Local variation in herbivore feeding activity on an inshore reef of the Great Barrier Reef

Threats to coral reefs may be manifested through an increase in macroalgae. Across the globe, phase-shifts from coral to macroalgal dominance have been reported from the Caribbean, Indian and Pacific Oceans. While the Great Barrier Reef (GBR) is in relatively good condition, inshore reefs may exhibit over 50% macroalgal cover. However, our understanding of the processes preventing the macroalgal expansion remains uncertain. Using a remote video bioassay approach, this study quantified herbivory in three bays along the leeward margin of Orpheus Island. Despite significant with-in bay variation in herbivory there was no detectable statistical difference in the rates of herbivory among bays. Furthermore, of the 45 herbivore species recorded from the island, only three played a significant role in bioassay removal, Siganus canaliculatus, Siganus javus and Kyphosus vaigiensis, with only one species predominating in each bay. Reefs of the GBR may therefore be more vulnerable than previously thought, with the removal of macroalgae depending on just a few species, which exhibit considerable spatial variability in their feeding behaviour.     

Cross-shelf variation in browsing intensity on the Great Barrier Reef

Herbivory is widely accepted as a key process determining the structure and resilience of coral reefs, with regional reductions in herbivores often being related to shifts from dominance by coral to leathery macroalgae. The removal of leathery macroalgae may therefore be viewed as a critical process on coral reefs. However, few studies have examined this process beyond a within-reef scale. Here, browsing activity was examined across the entire Great Barrier Reef shelf using bioassays of the leathery macroalga Sargassum to directly quantify algal removal. The assays revealed marked cross-shelf variation in browsing intensity, with the highest rates recorded on mid-shelf reefs (55.2–79.9% day⁻¹) and decreasing significantly on inner- (10.8–17.0% day⁻¹) and outer-shelf (10.1–10.4% day⁻¹) reefs. Surprisingly, the variation in browsing intensity was not directly related to estimates of macroalgal browser biomass; rather, it appears to be shaped primarily by the local environment and behaviour of the component species. Removal rates across the inner- and mid-shelf reefs appear to be related to the attractiveness of the assays relative to the resident algal communities. Controlling for the influence of the resident algal communities revealed a positive relationship between removal rates and the biomass of a single macroalgal browsing species, Naso unicornis. In contrast, the low removal rates on the outer-shelf reefs displayed no relationship to algal or herbivore communities and appeared to reflect a negative behavioural response by the resident fishes to a novel, or unfamiliar, alga. These findings not only highlight the complexities of the relationship between fish presence and ecological function, but also the value of examining ecological processes across broader spatial scales.     

Damselfish territories as a refuge for macroalgae on coral reefs

Herbivory is widely accepted as a key process determining the benthic community structure and resilience of coral reefs. Recent studies have mostly focused on the importance of roving herbivorous fishes in ecosystem processes. Here, we examine the role of territorial damselfish in shaping patterns of macroalgal distribution based on benthic surveys and macroalgal bioassays. The territory composition and effect of resident damselfish on the removal of Sargassum bioassays were quantified for six species of damselfish on Lizard Island, a mid-shelf reef in the northern Great Barrier Reef (GBR). The functional composition of algal communities within territories varied markedly among species. The territories of four species (Dischistodus perspicillatus, Dischistodus pseudochrysopoecilus, Plectroglyphidodon lacrymatus, and Stegastes nigricans) were characterized by algal turfs, while the territories of two species (Dischistodus prosopotaenia and Hemiglyphidodon plagiometopon) were characterized by foliose and leathery brown macroalgae. Sargassum, a generally rare alga on mid-shelf reefs, was a particularly common alga within D. prosopotaenia territories on the leeward side of the island but absent within their territories on the windward side of the island. D. prosopotaenia was the only species to retain the transplanted Sargassum, with only a minimal reduction in Sargassum biomass (1.1%) being recorded within their territories at both leeward and windward sites over a 24-h period. In contrast, reductions in Sargassum biomass were high in areas adjacent to D. prosopotaenia territories (83.8%), and within and adjacent to the territories of the five remaining damselfish species (76.2–92.5%). Overall, only one of the six damselfish species provided a refuge for leathery brown macroalgae and may facilitate the development of this macroalgae on mid-shelf reefs of the GBR.     

