Temporal patterns in spawning and juvenile recruitment can have major effects on population size and the demographic structure of coral reef fishes. For harvested species, these patterns are crucial in determining stock size and optimising management strategies such as seasonal closures. For the commercially important coral grouper (Plectropomus spp.) on the Great Barrier Reef, histological studies indicate peak spawning around the summer new moons. Here we examine the timing of spawning activity for P. maculatus in the southern Great Barrier Reef by deriving age in days for 761 juvenile fish collected between 2007 and 2022, and back-calculating settlement and spawning dates. Age-length relationships were used to estimate spawning and settlement times for a further 1,002 juveniles collected over this period. Unexpectedly, our findings indicate year-round spawning activity generates distinct recruitment cohorts that span several weeks to months. Peak spawning varied between years with no clear association with environmental cues, and little to no alignment with existing seasonal fisheries closures around the new moon. Given the variability and uncertainty in peak spawning times, this fishery may benefit from additional and longer seasonal closures, or alternative fisheries management strategies, to maximise the recruitment contribution from periods of greatest reproductive success.

Harrison HB, Drane L, Berumen ML, Cresswell BJ, Evans RD, Galbraith GF, Srinivasan M, Taylor BM, Williamson DH, Jones GP. 2023 Ageing of juvenile coral grouper (Plectropomus maculatus) reveals year-round spawning and recruitment: implications for seasonal closures. Proc Roy Soc B.

Otolith preparation and age determination

The sagittal otoliths were extracted from a subset of individuals from each collection period (Table S1) to estimate age and age-length relationships of juvenile P. maculatus. The preparation of otoliths followed the methods described in (Taylor et al 2016) and is consistent with previous studies of early life history growth in P. maculatus from the southern GBR (Mannering et al. 2008; Harrison et al. 2012, 2020; Williamson et al. 2016). Briefly, one otolith from each juvenile fish was affixed to a glass microscope slide using thermoplastic glue (Crystalbond 509), with the primordium (nucleus) on the inside edge of the slide and sulcus ridge perpendicular to the slide edge to obtain a transverse section of the sagittal otolith. Each otolith was ground to the edge of the slide using a GEMMASTA lapping wheel with a 1200 grit diamond sanding wheel. The otoliths were then removed and affixed to a clean labelled slide, with the ground surface down, and polished using the same grit to a thin (≃150 μm) transverse section that intercepts the nucleus. Successive polishing was then carried out with 9, 3, and 0.3 micron lapping film until daily growth increments were of optimal clarity. Sectioned otoliths were then coated in immersion oil and photographed under 200x and 400x magnification.

Taylor BM, Gourley J, Trianni MS. 2016 Age, growth, reproductive biology and spawning periodicity of the forktail rabbitfish (Siganus argenteus) from the Mariana Islands. Mar Freshw Res 68, 1088–1097. (doi:10.1071/MF16169)

Harrison HB, Bode M, Williamson DH, Berumen ML, Jones GP. 2020 A connectivity portfolio effect stabilizes marine reserve performance. Proceedings of the National Academy of Sciences 117, 25595–25600. (doi:10.1073/pnas.1920580117)

Mannering TD, Jones G. 2008 Benefits of marine protected areas beyond boundaries: an evaluation for two coral reef fishes. MSc, James Cook University, Townsville.

Williamson DH et al. 2016 Large-scale, multidirectional larval connectivity among coral reef fish populations in the Great Barrier Reef Marine Park. Mol Ecol 25, 6039–6054. (doi:10.1111/mec.13908)

Please see the README document ("README.md") and the accompanying published article: Harrison HB, Drane L, Berumen ML, Cresswell BJ, Evans RD, Galbraith GF, Srinivasan M, Taylor BM, Williamson DH, Jones GP. 2023 Data from Ageing of juvenile coral grouper (Plectropomus maculatus) reveals year-round spawning and recruitment: implications for seasonal closures. Proc Roy Soc B, Accepted DOI: https://doi.org/10.1098/rspb.2023.0584