Lobster Density Research Articles

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.

Kelp beds as habitat for American lobster Homarus americanus

The American lobster Homarus americanus and kelp Laminaria longicruris and L. saccharina are prominent and often intimately associated members of the subtidal community in the western North Atlantic Ocean. However, no one has identified the nature of this relationship or specifically investigated whether kelp beds are a superior habitat for lobsters. We conducted field studies in 1990 and 1991 at a coastal site centrally located along the Gulf of Maine. USA, to determine how lobsters use kelp beds as habitat. Identically sized and spaced plots of live and artificial (plastic) kelp were established and monitored for lobster population densities. Adjacent featureless sediment plots of identical size served as controls. Lobster population density and biomass were significantly higher in both real and artificial kelp treatments than in non-kelp control plots (p < 0 0001). The change in lobster density was apparent the day following placement of the expenment, so a secondary trophic effect such as attracting prey into treatments is unlikely to have occurred. Thus, kelp beds can affect local lobster population dens~ties by providing shelter for lobsters, thereby concentrating individuals and increasing the local carrying capacity of potential lobster habitats The effect of kelp beds on the local carrying capacity of lobster habitats was further explored by test~ng how lobsters respond to differing patch sizes. A graded size series of circular patches of artificial kelp was established, in which kelp blade density and total area were held constant for each treatment. Treatments were subdivided into four 1 m', two 2 m*, or one 4 m2 patches. Experiments were surveyed for lobster population density and size structure to determine if statistical differences existed among treatments. Lobster density was significantly greater in the smallest patches (p c 0.001). Moreover, lobsters typically occupied the edges of kelp beds, and their abundance within kelp patches corresponded to the patch's perimeter-to-area relationship. This suggests that 'edge effects' influence the local carrying capacity for lobsters by influencing the lobsters' choice of kelp beds as habitat.

Shelter Selection by Spiny Lobster Under Variable Predation Risk, Social Conditions, and Shelter Size

Shelter use patterns of den—dwelling Caribbean spiny lobster, Panulirus argus, appear to be regulated by predation risk. The risk of predation may be modified by (1) social structure, which alters the effectiveness of communal defense, and (2) the scaling between lobster size and shelter size, which enhances the protective capacity of the den. These hypotheses were tested with field enclosure experiments using artificial lobster shelters, which examined the effects of predation risk (i.e., presence or absence of a major predator, the nurse shark Ginglyostoma cirratum), spiny lobster size, social condition (i.e., presence or absence of conspecifics), and shelter size upon den choice by juvenile and adult P. argus. To corroborate the findings of the enclosure experiments we also quantified seasonal, size—specific abundance patterns of P. argus in the field by deploying artificial lobster shelters (casitas) of different sizes in two habitats that differed primarily in the potential for gregarious interactions: an inner—bay, sand seagrass flat with high lobster densities, and an outer—bay, seagrass bed adjacent to coral reefs with sparsely distributed lobsters. The experimental and observational field results were strikingly similar–social condition and the scaling of lobster size to shelter size jointly regulated den choice patterns of adult and juvenile Panulirus argus, particularly under high predation risk. When conspecific density and predation risk were low, lobsters resided primarily in shelters whose dimensions were scaled to their own; when conspecific density was high and predation risk was low, lobsters resided predominantly in large shelters offering the highest potential for gregariousness; when conspecific density and predation risk were high, lobsters shifted to gregarious habitation in smaller, scaled shelters; and, when predation risk was high and conspecific density was low, lobsters occupied smaller shelters. The frequency of gregariousness in the field was much higher at the inner—bay site, where lobsters were dense, than at the outer—bay site, where lobsters were sparse, even accounting for the difference in lobster density between sites. This study indicates that the density of conspecifics in a given habitat can enhance gregariousness in spiny lobsters, which in turn influences the relative impact of lobster size, shelter size, and predation risk upon den choice. In defining the critical determinants of den choice for P. argus, we also provide an empirical and conceptual framework for identifying how variation in the availability of resources, such as conspecifics and appropriately scaled refuges, influence the distribution and abundance of social, shelter—dwelling species.

Recruitment habitats and nursery grounds of the American lobster Homarus americanus: a demographic bottleneck?

