Age and growth of co-occurring larvae of two flounder species, Rhombosolea tapirina and Ammotretis rostratus

Abstract

Daily growth increments on otoliths were used to age larvae of the pleuronectid fluonders Rhombosolea tapirina Gunther and Ammotretis rostratus Gunther, collected from Port Phillip Bay, Victoria, Australia, in winter 1984. Daily formation of growth increments was confirmed for R. tapirina by examining the growth of the marginal increment on otoliths of larvae collected over two 24h periods in winter 1985. The first distinctive growth increment was laid down approximately 5 d after hatching, at the onset of external feeding. Growth of flounder larvae was exponential from an early feeding stage to notochord flexion at approximately 30 d after hatching. The specific growth rate was very similar for the two species, at slightly over 4% of standard length per day. Predicted absolute growth rate of R. tapirina larvae increased from approximately 0.10 mm d-1 in early feeding larvae to approximately 0.23 mm d-1 in flexion-stage larvae, compared with 0.12 to 0.28 mm d-1 for A. rostratus larvae of equivalent ages. Exponential models did not adequately describe growth of first-feeding larvae, which was slower than predicted. Growth in the field was faster than that recorded for the same species in the laboratory at higher water temperatures and prey abundances. Otolith growth accelerated markedly in relation to growth in length at the beginning of metamorphosis, causing a significant alteration in the morphology of growth increments, and eventually leading to the cessation of production of visible increments.