Geographic variation in size at age of the coral reef fish, Thalassoma lunare (family: Labridae) : a contribution to life history theory.

Ackerman, J. L. (2004) Geographic variation in size at age of the coral reef fish, Thalassoma lunare (family: Labridae) : a contribution to life history theory. PhD thesis, James Cook University.

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Abstract

Key life history traits such as age at maturity, longevity and mortality have all shown correlations with body size. However, the relationship between age and size is particularly important as growth rates (size per time) can have a profound influence on reproductive capabilities and survival. This is especially so in the context of trade-offs between traits, where fast growth may reduce survivability. One way to test trade-offs between life history traits such as growth, reproduction and survival is to examine phenotypic correlations between distinct populations. Life history attributes of populations have been shown to vary over large geographical ranges, in particular those involving a latitudinal or altitudinal gradient. Temperature is likely to be a particularly important factor influencing life histories at these scales, especially for those animals that rely on external sources for body heat. Numerous studies of terrestrial ectotherms have shown that at higher latitudes (or altitudes) body size is often larger than that of lower latitudes. Furthermore, life history theory predicts that at higher latitudes (lower temperatures) individuals should exhibit larger final body sizes but smaller size at age (i.e. slower growth). With slower growth there is likely to be a trend towards later maturation, but greater longevity. With later maturation (at larger sizes) reproductive effort is delayed, however there is the potential for greater fecundity (assuming fecundity increases with body size). However, these theoretical predictions were yet to be tested in coral reef fish. Therefore, the objective of this project was to examine age-based parameters in discrete populations of Thalassoma lunare, a widespread and opportunistic wrasse, to evaluate whether reef fish fit predictions of traditional life history theory. Addressing this question is fundamental to our understanding of reef fish populations, the dynamics of interactions and the diversity of coral reef fish. Thalassoma lunare is widely distributed, found along much of the East Australian coastline, surrounding Pacific Islands and Indian Ocean. Thalassoma lunare is found throughout both inshore, midshelf and outer shelf reefs on the Great Barrier Reef, temperate rocky reef systems and exposed, sheltered and lagoonal sites. Therefore, it was an ideal study organism to look at regional scales for the determination of life history trade-offs in coral reef fishes. The broad aims of this project, therefore, were to 1) describe the age-based biology of T. lunare, and 2) determine whether life history trade-offs exist between growth, reproduction and longevity in this species by comparing populations along a latitudinal gradient within two oceanic basins. Specifically, the variables to be studied were size at age, lifetime growth trajectories, initial growth rates, age and size at maturity, age and size specific sex distributions, mortality and life span. To describe the age-based biology of T. lunare, samples were collected at temporal intervals for a period of two years from the Palm Group of islands, including Pelorus and Orpheus Islands. Age estimates were determined from otolith samples and reproductive parameters were gathered from both macroscopic and histological techniques. To determine whether life history trade-offs exist, geographically separate populations along two latitudinal gradients were examined, one each in the Indian and Pacific Basins, ranging from 4oS to 32oS. Otolith samples for age estimation and gonads for reproductive variables were collected in the East Indian Ocean from Scott Reef (14oS), Tantabiddi, Ningaloo Reef North (22oS), Coral Bay, Ningaloo Reef South (23oS), Beacon Island, the Houtman Abrolhos Islands (29oS) and Rottnest Island (32oS). In the West Pacific Basin samples were collected from Kimbe Bay, Papua New Guinea (PNG; 5oS), Roviana Lagoon, Solomon Islands (8oS), Lizard Island (14oS), the Palm Group of islands (19oS), One tree Island (23oS) and Cabbage Tree Island, Port Stephens (32oS). In addition, two exposed outer shelf reefs were sampled along this latitudinal gradient to account for habitat variation, one at 14oS (approximately 30km east of Lizard Island) and the other at 23oS (exposed outer shelf reef surrounding the One Tree Island Lagoon). Coral reef fish have been shown to exhibit variation in life history traits due to habitat variation. This can be caused by environmental differences in biotic factors such as predation or competition or abiotic factors such as wave action or habitat structure. Therefore, it was imperative that local scale variation (habitat) in life history parameters was examined in conjunction with the large geographic scale study. A gradient of increasing size, survivability and longevity and decreasing mortality was expected from exposed habitats through sheltered to lagoonal habitats. Therefore, three regional sites were chosen along the latitudinal gradients, two located within the Great Barrier Reef (GBR) and one in the Indian Ocean for the examination of local scale life history differences. These included the Lizard Island Region and the Palm Group of Islands within the GBR and Scott Reef in the Indian Ocean. At Lizard Island, 3 habitat types were identified, exposed reef, sheltered reefs and a lagoon. The Palm Group of islands consisted of 4 sheltered locations and Scott Reef consisted of an outer exposed reef habitat, and two sheltered habitats, an inner reef and the leeward side of a sandy cay. Otolith increments in T. lunare were found to be yearly in formation with no evidence of fish living beyond that of 7 years of age. The plot of size against age showed similar patterns of initial rapid increases followed by a tapering growth trajectory similar to those of other labroid fishes. Whilst mature females were evident in the population at the age and size of 1 year and 70mm Standard Length (SL) respectively, 50% maturity did not occur for females until approximately 2 years of age and 90mm SL. Furthermore, no females were found beyond 5 years of age and 130mm SL, whilst males were only evident in the population after 2 years of age and 90mm SL. This is typical of a protogynous hermaphrodite, where individuals change sex