1. To accurately predict maturation rates and optimize photoperiod regimes to prevent early maturation.
Ambient environmental conditions mean that the Tasmanian salmon industry will always suffer from high maturation rates due to its high water temperatures and increased light intensity. Additional artificial lighting in Tasmania has been shown to reduce maturation by up to 30%; increase growth rates significantly; and delay maturation by 8 weeks (Porter et al., unpublished). While these strategies have improved seasonal production and have been estimated to be worth $8-16 million per year (TSGA report), seasonal fluctuations in environmental conditions and fish stocks still produce variability within their effectiveness. Trials to date have highlighted the need for increased light intensities both between sites and seasons depending on the results required. Therefore further work into the use of increased light intensity and plasma melatonin production is required.
2. To better understand the timing of oocyte maturation in relation to varied environmental conditions.
a) The development of oocytes within the gonads needs to be initiated well in advance of the fish spawning. At present it is unknown precisely when this occurs and what physiological parameters are required to allow maturation to proceed. The timing of this “gating” period will be determined as this is undoubtedly the most effective time with which to apply environmental manipulations to inhibit the maturation process to continue. The “gating mechanisms” i.e. size and energetic status will be investigated to more accurately determine the timing and duration of the application of artificial lights.
b) One hormonal candidate for the transduction of information between growth and reproductive processes is insulin-like growth factor I (IGF-I). The majority of research has investigated the interactions between the GH/IGF system and maturation during the latter stages of oocyte maturation i.e. secondary oocyte growth and development. Consequently there is a paucity of information on the impact of IGF-I on the initiation of oocyte maturation and primary oocyte growth and development. Determining the role of IGF-I at this stage would assist in our understanding of the interaction between growth and reproductive processes, and thus provide additional tools to control the timing of maturation in commercial operations.
At present the culture of Atlantic salmon within Australia produces approximately 26,000 t of fish per annum and is a direct employer of over 1100 workers with the majority of farmed fish sold nationally and only 12% exported.
Environmental conditions, such as increased temperatures and high light intensities, found within Tasmania, provide exceptional growing conditions for Atlantic salmon with growth rates far exceeding other salmon producing nations. However, the life history strategy of Atlantic salmon has evolved to ensure maximum opportunity for reproduction which means that both male and female fish are able to initiate maturation and reproduce at several times during their life cycle if environmental and nutritional factors are favourable (Thorpe et al., 1990). Consequently salmon farmed within Tasmania have a far greater rate of early maturation resulting in a percentage of the stock being unsaleable due to poor flesh quality.
Previous work has resulted in the development of artificial lighting strategies to alter the timing and/or completely inhibit these reproductive events on farms and has been successful in providing harvestable fish year-round. The current study seeks to build on the previous work to ultimately gain a greater understanding of the physiological mechanisms responsible for maturation in salmonids.
In a tank trial with rainbow trout it was determined that elevated winter water temperature appears to have a significant impact on maturation by increasing the number of maturing individuals in photoperiod advanced stocks. This finding has implications for farms which may experience variations in water temperatures from year to year and has the potential to negatively affect the efficiency of lighting strategies to inhibit maturation. Variations in endocrine growth factor levels (insulin-like growth factor-I) between maturing and immature individuals suggest that this hormone plays a role in the link between growth and reproduction.
A population of female Atlantic salmon were investigated over a 2.5 year period at the Saltas Ltd. hatchery site (Wayatinah, Tasmania). This work determined that growth, condition and lipid levels of Tasmanian grown Atlantic salmon are factors which exhibit key differences when compared to Northern hemisphere stocks. It was apparent from the accelerated growth and completion of maturation before reaching 3 years of age, that the rate of development is significantly advanced in Tasmania compared to traditional stocks in the Northern hemisphere. It was also evident that the key period of interest with regards to further exploring the physiological factors responsible for precocious maturation in Tasmania is during the parr phase, prior to seawater transfer. Furthermore, the contrast in condition and energy resources accumulated as lipids in the muscle tissue of parr and juvenile Atlantic salmon compared with Northern hemisphere stocks supports the proximate life-history model proposed by Thorpe et. al. (1998) which designated lipid levels as a determining factor in the decision to mature.