The role of tabular structure in the ecology of large reef fishes

Kerry, James Timothy (2015) The role of tabular structure in the ecology of large reef fishes. PhD thesis, James Cook University.

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Abstract

The ecology of an organism is defined by its relationship with the environment. In many ecosystems, the structural complexity (physical, three-dimensional structure) of the environment is a major facet that determines the nature of this relationship. For example, on coral reefs, structural complexity provides reef associated fishes with numerous functions including, access to food, shelter, and recruitment habitat.

The vulnerability of coral reefs to degradation and loss of structure, particularly in the face of climate change, adds impetus to understand the relationship between structural complexity and coral reef fishes. This is especially important given that numerous studies have documented positive relationships between the structural complexity of coral reefs and the abundance and diversity of associated coral reef fishes. However, few studies have been able to identify the specific functional properties of structural complexity that drive these associations. This thesis, therefore, endeavoured to examine the functional importance of a particular morphological structure on coral reefs – tabular structure – and to assess its relevance to a diverse assemblage of large reef fishes, which had been previously identified to strongly associate with this structure on coral reefs.

Although previous studies had hypothesised that declines in the abundance and diversity of large reef fishes might be seen following the loss of tabular structures, no studies had experimentally demonstrate this relationship. The first data chapter (Chapter 2), therefore, examined this outcome by experimentally excluding access of large reef fishes to the understory of tabular structures. The exclusion of large reef fishes from tabular structures using mesh wire had a significant effect on the distribution of large reef fishes, even though these structures only constituted a small fraction (4%) of the benthic cover. Reduction in the availability of tabular structures at this spatial scale (200 m2) shifted activity spaces of large reef fishes away from these localities. Importantly, the observed movement of large reef fishes is likely to occur at reef wide scales during disturbance events. Indeed, if large reef fishes abandon areas of impacted reefs, there is a strong possibility that this may severely impact ecosystem function in those areas.

In the second data chapter (Chapter 3), video analysis of fishes sheltering under tabular structures at midday and at sunset was used to test two alternate hypotheses to explain sheltering behaviour: avoidance of predation or avoidance of solar irradiance. Patterns of shelter use offered minimal evidence in support of the predation avoidance hypothesis, with usage of tabular structures being low during the sunset period when predators of large reef fishes were likely to be most active. However, tabular structure usage reached a peak during the middle of the day when incident solar irradiance would reach its zenith. As such, the data suggest that large reef fishes are primarily sheltering beneath tabular structure to protect themselves from harmful UV irradiance, which can be extreme in shallow, oligotrophic waters. Furthermore, given the significant costs of producing UV-blocking mycosporine-like amino acids, fishes may achieve considerable energetic savings by sheltering beneath tabular structure.

Intense use of tabular structures by large reef fishes suggests that these structures may already be a limited resource on some coral reefs. The third data chapter (Chapter 4), therefore, used video analysis to record competition for access to tabular structures by large reef fishes. Examination of 26 tabular structures revealed an exceptionally high mean biomass of sheltering large reef fishes (4.71 kg m-2), which is one or two orders of magnitude greater than typical coral reef biomass estimates between (0.031 to 0.1 kg m⁻²). There was also strong interactions among the 30 species of large reef fishes observed using tabular structures for shelter, which resulted in displacement and exclusion of losing individuals. This behaviour generated a dominance hierarchy for access to tabular structures, with several species of functionally-important large reef fishes being located in the lower ranks of the hierarchy. This raises cause for concern, given the likelihood of a future reduction in the availability of tabular structures, which may mean that these species are excluded from these shelter by more dominant species.

The fourth data chapter (Chapter 5), evaluated the role of multiple environmental factors in driving sheltering behaviour by large reef fishes under tabular structure. Patterns of tabular structure usage by large reef fishes were documented through video analysis and compared to six environmental variables that were collected in situ and from weather stations at the study sites. Two environmental factors emerged as key drivers of sheltering behaviour in large reef fishes: increased visibility (decreasing turbidity) and increasing wind speed (increasing wave energy). Turbidity correlates negatively with in-water irradiance and, therefore, as water clarity improves large reef fishes are more likely to seek out shelter to avoid UV irradiance, as observed in Chapter 3. Increasing wave energy places energetic costs on swimming or station holding in fishes. Fishes therefore seek refuge from water movement. Tabular structures likely reduce water momentum by creating drag and also reduce turbulence by preventing mixing of surface flow with understory flow, providing station holding large reef fishes with a potential refuge from energetic swimming. Chapter 5, therefore, provides further evidence that tabular structures are important energetic refuges for large reef fishes, offering shade from down-welling UV-irradiance and respite from wave energy.

Overall, the sheltering behaviour of large reef fishes beneath tabular corals appears to be driven by the energetic savings that these structures can provide for fishes. Both in the form of providing shade from harmful UV-irradiance and in offering respite from wave energy. Movement from of reef localities where tabular structure are unavailable suggests that multiple species of large reef fishes place a high priority on the energetic savings that these structures provide. This observation is further supported by the strong competitive interactions that were observed to occur for access to tabular structures. If, as predicted, the availability of tabular structures on coral reefs declines in the future, there may be associated losses of large reef fishes and an accompanying loss in ecosystem functionality. The importance of tabular structures, demonstrated herein, highlights the need for conservation planning to preserve these key structures, or for consideration of alternative structural restoration where this is no longer possible.

Item ID: 45254
Item Type: Thesis (PhD)
Keywords: ecology; fish behavior; fish behaviour; large reef fishes; sheltering; shelters; structure; table corals; tabular corals; tabular structures
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Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Kerry, J.T., and Bellwood, D.R. (2015) Do tabular corals constitute keystone structures for fishes on coral reefs? Coral Reefs, 34 (1). pp. 41-50.

Chapter 3: Kerry, J.T., and Bellwood, D.R. (2015) The functional role of tabular structures for large reef fishes: avoiding predators or solar irradiance? Coral Reefs, 34 (2). pp. 693-702.

Chapter 4: Kerry, J.T., and Bellwood, D.R. (2016) Competition for shelter in a high-diversity system: structure use by large reef fishes. Coral Reefs, 35 (1). pp. 245-252.

Date Deposited: 30 Aug 2016 06:47
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 33%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 34%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 33%
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