Ecology of tailor, Pomatomus saltatrix, in eastern Australia

• Main Author: Schilling, Hayden
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: UNSW Sydney 2019
• Subjects: Tailor; Pomatomus; Bluefish; Fisheries science; Otolith chemistry; Larval dispersal; Diet; Reproduction; FOS Medical biotechnology; Indian; Pacific; Northwest Atlantic
• Online Access: https://dx.doi.org/10.26190/unsworks/3886; http://hdl.handle.net/1959.4/64915

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 classification tree analysis showed that body size and latitude had the greatest influence on the diet of P. saltatrix, with significant ontogenetic diet shifts occurring at 8 and 30 cm fork length (FL). While piscivory is evident in the majority of P. saltatrix diets by ~8 cm FL, crustaceans are almost entirely absent from the diet after ~30 cm FL. The importance of latitude was likely related to the broad-scale oceanography in the study region, including the East Australian Current and its separation from the continental shelf. Otolith chemistry was used to evaluate the use of estuaries and the coastal marine environment by juvenile P. saltatrix in eastern Australia. Otolith chemical signatures of juveniles from 12 estuaries, spanning 10° of latitude, were characterised using laser ablation-inductively coupled plasma-mass spectrometry. Based upon multivariate otolith elemental signatures, fish collected from most estuaries could not be discriminated from one another. This was attributed to the varying influence of marine water on otolith elemental composition in fish from all estuaries. Using a reduced number of estuarine groups, the multivariate juvenile otolith elemental signatures and univariate Sr:Ca ratio suggest that between 24 and 52 % of adult P. saltatrix had a juvenile period influenced by the marine environment. Elemental profiles across adult (age-1) otoliths highlighted a variety of life history patterns, not all consistent with a juvenile estuarine phase. Furthermore, the presence of age-0 juveniles in coastal waters was confirmed from historical coastal trawl data. Combining multiple lines of evidence suggests considerable plasticity in juvenile life history for P. saltatrix in eastern Australia, through their utilisation of both estuarine and coastal nurseries. A reproductive biology survey across the whole population and an analysis of historical larval fish abundance, found a second distinct spawning period in late summer in NSW in addition to the recognised spring spawning in QLD. Ovaries displayed asynchronous oocyte development suggesting fish spawn multiple times per season. Fecundity showed an exponential relationship with fish length with estimates of batch fecundity ranging from 99,488 to 1,424,425 eggs per fish. When combined with a length frequency of the population, the majority of eggs were produced by fish < 40 cm fork length. Length at 50 % maturity (L50) was estimated at 30.2 and 31.5 cm FL for male and female tailor respectively, suggesting that raising the NSW minimum legal length (MLL) from 30 cm total length TL to 35 cm TL to match the QLD MLL could significantly improve recruitment. The sex ratio was found to have shifted in the last 40 years to a female dominated population (1.58 females :1 male). An oceanographic particle tracking model revealed that larvae from the different spawning events are dispersed differently, with the late summer spawning period supplying the most recruits to the southern portions of the species distribution, suggesting that the multiple spawning periods have developed as a result of selection over time as surviving larvae from the different spawning periods are transported differently by the ocean currents. P. saltatrix in the southwest Pacific (eastern Australia) show a life history pattern generally consistent with most other global populations of P. saltatrix except the northwest Atlantic population which grows larger, older and matures later.