Relationship between fork length and spawning duration (A) and resting duration (B).

<p>90% confidence intervals of the estimate are shown.</p

Histological sections of ovaries showing examples of the reproductive phases.

<p>(A) non-spawning female with advanced yolked (AY) oocytes, alpha (α) atresia, but no evidence of imminent or recent spawning activity; (B) actively spawning female with AY oocytes, post-ovulatory follicles (POFs) but no α atresia; (C) regressing female with unyolked (UY) oocytes and massive atresia of AY oocytes. EY = early yolked oocytes, β = beta atresia. White circles indicate postovulatory follicles. The white scale bars is 500 μm.</p

Relationship between ovary weight and fork length (natural log transformed) for southern bluefin tuna.

<p>Crosses/dotted line are ovaries with advanced yolked oocytes and new or <12 hour POFs (just spawned—<i>after</i> ovaries; n = 176), and dots/solid line are ovaries with hydrated oocytes without new POFs (about to spawn—<i>before</i> ovaries; n = 52).</p

Spawning Dynamics and Size Related Trends in Reproductive Parameters of Southern Bluefin Tuna, <i>Thunnus maccoyii</i>

<div><p>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) <i>Thunnus maccoyii</i> 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.</p></div

Number of females classified by reproductive phase/subphase and level of alpha stage atresia (α).

<p>¹ Reproductive phase could not be determined because the ovaries contained advanced yolked oocytes but postovulatory follicles were indistinguishable due to tissue decay.</p><p>Number of females classified by reproductive phase/subphase and level of alpha stage atresia (α).</p

Estimated batch fecundity estimate relative to a 190 cm southern bluefin tuna.

<p>Estimated batch fecundity estimate relative to a 190 cm southern bluefin tuna.</p

Number of ovaries classified by the most advanced group of oocytes and postovulatory follicle stage present.

<p>MAGO = most advanced group of oocytes, AY = advanced yolked, MN = migratory nucleus, H = hydrated, NA = indistinguishable, POF = postovulatory follicle.</p><p>Number of ovaries classified by the most advanced group of oocytes and postovulatory follicle stage present.</p

Seasonal variation in mean relative condition factor (<i>Kn</i>) (A) and gonad index (<i>GI</i>) (B).

<p>Non-spawning females = ●, actively spawning females = ○. Results are not shown for months where n ≤ 10. Females with hydrated oocytes were excluded from the GI analysis to remove the effect of heavy hydrating ovaries. Dashed line indicates the mean for all females. Error bars are ± 2 S.E.</p

Proportion of females in each reproductive phase and spawning fraction by month.

<p>A-S = actively spawning, N-S = non-spawning, R = regressing, SF = spawning fraction. Sample sizes are indicated at the top. The results are only shown if n ≥ 5.</p

Variation in relative condition factor (<i>Kn)</i> (A) and gonad index (<i>GI</i>) (B) with reproductive phase.

<p>N-S = non-spawning, A-S = actively spawning, and R = regressing. Females with hydrated oocytes were excluded from the GI analysis. The box represents the median and interquartile range, and the vertical lines represent the 95% confidence interval for the median.</p