The influence of pelagic life history on the quality of tropical goatfish (family Mullidae) at settlement

McCormick, Mark Ian (1992) The influence of pelagic life history on the quality of tropical goatfish (family Mullidae) at settlement. PhD thesis, James Cook University.

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High variability in the replenishment of coral reef fish populations by pelagic larvae has been extensively documented. Little is known of the mechanisms underlying this variability or the processes within the plankton that determine the growth and developmental rates of larval fish. Ultimately, these processes will not only influence numerical abundance but also the quality of the recruiting larvae. A common assumption is that fish settle to a reef with similar probabilities of survival and success. The physical condition (e.g. size, biochemical composition and muscle development) of the settling reef fish, and how that interacts with their performance (e.g swimming speed), may influence which individuals survive to join the reproductive population. This study examines how processes within the pelagic life stage influence the quality of newly settling reef fish and explores the ramifications of variable quality to the dynamics of reef fish populations.

The ecology of the pelagic life-stages of goatfish (family Mullidae) was investigated from samples collected in the Lizard Island section of the northern Great Barrier Reef (GBR) (1986 - 1991). A plankton-mesh purse seine was used to collect pelagic stage fish from around small aggregation rafts (1 x 1m). Patterns of distribution around the Lizard Island fringing reef and across the GBR lagoon were examined. Five species were caught, the most abundant being Upeneus tragula and U. moluccensis. Pelagic stage mullids were found to form large schools (1000+ individuals) that comprised of a number of species (as many as 5), each with a broad size distribution. The largest individuals of each species caught were usually competent to settle.

A distinct metamorphosis accompanied settlement to the reef. Fish changed pigmentation from their silvery pelagic colouration to a mottled cream. Over the same 6 - 12 h period, major restructuring occurred to the barbel sensory system. These are outgrowths of the gustatory system, consisting of batteries of tastebud cells, and are important for the detection and capture of prey items after settlement. Barbel length abruptly increased (up to 52 %) as did mean taste-bud size (up to 100%). Barbels were found to vary markedly in their state of development at settlement.

Age, size and body mass at settlement were examined for goatfish, Upeneus tragula, among five stations across the GBR lagoon, over time periods ranging from days to three years. Larval durations and fish length showed equally high levels of variability (ranges: 25 - 37d, 19 - 31 mm standard length respectively). Significant differences in age, length and weight at settlement were found at all spatial and temporal scales. Growth rates averaged over the whole larval period ranged from 0.55 - 1.0 mm/d. Similarly high levels of variability were found in the biochemical composition of newly settled U. tragula over ten samples collected over two summers (1990/91, 1991/92). Relationships of nine commonly used measures of fish condition with fish length were very poor. Maximum non-sustainable swimming speed and biochemical composition were particularly poorly correlated with fish size (r < 0.2).

Two experiments examined the extent to which food availability and water temperature during the late larval stages influenced the high levels of variability in age and body characteristics of U. tragula at settlement. Feeding history influenced size, age, muscle development and body composition of fish at settlement. Water temperature (25 - 30 °C range) significantly influenced only age at settlement. These experiments suggested that the processes of growth and differentiation for late pelagic stage fish dissociated from one another under certain environmental conditions. Over a natural range, water temperature did not dissociate growth and developmental rates. Food availability strongly influenced the relationship between these processes and ultimately governed the age and condition of the fish at metamorphosis. Differences in the temperature regime and feeding history within the pelagic life-stage accounted for much of the variability in growth characteristics among samples.

Two preliminary experiments examined whether the size or body condition at which a fish settled biased the probability of it surviving the initially high predation pressure. Results suggest that predation by the lizardfish, Synodus variegatus, was random with respect to both size and biochemical composition of the newly settled goatfish, U. tragula. This suggests that the influence of the pelagic life history will extend well into the post-settlement stages and may ultimately influence which individuals join the reproductive population.

Item ID: 24117
Item Type: Thesis (PhD)
Keywords: age; barbels; goatfish; larval settlement; life histories; life history; Lizard Island; marine fishes; pelagic larvae; size; Upeneus; variability; water temperature
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Date Deposited: 14 Dec 2012 03:33
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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