Population dynamics and fishery management of the abalone, Haliotis iris

The dynamics of an unfished population of the abalone Haliotis iris Martyn in Peraki Bay, Banks Peninsula, were studied from 1973 to 1976 and the population parameters interpreted with respect to exploitation of the species. As Haliotis iris cannot yet be aged, a size-class population model was developed along the lines of the Leslie (1945, 1948) matrix model. The population projection matrix incorporates mortality, recruitment and individual growth components, and operates on a population divided into size classes. The number of animals in the study population, and its size structure, were determined using transects and a stratified random sampling design. In February 1976 the population consisted of about 179000 animals on 4.5 km of coastline, and the length frequency distribution was strongly skewed to the left, indicating an accumulation of old individuals and a recent history of low recruitment. Growth of animals < 70 mm long was determined from changes in length frequency, and of larger animals by the analysis of tag returns. Equations which incorporate individual variability of growth parameters were developed for the analysis of tag return data. The mean parameters of the Von Bertalanffy growth equation for the study population are K = 0.1636 and L = 131.9 mm. Both are lower than those reported for abalones in a more typical habitat near Kaikoura, probably because of differences in the availability and quality of algal food species. Annual instantaneous natural mortality, as estimated by several methods from population size structure and individual growth rate data, is about 0.1, suggesting that some individuals live in excess of 30 years. The main cause of death is burial by sand during storms. The reproductive cycle was examined. H. iris failed to spawn during 2 of the 4 years monitored and the failure during one year was geographically widespread. Egg production per female in the study area increases with body length up to a maximum of 1-2 million in 115-130 mm individuals; whereas at Kaikoura it continues to increase with length and reaches 11 million per 150 mm female. Recruitment, as estimated from population sampling, was 2000-5000 two year olds per year and seemed independent of local spawning success. Surface current systems could transport planktonic H. iris larvae far from a parent stock. Simulations, using the model developed, suggest that the population size structure in February 1976 resulted from a short (about 5 year) period of high recruitment, followed by 10-15 years of low recruitment. The present minimum legal size for the fishery (127 mm) provides close to the maximum yield per recruitment for most stocks, although some local reductions could be warranted where individual growth rate and asymptotic size are low or where the natural mortality is greater than 0.15