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Patterns of abundance and movement of the spiny lobster Jasus edwardsii (Hutton) (F. Palinuridae) were examined within a coastal reef system. Overall population size remained relatively stable over a 3 yr study period. Marked differences in density, sex ratio and size frequency of spiny lobsters occurred between depths on the reef over a horizontal scale of 300 m. These differences varied seasonally and were related to moulting, reproductive and feeding cycles. They were not directly Linked to fluctuations in sea surface temperature, sea surge or photoperiod. Similar annual trends occurred at other sites. Highest densities of females in shallow (< 10 m) water coincided with moulting in May. Males moved inshore only briefly to moult in October-November often with no concomitant increase in male density in shallow (< 10 m) water. Increases in density of females at the deep (25 m) seaward edge of the coastal reef occurred during the egg-bearing period. In summer (December, January) the density of large males at a deep patch reef (25 to 30 m) increased. These males foraged nocturnally for bivalves on the surroundmg sand flat. Resighting of tagged individuals confirmed that the observed fluctuations in density, sex ratio and size frequency of spiny lobsters at different depths were caused by males and females moving inshore-offshore at different times of the year.

Ontogenetic changes in the behavior, spatial distribution, or habitat use of a species are presumably adaptations to ecological forces that dlffer in their effect on various life stages. The New Zealand rock lobster Jasus edwardsii is one of several species of spiny lobster that exhibits dramatic ontogenetic shifts in sociality and spatia.1 distribution, and we tested whether such changes are adaptive. We first surveyed several natural populations of J. edwardsii to document size-speclfic differences in aggregation. To determine i f chemical cues discharged by conspecifics promote aggregation of certain ontogenetic stages, we tested the responsiveness of lobsters of 3 ontogenetic stages (early benth~c juvenile, juvenile, and subadult) to the chemical cues produced by conspecifics of different sizes. Finally, we tethered lobsters of different ontogenetic stages alone and in groups to test the effect of lobster size and aggregation on mortality. Our results offer compelling evidence that pre-reproductive J. edwardsii undergo an ontogenetic change in sociality that alters their spatial distribution and sunrival. Our field surveys show that J. edwardsii are solitary as early benthic juveniles and become social and aggregate as they grow larger. We then demonstrate, using laboratory experiments, that there is a sizespecific increase in the response of pre-reproductive J. edwardsii to the chemical cues of larger conspecifics which facilitates these ontogenet~c changes in aggregation. Finally, our tethering results confirm that this change in social condition is selectively advantageous: aggregation does not increase the survival of small lobsters, but larger lobsters survive better in groups. Thus, in this study we demonstrate the linkage between ontogenetic changes in the spatial distribution of a species, the behavioral process that creates the pattern, and the selective advantage conferred by these developmental changes.

Interrelations between zooplanktivorous fish and zooplankton were examined at the Poor Knights Islands 20 km off the east coast of Northland, New Zealand from 1980 to 1983. The pomacentrid Chromis dispilus was the most abundant planktivore at all locations; high densities of other planktivores were also found. The rankings of these species varied considerably among locations. These differences may have been due in part to Caprodon longimanus (Serranidae), Scorpis violaceus (Kyphosidae), and Decapterus koheru (Carangidae) malung forays for food away from the immedate vicinity of rocky reefs. The hypothesis that fish have a localised effect on zooplankton was investigated in detail within a small reef area (-2500 m') on 7 separate occasions. Distribution patterns of planktivorous fish changed according to current direction. Fish were always most abundant on the incurrent side of the reef and within an archway during the day. Large differences in densities of zooplankton were detected along a 200 m transect where samples were taken upcurrent, within, and downcurrent of the archway during the day. Lowest zooplankton densities were usually found in the archway where planktivorous fish were abundant. At night when fish were absent from the water column, there was a trend for highest abundances of plankton within the arch, relative to upcurrent and downcurrent sites. A similar 200 m transect parallel to the arch, but 1 km offshore where planktivorous fish were absent, showed no significant differences in density of plankton along its length during the day or night. Zooplankters that showed greatest reductions in density in the vicinity of feeding fish were most abundant in the guts of C. dispilus sampled from the arch. Estimates of removal rates of zooplankters by fish based on concurrent estimates of fish densities, plankton, feeding rates, diet of fish and current speeds suggest that fish were capable of causing the measured reductions in zooplankton concentrations through the archway. The diets of C. dispilus changed among sampling occasions and corresponded partly to changes in the composition of zooplankton captured in nets. Accordingly, any effect that fish have on zooplankton may change between times. Ecological consequences of predation by fish are discussed in terms of zooplankton communities, fish, and rocky reef environments.

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