Reproductive Biology of the Giant Kokopu Galaxias argenteus

Abstract

Aquaculture is a vital part of New Zealand’s future and the industry is striving to become a billion dollar business by the year 2025. Despite New Zealand finfish farming being in its infancy compared to developments overseas, there is still room for improvement to develop, diversify and expand an already well managed finfish industry.

The whitebait fishery is a distinctive fishery in New Zealand, characterised by its shoals of translucent juveniles that belong to the genus Galaxias. It is a seasonal and valuable fishery. However, the commercial production of whitebait using inanga (G. maculatus), the most abundant galaxiid species, has been slow due to technical constraints (e.g. low fecundity and poor larval survival). The giant kokopu (G. argenteus), the world’s largest galaxiid, is a promising alternative, although it is considered threatened. Specimens have been recorded to have far greater fecundity and egg diameters compared to inanga. However, no observations of spawning or egg deposition have been documented.

Due to the incomplete understanding of giant kokopu reproduction, ovarian biopsies and blood samples were collected at monthly intervals using non-lethal methods throughout the reproductive cycle of wild giant kokopu until ovulation. Oocyte diameters and levels of plasma estradiol-17β (E2) were described at each sampling point, as well as ovarian mRNA levels of cathepsin D (ctsd), a protein associated with yolk processing in developing oocytes. In order to assess the potential of this fish for cultivation, the same variables were measured in fish held in captivity by repeated biopsies. Outcomes were compared, and where possible, gametes were non-lethally stripped from ripe wild and captive fish to obtain data on embryonic development and hatching when eggs were incubated under different conditions that they may naturally encounter.

Cathepsin D mRNA levels were quantified using real-time PCR, and plasma E2 levels were measured using radioimmunoassay. Eggs were dry-fertilised with milt from wild fish prior to activation of gametes in water of different salinities (0 ppt, 9 ppt, 15 ppt and 30 ppt). Subsequently, eggs were either incubated semi-dry (simulated terrestrial oviposition) or submerged (simulated aquatic oviposition) under controlled conditions.

This study identified characteristics of the giant kokopu that make it an appealing candidate for commercial cultivation and provides a foundation from which brood stock management and optimal culture protocols can be developed, for both conservation and aquaculture purposes. A relatively non-invasive biopsy method was developed to collect ovarian tissue with a low ethical cost and without the need to sacrifice fish. Alternatively, plasma E2 levels described throughout the reproductive cycle could be a biochemical tool to estimate the reproductive state by blood sample.

Maintenance of captive fish under a simulated photo-thermal regime was effective in inducing oogenesis. Spontaneous ovulation commenced in late June in wild fish and early August in captive fish. Progression of oocyte growth, levels of plasma E2 and ctsd mRNA throughout oogenesis were generally consistent between wild and captive fish. Significant differences in E2 detected during mid-vitellogenesis may reflect an environmental stressor under captive conditions. Hatching occurred in eggs incubated semi-dry and fully submerged in water, with highest hatch rates from eggs submerged during incubation. Hatch rates between wild and captive eggs were similar.