The use of ULVA as a feed supplement in the development of an artificial diet and feeding regimes to produce export quality roe from the sea urchin Tripneustes gratilla (Linnaeus)

Tripneustes gratilla is a fast growing, shallow water echinoid, which occurs across the IndoPacific, with its south-western limit in warm, temperate waters of South Africa. The success of T. gratilla cultivation depends, in part, on the development of a high quality, costeffective, gonad-conditioning diet that can produce large, marketable quality gonads. The aim of this research was to determine whether Ulva supplementation would improve palatability, consumption and digestibility of an artificial feed administered to T. gratilla and optimise gonad production and quality. At an inclusion level of 20% (20U), Ulva was shown to significantly (p < 0.05) improve the attractiveness and palatability of a formulated feed, compared to a nutritionally equivalent feed that had not been supplemented with dried Ulva (0U). Food consumption rates (FCR) and apparent digestibility coefficients (ADC %) for protein and energy, using insoluble ash as an indigestible marker, were measured for all experimental diets. FCR was significantly higher (p < 0.05) for urchins fed artificial feeds supplemented with Ulva, when compared to urchins fed non-supplemented feeds, suggesting that the inclusion of Ulva into the artificial diets acts as a feeding stimulant.Increased palatability and consumption subsequently led to significantly increased protein retention in urchins fed the 20U diet. The addition of Ulva to artificial feeds also significantly improved gonad colouration duringgonad enhancement trials. Gonad lightness (L*), redness (a*) and yellowness (b*) were quantified using a hand-held reflected-light, fibre-optic spectrophotometer and showed that, at an inclusion level of 20%, Ulva produced gonads that did not differ in colouration from those fed a natural diet of fresh Ulva (FU), and significantly improved gonad colour, particularly gonad yellowness (b*), compared to the same artificial diet without Ulva supplementation. This response was likely related to Ulva’s high βcarotene content. The Ulva content of artificial feed significantly improved gonad colouration, while the formulated portion of the feed produced significantly larger gonads than those produced using a natural diet of fresh Ulva. The 20% Ulva inclusion diet, in particular, increased the gonad somatic index (GSI) by 205%, in just nine weeks (7.6 to 23.3%), compared to a 57% increase in the control group (fresh Ulva). Full life-cycle growth trials were also conducted using two of theformulated feeds (20U & 0U), as well as fresh Ulva, to establish appropriate feeding regimes thatcould produce a harvestable product in the shortest time. During grow out, juvenile somatic growth needs to be maximised until an individual reaches marketable size and sexual maturity, which is followed by gonad enhancement, through nutritive cell development,aimed to maximise gonad yield and quality before harvest. The effects of 5 different feeding regimes on somatic and gonadal growth of juvenile T. gratilla were investigated, over a 32 week period. The feeding regimes used were: fresh Ulva (FU) only; fresh Ulva for 20 weeks and the 20U diet for 12 weeks (FU-20U); fresh Ulva – 0% Ulva (FU-0U); 20% Ulva – fresh Ulva (20U-FU) and 0% Ulva – fresh Ulva (0U-FU). Somatic growth was largely effected by the presence of Ulva within a diet. Similar growth, in diameter, of juvenile urchins was achieved using either an artificial diet containing Ulva (20U) or fresh Ulva (FU), during the somatic growth phase.An artificial diet with the same nutritional properties but without Ulva supplementation (0U) produced urchins that were significantly (p > 0.05) smaller. Gonad production in the somatic growth phaseof the trial was higher for urchins fed with artificial diets (0U & 20U), but the reduced size of urchins in the 0U diets significantly reduced gonad mass. After the diets were changed from artificial feeds (0U & 20U) to fresh Ulva (FU) and vice versa (week 20), both gonad size and colour were affected, with artificial diets promoting gonad growth, while FU improved gonad colour. At the end of the study, all feeding regimes produced similar amounts of gonad, except for those individuals that were fed the 0U feed during the somatic growth phase (which were significantly (p > 0.05) smaller). Gonad colourationof all treatments at the end of the study was of marketable quality. From these results it is recommendedthat FU be fed inthe somatic growth phase, while the 20U artificial diet should be used to increase gonad size, and optimise gonad colour, in the gonad enhancement phase. Through the use of stable isotope mass spectrometry and IsoSource, a mixing and mass balance model, the relative contribution and importance of specific feed ingredientsto gonad production was determined. Ulva was shown to be an important isotopic source for gonad production, accounting for an average of 33% of the isotopic signal across all Ulvacontaining, diets at the end of the trial. The final section of work focused on efforts to manipulate the gametogenic cycle of T. gratilla, by altering daylength, to attempt to reduce the production of large amounts of gametes, which would decrease gonad value. Histology indicated that urchins exposed to a Short day (8:16 h) were significantly more advanced reproductively, with the majority of urchins in a mature or spent state compared to urchins exposed to a Long day (16:8 h), which were mostly premature. Nutritive phagocyte (NP) density within the gonads supported the findings from histology, as gonads from urchins exposed to a short day had significantly less NP‟s (21.58 ± 4.35%), compared to the Long day treatment (65.26 ± 3.09%). The results from this study suggest that urchins of this species exposed to Long days progress through gametogenesis more slowly than those exposed to Short days, which, appeared to mature more rapidly. These findings could allow for the production of a high quality product for longer periods of the year, without the onset of gonad maturation. The research in this thesis clearly shows that the use of the macroalga Ulva as a feed, or feed additive, to artificially formulated, high protein feeds can have a number of significant benefits in echinoculture. The use of the artificial feeds and feeding regimes developed in this work, along with the ability to manipulate daylenth to suspend gametogenisis, could greatly facilitate the success of the newly developing T. gratilla industry, both in South Africa and worldwide.