Pomatomus Saltatrix, in Eastern Australia
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Ecology of tailor, Pomatomus saltatrix, in eastern Australia Hayden Thomas Schilling A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy School of Biological, Earth and Environmental Sciences Faculty of Science University of New South Wales, Australia December 2019 Thesis/Dissertation Sheet Surname/Family Name : Schilling Given Name/s : Hayden Thomas Abbreviation for degree as give in the University : PhD calendar Faculty : Faculty of Science School : School of Biological, Earth and Environmental Sciences Thesis Title : Ecology of tailor, Pomatomus saltatrix, in eastern Australia Abstract (350 words maximum): Pomatomus saltatrix is a highly popular and heavily fished globally distributed mesopredatory fish known by a variety of common names (tailor, bluefish, elf, enchova and shad). The southwest Pacific (eastern Australian) population of P. saltatrix has been the subject of anecdotal declines in size, abundance and catch rates. Despite this there is a lack of ecological knowledge for this population. The aim of this research is to increase knowledge of the understudied southwest Pacific Ocean population of P. saltatrix. Samples were collected over 3 years from commercial and recreational fishing sectors. P. saltatrix in the southwest Pacific displayed fast, almost linear early growth with no visible asymptotic length. The Schnute growth equation provided the best fit and yielded parameter values of a = 0, b = 2.49, size at age 1 = 25.46 cm fork length (FL) and size at age 4 = 46.34 cm FL. The instantaneous total mortality rate (Z) was estimated to be 1.62 and the instantaneous natural mortality rate (M) was estimated to be 0.82. We explored the dietary variation using a classification tree analysis. Body size and latitude had the greatest influence on the diet of P. saltatrix, with significant diet shifts occurring at 8 and 30 cm FL. The importance of latitude was likely related to the East Australian Current and its separation from the continental shelf. We used otolith chemistry to evaluate the use of estuarine and marine environments by juvenile P. saltatrix. It was found that 24 – 52% of adult P. saltatrix had a juvenile period characterised by the marine environment. This suggests that P. saltatrix show considerable plasticity in juvenile habitat use. It was found that P. saltatrix have asynchronous oocyte development and may spawn multiple times during two distinct spawning events. The late summer spawning event in northern NSW was shown to be highly important for larval transport to the southern portion of the species distribution. P. saltatrix in the southwest Pacific show a life history pattern generally consistent with most other global populations of P. saltatrix except the northwest and east Atlantic populations. 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Name: Signature Date (dd/mm/yy) Hayden Schilling Abstract Pomatomus saltatrix is a globally distributed pelagic mesopredator which is exploited heavily throughout its range. Despite the implementation of strict management strategies, the southwest Pacific Ocean (eastern Australian) population has limited published information about the life history of this important fishing species. This thesis has 4 main chapters which investigate growth and mortality, diet, juvenile habitat use and the reproductive biology of the southwest Pacific Ocean population of P. saltatrix, locally known as tailor. By validating a whole otolith ageing technique and using a model selection process with 8 candidate growth models, I showed that the Schnute growth equation provided the best fit and yielded parameter values of a = 0, b = 2.49, size at age 1 = 25.46 cm fork length (FL) and size at age 4 = 46.34 cm FL. P. saltatrix in the southwest Pacific had a similar growth rate to other global populations, with the exception of the northwest Atlantic population which shows a considerably faster growth rate. Using a length frequency representative of harvested P. saltatrix and a catch curve analysis, instantaneous total mortality rate (Z) for the southwest Pacific population was estimated to be 1.62. Comparison with other global populations showed the northwest Atlantic population to have both the lowest natural mortality (M) and Z, together with the largest maximum biological age (14 y). All other populations were similar with higher mortality and maximum ages of 6 – 10 years. Z was driven largely by M for most populations, although fishing mortality (F) was highest in the east Indian and southwest Pacific populations. The dietary niche of P. saltatrix in eastern Australia was explored using a classification tree analysis to identify key factors driving diet variation. P. saltatrix was shown to be an opportunistic generalist predator which exhibited increased baitfish consumption, and decreased crustacean consumption, with increasing size. The