Spatial and temporal dynamics of Western Australia’s commercially important sharks

This project took advantage of an unprecedented deployment of acoustic receiver infrastructure around the Western Australian coast to monitor a large number of sharks implanted with acoustic tags. This research was undertaken to better understand the temporal and spatial dynamics of commercially-important sharks in Western Australia, which has enabled a re-evaluation of the risks of cryptic sources of catch and bycatch to dusky and sandbar shark stocks and development of population simulation models with which to test the implications of spatially-different histories of fishery management, as well as future spatial-management arrangements for whiskery and gummy sharks stocks

Resource Assessment Report Temperate Demersal Elasmobranch Resource of Western Australia

Whiskery (Furgaleus macki), gummy (Mustelus antarcticus), dusky (Carcharhinus obscurus) and sandbar (C. plumbeus) sharks are the main (~80% of the shark catch) shark species of the Temperate Demersal Elasmobranch Resource (TDER)

Ecological Risk Assessment for the Temperate Demersal Elasmobranch Resource

In March 2021, the Department of Primary Industries and Regional Development (Department) convened an ecological risk assessment (ERA) of the fisheries that access the Temperate Demersal Elasmobranch Resource (Resource). ERAs are conducted by the Department as part of its Ecosystem Based Fisheries Management framework

Global Phylogeography of the Dusky Shark Carcharhinus obscurus: Implications for Fisheries Management and Monitoring the Shark Fin Trade

Genetic stock structure information is needed to delineate management units and monitor trade in sharks, many of which are heavily exploited and declining. The dusky shark Carcharhinus obscurus is a large apex predator that is sought after for its fins and is considered highly susceptible to overexploitation. The International Union for the Conservation of Nature (IUCN) classifies this species as ‘Vulnerable’ globally and ‘Endangered’ in the northwest Atlantic. We make the first assessment of global stock structure of C. obscurus by analyzing part of the mitochondrial control region (mtCR) in 255 individuals sampled from 8 geographically dispersed locations. We found 25 mtCR haplotypes and rejected a null hypothesis of panmixia (analysis of molecular variance, ΦST = 0.55, p \u3c 0.000001), detecting significant differentiation between 3 management units: US Atlantic (USATL), South Africa (SAF), and Australia (AUS). We also found preliminary evidence of population structure between the USATL and southwest Atlantic (Brazil). There were no shared haplotypes between the western Atlantic and Indo-Pacific. These analyses suggest that replenishment of the collapsed USATL management unit via immigration of females from elsewhere is unlikely. Mixed stock analysis (MSA) simulations show that reconstruction of the relative contributions of USATL, SAF, and AUS management units to the Asian fin trade is possible using these mtCR sequences. We suggest avenues for obtaining samples to conduct MSA of the shark fin trade, which could enhance management of dusky sharks and other species that are exploited for their fins

High Post-Capture Survival for Sharks, Rays and Chimaeras Discarded in the Main Shark Fishery of Australia?

Most sharks, rays and chimaeras (chondrichthyans) taken in commercial fisheries are discarded (i.e. returned to the ocean either dead or alive). Quantifying the post-capture survival (PCS) of discarded species is therefore essential for the improved management and conservation of this group. For all chondrichthyans taken in the main shark fishery of Australia, we quantified the immediate PCS of individuals reaching the deck of commercial shark gillnet fishing vessels and applied a risk-based method to semi-quantitatively determine delayed and total PCS. Estimates of immediate, delayed and total PCS were consistent, being very high for the most commonly discarded species (Port Jackson shark, Australian swellshark, and spikey dogfish) and low for the most important commercial species (gummy and school sharks). Increasing gillnet soak time or water temperature significantly decreased PCS. Chondrichthyans with bottom-dwelling habits had the highest PCS whereas those with pelagic habits had the lowest PCS. The risk-based approach can be easily implemented as a standard practice of on-board observing programs, providing a convenient first-step assessment of the PCS of all species taken in commercial fisheries

Ecological roles and importance of sharks in the Anthropocene Ocean

In ecosystems, sharks can be predators, competitors, facilitators, nutrient transporters, and food. However, overfishing and other threats have greatly reduced shark populations, altering their roles and effects on ecosystems. We review these changes and implications for ecosystem function and management. Macropredatory sharks are often disproportionately affected by humans but can influence prey and coastal ecosystems, including facilitating carbon sequestration. Like terrestrial predators, sharks may be crucial to ecosystem functioning under climate change. However, large ecosystem effects of sharks are not ubiquitous. Increasing human uses of oceans are changing shark roles, necessitating management consideration. Rebuilding key populations and incorporating shark ecological roles, including less obvious ones, into management efforts are critical for retaining sharks’ functional value. Coupled social-ecological frameworks can facilitate these efforts

Assessment of ecological risks from effects of fishing to Piked Spurdog ( Squalus megalops ) in South - Eastern Australia

