AGE, GROWTH AND BACK-CALCULATED BIRTH-DATE DISTRIBUTIONS OF LARVAL JACK MACKEREL, TRACHURUS-DECLIVIS (PISCES, CARANGIDAE), FROM EASTERN TASMANIAN COASTAL WATERS

Otolith microstructure of the larvae of jack mackerel, Trachurus decli vis, was examined to derive estimates of age, growth and back-calculat ed 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 increme nts with the sampling interval. Otoliths examined under the scanning e lectron microscope confirmed the otolith structure and increment count s obtained with the light microscope. Initial increments were shown to form at the time of first feeding. Estimated ages of larvae ranged fr om 2 to 25 days after first feeding, and larvae showed exponential gro wth in all three years. Interannual variations in larval growth were e vident, with growth being significantly slower in 1991 (4.6% of standa rd 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-t han-normal water temperatures over the study area in these years assoc iated with a major La Nina event. Back-calculated birthdate distributi ons showed that spawning occurred during summer (mid December to mid F ebruary) in all three years, with indications of semilunar spawning ac tivity in which peaks were apparently associated with both full and ne w moons.