Identifying appropriate length-based indicators (LbI) and an alternative fishing gear application for giant freshwater prawns to achieve the preferred size of first capture

Abstract

The recently introduced GFP (M. rosenbergii) in Sri Lanka's CBF reservoirs is becoming a high-valued CBF product with export potential by providing a high margin income to fishers. Extensive stocking of GFP is done together with tilapia and carp species and regular stocking of carp and GFP is necessary for the development of CBF in reservoirs as they do not breed naturally in Sri Lankan reservoirs. Therefore, some length-based indicators (LbI) are not appropriate for assessing the current status of Sri Lankan CBF irrigation reservoirs and this study was conducted to determine the optimal sizes of first capture and the sizes of consume acceptability of CBF species rather than using exploitation status. Length-weight sampling was conducted from January 2023 to December 2024 in selected perennial irrigation reservoirs and the results showed that the capture of undersized species (below marketable size: LMK ) reached maximum up to 30%, 36%, 30% and 60% for O. niloticus, L. rohita and M. rosenbergii male and female, respectively. To avoid undersized GFP at the landing sites, an application of trap size selectivity using experimentally selected opera house trap (OHT) was also conducted at Vendrason reservoir, Kantale, Sri Lanka. The experiment included treatments with 8 cm, 11 cm and 13 cm diameter aperture size and deployed in 24 hours, and the results showed that the highest catch per unit effort (CPUE) (0.038 kg hr-1) and mean weight (0.69 kg) were recorded for 13 cm and 11 cm diameter OHT, respectively.