Age, growth and back-calculated birthdate distributions of larval jack mackerel, Trachurus declivis (Pisces: Carangidae), from eastern Tasmanian coastal waters

Abstract

Otolith microstructure of the larvae of jack mackerel, Trachurus declivis, was examined to derive estimates of age, growth and back-calculated birthdate distributions. Larvae were collected from shelf waters of the eastern coast of Tasmania between January and March, 1989 to 1991. Daily increment formation was confirmed by successive sampling of a cohort of individuals, correlating the change in the number of increments with the sampling interval. Otoliths examined under the scanning electron microscope confirmed the otolith structure and increment counts obtained with the light microscope. Initial increments were shown to form at the time of first feeding. Estimated ages of larvae ranged from 2 to 25 days after first feeding, and larvae showed exponential growth in all three years. Interannual variations in larval growth were evident, with growth being significantly slower in 1991 (4.6% of standard length per day) than in 1989 (5.2% SL day-1) and 1990 (5.4% SL day-1). The faster growth in 1989 and 1990 may be attributable to warmer-than-normal water temperatures over the study area in these years associated with a major La Nina event. Back-calculated birthdate distributions showed that spawning occurred during summer (mid December to mid February) in all three years, with indications of semilunar spawning activity in which peaks were apparently associated with both full and new moons.