No abstract

The subtidal region at Cape Banks, New South Wales, Australia, is characterized by large areas covered solely by encrusting coralline algae in association with large densities of invertebrate grazers. Orthogonal removals of these grazers: the sea urchin Centrostephanus rodgersii; the limpets Patel/oida a/ticostata, P. mufria, and Cel/ana tramoserica; and the turbinids Australium tentiforme and Turbo torquata, were done at two sites and two times to investigate the interactions among these species and their effects on the algal community. These manipulations had large effects on both the algal and animal communities, although the type and intensity of these interactions altered at different temporal and spatial scales.Initially, the removal of urchins and/or all species oflimpets resulted in a rapid increase in the amount ofnoncrustose algae in these areas. Removal ofturbinids, however, had no significant effect. Where both limpets and urchins had been removed, the cover of foliose algae continued to increase quickly and was ~80-100% after 12 mo. In areas where only urchins were removed, the increase in the cover of algae was slower, and only approached 100% after 18-24 mo. Where only limpets had been removed, 3 mo after an initial increase in the cover of filamentous algae the cover declined to be only slightly more than in control areas. These results were similar at all sites and times. In one series of removals, the effects of some of the species of limpets were separated. In this case, the increase in the cover of algae was found to be fastest when only P. mufria was left (the urchins and other limpets removed), than when all limpets and urchins had been removed.Initially, the removal of the urchins generally resulted in a large increase in the recruitment and density of the limpets. Approximately 1 yr following the removal of the urchins, however, the density of adult and juvenile limpets of all species had declined to near zero. Removal of the larger species oflimpets, P. a/ticostata and Cel/ana, also increased the rate of recruitment of the smaller species, P. mufria.Thus, these invertebrate grazers, especially the sea urchins, appeared to be necessary for the maintenance of the areas of crustose algae. Similarly, the sea urchins were found to be necessary for the continued presence of the limpets within these areas. The strong effects of the urchins on this community, and the application of the situation to theories concerning keystone species and facilitation, are discussed in relation to the important but more subtle effects of the limpets.

The relatively high catch of invertebrate species compared with finfish off Western Australia is in sharp contrast to other regions of the world, where finfish production usually dominates. This low level of finfish production is primarily due to the Leeuwin Current, which consists of warm, low-nutrient waters flowing south along the edge of the continental shelf of the Western Australian coast. In contrast, the other eastern boundary currents in the Southern Hemisphere (Humboldt and Benguela) are associated with upwelling of cool, nutrient-rich waters flowing north and the high rates of primary production resulting in a large finfish production. The Leeuwin Current, being the dominant oceanographic feature off Western Australia, has a major influence on the abundance of many species. The larval phase is the stage mainly affected by the current, but not always with the same result. For example, the strength of the Leeuwin Current has a significant positive influence during the larval stage of the western rock lobster (Panulirus cygnus). However, the current has a negative influence during the larval life of the scallop, Amusium balloti, in Shark Bay. Similarly for the pelagic finfish species, the current has a negative effect on larval survival of pilchards (Sardinops sagax neopilchardus) but a positive impact for whitebait (Hyperlophus vittatus). Possible mechanisms for the effect of the current include transportation of larvae and temperature effects on spawning success and on survival and growth of larvae.

A sample of 60 fish revealed a near-linear relationship between otolith weight and the estimated age (determined from scale and otolith annuli) for the pilchard Sardinops neopilchardus, with little overlap of otolith weight values between age classes. In an additional sample of 612 fish where only otolith weight and length were measured, only two clear length modes were found, but five modes using an otolith weight frequency distribution could be distinguished. The means of the five modes were similar to that predicted from the mean weights for each age class in the original sample of 60 fish. These five modes were also present in a subsample of the 612 fish, chosen such that their lengths formed a uniform distribution (i.e. no modes). These results were consistent with a direct relationship between otolith weight and age, but further validation is required before this variable can be used as the sole determinant of age.

No abstract