The chondrichthyan (shark, batoid and holocephalan) bycatch of the Queensland East Coast Trawl Fishery (ECTF) was examined through a series of fishery-independent trawl surveys, together with fishery-dependent (opportunistic) sampling. Project aims were to document the chondrichthyan bycatch composition in order to test the effectiveness of turtle exclusion devices (TEDs) and bycatch reduction devices (BRDs), to examine biological aspects of bycatch species, and to combine data collected through these parts to assess the conservation status and sustainability of bycatch species. A total of 37 chondrichthyan species (one holocephalan, 19 batoids and 17 sharks) from 18 families were recorded in the bycatch of the fishery. The most speciose families recorded were the stingrays (Dasyatidae; 7 species), the requiem sharks (Carcharhinidae; 5 species), the catsharks (Scyliorhinidae; 4 species) and the stingarees (Urolophidae; 3 species). Chondrichthyan bycatch was highly variable between fishery sectors; catch rates were low in the tiger/Endeavour prawn sector (north Queensland; 0.02–0.12 individuals ha-1 trawled) and in the eastern king prawn (deepwater) sector (southern Queensland; 0.08 individuals ha-1 trawled), intermediate in Hervey Bay (southern Queensland; 0.25 individuals ha-1 trawled) and in the scallop sector (central Queensland coast; 0.31 individuals ha-1 trawled) and highest in the eastern king prawn (shallow water) sector (southern Queensland; 0.96 individuals ha-1 trawled). Chondrichthyan bycatch in the eastern king prawn (shallow water) sector was dominated by the three batoids Aptychotrema rostrata, Trygonoptera testacea and Urolophus kapalensis (~92% of the chondrichthyan bycatch by number), in the eastern king prawn (deepwater) sector by the skate Dipturus polyommata and the two catsharks Asymbolus rubiginosus and Figaro boardmani (~83% of the chondrichthyan bycatch by number), in the scallop sector by the three batoids A. rostrata, Neotrygon kuhlii and Neotrygon picta (~91% of the chondrichthyan bycatch by number), and in the tiger/Endeavour prawn sector by the two batoids Himantura astra and Gymnura australis and the two sharks Chiloscyllium punctatum and Hemigaleus australiensis (~67% of the chondrichthyan bycatch by number). The testing of TEDs and BRDs, which are mandatory throughout the fishery, demonstrated only a limited ability to reduce chondrichthyan bycatch in the ECTF, which is comprised mainly of relatively small species. The shorter trawl durations of the surveys compared with normal commercial activities may have under-represented larger species. No significant reductions in chondrichthyan bycatch were found using a TED and a radial escape section BRD in the eastern king prawn (shallow water) sector, using a TED and a square-mesh codend BRD in the eastern king prawn (deepwater) sector, or using a TED and a fisheye BRD in Hervey Bay. There was however, a significant difference in the probability of capturing the group ‘sharks and guitarfishes’ (comprised largely of A. rostrata) between codend types in the scallop sector, with the lowest probability of capture in nets fitted with both a TED and a square-mesh codend BRD (the difference was largely attributed to the effects of the TED). In the tiger/Endeavour prawn sector, in which three different BRDs were trialed (fisheye, square-mesh codend, square-mesh panel), the probability of capturing chondrichthyans was significantly lower in nets fitted with a fisheye BRD than in the standard (control) net, and the probability of capturing batoids was significantly lower in nets fitted with a fisheye BRD or with a square-mesh codend BRD than in the standard (control) net. The small sample size of chondrichthyan catches in some sectors may have reduced to power to detect bycatch reduction. The biology of several bycatch species from the families Rajidae, Rhinobatidae, Urolophidae and Scyliorhinidae was examined. For D. polyommata, size at birth was estimated at ~100–110 mm total length (LT), size at first feeding at ~105–110mm LT, size at 50% maturity (LT50 and 95% CI) at 321 (305–332) mm LT for females and 300 (285–306) mm LT for males. Diet (described by the index of relative importance as a percentage) was predominantly crustacean based, with carid shrimps (53.6%) and penaeoid prawns (23.3%) being the most significant prey groups. For