There are two major aspects:
1. Importance to industry of control of precocious sexual maturation
Tasmanian salmon typically mature after only one winter at sea, in contrast to northern- hemisphere populations where the majority take two ‘sea-winters’ to mature. The Tasmanian fish still reach 3 – 5 kg during this period due to the favourable effects of higher temperature on growth, however, the less desirable outcome of early maturation is the compression of the harvest season. Strategies designed to improve seasonal production have been estimated to be worth $A 8 – 16 million per year to the Tasmanian salmon industry (confidential industry estimate prepared for CRC for Aquaculture in 1998).
2. Requirement to conduct research in Tasmania
Functional photoperiod manipulation techniques have been developed overseas so why do it in Tasmania rather than simply import solutions? Overseas protocols have been trialled by the Tasmanian industry but have given negative or unpredictable results. Confounding factors which require consideration before overseas protocols can be applied successfully and predictably in Tasmania are:
A. Light intensity.
Due to Tasmania’s low latitude and high number of sunshine hours relative to the majority of northern hemisphere salmon farming areas, it is likely that salmon farmed in Tasmania are exposed to higher daytime light intensities, particularly at the equinoxes. Relative light intensity is a critical factor for the success of photoperiod manipulation practices and it is expected that higher, yet to be determined, levels of nighttime illumination will be required under Tasmanian conditions.
Overseas scientists report increased melatonin secretion (up to approx. 30%) at summer temperatures relative to winter temperatures in Atlantic salmon maintained under identical photoperiods. Tasmania’s relatively high water temperatures suggest that a further increase in light intensity will be required to reduce plasma melatonin levels below the putative threshold required to ensure that the fish perceive any modification to photoperiod.
C. Seasonal and individual variation.
Assuming that the preceding factors can be adequately clarified, it will be necessary to account for the possible effects of differences between seasons (both within and between years) and individual variation within fish populations in relation to the response of stocks to photoperiod manipulation. Relative to overseas salmon farming areas, Tasmania tends to have a short, mild winter, an early, warm spring and a long, hot summer. Thus timing of the critical “gate-open” decision period for maturation has yet to be determined under Tasmanian conditions.