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

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

Dietary overlap of carcharhinid sharks in the Gulf of Papua

Assessing the feeding patterns of sharks provides insight into ecological interactions. Three coastal sharks are common by-catch in the Gulf of Papua prawn fishery in Papua New Guinea. The diets of Carcharhinus coatesi (n = 122), Rhizoprionodon acutus (n = 83) and Rhizoprionodon taylori (n = 177) were assessed using stomach content analysis. Teleosts, crustaceans and molluscs were the main prey. Percentage frequency of occurrence (%FO) and percentage frequency by number (%N) were computed to describe dietary compositions. Non-metric multidimensional scaling and Morisita Index determined the level of feeding overlap. Rhizoprionodon taylori was a generalist feeder having the broadest diet, R. acutus was the most selective feeder, preying predominantly on teleosts and C. coatesi consumed the greatest proportion of crustaceans that increased with size. The pairwise ANOSIM tests showed significant difference in dietary compositions of R. acutus and R. taylori (P = 0.1%, R = 0.318) and R. acutus and C. coatesi (P = 0.1%, R = 0.589), which indicate potential resource partitioning. Further work should aim to adequately characterise diets, improve prey identification and investigate spatial and temporal resource use patterns. Understanding ecological processes informs ecosystem approaches fisheries management

Age, growth and maturity of oceanic whitetip shark (Carcharhinus longimanus) from Papua New Guinea

Oceanic whitetip sharks (Carcharhinus longimanus) in the Western Central Pacific have been overfished and require improved assessment and management to enable planning of recovery actions. Samples from 103 individuals (70 males and 33 females; 76.0-240- and 128-235-cm total length (TL) respectively) were used to estimate age, growth and maturity parameters from sharks retained by longline fisheries in Papua New Guinea. Back-calculation was used because of the low number of juveniles and a multimodel framework with Akaike's information criterion corrected for small sample size (AIC(c)) estimated growth parameters. The von Bertalanffy growth model provided the best fitting growth model for both sexes. Parameter estimates for males were: asymptotic length (L-infinity) = 315.6 cm TL; growth coefficient (k) = 0.059 year(-1); and length at birth (L-0) = 75.1 cm TL. For females, the parameter estimates were: L-infinity = 316.7 cm TL; k = 0.057 year(-1); and L-0 = 74.7 cm TL. Maximum age was estimated to be 18 years for males and 17 years for females, with a calculated longevity of 24.6 and 24.9 years respectively. Males matured at 10.0 years and 193 cm TL, whereas females matured at 15.8 years and 224 cm TL. C. longimanus is a slow-growing, late-maturity species, with regional variation in life history parameters, highlighting increased vulnerability to fishing pressure in this region

Rediscovery of the Threatened River Sharks, Glyphis garricki and G. glyphis, in Papua New Guinea

Recent surveys of the shark and ray catches of artisanal fishers in the Western Province of Papua New Guinea (PNG) resulted in the rediscovery of the threatened river sharks, Glyphis garricki and Glyphis glyphis. These represent the first records of both species in PNG since the 1960s and 1970s and highlight the lack of studies of shark biodiversity in PNG. Two individuals of G. garricki and three individuals of G. glyphis were recorded from coastal marine waters of the Daru region of PNG in October and November 2014. The two G. garricki specimens were small individuals estimated to be 100–105 cm and ~113 cm total length (TL). The three G. glyphis specimens were all mature, one a pregnant female and two adult males. These are the first adults of G. glyphis recorded to date providing a more accurate maximum size for this species, i.e. ~260 cm TL. A single pup which was released from the pregnant female G. glyphis, was estimated to be ~65 cm TL. Anecdotal information from the fishers of pregnant females of G. glyphis containing 6 or 7 pups provides the first estimate of litter size for this species. The jaws of the pregnant female G. glyphis were retained and a detailed description of the dentition is provided, since adult dentition has not been previously documented for this species. Genetic analyses confirmed the two species cluster well within samples from these species collected in northern Australia

Papua New Guinea: a potential refuge for threatened Indo–Pacific river sharks and sawfishes

The conservation of threatened elasmobranchs in tropical regions is challenging due to high local reliance on aquatic and marine resources. Due primarily to fishing pressure, river sharks (Glyphis) and sawfishes (Pristidae) have experienced large population declines in the Indo-Pacific. Papua New Guinea (PNG) may offer a refuge for these species, as human population density is low, and river shark and sawfish populations are thought to persist. However, few data are available on these species in PNG, and risk posed by small-scale fishers is poorly understood. This study observed elasmobranch catches in small-scale fisheries in riverine and coastal environments in the East Sepik (northern region), Gulf, and Western Provinces (southern region) of PNG. Surveys were conducted over a period of weeks to months in each region, during the dry season across seven field trips from 2017 to 2020. We observed a total of 783 elasmobranchs encompassing 38 species from 10 families. River sharks made up 29.4% of observations in the southern region, while sawfishes made up 14.8 and 20.3% in the northern and southern regions, respectively. River sharks were commonly caught by small-scale fishers in lower riverine environments in southern PNG, while sawfishes were generally less common and mainly observed through dried rostra. The primary threat to river shark and sawfish populations is their capture by small-scale fishers targeting teleosts for swim bladder. Persisting populations of river sharks and sawfishes indicate that PNG is the second known nation with viable populations of multiple species in the Indo-Pacific. However, populations are declining or at high risk of decline, and fisheries management and conservation are required to realize the potential of PNG as a long-term refuge

