J.D. Prince

Experimental evidence for limited dispersal of haliotid larvae (genus Haliotis; Mollusca: Gastropoda)

The literature on haliotids generally assumes that these organisms have a short pelagic larval life with dispersal powers limited only by the time available before settlement. The results of an experiment designed to test this assumption are presented. The density of mature Haliotis rubra Leach was experimentally decreased along a 90 m section of shoreline. The resulting effect on recruitment patterns was examined in comparison with previous recruitment patterns. The results indicate that haliotid larvae may be strongly benthic with limited patterns of dispersal.

Confirmation of a relationship between the localized abundance of breeding stock and recruitment for Haliotis rubra Leach (Mollusca: Gastropoda)

The hypothesis that for Haliotis rubra Leach the localized abundance of mature abalone determines the abundance of recruitment was tested with experimental reductions of breeding stock at four sites. At each site there was an experimental area 60 m in length, from which adult abalone were removed, and two control areas 60 m either side of the experimental area. The abundance of recruitment was found to vary significantly between the four sites and between control areas within three of the sites. Observations which are consistent with recruitment being highly localized. The overall density of recruitment to the experimental areas was significantly lower than in the control areas indicating that the abundance of breeding animals determines the abundance of recruitment. When these data were combined with data from the studies of Prince et al. (in press) for the same sites, a relationship between breeding stock and recruitment is clearly evident, confirming that the density of breeding stock was the major determinant of recruitment abundance. These results show that for this species, on a spatial scale measured in 10s of meters, the abundance of recruitment is directly related to the abundance of mature abalone.

Stock assessment of school shark, Galeorhinus galeus, based on a spatially explicit population dynamics model

The school shark (Galeorhinus galeus) resource off southern Australia is assessed by use of an assessment approach that takes account of the spatial structure of the population. The population dynamics model underlying the assessment considers the spatial as well as the age-specific characteristics of school shark. It allows for a series of fisheries (each based on a different gear type), explicitly models the pupping/recruitment process, and allows for multiple stocks. The values for the parameters of this model are determined by fitting it to catch-rate data and information from tagging studies. The point estimates of the pup production at the start of 1997 range from 12% to 18% of the pre-exploitation equilibrium size, depending on the specifications of the assessment. Allowing for spatial structure and incorporating tag release–recapture data lead to reduced uncertainty compared with earlier assessments. The status of the resource, as reflected by the ratio of present to virgin pup production and total (1+) biomass, is sensitive to the assumed level of movement between the stocks in New Zealand and those in Australia, with lower values corresponding to higher levels of movement.

Recruitment, growth, mortality and population structure in a southern Australian population of Haliotis rubra (Mollusca: Gastropoda)

A population of Haliotis rubra Leach at Blubber Head in south-eastern Tasmania, Australia (43°19′S; 147°04′E) was sampled between February 1984 and October 1985. Relatively large numbers of 30 to 45 d-old new recruits were observed entering the population between October and February of each year. The annual instantaneous rate of natural mortality, Z, was 0.70 during the second year of life. The rate of natural mortality declined with age. The onset of sexual maturity began at 4 to 5 yr of age or an approximate maximum length of 90 mm, and most abalone in the population were mature at 6 to 7 yr of age or at lengths>110 mm. The onset of sexual maturity coincided with a decline in growth rates and the emergence of abalone from the subboulder habitat. These observations are discussed in the light of previous observations of population structure in haliotid populations.

A management strategy for sedentary nearshore species that uses marine protected areas as a reference

Abstract Classical approaches to fisheries stock assessment rely on methods that are not conducive to managing data-poor stocks. Moreover, many nearshore rocky reef species exhibit spatial variation in harvest pressure and demographic rates, further limiting traditional stock assessment approaches. Novel management strategies to overcome data limitations and account for spatial variability are needed. With the ever-increasing implementation of no-take marine protected areas (MPAs), there is great potential for improving decision making in management through comparisons of fished populations with populations in MPAs at spatially explicit scales. We developed a management strategy that uses a combination of data-based indicators sampled inside and outside of MPAs as well as model-based reference points for data-poor, sedentary nearshore species. We performed a management strategy evaluation of this MPA-based decision tree model for a hypothetical population of grass rockfish Sebastes rastrelliger in California. We introduced process, observation, and model uncertainty in numerous scenarios and compared these scenarios with the precautionary approach currently used to manage data-poor species. Our model consistently improved total catches while maintaining the biomass and spawning potential ratio at levels well within acceptable thresholds of management. We suggest further exploration of this MPA-based management approach, and we outline a collaborative research program in the California Channel Islands that may well be suited for testing an experimental management procedure.

