Richard A. Wahle

Habitat restrictions in early benthic life : experiments on habitat selection and in situ predation with the American lobster

In shallow marine environments, many animals that eventually attain large body size and range widely are restricted to refugia early in life. A prime example is the American lobster Homarus americanus Milne Edwards. During the first few years of benthic life, lobsters are most strongly associated with shelter providing habitats (e.g., cobble) but this association is less frequent as they grow. The restricted distribution of such early benthic phase lobsters (5- ≈ 40 mm carapace length, CL) may be reinforced by predation, but experiments to quantify predators and predation rates in situ have not been reported. This study confirms previous habitat selection studies in showing that shelter-seeking behavior is likely the proximate cause of the association, but that predation probably reinforces the association until lobsters outgrow their most vulnerable size. Field predation experiments and video observation show that tethered early benthic phase lobsters were attacked by demersal fishes and crabs significantly more often when unsheltered by cobble, and that this vulnerability declines dramatically with increasing body size. Moreover, many of the species of fish and crab predators observed by video were common at the five sites censused in mid-coast Maine, and occur throughout the range of the American lobster. There is strong evidence that lobsters and their macruran (large abdomened) allies in shallow marine and aquatic environments are similarly restricted to shelter-providing habitats early in their benthic life, possibly because of their inability to avoid predators by rapidly burying themselves in sediment.

Body-size dependent anti-predator mechanisms of the American lobster

Like many lobsters and crayfish, the American lobster Homarus americanus exhibits an ontogenetic shift from a strong, apparently predator reinforced association with sheltering habitats early in life to an increasingly wide ranging existence as it grows. At least in part, this shift may be related to a change in behavioral responses to predators, but neither the sensory capabilities nor the size-specific responses of lobsters to predators are well studied. I conducted laboratory experiments to determine the response of early benthic phase lobsters (ca. 5 to 40 mm carapace length) to the physical and chemical presence of fish predator

Pre- and post-settlement processes in recruitment of the American lobster

Abstract In censuses conducted between 1989 and 1993 the cause of consistent local differences in benthic recruitment of the American lobster ( Homarus americanus Milne-Edwards) to coastal sites in Maine was unclear. Field experiments were conducted to assess the role of pre- and post-settlement processes in causing high and low extremes in recruitment on opposite sides of an outer coastal island in our study area. The west side of the island has some of the highest population densities measured in New England. Our results indicate that postlarval supply determines these differences in recruitment between the two sites. Standardized replicate cobble plots deployed on each side of the island ruled out habitat differences as an explanation for these differences, because they exhibited the same east-west difference in recruitment as the natural habitat. We also ruled out differing rates of post-settlement loss because we recovered previously marked and released settlers in equal numbers in similar plots from both sites. The distribution of neustonic postlarvae and hydrographic evidence indicate that wind-driven surface transport produces an asymmetric postlarval supply to the two sides of the island during the settlement season. Differences in the degree of asymmetry from year to year correspond to differences in the magnitude of eastward transport. We also conducted experiments at the site receiving high recruitment to assess whether new recruits or older year classes were near the saturation of cobble habitat for these animals. The combination of saturation seeding trials, using hatchery-reared lobsters, and weekly counts of natural recruits and immigrants suggests that lobsters may become increasingly subject to crowding as they grow. Movements away from the initial settlement site, probably in part caused by crowding, tend to smooth the inequality in population density which is set initially by postlarval supply.

