Climate change impact on the feeding habits of Indian mackerel observed along the Kerala coast

Abstract

While most food and feeding research in fisheries emphasize the feeding habits and diet components of the fish, the aim of the present work was to assess any change in the diet composition of Indian mackerel Rastrelliger kanagurta (Cuvier, 1816) and identify the role of climate change in effecting it. Non-parametric statistical technique, the Generalized Additive Model (GAMs) was used for modeling the causal link of diet changes in relation to climate change using mgcv package of R software. Monthly Index of Relative Importance (IRI) was calculated for 36 months from January 2013 to December 2015 and major prey items in the diet were identified. Climatic variables - Sea Surface Temperature (SST), precipitation (Pr), Chlorophyll a (Chl a), salinity and Ekman transport or coastal upwelling index (CUI)) pertaining to the study area were extracted for the period. Length-wise IRI was compared to check the existing patterns in the feeding habits of the fish. The dominant prey items observed in the gut of Indian mackerel during the study period were diatoms ( Fragillaria sp., Nitzchia sp., Thalassiothrix sp., Thalassiosira sp. and Coscinodiscus sp.), dinoflagellates (Ceratium sp., Ornothocercus sp., Dinophysis sp. and Prorocentrum sp.), copepods (Calanoida, Cyclopoida and Harpacticoida), decapods (Acetes) and tintinnids (Tintinnopsis sp., Codenellopsis sp. and Flavella sp). GAM models were fitted with monthly IRI of the major prey items (copepods, diatoms, dinoflagellates, Acetes and tintinnids) in the gut and climatic variables. Model selection was done in terms of sharpening the relation between the predictors and the response variable using Akaike information criterion, R-squared and F-statistics. GAM model results revealed that occurrence of prey items in the diet of Indian mackerel were influenced by environmental variables. An increase in the relative importance of dinoflagellates, Acetes and tintintids in the diet over the historic period was observed. The study also revealed a shift in the diet composition of younger fishes. The results of the current study provide a more in-depth assessment of the nonlinear relationship between climatic variables and diet composition of Indian mackerel.