Inferring spawning migrations of orange roughy (Hoplostethus atlanticus) from spawning ogives

Abstract

Orange roughy (Hoplostethus atlanticus) occurs throughout New Zealand waters at depths between 700 and 1500 m. Most spawning occurs in a few localized aggregations in winter and it appears that fish migrate substantial distances (hundreds of kilometres) to and from these aggregations. However, there is no direct evidence of migration. Indirect evidence of migration of females is presented in the form of trends in L₅₀, the mid point of the spawning ogive (i.e. the length at which 50% of mature females are spawners). Data from 21 research surveys carried out between March and August over an eleven-year period show consistent geographical trends in L₅₀ on the east coast and the Chatham Rise. L₅₀ was consistently low (28–32 cm) near major spawning grounds and increased to high values (>34 cm) far from these grounds. These trends increase the knowledge of orange roughy spawning migrations in two important fishing areas.