Morphometric and energetic development of artificially reared giant kōkopu (Galaxias argenteus)

Abstract

The larvae of some freshwater fish species of the Galaxiidae family are highly-prized in local cuisines in parts of the Southern Hemisphere, however, supplies from wild fisheries are limited. In New Zealand, the larval form of giant kōkopu, Galaxias argenteus, has long been held as a potentially viable aquaculture species due to its high market price (US$70–115 kg−1) and the limited supply of ‘whitebait’ from the wild fishery. However, little is known about the larval stages of this species and the current culture techniques result in poor growth performance and high mortality. This study describes aspects of the morphological development, and larval feeding requirements of giant kōkopu to assist in advancing their aquaculture. Morphometric characteristics of larval giant kōkopu that are critical to the success of larviculture were measured over the 77 day production period, from hatch to harvest. The initial slow growth of the larvae appears to be a result of rapid depletion of endogenous energy reserves prior to initiation of first feeding, while the extent of eye pigmentation and the presence of an open mouth upon hatching indicate that initiating feed provision should be brought forward from 3 DAH. Mouth gape width at hatch was 348 μm and increased rapidly in the first four weeks to 651 μm suggesting that larger live feed items could be used as the initial feed, such as instar-II Artemia. At harvest, the larvae varied enormously in size and condition, with larger fish showing early signs of metamorphosis, such as increasing opacity of flesh and coloration of internal organs. The extent of this variability reduces productivity, increases waste and results in a lower quality product. The results confirm that an improved understanding of the developmental biology of the larvae can assist in providing a more effective feeding regime, particularly for determining the timing of first feed provision and the feed particle size requirements throughout larval development. Consequently, heterogenous growth performance is an aspect of production that needs further attention for developing the efficient aquaculture production of this species.