Nathan R. Geraldi

Evaluating local population dynamics of the American lobster, Homarus americanus, with trap‐based mark‐recapture methods and seabed mapping

Abstract Seabed mapping, spatially referenced trapping, and mark‐recapture methods have all been useful tools in ecological studies of lobsters and other benthic animals. Here we integrate the three methods to evaluate local population dynamics and movements of American lobsters, Homarus americanus, in coastal fishing grounds in Maine, United States. The study was conducted on five study areas of different size, and used two different sampling protocols. At one site (1 km2 in area) we used a monthly mark‐recapture sampling interval over a 6‐month period, only tagging a subsample of the catch. At four smaller sites (0.3 km2) we used a shorter‐term approach, sampling at 3–4‐day intervals for a 2‐week duration, tagging the entire catch. Tagging data were analysed with a modified Jolly‐Seber model adapted for continuous sampling to estimate population abundance, gains (immigration), and losses (emigration and mortality). Side‐scan sonar surveys of the seabed combined with diver‐based population surveys, stratified by substrate type, provided an independent comparison to mark‐recapture‐based estimates of abundance over the same areas. Spatial referencing of trap catch also allowed us to relate catch rates and lobster movements directly to seabed features. The longer‐term tagging data on the larger study area provided abundance estimates that were more consistent with the diver observations, and estimates of gains and losses statistically more robust, than those derived from the shorter‐term effort on the smaller sites. The flux of lobsters followed the well known seasonal movements on these fishing grounds, with gains and losses from the larger study area ranging over 1000 individuals per day, and an estimated mid‐summer peak density of >65 000 lobsters per km2 (individuals >50 mm carapace length). This approach may lend itself to broader application with the American lobster.

Habitat effects on American lobster (Homarus americanus) movement and density : insights from georeferenced trap arrays, seabed mapping, and tagging

Understanding the influence of heterogeneous marine landscapes on the movements of benthic megafauna is often hampered by limited spatial resolution and insufficient sample size. Here, we combined the benefits of seabed mapping, georeferenced trap arrays, and conventional tagging methods to quantify the effect of substrate on movements of the American lobster (Homarus americanus). In total, 21 848 lobsters were tagged, and movements were tracked among spatially referenced research and commercial traps. We found that lobster densities from diver surveys were highest on rocky habitat, but catch rates in traps were highest on unstructured sediment, resulting in traps on level bottom having a larger effective fishing area than traps on structurally complex habitat. Moreover, tag returns indicated that lobsters initially caught and released on sediment moved farther and faster than those initially caught in traps on rocky substrate. These observations are consistent with previous reports of the existence of a ...

Enhancing the potential for population recovery: restoration options for bay scallop populations, Argopecten irradians concentricus, in North Carolina.

ABSTRACT Historically depressed bay scallop populations in North Carolina have retained some capacity to replenish themselves. However, continued abnormally high predation by cownose rays, and growing degradation of seagrass beds may limit bay scallop population recovery. We modified existing methods for protecting spawning adult scallops from cownose rays (stockades) and enhancing scallop set (spat collectors in the natural habitat and in managed shore-side ponds) to determine whether the population growth of bay scallops could be enhanced. We found, with one important exception, that spawner sanctuaries inside stockades could be used to concentrate and protect adult scallops during a time when they are susceptible to ray predation. Spat collector bags proved effective not only in gathering scallop spat but serving as nurseries for juvenile scallop grow-out. The results from our pond experiments were mixed: spat collector bags did not do well in the pond, but an alternative collector design holds promise for making shore-side ponds important sources for inexpensive scallop seed. The simple techniques examined in this study can be used to enhance the potential of bay scallop populations to recover from low abundances, and were applied successfully to enhance North Carolina bay scallops during the two years of this project.

Restricting Prey Dispersal Can Overestimate the Importance of Predation in Trophic Cascades

Predators can affect prey populations and, via trophic cascades, predators can indirectly impact resource populations (2 trophic levels below the predator) through consumption of prey (density-mediated indirect effects; DMIEs) and by inducing predator-avoidance behavior in prey (trait-mediated indirect effects; TMIEs). Prey often employ multiple predator-avoidance behaviors, such as dispersal or reduced foraging activity, but estimates of TMIEs are usually on individual behaviors. We assessed direct and indirect predator effects in a mesocosm experiment using a marine food chain consisting of a predator (toadfish – Opsanus tau), prey (mud crab - Panopeus herbstii) and resource (ribbed mussel – Geukensia demissa). We measured dispersal and foraging activity of prey separately by manipulating both the presence and absence of the predator, and whether prey could or could not disperse into a predator-free area. Consumption of prey was 9 times greater when prey could not disperse, probably because mesocosm boundaries increased predator capture success. Although predator presence did not significantly affect the number of crabs that emigrated, the presence of a predator decreased resource consumption by prey, which resulted in fewer resources consumed for each prey that emigrated in the presence of a predator, and reduced the overall TMIE. When prey were unable to disperse, TMIEs on mussel survival were 3 times higher than the DMIEs. When prey were allowed to disperse, the TMIEs on resource survival increased to 11-times the DMIEs. We found that restricting the ability of prey to disperse, or focusing on only one predator-avoidance behavior, may be underestimating TMIEs. Our results indicate that the relative contribution of behavior and consumption in food chain dynamics will depend on which predator-avoidance behaviors are allowed to occur and measured.

Prey size structure diminishes cascading effects by increasing interference competition and predation among prey

The size of an organism can change by orders of magnitude during its lifespan. Size can determine whether an individual consumes, is consumed, competes, or avoids individuals of the same or different species. Two complementary mesocosm experiments with a tri-trophic food chain (top predator, toadfish, Opsanus tau; intermediate prey, mud crab, family Xanthidae; basal resource, oyster, Crassostrea virginica) were conducted to measure how the size of both the top predator and the intermediate prey affects consumptive and behavioral interactions in trophic cascades. In the first experiment, I systematically varied the sizes of predators and prey, respectively. The amount of crab biomass consumed was dependent on crab size and not toadfish size, but the effect of crab size did not cascade to alter oyster survival. Increased oyster survival from crab interference competition in the absence of toadfish was similar to oyster survival,from predator-avoidance behavior in the presence of a toadfish. When all crab size classes were present, crab mortality was similar in the presence and absence of toadfish, highlighting the importance of intraguild predation in food-web dynamics. The second experiment separated crab mortality by other crabs from crab mortality by predatory toadfish and found that crab mortality generally switched from intra- to interguild predation when a toadfish was present. In addition, field surveys indicated mud crab abundance and size was primarily influenced by mud crab recruitment, but not by toadfish abundance, which supports our experimental results that interactions among mud crabs have similar effects to predator-prey interactions. These findings indicate that changes in size or abundance of intermediate prey may be comparable to changes in top predator abundance in terms of trophic interactions and their transmission to lower levels, which suggests that certain types of relatively simple food chains can be resilient to the loss of higher trophic levels.