Predictive models for penaeid shrimp abundance in the seas off Negombo and Hendala, Sri Lanka

The present investigation was undertaken to study the shrimp trawl fishery in the west coast of Sri Lanka with special emphasis on formulation of predictive models for shrimp abundance. The shrimp trawling in the shallow seas off west coast of Sri Lanka is the major fishing activity exploiting the parent population of the penaeid shrimps (comprised mainly of Metapenaeus dobsoni, Parapenaeopsis coromandelica, Penaeus indicus and Penaeus merguiensis) which utilize the Negombo Lagoon as the nursery environment. Trawling is conducted by two types of craft in the west coast: mechanized 3.5 tonnes wooden boats are operating in the seas off Hendala and non-mechanized traditional crafts are operating in the northern Negombo trawling ground. Variation of the abundance of shrimp was studied over time, including the seasonal variations, the capability of non-mechanized and mechanized trawlers to exploit the resources and the other parameters influencing the shrimp abundance. The present study revealed that the period from May–June until August–September is the best fishing season for shrimp in the west coast, which apparently coincided with the Southwest monsoon season in the island. Three statistical model fitting techniques (Multiple linear regression models, Generalized additive models and Regression tree models) were employed to assess and predict the shrimp abundance in the west coast of Sri Lanka using data on commercial fisheries and environmental variables. In terms of reliability and accuracy for predictions the tree models were found to be the most competent. The impacts of the predictors determining the shrimp abundance were discussed in detail. The operational characteristics—number of hauls, true fishing time and the ratio of operated boats used for model formulation were more useful as predictor variables than the environmental variables. The study also emphasized the importance of formulation of an effective mechanism for optimal exploitation of the shrimp resources in the west coast of Sri Lanka.