Growth, age structure, and productivity of jack mackerels (Trachurus spp.) in New Zealand waters

Abstract

Growth of jack mackerels, Trachurus novaezelandiae and Trachurus declivis, off the central west coast and in the Bay of Plenty, New Zealand, was determined by counting growth check rings in otoliths. One growth check ring appeared to be laid down each year. Peaks in length‐frequencies corresponded to calculated mean lengths at age, and strong or weak age classes progressed through the population at a rate concurrent with the annual deposition of rings. No significant sexual differences in growth were apparent for either species but geographical differences are indicated. The von Bertalanffy parameters l ₈, k, and t ₀ that are considered to describe growth best are 37.7 cm, 0.23 yr⁻¹, ‐2.12 yr for T. novaezelandiae in the Bay of Plenty, 35.3 cm, 0.31 yr⁻¹, ‐0.59 yr for T. novaezelandiae off the central west coast, and 45.8 cm, 0.28 yr⁻¹, ‐0.74 yr for T. declivis off the central west coast, respectively. The best estimate of instantaneous natural mortality is 0.18 for both species. Instantaneous fishing mortality in the trawl fishery off the central west coast is probably < 0.1; yield per recruit analyses indicate that the fishery could expand.

Keywords:

• Trachurus novaezelandiae
• Trachurus declivis
• age determination
• age validation
• growth
• mortality
• yield per recruit