Laboratory analysis of predation by cyclopoid copepods on first-feeding larvae of cultured Brazilian fishes

摘要

Predation by a tropical cyclopoid copepod, Thermocyclops decipiens, on first-feeding larvae of cultured Brazilian fishes, Piaractus mesopotamicus and Colossoma macropomum, was studied in the laboratory. The experiments were designed to quantify the predatory effects of cyclopoid on larval mortality, fin damage and growth in response to a broad range of zooplankton that varied with respect to composition, size structure and abundance. The field-collected zooplankton consisted mainly of predaceous cyclopoid and cladocerans, an alternative prey to cyclopoid and principal first food for both fish species. The results indicate that T decipiens can adversely affect first-feeding larvae of P. mesopotamicus and C macropomum. Dead larvae were found partly or completely eaten and surviving larvae had parts of the lobe of fins damaged at the end of the experiments. Only adult and later stage copepodid females were associated with damage to larvae. Immediate effects of cyclopoid attacks were evaluated by larval mortality rate and fin damage magnitude of survivors. Both were found to be positively predator density-dependent and significantly suppressed by alternative prey presence, especially at high proportion of cladoceran to cyclopoid. When there was no immediate larval mortality, cyclopoid presence was related with a reduced larval growth over the 3.5 initial days of feeding. Larval growth was more negatively affected by predator density than positively affected by larval food density, a possible reflection of stress caused by successive cyclopoid attacks. The results suggest that cyclopoid and larval food densities are the most important determinants of the degree of damage caused by cyclopoid attack on fish larvae in the period of first-feeding and have important applied implications.