Automatic Bluefin Tuna Sizing with a Combined Acoustic and Optical Sensor.

A proposal is described for an underwater sensor combining an acoustic device with an optical one to automatically size juvenile bluefin tuna from a ventral perspective. Acoustic and optical information is acquired when the tuna are swimming freely and the fish cross our combined sensor's field of view. Image processing techniques are used to identify and classify fish traces in acoustic data (echogram), while the video frames are processed by fitting a deformable model of the fishes' ventral silhouette. Finally, the fish are sized combining the processed acoustic and optical data, once the correspondence between the two kinds of data is verified. The proposed system is able to automatically give accurate measurements of the tuna's Snout-Fork Length (SFL) and width. In comparison with our previously validated automatic sizing procedure with stereoscopic vision, this proposal improves the samples per hour of computing time by 7.2 times in a tank with 77 juveniles of Atlantic bluefin tuna (Thunnus thynnus), without compromising the accuracy of the measurements. This work validates the procedure for combining acoustic and optical data for fish sizing and is the first step towards an embedded sensor, whose electronics and processing capabilities should be optimized to be autonomous in terms of the power supply and to enable real-time processing.

Age and growth of slender tuna (Allothunnus fallai) in an unexploited temperate population.

Many large predatory fishes are in decline and tuna sustainability is high on the global agenda. Slender tuna (SLT), Allothunnus fallai, is data-poor and a rare contemporary example of a globally unexploited temperate tuna. This study analysed 214 otoliths for age and growth of fish collected in the South Atlantic. Observed ages varied between 9 and 42 years for a size range of 68-90 cm fork length. We reveal important life history data for SLT before exploitation and underline the relevance of data-poor stocks in understanding wider questions for exploited tuna.

Biomechanical Analysis of the Slow-Twitch (Red) Muscle Force Transmission Pathways in Tunas.

In tunas, the slow-twitch red muscle, which has an elevated temperature, powers thunniform locomotion, a stiff-bodied swimming style. The anatomical placement and operating temperatures of red muscle vary widely among teleosts: in tunas, the red muscle is located centrally in the body, adjacent to the spine, and maintains an elevated temperature. In the majority of ectothermic teleosts, red muscle is located laterally in the body, adjacent to the skin, and operates at ambient temperature. The specialized physiology and biomechanics of red muscle in tunas are often considered important adaptations to their high-performance pelagic lifestyle; however, the mechanics of how muscular work is transmitted to the tail remains largely unknown. The red muscle has a highly pennate architecture and is connected to the spine through a network of bones (epicentral bones) and long tendons (posterior oblique tendons). The network of long tendons has been hypothesized to enhance the power transmitted to the tail. Here, we investigate the morphology and biomechanics of the tuna's red muscle and tendons to determine whether elasticity is exploited to reduce the cost of transport, as is the case in many terrestrial vertebrates. To address this question, we evaluate two hypotheses: (1) tendons stretch during red-muscle-actuated swimming and (2) tendons comprise the primary load transmission pathway from the red muscle to the spine. To evaluate these hypotheses, we measured the mechanical properties of the posterior oblique tendons and performed novel dissections to estimate the peak force that the red muscle can generate. The force-generating capacity of the red muscle is calculated to be much greater than the load-bearing capacity of the posterior oblique tendons. Thus, the long tendons likely stretch under force from the red muscle, but they are not strong enough to be the primary force transmission pathway. These results suggest that other pathways, such as serial load transmission through the red muscle myomeres to the great lateral tendon and/or the anterior oblique tendons to the skin, transmit appreciable force to the tail.

Tunas as a high-performance fish platform for inspiring the next generation of autonomous underwater vehicles.

Tunas of the genus Thunnus are a group of high-performance pelagic fishes with many locomotor traits that are convergently shared with other high-performance fish groups. Because of their swimming abilities, tunas continue to be an inspiration for both comparative biomechanics and the design of biomimetic autonomous underwater vehicles (AUVs). Despite the strong history of studies in tuna physiology and current interest in tuna biomechanics and bio-inspired design, we lack quantitative data on the function of many features of tunas. Here we present data on the morphology, behavior, and function of tunas, focusing especially on experimentally examining the function of tuna lateral keels, finlets, and pectoral fins by using simple physical models. We find that both triangular lateral keels and flexible finlets decrease power requirements during swimming, likely by reducing lateral forces and yaw torques (compared to models either without keels or with rectangular keels, and models with stiff finlets or strip fins of equal area, respectively). However, both triangular keels and flexible finlets generate less thrust than other models either without these features or with modified keels or finlets, leading to a tradeoff between power consumption and thrust. In addition, we use micro computed tomography (µCT) to show that the flexible lateral keels possess a lateral line canal, suggesting these keels have a sensory function. The curved and fully-attached base of tuna pectoral fins provides high lift-to-drag ratio at low angles of attack, and generates the highest torques across speeds and angles of attack. Therefore, curved, fully-attached pectoral fins grant both better gliding and maneuvering performance compared to flat or curved, partially-attached designs. We provide both 3D models of tuna morphology derived from µCT scans and conclusions about the performance effects of tuna-like features as a resource for future biological and engineering work for next-generation tuna-inspired AUV designs.

Evidence of spawning by hermaphroditic skipjack tuna Katsuwonus pelamis.

Hermaphroditic gonads were collected from three skipjack tuna Katsuwonus pelamis in the western and central Pacific Ocean. Both ovarian and testicular tissues were found in the same gonad lobe for each of the three fish. Histological sections of the hermaphroditic gonad from one K. pelamis showed postovulatory follicles, which is evidence of spawning.

Relationship between weight and linear dimensions of Bluefin tuna (Thunnus thynnus) following fattening on western Mediterranean farms.

