Minimum legal size of commercially important groupers in the Coral Triangle Sulawesi Sea, Indonesia

Abstract

Major challenges facing grouper fisheries in Kwandang Bay within the Coral Triangle include declines in potential spawners and as well as in the abundance of juvenile fish. One measure that can be taken to maintain valuable fisheries stocks such as groupers is to regulate the minimum legal size (MLS). The aim of this research was to identify and analyse the mean size at first maturity (Lm50) as a basis for setting an MLS for three grouper species of commercial importance. A total of 525 grouper specimens (307 Variola albimarginata, 90 Variola louti, and 128 Plectropomus leopardus) were collected from January to October 2022 from grouper traders, traditional markets, and the Kwandang Fishing Port. The Lm50 for each species was calculated using the Spearman-Karber formula. The Kwandang Bay catch was dominated by immature juvenile fish and the size class contributing the most individuals was below the Lm50 for all three grouper species. The MLS should be at least 32 cm for V. albimarginata, 33 cm for V. louti and 35 cm for P. leopardus in order to enable sufficient fish to spawn at least once, increase the number of spawners, and thereby maintain stock abundance.

Population dynamics of Variola albimarginata Baissac, 1953 and Variola louti Fabricius, 1775 (Perciformes: Serranidae) in Sulawesi Sea, Indonesia

“…Fisheries and conservation management measures that could be applied to rebuild and maintain the V. louti and V. albimarginata populations in Kwandang Bay include setting size limits and spatio-temporal closures. Based on life-history data, setting minimum legal sizes at or above 33 cm for V. louti and 32 cm for V. albimarginata would enable most fish to reproduce (as protogynous females) before capture (Achmad et al 2023b). In practice, this could require gear restrictions or modifications, for example, hook size, as well as monitoring of fish landed or traded.…”

“…Temporary or permanent spatial fishery closures are another option. This could involve temporary closure of the fishery in Kwandang Bay, as life history data indicate that restricting or closing the fishery for one in May or June should substantially increase the natural recruitment of several grouper species, including the lyretail groupers V. louti and V. albimarginata (Achmad et al 2022(Achmad et al , 2023b. If the required data were available, a more finely targeted approach would be possible; areas known or suspected as spawning aggregation sites could be closed to fishing, either temporarily during the peak spawning season or permanently (Sadovy 2016).…”