, with the greatest reductions within Delaware Bay states. Harvest regulations in Delaware Bay starting in the late 1990s, such as harvest quotas, seasonal closures, male-only harvest, voluntary use of bait-saving devices, and establishment of the Carl N. Shuster Jr. Horseshoe Crab Reserve, were followed by stabilization and recent evidence of increase in abundance of horseshoe crabs in the region. However, decreased harvest of the Delaware Bay population has redirected harvest to outlying populations, particularly in New York and New England. While the recent Delaware Bay assessments indicate positive population growth, increased harvest elsewhere is believed to be unsustainable. Two important considerations for future assessments include (1) managing Delaware Bay horseshoe crab populations within a multi-species context, for example, to help support migratory shorebirds and (2) anticipating the potential for harvest restrictions within Delaware Bay to redirect harvest onto outlying populations that cannot sustain the increased harvest.

An Age-structured Population Model for Horseshoe Crabs in the Delaware Bay Area to Assess Harvest and Egg Availability for Shorebirds

The objective of this simulation study was to create an age-structured population model for horseshoe crabs (Limulus polyphemus) in the Delaware Bay region using best available estimates of age-specific mortality and recent harvest levels. Density dependence was incorporated using a spatial model relating egg mortality with abundance of spawning females. Combinations of annual female harvest (0, 50, 100, and 200 thousand), timing of female harvest (before or after spawning), and three levels of density-dependent egg mortality were simulated. The probability of the population increasing was high (>80%) with low and medium egg mortality and harvest less than 200 thousand females per year. Under the high egg mortality case, the probability of the population increasing was <50% regardless of harvest. Harvest occurring after spawning increased the probability of population growth. The number of eggs available to shorebirds was highest when egg mortality was lowest and female abundance was at its highest levels. Although harvest and egg mortality influenced population growth and food availability to shorebirds, sensitivity and elasticity analyses showed that early-life stage mortality, age 0 mortality in particular, was the most important parameter for population growth. Our modeling results indicate areas where further research is needed and suggest effective management will involve a combination of harvest management and actions to increase early juvenile survival.

Predation by Captive Wild Brook Trout on Calcein-Marked versus Nonmarked Atlantic Salmon Fry

Abstract Juvenile Atlantic salmon Salmo salar and other fish species marked with the fluorochrome dye calcein exhibit a green fluorescence in fin rays and other calcified structures under specific optical conditions. To determine whether brook trout Salvelinus fontinalis would preferentially prey upon calcein-marked individuals, we introduced calcein-marked and nonmarked Atlantic salmon fry simultaneously to captive wild brook trout in four controlled indoor raceway trials. Each trial consisted of 2 brook trout and 100 each of calcein-marked and nonmarked Atlantic salmon fry; no individuals were used in more than one trial. At the termination of each 3-d trial, predators were removed from raceways, and surviving Atlantic salmon fry were examined with a calcein detection device to tally numbers of marked and nonmarked individuals. In individual trials, 2 brook trout consumed between 20 and 99 Atlantic salmon fry over a 3-d period (10–49% of available prey). Replicated goodness-of-fit (G-statistic) analysis...

Evaluation of Removal Sampling for Basinwide Assessment of Atlantic Salmon

Abstract Removal estimators for stream fish abundance are widely used but can result in biased population estimates at the site level. We conducted computer simulations to examine how the Carle and Strub (1978) estimator, coupled with variation in catchability, influences the accuracy of population estimates at the site level. Site-level population estimates were then used to examine what effect potential bias in the population estimate at a site had on basinwide abundance estimates. Historic electrofishing data collected from Atlantic salmon Salmo salar in the Narraguagus River, Maine, were used as the baseline for construction of these simulations. At the site level, mean percent bias of population estimates was −23% when catchability was low (0.30–0.40) and when the true population was low (1–20 fish). Bias was reduced as the true population size increased and catchability increased. The negative bias at the site level affected total population estimates for the entire river basin. Under current sampli...

The Effect of Rearing Density on Growth, Survival, and Feed Conversion of Juvenile Atlantic Sturgeon

Abstract We investigated the effect of rearing density on the growth and survival of juvenile Atlantic sturgeon Acipenser oxyrinchus using initial rearing densities ranging from 10 to 30 fish per tank or 3.6–10.9 kg/m2 of substrate. The mean weight of fish at the start of the trial was 368.7 g. After 7 weeks of rearing, density ranged from 6.5 to 16.3 kg/m2 (14.7–36.6 g/L). Fish reared at the lowest density had significantly higher mean weight and length at the end of the trial. Growth over the 7-week experiment was modeled as a function of time, and growth rate was inversely proportional to density. The mean increase in total biomass was 63.14%, and the mean daily length gain was 0.23%. Overall survival was 99.7%, and calculated feed conversion rates ranged from 1.93 to 2.65. However, feed conversion rates were likely not at maximum levels due to the selection of a relatively high experimental feeding rate (2.5% of body weight per day).

