Scarus Rivulatus Research Articles

Size-dependent variation in the functional role of the parrotfish Scarus rivulatus on the Great Barrier Reef, Australia

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 360:237-244 (2008) - DOI: https://doi.org/10.3354/meps07413 Size-dependent variation in the functional role of the parrotfish Scarus rivulatus on the Great Barrier Reef, Australia Roberta M. Bonaldo*, David R. Bellwood Australian Research Council Centre of Excellence for Coral Reef Studies, and School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia *Email: roberta.bonaldo@jcu.edu.au ABSTRACT: Biodiversity loss and fishing-induced changes in the size distributions of fishes can impact ecosystem function on coral reefs. These changes have led to an urgent need for studies on the particular roles of species to underpin effective coral reef management. The present study focuses on the feeding ecology of 6 size classes (from 2.5 to 30 cm total length) of Scarus rivulatus (family Labridae), one of the most abundant parrotfishes on the Great Barrier Reef, Australia. Individuals in all 6 size classes strongly selected the epilithic algal matrix for foraging and rejected other substratum types, including coral, macroalgae and sand. The 6 size classes also had similar feeding rates (bites min–1) and diel feeding patterns, with higher feeding activity during the afternoon. However, the size of grazing scars by S. rivualtus differed significantly among the 6 size classes, with small individuals scraping a greater substratum area per unit biomass and larger individuals taking a greater volume of material per unit biomass. Thus, biomass cannot be not be viewed as a proxy for ecosystem impact; different sized individuals of S. rivulatus, and probably other parrotfish species, have a markedly different impact on the reef substratum. Selective harvesting of large individuals will therefore change the functional role of this species. These results emphasize the importance of considering the size of individuals when evaluating the role of reef species in ecosystem process. These intraspecific functional differences will be critical when formulating reef management strategies and evaluating the impact of fishing activity on reef ecosystems. KEY WORDS: Herbivory · Coral reef fish · Resilience · Fishing impacts · Parrotfish · Functional redundancy Full text in pdf format PreviousNextCite this article as: Bonaldo RM, Bellwood DR (2008) Size-dependent variation in the functional role of the parrotfish Scarus rivulatus on the Great Barrier Reef, Australia. Mar Ecol Prog Ser 360:237-244. https://doi.org/10.3354/meps07413 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 360. Online publication date: May 22, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Direct versus indirect methods of quantifying herbivore grazing impact on a coral reef

Two methods were used to assess the grazing impact of roving herbivorous fishes across a coral reef depth gradient within Pioneer Bay, Orpheus Island, Great Barrier Reef. The first technique employed was a method traditionally used to quantify herbivory on coral reefs via the (indirect) inference of herbivore impact from biomass estimates and reported feeding rates. The second method (one of a range of direct approaches) used remote underwater video cameras to film the daily feeding activity of roving herbivores in the absence of divers. Both techniques recorded similar patterns and relative levels of herbivore biomass across five reef zones at the study site. Indirect estimates of the grazing impact across the reef depth gradient of the three predominant species of herbivore broadly coincided with levels quantified directly by remote underwater video, indicating that, to a large extent, presence does correspond to function. However, the video data suggested that, for individual species in particular reef zones, the absolute level of impact may be less than that inferred from presence. In the case of the parrotfish Scarus rivulatus, the video recordings suggested that, at the reef crest, an average of 52% (±18 SE) of each m2 area of reef would be grazed each month, compared with an area of 109% (±41 SE) suggested by inferring grazing activity from presence alone. Potential biases associated with remote video recorders may explain some of the discrepancy between values. Overall, the results suggest that, for some fish groups, the indirect method of inferring function from presence can provide a good indication of relative levels of herbivore impact across a coral reef.

