Does structural change in the zooplankton community affect larval fish feeding in anthropogenically disturbed tropical waters?

Anthropogenic perturbations and climate change have altered the zooplankton community structure in the Klang Strait during the past 30 years, in that the taxa of large-bodied crustaceans (Acartiidae, Calanidae, Pseudodiaptomidae) are being replaced by those of small-bodied crustaceans (Oithonidae, Ectinosomatidae), gelatinous jellyfish, and appendicularians. Since zooplankton constitutes the main larval food, we questioned: have bottom-up effects impacted larval fish feeding via the food chain? Larval fish that were sampled previously (1985–1986) and nearly three decades thereafter (2013–2014) were analysed for their dietary composition. Despite the dramatic replacements of zooplankton taxa due to escalating anthropogenic disturbances, the dominant copepod families, Paracalanidae, Oithonidae and Euterpinidae, remain the major prey for fish larvae. Dietary shifts in prey composition from before to after impact depend on the larval fish family and their ontogenetic stage. Dietary changes are observed in the Bregmacerotidae, Engraulidae, Gobiidae and Sciaenidae that opportunistically feed on the small-bodied copepods (oithonids and Parvocalanus crassirostris ), whereas the Callionymidae, Clupeidae and Cynoglossidae naturally feed on these copepods even before these prey become numerically dominant with anthropogenic disturbance. There is no dietary shift in the Leiognathidae, exceptional in that they are specialists feeding mainly on detritus and polychaete larvae. Since the bottom-up effects are not comprehensive among fish families and dietary plasticity is evident, it is postulated that only the intolerant or non-adaptable larval species are adversely affected by the environmental perturbations.