Visual acuity and feeding in larval Premnas biaculeatus

Ontogenetic change in the visual acuity of Premnas biaculeatus larvae was determined both behaviourally and anatomically. Visual acuity improved substantially between early feeding (day 3 post-hatch) and the pre-settlement (day 10 post-hatch) larvae but, at both ages, the anatomically-measured visual acuity was greater than that determined behaviourally. It appears that estimated anatomical visual acuity values substantially over-estimate the functional visual acuity realized under normal conditions. The distribution of the reactive angles indicated that most frequently prey within 0 to 9° of the longitudinal larval axis elicited a feeding response at both larval ages. This suggests that stereoscopic vision is used extensively during feeding in this species. The prey capture success with rotifers ranged from 96% at 3 days post-hatch to 100% at 10 days post-hatch. These values differ markedly from previous studies on temperate species and highlight the well developed abilities of larval P. biaculeatus at a given     

The effect of a minimum length specification on visual estimates of density and biomass of coral reef fishes

The effects of fish length on visual estimates of fish abundance and biomass are assessed based on analyses of size frequency distributions. A 2 cm change in the specified minimum size of fish surveyed produced a calculated change of 14.5 to 126% in numerical estimates. A change of 28.4 to 411% was calculated for a 4 cm change. The extent of the change in the numerical estimate varies with the family considered but, in all cases, was most marked if the specified minimum size was decreased. The effects upon biomass estimates are less marked. In the Scaridae, for example, there is a calculated change of 4.1 to 7.4% for a 2 cm change in the minimum specified size. The changes in the estimates are discussed in relation to the use of visual survey techniques for assessing reef fish standing stocks.     

Dangerous demographics: the lack of juvenile humphead parrotfishes Bolbometopon muricatum on the Great Barrier Reef

The humphead parrotfish, Bolbometopon muricatum, the largest of all parrotfish species, is heavily fished throughout most of its range. In remote and heavily protected locations, such as the Great Barrier Reef (GBR), it is a major component of parrotfish biomass and plays a critical role in ecosystem processes. However, extensive surveys of GBR populations have revealed a striking lack of juveniles. Of 633 individuals censused, just four were juveniles. This represents 0.6% juveniles and contrasts markedly with the 20.2–40.2% juveniles recorded in eight other medium to large parrotfish species. These low values in Bolbometopon are corroborated by over 5,000 h of independent observations and extensive museum collections. Whilst there is no evidence to suggest that this is an extraordinary new condition for GBR Bolbometopon, it may nevertheless expose them to special risks in a changing and unpredictable world. Despite excellent management on the GBR, Bolbometopon populations may be more vulnerable than previously thought.     

Seasonal changes in the size and composition of the fish yield from reefs around Apo Island, Central Philippines, with notes on methods of yield estimation

Fish yields from Apo Island, Central Philippines were assessed based on detailed quantitative analyses of fish landings, with specific reference to seasonal changes in the size and nature of the yield in terms of familial composition and degree of reef association of contributing species. The total estimated annual yield is 24-861 km⁻² year⁻¹ (±12.75, 95% C.I.)- Of the 39 families harvested, yields are dominated throughout the year by two, the Carangidae (40%) and the Acanthuridae (23.6%). The yield is likewise consistently dominated by two specific groups: reef-associated, 39.1%, and reef planktivores, 23.1%. Reef species comprise only 17-1%. Seasonal differences in yields were only detected in the Carangidae and the reef-associated species group. Apo Island reefs appear to be highly productive. Possible reasons for this are discussed. The variety of methods used for estimating reef fish yields, however, make comparisons difficult. Standardized procedures are therefore suggested.     

Trophic separation in planktivorous reef fishes: a new role for mucus?

Oecologia - The feeding apparatus directly influences a species’ trophic ecology. In fishes, our understanding of feeding modes is largely derived from studies of rigid structures (i.e....     

The functional roles of surgeonfishes on coral reefs: past,present and future

Reviews in Fish Biology and Fisheries - Surgeonfishes have had a long evolutionary history that has been closely linked with coral reef ecosystems. Today they are a key component of reef fish...