Spatial variation in female reproduction for an exploited coral reef fish, Plectropomus leopardus

Carter, Alex B. (2015) Spatial variation in female reproduction for an exploited coral reef fish, Plectropomus leopardus. PhD thesis, James Cook University.

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No–take marine reserves have the potential to benefit fished species via the net export of adult fish (spillover) and/or eggs and larvae (recruitment subsidy) of target species from protected to fished areas. Recruitment subsidy from reserves is expected because fisheries generally remove the Big Old Fat Fecund Females (BOFFFs) selectively from a population. BOFFFs are expected to contribute disproportionately to reproductive output of populations by producing relatively better quality eggs, in greater quantities, compared with smaller and younger females. Quantifying the relative contribution of reserves to egg production is a crucial step in evaluating the potential for recruitment subsidy to occur.

It is important when assessing any effect of reserves that appropriate spatial scales are used whereby, ideally, multiple reserves within the same ecosystem are assessed. This is important in egg production per unit area (EPUA) calculations because the reproductive dynamics of species that occur across broad spatial ranges often vary at a regional scale due to variation in environmental conditions. Such variation may complicate fisheries management if reserves created with the objective of enhancing EPUA of the target species are placed in areas where reproductive output is naturally limited, or where high adult fish densities represent larval sinks, not larval sources. Furthermore, comparisons of EPUA between fished and reserve reefs may be confounded where reproductive data are collected only from fished areas, thereby ignoring potential fishing-related effects such as density-dependent reproduction, Allee-type depensation, altered spawning behaviour, and skewed sex ratios for hermaphroditic species.

In this thesis, I present extensive information on the reproductive biology of the common coral trout Plectropomus leopardus [Lacépède 1802], on Australia's Great Barrier Reef (GBR). P. leopardus is a valuable food fish in the tropics and the major target species of both recreational and commercial line fisheries on the GBR. The species occurs throughout the Great Barrier Reef Marine Park (GBRMP) across 14° of latitude and is a protogynous hermaphrodite. The objectives of this thesis were to: determine the relationships between maternal traits of P. leopardus and egg quantity and quality; compare these relationships at an appropriate spatial scale; test for the existence of BOFFFs; and estimate EPUA for P. leopardus along the GBR. The GBRMP is an excellent study site as the extensive network of no-take reserves offers access to comparable fished and reserve (protected) reefs. The vast area also allows environmental and fishing pressure influences to be tested on a regional scale.

In Chapter 2, I quantify the effects of length and age on the sex ratio, proportion of vitellogenic females, and spawning frequency of P. leopardus on fished and reserve reefs in the northern, central and southern GBR. In Chapter 3, I outline a new method that uses the ratio of the number of migratory nuclei to the number of hydrated oocytes to estimate batch fecundity of P. leopardus. Previous estimates of batch fecundity were biased due to a small sample size, reliant on sampling the few hours prior to dusk spawning when the maximum number of hydrated oocytes is present in the ovary. My new method increases the time during the day over which samples can be collected and, therefore, increases the sample size available and reduces biases in batch fecundity estimates. This method was used in Chapter 4 to quantify the relationships between length, weight and age with batch fecundity of female P. leopardus on fished and reserve reefs in the northern, central and southern GBR. Maternal effects also are widely known to influence egg quality, and therefore the early life history traits of offspring. In Chapter 5, I examined the maternal effects of length, weight, age, and hepatosomatic index with indicators of egg quality (egg size, oil droplet size, total lipid content and lipid classes) for P. leopardus. Chapter 6 combined a 10-year length-density data set with reproductive data from Chapters 2 – 4 of this thesis to quantify EPUA of P. leopardus in fished and reserve reefs in the northern, central and southern GBR.

Despite the broad distribution of P. leopardus along the GBR, reproductive potential of females was maximized in the central section, particularly in the absence of fishing. Length, weight, and age had positive effects on batch fecundity of females from northern and central reefs but negligible effects on spawners from southern reefs. The proportion of vitellogenic females increased with length and age, as did the proportion of males in the population (as expected for a protogyne). However, female-male sex change occurred at smaller sizes and younger ages in the southern GBR, particularly on fished reefs. P. leopardus spawned most frequently on central reserve reefs (every 2.3 days during the spawning season) and as infrequently as once every 2 – 3 months in the southern GBR regardless of reserve status. No effect of length on spawning frequency was detected. Maternal length and/or weight also had positive effects on indicators of egg quality, including egg size, oil droplet size, and the proportion of long-term storage lipid wax esters within the egg. Large female P. leopardus in the central GBR, therefore, generally produce more eggs, and eggs with enhanced provisioning, providing a potential advantage for larvae during the critical transition to successful exogenous feeding.

