Chondrichthyans (i.e., sharks, rays, and chimaeras) are susceptible to over-exploitation due to life history characteristics such as slow growth, late maturity, and low fecundity. Approximately one-third of all chondrichthyans have an elevated risk of extinction due to capture in fisheries targeting other species. Chondrichthyans are caught incidentally in penaeid trawls which are characterised by small mesh sizes and are towed on the seafloor. Previous research has shown that small chondrichthyans are found in the discarded portion of catches from the Queensland east coast otter trawl fishery (QECOTF), Australia’s largest penaeid-trawl fishery.
The current study quantified the ecological risk posed to 48 chondrichthyan species discarded by trawlers operating in the QECOTF south of the Great Barrier Reef Marine Park (GBRMP, >24.5°S) in the 2019 fishing year. Risk was assessed using the Sustainability Assessment for Fishing Effects (SAFE) method, a quantitative approach that compares the instantaneous fishing mortality rate (F) of each species to its maximum sustainable fishing mortality (Fmsm). Fishing mortality is a function of the area trawled within each species’ distribution, offset by the survival of discarded animals and escape from trawls via turtle excluder devices (TEDs). The SAFE analysis indicated that the level of fishing effort in the 2019 fishing year posed low risk to the long-term sustainability of all but two of the 48 species assessed. Overfishing of two species, Squalus megalops and Dentiraja australis, was occurring. Management changes introduced in the early 2000s resulted in significant reductions in nominal fishing effort in the study area. This, combined with the mandated use of TEDs and the prohibition of the retention of chondrichthyan products, has led to a reduction in the fishing mortality of chondrichthyans in the study region and throughout the QECOTF. Fishing impacts can be further mitigated by reducing maximum bar space, the distance between adjacent vertical bars in a single grid hard TED, to increase the escape of chondrichthyans. Additionally, fishers can increase post-release survival of chondrichthyans by reducing trawl duration and returning individuals to the sea as soon as practicable to reduce exposure to air.
The majority of the 48 species assessed in this study lack life history information, which is required to estimate maximum sustainable fishing mortality (Fmsm). As such, growth was estimated to improve the risk categorisation for Aptychotrema rostrata, the most common chondrichthyan found in penaeid-trawl discards in the study area (see Table 16). Growth parameters were derived from vertebral sections and estimated in a Bayesian framework with informative priors, to minimise bias resulting from the undersampling of older, larger animals. The number of length-at-age observations was increased via backcalculation. A total of 212 length-at-age observations were used to estimate the growth parameters which, after back-calculation, increased to 1112 length-at-age measures. The von Bertalanffy growth function (VBGF) was found to best fit the A. rostrata length-at-age data. With the sexes combined, the estimated VBGF parameters were L∞ = 923 mm TL, L0 = 193 mm TL and k = 0.08 year–1 . Estimates of v L∞ and L0 were higher for females (1141 and 193 mm, respectively), compared to males (813 and 187 mm, respectively). The growth coefficient for females (k = 0.05 year–1 ) was half that of males (k = 0.10 year–1 ).
The escape of chondrichthyans from penaeid trawls, via TEDs, was estimated using data collected by observers on-board commercial trawlers operating in Australia’s northern prawn fishery (NPF) during 2001 (Chapter 4). Generalised linear mixed modelling was used to quantify factors affecting the escape of Carcharhiniformes, Myliobatiformes, Orectolobiformes and Rhinopristiformes. Fish size, bar space, TED orientation (top- or bottom-shooter), grid size and grid shape were among the factors tested. More than 6,200 individuals were caught during the sampling conducted in the NPF to quantify escape via TEDs. The catch of large elasmobranchs was lower from nets containing TEDs: increasing fish size was found to result in higher escape for all taxonomic orders. Top-shooter TEDs increased the escape of Carcharhiniformes, while bottom-shooter TEDs facilitated greater escape of Myliobatiformes. Grid orientation had no effect on the escape of Orectolobiformes or Rhinopristiformes. Decreasing bar space was found to increase the escape of the Australian blacktip shark (Carcharhinus tilstoni). The TEDs facilitated the escape of several species of conservation interest including the scalloped hammerhead (Sphyrna lewini) and zebra shark (Stegostoma fasciatum). The rostrum of the narrow sawfish (Anoxypristis cuspidata), however, inhibited escape of this globally important species.
To improve estimates of the risk posed to the two most common chondrichthyans caught by trawlers operating in the QECOTF, the post-trawl survival (PTS) rate of the common stingaree (Trygonoptera testacea) and the eastern shovelnose ray (A. rostrata), was quantified experimentally, using on-board tanks to house animals up to three days post-capture. A total of 155 A. rostrata and 187 T. testacea were assessed for PTS. The experiments revealed that A. rostrata were more resilient to trawl catch-andrelease than T. testacea. For both species, survival was found to increase with size, whereas increasing time on deck resulted in lower survival. Female T. testacea were found to be more resilient than males, and increased tow duration resulted in lower survival for A. rostrata.
The mean (± s.e.m.) PTS for female and male T. testacea was 33.5 ± 6.0% and 17.3 ± 5.5% respectively, compared with a mean PTS for A. rostrata of 86.8 ± 3.2%. The risk assessment results were limited by a lack of life history information for 26 of the species assessed and this requires attention to improve future assessments. Region-specific life history metrics are also desirable. Future assessments will benefit from improved estimates of escape via TEDs and the survival of released individuals, particularly for those species for which these metrics are currently lacking.
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