Macroalgal grazing selectivity among herbivorous coral reef fishes

Despite growing interest in examining functional groups among coral reef species, few studies have examined the relative functional impacts of individual herbivorous fish species in coral reef ecosystem processes. We investigated potential functional roles within an herbivorous reef fish assemblage by assessing the feeding selectivity of fishes on macroalgae. Transplanted multiple-choice algal assays and remote stationary underwater digital video cameras were used to quantify the feeding selectivity of 6 herbivorous reef fish species: Siganus doliatus and S. canaliculatus (Siganidae); Chlorurus microrhinos, Hipposcarus longiceps, and Scarus rivulatus (Labridae); and Pomacanthus sexstriatus (Pomacanthidae). The siganids strongly selected Sargassum sp. (Phaeophyta) and, to a lesser extent, Hypnea sp. (Rhodophyta), but strongly avoided calcified macroalgae. However, S. doliatus displayed strong selection for Hypnea sp., whereas S. canaliculatus exhibited stronger selection for Sargassum sp. In contrast, the parrotfishes exhibited similar selectivities, with all 3 species selecting heavily calcified Halimeda spp. (Chlorophyta), but strongly avoiding Galaxaura sp. (Rhodophyta) and other red and brown macroalgal species. The pomacanthid was the least selective of the 6 fish species, exhibiting no clear selection or avoidance. These results emphasize the potential variation among species within presumed functional groups, and the need for care when defining functional groups at the family or the genus level. The extent of specialization, with strong selection and avoidance, suggests that functional redundancy among herbivores may be less than previously assumed.

Cross-shelf variation in the role of parrotfishes on the Great Barrier Reef

Herbivorous fishes are a key functional group on coral reefs. These fishes are central to the capacity of reefs to resist phase shifts and regenerate after disturbance. Despite this importance few studies have quantified the direct impact of these fishes on coral reefs. In this study the roles of parrotfishes, a ubiquitous group of herbivorous fishes, were examined on reefs in the northern Great Barrier Reef. The distribution of 24 species of parrotfish was quantified on three reefs in each of three cross-shelf regions. Functional roles (grazing, erosion, coral predation and sediment reworking) were calculated as the product of fish density, bite area or volume, bite rate, and the proportion of bites taken from various substrata. Inner-shelf reefs supported high densities but low biomass of parrotfishes, with high rates of grazing and sediment reworking. In contrast, outer-shelf reefs were characterised by low densities and high biomass of parrotfish, with high rates of erosion and coral predation. Mid-shelf reefs displayed moderate levels of all roles examined. The majority of this variation in functional roles was attributable to just two species. Despite being rare, Bolbometopon muricatum, the largest parrotfish species, accounted for 87.5% of the erosion and 99.5% of the coral predation on outer-shelf reefs. B. muricatum displayed little evidence of selectivity of feeding, with most substrata being consumed in proportion to their availability. In contrast, the high density of Scarus rivulatus accounted for over 70% of the total grazing and sediment reworking on inner-shelf reefs. This marked variation in the roles of parrotfishes across the continental shelf suggests that each shelf system is shaped by fundamentally different processes.

Quantifying herbivory across a coral reef depth gradient

Despite the widely accepted importance of herbivory as a determinant of reef benthic community structure, few studies have examined the relative contributions of individual species to ecosystem processes at the whole reef scale. This study quantifies the grazing impact of individual species of roving herbivorous fishes across an inner shelf fringing reef at Orpheus Island, Great Barrier Reef, Australia. Estimates of roving herbivore impact based on dawn to dusk observations of feeding rates, measurement of bite sizes and relative abundance revealed that the Orpheus Island system was dominated by 3 species: Scarus rivulatus, Chlorurus microrhinos and Siganus doliatus. The estimated impact of all 3 species varied significantly across the reef depth gradient, with the rate of disturbance peaking at the crest and decreasing significantly down the slope and across the reef flat. The estimated species-specific disturbance levels suggested that during the course of a single month 104% of a square metre area of the reef crest is grazed by S. rivulatus while 40% is subject to grazing by C. microrhinos. A total of 26 cm3 of algal material is removed from the same area by S. doliatus. Overall, there was a 240-fold decrease in grazing activity across the reef flat from that at the crest. The pattern of grazing impact of the numerically dominant siganid and scarid fishes was negatively correlated with the distribution of macroalgae across the same reef gradient. The results of the current study provide support for the hypothesis that algal community structure is shaped by levels of herbivory.