Male biased sex ratios and low individual female fecundity in the southern GBR limited the reproductive benefits expected from higher population densities or larger fish in reserves. Egg production per unit area was greatest on reserve reefs in the central GBR where on average an estimated ~800 000 oocytes 250 m⁻² year⁻¹ were produced, and lowest on southern GBR reefs where average EPUA was approximately ~30 000 oocytes 250 m⁻² year⁻1. The effect of reef protection on EPUA was inconsistent among regions: EPUA was 180% greater on reserve than fished reefs at Townsville and Cairns in the central GBR and 16% greater on reserve than fished reefs in the southern GBR, but 88% greater on fished reefs in the northern GBR. Individual fecundity had a consistent, positive effect on reef EPUA and was greatest on reserve reefs in the central GBR (>1.7 million oocytes female⁻¹ year⁻¹), and much reduced in the southern GBR (~9 000 oocytes female⁻¹ year⁻¹). I hypothesize that regional variation in reproduction is most likely driven by water temperature. The presence of BOFFFs in the northern and central GBR meant that EPUA increased with density at a much steeper rate than in the southern GBR, despite greater densities of P. leopardus in the south. Fork length had a positive effect on EPUA until fish reached ~400 mm FL, at which size EPUA became asymptotic (northern and central GBR) or decreased slightly (southern GBR).

The spatial variation in reproductive biology of P. leopardus identified in this thesis demonstrates the varied outcomes that reserve reefs can have on EPUA when the distribution of a target species spans a broad geographic range. The presence of BOFFFs on central GBR reserve reefs may have important implications for recruitment subsidy at a regional scale (north-to-south recruitment subsidy) as well as the local scale (reserve-to-fished reef recruitment subsidy). Male bias and lack of spawning activity on southern GBR reefs, where densities of adult P. leopardus are highest, xv suggests recruits may be supplied from the central GBR. This thesis highlights the need for further research on reproductive responses of target species to fishing at appropriate spatial scales, particularly hermaphroditic species, and careful consideration of the suitability of single conservation or fishery management strategies for species distributed across large and diverse spatial scales.

Item ID: 46883
Item Type: Thesis (PhD)
Keywords: batch spawn, common coral trout, coral reef fish, egg size, fecundity estimates, fertility, fisheries biology, fishing, Great Barrier Reef, hydrated oocyte, lipid, marine reserve, maternal effect, migratory nucleus, Plectropomus leopardus, reproduction, Serranidae, spatial variation
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Carter, A.B., Russ, G.R., Tobin, A.J., Williams, A.J., Davies, C.R., and Mapstone, B.D. (2014) Spatial variation in the effects of size and age on reproductive dynamics of common coral trout Plectropomus leopardus. Journal of Fish Biology, 84 (4). pp. 1074-1098.

Chapter 3: Carter, A.B., Williams, A.J., and Russ, G.R. (2009) Increased accuracy of batch fecundity estimates using oocyte stage ratios in Plectropomus leopardus. Journal of Fish Biology, 75 (3). pp. 716-722.

Chapter 4: Carter, Alex B., Davies, Campbell R., Mapstone, Bruce D., Russ, Garry R., Tobin, Andrew J., and Williams, Ashley J. (2014) Effects of region, demography, and protection from fishing on batch fecundity of common coral trout (Plectropomus leopardus). Coral Reefs, 33 (3). pp. 751-763.

Chapter 5: Carter, Alex B., Carton, Alexander G., McCormick, Mark I., Tobin, Andrew J., and Williams, Ashley J. (2015) Maternal size, not age, influences egg quality of a wild, protogynous coral reef fish Plectropomus leopardus. Marine Ecology Progress Series, 529. pp. 249-263.

Date Deposited: 17 Jan 2017 02:42
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060803 Animal Developmental and Reproductive Biology @ 50%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 50%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 50%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
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