We have identified benthic recruitment habitats and nursery grounds of the American lobster Homarus americanus Milne Edwards in the coastal Gulf of Maine, USA, by systematically censusing subtidal sediment, cobble, and ledge substrata. We distinguish lobsters between settlement size (5 mm carapace length (CL)) to ca 40 mm CL as the 'early benthic phase' (EBP) because they are ecologically and behaviorally distinct from larger lobsters EBP lobsters are cryptic and apparently restricted to shelter-providing habitats (primarily cobble substratum) in coastal Gulf of Maine. In these habitats we found average population densities of EBP lobsters as high as 6.9 m-2 EBP lobsters were virtually absent from ledge and sedimentary substrata devoid of vegetation although larger lobsters are commonly found there. It is possible that the requirement for shelter-providing substrata by this life phase creates a natural demographic 'bottleneck' to benthic recruitment for the species. Prime cobble recruitment habitat is relatively rare and comprises ca 11 % of the 60.2 km of shoreline at our study area in midcoast Maine. If this low availability of cobble exists throughout the Gulf of Maine, as other studies indicate, it could limit lobster production potential. We verified the geographic extent of recruitment to cobble habitats censused in 3 of 4 regions spanning ca 300 km of the coastal Gulf of Maine (from Nahant, Massachusetts to Swans Island, Maine). Early benthic phase lobsters were absent from cobble censused in the northeastern extreme of our survey (Swans Island). This pattern is consistent with earlier speculation that relatively cool water temperatures may limit larval settlement in this region.

Estimating the Density and Mortality of Juvenile Western Rock Lobsters (Panulirus cygnus) in Nursery Reefs

Studies of juvenile rock lobsters (Panulirus cygnus) in the nursery reefs of Western Australia indicated that density-dependent processes on the reefs were the major determinant of year-class strengths of recruitment to the fishery. Reassessment of early data, plus additional information, indicates that these estimates may have been inaccurate. Alternative methods of estimating juvenile densities were examined; application of length–frequency analysis to calculate the age-classes; visual estimates, and single-night trapping. Length–frequency analysis indicated that either immigration onto the test reefs had occurred, or the data were inaccurate. Visual estimates and single-night trapping were more rapid and used less manpower and resources than mark–recapture methods, but the estimates were of unknown accuracy. No truly satisfactory method of making reliable estimates of either the density of natural mortality rate of juveniles in the nursery reefs is yet available. Nevertheless, the data on the levels of larval settlement and subsequent catches, suggest that density dependent processes probably have a significant effect on the levels of recruitment to the fishery, although the upper level to this recruitment has not been reached over the past 20 yrs.

Use of Salt-Marsh Peat Reefs by Small Juvenile Lobsters on Cape Cod, Massachusetts

The habitats utilized by small juvenile (<40 mm carapace length, CL) lobsters (Homarus americanus) are poorly known. We discovered and studied an undescribed juvenile lobster habitat in Nauset Marsh, Cape Cod. Juvenile lobsters (X=26.7 mm carapace length, 6 to 72 mm, n=38) were collected from suction samples primarily 0144 01 in “peat reef” habitats during the period from August 1985 through October 1986. The reefs consisted of large blocks ofSpartina alterniflora peat that had separated from the marsh surface and fallen into adjacent subtidal marsh channels. The smallest lobsters (6 to 7 mm CL) were collected from peat reefs in October 1985, and April and July 1986. In these habitats, juvenile lobster density averaged 2.5 individuals m−2 (range 0–5.7) in suction samples. Peat reef habitats occur in other salt marshes in the northeastern United States and may be an important nursery habitat for small juvenile lobsters.

Effect of a baited trap on the foraging movements of juvenile western rock lobsters, Panulirus cygnus George

The influence of a commercial trap on the foraging movements of juvenile P. cygnus was determined by tracking animals to which electromagnetic tags had been attached. Catch rates were low. Tagged lobsters were caught on eight occasions; on 14 occasions, tagged lobsters visited the trap and stayed for several hours during the night, but left by morning. The greatest distance travelled by a lobster from down stream to the baited trap was 120 m. On 19 occasions lobsters upcurrent from the trap passed as close as 20 m without going to the trap, although other lobsters as far away as 75 m upcurrent visited the trap. Previous tracking records of individuals foraging upcurrent of the trap suggest that these lobsters' visits may have been due to the trap's location within their foraging range rather than to detection of the bait. Present trapping techniques used to estimate the densities of juvenile lobsters on individual patch reefs may be biased because lobsters on neighbouring reefs as far away as 120 m could be attracted to the baited trap. The present results also suggest that some lobsters may leave the traps before sunrise, and that traps set closer than 120 m apart may fish competitively for the same individuals.

