The biology and fisheries of angel sharks and sawsharks in south-eastern Australia
thesisposted on 2022-03-29, 01:21 authored by Vincent Raoult
Shark populations are under threat from commercial fisheries, and many studies suggest that shark fisheries are not sustainable. Angel sharks (Squatina australis, Squatina albipunctata) and sawsharks (Pristiophorus nudipinnis, Pristiophorus cirratus) have been caught by commercial fisheries in south-eastern Australia since the beginning of the 20th century. Despite indications from research trawlers that their populations have declined, no studies have been conducted to examine the effects of commercial fisheries or the biology of these sharks. This project aimed to examine the impacts of commercial fisheries on sawshark and angel shark populations, and to contribute to a better understanding of the life-history traits of these sharks. With this information, it should be possible to assess the sustainability of sawshark and angel shark fisheries in south-eastern Australia. A collection of commercial and scientific fishery data sets were combined to determine whether fisheries have had an impact on angel shark and sawshark populations. Data sets were also used to re-define the distributions of the four species that occur in the area. In both cases, it appears that current fishing rates are sustainable, but shark populations are at significantly lower levels than when fisheries commenced. It is therefore recommended that all four species are classified as ‘sustainably overfished’ and that they require ongoing monitoring. Historically, angel sharks have been difficult to age due to their unique vertebral structure. Sawsharks have similar vertebrae with no apparent band deposition. A scanning x-ray fluorescence microscope (SXFM) was used to observe the elemental distribution of shark vertebrae, and to determine the age of angel sharks and sawsharks. Traditional age bands were more correlated with strontium rather than calcium, suggesting that strontium is the driver for band deposition. These results suggest that vertebral banding may be related to environmental variables such as salinity. Due to the difficulty of accessing SXFMs and their rarity, it would be beneficial if other, more accessible ageing techniques were developed that yielded similar results. A micro computed tomography (microCT) ageing technique was developed that successfully replicated the results of the SXFM. These results suggest that microCT could be further developed into a widely-applicable elasmobranch ageing tool, which replicates results from a less-accessible SXFM and enables analytical ageing analysis. Morphometrics and reproductive data were collected for angel sharks and sawsharks caught by fisheries in south-eastern Australia. Female-biased dimorphism was apparent, as well as similar male and female growth curves. Morphometrics suggest that larger S. albipunctata have larger eyes than S. australis, and may be the reason S. albipunctata are more suited to deeper waters. Results also suggest that anterior pectoral margins can be used as a proxy for total length when examining truncated specimens. Muscle samples were analysed for isotopic analysis to determine the trophic levels of sawsharks, and whether co-occurring P. cirratus and P. nudipinnis avoided competition Results suggest that these two sawshark species avoid competition through resource partitioning, and that they have significantly different trophic levels. Isotope level comparison to other studies suggests that P. cirratus has an invertebrate diet, while P. nudipinnis has a piscivorous diet. For both species, diet shifted to higher trophic levels during ontogeny. This study suggests that fisheries have had a significant impact on the populations of sawsharks and angel sharks, but that current fishing levels can continue if the objective is not to return shark populations to their pre-industrial levels. Results are also of use to increase the effectiveness of management protocols.