Jessica H. Farley

Age, growth and preliminary estimates of maturity of bigeye tuna, Thunnus obesus, in the Australian region

Biological parameters such as age, growth and age (or size) at maturity are vital for accurate stock assessments and management plans to ensure that fisheries develop sustainably. Despite this, very few validated age studies have been conducted for large tropical pelagic species within the Australian region. Age and growth parameters were estimated for bigeye tuna, Thunnus obesus (Lowe, 1839), sampled from longline fisheries in the Australian region using validated techniques based on counts of annual increments. Poor increment clarity reduced the number of otoliths included in the final analysis to only 50% of the 3200 selected for reading (39–178-cm fork length). Microincrement analysis confirmed the position of the first two annual increments in these otoliths.A maximum age of 16 years was obtained, but over 80% of fish in the Australian catch were <5 years old. Growth is most rapid in the first few years of life and asymptotic length is reached at about age 9 to 10 years. The von Bertalanffy growth parameters were estimated at L∞ =169.09, k=0.238, and to=−1.706 for the south-west Pacific Ocean and L∞ =178.41, k=0.176, and to=−2.500 for the eastern Indian Ocean. These parameters were significantly different, suggesting that there is little mixing between populations in the Pacific and Indian Oceans. Length at 50% maturity for females sampled in northern Queensland was estimated to be 102.4-cm fork length.

Spatial and Sex-Specific Variation in Growth of Albacore Tuna (Thunnus alalunga) across the South Pacific Ocean

Spatial variation in growth is a common feature of demersal fish populations which often exist as discrete adult sub-populations linked by a pelagic larval stage. However, it remains unclear whether variation in growth occurs at similar spatial scales for populations of highly migratory pelagic species, such as tuna. We examined spatial variation in growth of albacore Thunnus alalunga across 90° of longitude in the South Pacific Ocean from the east coast of Australia to the Pitcairn Islands. Using length-at-age data from a validated ageing method we found evidence for significant variation in length-at-age and growth parameters (L ∞ and k) between sexes and across longitudes. Growth trajectories were similar between sexes up until four years of age, after which the length-at-age for males was, on average, greater than that for females. Males reached an average maximum size more than 8 cm larger than females. Length-at-age and growth parameters were consistently greater at more easterly longitudes than at westerly longitudes for both females and males. Our results provide strong evidence that finer spatial structure exists within the South Pacific albacore stock and raises the question of whether the scale of their “highly migratory” nature should be re-assessed. Future stock assessment models for South Pacific albacore should consider sex-specific growth curves and spatial variation in growth within the stock.

Reproductive Dynamics and Potential Annual Fecundity of South Pacific Albacore Tuna (Thunnus alalunga)

The reproductive biology of albacore tuna, Thunnus alalunga, in the South Pacific Ocean was investigated with samples collected during broad-scale sampling between 2006 and 2011. Histology was done in a single laboratory according to standard protocols and the data analysed using generalized linear mixed-effects models. The sex ratio of albacore was female biased for fish smaller than approximately 60 cm FL and between 85 and 95 cm, and progressively more male biased above 95 cm FL. Spawning activity was synchronised across the region between 10°S and 25°S during the austral spring and summer where sea surface temperatures were ≥24 °C. The average gonad index varied among regions, with fish in easterly longitudes having heavier gonads for their size than fish in westerly longitudes. Albacore, while capable of spawning daily, on average spawn every 1.3 days during the peak spawning months of October to December. Spawning occurs around midnight and the early hours of the morning. Regional variation in spawning frequency and batch fecundity were not significant. The proportion of active females and the spawning fraction increased with length and age, and mature small and young fish were less active at either end of the spawning season than larger, older fish. Batch fecundity estimates ranged from 0.26 to 2.83 million oocytes with a mean relative batch fecundity of 64.4 oocytes per gram of body weight. Predicted batch fecundity and potential annual fecundity increased with both length and age. This extensive set of reproductive parameter estimates provides many of the first quantitative estimates for this population and will substantially improve the quality of biological inputs to the stock assessment for South Pacific albacore.