Ecological risk assessment of elasmobranchs caught in the Gulf of Papua prawn fishery

One-third of all elasmobranch species currently known to occur in Papua New Guinea are taken as bycatch in the Gulf of Papua trawl fishery. An ecological risk assessment was conducted on the 16 species of sharks and 23 species of rays caught by the fishery. Eight species were classified to be at low risk, 28 species were at medium risk while three species – Rhynchobatus palpebratus (eyebrow wedgefish), Carcharhinus coatesi (Australian blackspot shark) and Maculabatis astra (blackspotted whipray), all endemic to New Guinea and northern Australia – faced the highest level of risk from the fishery at current fishing levels. There is potential for more vessels to enter the fishery in future, which is likely to elevate the risk for a greater proportion of elasmobranch species. Ongoing monitoring of bycatch levels, supported by biological and ecological studies on frequently caught species, is required to assess and manage risk in the future. The development and use of trawl excluder devices or bycatch reduction devices in this fishery have the potential to reduce bycatch levels and reduce risk levels

Age, growth and maturity of the Australian sharpnose shark Rhizoprionodon taylori from the Gulf of Papua.

Coastal sharks with small body sizes may be among the most productive species of chondrichthyans. The Australian sharpnose shark (Rhizoprionodon taylori) is one of the most productive members of this group based on work in northern and eastern Australia. However, life history information throughout the remainder of its range is lacking. To address this knowledge gap, the age, growth and maturity of R. taylori caught in the Gulf of Papua prawn trawl fishery in Papua New Guinea, were studied. One hundred and eighty six individuals, comprising 131 females (31-66 cm TL) and 55 males (31-53 cm TL) were aged using vertebral analysis and growth was modelled using a multi-model approach. 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 (L∞ = 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 (L∞ = 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 Gulf of Papua prawn trawl fishery. 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 and its vulnerability to fishing in the Gulf of Papua

Preliminary age and growth estimates of the blue shark (Prionace glauca) from Papua New Guinea

Blue sharks (Prionaceglauca) are recognised as one of five key pelagic shark species in the Western Central Pacific Ocean (WCPO) due to their frequent incidental catch in tuna and billfish longline fisheries. Given their importance in the region, the aim of this study was to investigate the life history of this species for use in future population assessments in Papua New Guinea (PNG). Eighty-one vertebral samples were examined to provide preliminary age and growth estimates for P.glauca caught by commercial longline vessels operating in the Bismarck and Solomon seas. Ages ranged from 10 to 25 years. A Bayesian approach using Markov Chain Monte Carlo (MCMC) was used to estimate growth parameters. The von Bertalanffy model provided the best fitting growth model (males: L∞ = 379 cm, L0 = 45.8 cm, k = 0.07 year−1; females: L∞ = 329.2 cm, L0 = 45.8 cm and k = 0.08 year−1; combined sexes: L∞ = 350.8 cm, L0 = 45.8 cm, k = 0.07 year−1). The asymptotic length parameter estimate for the male P.glauca population from PNG was the largest reported. Our results demonstrate that intraspecific variation in life history traits of P.glauca across its entire distribution is likely due to differences in methodology, sample size and interpretation of growth bands rather than regional differences in growth. This study takes an important step towards facilitating management strategies for P.glauca in PNG by producing preliminary growth estimates for the species. However, further studies with larger sample sizes are required to conduct age validation and refine the life history information for this highly migratory species in PNG

Age, growth and maturity of the Australian blackspot shark (Carcharhinus coatesi) in the Gulf of Papua

Small-bodied coastal sharks are often caught as by-catch in fishing operations. Life-history information for these sharks is needed to ascertain the level at which these populations are potentially affected by fishing. This study determined the age, growth and maturity of Carcharhinus coatesi captured by prawn trawlers in the Gulf of Papua. Using vertebral aging and an information-theoretic multimodel approach, the von Bertalanffy growth model fit the data best; parameters were L0 = 40.6 cm ± 0.8, L∞ = 74.8 cm ± 2.1, k = 0.33 year−1 ± 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 (CI: 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), A95 = 7.4 years (CI: 3.6, 8.8)) reach maturity at about the same age. This study addresses the need for specific life-history information for a data-deficient species in a region that has remained relatively under-researched but in need of ongoing improvements to fisheries management