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Skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tuna are the target species of tropical tuna fisheries in the Indian Ocean, with high commercial value in the international market. High fishing pressure over the past three decades has raised concerns about their sustainability. Understanding life history strategies and stock structure is essential to determine species resilience and how they might respond to exploitation. Here we provide a comprehensive review of available knowledge on the biology, ecology, and stock structure of tropical tuna species in the Indian Ocean. We describe the characteristics of Indian Ocean tropical tuna fisheries and synthesize skipjack, yellowfin, and bigeye tuna key life history attributes such as biogeography, trophic ecology, growth, and reproductive biology. In addition, we evaluate the available literature about their stock structure using different approaches such as analysis of fisheries data, genetic markers, otolith microchemistry and tagging, among others. Based on this review, we conclude that there is a clear lack of ocean basin-scale studies on skipjack, yellowfin and bigeye tuna life history, and that regional stock structure studies indicate that the panmictic population assumption of these stocks should be investigated further. Finally, we identify specific knowledge gaps that should be addressed with priority to ensure a sustainable and effective management of these species., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Southern bluefin tuna (SBT) is a valuable species that has been subject to high exploitation rates since the 1950s. In 2011, the spawning stock biomass was estimated to be at a historically low level, at only 5% of pre-fished biomass. A key component for managing and rebuilding the stock is having reliable, fishery-independent estimates of juvenile abundance. This paper describes how such estimates have been constructed from aerial surveys of juvenile (age 2-4) SBT conducted annually in the Great Australian Bight from 1993-2000 and 2005-2009. During these surveys, observers flew along pre-set transect lines searching for surface schools of SBT. Data were collected on the location and biomass of SBT sightings, and on the environmental conditions present during the survey. Sea surface temperature (SST) was found to correlate with the size (biomass) of schools, and several environmental variables, SST and wind speed in particular, were found to correlate with the number of sightings (presumably by affecting the ability of observers to see surface schools as well as whether fish were present at the surface). In addition, observers changed over time and differed in their aptitude for spotting tuna. Thus, generalized linear mixed models (GLMMs) were used to standardize the sightings and biomass data to a common set of observers and environmental conditions in order to produce an annual time series of relative abundance estimates. These estimates, which form one of two key inputs to the management procedure used by the international Commission for the Conservation of Southern Bluefin Tuna to set the global catch quota, suggest juvenile abundance was highest in the first years of the survey (1993-1996), after which it declined and fluctuated around a level about four times lower., Competing Interests: The authors have declared that no competing interests exist.

Fatty acids are among the least understood nutrients in marine environments, despite their profile as key energy components of food webs and that they are essential to all life forms. Presented here is a novel approach to predict the spatial-temporal distributions of fatty acids in marine resources using generalized additive mixed models. Fatty acid tracers (FAT) of key primary producers, nutritional condition indices and concentrations of two essential long-chain (≥C20) omega-3 fatty acids (EFA) measured in muscle of albacore tuna, Thunnus alalunga, sampled in the south-west Pacific Ocean were response variables. Predictive variables were: location, time, sea surface temperature (SST) and chlorophyll-a (Chla), and phytoplankton biomass at time of catch and curved fork length. The best model fit for all fatty acid parameters included fish length and SST. The first oceanographic contour maps of EFA and FAT (FATscapes) were produced and demonstrated clear geographical gradients in the study region. Predicted changes in all fatty acid parameters reflected shifts in the size-structure of dominant primary producers. Model projections show that the supply and availability of EFA are likely to be negatively affected by increases in SST especially in temperate waters where a 12% reduction in both total fatty acid content and EFA proportions are predicted. Such changes will have large implications for the availability of energy and associated health benefits to high-order consumers. Results convey new concerns on impacts of projected climate change on fish-derived EFA in marine systems.

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.

Commercial aerial spotting of surface schools of juvenile southern bluefin tuna (SBT), Thunnus maccoyii, is conducted as part of fishing operations in the Great Australian Bight in summer. This provides the opportunity to efficiently collect large amounts of data on sightings of SBT. The data can potentially be used to construct a time-series index of relative abundance by standardising the data for issues such as weather, spotter ability and ocean conditions. Unlike a statistically designed survey, the commercial spotting is governed by business considerations and fishing operations. The SBT dataset is therefore highly unbalanced with regard to spotters operating in each season. This complicates the standardisation of the data, particularly with regard to interactions between covariates. We show how a generalized additive model with random effects can simplify both the fitting of the model and the construction of an index, while also avoiding the need to leave out strata or interaction terms that are important. The approach is applicable to standardisation of more traditional catch and effort data.

