Reproductive biology of two small-bodied sharks, Carcharhinus coatesi and Rhizoprionodon acutus, in the Northern Territory, Australia

Brief description

Sharks and rays are widely perceived to have slow reproductive rates and, thus, are often considered highly suspectable to population declines from commercial fishing. However, the reproductive biology of sharks is highly diverse, and some species can sustainably support significant commercial harvest. Small sharks are commonly caught and discarded as bycatch in several Australian and international fisheries. This represents either a wasted resource that could potentially be sustainably utilised, or a potential conservation issue if overharvesting occurs. This study examines the reproductive biology of two small-bodied sharks in the Northern Territory, Australia. Australian Carcharhinus coatesi (n = 726) and Rhizoprionodon acutus (n = 179) were collected between May 2018 and November 2019 from commercial trawl fisheries. Metadata record contains life history information for two shark species: C. coatesi and R. acutus. It contains length measurements, weights and measurements for reproductive biology. It also contains spatial and temporal information from approximately where sharks were caught.

Lineage

Statement: Carcharhinus coatesi and R. acutus individuals were collected between May 2018 – November 2019 from the NT commercial fisheries, which operates in the Exclusive Economic Zone (EEZ) of the Northern Territory (NT). Date of catch were recorded onboard commercial fishing boats and individuals were frozen until collection from the wharf in Darwin. Sharks were defrosted and the stretched total length (referred to from now as total length, TL) in millimetres, and weight in grams were recorded for each individual. The sex of each shark and reproductive stage was determined by assessing the state of the gonads and classifying them according to the criteria of Harry et al 2013. In male sharks, inner and outer margins of each clasper were measured. For the purpose of this analysis, gonad stages were converted to binary format (immature = 0, mature =1). For mature females with oocytes in the ovaries, one of the two ovaries was selected at random, and the largest ovarian follicle was measured in millimetres to determine the maximum ovum diameter (MOD). For pregnant females (Stage 5) the number of embryos was counted, and the size, weight and sex of each embryo recorded. Litter size was determined by counting the number of visible embryos in utero.The size at birth was determined using the smallest free-swimming juveniles and largest, developed embryos. For C. coatesi there were 3 neonates with open umbilical scars. The gut, liver and gonads were weighed using a scale accurate to 0.5 gram. The head, gut, liver and gonad were all stored at -20°C for later processing. For further details, please consult the research paper.

Statement: All animals were collected under section 17 of the Northern Territory of Australia Fisheries Act. Charles Darwin University advised that Animal Ethics approval by their Committee is not required for the opportunistic collection of deceased animals that are caught as part of a commercial fishing operation.

Notes

Credit
This research was funded by CDU HDR scholarship.

Credit
In-kind support from the Northern Territory Government Department of Industry, Tourism and Trade Fisheries Division