Juvenile Western Rock Lobsters Research Articles

Chemotaxis is important for fine scale habitat selection of early juvenile Panulirus cygnus

The western rock lobster (Panulirus cygnus [George 1962]) is the basis for Australia's most valuable single species wild caught fishery. The ability of early juvenile western rock lobster to select and settle onto suitable habitats is critical for the recruitment of this commercially important species. Many marine organisms, including the western rock lobster, have highly complex olfactory systems that allow for various behaviours including habitat selection and the location of food. We tested the active habitat selection of early juvenile western rock lobsters using only chemotaxis in scent based trials. We used a Y-maze bio-assay with which we were able to present individuals with several different natural scent stimuli. This study used three commonly observed broad habitat complexes followed by three fine scale seagrass assemblages to provide different scent stimuli. Seagrass habitats were chosen significantly more often by the juvenile lobsters in the broad habitat scale trials over bare sand and turf habitats. At a finer scale, lobsters showed a mixed response but tended to choose the canopy forming seagrass species, Amphibolis antarctica [(Labillardière) Sonder & Ascherson ex Ascherson, 1868] and Posidonia australis [Hooker, 1858] significantly more often than the other scents provided. This study developed a Y-maze chemotaxis assay for P. cygnus and confirmed that they respond to habitat related scent cues. These findings suggest the importance of seagrass habitats to early juvenile western rock lobsters and the potential impacts that habitat change may have on the successful recruitment of this commercially important species.

Climate variability affecting the contranatant migration of Panulirus cygnus, the western rock lobster

Large-scale animal migrations have high energetic costs and are only undertaken for specific biological outcomes, including opening up habitat resources or for reproduction. Juvenile western rock lobsters (Panulirus cygnus) migrate firstly offshore and then northwards from their inshore nursery areas (<40 m) into breeding grounds (40–100 m). In the 2000s, catches of migrating lobsters in the northern part of the western rock lobster fishery declined and concerns were raised as to the sustainability of this stock. This study has documented the substantial inter-annual variation in the latitudinal distribution (between latitudes 27° and 30° S) of migrating lobsters in deep water. This distribution was found to be highly correlated with the sea-floor meridional current strength in January and mean latitude and abundance of puerulus (post-larvae) settlement 3–4 years previously. Combined, these indices explained 92 % of the annual variation in the latitude of migration, and the majority of the variation in commercial catches towards the north of the fishery. These indices are all either directly or indirectly impacted by the south-flowing Leeuwin Current. This current, which is influenced by ENSO conditions, has increased in strength over the last 20 years, a trend that has been identified as a relatively short-term decadal shift, while climate change models project a progressive reduction in the strength of this current in the longer term. This may result in greater northward lobster migrations and a shift in the biomass of lobsters more towards the northern end of the fishery.

Density and growth in populations of juvenile western rock lobsters, Panulirus cygnus (George)

SCUBA divers surveyed populations of juvenile Panulirus cygnus on limestone reefs at Seven Mile Beach and Cliff Head, Western Australia, between 1987 and 1989. Estimates of the density and size structure of populations of early-juvenile P. cygnus had high precision, but their accuracy was lower because visual surveys underestimated the population size. Nevertheless, these estimates may be useful indices of puerulus settlement and the growth, habitat use and population densities of juvenile P. cygnus. Puerulus settlement on collectors correlated well with natural densities of early juveniles on reefs. Densities of juveniles were at least three times greater at Seven Mile Beach than at Cliff Head. However, juveniles grew faster at Cliff Head than at Seven Mile Beach. The density of juveniles in reef ledges was twice that in caves; the density in caves was 10 times that on the reef face.

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.

