Abstract:
Red gurnard were sampled monthly over 16 months from four areas in the Hauraki Gulf. Ageing by otoliths showed that males have a slower growth rate and smaller asymptotic length than do females. Average von Bertalanffy growth equations for each sex are:
Female lt = 36.35 1 – e-0.6413(t – 0.1886) (cm)
Male lt = 28.77 1 – e-0.5686(t + 0.5521) (cm)
The average length-weight relationship for both sexes is:
W = 0.009982 L2.9937 (gm)
Relative condition varies greatly with stage of gonad maturity, especially for females. Peak condition occurs during the late pre-spawning stages of maturity.
Sex ratios change greatly with time of year and depth, males occurring predominantly in deeper water. An offshore spawning ground in Cradock Channel has been established and a spawning migration from the inshore areas to the spawning ground takes place during the spring and summer, the spawning season being an extended one. Males appear to be at least partly resident on the spawning ground. Red gurnard are probably partial spawners. The offshore spawning ground is associated with an inshore nursery ground through the tide and current system of the area. Some fish become sexually mature in their second summer after being spawned and 90% are sexually mature in their third summer after being spawned.
Males are fully recruited on to the trawling grounds of the Hauraki Gulf at age two years and females at age three years. Catch curves suggest a sudden increase in fishing intensity occurred during 1964 with the present total instantaneous mortality coefficient, Z, being 0.921 for females and 0.737 for males. Results of yield-per-recruit calculations suggest that present levels of fishing intensity are appropriate for maximum return for effort expended on the species in the Hauraki Gulf.
