Feeding ecology and interannual variations in diet of larval jack mackerel, Trachurus declivis (Pisces: Carangidae), from coastal waters of eastern Tasmania

Abstract

Crustacean microzooplankton were the main prey of larval Trachurus declivis collected in the summers of 1988, 1989 and 1990 from coastal waters of eastern Tasmania. The diet was dominated by harpacticoids (Microsetella rosea), cyclopoids (mainly Oithona spp.), calanoids and the calyptopis stage of the euphausiid Nyctiphanes australis. Bivalve veligers were occasionally eaten. Diets of larvae were affected by interannual variations in plankton composition, particularly in 1989 when intrusions of low-nutrient subtropical water exclused large zooplankters (e.g. N. australis) from the study area. Larvae≤6 mm selected for copepod nauplii; all larvae selected for M. rosea, cyclopoids, and the calyptopis stage of N. australis. Even though calanoids were a major prey taxon, there were proportionally fewer eaten than were present in the environment. In all, 78% of larvae taken during the daytime had food in their stomachs, as opposed to 38% of the larvae from night samples. Feedmid-morning and late afternoon. A gut evacuation rate of ∼4 to 6 h was estimated. We calculated that the larvae ate between 9 and 13% of their body weight in food per day. The larvae of T. declivis in this study were not sufficiently abundant to have an impact on their prey.