Habitat preferences and associations of juvenile algal-associated fishes on coral reefs

Waldeland, Oda Ravnaas (2017) Habitat preferences and associations of juvenile algal-associated fishes on coral reefs. Masters (Research) thesis, James Cook University.

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Macroalgae are an important and diverse component of tropical inshore reefs, providing a range of ecological and functional services. Most of the research emphasis has focused on the negative effects of increased macroalgae on coral reefs to the growth and persistence of corals and fishes. However, on undisturbed reefs, macroalgal beds provide important habitats for newly settled fishes, many of which have ontogenetic changes to coral reefs and replenish adult populations that help control macroalgal growth. The few studies that have investigated macroalgal associations of juvenile fishes have treated macroalgae as a single entity. In doing so, any associations of fishes with taxonomic or functional groups cannot be discerned. The objective of this thesis was to undertake observational and experimental studies to investigate the patterns and mechanisms of habitat selection and preferences in algal-associated juvenile fishes on coastal reefs.

In Chapter 2, I investigated the availability, use and selectivity of microhabitats on the reef flat of an inshore reef by recently settled coral reef fishes. Fish and benthic communities were quantified along three 50m transects at each of six reef flat sites along the leeward side of Orpheus Island, an inshore island on the central Great Barrier Reef (GBR). Benthic communities were quantified using the point-intercept method, recording the benthos directly underneath the transect every 0.5m. Recently settled fish were surveyed within a 2m belt along each transect. All recently settled fishes were identified to species, and the microhabitat they were associated with was recorded. A total of 22 species of recently settled fish were recorded across all transects, with the majority of the species being herbivorous as adults. Examination of the microhabitat preferences of the six most abundant species revealed that the two herbivorous rabbitfish species, Siganus lineatus and S. canaliculatus, appeared to associate with large fleshy macroalgae (namely Padina and Sargassum). In contrast, three herbivorous damselfishes, Pomacentrus wardi, P. adelus and P. chrysurus, and the invertebrate feeding cardinalfish Apogon cookii showed no apparent selection for any of the microhabitats examined. These findings suggest that macroalgae are important habitats for some newly settled coral reef fishes, and the availability and composition of macroalgae may shape juvenile populations. Both habitat selection at settlement and/or post-settlement processes may contribute to the establishment of these patterns.

The objective of Chapter 3 was to investigate the specific habitat and olfactory preferences of two rabbitfishes (S. lineatus and S. canaliculatus) and two herbivorous damselfishes (P. wardi and P. adelus) at settlement. Naïve settlement-stage fishes were collected in light traps and used in a series of aquarium-based settlement choice experiments in which an individual larva was introduced into the centre of a 500L aquarium and allowed to choose among four habitat options. The fishes were released overnight and their associations with the four habitats recorded the following morning.

Four separate habitat-choice experiments were conducted, each with four different substratum options: (1) benthic habitats (live coral, dead coral, macroalgae, coral rubble), (2) macroalgal species (Sargassum, Padina, Galaxaura, coral rubble), (3) macroalgal density (8, 4, 1, 0 Sargassum thalli) and (4) macroalgal height (30, 20, 10, 0cm Sargassum). In addition, the role of olfaction was investigated using a 2 channel-choice flume chamber, pairing a benthic substrate water cue to the off-reef control cue (1 km from the nearest reef). The aquarium trials showed that both rabbitfish species preferred macroalgae in experiment 1 and the highest density Sargassum patch in experiment 3. The two rabbitfishes also differed in their habitat preferences as only S. canaliculatus preferred Sargassum in experiment 2 and S. lineatus preferred the tallest Sargassum patch in experiment 4. In contrast, neither damselfish species displayed any preferences among the habitats present. The olfactory trials revealed significant but weak attractions to various chemical cues from benthic microhabitats, however, these varied among species and are unlikely to be biologically significant. It is apparent that juvenile rabbitfishes have strong innate preferences for macroalgae at settlement, however, species-specific preferences for macroalgal species and habitat features may affect the distribution of juveniles on the reef.

In summary, macroalgae appear to provide important settlement and juvenile habitats for at least the two rabbitfishes species examined herein, and given the prevalence of juvenile fishes in this habitat, it is likely that such preferences may be more widespread. It is unclear why these early life stages are using macroalgae, but they may be associated with the provision of refugia from predation, or enhanced food supply as these fishes transition from a carnivorous to herbivorous diet. Interestingly, many studies have focused on the role of herbivorous rabbitfishes in removing macroalgae from coral reefs, yet the results of this thesis highlight that these same fishes are dependent on macroalgae at a critical life stage. Clearly, further investigations are required to elucidate the importance of macroalgae to reef fishes at settlement, and potentially other life stages.

Item ID: 51630
Item Type: Thesis (Masters (Research))
Keywords: algal-associated, coastal reefs, coral reef fishes, Great Barrier Reef, habitat selection, juvenile fishes, macroalgae, Orpheus Island, tropical reefs
Date Deposited: 22 Nov 2017 05:15
FoR Codes: 06 BIOLOGICAL SCIENCES > 0699 Other Biological Sciences > 069902 Global Change Biology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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