Reproductive physiology of wreckfish (hāpuku) Polyprion oxygeneios in captivity

Abstract

Summary New Zealand’s flagship aquaculture species are currently the green-lipped mussel (Perna canaliculus), the Pacific oyster (Crassostrea gigas) and the chinook salmon (Oncorhynchus tshawytscha). However, important drivers to achieve growth of the aquaculture sector have been identified through the diversification of species being cultured and the development of innovative products. In response to this, the New Zealand National Institute of Water and Atmospheric Research (NIWA) has responded by conducting extensive research into the culture of the wreckfish species Polyprion oxygeneios – locally known as hāpuku or groper.

Information is limited the reproductive biology, behaviour and life histories of both wild hāpuku and those maintained in captivity. In the wild, it is believed that breeding individuals migrate to deep and in many cases unfishable areas to spawn while wild-caught captivity-acclimated broodstock maintained under photo-thermal control communally spawn without hormonal intervention. However, despite the development of hatchery technologies to raise hāpuku larvae by NIWA, seed supply is still reliant on these wild-caught captivity-acclimated broodstock and significant issues bottlenecking production include variable egg quality and poor larval survival.

It is now evident that the development of appropriate broodstock management strategies for an emerging aquaculture species like hāpuku requires the ability to identify, monitor and control ovarian growth, oocyte maturation and ovulation in stock. Research within this thesis makes a significant contribution to such requirements by studying first filial (F1) fish in captivity at various stages of the reproductive cycle.

The first study has a basic focus examining ovarian development by RNA-seq on tissues at different stages oogenesis (see Chapter 2 for details). The aim of this study was provide a snapshot of the composite changes in gene expression associated with oocyte growth in hāpuku – particularly during the transition from previtellogenic to early vitellogenic stages of ovarian growth. Thirty-five differentially expressed genes were identified and upregulated between previtellogenic and early vitellogenic stages of development. Once these genes were grouped according to tentative functions, those associated with the respiratory electron transport chain, lipid metabolism, steroidogenesis and mineral/ solute transport were apparent.

The second study centred around advancing puberty by examining the effects of several sex steroid and neuropeptide treatments on the pituitary-gonadal axis of pre-pubertal hāpuku in vivo (see Chapter 3 for details). While puberty could not be advanced, outcomes from this study included insights into the negative feedback effects of androgens on the pituitary-gonadal axis of pre-pubertal hāpuku and directions for future research towards the exploration of alternative treatments to advance puberty.

In order to develop more appropriate broodstock management strategies, Chapter 4 aimed to describe ovarian development of captive F1 hāpuku in relation to temperature regime. This was achieved by employing repeat-measurement approaches to document sequential changes in oocyte development, the relative transcript abundance of several genes associated with oogenesis and estimate E2 levels in plasma of individual females during their maiden spawning cycles, while being maintained under constant or varying water temperature regimes. The first description of a reproductive cycle in F1 female hāpuku was achieved. Females reached sexual maturity at the age of five years and displayed distinct seasonality to their breeding cycle when maintained on a simulated-natural photoperiod at constant or variable temperatures. To add, the adverse effects of constantly elevated water temperatures on ovarian development were highlighted by the low number of females reaching the advanced stages of vitellogenesis in this group relative to females maintained at cooler varying temperatures.

The fourth and final study aimed to induce spawning on demand in F1 fish from the latter study by trialling a gonadotropin-releasing hormone analogue (GnRHa) as a reproductive therapy (see Chapter 5 for details). Second filial (F2) progeny were produced and the life-cycle of hāpuku was completed for the first time in the world – not only for hāpuku but any species within the genus Polyprion. Surviving F2 offspring originated from gametes of GnRHa-treated broodstock collected by strip-spawning; no F2 larvae survived from gametes produce by communal spawning events of GnRHa-treated broodstock. Spawning was not detected from blank-treated broodstock or those treated with low doses of GnRHa. The production of F2 offspring is a critical milestone to overcome as it allows domestication to begin and opens avenues to improve the biological performance of stock through advances in selective breeding.