Abstract
Knowledge of spawning behaviour and fecundity of fish is important for estimating the reproductive potential of a stock and for constructing appropriate statistical models for assessing sustainable catch levels. Estimates of length-based reproductive parameters are particularly important for determining potential annual fecundity as a function of fish size, but they are often difficult to estimate reliably. Here we provide new information on the reproductive dynamics of southern bluefin tuna (SBT) Thunnus maccoyii through the analysis of fish size and ovary histology collected on the spawning ground in 1993–1995 and 1999–2002. These are used to refine previous parameter estimates of spawning dynamics and investigate size related trends in these parameters. Our results suggest that the small SBT tend to arrive on the spawning ground slightly later and depart earlier in the spawning season relative to large fish. All females were mature and the majority were classed as spawning capable (actively spawning or non-spawning) with a very small proportion classed as regressing. The fraction of females spawning per day decreased with fish size, but once females start a spawning episode, they spawned daily irrespective of size. Mean batch fecundity was estimated directly at 6.5 million oocytes. Analysis of ovary histology and ovary weight data indicated that relative batch fecundity, and the duration of spawning and non-spawning episodes, increased with fish size. These reproductive parameter estimates could be used with estimates of residency time on the spawning ground as a function of fish size (if known) and demographic data for the spawning population to provide a time series of relative annual fecundity for SBT.
Highlights
Total egg production is considered a more accurate representation of the reproductive potential of a species than spawning stock biomass as the size composition of the spawning population, and changes in annual fecundity with fish size/age, are taken into account in the PLOS ONE | DOI:10.1371/journal.pone.0125744 May 18, 2015Southern bluefin tuna (SBT) Spawning DynamicsDevelopment Corporation (Project #1999/106)
The largest females (!180 cm fork length (FL)) were sampled over a broader 6 month period from October through to March, with a small proportion caught both prior to (August and September) and after (April and May) the main spawning period (Fig 2D).These results suggest that the smaller individuals tend to arrive slightly later and depart earlier in the spawning season than the largest individuals
This study found that large females are caught on the spawning ground over a longer period relative to smaller fish; large fish appeared as early as August/September albeit in relatively small numbers
Summary
Total egg production is considered a more accurate representation of the reproductive potential of a species than spawning stock biomass as the size (or age) composition of the spawning population, and changes in annual fecundity with fish size/age, are taken into account in the PLOS ONE | DOI:10.1371/journal.pone.0125744 May 18, 2015. An understanding of the relationships between reproductive parameters, such as spawning frequency, batch fecundity and spawning duration, with fish length are required to estimate potential annual fecundity at size for a multiple-spawning species with indeterminate fecundity These data are rarely available for highly migratory species, such as tunas, and are difficult to estimate reliably for species where individuals migrate to and from spawning areas at different times during the spawning season [3, 4]. Very little information was obtained on the smaller SBT that appear on the spawning ground It is these fish that are in the process of recruiting into the spawning population and are likely to provide influential data on the relationship between fish size and egg production. The increased number of small SBT in the catches on the spawning ground from the early-2000s meant that it was possible to obtain sufficient numbers of small fish to improve parameter estimation for these smaller size classes
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