ALBERT

Notes: Abstract Abundances of the surgeon fish Acanthurus lineatus (Linnaeus) within a single reef-system were estimated using a hierarchical sampling design during 1982. Additional sampling was carried out during 1983 and 1984 at a restricted number of sites. This species is aggressive toward other herbivorous fishes and is strongly site-attached. On the study reef (Lizard Island; 14°40′S; 145°28′E), A. lineatus was restricted to reef-crest sites below mean low water, mainly on reefs not directly exposed to prevailing winds. Within this reef zone and depth-defined stratum, A. lineatus was distributed heterogenously with high abundance, (approximately 14 fish per 300 m2) at a single sample-locality covering 600 m of reef crest. Abundances of herbivorous species (members of the families Acanthuridae and Scaridae) at other localities did not correlate with abundance patterns of A. lineatus. Subsampling within study localities revealed considerable heterogeneity in the abundance patterns of herbivorous fishes, especially within the area of high A. lineatus density. A detailed behavioural study of interactions among herbivorous fishes at two adjacent sites within the locality of high A. lineatus abundances revealed a complex pattern of site-general and sitespecific features. A. lineatus excluded smaller scarids from its feeding territories at one site, but not at another. Scarids attaining large size (〉350 mm standard length) were present at one site and consistently fed within A. lineatus territories; large scarids were rare at the second site, even though the distances involved were small. In addition, the small surgeon fish A. nigrofuscus, a consistent target for A. lineatus aggression, was rare at one site but moderately common at the other. Finally, the abundant surgeon fish Ctenochaetus striatus was present at high densities at both sites and fed within A. lineatus territories. This species was not attacked by A. lineatus nor did it attack other herbivorous fishes within the vicinity. Small but consistent differences in reef structure were detected at each site. Local-scale heterogeneity in these interactions makes it difficult to develop generalizations concerning the role of territorial herbivores such as A. lineatus within reef systems. We hypothesize that very local differences in the within-habitat component of acanthurid and scarid abundances and distributions may reflect site-associated variability in recruitment patterns, post-recruitment mortality or behaviour that is independent of A. lineatus activities. Replicated removal experiments which include explicit tests for local site-effects and better descriptions of recruitment in larger herbivorous fishes are required before these interpretations can be evaluated.

Notes: Abstract The occurrence of unusual symbiotic microorganisms was examined in the intestines of a range of fish from the Great Barrier Reef, Australia. The fish taxa examined included 26 species of the family Acanthuridae, as well as representatives of phylogenetically related and herbivorous taxa. The microorganisms, referred to as protists, were only found in herbivorous and detritivorous members of the Acanthuridae. Protists were not found in planktivorous acanthurids, nor in any members of the families Kyphosidae, Pomacentridae, Scaridae, Zanclidae, Siganidae and Bleniidae we examined. In addition, protists were absent from the herbivorous acanthurids A. xanthopterus and A. nigricans. A range of protist forms, characterized by differences in size (8 to 417 μm), shape and mode of cell division (daughter-cell production and binary fission), was observed. The occurrence of these forms appeared to be correlated with host feedingecology. Large forms (〉100 μm) of the protists were only found in acanthurids which fed over hard-reef substrata. Smaller forms were found in sand-grazing and detritivorous species. One of the protist forms appears identical to protists previously reported from Red Sea acanthurids.

Notes: Abstract The distribution patterns of presettlement reef fish and how they were influenced by the proximity of reefs were investigated off the coast of Northland, New Zealand, from 1981–1986. We used ichthyoplankton tows and visual counts of fish. Distributions of presettlement fish of some species were influenced by the proximity of reefs, regardless of whether reefs were on the coast of the mainland or islands across the shelf. Presettlement fish of families that lay demersal eggs were most abundant near reefs: Gobiescocidae, Acanthoclinidae, Tripterygiidae, Eleotridae, and Gobiidae. The distribution of presettlement sparids, mullids (pelagic eggs), and blenniids and monacanthids (demersal eggs) was not determined in a predictable way by the proximity of reefs. High-frequency sampling over three days suggested that patches of presettlement sparids of 1 to 2 km in dimension may move quickly through a study area. High abundance of presettlement gobiescocids and tripterygiids were found in 0 to 2 m of water over rocky reefs at high and low tides. Presettlement eleotrids were associated with reefs in deeper water (3 to 20 m) and in some habitats with aggregations of mysids. The lack of general patterns of distribution for presettlement reef fish suggests that modelling patterns of drift of these fish as a single group is inappropriate; this concurs with evidence from tropical waters.