Direct evaluation of macroalgal removal by herbivorous coral reef fishes

Few studies have examined the relative functional impacts of individual herbivorous fish species on coral reef ecosystem processes in the Indo-Pacific. This study assessed the potential grazing impact of individual species within an inshore herbivorous reef fish assemblage on the central Great Barrier Reef (GBR), by determining which fish species were able to remove particular macroalgal species. Transplanted multiple-choice algal assays and remote stationary underwater digital video cameras were used to quantify the impact of local herbivorous reef fish species on 12 species of macroalgae. Macroalgal removal by the fishes was rapid. Within 3 h of exposure to herbivorous reef fishes there was significant evidence of intense grazing. After 12 h of exposure, 10 of the 12 macroalgal species had decreased to less than 15% of their original mass. Chlorodesmis fastigiata (Chlorophyta) and Galaxaura sp. (Rhodophyta) showed significantly less susceptibility to herbivorous reef fish grazing than all other macroalgae, even after 24 h exposure. Six herbivorous and/or nominally herbivorous reef fish species were identified as the dominant grazers of macroalgae: Siganus doliatus, Siganus canaliculatus, Chlorurus microrhinos, Hipposcarus longiceps, Scarus rivulatus and Pomacanthus sexstriatus. The siganid S. doliatus fed heavily on Hypnea sp., while S. canaliculatus fed intensively on Sargassum sp. Variation in macroalgal susceptibility was not clearly correlated with morphological and/or chemical defenses that have been previously suggested as deterrents against herbivory. Nevertheless, the results stress the potential importance of individual herbivorous reef fish species in removing macroalgae from coral reefs.

Daily feeding rates in herbivorous labroid fishes

Estimates of daily feeding rates were obtained for two groups of herbivorous labroid fishes, one confined to cold water and the other to tropical reef environments. These were the family Odacidae, represented by Odax pullus from New Zealand waters, (Goat Island Bay: Latitude 36° South; on the northeastern coast of New Zealand) and the family Scaridae, represented by Scarus rivulatus, S. schlegeli and S. sordidus from the northern Great Barrier Reef (Lizard Island; a mid-shelf reef at 14° South latitude). Observations on the odacid were made in 1984 and in 1992, and on the scarids in 1984 and 1988. O. pullus displayed a diurnal feeding pattern in which the rates (expressed as bites min-1) are greatest early in the day. The mean combined feeding rate for three size groups (juveniles, subadults and adults) peaked (average of 2.9 bites) from 06.00 to 08.00 hrs and declined fourfold to a combined average of 0.7 bites min-1 by midday. The greatest mean feeding rate recorded was 3.7 bites min-1, with an overall mean of 1.8 bites min-1. For subadults and adults there were consistent trends in feeding, with subadults feeding at a greater rate than adults and both groups displaying a decline in feeding rate during the day. The change in feeding rate with time of day was statistically significant in both groups. The pattern for juvenile O. pullus was different from that in the two larger size groups in that juveniles did not show a uniform decline in feeding with time of day. For scarids, the daily feeding rate varied by site, but the pattern was similar for all species, characterised by initial low rates increasing to higher but variable levels by midday. The influence of both site of feeding and time of day on feeding rate was confirmed by analysis. The overall mean values for each species were 20.1 bites min-1 for S. rivulatus, 19.7 bites min-1 for S. schlegeli and 14.9 bites min-1 for S. sordidus. For scarids, the peak feeding rates varied from 19.3 to 32.8 bites min-1, with overall rates from 14.9 to 21.1 bites min-1. Estimates of activity and movement patterns during feeding were obtained for O. pullus. Distance moved per unit time was highly variable, 0.1 to 47.5 m min-1, with a mean of 8.5 m min-1 (SD=9.9). Trends in movement among sexes and size classes were obscured by the variable movement patterns of individual fishes.

Growth in juvenile Scarus rivulatus and Ctenochaetus binotatus: a comparison of families Scaridae and Acanthuridae

Daily lapillus growth increments were used to determine age and growth parameters for Scarus rivulatus (Valenciennes) and Ctenochaetus binotatus (Randall) from reefs off Lizard Island in ihe northern Great Barrier Reef. The growth of S. rivulatus increased after settlement whilst growth of C. binotatus decreased after settlement. A check found in the otoliths of both species, and attributed to settlement, was estimated to be from 28 to 47 days for S. rivulatus and from 47 to 74 days for C. binotatus.