Multi-mesh gillnet selectivity of Oreochromis mossambicus and O. niloticus (Cichlidae) in the fishery of three large perennial reservoirs in Sri Lanka

R P Prabath K Jayasinghe,Upali S Amarasinghe,Jacques Moreau

doi:10.4038/sljas.v22i1.7517

Abstract

As in many tropical reservoir fisheries, the major fishing gear in the reservoirs of Sri Lanka is gillnet. Gillnets of a wide range of stretched mesh sizes (6.9 - 11.4 cm) are used in individual boats in Sri Lankan reservoirs targeting mainly two exotic cichlid species, Oreochromis mossambicus and O. niloticus , which dominate the fisheries accounting for over 80% of the landings. Although the filament characteristics and dimensions of gillnets of different mesh sizes are uniform, their mesh composition varies from boat to boat making catch samples in individual boats are under the influence of different selectivity patterns. As such, an approach is presented for constructing the overall selection curves in the sampled boats during different sampling occasions for the two cichlid fish species in the three reservoirs. For this purpose, gillnet selectivity of each mesh size was determined using Baranov-Holt method. Probabilities of capture of mesh-wise gillnet selection curves were then summed up giving weight to the contribution of each mesh size to the total number of net pieces in the sampled boat, to determine the overall gillnet selection from gillnets of all mesh sizes. The importance of the findings of the present study for length-based stock assessment methods and for imposing mesh regulations to the gillnet fisheries in reservoirs of Sri Lanka is discussed.

Highlights

Tropical lowland reservoirs support profitable fisheries and are affordable sources of animal protein for rural communities in developing countries (Fernando and Holčik 1991; Welcomme 2001; Amarasinghe and De Silva 2015)
Even when filament characteristics and dimensions of gillnets used by fishers in individual boats are more or less similar, as is the case of the reservoir fishery of Sri Lanka, selectivity patterns may vary from season to season depending on the use of various mesh gillnets
Number of fish measured for determining mesh-wise length frequency data ranged from 103 in mesh size 11.4 cm in Udawalawe to 672 in 10.2 cm mesh size in Minneriya for O. mossambicus and from 103 in 11.4 cm mesh size in Udawalawe to 491 in 8.4 cm mesh size in Udawalawe for O. niloticus (Table 1)

Summary

Introduction

Tropical lowland reservoirs support profitable fisheries and are affordable sources of animal protein for rural communities in developing countries (Fernando and Holčik 1991; Welcomme 2001; Amarasinghe and De Silva 2015). Even when filament characteristics and dimensions of gillnets used by fishers in individual boats are more or less similar, as is the case of the reservoir fishery of Sri Lanka, selectivity patterns may vary from season to season depending on the use of various mesh gillnets. Correction of length frequencies using a mean selection curve is not adequate to investigate the status of the fisheries of commercially exploited species by multi-mesh gillnets using length-based stock assessment methodologies. When the lengthbased stock assessment methodologies (Sparre and Venema 1998; Gayanilo et al 2005) are employed in such multi-mesh gillnet fisheries, pooling of length data of gillnet catches of a fleet of mesh sizes assuming that selectivity effects may be negligible, may introduce bias into growth and mortality estimates

Methods

Results

Conclusion