Simon de Lestang

Acoustic tracking: issues affecting design, analysis and interpretation of data from movement studies

Acoustic telemetry systems are an increasingly common way to examine the movement and behaviour of marine organisms. However, there has been little published on the methodological and analytical work associated with this technology. We tested transmitters of differing power outputs simultaneously in several trials, some lasting ~50 days, to examine the effects of power output and environmental factors (water movement, temperature, lunar cycle and time of day). There were considerable and volatile changes in detections throughout all trials. Increased water movement and temperature significantly reduced detection rates, whereas daytime and full-moon periods had significantly higher detection rates. All nine transmitters (from seven transmitter types tested) showed a sigmoidal trend between detection frequency and distance. Higher-powered transmitters had a prolonged detection distance with near-maximal detections, whereas lower-powered transmitters showed an almost immediate decline. Variation of detection frequency, transmitter type and the modelled relationship between distance and detection frequency were incorporated into a positioning trial which resulted in markedly improved position estimates over previous techniques.

Influence of a deep artificial entrance channel on the biological characteristics of the blue swimmer crab Portunus pelagicus in a large microtidal estuary

The opening of a deep and permanent artificial entrance channel into the Peel–Harvey Estuary in southwestern Australia in 1994 provided an excellent opportunity to determine the biological characteristics of the blue swimmer crab Portunus pelagicus in a large microtidal estuary, after tidal exchange with the ocean had increased markedly and salinities recovered more rapidly from their winter minima. P. pelagicus was collected from the original entrance channel, which is located 15 km to the north of the artificial channel, the two large basins (Peel Inlet and Harvey Estuary) and the saline lower reaches of a major tributary at regular intervals in 1995–1998. The resultant information on densities, size and age compositions, growth rates and reproductive biology were compared with those derived from data recorded by Potter et al. [Mar. Biol. 78 (1983) 75] for P. pelagicus in the Peel–Harvey Estuary in 1980 and 1981. This led to the following conclusions regarding the ways in which this portunid has responded to the impacts of the construction of an artificial entrance channel in this estuary. (i) Late 0+ crabs enter earlier than previously the Peel Inlet and both the Harvey Estuary, which now opens directly to the sea, and a tributary river in which salinities likewise rise more rapidly in the spring. (ii) Female crabs become ovigerous earlier, in concert with the earlier restoration of high salinities in the spring. (iii) Ovigerous crabs emigrate to the ocean sooner, presumably in response to the greater stimulus provided for emigration by the marked increase that has occurred in tidal flow. (iv) The growth rate during the first few months of life has increased, possibly as a consequence of a decline in the density of crabs within the estuary during those months. (v) A cohort of small 0+ crabs, not previously found in the estuary and almost certainly derived from population(s) spawning further south, is present for a few months but then disappears from the estuary. Although large crabs do not now appear to grow towards their maximum size as rapidly as previously, this is an artefact brought about by the culling of large crabs through the marked increase that has occurred in fishing pressure during recent years.

Comparisons of the Growth and Gonadal Development of Otolith-Stained, Cultured Black Bream, Acanthopagrus butcheri, in an Estuary with Those of Its Wild Stock

Acanthopagrus butcheri was cultured from broodstock from a southwestern Australian estuary in which its abundance had declined. After marking their otoliths with alizarin complexone, the cultured juveniles were released into this estuary at 6 months of age. Regular sampling demonstrated that this stain persisted throughout the next 3.5 years and that, during the last two of those years, cultured fish dominated the black bream assemblage. Cultured fish grew slower than wild fish, with females reaching total lengths of about 182, 220, and 243 mm at 2, 3, and 4 years, respectively, compared with about 199, 248, and 286 mm in wild fish. However, cultured A. butcheri grew faster than wild A. butcheri in most other estuaries studied. By 3 years of age, essentially all wild fish, but only some restocked fish, had matured. Cultured females matured at a similar length but at an older age than wild females, and cultured males matured at both a greater length and older age than wild males. We conclude that cultured A. butcheri can be used to increase the abundance of a depleted stock, but the reason for the slightly reduced performance of cultured fish should be elucidated and overcome.

