Environmental and fishery-induced effects on life histories and assemblages of parrotfishes

Taylor, Brett McCully (2015) Environmental and fishery-induced effects on life histories and assemblages of parrotfishes. PhD thesis, James Cook University.

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Abstract

Characteristics of fish communities that are important to fishery managers include the life histories of targeted species, the spatial variation in community dynamics (e.g., species' abundance and demographic variation), and the spatial scales at which communities are critically observed. At present, our understanding of life history and demographic variability for many exploited coral-reef fishes is poor, and this hinders our ability to interpret fishery dynamics and develop sound conservation policies. Across Indo-Pacific coral reefs, parrotfishes (Labridae: tribe Scarinae) in particular are a ubiquitous and diverse component of reef-fish assemblages that are increasingly prevalent in commercial and artisanal fisheries harvests. The objective of this thesis was to examine patterns of demographic variation among and within species across several spatial scales and relate observations to environmental and anthropogenic factors.

As anthropogenic and environmental factors interact across spatial scales, sampling programs designed to disentangle these are required by both ecologists and resource managers. Hence, I have combined a variety of methods and analyses to explore patterns of demographic and community-level variation in parrotfish assemblages from Micronesia. First, I used fishery-independent sampling coupled with standard ageing (using sagittal otoliths) and gonad histology techniques to determine growth patterns, life spans, mortality rates, and timing of maturation and sex change for twelve common parrotfishes representing five genera in Micronesia. Interspecific variation in key life-history traits was explored using multivariate analysis. I then applied these trait data to examine the effect of life histories on vulnerability to overexploitation across the twelve species. Vulnerability for each species was derived from independent measures associated with both temporal (20-year catch records) and spatial abundance datasets. Subsequent studies were based on parrotfish abundance, biomass and body length estimates derived from stereo-video surveys spanning eight islands across Micronesia. I first examined variation in timing of sex change for exploited parrotfishes using a novel method across a range of environmental, anthropogenic and geographic factors. Finally, parrotfish assemblage structure, size distribution and phylogenetic diversity were assessed in a multi-scale context against potential environmental, biogeographic and anthropogenic correlates.

Across all species examined, there was a high level of correlation among life-history traits. However, relationships between length-based and age-based traits were weakest, with a tenuous link between maximum body size and life span. Species were significantly grouped at two major levels based on multivariate analysis of similarities among life-history traits. The first grouping was driven by length-based parameters (lengths at maturity and sex change, maximum length) and separated the small- and large-bodied species. Within these, species were grouped by age-based parameters (age at maturity, mortality, life span). Groupings based on demographic and life-history features were independent of phylogenetic relationships at the given taxonomic level. Further, Most traits examined were significant predictors of vulnerability across species, but their relative utility differed considerably. Length-based traits (e.g., lengths at maturity and sex change, maximum length) were superior to age-based traits (e.g., life span), but one age-based trait, age at female maturation, emerged as the optimal overall. The results reiterate that body size is an important character differentiating species, but interspecific variation in age-based traits complicates its use as a life-history proxy. Easily-derived metrics such as maximum length can be effective management tools when applied to phylogenetically-related multispecies assemblages, but more holistic and comprehensive demographic data should be sought, especially in data-deficient and heavily impacted regions. Results from the sex change analysis were highly dependent on spatial scale. Fishing pressure was the most influential factor determining length at sex change at the within-island scale where a wide range of anthropogenic pressure existed. Sex transition occurred at smaller sizes where fishing pressure was high. Among islands, however, differences were overwhelmingly predicted by reefal-scale structural features, a pattern evident for all species examined. For the most abundant species, Chlorurus spilurus, length at sex change increased at higher overall densities and greater female-to-male sex ratios at all islands except where targeted by fishermen; here the trend was reversed. This implies differing selective pressures on adult individuals can significantly alter sex change dynamics, highlighting the importance of social structure, demography and the selective forces structuring populations. For community assemblages, results indicated that correlates of assemblage structure are also scale-dependent; biogeographic distributions of species and island geomorphology hierarchically influenced community patterns across islands whereas biophysical features and anthropogenic pressure influenced community assemblage structure at the within-island scale. Species richness and phylogenetic diversity increased with greater broad-scale habitat diversity associated with different island geomorphologies. However, within-island patterns of abundance and biomass varied in response to biophysical factors and levels of human influence unique to particular islands. While the effect of fishing activities on community composition and phylogenetic diversity was obscured across island types, fishing pressure was the primary correlate of mean parrotfish length at all spatial scales.

In these analyses, considerable life-history and community-level responses to exploitation were observed, but results suggest potential fishing effects on demography and assemblage structure may be obscured by natural variation at biogeographic scales. Hence, despite widespread fishery-induced pressure on Pacific coral reefs, the structuring of parrotfish assemblages at broad spatial scales remains a story largely dependent on habitat; that is, the geomorphological features of islands that facilitate spatial heterogeneity in coral reef communities. Biological communities across islands will likely respond to exploitation differently, and therefore equations for biodiversity conservation and fisheries management should integrate inherent features of island geomorphology alongside human impact levels.

Item ID: 41171
Item Type: Thesis (PhD)
Keywords: age at maturity; effect of fishing; fishing effects; geographical distribution; Labridae; life history; life history; multispecies fisheries; otoliths; parrot fish; parrotfishes; phylogeny; protogyny; sex change; spatial scale; vulnerability
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Taylor, B.M., and Choat, J.H. (2014) Comparative demography of commercially important parrotfish species from Micronesia. Journal of Fish Biology, 84 (2). pp. 383-402.

Chapter 3: Taylor, Brett M., Houk, Peter, Russ, Garry R., and Choat, J. Howard (2014) Life histories predict vulnerability to overexploitation in parrotfishes. Coral Reefs, 33 (4). pp. 869-878.

Chpater 4: Taylor, Brett M. (2014) Drivers of protogynous sex change differ across spatial scales. Proceedings of the Royal Society of London Series B, Biological Sciences, 281 (1775). pp. 1-8.

Chapter 5: Taylor, Brett M., Lindfield, Steven J., and Choat, J. Howard (2015) Hierarchical and scale-dependent effects of fishing pressure and environment on the structure and size distribution of parrotfish communities. Ecography, 38. pp. 520-530.

Date Deposited: 02 Dec 2015 04:48
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 33%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 33%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 33%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 33%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 34%
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