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.
Highlights
The bluefin tunas (Thunnus thynnus, T. orientalis, and T. maccoyii) are often regarded as the emblematic ocean wanderers amongst teleosts
If we assume that Southern bluefin tuna (SBT) classified as likely non-spawners had undertaken migrations with a comparable duration to those SBT whose spawning migrations were observed, arrival in the region of the spawning ground was estimated to have occurred between March and June (Table 1)
Movement patterns of SBT tagged in the Tasman Sea region are clearly highly variable: individuals likely to be mature undertook spawning migrations, and remained in the Tasman Sea throughout the year
Summary
The bluefin tunas (Thunnus thynnus, T. orientalis, and T. maccoyii) are often regarded as the emblematic ocean wanderers amongst teleosts. Tropical conditions that favour larval thermal requirements may involve physiological challenges for reproducing individuals. The cold-adapted physiology of bluefin tunas may result in individuals undergoing significant thermal stress when inhabiting tropical spawning habitats [7,8] adding to the challenges imposed on individuals associated with migration. As long-lived species (up to 40 yr; [1,9,10]), the life history strategy of bluefin tunas involves trade-offs between energetically costly spawning migrations and expected life-time reproductive output [11]. It could be expected that reproductive schedules in mature individuals might be somewhat flexible in their timing in response to individual condition. Empirical data on spawning migrations to confirm or refute such a strategy has been lacking
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