Using growth band autofluorescence to investigate large-scale variation in growth of the abalone Haliotis midae

Growth of the abalone, Haliotis midae, was investigated at Port Alfred, on the south coast of South Africa, using both new and established techniques. A new method for aging animals is described, which makes use of shell autofluorescence under UV light to visualise internal growth bands. The deposition of growth bands was validated using measurements from shells of known age and, at one site, comparing growth estimates to those from cohort analysis undertaken at the same site. The new technique is far less time consuming and labour intensive than previously described methods; it is also non-destructive and proved to have potential for the reliable and rapid assessment of growth in large-scale studies. Growth of H. midae was also investigated at nine other sites, incorporating the full distribution range of the species. Systematic geographic variation in growth was observed along the South African coastline. Statistically significant differences existed among sites in growth rates for animals < 4 years and between 4 and 6 years and in the mean maximum sizes attained. Generally, H. midae from the south/southeast coast were found to have faster growth rates, smaller mean maximum sizes and were assumed to attain sexual maturity (determined in previous studies) earlier than those along the southwest/west coast. The geographic differences in estimates of growth observed have significant implications for future modelling approaches and indicate that present national management strategies are not appropriate as they fail to take regional variability into account.