The ecology of Jasus edwardsii (Hutton) (Crustacea: Palinuridae)

Abstract

The rock lobster Jasus edwardsii (Palinuridae) was studied, for three years, in the Cape Rodney - Okakari Point Marine Reserve, north east New Zealand. The purpose was to determine the static and dynamic patterns of distribution of this rock lobster, and then to relate these to patterns to features of its life history and environment. This required a comparison of densities, sex ratios and size frequencies, between and with in areas, over periods ranging from hours to years, and an investigation of the biology and timing of moulting and reproduction. These comparisons, in turn, required the development of a visual census technique, which would provide unbiased, precise estimates of density, and a tag-resight programme with which to investigate suspected patterns of movement. A large population of rock lobsters, comprised mainly of adults was found in the Marine Reserve. The adult population increased very little over the three years of study, but the numbers of juveniles increased substantially. Settlement of puerulus larvae in collectors was low throughout the study, and it is therefore suggested that increases in the numbers of juveniles may have been due to immigration. Differences in the distribution of rock lobsters between habitats were broadly related to the abundance of shelter, however this relationship was confounded by two effects related to depth. Firstly, juveniles (‹85mm Carapace Length (C.L.)) were more abundant in the shallow (‹10m) rather than deep habitats, which probably reflects patterns of settlement. Secondly, seasonal changes in the density, sex ratio and size frequency of rock lobsters occurred at particular depths. These changes were caused by inshore - offshore movements of mature (›85m C.L.) male and female animals at specific times of the year. Movements were not directly related to sea-surface temperature, sea-surge or photoperiod, but were closely linked to moulting and reproductive cycles. The highest densities of both mature (›85mm C.L.) females and males in shallow (‹10m) water coincided with their respective periods of moulting. However inshore movement to moult could not be explained in terms of predator avoidance, optimal temperature or shelter availability. Offshore movement of ovigerous females to the seaward edge of the coastal reef where tidal currents are strongest, occured during the 103 day period for which the females carried external eggs. Consequently, it is suggested that, this movement resulted in more rapid dispersal of larvae at hatching, and in lower predation of larvae by planktivorous reef fish, than if hatching had taken place where currents were absent nearshore. Seasonal and cyclical changes in the dispersion of rock lobsters within a site at a particular depth occurred, and were related to seasonal changes in density or behaviour. Seasonal decrease in the mean group size of large (›140m C.L.) males occured independently of density but was accompanied, and probably caused, by a rapid rise in inter-male aggression. This aggressive behaviour was closely linked to the reproductive success of males. In the field, females only mated with males ›140mm C.L. although all males ›85mm C.L. were reproductively mature. A laboratory experiment suggested that aggressive interactions between males prevented males smaller than 140m C.L. from participating in reproductive activities. In the field, the largest males tended to shelter with the greatest number of mature females, suggesting a polygamous mating system. After the reproductive period, aggressive behaviour between males diminished rapidly, and was accompanied by a density independent rise in their mean group size, which peaked during the male moulting period, four to five months after reproduction. It is suggested that broadly similar cyclical changes in depth distribution and dispersion of rock lobsters occur throughout New Zealand, and in all species of the Jasus lalandii group. However, in other areas, because of latitudinal and size dependent variations in the timing of moulting and reproduction, and differences in the depths to which coastal reefs extend, the exact timing of these cycles, and the depth range over which they occur, will be different from those observed in the Marine Reserve. Comparison of rock lobster populations between adjacent localities in north east New Zealand suggests that the greater abundance and larger size of animals in the Marine Reserve was due both to relatively high recruitment of juveniles and to protection from commercial and recreational fishing. This study describes, for the first time, the detailed patterns of distribution of Jasus edwardsii. This provides a vital framework within which broad questions concerned with fisheries biology and community ecology can be addressed.