Predator-prey interactions in seagrass beds. I. The influence of macrofaunal abundance and size-structure on the diet and growth of the western rock lobster Panulirus cygnus George

Juvenile western rock lobsters Panulirus cygnus George at two Western Australian sites, Seven Mile Beach and Cliff Head, showed a range of diets and feeding strategies. Rock lobsters at Seven Mile Beach, an area with comparatively few animal prey, ingested large quantities of epiphytic coralline algae. Rock lobsters at Cliff Head primarily consumed invertebrates, with less food being ingested overall than at Seven Mile Beach. The diets of rock lobsters also varied greatly between different seasons, and between rock lobsters feeding in different habitats at Seven Mile Beach. On all sampling occasions, the diet of P. cygnus reflected the abundance and size distribution of benthic macrofauna. Selection of foraging habitat was not, however, found to be related to food availability. Rock lobsters consumed slow-moving, benthic prey, particularly molluscs, within the size range 2–5.6-mm sieve size. When such prey were in low numbers, rock lobsters incorporated large quantities of plant material into their diet. The dietary differences between sites were largely attributable to the trochid mollusc Cantharidus lepidus locally settling in extremely high densities (> 10000· m −2) at Cliff Head in late summer. Rock lobsters at Cliff Head grew nearly twice as rapidly as rock lobsters at Seven Mile Beach. The dietary and prey density data support the hypothesis of Chittleborough (1976) that these differences in growth between sites are associated with diet. Chittleborough's hypothesis can, however, now be refined: the localized recruitment of C. lepidus is postulated to be the primary cause of the high rock lobster growth rates at Cliff Head.

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.

Foraging patterns of juvenile western rock lobsters Panulirus cygnus George

Foraging patterns of juvenile western rock lobster Panulirus cygnus George were investigated in the field by tracking lobsters with electromagnetic tags for up to 3 wk. Most foraging activity begins in response to changes in light levels associated with dusk and not by diurnal changes in water temperature or currents. Similarly, lobsters return from foraging around dawn when light levels start to increase. Foraging activity was constant throughout the night, and males were more active than females. Lobsters usually moved at a rate of ≈ l m · min −1 at night, although sometimes they travelled up to 18m· min −1 when walking over bare sand. They often lived on one reef but travelled to forage on seagrass beds in front of another reef, sometimes visiting a den on that reef. Some lobsters used several dens on different reefs during the 3 wk of observation. Although lobsters foraged in both of the two main structural varieties of seagrass beds, Amphibolis and Heterozostera with Halophila, they foraged for longer periods in the Heterozostera with Halophila beds. Foraging ranges varied within and between individuals although an individual lobster might forage in the same general area on a few successive nights or even up to several weeks.

An electromagnetic tracking system for use in shallow water

Modifications to an electromagnetic method of tracking juvenile western rock lobsters Panulirus cygnus George are documented. The receiving aerials were made more robust to increase their useful life, and an automated tracking system was developed. The system continuously collects data on the movements of up to 14 tagged lobsters simultaneously. The feasibility of the method and its problems are discussed and suggestions are made for its improvement. The susceptibility to wave action of the aerials and the main communication cable that brings the signals to shore suggest that electromagnetic tracking is feasible only in calm and shallow environments.

A quantitative study of nocturnal foraging distances of the western rock lobster Panulirus cygnus George

An electromagnetic tracking device was used to study the distances over which juvenile western rock lobsters Panulirus cygnus George forage at night. Up to 14 tagged lobsters were tracked continuously for up to 3 wk with the aid of an automatic tracking system. A total of 460 “lobster nights” (data on one lobster for 1 night) was recorded during the study. On 48 of these “lobster nights”, the lobsters did not forage. Approximately half of the remaining data were incomplete or “censored” due to lobsters moving outside the tracking area for part of the night. A Weibull distribution showed an extremely close fit to the data after adjustment for the bias of censoring. Through analyses of deviance, comparisons were made of the effect on distance travelled of sampling time, sex, and time since tagging in a way that was analogous to an analysis of variance. The results of the study indicate that nocturnal foraging distances were variable: 95% of the population covered between 72.5 and 585 m · night −1. These distances, which are much greater than previously reported, show that the animals have an unexpectedly large foraging area. The effect on survival of P. cygnus of limited food supply due to small foraging distances may not be as important as previously believed.