A. rostrata, size at birth was estimated at <170 mm LT, size at 50% maturity (LT50 and 95% CI) at 640 (618–663) mm LT for females and 597 (551–649) mm LT for males, and litter size was 9–20 (n = 9; mean ± S.E. = 15.1 ± 1.2). For T. testacea, size at birth was estimated at 77–100 mm disc width (WD), size at 50% maturity (WD50 and 95% CI) at 163 (156–169) mm WD for females and 146 (140–150) mm WD for males, and litter size was always one (n = 6). For U. kapalensis, size at birth was estimated at 75–100 mm WD, size at 50% maturity (WD50 and 95% CI) at 154 (145–160) mm WD for females and 155 (149–159) mm WD for males, and litter size was always one (n = 16). The catsharks A. analis, A. rubiginosus and F. boardmani were all confirmed as single oviparous species (carrying only one egg case in each uterus at one time). Ovarian fecundity (the number of vitellogenic follicles) averaged 13.6 (range 13–20) in A. analis, 13.5 (range 5–23) in A. rubiginosus and 10.4 (range 9–13) in F. boardmani. While only limited data were available from southern Queensland, several indicators suggest that Asymbolus catsharks are reproductively active year-round. A general lack of small-sized or immature catsharks captured during the study made assessments of size at maturity difficult for these species. The conservation status of ECTF bycatch species was examined through the application of the IUCN Red List of Threatened SpeciesTM Categories and Criteria, which considers extinction risk at the global level. Of the 24 ECTF chondrichthyan bycatch species evaluated against the IUCN Red List Categories and Criteria, four have been assessed globally as Vulnerable (a threatened category indicating that a species is ‘facing a high risk of extinction in the wild’), seven as Near Threatened, 11 as Least Concern and two as Data Deficient. While the four globally threatened species (A. nichofii, Heteroscyllium colcloughi, Rhynchobatus australiae and Urolophus sufflavus) were only minor components of the ECTF bycatch, their global conservation status warrants that fisheries management and industry should act to ensure minimal impacts on these species. An ecological risk assessment method (Susceptibility-Recovery Analysis) was used to assess the relative sustainability or risk of individual species to the fishing activities of the ECTF. Two separate approaches were taken to the technique, which considers sustainability to be dependent on the susceptibility of a species to the fishery and the recovery potential of a species after depletion by fishing activities. The first approach applied the precautionary principal when data were lacking for the calculation of recovery attributes, while the second used biological data from similar species when species-specific data were lacking. The precautionary approach tended to overestimate risk to poorly known oviparous species. The biological approach suggested that A. nichofii, F. boardmani, Rhizoprionodon acutus, Rhizoprionodon taylori and Rhynchobatus palpebratus face the least risk (i.e. were the most sustainable) while several medium-large batoids and the sharks Loxodon macrorhinus and Heteroscyllium colcloughi were the species most at risk (i.e. least sustainable). Demonstrating ecological sustainability of the ECTF will need to be a continued management objective into the future. To further improve the ecological sustainability of the fishery in relation to sharks, batoids and holocephalans, a number of management recommendations are proposed: (1) give warranted conservation consideration to listed threatened species as well as species identified as being at risk; (2) expand required logbook information on chondrichthyan species to include recording of catches of these species; (3) encourage safe release practices for all chondrichthyans to maximise survivorship of discards; (4) initiate research into the survivorship of discards; (5) ensure long-term observer coverage on commercial vessels to monitor bycatch levels; and, (6) test and quantify reduced TED bar spacings (presently 120 mm) in fishery sectors which show the highest chondrichthyan bycatch levels, that is, the eastern king prawn (shallow water) and scallop sectors.
| Identifer | oai:union.ndltd.org:ADTP/287459 |
| Creators | Peter Kyne |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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