Fish predators of the western rock lobster (Panulirus cygnus George) in a nearshore nursery habitat

Nearshore limestone reefs of Western Australia represent an important nursery habitat for juveniles of the western rock lobster (Panulirus cygnus). As part of a trophic study of the fish community associated with these reefs, fishes were examined for evidence of predation on western rock lobsters. Six fish species were found to have ingested P. cygnus. Of these, Psammaperca waigiensis (sand bass), Pelsartia humeralis (sea trumpeter), Pseudolabris parilis (brown-spotted wrasse) and Plectorhyncus flavornaculatus (gold-spotted sweetlips) were among the ten most abundant fish in gill-net collections from the study site. P. cygnus formed a significant portion of the diet of P. waigiensis, but was &lt; 5% of the dietary volume of other species. Predation occurred at all times of the day and was concentrated on small, newly-settled P. cygnus (8-15 mm carapace length). The vulnerability of lobsters to predation appears to be strongly related to size, and the cryptic habits that are unique to newly-settled stages can be explained in terms of the risks of predation. Assessment of the proportion of mortality of P. cygnus due to predation is hampered by a current lack of information on the natural densities of both fishes and small rock lobsters. However, conservative estimates indicate that an annual removal by fish of thousands of lobsters per hectare is likely, suggesting that predation is a major factor affecting the survival of P. cygnus on nursery reefs.

Ecology and Growth of Postlarval and Juvenile Lobster, Homarus americanus, off Îles de la Madeleine (Quebec)

Lobsters, Homarus americanus were studied in various habitats off Îles de la Madeleine, Quebec, from June to October 1985. Density, biomass, and size structure were estimated quantitatively using underwater corrals (40 m2) on rocky bottoms or transects (200 m2) on sandy bottoms and eelgrass beds. On the basis of behavioural observations during capture, lobsters were divided into postlarvae (&lt;25 mm carapace length, CL), juveniles (25–76 mm CL), and adults (&gt;76 mm CL). Density, biomass, and size structure were closely related to the coarseness of the bottom. Density and biomass were greater and mean size was larger on algae-covered boulders than on bare stones. Postlarval and juvenile lobsters were most numerous on shallow rocky bottoms where high densities (0.8–3.8 ind∙m−2) of small lobsters (mean size from 23.8 to 35.5 mm CL) were captured. Low density of juveniles and adults occurred on sand covered with eelgrass. No resident lobsters were found on bare sand. Temporal variation of postlarval density was synchronous at the three stations investigated. Postlarval density decreased during the summer until the settlement of planktonic postlarvae in mid-August. Moulting activity occurred throughout the summer, with a peak in early July. Mean size values for stages V–XIV were fitted to the cumulated size distribution and were used to calculate the percent increment for each moult stage (growth factor) and the expected duration of each stage. The 2-mo duration of the stage IV settlement period could result in a size range of 6–21 mm CL (mean size 14.5 mm CL, stage VIII) for a cohort by the end of its first season of benthic growth. The transition from planktonic to benthic life was characterized by a decrease of the growth factor at stage V, which was subsequently compensated at stages VI and VII by high growth factor values. In the first three growth seasons, spring and summer moults exhibit higher growth factor values than fall moults. These seasonal variations result in regular oscillations of the growth factor dampening progressively between stages V and XIV, instead of the smooth decline hypothesized in previous studies. A combination of factors, such as cryptic behaviour, high substrate selectivity, gradual settlement over the late summer months, and variability in growth factor, reflect the very fine tuning of the lobster's early benthic life stages to the constraints of its physical environment.

Predation of Sea Scallops (Placopecten magellanicus) by Lobsters (Homarus americanus) and Rock Crabs (Cancer irroratus) in Underwater Cage Enclosures

Predation studies in underwater cage enclosures on natural substrates were conducted in 1979 and 1980 to assess the propensity of adult American lobsters, Homarus americanus, and rock crabs, Cancer irroratus, to prey upon sea scallops, Placopecten magellanicus. In 1979, the initial prey by both predator species was large, embedded horse mussels (Modiolus modiolus). In the enclosure containing lobsters, this was followed by lobster cannibalism and, finally, when lobster density was reduced, predation on large sea scallops (80–110 mm shell height). No cannibalism or predation on large scallops occurred in the rock crab enclosure. In 1980, small scallops (40–55 mm shell height) were presented to lobsters and rock crabs; predation occurred regardless of predator density. Large sea scallops and unembedded horse mussels were consumed when crustacean densities approached natural levels. These studies demonstrate that small sea scallops and embedded horse mussels can be readily consumed by lobsters and rock crabs under field conditions, but that natural predation of large scallops by either rock crabs or lobsters is of lesser magnitude.Key words: sea scallops, crabs, lobsters, predation, prey selectivity

Predator-prey relationships between the rock lobster Jasus lalandii and the mussel Aulacomya ater at Robben Island on the Cape West Coast of Africa

