Demographic and reproductive characteristics of the squaretail coralgrouper (Plectropomus areolatus) in Solomon Islands

Hughes, Alec Tamana (2017) Demographic and reproductive characteristics of the squaretail coralgrouper (Plectropomus areolatus) in Solomon Islands. PhD thesis, James Cook University.

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Abstract

The overall goal of this study was to examine the age-based demographics and aggregating behaviour of Plectropomus areolatus in Solomon Islands, so that culturally appropriate management strategies could be developed for this species. To place this work within a regional context I began by examining the abundance patterns of Plectropomus species in Australia, Papua New Guinea and Solomon Islands. The analyses show that P. leopardus and P. laevis dominate the reefs of north-eastern Australia while P. areolatus is the most prevalent species in the western Solomon Islands. A more in-depth look at P. areolatus abundance was made possible by examining populations at three locations in western Solomon Islands that had experienced different levels of historical fishing pressure. This showed that habitat structure was a greater driver of P. areolatus abundance than fishing pressure. The most abundant P. areolatus populations occurred in habitats that provided for recruitment (inshore reefs), juvenile growth (lagoonal habitats) adult habitats (barrier reef systems) and spawning sites (well flushed reef passes linking the ocean environment with the lagoonal habitat).

The age-based investigation of the study focused on four locations in western Solomon Islands. Growth analysis revealed that this species is relatively fast growing, with most somatic development taking place in the first quarter of a moderate life-span of 12 years. There were sex-specific differences in growth with males growing faster and reaching larger sizes than females. Patterns of growth varied between study locations, with the variation observed explainable in terms of historical fishing pressure. An investigation of the reproductive biology of P. areolatus indicated that the sexual pattern of P. areolatus was gonochoristic, which differs from the pattern of protogynous reproductive development previously suggested for this genus. Males pass through an immature bisexual phase before maturing and recruiting into the population. In the western Solomon Islands heavily fished populations reached sexual maturity at an earlier age than lightly fished populations.

A multi-year investigation on the spatial and temporal dynamics of a grouper spawning aggregation was carried out at Ghizo where the brown-marbled grouper, Epinephelus fuscoguttatus, camouflage grouper, Epinephelus polyphekadion, and squaretail coralgrouper, Plectropomus areolatus are known to aggregate. Both Epinephelus species aggregated from February to July each year, whereas P. areolatus formed aggregations in every month of the year. Aggregations of all three groupers peaked between March and June. Spawning aggregations were tightly linked to the lunar cycle, with peak densities of all species seen several days prior to the new moon, the lunar period when the gonadosomatic index for P. areolatus also peaked. P. areolatus densities were higher in the shallower depths, with the reverse pattern seen in Epinephelus species. A marked drop in mean aggregation densities for P. areolatus towards the end of the 5-year monitoring period appears to be due to excessive fishing pressure at this aggregation site. The findings of this research demonstrate that P. areolatus, an economically important species in Solomon Islands, is particularly vulnerable to overfishing due to its predicable aggregating behaviour. As a result of this study, management regulations for aggregating groupers are currently being incorporated into the Western Province Fisheries Ordinance and the Solomon Islands 2016 Fisheries Act.

Item ID: 52614
Item Type: Thesis (PhD)
Keywords: aggregation densities, groupers, growth, Plectropomus areolatus, reproduction, sex differences, Solomon Islands, squaretail coralgrouper
Date Deposited: 21 Feb 2018 01:42
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
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