Red Sea butterflyfish responds to changing coral cover
A new study into changing Red Sea coral and its effects on the butterflyfish shows a significant variation in behaviour. The research found increased feeding rates, aggressive encounters and territory sizes where there was lower coral cover, which could be an informative bio-indicator.
Red Sea coral reefs exhibit substantial ecological, economic, and cultural functions. The stability of coral reef ecosystems, however, has been challenged in the last decades by anthropogenic impacts through tourism, nitrification, elevated atmospheric CO2-input, and globally rising water temperatures. These threats have generated rising awareness that substantial management efforts are required to maintain coral reef ecosystems worldwide.
While knowledge about anthropogenic impacts on coral communities such as the coverage of living coral and other substrate is rife, indirect impacts via coral growth on species at higher trophic levels within the community remain much less understood. Corallivorous butterflyfish (Chaetodontidae) directly rely on the availability of live coral food and may thus be strongly affected by changes in coral reef condition. Their abundance is known to tightly correlate with the spatial distribution of specific coral species.
The blacktail butterflyfish
This study supplements current knowledge on the effects of changes in coral cover on butterflyfish using the Blacktail Butterflyfish Chaetodon austriacus (see image above ) as a study system. The species is highly abundant throughout its range, strictly corallivorous, and shows diurnal activity, pronounced site fidelity, and strong territoriality.
We specifically investigated the link between small scale field-variation in live coral coverage and three target variables: feeding activity, territory size, and intra-specific aggression.
Field observations were conducted at the fringing reefs at Mangrove Bay (Sharm Fugani, Egypt). Data were collected at 0.3 to 5 m depth while snorkelling along the reef-flat, reef-crest, and reef-slope. Territories in deeper water were not taken into account since depth is assumed not to alter the behaviour of C. austriacus.
Corallivorous butterflyfish directly rely on the availability of live coral food and may thus be strongly affected by changes in coral reef condition
Analogous to other studies, the behaviour of a single focal individual within each pair was recorded, assuming that the behaviour of one individual is representative of both. Each focal was recorded for 30 minutes while maintaining a minimum and apparently non-disturbing distance of 2m. Feeding rate was recorded as the total number of feeding bites per individual on living coral. Aggressive encounters were defined as rapid and directional movement towards conspecifics. The total number of aggressive encounters per individual during 30 minutes was used to quantify the level of agonistic aggression. Territory size of each pair was assessed based on hand-drawn territory boundaries, defined as the polygon joining the outermost locational observations within a one-hour period as localised using prominent features of the reef landscape. The fish typically patrolled their almost circular territories whilst foraging, with pairs moving along their territory border and completing several ‘territory circuits’.
Proportional coral cover was quantified using the Quadrat Grid Transect method. For each recorded focal fish, the two by two metre grid was placed at a single spot within the territory that appeared representative for the overall occurrence of the three differentiated substrate categories. At each of 121 grid intersections, the reef surface was then categorized in living coral versus dead coral (bleached and/or covered by algae) and other biogenic substrate. This enabled the proportion of live coral cover to be calculated.
Data was normally distributed and regression analysis used to define the relationship between coral cover and behavioural response variables.
Results and discussion
Field observations revealed a negative relationship between live coral cover and feeding rate (Fig a). Moreover, as predicted, both territory size (Fig b) and the number of agonistic encounters (Fig c) decreased when living coral cover increased.
Our study documented feeding rates and aggressive encounters in unmanipulated environments, where fish had time to adapt their behaviour to the given set of conditions.
The observed intensified competition for space is likely to be affected by the need to enlarge territory size. We presume that low coral cover drove fish to cross the determined territory boundaries more often to compensate for the decreased food availability within their own territories. Since all observed territories were directly adjacent, this behaviour resulted in a greater territory overlap and thus in more aggressive interactions.
Combined, our findings show that feeding rate, territorial behaviour, and territory size of C. austriacus substantially vary with live coral cover. Our study thus exemplifies the indirect impact of variation in coral cover on higher trophic levels in coral reef communities. Moreover, linking the behaviour of C. austriacus to coral cover, the reported findings validate the earlier proposition that this species renders as an informative indicator species for monitoring schemes in Red Sea coral reef ecosystems. Specifically, longitudinal studies that find increasing feeding rates, territory sizes, and agonistic interactions in C. austriacus would strongly indicate gradual degradation of the coral reef community.
We wish to thank David Righton for suggestions on an earlier version of this manuscript. M Herberich, R Ratzbor, and C Zell supported coral cover surveys. We are further grateful to Ducks Dive Centre at Mangrove Bay Resort for logistic support.