Spatial and temporal variation in growth and age composition of the temperate wrasse Notolabrus fucicola in Tasmanian waters

The mechanisms that determine the age and size structure of a population are not well understood for temperate reef species and are functionally divided into pre-settlement and post-settlement processes. Their relative importance in structuring populations can be inferred from spatial comparisons of adult age composition and growth rates, provided factors that obscure or bias the ability to detect signals from early life history are considered. These factors include migration, fishing mortality and methodological issues such as ageing errors and size-selectivity of sampling gears. The biological features that promote Notolabrus fucicola as a suitable species for demographic research and the highly variable hydrology of the east and southeast coasts of Tasmania, provide an opportunity for examination of the relative roles of pre-and post-recruitment processes in structuring temperate reef populations. In addition, N. fucicola is the primary target of a commercial live wrasse fishery that has developed over the last decade in Tasmania, necessitating an understanding of its population dynamics for effective management. Notolabrus fucicola were collected using baited traps from six locations around the east and southeast coasts of Tasmania. A methodology for age estimation was developed using thinly sliced transverse sagittal otolith sections and the periodicity and timing of increment formation, location of the first annual increment and precision of age estimations were validated. A size-selectivity function for the traps was calculated directly from sampling an isolated population of N. fucicola of known size structure, generated from a simulated fish-down using tags. The selectivity function was used to reconstruct the raw size and age structures from which fishing mortality was estimated. Fishing mortality varied spatially, with strong mortality prior to the minimum legal size limit implied at some sites. Spatial comparisons of growth were not biased by fishing mortality and indicated that growth rates followed the sea surface temperature gradient, but did not follow trends in fishing mortality, diet or reef structure. Given that growth is sensitive to resource limitation, the spatial trend in growth rates suggested that populations of N. fucicola were not structured by post-settlement density-dependent mortality. Spatial and temporal comparisons of selectivity-corrected age compositions, whilst primarily detecting differences in the level of fishing mortality between sites, indicated low variation in relative year class strength. This result is consistent with recent findings suggesting that N. fucicola are well adapted to maximize larval survival despite the highly variable coastal hydrography. Thus, it is likely that populations of the temperate wrasse N. fucicola are primarily structured by larval supply.