Year

2004

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences - Faculty of Science

Abstract

Burrowing ghost shrimp, Trypaea australiensis Dana 1852, are a popular bait organism harvested from estuarine intertidal sediments in eastern Australia by recreational and commercial fishers. Previous investigations concerning the biology and ecology of this species have been limited in this region, particularly for south-eastern Australia. Despite considerable public concern about the species, catches of T. australiensis are largely unregulated, at least partly because there has been virtually no data to support the development of appropriate management strategies. The present research was initiated to address the lack of baseline data on the fisheries biology, ecology and recreational harvesting of T. australiensis, to assist in providing a more sustainable approach to management and conservation. Preliminary studies comparing different quantitative sampling methods demonstrated that manual suction pumps (i.e., yabby pumps) were a more efficient and representative method of sampling T. australiensis than coring devices. Following the determination of a suitable sampling methodology, a hierarchical sampling program was established in south-eastern Australia to investigate spatial and temporal patterns of abundance, reproduction and growth. This involved sampling three estuaries spanning 300 km of coastline, namely Port Hacking, Shoalhaven River and Moruya River. The experimental design incorporated nested spatial scales ranging from hundreds of kilometres (between estuaries), kilometres (between sites), tens to hundreds of metres (between plots) and metres (between quadrats). Sampling was conducted over a two-year period, monthly for the first year and quarterly for the second. One site was sampled monthly for the entire study period to investigate differences between years. Overall, abundances were more variable on smaller spatial scales between sites within estuaries and between plots within sites. However, it was evident that these spatial patterns also changed through time. Increases in abundance across all sites during spring/summer periods, resulted in an increased patchiness in abundance within sandflats at a scale of tens to hundreds of metres (between plots), and less patchiness at the smallest spatial scales of metres (between replicates). This result suggested that increases in abundance might be explained by a redistribution of animals due to spawning aggregations, rather than nett increases in shrimp numbers. Furthermore, while recruitment occurred during spring and summer months, increases in numbers and biomass during these periods were greater than would be expected from recruitment alone. Increases in temperature and salinity may have initiated hypothesized breeding aggregations, although relationships between environmental variables and abundance were not significant even though general trends were evident. Timing of recruitment was consistent between years and included a latitudinal pattern of earlier recruitment in southern estuaries, which was probably related to earlier breeding periods at these locations. Despite this, the supply of new recruits was consistent over large geographic scales and patchy within and between sites within estuaries, suggesting distribution and survival of larvae may be influenced by local hydrology and environmental conditions. Stronger recruitment in consecutive years may have been related to prevailing drought conditions, enhancing survival of larvae due to consistently higher salinities resulting from reduced freshwater inputs. Relationships between number of burrows and abundance of T. australiensis were investigated, since burrow counts are often used to estimate densities of ghost shrimp. Some significant relationships were observed between number of burrows and abundance of T. australiensis. However, spatial and temporal inconsistencies indicated that counting of burrow openings was not always a reliable predictor of relative abundance. Estimates of the magnitude of populations suggested that standing stocks of T. australiensis at individual sites were in the order of millions of individuals. Sex ratios were significantly biased towards females following sexual maturity and this pattern was spatially and temporally consistent at all locations. Hypotheses explaining female-biased sex ratios remain speculative for T. australiensis and other thalassinids and further research is required. Despite consistencies related to female-biased sex ratios, there was considerable intraspecific variability in the reproductive biology of T. australiensis over different spatial and temporal scales. Female size at maturity, based on ovigerous females, was smaller for populations in southern estuaries. However, the specific roles of environmental and/or density-dependent factors such as population abundance, temperature and availability of food in determining these patterns are unclear. Similarly, these factors may also influence commencement of breeding seasons, which was asynchronous and occurred progressively earlier in southern populations. Unimodal breeding seasons generally lasted 5 to 6 months, with ovigerous females occurring from mid-summer through to autumn across most sites. Fecundity increased linearly with female size across all sites and decreased with increasing latitude. Estimates of reproductive output also indicated that fecundity was higher for females carrying late stage embryos, compared to freshly extruded eggs, which suggested that studies counting only uneyed embryos may underestimate fecundity. Measurements of embryos indicated that T. australiensis employs a strategy of high fecundity and small egg size relative to other thalassinids. Number of broods per female was not experimentally determined, although theoretical maximums of between 3 and 5 broods per season were estimated. Differences in patterns of relative growth between males and females explain sexual dimorphism in size and shape of primary chelae. Males also appeared to undergo a puberty moult at smaller sizes than females. The enlarged cheliped of male T. australiensis and other ghost shrimp is generally believed to be used in competitive interactions with other males, although more research is required to validate this hypothesis. Size at maturity as determined from analyses of relative growth was significantly related to latitude, with males and females from southern estuaries maturing at smaller sizes. However, estimates of female size at maturity from relative growth analyses did not compare well to sizes of ovigerous females, suggesting that the puberty moult may not coincide with gonad development. Future studies should attempt to determine male size at maturity based on reproductive condition rather than relying on analyses of relative growth data. Length frequency analyses indicated that T. australiensis is a fast growing species with a life-span of 3 to 4 years. Results suggesting a longer life-span at some sites are likely to be an artefact of the ELEFAN procedure. There were no consistent geographic trends in growth parameters, although lower growth rates and asymptotic lengths at sites in the Moruya River, probably reflect the smaller size of individuals in this estuary. Patterns of fishing mortality and exploitation ratios determined from stock assessment procedures suggest that populations across all sites are either currently under-exploited or approaching optimal harvesting rates. However, theses results are of a preliminary nature, given questions regarding the reliability of ELEFAN for some data sets. Recreational creel surveys at three locations (Maianbar, Shoalhaven Heads and Garlandtown) demonstrated that individual anglers frequently harvested large numbers (> 200) of T. australiensis during single fishing trips. There were no differences in mean harvesting time between locations, with differences in CPUE therefore reflecting fishing quality. Boat-based anglers spent significantly less time harvesting than shore-based anglers. High rates of refusal to either participate in the survey, or allow catches to be handled, were consistent with other studies of T. australiensis in Australia and are related to time constraints and fragility of the organism. Anglers commonly underestimated number of shrimp harvested and over-estimated time spent harvesting, which has consequences for studies that rely on data from telephone surveys or diaries. Anglers removed large numbers of T. australiensis from individual locations during single low-tide periods. For example at the Maianbar site, mean estimated catch during a 6-hour low-tide event occurring on a summer weekend/holiday day, was over 4,500 individuals. Although total estimated harvests were considerable during the 6-month study period, recreational catches represented fewer than 2% of estimated stock sizes across all sites. A doubling of these estimates for an entire year was considered an overestimate of annual harvest, given that most angers claimed they did not harvest during winter months. The results of this research have been discussed in relation to the life-history of T. australiensis in south-eastern Australia, the harvesting of the species, the objectives of sustainable management of populations and scope for further research. Specifically, restrictive management is not advocated for T. australiensis in NSW. However, continued monitoring and research is necessary to determine the long-term ecological impacts of harvesting activities on populations, communities and habitats, as well as experimental tests of hypotheses constructed to explain observed patterns.

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