Relationship Between Indices of Juvenile Abundance and Recruitment in the Western Rock Lobster (Panulirus cygnus) Fishery

Indices of abundance of 3- and 4-yr-old juvenile western rock lobsters (Panulirus cygnus) were used to determine the status of the stocks between the puerulus stage and subsequent recruitment to the fishery. These indices were based on logarithmically transformed catch rate data of undersize lobsters obtained from a monitoring program which commenced in 1971–72. The juvenile indices were determined from regression coefficients associated with the factor "year", adjusted for other factors, e.g. location, month, depth, soak time, which affect juvenile catch rates. A revised index of recruitment was based on logarithmically transformed catch rates, in depths up to 18 m. A 7-d moving average was used to smooth out daily fluctuations to enable the "best" 60-d catch rate over the period of recruitment to the fishery (November–January) to be determined so that differences in the time of recruitment may be taken into account in quantifying the level of recruitment. This index is compared with a previous recruitment index based on total catch divided by total effort during November–December. The relationships between the abundance indices of 3- and 4-yr-old male juveniles and the recruitment index resulted in correlations of 0.95 and 0.93, respectively, from the log–log analyses of 11 year-classes with a multiple correlation of 0.97 when the 3- and 4-yr-old indices were used in a multiple regression. These indices provide an independent method to support or modify the prediction of recruitment based on an index of abundance of settling puerulus (aged 9–11 mo) obtained 4 yr before they recruit into the fishery.

Natural diet and growth of juvenile western rock lobsters Panulirus cygnus George

The natural diet of juvenile western rock lobsters ( Panulirus cygnus George) at two sites, one of high and one of low growth rate, was examined to determine if there was a relationship between natural diet and growth rate. The diet of P. cygnus at the high growth rate site (Cliff Head in Western Australia) encompassed a wide range of animal and plant material, but was dominated by molluscs. There were significant variations in both the proportions of the different dietary items and in the degree of foregut filling with time of year. At the low growth rate site (Seven Mile Beach) the diet covered a similar range of animal and plant material as at Cliff Head, but was dominated by two species of foliose coralline algae. There was little variation in the proportions of the various dietary items with time of year, but the degree of foregut filling showed significant seasonal variation. The dietary data did not provide any evidence to support the hypothesis of Chittleborough that limited food supply is the cause of differences in growth between the sex/age-class groups, although there was an obvious association between the animal-rich diet at Cliff Head and the plant-rich diet at Seven Mile Beach and annual growth at the two sites. The nutritional role of the various dietary items, particularly the plant material, requires further evaluation. The allocation of nutritional resources to competitors also requires examination. The broad dietary spectrum of P. cygnus and their high densities in the coastal reef ecosystem suggest a significant role of grazing and predation by this species in the control of shallow reef communities of Western Australia.

Migrations of pre-adult western rock lobsters, Panulirus cygnus, in Western Australia

In September 1978, 5 034 juvenile western rock (spiny) lobsters (Panulirus cygnus George) were tagged on a shallow coastal reef near Cliff Head in Western Australia. Of these, 26% were recaptured and removed from the population during the subsequent commercial fishery between 15 November 1978 and 30 June 1979. A resurvey of the tagging site in September 1979 found that size structure of the population was similar to the tagged population in September 1978 but that the 1979 population was composed of almost completely different individuals and only 2% of the rock lobsters tagged in September 1978 were recaught at this time. A wide range of movement was indicated by the recapture positions of 354 rock lobsters caught off the tagging site. Most travelled almost directly offshore but some also travelled offshore in a northwesterly direction. Both trends were shown in the data from the recaptures of 18 rock lobster released farther offshore from Cliff Head in September 1981, and of 19 released at a close inshore site in earlier years. Maximum offshore rates of movement of up to 622 m d-1 were recorded for distances up to 37 nautical miles (ca. 68 km). Growth rates for the tagged individuals which moved off, and those that were caught on, the tagging site were similar and close to growth rate under optimal conditions for this species. Under the present high levels of exploitation few, if any, of the rock lobsters remain to become permanent residents in the coastal reefs. It is believed that the north-westerly movements of some of the pre-adult rock lobsters, on their way to the breeding grounds on the outer edge of the continental shelf, are due to the upstream movement of these rock lobsters in a current from the north. The north-westerly movements allow the rock lobster population increased utilization of the food supplies of the coastal area.