CHANGES IN DENSITY, AGE COMPOSITION, AND GROWTH RATE OF PORTUNUS PELAGICUS IN A LARGE EMBAYMENT IN WHICH FISHING PRESSURES AND ENVIRONMENTAL CONDITIONS HAVE BEEN ALTERED

Abstract Commercial and recreational catches of the blue swimmer crab Portunus pelagicus in Cockburn Sound, a large marine embayment on the west coast of Australia, have risen markedly over the last 20–30 years. However, because commercial fishers changed from using tangle nets to traps to catch crabs during this period, the annual catch per unit effort data for the commercial fishery throughout this period are not directly comparable and cannot thus be used to elucidate whether the increased catches reflected an increase in crab density. Trawling was thus undertaken to estimate the densities of P. pelagicus in Cockburn Sound in the late 1990s to facilitate comparisons with those we estimated from trawl catch rates for this species in that embayment during the early 1970s. The comparisons demonstrate that, despite increases in commercial and recreational crab catches, the densities of P. pelagicus in Cockburn Sound have risen markedly between the above two periods. This change is probably related to a decline in the abundance of the large piscivorous predators of P. pelagicus as a result of heavy fishing pressure and possibly also to an increase in the abundance of the prey of this portunid. Size composition data demonstrate that appreciable numbers of crabs survived in Cockburn Sound until the end of their second year of life and even beyond during the early 1970s, whereas the vast majority of 1+ crabs were removed by heavy fishing pressure by the month (June) that they had reached 18 months in age in the late 1990s. The fact that, in the late 1990s, few legal-sized crabs still remained for fishing between July and December and the 0+ age class increased in number and size throughout the year accounts for the broad estimates of biomass becoming far greater in these months in the 1990s than in the corresponding months in the early 1970s. Growth during the first eleven months of life, i.e., in the period leading up to the age at which crabs reach the minimum legal size for retention, was significantly faster in the early 1970s than in the late 1990s when crab densities were much greater. The slower growth rate and the reduced longevity through heavy fishing pressure in the latter period would help account for females becoming mature at a smaller size and for ovigerous females being represented by one rather than two substantial size cohorts, respectively. The essentially single size cohort in the late 1990s and first cohort in the early 1970s correspond mainly to crabs in their first maturity instar, whereas the second cohort in the early 1970s predominantly represented crabs in their second maturity instar.

Catch Predictions in Stock Assessment and Management of Invertebrate Fisheries Using Pre-Recruit Abundance—Case Studies from Western Australia

Catch predictions based on pre-recruit abundance have proven valuable in the stock assessment and management of major invertebrate fisheries in Western Australia for western rock lobster (Panulirus cygnus), pearl oyster (Pinctada maxima), abalone (Haliotis roei), prawn (Penaeus esculentus), scallop (Amusium balloti), and blue swimmer crab (Portunus armatus). Predictions are based on puerulus stage (post-larval) rock lobster from artificial collectors; 0+ and 1+ pearl oyster spat attached to commercial shell; dive survey of Roes abalone; and trawl surveys of prawn (shrimp), scallop, and crab. These reliable predictions (R2 = 0.67–0.97) are used in stock assessment and management harvest strategies to ensure that fishing effort or catch quotas are set so that adequate breeding stocks are maintained. This pro-active management based on predicted abundance avoids the pitfall of heavy fishing on poor recruit classes, which is a common cause of recruitment overfishing. The catch-pre-recruit relationship provides valuable information on density-dependent effects between the two life-history stages, with high levels evident in rock lobster and little evidence for prawns and crabs, which reflects the stage at which pre-recruits are measured. The review emphasizes the advantages of catch predictions based on pre-recruit abundance relative to environmental variables and the need for cost-effective pre-recruit monitoring to ensure long-term data. The pre-recruit abundance is valuable in investigating factors affecting year-class strength, including environmental factors and overfishing. The pre-recruit abundance is probably the most valuable information under climate change situations as it provides firm evidence of a change in abundance to justify management action before the change is reflected in the fishery.

NEITHER MATERNAL SIZE NOR SITE OF SPAWNING INFLUENCES LARVAL COMPETENCY IN WESTERN ROCK LOBSTER PANULIRUS CYGNUS GEORGE

Abstract The size of the western rock lobster broodstock is variable over the range of the Western Australian fishery, due to regional differences in the density and length structure of the stock and the females size at maturity. Management regulations do not currently discriminate in the protection afforded to the broodstock in the different regions. This study has examined whether regional and size differences in early egg diameter, phyllosoma length at hatch and phyllosoma competency, are related to lipid class and fatty acid composition, protein composition and water content in late stage eggs and early stage phyllosoma larvae. A positive relationship, particularly to phyllosoma competency, would have management implications for the protection of broodstock in different parts of the fishery. Females were sampled in three size groups captured in two sampling regions (Abrolhos Islands 60-70 and 80-90 mm CL; coastal 80-90 and > 95 mm CL). Mostly, eggs and larvae from different maternal size classes and regions were not significantly different with respect to lipid class and fatty acid composition, protein composition and water content, and larval competency. The lipid classes of eggs differed significantly (P < 0.02) between the three maternal size-classes; fatty acids differed significantly between both eggs and phyllosoma grouped by maternal catch site and size-class (P = 0.02 and 0.003, respectively); and there was no correlation between biochemical differences and larval competency. These results imply that maternal identity and subsequent diet may have a greater influence on larval competency in P. cygnus than the maternal attributes (size and capture site) that were tested.