The demographics of the southern bluefin tuna (SBT) Thunnus maccoyii spawning stock were examined through a large-scale monitoring program of the Indonesian longline catch on the spawning ground between 1995 and 2012. The size and age structure of the spawning population has undergone significant changes since monitoring began. There has been a reduction in the relative abundance of larger/older SBT in the catch since the early 2000s, and a corresponding decrease in mean length and age, but there was no evidence of a significant truncation of the age distribution. Pulses of young SBT appear in the catches in the early- and mid-2000s and may be the first evidence of increased recruitment into the spawning stock since 1995. Fish in these two recruitment pulses were spawned around 1991 and 1997. Size-related variations in sex ratio were also observed with female bias for fish less than 170 cm FL and male bias for fish greater than 170 cm FL. This trend of increasing proportion of males with size above 170 cm FL is likely to be related to sexual dimorphism in growth rates as male length-at-age is greater than that for females after age 10 years. Mean length-at-age of fish aged 8-10 years was greater for both males and females on the spawning ground than off the spawning ground, suggesting that size may be the dominant factor determining timing of maturation in SBT. In addition to these direct results, the data and samples from this program have been central to the assessment and management of this internationally harvested stock.

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).

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.

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.

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). [ABSTRACT FROM AUTHOR]

Highlights: [•] We measured trace element concentrations in South Pacific albacore otoliths. [•] Classification accuracy to geographically distant capture locations was high. [•] Some evidence of different larval sources for the west and east South Pacific. [•] Cycling in otolith Sr:Ca and Ba:Ca was observed across life-history transects. [•] Patterns may reflect seasonal north-south movements across oceanic fronts. [Copyright &y& Elsevier]

Williams, A. J., Leroy, B. M., Nicol, S. J., Farley, J. H., Clear, N. P., Krusic-Golub, K., and Davies, C. R. Comparison of daily- and annual-increment counts in otoliths of bigeye (Thunnus obesus), yellowfin (T. albacares), southern bluefin (T. maccoyii) and albacore (T. alalunga) tuna. – ICES Journal of Marine Science, 70: .Information on the age of individuals is often required for models assessing the status of stocks. Techniques used to estimate age of tuna have varied across species and agencies, precluding meta-analyses of age and growth. We compared age estimates obtained from commonly used ageing techniques for four important tuna species: bigeye tuna, yellowfin tuna, southern bluefin tuna, and albacore tuna. Estimates of age from counts of annual increments in transverse-sectioned otoliths were generally higher than those from counts of daily increments in transverse and longitudinal sections for all species, particularly for fish older than two years. However, annual counts produced younger estimates, on average, relative to daily counts for bigeye and yellowfin tuna younger than one year. Estimates derived from daily increments in longitudinal and transverse sections were generally similar, although longitudinal sections produced relatively older age estimates for individuals older than two years. A linear or non-linear increase in the magnitude of differences between ageing methods was the best-approximating model in all cases except when comparing daily-increment counts between transverse and longitudinal otolith sections for southern bluefin tuna. These observations are consistent with a narrowing of daily increments with increasing age, resulting in underestimates of age relative to those derived from annual increments. We conclude that (i) daily increments are unsuitable for ageing individuals over two years, especially for southern bluefin and albacore, (ii) longitudinal sections are more precise and produce older age estimates than transverse sections for daily-age estimates, (iii) there are considerable differences in these trends between species, likely dependent on longevity, and (iv) parameter estimates and/or conclusions based on meta-analyses using age data derived from different ageing methods are likely confounded with methodological biases. This result demonstrates that greater effort is required to provide consistent, validated methods for routine age determination to support the assessment and management of these valuable populations. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Otolith-based age estimates were obtained for 2769 southern bluefin tuna (SBT) (Thunnus maccoyii) caught on feeding grounds in the southern oceans and their spawning ground in the north-east Indian Ocean between 1985 and 1998. Ages ranged from 0 to 41 years, and males were on average larger at age than females after age 6 for fish sampled in the southern oceans. This sexual dimorphism in growth contributed to the sex ratio being biased toward males for length classes ≥170cm FL. The estimated age composition of catches, derived from age-length keys applied to length frequency data, showed clear differences between fisheries and fishing grounds that are consistent with seasonal and ontogenetic changes in migration patterns. Specific information that can be gained on the distribution and migration of juveniles from their relative abundance in catches is limited because of unknown targeting or discarding practices on some fishing grounds. However, the occurrence of 2–4-year-old SBT on opposite sides of the Indian Ocean during the austral summer confirms that juveniles are not restricted to the southern coastal waters of Australia, and that a divergent migration path must exist possibly near the southern west coast of Australia. During the winter months, SBT aged 2–4 years were caught on all fishing grounds south of the spawning ground, confirming that they are capable of extensive migrations. By age 5, SBT were rarely caught north of about 35°S, except in waters adjacent to Australia''s west coast where adults were caught migrating to/from the spawning ground. The commercial catch of SBT in the southern oceans was dominated by juveniles and sub-adults on most fishing grounds examined. Off north-east New Zealand, however, the majority of fish caught were of spawning age (61% were >10-year old) suggesting that the Tasman Sea may form the eastern boundary for juvenile migration. SBT caught on the spawning ground ranged in age from 8 to 34 years with the majority being between 15- and 25-years old. [Copyright &y& Elsevier]

Skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tuna are the target species of tropical tuna fisheries in the Indian Ocean, with high commercial value in the international market. High fishing pressure over the past three decades has raised concerns about their sustainability. Understanding life history strategies and stock structure is essential to determine species resilience and how they might respond to exploitation. Here we provide a comprehensive review of available knowledge on the biology, ecology, and stock structure of tropical tuna species in the Indian Ocean. We describe the characteristics of Indian Ocean tropical tuna fisheries and synthesize skipjack, yellowfin, and bigeye tuna key life history attributes such as biogeography, trophic ecology, growth, and reproductive biology. In addition, we evaluate the available literature about their stock structure using different approaches such as analysis of fisheries data, genetic markers, otolith microchemistry and tagging, among others. Based on this review, we conclude that there is a clear lack of ocean basin-scale studies on skipjack, yellowfin and bigeye tuna life history, and that regional stock structure studies indicate that the panmictic population assumption of these stocks should be investigated further. Finally, we identify specific knowledge gaps that should be addressed with priority to ensure a sustainable and effective management of these species. [ABSTRACT FROM AUTHOR]

Broad-scale food web inferences of 534 albacore tuna, Thunnus alalunga , in the south-west Pacific Ocean in 2009 and 2010 were made using bulk nitrogen (δ 15 N) and carbon (δ 13 C) stable isotopes. Condition was also examined for the same fish using C:N ratios. A Generalized Additive Modeling (GAM) approach was used to analyze spatio-temporal, biological and environmental drivers that impact the distribution of tuna isotopes and to create oceanographic maps. Based on model formulations, five bioregions with distinct isotopic signatures were identified and were related to known biological, nutrient cycling and oceanographic (temperature, primary productivity and eddy) features associated with the East Australian Current. δ 15 N values showed a large-scale, uniform latitudinal gradient decreasing from the south to north, in a region encompassing oligotrophic waters in the Coral Sea. In contrast, δ 13 C values were lower in the nutrient rich Tasman Sea waters and offshore East Australia. C:N ratios suggested that albacore occupying southern and offshore waters were in better condition. Ontogenetic trends in all three biochemical parameters were identified and related to differences in size distribution. Regional-specific temporal variations were detected including similar monthly changes for both isotopes and significantly less enriched δ 13 C (by 1.9‰) than in previous work undertaken in 2006, potentially signifying a substantial shift in the carbon cycle that supports food webs off central east Australia. Our results showed that isotopic measurements in tuna and the GAM framework provide powerful tools to assess ecosystem functioning and change by linking sources of nutrients and organic matter to local food web assembly. Such knowledge is vital to support an ecosystem based approach to fisheries management including biogeochemical whole-of-ecosystem models and monitoring programs at regional and landscape-scales. [ABSTRACT FROM AUTHOR]