Sawfish (Pristidae) of the Gulf of Carpentaria, Queensland, Australia

Peverell, Stirling Charles (2010) Sawfish (Pristidae) of the Gulf of Carpentaria, Queensland, Australia. Masters (Research) thesis, James Cook University.

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Abstract

The sawfish group Pristidae are relatively rare and are critically endangered in many habitats around the world. Information on their distribution and life history is limited. This study has improved the knowledge of pristid distribution and abundance within the inshore and offshore set net fisheries of the Gulf of Carpentaria (GoC), Queensland (Qld). Complementing this is information on the life history and biology of each of the species with recommendations for future management strategies. Pristis microdon, P. zijsron, P. clavata and Anoxypristis cuspidata ocurr throughout the Northern, Southern and Western Qld regions of the GoC, Australia.

Abundance and Distribution

This study showed that A. cuspidata was the most abundant species and was recorded in both the inshore and offshore set net fisheries in both its mature and immature life stages. Anoxypristis cuspidata abundance appeared to be greatest in the Northern region of the Gulf with a maximum catch per unit effort (CPUE) of 0.83 sawfish per 500m net day ⁻¹. The size distribution and catch locations of A. cuspidata suggest that the inshore area to a depth of ten metres may be the preferred habitat for juveniles, while adults primarily occur offshore. Pristis microdon, P. zijsron, and P. clavata were recorded only in the inshore fishery with catches dominated by immature animals. The abundance of P. microdon and P. zijsron was extremely low with a maximum CPUE of 0.1 and 0.2 sawfish per 500m net day ⁻¹ respectively and their distribution patchy. The maximum CPUE for P. clavata was 0.83 sawfish per 500m net day ⁻¹, however unlike A. cuspidata, their distribution was more restricted. The incidental catches of P. microdon in the set net fisheries of the GoC appear to be seasonal. This species was predominantly caught in the inshore fishery late in the monsoonal wet season (February to April) and inhabited both freshwater and estuarine environments. These findings are supported by tag recapture and vertebral microchemistry (LA-ICPMS) analysis.

Tagging

The tag and recapture data demonstrates that P. microdon is capable of moving along the coastal foreshore between estuaries, and juveniles migrate upstream following the receding freshwater and downstream with the floodwaters. The findings from the LAICPMS analysis support the hyppothesis that P. microdon utilise freshwater, estuarine Sawfish (Pristidae) of the Gulf of Carpentaria, Queensalnd Australia vii and marine habitats during different stages of their life history. High Sr88 to Ca43 ratios indicative of a marine environment were recorded in the section of vertebrae representative of the mature life stage in P. microdon. Low ratios indicative of a freshwater environment were recorded during the juvenile life stages. This habitat preference demonstrated by juvenile P. microdon is possibly predator avoidance behaviour. Although P. microdon was not represented in the incidental catch of the offshore gillnet fishery, it is highly likely they do inhabit this fishing area based on the findings of the tag and release information and LA-ICPMS data from this study. Furthermore, P microdon is a bycatch species in the Northern Prawn Fishery (NPF) thereby giving credibility to the hypothesis that this species inhabits deeper offshore waters of the GoC. Unlike the commercial catch of P. microdon, there appeared to be no seasonal trend in the catches of the other three sawfish species. Pristis clavata, P. zijsron and A. cuspidata were recorded throughout the commercial set net fishing season. Information obtained from tag and recapture of P. clavata, and A. cuspidata and LA-ICPMS analysis and short term acoustic tracking of P. zijsron indicate that these specimens may have restricted site fidelity. Observations of the reproductive organs and the capture of neonate specimens indicate that in all four pristids, pupping occurred through the wet season until the beginning of the dry season in May.

Age and Growth

The age at maturity estimates of the four pristid species in this study were similar between genera. The number of growth bands on cross-sectioned vertebrae and observations made of reproductive organs, the age at maturity for P. microdon, P. clavata and P. zijsron was between 8 and 10 years. For A. cuspidata with age estimates based from growth bands on branchial vertebrae sections and macro-staging of reproductive organs, the age at maturity was approximately 3 years. In this study, size at maturity for female P. microdon was 300cm TL. The observed size at maturity of male P. clavata and female P. zijsron in this study was 295cm TL and 380cm TL respectively. The observed size at maturity for A. cuspidata was considerably smaller than for Pristis spp. at 203cm TL for males and 225cm TL for females. All GoC pristid species had a rapid growth rate in the first twelve months of development. This first year increase in size in P. microdon was 52cm, in P. clavata it was 41cm, in P. zijsron it was 52cm and in A. cuspidata it was 82cm. In all GoC pristids the growth rate in the mature stages decreased to a total growth over the last 10 years of only 20cm in P. microdon, 14cm in Sawfish (Pristidae) of the Gulf of Carpentaria, Queensalnd Australia viii P. clavata, 16cm in P. zijsron and 4cm in A. cuspidata. The maximum ages of GoC pristids ranged from 35 years (P. microdon), 34 years (P. clavata), 24 years (P. zijsron) and to at least 9 years (A. cuspidata).

Diet

Prey items found in the stomachs of Pristis included telesot fishes and Crustacea. Prey items of freshwater origin were only recorded in the stomachs of P. microdon. Anoxypristis cuspidata appeared to have a more non-selective preference for prey items compared to Pristis. Prey items in the stomach of A. cuspidata included benthic teleost species such as Platycephalus spp. and pelagic species including squid Photololigo chinensis. It was therefore concluded that A. cuspidata have a benthopelagic diet.

Item ID: 29567
Item Type: Thesis (Masters (Research))
Keywords: sawfish; Pristidae; distribution; life history; Gulf of Carpentaria, Queensland; management strategies
Date Deposited: 08 Oct 2013 06:31
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 40%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 40%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 20%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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