Notes: Abstract The surface slicks of internal waves were sampled in continental shelf waters off the east coast of Northland, New Zealand, from 1982–1985. Densities of small fish and zooplankton from surface waters were higher in slicks than in the rippled water adjacent to them. Presettlement fish, of species found as adults nearshore, were abundant in ichthyoplankton samples and visual counts from slicks. These fish ranged from fish with primordial fins to those with adult fin-ray counts. Some small reef fish aggregated around drift algae. Drift algae were also abundant in slicks. Slicks moved at 0.5 to 1.25 km h-1 in the direction of shore. Hence, a consequence of aggregation in slicks is that presettlement fish may be transported onshore. When slicks were absent, drift algae were found in scattered patches at different distances from shore. In the presence of slicks algae were aligned in them. Internal waves, therefore, may have the ability to turn scattered distributions into regular patterns. Because the surface slicks that internal waves produce are common coastal features, we should modify our perspective of how small fish and zooplankton are distributed in local water masses. Furthermore, the occurrence of internal waves should influence the way in which surface waters are sampled. We suggest that fish settlement patterns need to be investigated concurrently with measurements of physical mechanisms which may promote aggregation, and onshore movements of small fish (e.g. internal waves). These waves may contribute to the variability in settlement rates of small fish.

Notes: Abstract Total abundance estimates for the large, common, reef fish Cheilodactylus spectabilis (Hutton) were obtained for a marine reserve and adjacent section of coast in north-eastern New Zealand during 1985. Visual strip-transects were used to estimate abundance and size structure in both areas. The accuracy, precision and cost efficiency of five transect sizes (500, 375, 250, 100, 75 m2) were examined over three times per day (dawn, midday and dusk), by simulating transects over mapped C. spectabilis populations. Two transect sizes showed similarly high efficiency. The smaller of the two (20x5 m) was chosen for the survey because of the general advantages attributable to small sampling units. Biases related to strip-transect size are discussed. Preliminary sampling indicated that C. spectabilis was distributed heterogeneously, and that density was habitat-related. An optimal stratified-random design was employed in both locations, to obtain total abundance and size-structure estimates. This reduced the between-habitat source of variability in density. The total number of sampling units used was governed by the time available. The resulting total abundance estimates obtained were 18 338±2 886 (95% confidence limit) for the 5 km marine reserve, compared to 3 987±1 117 for an adjacent, heavily fished 4 km section of coast. When corrected for total area and habitat area sampled, this represented a 2.3-fold difference in abundance. If sampling had been designed to detect an arbitrary 10% difference in abundance within each habitat, an infeasible 440 h of sampling would have been required. Size-frequency distributions of C. spectabilis at the reserve had a larger model size class than distributions from the adjacent area. The data suggest that reserve status is causal in these differing abundance and size structure estimates.

Notes: Summary The subtidal coralline flats of northeastern New Zealand support a characteristic guild of grazing herbivores. The most important members of this guild are an echinometrid echinoid, patellid, turbinid and trochid gastropods. Densities of these herbivores fluctuate through time. Interactions within and among the different species of echinoids and gastropods were investigated experimentally. Different combinations of species were caged at densities up to 5 times that of ambient for a 24 week period in an experiment designed to differentiate between intra- and interspecific competition. The echinoidEvechinus chloroticus and the turbinid gastropodCookia sulcata exhibited reduced mean dry weight with increasing intraspecific densities. There was little evidence of density-related mortality in these species. The limpetCellana stellifera showed comparatively large losses of weight and enhanced mortalities in intraspecific experimental treatments but this was not related to density. Investigation of interspecific interactions amongstEvechinus andCookia revealed no evidence of a negative influence of one species on the other. In terms of dry weight,Cookia was indifferent to the presence ofEvechinus, andEvechinus benefited in the reciprocal interaction.Cookia also enjoyed an enhanced mean dry weight when in the presence ofCellana compared to the equivalent intraspecific treatments. There were no coherent trends in proportional mortality in any treatments with enhanced interspecific densities. Cellana, in the presence ofCookia, exhibited a dramatic decrease in mortality rate and increase in mean dry weight. The presence of the turbinid gastropod was clearly beneficial to the limpet when compared to the intraspecific treatments with enhanced intraspecific densities and the control cages containingCellana at ambient density. We suggest that subtidal areas constitute poor habitats for limpets in the absence of agents such asCookia which may provide or maintain suitable sites for attachment and grazing. For the combinations of densities and species investigated there was a consistent trend towards positive interspecific interactions. It seems unlikely that at the sites investigated interspecific competition could act to restrict distributions, or limit abundances of species.