Early life-history, settlement dynamics and growth of the temperate wrasse, Notolabrus fucicola (Richardson 1840), on the east coast of Tasmania
Creators: name: Welsford, DC
Publisher: name: University Of Tasmania
Year: 2023

description: Temperate reef fishes in general have received less attention than their tropical counterparts, despite their prominence in shallow reef ecosystems and developing fisheries. The temperate wrasses are a case in point, where a commercial fishery in southeastem Australia has developed over the last decade, targeting the large species in the genus Notolabrus. More research directed at the biology and ecology of temperate wrasses is required to improve the understanding of wrasse population dynamics and inform the management of fisheries targeting these species. This thesis describes original studies investigating the biology and ecology of all life-history stages of Notolabrus spp. in Tasmania. The development of Notolabrus spp. is described from eggs to post-settlement juveniles, based on reared larvae and wild caught pre- and post-settlement specimens. The early life stages of the two species of Tasmanian Notolabrus are morphologically very similar and cannot be readily identified by traditional methods, such as meristic or pigmentation pattern analyses. A random fragment length polymorphism (RFLP) assay is developed to discriminate all the 5 Tasmanian species in the pseudolabrine group (Notolabrus and closely related genera). Ageing fish using otolith microstructure requires species-specific validation of the onset and periodicity of increments. The presence of druly increments in the otoliths of N. fucicola and N. tetricus is validated using oxytetracycline (OTC). The initiation of daily increments is determined, based on comparisons of the sagittal otoliths of reared larvae and the primordial region in wild caught post-settlement specimens. Daily increments are initiated in the larval otolith at yolk sac absorption, which may occur up to 14 days after fertilisation at 10 ¬¨‚àûC. Otolifh microstructure is used to determine spawning and settlement dates of N. fucicola and N. tetricus at three sites on the east coast of Tasmania, over three years. The planktonic larval duration of N. facicola varies from 40 - 87 days, back-calculated from the settlement check observed in sagittal otoliths. The settlement season for these species extends for more than 100 days over spring and summer, a period of high variability in the oceanographic conditions off the Tasmanian coast. Patterns of sea surface temperature (SST) are correlated with peaks of spawning dates, larval duration, and post-settlement growth. Settlers show earlier spawning dates in years with higher SSTs. Mean larval duration decreases as SST increases in each year. Growth is significantly faster in later settlers within a year class. A trade-off is apparent between earlier settlers spending longer in the risky planktonic phase, but arriving on the reef earlier and hence having longer to grow in the post-settlement habitat before winter, as against later settlers' shorter exposure to the planktonic environment but shorter growth phase post-settlement before water temperatures and productivity plunge. The growth of demersal juvenile and adult N. facicola is modelled using otolith and tag- recapture data. Robust methods of comparing growth models are developed, involving likelihood ratio tests, and percentile confidence intervals and 2D plots of parameters estimates derived using bootstrapping. Traditional models for describing growth in fishes, such as the von Bertalanffy growth function show high levels of correlation in their parameter estimates, and are best compared using likelihood ratio tests, and inspection of parameter plots. Notolabrus facicola growth varies seasonally, between sites, and at one site growth difference between sexes is detected. Such growth variability poses a challenge to management based on models assuming uniform growth. Growth variability in N. facicola may explain why the species is a gonochorist, rather than a protogynous hermaphrodite as in most other wrasses species. Sex change may have been abandoned because attaining and/or maintaining a clear size advantage over other members of a social group to control mating opportunities may be impossible under unpredictable growth conditions.