EARLY LIFE HISTORY AND RECRUITMENT PROCESSES OF CLAWED LOBSTERS

This review focuses on the larval, postlarval, and early benthic life of Homarus and Nephrops, the two best studied, and commercially most important, genera of the Nephropidae. Processes acting during this time of the life cycle may be critical of the fate of a cohort. In the past decade, significant advances have been made in understanding events before and after settlement. Nephrops and Homarus are very similar with respect to the processes affecting the distribution of the pelagic larvae and postlarvae: wind, currents and larval behavior play a significant role. Duration of the pelagic phase is determined by temperature, timing of settlement, and perhaps nutrition. Both genera probably arc quite selective of substratum during settlement, but this has been investigated only in Homarus, where substratum type, odor, and predator presence affect choice of habitat. The two genera contrast, however, with respect to some of the processes occurring during and after postlarval settlement. Although both are cryptic, newly settled Homarus are found in shallow, rocky habitats, while settled Nephrops arc found in deep water, in burrows they construct in cohesivc mud. Homarus undergoes a developmental change in behavior within the first few years of benthic life that causes an increasingly wide range of movement; sexually mature H. americanus can move hundreds of kilometers in a year. Nephrops emerges more as it grows, but appears to be far more sedentary than Homarus. Thus in contrast to Homarus, a number of distinct populations of Nephrops exist within the species range that are clearly defined by habitat boundaries. New techniques now permit routine census and tagging of early benthic phase Homcarus, making it possible to follow year classes from the time of settlement. It is the years immediately after settlement that a cohort may be subject to density-dependent controls, a key issue facing workers on both groups.

Recruitment to American Lobster Populations Along an Estuarine Gradient

This study evaluates patterns in the distribution and abundance of newly recruited (young-of-the-year) and older American lobster (Homarus americanus Milne Edwards) along a 22 km length of the Narragansett Bay estuary, Rhode Island, with particular attention to substratum associations. This not only represents the first assessment of benthic recruitment of this species along an estuary, but it is also the first study of lobster recruitment in southern New England. Censuses were conducted by divers in a substratum-specific manner. in cobble-boulder habitat, with the aid of a diver-operated suction sampler, I found newly recruited (5–10 mm carapace length) lobsters to be most abundant on the open coast, with numbers diminishing to zero in the upper bay. Visual censuses of older lobsters in the same habitat revealed a similar pattern. On featureless sedimentary habitats new recruits were absent and lobster densities were at least two orders of magnitude lower than in rocky habitats. In Narragansett Bay, rocky habitats comprise a small proportion of the bottom. The availability of such habitats, the relative importance of larval supply and potential physiological stress in limiting recruitment up-bay remain unclear.

The Feeding Ecology of Crangon Franciscorum and Crangon Nigricauda in San Francisco Bay, California

ABSTRACT The distribution, abundance, and diet of Crangon franciscorum and C. nigricauda were estimated from 15 monthly samples from the channel and shallows of San Pablo Bay. Crangon franciscorum outnumbered C. nigricauda by an overall ratio of 10:1. The majority of the C. nigricauda (85%) were taken in the high salinity waters of the channel. Most of the C. franciscorum catch (68%) came from the low salinity shallows. This species concentrated only in the channel when reproductive in the winter. Crangon franciscorum and C. nigricauda ate prey in common, although relative abundance of foods in the diet varied between species. Crangon nigricauda concentrated its diet on amphipods, while C. franciscorum partitioned its diet more evenly among several prey. Amphipods, bivalves, and foraminiferans were eaten significantly more often in the food-rich shallows than in the channel. These foods and others varied seasonally in the diet. Ontogenetic shifts in types and sizes of prey taken were observed, with crangonid shrimp remains occurring in large C. franciscorum (≧40 mm). Neomysis spp., abundant in the diets of caridean shrimps of the upper reaches of the estuary, are notably rare in the crangonids of San Pablo Bay. The diets of C. franciscorum and C. nigricauda are heavily influenced by predator size, temperature-salinity preferences, and prey availability.

Substratum constraints on body size and the behavioral scope of shelter use in the American lobster

Abstract For large-bodied crustaceans like the American lobster Homarus americanus Milne Edwards, substratum can set upper limits to the sizes of the occupants. Based on prior field observations, laboratory experiments were designed to examine a simple conceptual model that predicts the maximu lobster body size that can occupy the interstitial spaces among cobbles of a given diameter. Laboratory behavioral observations indicated that although lobsters must find larger shelters as they grow, they acquire the ability to manipulate an increasingly wide spectrum of substratum sizes, thereby preventing strict agreement with the model. On the basis of these observations, a graphic model of lobster-substratum size-scaling is presented and the potential demographic consequences of shelter constraints in nature is discussed.