Growth of juvenile Panulirus longipes cygnus George on coastal reefs compared with those reared under optimal environmental conditions

The growth of a group of six juvenile western rock lobsters raised for 6 years in aquaria was well represented by the von Bertalanffy equation lt = 113.47{1-exp{-0.459(t-1.045)}] Using single-moult increments as well as annual growth (determined by shift in modal length and returns of tail-punched and tagged individuals) growth of juveniles in the wild population was compared with that of those reared under near-optimal conditions. At three of the four localities studied, growth was retarded. Growth was most suppressed at the locality where the density of juveniles was highest. However, at a locality where nursery reefs carried relatively low densities in recent years, growth was still well below that achieved in aquaria. Limited food resources on shallow nursery reefs was suggested as the main reason for the retardation of growth. Where the shortage was less severe, competition amongst juveniles resulted in the growth of younger age-groups (2+ and 3+ years) being more retarded than older juveniles, and that of females being more retarded than that of males. Upon emigrating to deeper waters (mainly at 5 and 6 years of age), juveniles showed a marked acceleration in growth rate. As juveniles have distinct growth phases with different environmental pressures applying at each, growth in the wild population at most localities is not well represented by a single von Bertalanffy equation.

Environmental factors affecting growth and survival of juvenile western rock lobsters Panulirus longipes (Milne-Edwards)

The effects of temperature, photoperiod, oxygen, food supply, crowding, and autotomy of limbs have been measured under controlled conditions in aquaria. Growth rate increased with temperature to a maximum 26� C above which both growth rate and survival declined. Varying the length of photoperiod did not affect growth rate or survival, except that the growth rate was depressed significantly in continuous darkness. A mild deficiency of oxygen (60-67 % saturation) resulted in a smaller size increment at a moult; depression to 47-55 % saturation caused deaths at ecdysis. Daily feeding was necessary to maintain maximum rate of growth. The fist response to decreased food supply was a reduction in frequency of moulting. More severe shortage of food also depressed growth increment per moult. Feeding rates and conversion ratios have been measured. Frequency of moulting (and hence growth rate) was depressed markedly when juveniles were held in isolation. At a moult replacing two lost limbs, the growth rate was not affected; replacement of four limbs reduced that moult increment. Single loss of up to four limbs did not result in an earlier moult, but repetitive loss of two or more limbs at or immediately after each ecdysis led to precocious moulting. The impact of these and other components of the environment (shelter, salinity, turbidity, competitors and predators) upon juveniles during the 4 years spent on shallow coastal reefs is discussed. Food supply is emerging as the dominant factor determining growth and survival in the wild population.

Home range, homing and dominance in juvenile western rock lobsters.

Home ranges of juvenile western rock lobsters (Panulirus longipes cygnus George), measured on a relatively uncrowded reef over a period of one year, did not exceed a radius of 15 m. Tagged juveniles transferred to similar reefs seldom remained on the new sites. Some found their way back to the home reef (from distances of up to 400 m), while many dispersed at random, travelling up to 8.3 km in 9 weeks. Dominance for food is displayed amongst juveniles (aged 2-5 years), especially under crowded conditions. The significance of these behaviour patterns is discussed in relation to research and management problems.