The biology and ecology of carcharhiniform sharks in the Gulf of Papua prawn trawl fishery

Baje, Leontine Immoine (2019) The biology and ecology of carcharhiniform sharks in the Gulf of Papua prawn trawl fishery. PhD thesis, James Cook University.

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The elasmobranch fauna of Papua New Guinea (PNG) and its interaction with fisheries has been poorly studied in the past. Fisheries generally adversely impact elasmobranchs due to their low productivity life histories. Without fishery and region specific data on elasmobranchs the impact on their populations cannot be fully understood and subsequent development of appropriate fisheries management and conservation measures cannot be achieved. The objectives of this thesis were to address some of these data gaps for the Gulf of Papua Prawn Fishery (GoPPF) in PNG through the assessment of biological and ecological parameters of species caught as bycatch and the development of an ecological risk assessment for all elasmobranch species caught in the fishery.

The ecological component of this work focused on the feeding relationships among the Australian blackspot shark Carcharhinus coatesi, the milk shark Rhizoprionodon acutus and the Australian sharpnose shark Rhizoprionodon taylori. Rhizoprionodon acutus had a more specific diet compared to the other species, feeding almost exclusively on teleosts while C. coatesi and R. taylori had more diverse diets that had greater overlap. The limited sampling in this study did not fully characterise the diets of the three species, however, it does provide the first empirical evidence of trophic relationships between these sympatric sharks and their prey for the Gulf of Papua.

The biology of R. taylori and C. coatesi was investigated through determination of their age, growth and maturity. Ages were determined from vertebrae samples. Length at age data were fitted to several models in a multi-model information theoretic approach to determine which model provided the best fit. Maturity was analysed using logistic regression of maturity categories recorded from samples combined with size and age data. These studies provide an understanding of the growth rate and pattern of each species and the length and age which males and females of each species reach reproductive maturity.

To assess the biology of R. taylori, 186 samples were collected comprising 131 females (31-66 cm TL) and 55 males (31-53 cm TL). The lack of small individuals close to the size at birth made fitting of growth curves more difficult, two methods (fixed length at birth and additional zero aged individuals) accounting for this were trialled. The von Bertalanffy growth model provided the best fit to the data when used with a fixed length-at-birth (L0 = 26 cm TL). Males (????∞= 46 cm TL, k = 3.69 yr-1, L50 = 41.7 cm TL and A50 = 0.5 years) grew at a faster rate and matured at smaller sizes and younger ages than females (????∞ = 58 cm TL, k = 1.98 yr-1, L5o = 47.0 cm TL and A50 = 0.93 years). However, none of the methods to account for the lack of small individuals fully accounted for this phenomenon, and hence the results remain uncertain. Despite this, the results reaffirm the rapid growth of this species and suggest that the Gulf of Papua population may grow at a faster rate than Australian populations. Rhizoprionodon taylori is possibly well placed to withstand current fishing pressure despite being a common bycatch species in the GoPPF. However, further research needs to be undertaken to estimate other key life history parameters to fully assess the population status of this exploited shark species.

Carcharhinus coatesi is a similar small bodied coastal shark to R. taylori but some differences were observed in its growth and maturity parameters. The von Bertalanffy growth model also fit the data best for C. coatesi; parameters were L0 = 40.6 cm ± 0.8, L∞ = 74.8 cm ± 2.1, k = 0.33 year1 ± 0.06. Length-at-maturity analysis indicated that males reach maturity at L50 = 66.3 cm (CI: 63.8, 71.4) and L95 = 71.6 (C1: 64.6, 74.2) cm while females matured at L50 = 71.4 cm (CI: 61.5, 72.01) and L95 = 72.5 cm (CI: 62.7, 74.0). Age-at-maturity estimates showed that both males (A50 = 5.1 years (CI: 4.6, 7.1), A95 = 6.4 years (CI: 5.1, 7.2) and females (A50 = 5.3 years (CI: 3.5, 8.7) and A95 = 7.4 (CI: 3.6, 8.8) years) reach maturity at about the same age, but in comparison to other small bodied carcharhinids, C coatesi has slower growth in early life stages and reaches maturity at a later age. This biological trait along with a small litter size indicates that the population of C. coatesi in the Gulf of Papua may be more susceptible to decline as a result of fishing.

An Ecological Risk Assessment (ERA) was conducted to estimate the susceptibility of species caught in the fishery and the potential for a species to recover from population declines due to fishing if they occur. Of the 39 elasmobranch species encountered as bycatch in the fishery 10 were classified as being at low risk, 26 subjected to medium risk and 3 at high risk. The species at high risk were the Australian blackspot shark C. coatesi, the eyebrow wedgefish Rhynchobatus palpebratus and the blackspotted whipray Maculabatis astra. This is the first ERA conducted for this fishery. The findings provide fishery managers with information to implement an ecosystem-based approach to managing the fishery to reduce bycatch and improve the sustainability of the GoPPF.

This thesis has provided new information on the diet, age, growth, maturity and the potential risk of species suffering population declines from being caught in the GoPPF. These outcomes have implications for fisheries management and conservation of species in PNG and the surrounding regions. The areas of study begin to address current data gaps for this fishery and also set the foundation for future work to improve fisheries management and protect the survival of species through conservation measures in PNG.

Item ID: 64388
Item Type: Thesis (PhD)
Keywords: Papua New Guinea, Gulf of Papua, Gulf of Papua Prawn Fishery (GoPPF), ecological risk assessment, elasmobranchs, sharks, rays, bycatch, fisheries, Australian blackspot shark, Carcharhinus coatesi, milk shark, Rhizoprionodon acutus, Australian sharpnose shark, Rhizoprionodon taylori, diet, age, growth, maturity, population declines, conservation measures
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Copyright Information: Copyright © 2019 Leontine Immoine Baje.
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Two publications arising from this thesis are stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs. The publications are:

Chapter 4: Baje, Leontine, Smart, Jonathan J., Chin, Andrew, White, William T., and Simpfendorfer, Colin A. (2018) Age, growth and maturity of the Australian sharpnose shark Rhizoprionodon taylori from the Gulf of Papua. PLoS ONE, 13 (10). e0206581.

Chapter 5: Baje, Leontine, Smart, Jonathan J., Grant, Michael I., Chin, Andrew, White, William T., and Simpfendorfer, Colin A. (2019) Age, growth and maturity of the Australian blackspot shark (Carcharhinus coatesi) in the Gulf of Papua. Pacific Conservation Biology, 25 (4). pp. 403-412.

Date Deposited: 18 Sep 2020 00:27
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070403 Fisheries Management @ 33%
06 BIOLOGICAL SCIENCES > 0608 Zoology > 060809 Vertebrate Biology @ 34%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 33%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 34%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 33%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 33%
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