Reproductive schedules in southern bluefin tuna: are current assumptions appropriate?

Abstract

Southern bluefin tuna (SBT) appear to comprise a single stock that is assumed to be both mixed across its distribution and having reproductive adults that are obligate, annual spawners. The putative annual migration cycle of mature SBT consists of dispersed foraging at temperate latitudes with migration to a single spawning ground in the tropical eastern Indian Ocean. Spawning migrations have been assumed to target two peaks in spawning activity; one in September-October and a second in February-March. SBT of sizes comparable to that of individuals observed on the spawning ground were satellite tagged in the Tasman Sea region (2003-2008) and demonstrated both migrations to the spawning grounds and residency in the Tasman Sea region throughout the whole year. All individuals undertaking apparent spawning migrations timed their movements to coincide with the second recognised spawning peak or even later. These observations suggest that SBT may demonstrate substantial flexibility in the scheduling of reproductive events and may even not spawn annually as currently assumed. Further, the population on the spawning grounds may be temporally structured in association with foraging regions. These findings provide new perspectives on bluefin population and spatial dynamics and warrant further investigation and consideration of reproductive schedules in this species.

Figure 1. Length frequencies of SBT caught by the Indonesian fishery on the spawning ground and SBT tagged in this study.

Figure 2. Movement paths of fish categorized by putative spawning behaviour

. (a) spawners showing movements from the tagging region to the spawning grounds (defined in blue) (b) likely spawners which made large westward migrations; (c) likely-non spawners remained in the Tasman Sea region until late in the spawning season and (d) non-spawners which remained resident in the Tasman for a full spawning cycle.