Biological disturbance was investigated in three subtidal encrusting communities in the warm temperate region of northeastern New Zealand. The mode of operation of the major dis· turbance agents was established and their effect on encrusting organisms quantified where possible. The three communities were all dominated by crustose coralline algae, sponges, and ascidians.The urchin Evechinus chloroticus was the most widely influential of the grazers in these encrusting communities. At densities >6-7 urchins/m 2 this species grazed over the entire surface area in any locality, modifying community structure and reducing diversity to form crustose coralline flats (often referred to as barren grounds). At densities below this level urchins usually confined their grazing to patches within the encrusting community. The mean rate of free space release by an individual urchin was different in the three communities; the rate was four times higher where the urchins occurred at high densities on a coralline flat community than in the other two areas. Larger urchins appeared to clear space at a rate 1.5 times higher than that of smaller urchins. A 2-yr exclusion experiment in a community heavily grazed by Evechinus resulted in an increase in cover of ephemeral algae and coralline turf algae, but no basic change in species composition.Large numbers of five species of herbivorous gastropods, Cellana stellifera, Cantharidus purpureus, Trochus viridis, Micrelenchus sanguineus, and Cookia sulcata were positively associated with high densities of Evechinus. Removal of high densities of Cellana resulted in an increase in the cover of ephemeral algae and coralline turf algae in spite of continued Evechinus grazing. At high densities the grazing fish Parika scaber occasionally fed in loose schools that removed large sponges and ascidians and released free space at a rate of =0.2% of the surface per fish per year. This species also grazed selectively on small encrusting organisms in which case free space was not released. Exclusion of high densities of Parika for 2 yr resulted in a major shift in community structure from sponge/ coralline domination to ascidian/sponge domination. Episodic outbreaks of a bacterial/fungal disease occasionally affected populations of long-lived sponges. In the summer of 1975, 18% of the population of Ancorina a lata was destroyed and in 1976, 45% of Polymastia fusca degraded completely. These two episodes were the only extensive outbreaks of this disease in 10 yr of observation.The abundant grazing species investigated are generalist feeders that are capable offeeding on the entire size range of most encrusting species. Hence there is no escape from biological disturbance either in small or large size for encrusting organisms in these areas. The populations of the common grazers frequently fluctuate in numbers in a local area and their effect on the encrusting communities is not constant through time. The selective consequences of these features for long-lived encrusting organisms are discussed. It is sugg...

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Networks of no-take marine reserves (NTMRs) are widely advocated for preserving exploited fish stocks and for conserving biodiversity. We used underwater visual surveys of coral reef fish and benthic communities to quantify the short- to medium-term (5 to 30 years) ecological effects of the establishment of NTMRs within the Great Barrier Reef Marine Park (GBRMP). The density, mean length, and biomass of principal fishery species, coral trout (Plectropomus spp., Variola spp.), were consistently greater in NTMRs than on fished reefs over both the short and medium term. However, there were no clear or consistent differences in the structure of fish or benthic assemblages, non-target fish density, fish species richness, or coral cover between NTMR and fished reefs. There was no indication that the displacement and concentration of fishing effort reduced coral trout populations on fished reefs. A severe tropical cyclone impacted many survey reefs during the study, causing similar declines in coral cover and fish density on both NTMR and fished reefs. However, coral trout biomass declined only on fished reefs after the cyclone. The GBRMP is performing as expected in terms of the protection of fished stocks and biodiversity for a developed country in which fishing is not excessive and targets a narrow range of species. NTMRs cannot protect coral reefs directly from acute regional-scale disturbance but, after a strong tropical cyclone, impacted NTMR reefs supported higher biomass of key fishery-targeted species and so should provide valuable sources of larvae to enhance population recovery and long-term persistence.

Thinly encrusting species of subtidal sponge grow at slow but measurable rates over natural surfaces by lateral spreading. Of the eleven species studied here, Aplysilla rosea had the highest undisturbed rate of growth and Microciona sp. the lowest with an overall negative change in size. Using the mean growth rate it can be estimated that the largest sponge patches observed in the field may be over seventy years old. Growth rates of individual patches were varied but this variation was not synchronous within a species nor did it show any regular temporal pattern. Similarly, no relation between the normal thickness of the species, the wet weight, or true organic content of the species with undisturbed rates of growth could be found. However, the mean patch size of the species was correlated with the undisturbed growth rates. If the tissues of the sponges were damaged, rapid regeneration was initiated at rates many times greater than the undisturbed growth rate of the species. It was also found that even very small sponge patches could recover after almost all living tissue was scraped from the rock.

The application of no-take marine reserve status to an area is expected to increase abundance and average size of individuals of species targeted by fisheries. The majority of the evidence supporting such expectations still involves comparisons of abundance at the one time of sites with and without marine reserve protection. Very few studies have data on the abundance and size structure of species targeted by fisheries in an area before reserve status is applied. Quantitative estimates of density and biomass of coral trout, Plectropomus spp., the major target of the hook and line fisheries on the Great Barrier Reef (GBR), Australia, on inshore fringing reefs of the Palm and Whitsunday Island groups, central GBR, are provided for 3–4 years before (1983–1984), and 12–13 years after (1999–2000) the establishment of no-take reserves in 1987. Quantitative estimates of density and biomass of coral trout in areas open to fishing were also collected in 1999–2000 at these two island groups. Density and biomass of coral trout increased significantly (by factors of 5.9 and 6.3 in the Palm Islands, and 4.0 and 6.2 in the Whitsunday Islands) in the reserve sites, but not the fished sites, between 1983–1984 and 1999–2000. In 1999–2000, density and biomass of coral trout and a secondary target of the fisheries, Lutjanus carponotatus, were significantly higher in the protected zones than in the fished zones at both island groups. The density and biomass of non-target fish species (Labridae, Siganidae and Chaetodontidae) did not differ significantly between reserve and fished zones at either island group. This is the most convincing data to date that the management zoning of the world's largest marine park has been effective, at least for coral trout on inshore reefs.