Target species in some Australian shark fisheries are adequately managed, but there has been little attention given to non - target shark species and there is limited information on the biology of their local populations. Among this group of non-target species, the piked spurdog - Squalus megalops ) is of special interest because it is a dominant and ecologically important species with high natural abundance. Hence, the main purpose of the present research was to improve knowledge of the basic biology of this species and to provide essential data for its management, sustainable use and conservation. Squalus megalops had a complex population structure, segregating by sex, size and breeding condition. The sex ratio was biased towards females and there was sexual size dimorphism with females attaining a larger maximum size than males. Conversion factors from partial lengths to total length and from partial masses to total mass were determined due to the common commercial fishing practice of eviscerating, beheading and finning sharks. Comparisons of total and partial length - length and mass - length relationships between males and females using different ranges of size showed that there was no effect of size range on measurements reflecting only somatic growth ( fork and carcass lengths ; carcass, pectoral fin and caudal fin masses ). However, for variables reflecting somatic and reproductive growth ( total and liver masses ), different outcomes can be expected when different ranges of size are compared. Examination of dietary composition revealed that S. megalops is an opportunistic predator that consumes a wide range of prey items. High variability was found when overall importance of prey items was estimated. Dietary composition varied in space and time, exhibiting differences among regions, seasons and size classes. Therefore, the intrinsic natural variability in the dietary composition of S. megalops and its spatial and temporal variation in diet suggest that information on the ecological relationships among species is likely to be missed when predator - prey interactions are only inferred from overall diet. Reproductive parameters were determined for population assessment. For both sexes, length - at - maturity differed depending on the criterion adopted for defining maturity. Mature males are capable of mating throughout the year. Females have a continuous asynchronous reproductive cycle. The sex ratio of embryos is 1 : 1 and litter size and nearterm embryo size increase with maternal length. Females have an ovarian cycle and gestation period of two years. Although all females are mature at 600 mm, only 50 % of them are in maternal condition, contributing to annual recruitment each year. Hence, for chondrichthyan species with reproductive cycles of two, three or more years, if maturity ogives are used in population assessments instead of maternity ogives, the models will over - estimate recruitment rates. Age and growth information was also determined for population assessment. Precision estimates, the relationship between spine total length and body length, edge analysis, and agreement between counts on the inner dentine layer and the enameled surface support the use of the first dorsal fin spine for the age estimation of S. megalops. Based on goodness - of - fit criterion, the best growth model for males and females was a two - phase von Bertalanffy function. However, model selection cannot be based on quality of statistical fit only and results should be interpreted with caution. Regardless of the model used, the growth rate of S. megalops, particularly of females, is very low, even within the range of growth rates reported for shark species. A three - levelled hierarchical risk assessment approach was trialed to evaluate the suitability of the approach for S. megalops. Integration of qualitative, semi - quantitative, and quantitative biological and fishing impact data showed that S. megalops is potentially highly susceptible to the effects of fishing. A qualitative assessment indicated that the only fishing related activities to have moderate or high impact on S. megalops were those associated with ' capture fishing ' of the otter trawl, Danish seine, gillnet and automatic longline methods. A semi - quantitative assessment ranked S. megalops at risk because of its low biological productivity and, possibly, its catch susceptibility from cumulative effects across the separate fishing methods. Finally, a quantitative assessment showed that population growth is slow even under the assumption of density - dependent compensation where the fishing mortality rate equals the natural mortality rate. Therefore, conservation and management for sustainable use of S. megalops will require a close control of fishing mortality due to the low capacity of this species to withstand fishing pressure.Thesis (Ph.D.)--School of Earth and Environmental Sciences, 2006

Acoustic and conventional tagging support the growth patterns of grey nurse sharks and reveal their large-scale displacements in the west coast of Australia

The grey nurse shark, Carcharias taurus, is a globally vulnerable coastal species with aggregatory behaviour and low productivity, making it highly susceptible to overfishing. Little is known on the biology and movement for the population along the west coast of Australia. Here we use acoustic telemetry to show that C. taurus can undertake large-scale movements and potentially capitalise on seasonal prey aggregations. Conventional tagging provided evidence to support the growth parameter values used to represent the species' growth dynamics and considerably extended the species' maximum observed age. As maximum age is a proxy for productivity, our findings directly inform the recovery plan currently in place for Australian C. taurus

Data from: The spatial segregation patterns of sharks from Western Australia

The extent to which sharks segregate by size and sex determines the population structure and the scale at which populations should be managed. We summarized 20 years of fisheries-dependent and independent sampling to define the spatial patterns of size and sexual segregation for sharks in Western Australia. Carcharhinus obscurus and C. plumbeus showed a large-scale (more than 1000 km) latitudinal gradient in size. Large individuals occurred predominantly in the northwest and north whereas smaller individuals occurred predominantly in the southwest and south. Mustelus antarcticus and Furgaleus macki showed strong sexual segregation at very large scales. Females occurred predominantly in the west and southwest whereas the proportion of males in catches substantially increased in the southeast. The populations of other shark species did not show sex and size segregation patterns at very large scales; most species, however, showed varying degrees of segregation when data were analysed at a smaller scale. These findings highlight the importance of matching the scale of observation to the scale of the phenomenon observed. As many shark species are highly mobile, if sampling is opportunistic and constrained both temporally and spatially, the observed segregation patterns may not be representative of those at the population level, leading to inaccurate scientific advice

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