Biannual otolith zonation of Cape hake (Merluccius capensis) in response to fish physiology and environment in the northern Benguela

Understanding and validating annual structures in fish otoliths are important for stock assessments and fisheries ecology. Biannual translucent zone formation has been demonstrated for 3-21 months old Namibian shallow-water hake Merluccius capensis. This paper addresses the hypothesis that the pattern continues in older fish. Otolith zone periodicity was studied for four cohorts hatched in July of 1996, 1998, 2002 and 2005, based on modal progression analysis (n = 1059). Edge analysis and marginal increment analysis were performed on otoliths representing eleven months of the year from port samples (n = 1153) from the years 2007-2015. Two to three translucent (T)-zones are formed on the hake otoliths each year, one or two between austral summer and autumn (January to April) and one in winter-spring (between July and October). Annual zones are usually the T2, T5, T8 zones (likely every third T-zone). Annual zones are not distinguishable from pseudo-annuli under the light microscope, and therefore otolith length measurements (OL) at each T-zone should be used as a guide for assigning age (9, 15 and 19 mm OL for ages 1, 2 and 3 respectively). Otolith T-zones are associated with both warm and cold periods, when they are presumably limited by temperature or dissolved oxygen concentration as well as food availability and feeding efficiency at both ends of their tolerance range. The most noticeable factor influencing the formation of translucent zones in otoliths is fish condition, not always linked with spawning (not always linked with high GSI). M. capensis otolith zonation is thus linked with their endogenous adaptation to the ecosystem, their fish physiology and also regulated by environmental variability.