from females to males. Whilst no running ripe gonads were found in the winter months, they were found in April, October and November. In addition, spawning aggregations were observed in July and ripe gonads were found in August. It appears, therefore, that spawning events may occur throughout the year at the Palm Group of islands. Variations in age-based parameters of T. lunare populations across latitudinal gradients throughout the West Pacific Basin and East Indian Ocean were in stark contrast to the predictions. The expected gradient of increasing size and longevity and decreasing mortality was not always observed from lower to higher latitudes. Instead, reef exposure played a significant role in shaping life history parameters of these coral reef fish populations. Gradients from exposed reefs through to sheltered reefs were more likely to cause an increase in size and longevity and a decrease in initial growth rates and mortality. A general pattern of increasing longevity with increasing latitude was evident in the Indian Ocean but not in the Pacific Basin. Mean 20% longevity estimates from regions in the Indian Ocean were highest at Rottnest Is and the Abrolhos Is, in the south, being significantly greater than those estimates from Ningaloo South and Scott Reef. In the Pacific Basin, mean 20% longevity estimates followed a habitat cline more so than a latitudinal one. With the exception of One Tree Island Outer Reef, which exhibited large variation due to small sample sizes, the majority of regions with high estimates of longevity were those with relatively sheltered environments, such as the One Tree Island Lagoon and the inshore Palm Group. Growth at all regions showed similar patterns of initial rapid increases followed by a tapering growth trajectory as those found at the Palm Group of islands. However, both the Indian Ocean and Pacific Basins exhibited little evidence of predicted latitudinal gradients in growth trajectories. There was no evidence of differences in growth parameters between PNG and Port Stephens, two regions separated by the greatest distance. In addition, the Outer Shelf Reef at Lizard Island and the One Tree Island Outer Reef were similar to these two regions, indicative of a habitat difference rather than a large latitudinal gradient. Instantaneous growth rates revealed patterns inconsistent with the theory that higher latitudes (colder waters; 18–25oC) would exhibit slower initial growth rates compared with lower latitudes (warmer waters; 26–31oC). In both oceanic basins the slowest rates were found in the mid-latitudinal regions. Additionally, mortality estimates were also highest at the mid-latitudinal regions. In general, female fish were mature by 1 year of age, and few mature females remained in the population beyond 4 years of age, with the majority of regions exhibiting substantial numbers of males in the 2-year-old age class. Maturity ogives for the regions in the Indian Ocean show that 50% female maturity was at the lowest ages for the midlatitudinal regions and highest for the most southern region sampled. One Tree Island Outer Reef and Port Stephens, in the Pacific Basin revealed the lowest age at which 50% female maturity occurred, whilst the highest value occurred at the Palm Group. There was little evidence of a latitudinal pattern regarding sex change in the Indian Ocean; however, a pattern was evident in the Pacific. Populations at the PNG and Solomon Is regions showed the earliest age at sex change, whilst the population at Port Stephens showed a later age of 50% sex change. In general, 50% female maturity occurred at approximately 40% of a population’s maximum size and 10 to 20% of a population’s maximum age, whilst 50% sex change occurred at approximately 70% of a population’s maximum size and 30 to 50% of a population’s maximum age. For most populations, size and age at maturity was generally related to their final body size and size and age at sex change was generally related to size and age at maturity. However, in areas of relatively high densities, e.g. Ningaloo South, OTI lagoon, Solomon Is and PNG, there appeared to be a significantly shorter period between the age at which 50% female maturity occurred and the age at which 50% sex change occurred. There was significant variation in age-based demographic parameters between habitats, within regions. However, this variation was often not consistent with the predictions that a gradient would exist from the exposed through sheltered to lagoonal habitats of increasing size and age and reduced estimates of mortality, as observed habitat parameters differed between regions. Longevity estimates between habitats, within regions, were only evident at the Palm Group and Scott Reef regions. This is surprising, as these two regions were less heterogeneous in exposure compared with the surrounding habitats of Lizard Island. However, growth trajectories exhibited differences between sheltered and exposed locations of Lizard Island with little difference found between habitats within the other two regions. Mortality, whilst fitting predictions at one region, was generally not consistent between regions. Differences in mortality estimates were only evident between the exposed and sheltered reefs of Lizard Island. Whilst a gradient of slower growth, later maturation, larger final body sizes and increasing longevities have been shown to exist in terrestrial ectotherms, for coral reef fish this may not always be the case. Age-based parameters of T. lunare appear to be more susceptible to habitat variation than large regional scales and/or large temperature differences. Therefore, exposed outer reef habitats are more likely to impose smaller final body sizes, potentially reduced longevities and increased mortality in contrast to the more sheltered inshore environments. Further work is now necessary to determine the biotic and abiotic factors that affect populations of coral reef fish across these habitat scales, especially traits such as age at maturity, longevity and mortality. Additionally, research is needed to link these life history attributes with phylogenetic information. Finally, modelling the extent to which phenotypic flexibility exists amongst traits between genetically similar populations will increase our understanding of evolved strategies to account for habitat heterogeneity.

Item ID: 66
Item Type: Thesis (PhD)
Keywords: Age-based parameters, Reef fish populations, Thalassoma lunare, Life history theory,
Date Deposited: 20 Jun 2007
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 0%
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