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The feeding ecology of longtail tuna was studied in northern and eastern Australia. Diet biomass data were used to estimate daily ration and consumption of individual prey taxa, particularly penaeids targeted by Australia's valuable Northern Prawn Fishery (NPF). Overall, the 497 stomachs contained 101 prey taxa. In both regions, small pelagic and demersal fishes comprised the majority of the diet biomass. Fish in both regions showed a marked increase in prey diversity, variation in prey composition and stomach fullness index in autumn and winter (March-August). This increase in apparently opportunistic feeding behaviour and feeding intensity showed an inverse relationship with reproductive activity, indicating a possible energy investment for gonad development. Daily ration decreased with increasing fish size, while annual consumption by fish increased with size. Total prey consumption in the Gulf of Carpentaria was estimated at 148 178 t year −1 . This includes 599 t year −1 of penaeids, equivalent to 11% of the annual NPF catch. This study demonstrated that longtail tuna play an important ecological role in neritic ecosystems. Their interaction with commercial fisheries highlights the need for targeted dietary studies of high order predators to better understand trophic pathways to facilitate ecosystem-based fisheries management.

Griffiths, S. P., Kuhnert, P. M., Fry, G. F., and Manson, F. J. 2009. Temporal and size-related variation in the diet, consumption rate, and daily ration of mackerel tuna (Euthynnus affinis) in neritic waters of eastern Australia. – ICES Journal of Marine Science, 66: 720–733. The diet, food consumption, and ration of mackerel tuna (Euthynnus affinis) were studied in Australian neritic waters. Overall, 43 prey taxa were identified from 271 stomachs. The diet was primarily pelagic clupeoids (78% by wet weight, WW; 71% by frequency of occurrence, FO) and demersal fish (19% WW; 32% FO). Multivariate regression tree analysis revealed that temporal differences, followed by fish size, explained most of the variation in the diet composition. Autumn diets differed from those in other seasons because tuna ate virtually only engraulids then. During other seasons, engraulids were still the dominant taxon in the diet, but fish also consumed a greater variety of other prey. Small tuna seemed to target small pelagic crustaceans and teleosts, and medium and large tuna to consume larger pelagic and demersal teleosts. Prey consumption increased with tuna size from 26.42 to 108.03 g d−1 for small and large tuna, respectively. Conversely, daily ration decreased with increasing tuna size from 4.10 to 1.95% body weight per day for medium and large tuna, respectively. Mackerel tuna consumed an estimated 25 036 t year−1 in the study region (170 990 km2). Diet studies are becoming increasingly important in informing ecosystem models, and this study confirmed the need for sampling regimes to capture temporal and size-related variation in diet composition, to maximize the utility of data for use in such models.

Griffiths, S. P., Fry, G. C., Manson, F. J., and Lou, D. C. 2010. Age and growth of longtail tuna (Thunnus tonggol) in tropical and temperate waters of the central Indo-Pacific. – ICES Journal of Marine Science, 67: 125–134. Age and growth of longtail tuna (Thunnus tonggol) were assessed by examination of annual growth increments in sectioned sagittal otoliths from 461 fish (238–1250 mm fork length, LF) sampled from tropical and temperate waters in the central Indo-Pacific between February 2003 and April 2005. Edge and microincrement analyses (presumed daily increments) suggest that longtail tuna deposit a single annual growth increment mainly between August and October. Age was, therefore, estimated for all fish by counting assumed annual growth increments. Ages ranged from 154 d to 18.7 years, with most fish being 3–9 years. Five growth models were fitted to length-at-age data, all of which indicated that the species is relatively slow-growing and long-lived. Recaptures of two tagged fish at liberty for 6.2 and 10.5 years support this notion. A bias-corrected form of Akaike's Information Criterion determined that the Schnute–Richards model provided the best fit to length-at-age data, with model parameter estimates (sexes combined) of L∞ = 135.4 cm LF, K = 22.3 year−1, t0 = 0.120 years, δ = 150.0, v = 0.019, and γ = 2.7 × 10−8. There was no significant difference in growth between sexes. The results suggest that longtail tuna grow more slowly and live longer than other tuna species of similar size. Coupled with their restricted neritic distribution, longtail tuna may be vulnerable to overexploitation by fisheries, and caution needs to be exercised in managing the species until more reliable biological and catch data are collected to assess the status of the population.

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Coastal habitats are critical to the sustained production of many fisheries. It is important, therefore, that fishery managers obtain accurate estimates of the extent of these habitats. This study investigated three methods of estimating the linear extent and area of mangroves (commercially available topographic data, aerial photographs and Landsat Thematic Mapper satellite imagery) in two regions in northern Australia: the Joseph Bonaparte Gulf (in particular, the Berkeley and Lyne Rivers) with typically narrow fringes of mangroves (<50 m wide), and the Embley River on Cape York Peninsula, with much broader mangrove stands (50-1000 m wide). Ground-truthing verified that aerial photographs provided the most accurate estimates of extents of mangroves in all rivers, because of their high spatial resolution (2 m). Landsat Thematic Mapper imagery gave good estimates of the area of habitats, but, because of the 30 m pixel resolution, it underestimated the linear extent in places where the mangrove fringe was narrow. Topographic data gave good estimates of the extent of mangroves where the forests were more extensive and less linear in shape, but were very poor otherwise at this scale. These findings have implications for the use of remote sensing techniques in ecological studies in these regions.

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