Conservation Status of the American Horseshoe Crab, (Limulus Polyphemus): A Regional Assessment

Horseshoe crabs have persisted for more than 200 million years, and fossil forms date to 450 million years ago. The American horseshoe crab (Limulus polyphemus), one of four extant horseshoe crab species, is found along the Atlantic coastline of North America ranging from Alabama to Maine, USA with another distinct population on the coasts of Campeche, Yucatán and Quintana Roo in the Yucatán Peninsula, México. Although the American horseshoe crab tolerates broad environmental conditions, exploitation and habitat loss threaten the species. We assessed the conservation status of the American horseshoe crab by comprehensively reviewing available scientific information on its range, life history, genetic structure, population trends and analyses, major threats, and conservation. We structured the status assessment by six genetically-informed regions and accounted for sub-regional differences in environmental conditions, threats, and management. The transnational regions are Gulf of Maine (USA), Mid-Atlantic (USA), Southeast (USA), Florida Atlantic (USA), Northeast Gulf of México (USA), and Yucatán Peninsula (México). Our conclusion is that the American horseshoe crab species is vulnerable to local extirpation and that the degree and extent of risk vary among and within the regions. The risk is elevated in the Gulf of Maine region due to limited and fragmented habitat. The populations of horseshoe crabs in the Mid-Atlantic region are stable in the Delaware Bay area, and regulatory controls are in place, but the risk is elevated in the New England area as evidenced by continuing declines understood to be caused by over-harvest. The populations of horseshoe crabs in the Southeast region are stable or increasing. The populations of horseshoe crabs in the Florida Atlantic region show mixed trends among areas, and continuing population reductions at the embayment level have poorly understood causes. Within the Northeast Gulf of Mexico, causes of population trends are poorly understood and currently there is no active management of horseshoe crabs. Horseshoe crabs within México have conservation protection based on limited and fragmented habitat and geographic isolation from other regions, but elevated risk applies to the horseshoe crabs in the Yucatán Peninsula region until sufficient data can confirm population stability. Future species status throughout its range will depend on the effectiveness of conservation to mitigate habitat loss and manage for sustainable harvest among and within regions.

Effect of Iodophor Concentration and Duration of Exposure during Water Hardening on Survival of Atlantic Salmon Eggs

Abstract Because of disease transmission concerns, field studies to evaluate the impact of water-hardening eggs at different concentrations of polyvinylpyrrolidone iodine (iodophor) for various times of exposure have rarely used untreated controls. Additionally, U.S. Fish and Wildlife Service protocol requires a subsequent post–water-hardening surface disinfection for salmonid eggs transferred between stations. The cumulative impact of this second disinfection on survival has not been fully investigated for Atlantic salmon Salmo salar. This study compared the percent of eye-up Atlantic salmon eggs that had been water-hardened with iodophor treatments at 50, 100, and 150 mg active ingredient/L for 30, 60, and 90 min with that of untreated controls and also examined the impact on egg survival of a second iodophor disinfection 5 h after the initial exposure. No discernable mortality resulted from the second (10-min) disinfection. Nontreated eggs had significantly greater survival than any of the iodophor-tre...

Relative Abundance, Length Frequency, Age, and Maturity of Dolly Varden in Nearshore Waters of the Arctic National Wildlife Refuge, Alaska

Abstract Uncertainty about the environmental effects of oil development prompted a study of Dolly Varden Salvelinus malma in the nearshore waters of the Arctic National Wildlife Refuge, Alaska. Abundance of fish less than 400 mm fork length (FL), as indexed by tyke net catch per unit effort (CPUE), was significantly different among years, with the highest daily catch rates occurring in 1991, a year of heavy pack ice and relatively cold water temperatures. The CPUE for fish 400 mm or greater did not differ significantly among years. Within each sampling year, both large and small fish appeared to be more abundant in nearshore waters earlier in the summer. Lengths ranged from 53 to 734 mm. Growth was greater early in the season and in the years 1989 and 1990, and site-specific temporal changes in distribution were reflected in analysis of length data. Individual fish were aged at 1–13 years. A gonadosomatic index indicated that sexually mature fish were greater than 400 mm FL. Wide summer dispersal makes Do...

Mortality Associated with Catch-and-Release Angling of Striped Bass in the Hudson River

Abstract Catch-and-release fishing commonly occurs in recreational fisheries, including those for the striped bass Morone saxatilis of the Atlantic coast. The contribution of catch-and-release practices to overall fishing mortality is often not estimated. We estimated the catch-and-release mortality for the Hudson River spawning stock of striped bass in 2001. Volunteer anglers caught striped bass between April 30 and May 16, 2001. Fish were transferred to transport boats in live wells and placed in one of nine 15,000-L land-based holding tanks. Control fish were collected by electrofishing and otherwise handled similarly. Treatment and control fish were uniquely tagged and held together for 5 d. Hooking mortality was estimated via conditional rate and additive rates. These two estimation techniques partitioned total observed mortality into hooking mortality and handling mortality, the latter being estimated from control fish. Catch-and-release mortality for striped bass averaged 16% for traditional J hook...