The natural diet and mode of feeding of the rock lobster Jasus lalandii (H. Milne Edwards) was determined in a rock-lobster sanctuary near Cape Town, South Africa. Field observations were tested and confirmed by means of aquarium studies. Rock lobsters feed mainly upon ribbed mussels Aulacomya ater (Molina), which comprise the largest component of the sessile benthic fauna. Mussel remains were found as the major constituent in 97% of the rock-lobster stomachs examined. The density of rock lobsters averaged 8,100 per hectare (0.81 m-2), while mussel biomass averaged more than 5 kg (wet whole weight) m-2 within the same depth range (12 to 30 m). More than 80% of the mussel biomass comprised large individuals between 60 and 90 mm in length. Large rock lobsters (mainly males) were capable of feeding on all sizes of mussels, although many of these were inaccessible to predation. Smaller rock lobsters became progressively more limited in the size range of mussels which they could crack open and consume. Competition between rock lobsters for small mussels appeared to be intense, as mussels of suitable size for feeding were generally in short supply to most of the rock-lobster population. Hence, feeding and growth rates of rock lobsters are likely to be affected by the relative population densities of predator and prey. Growth rates of rock lobsters could be limited by food supplies even in areas where mussel biomass is comparatively large.

COMPONENTS OF GROWTH RATE VARIATION AMONG LABORATORY CULTURED LOBSTERS (Homarus)

AbstractLaboratory‐reared juvenile lobsters (Homarus americanus) vary considerably in growth rate. Up to 30% of this variation appears to be heritable, but culture system, family by system interaction, and density differences within and between systems also contribute substantial growth rate variance. Results from split‐plot analyses of growth variance among 8 families of full‐sib juveniles grown for 90 days on live, adult brine shrimp at 3 temperatures (14°C ambient, 18°C, and 21°C; 2 replications) are in complete agreement with previous findings. Family, and by implication genotype, is a significant component of growth rate variance. Environment, however, plays an even more significant role. Density of lobsters in a system at the time of stocking new families affects growth of the younger juveniles adversely. Temperature produces expected fixed effects on growth rate with no evidence of statistical interaction between family and temperature. Replication over similar, but independent, experimental grow‐out systems reveals added growth variance apparently due to uncontrolled culture system variables. Significant culture system x genotype interactions are also observed.Maternal influences and environmental effects arising from separate rearing of families until stage IV are confounded in the family differences observed in our growth studies. Two preliminary observations bearing on this problem show no evidence for a strong influence of larval history on juvenile growth.Implications from this study are (1) that selective breeding for faster growth of lobsters would probably be successful although genotype x environment interactions might make the breeding program more complex and costly; (2) that important, possibly density‐dependent, environmental factors have not been identified or controlled in studies of lobster growth; and therefore (3) that much more research is needed to verify the biological validity of current concepts of high density, commercial Lobster aquaculture.

Contrasts between the benthic communities of subtidal hard substrata at Marcus and Malgas islands: a case of alternative stable states?

The fauna and flora of two sublittoral communities situated in close proximity on the west coast of South Africa are described and shown to be radically different, despite the fact that both experience similar physical conditions. One of these communities, at Marcus Island, has prolific beds of the black mussel Choromytilus meridionalis (35 954 g wet weight·m−2) accompanied by a large number of other species, notably sea urchins, holothurians, brittle stars, whelks and barnacles. In contrast, the other site, Malgas Island, is dominated by a large population of rock lobsters Jasus lalandii together with a dense seaweed flora (3 866 and 4 402 g wet weight·m−2 respectively). The numbers of species are similar at both sites (102 at Malgas Island and 107 at Marcus Island) but only 34 per cent are common to both localities. Although preliminary research indicates that the two sites have fairly similar physical conditions and bottom topography, they maintained their contrasting communities for the whole research period (1983–1986) and have probably done so for at least nine years, suggesting that they represent two stable alternative states of the same ecosystem. In the absence of rock lobsters at Marcus Island, wave action and inter/intraspecific competition are the main forces structuring the benthic community. Stability is maintained here by the resilience and rapid recovery of the mussel population after disturbance by storms. Where rock lobsters are abundant, as at Malgas Island, intense predation pressure is the main force controlling the structure of the benthos, and it is continually maintained because the rock lobsters are resistant to the effect of storms. Therefore, the presence or absence of high densities of rock lobsters is fundamental to the maintenance of these two contrasting communities.

Density of Crabs and Lobsters

RECENTLY the densities of a large male crab, Cancer pagurus L., and a large male lobster, Homarus vulgaris Milne-Edwards, were found at Plymouth using the displacement method, and since— to use a popular phrase—these are the heaviest animals which move freely in sea-water the results may be of general interest. Both specimens were known to be in the late inter-moult stage. The density of the crab with a width of carapace 25 cm. was found to be 1.2784 at a temperature of 8.0° C., which gives a sinking factor of 1243. The density of the lobster 39.5 cm. long was found to be 1.1871 at the same temperature, which gives a sinking factor of 1155. Thus the crab is considerably the denser.