Growth and survival of Haliotis Iris in northern New Zealand, with special reference to aquaculture

Abstract

Environmental conditions have been shown to have a significant effect on the ecology of abalone in both the laboratory and field. In northern New Zealand, near the latitudinal limits of H. iris, growth rates were estimated to be 10-30% faster than for juveniles of the same age in southern populations (Hooker et al. 1997). As a result of these findings there has been considerable interest in culturing H. iris in the warmer waters of northern New Zealand. Understanding variability in the growth of H. iris under different environmental conditions both in culture and in the field is vital to investigating the viability of culturing paua in northern New Zealand. To provide an insight into the growth and survival of paua under northern conditions, the growth and demography of three populations of wild H. iris were examined. Cohort analysis of size-frequency data estimated that juvenile H. iris at all sites grew up to a mean size of 30 mm in shell length during their first year, c. 31% faster than southern H. iris. In contrast, tag-recapture data were variable within and among populations. The estimate of growth for first year juveniles at Okakari agreed with the estimate from cohort analysis. However, the growth rates of older juveniles at Okakari and the growth of H. iris at Ti Point ant Takatu were slower and similar to growth estimated for southern populations. Growth of H. iris greater than 80 mm in length was negligible for all northern sites, with populations reaching a maximum size of 111-116 mm in shell length. Of the 5000 H. iris measured, none were larger than the legal minimum size of 125 mm in shell. The paucity of large-sized individuals suggested a high level of size-specific mortality was operating on H. iris populations in northern New Zealand. To assess the potential for growth of H. iris in aquaculture, wild H. iris were grown in tank culture. The resulting growth of H. iris less than 40 mm in length was highly variable, ranging between 12-94 μm per day (mean 52.2 μm ±11.0 s.e.). However, growth rates were within a range that suggested aquaculture was potentially viable under northern conditions. Survival of wild H. iris was low in tank culture, which indicated that successful aquaculture requires a greater knowledge of the factors affecting growth and survival under northern culture conditions. A series of manipulative experiments was carried out to ascertain the effect of diet, stocking density, temperature and growing environment on growth and survival rates of cultured H. iris in northern New Zealand. All factors were found to influence the growth of H. iris but growth and survival were not affected by high ambient seawater temperatures. However, the patterns of growth and survival were determined primarily by diet. H. iris fed diets manufactured specifically for abalone aquaculture had the best growth under ambient seawater temperatures (67.3 μm per day), but also the lowest survival (c. 43.3% after 441 days). Relatively, these growth rates were faster than those of H. iris cultured at l8°C for 90 days (59 μm per day) (Redfearn 1994). Growth of H. iris on the red alga Gracilaria was low (33.3 μm per day) and from an aquaculture perspective, disappointing. However, survival was significantly higher for H. iris fed macroalgae (64.4%) compared with animals fed manufactured diets. Differences in growth and survival among diet treatments indicated that manufactured diets had a negative effect on survival, possibly by decreasing water quality, and that survival was independent of nutrition. The consistently low survival rates of H. iris in all experiments were suggested to be a result of site specific factors. Site-specific variability in growth and survival rates of cultured paua was compared among sites in northern New Zealand, and between southern and northern locations. The growth and survival of H. iris cultured from southern and northern parent stock were also investigated to ascertain if northern paua grew better under local conditions. Although some variation in growth and survival was found between northern sites, greater differences were found between southern and northern locations. Northern H. iris had slower growth in southern New Zealand than under northern culture conditions (63.1 & 77.8 μm per day, respectively). However, survival was significantly lower for all H. iris cultured at northern sites, irrespective of origin. H. iris of southern origin had faster growth in both southern and northern locations (77.8 & 80.9 μm per day, respectively). Although H. iris cultured from northern parent stock grew slower than southern juveniles, northern H. iris had better survival under northern culture, indicating that northern H. iris are better suited for northern aquaculture. However, the low survival of both stocks in northern aquaculture indicate that untested factors, such as water quality, need to be addressed before undertaking aquaculture in northern New Zealand. Genetic diversity was investigated among populations of H. iris and among abalone species, using mitochondrial DNA. The investigation of H. iris population genetics proved problematic, due to the inability of polymerase chain reaction primers to amplify genetically informative loci. A partial sequence of the ribosomal l6s (large subunit) gene was amplified and analysed for all New Zealand species of the genus Haliotis. Haliotis iris and H. virginea subspecies were placed together on a phylogenetic tree, in a position close to Japanese and American Haliotid species. However, H. australis was separated from this clade and placed within the Australian / South African clade. Analyses of the four subspecies of H. virginea gave equivocal results. Limited support was given for H. v. morioria being treated as a subspecies, but the three other subspecies were found to share haplotypes and formed a polyphyletic clade. Further research is required to develop methodology that will address genetic diversity at the population level. This thesis has provided some insight into the various factors that influence growth and survival of H. iris in northern New Zealand. Results have suggested H. iris cultured under northern conditions have faster growth. Faster growth rates can translate into a fiscal advantage for abalone farmers, in an industry that is capital intensive with no guarantee of a return on their investment. However, before this advantage is realised further research is needed to address the low survival that was pervasive in northern culture experiments.