Early Life History and a Modeling Framework for Lobster (Homarus Americanus) Populations in the Gulf of Maine

Abstract Beginning in the late 1980s, lobster (Homarus americanus) landings for the state of Maine and the Bay of Fundy increased to levels more than three times their previous 20-year means. Reduced predation may have permitted the expansion of lobsters into previously inhospitable territory, but we argue that in this region the spatial patterns of recruitment and the abundance of lobsters are substantially driven by events governing the earliest life history stages, including the abundance and distribution of planktonic stages and their initial settlement as Young-of-Year (YOY) lobsters. Settlement densities appear to be strongly driven by abundance of the pelagic postlarvae. Postlarvae and YOY show large-scale spatial patterns commensurate with coastal circulation, but also multi-year trends in abundance and abrupt shifts in abundance and spatial patterns that signal strong environmental forcing. The extent of the coastal shelf that defines the initial settlement grounds for lobsters is important to future population modeling. We address one part of this definition by examining patterns of settlement with depth, and discuss a modeling framework for the full life history of lobsters in the Gulf of Maine.

Effects of habitat patchiness on American lobster movement across a gradient of predation risk and shelter competition

The influence of landscape structure on marine ecological processes is receiving increasing attention. However, few studies conducted in coastal marine habitats have evaluated whether the effects of landscape structure on species interactions and organismal behavior are consistent across the range of an organism, over which landscape context and the strength of species interactions typically vary. American lobster (Homarus americanus) juveniles seek refuge from predators within shallow rocky habitat but make short-distance movements to forage outside of shelter. We evaluated how the patchiness of cobble habitat influences juvenile lobster movement by conducting mark-recapture experiments on lobsters placed within patchy and contiguous cobble plots in three regions of New England among which risk of predation and intraspecific shelter competition vary (Rhode Island, mid-coast Maine, and eastern Maine, USA). We also evaluated whether habitat patchiness influenced lobster colonization of plots and whether lobster fidelity to individual shelters corresponds to variability in predator abundance and conspecific density among regions. Cobble patchiness reduced rates of lobster movement in all three regions in 2004 and in two of three regions in 2005, despite large differences in landscape context among regions. Region had much larger effects on lobster colonization than did patchiness, but patchy plots were colonized at higher rates than were contiguous plots where lobster densities were highest. Fidelity to shelter was higher in regions with low conspecific density (Rhode Island and eastern Maine) than in mid-coast Maine where conspecific density is high and where unmarked lobsters often occupied shelters vacated by marked lobsters. Our results indicate that cobble patchiness influences juvenile lobster movement at small scales, but that the effects of patchiness on movement were consistent across much of the range of the American lobster despite strong regional variation in predator abundance and conspecific density.

A vessel‐deployed passive postlarval collector to assess settlement of the American lobster Homarus americanus

Abstract Passive collectors are used widely in postlarval settlement and recruitment monitoring of spiny lobsters and crabs, but they have only been used in a limited way with clawed lobsters. For nearly two decades, diver‐based suction sampling has served to monitor spatial‐temporal patterns of American lobster (Homarus americanus) postlarval settlement and early juvenile abundance in shallow near‐shore nurseries. Collectors could reveal settlement patterns in zones beyond the practical limits of diving. In 2005, we launched a fisher‐scientist collaboration to evaluate the performance of passive collectors designed to extend the reach of sampling, and to be deployable from a vessel equipped with a standard pot‐hauler. Building on previous designs, our collectors comprised wire mesh trays lined with fine screening on the floor and walls and filled with cobble to simulate natural nursery habitat. Results indicate that no newly settled lobsters were lost during the retrieval process, and densities of young‐of‐year lobsters found in the collectors were similar to those in directly adjacent natural cobble habitat sampled by divers with suction samplers. The collectors also proved to be effective samplers of juvenile fish and crabs, suggesting a possibility for wider application. This success bodes well for expanded deployment of cobble collectors to broaden our understanding of the recruitment processes of lobster and other cobble‐dwelling fauna along the coast of New England, United States and Atlantic Canada.