Quantifying teleost discards in the shark fisheries of Western Australia.

Quantifying discards is essential for assessing the impact of fisheries on non-target species and the ecosystems in which these fisheries operate. In Western Australia (WA), fishers are required only to report catches of retained species. For the currently operating shark fisheries of WA, the authors quantified catch time series of discarded teleosts using data from at-sea observers collected since 1993. Sixty-two teleost species were observed in the catch of which 20 were routinely discarded. The most commonly discarded teleosts were western buffalo bream/silver drummer, Kyphosus cornelii/K. sydneyanus, and dusky morwong, Dactylophora nigricans. Annual discards peaked in the 1990s; nonetheless, current discard levels, 36.9 ± 2.1 (S.E.) t year-1 , are much lower than the overall annual retained catches (c. 1000 t year-1 ). The catch time series reconstructed in this study are important inputs for assessing the ecosystem-based approach used for managing WA's shark fisheries.

Potential expansion in the spatial distribution of subtropical and temperate west Australian sharks.

Fishery-dependent and -independent data collected since 1975 were examined to explore the spatial distribution of 30 shark and ray species in the west coast of Australia. Bigeye sixgill (Hexanchus nakamurai), tiger (Galeocerdo cuvier) and spinner (Carcarhinus brevipinna) sharks, and scalloped hammerhead (Sphyrna lewini) were observed >1000 and 300 km to the east of the edge of their reported distributions. Broadnose sevengill sharks (Notorhyncus cepedianus) and southern sawsharks (Pristiophorus nudipinnis) were observed >1000 km to the west of the edge of their reported distributions. Our study highlights the value of collecting and examining long time-series of data for understanding the spatial distribution of large marine predators.

Quantifying the unreported and unaccounted domestic and foreign commercial catch of sharks and rays in Western Australia.

Reliable catch information is scarce for most sharks and rays worldwide, with almost half of the stocks considered to be Data Deficient due to limited species-specific catch statistics. Western Australia (WA) hosts a diverse number of shark and ray species, some of which are considered to be threatened with extinction at a global level. Commercial catch statistics only account for shark and ray landings. The present study used the best available information to reconstruct unaccounted and unreported catches for 47 shark and ray taxa to better understand the impact of fishing. For some species, there was good agreement between reconstructed catches and reported landings, but overall reconstructed catches were 57% higher than those derived from official statistics alone, underestimating the actual extraction level for many species. The reconstructed catch time series provide the basis for the assessment of all species of sharks and rays captured in WA, including protected species that interact with commercial and recreational fisheries.

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.

Growth of tropical Dasyatid rays estimated using a multi-analytical approach.

We studied the age and growth of four sympatric stingrays: reticulate whipray, Himanutra uarnak (n=19); blue mask, Neotrygon kuhlii (n=34); cowtail, Pastinachus atrus (n=32) and blue-spotted fantail, Taeniura lymma (n=40) rays at Ningaloo Reef, a fringing coral reef on the north-western coast of western Australia. Age estimates derived from band counts within sectioned vertebrae ranged between 1 and 27 years (H. uarnak, 1 - 25 yrs.; N. kuhlii, 1.5 - 13 yrs.; P. atrus, 1 - 27 yrs. and T. lymma, 1 -11 yrs.). Due to limitations of sample sizes, we combined several analytical methods for estimating growth parameters. First, we used nonlinear least squares (NLS) to identify the growth model that best fitted the data. We then used this model, prior information and the data within a Bayesian framework to approximate the posterior distribution of the growth parameters. For all species the two-parameter von Bertalanffy growth model provided the best fit to size-at-age datasets. Based on this model, the Bayesian approach allowed the estimation of median values of W(D∞) (cm) and k (yr(-1)) for the four species (H. uarnak: 149 and 0.12; N. kuhlii: 42 and 0.38; P. atrus 156 and 0.16, and T. lymma 33 and 0.24, respectively). Our approach highlights the value of combining different analytical methods and prior knowledge for estimating growth parameters when data quality and quantity are limited.

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.