Reproductive dynamics of broadbill swordfish, Xiphias gladius, in the domestic longline fishery off eastern Australia

The reproductive ecology of broadbill swordfish, Xiphias gladius, was evaluated based on 1437 fish (size range 50–300 cm orbital fork length [OFL]) caught in the domestic longline fisheries off eastern Australia and New Zealand between May 1999 and March 2001. Reproductive activity was assessed using histology, a gonadosomatic index, and maximum oocyte size. Males were significantly smaller than females and represented less than one third of the sampled fish. Sex ratio differed significantly with respect to fish size and time of year. Females began maturing at 150-cm OFL and spawned from September to March, with the greatest activity from December to February. Males matured at 90-cm OFL; ripe males were found from January to March, but also in May and October, suggesting an extended reproductive period. During the spawning period the proportion of spawning to inactive mature sized females was significantly higher in waters west of longitude 158°E than in waters to the east. Further to the east, samples taken from the New Zealand fishery showed no actively spawning fish during the main spawning period. Females were increasingly reproductively active as water temperature increased beyond 24°C and sea surface chlorophyll a decreased below 0.2 μg L–1. Batch fecundity was linearly related to fish length with a mean batch fecundity of 1.66 million oocytes for females ranging in size from 173- to 232-cm OFL. The presence of hydrated oocytes and post-ovulatory follicles (POFs) in the same ovaries indicated multiple spawnings. Depending on the time taken for POFs to degrade, these may have been daily at the height of the spawning season.

Age estimation and validation for South Pacific albacore Thunnus alalunga

Validated estimates of age are presented for albacore Thunnus alalunga, sampled from a large part of the south-western Pacific Ocean, based on counts of annual opaque growth zones from transverse sections of otoliths. Counts of daily increments were used to estimate the location of the first opaque growth zone, which was completed before the first assumed birthday. The periodicity of opaque zones was estimated by marginal increment analysis and an oxytetracycline mark-recapture experiment. Both validation methods indicated that opaque zones formed over the austral summer and were completed by autumn to winter (April to August). The direct comparison of age estimates obtained from otoliths and dorsal-fin spines of the same fish indicated bias, which was assumed to be due to poor increment clarity and resorption of early growth zones in spines, resulting in imprecise age estimates. As such, age estimates from otoliths are considered to be more accurate than those from spines for T. alalunga. This is consistent with results for a growing number of tropical and temperate tuna Thunnini species. It is recommend that validated counts of annual growth zones from sectioned otoliths is used as the preferred method for estimating age-based parameters for assessment and management advice for these important stocks.

Maturity Ogives for South Pacific Albacore Tuna (Thunnus alalunga) That Account for Spatial and Seasonal Variation in the Distributions of Mature and Immature Fish

Length and age at maturity are important life history parameters for estimating spawning stock biomass and reproductive potential of fish stocks. Bias in estimates of size and age at maturity can arise when disparate distributions of mature and immature fish within a population are not accounted for in the analysis. Here we investigate the spatial and temporal variability in observed size and age at maturity of female albacore tuna, Thunnus alalunga, using samples collected across the South Pacific. Maturity status was identified using consistent histological criteria that were precise enough to allow for mature but regenerating females to be distinguished from immature females during the non-spawning season, permitting year-round sampling for maturity estimation in albacore. Using generalised linear mixed models, we found that the proportion of mature females at length varied significantly with latitude and time of year. Specifically, females at northern latitudes (∼10–20°S, where spawning occurs) were mature at significantly smaller lengths and ages than females at southern latitudes (∼20–40°S), particularly during the spawning season (October–March). This variation was due to different geographic distributions of mature and immature fish during the year. We present a method for estimating an unbiased maturity ogive that takes into account the latitudinal variation in proportion mature at length during a given season (spawning or non-spawning). Applying this method to albacore samples from the western region of the South Pacific gave a predicted length at 50% mature of ∼87 cm fork length (4.5 years).