Biannual otolith-zone formation of young shallow-water hake Merluccius capensis in the northern Benguela: age verification using otoliths sampled by a top predator

摘要

Otoliths collected at least monthly from scat samples of Cape fur seals Arctocephalus pusillus are used to show that shallow-water hake Merluccius capensis from the northern Benguela develop three translucent zones in their first 15 years of life. The novel sampling approach provided otoliths that belonged to four M. capensis cohorts of approximate known age (hatched in 1996, 1998, 2002 and 2005), allowing age verification. Following spawning in austral winter, translucent zones consistently formed first in summer and autumn (T1), then in winter and spring (T2) and again in summer and autumn (T3), with no difference in appearance of the zones (biannuli) for the four cohorts considered. The second translucent zone is usually the first true annulus (year mark). It forms during July to September in fish of 15-20 cm total length (L-T). Formation of the translucent zones appears to be determined by fish length or age, rather than by exogenous cues. It is suggested that length measurements should be used to help determine the first age group; fish with a translucent zone marked at otolith lengths >75 mm should be termed 1 year-old fish. Ages of M. capensis used in previous stock assessment models have been overestimated. Biannuli are an unusual occurrence in fish otoliths in general, but have been observed in other Merluccius species.