A stock reduction analysis of the Mexican abalone (Haliotid) fishery

The results of a stock-reduction analysis of two zones within the Mexican abalone fishery are presented. A reconstruction of the stock dynamics in these zones using these results suggests that stock abundance and levels of recruitment declined relatively constantly between the 1960s and mid-1980s. Results of our analysis suggest that the relationship between productivity and biomass in these fisheries has been relatively linear. This could indicate that these stocks are extremely unproductive, producing little surplus yield. An alternative hypothesis is also presented, which is that serial over-exploitation of small units of stock within these fisheries resulted in an apparent lack of productivity which may not be indicative of the true potential productivity of haliotid stocks.

Ecosystem of the South East Fishery (Australia), and fisher lore

A description of the marine ecosystem that sustains fisheries production around south-eastern Australia is based on a synthesis of fisher lore of the demersal trawling sector, a review of the literature and a decade of the authors field observations. A wide range of species are fished demersally in the South East Fishery (SEF). Until recently, managers and researchers have often implicitly assumed that many of these demersally caught species were neritic and lived in close association with the seabed. In contrast, fisher lore emphasizes the pelagic and oceanic nature of the commercial resource together with its environmentally forced variability. This paper substantially supports the views of the fishers. Up to 90% of the primary production of the SEF ecosystem may be garnered by fish foraging through extensive, but relatively sparse, oceanic phytoplankton and gelatinous zooplankton communities. Sporadically, climatic conditions cause oceanographic features to interact with shelf-break features and create ephemeral hotspots of primary production along the shelf break. Fish of the SEF take advantage of these productivity events to aggregate for feeding and breeding and their episodes of aggregation and dispersion cause the large seasonal variations in catchability observed with the shelf-break species. Implications for ecosystem management are briefly discussed.

Atherinosoma wallacei, a new species of estuarine and inland water silverside (Teleostei : Atherinidae) from the Swan-Avon and Murray rivers, Western Australia

Atherinosoma wallacei n.sp., is a common estuarine and inland water silverside or hardyhead having close affinities with the marine Atherinosoma presbyteroides. It is placed in Atherinosoma after examination of all the 18 existing type specimens representing the species found in this genus in Western Australia. Priority of Atherinosoma is confirmed.

Assessing the management-related benefits of fixed-station fishery-independent surveys in Australia's Southern Shark Fishery

Monte Carlo simulation is used to assess the benefits of different levels of survey intensity for a programme of fixed-station surveys using gill-nets, in terms of the ability to satisfy the management objectives for gummy shark, Mustelus antarcticus, in Bass Strait, Australia. The simulations consider the entire management process: data collection, data analysis, stock assessment, and setting of total allowable catches (TACs). The use in assessments of survey results rather than future commercial catch rates leads to a marked improvement in the ability to satisfy the legislative management objective for Australian fisheries of Ecological Sustainable Development, especially if future catch rates are not related linearly to abundance or if catchability is density-dependent. The benefits of more than six stations per survey site appear limited, however, because of the possible impact of variability in survey results due to factors unrelated to sampling.

Spawning dynamics of the eastern gemfish (Rexea solandri) in relation to regional oceanography in south-eastern Australia

Experienced fishers believe the winter aggregations of gemfish are influenced by the following: cold bottom currents from the south and east flowing up onto the shelf; the edge of warm-core eddies; and topographic features along the shelf break. These claims were tested through observations made at sea during industry surveys of the winter gemfish seasons 1996-98, and the study of 250 m isotherm charts, sea surface temperature data, and historic catch data. The study generally confirmed the anecdotal information collected from the fishers. The first gemfish aggregations of the winter season often form on the southern edge of a warm-core eddy. Secondarily, aggregations tend to form around the northern edge of the warm-core eddy if it is interacting with the shelf break. Finally, gemfish aggregate around the point at which the main flow of the East Australian Current (EAC) detaches from the continental shelf. This timing and location apparently coincides with conditions conducive to the production of subsurface plumes of nutrient-rich deep Sub-Antarctic mode Water. These plumes lead to seasonal enhancement of phytoplankton growth along the edge of the continental shelf, which may offer the adaptive advantage of enhancing the survival and growth of larval gemfish.