This study presents various models based on formulae relating weight and dimensions (length, height and width) of Bluefin tuna, Thunnus thynnus (L.), fattened in captivity. The main aim of establishing these expressions is to design tools for indirectly predicting the weight of a Bluefin tuna from measurements of one or more dimensions obtained using non-invasive methods such as stereoscopic cameras. Measurements of maximum length, height and width following slaughter were taken of fish fattened in captivity (n = 2078). Different relationships drawn from the dimensions of the tuna against their weight are fitted with part of the data collection and later checked against a reserved sample set. The resulting formulae are compared with the formulae most commonly used in the case of wild tuna. The results of this study confirm that, for tuna fattened in cages, the availability of more than one dimension to estimate weight improves the predictive power of the model and reduces error in the estimate.

Micro-anatomical structure of the first spine of the dorsal fin of Atlantic bluefin tuna, Thunnus thynnus (Osteichthyes: Scombridae).

The first spine of the first dorsal fin (FS) of the Atlantic bluefin tuna (ABFT), Thunnus thynnus, is customarily used in age determination research because its transverse sections display well-defined growth marks. In this paper the FS structure was studied to explain its known dramatic age- and season-related morphological modifications, which are evidently caused by bone remodeling. Cross sections of samples from six adult ABFT were in part decalcified to be stained with histological, histochemical and immunohistochemical methods, and in part embedded in methyl-methacrylate to be either observed under a linear polarized light or microradiographed. FS showed an external compact bone zone and an inner trabecular bone zone. The compact bone zone consisted of an outer non-osteonic primary bone layer (C1) and an inner osteonic bone layer (C2). C1 was in turn characterized by alternate translucent and opaque bands. Evidence of spine bone remodeling was shown by the presence of osteoclasts and osteoblasts as well as by tartrate-resistant acid phosphatase (TRAP) positive bands at the boundary between old and newly formed bone. The examination of plain, i.e. not-fixed and not-decalcified, FS from 28 ABFT showed that the average thickness of C1 remained fairly constant during fish growth, whereas C2 increased significantly, indicating that the periosteal primary bone apposition is counterbalanced by the parallel bone remodeling occurring inside the compact bone zone. The present study revealed the structure of the ABFT FS and the pattern of its bone remodeling. Both of them underlay phenomena, never examined in detail before, such as the appearance followed by the progressive disappearance of growth bands.

Scale diversity in bigeye tuna (Thunnus obesus): Fat-filled trabecular scales made of cellular bone.

Tunas of the genus Thunnus possess many morphological and physiological adaptations for their high-performance epipelagic ecology. Although Thunnus anatomy has been studied, there are no quantitative studies on the structure of their scales. We investigated the scales of bigeye tuna (Thunnus obesus) from ten regions of the body using micro computed tomography (µCT)-scanning and histology to quantitatively and qualitatively compare regional scale morphology. We found a diversity of scale sizes and shapes across the body of bigeye tuna and discriminant function analysis on variables derived from µCT-data showed that scales across the body differ quantitatively in shape and size. We also report the discovery of a novel scale type in corselet, tail, and cheek regions. These modified scales are ossified shells supported by internal trabeculae, filled with fat, and possessing an internal blood supply. Histological analysis showed that the outer lamellar layers of these thickened scales are composed of cellular bone, unexpected for a perciform fish in which bone is typically acellular. In the fairing region of the anterior body, these fat-filled scales are stacked in layers up to five scales deep, forming a thickened bony casing. Cheek scales also possess a fat-filled internal trabecular structure, while most posterior body scales are more plate-like and similar to typical teleost scales. While the function of these novel fat-filled scales is unknown, we explore several possible hypotheses for their function.

Hydraulic control of tuna fins: A role for the lymphatic system in vertebrate locomotion.

The lymphatic system in teleost fish has genetic and developmental origins similar to those of the mammalian lymphatic system, which is involved in immune response and fluid homeostasis. Here, we show that the lymphatic system of tunas functions in swimming hydrodynamics. Specifically, a musculo-vascular complex, consisting of fin muscles, bones, and lymphatic vessels, is involved in the hydraulic control of median fins. This specialization of the lymphatic system is associated with fish in the family Scombridae and may have evolved in response to the demand for swimming and maneuvering control in these high-performance species.

Bycatch in the Maldivian pole-and-line tuna fishery.

Tropical tuna fisheries are among the largest worldwide, with some having significant bycatch issues. However, pole-and-line tuna fisheries are widely believed to have low bycatch rates, although these have rarely been quantified. The Maldives has an important pole-and-line fishery, targeting skipjack tuna (Katsuwonus pelamis). In the Maldives, 106 pole-and-line tuna fishing days were observed between August 2014 and November 2015. During 161 fishing events, tuna catches amounted to 147 t: 72% by weight was skipjack, 25% yellowfin tuna (Thunnus albacares) and 3% other tunas. Bycatch (all non-tuna species caught plus all tuna discards) amounted to 951 kg (0.65% of total tuna catch). Most of the bycatch (95%) was utilized, and some bycatch was released alive, so dead discards were particularly low (0.02% of total tuna catch, or 22 kg per 100 t). Rainbow runner (Elagatis bipinnulata) and dolphinfish (Coryphaena hippurus) together constituted 93% of the bycatch. Live releases included small numbers of silky sharks (Carcharhinus falciformis) and seabirds (noddies, Anous tenuirostris and A. stolidus). Pole-and-line tuna fishing was conducted on free schools and schools associated with various objects (Maldivian anchored fish aggregating devices [aFADs], drifting FADs from western Indian Ocean purse seine fisheries, other drifting objects and seamounts). Free school catches typically included a high proportion of large skipjack and significantly less bycatch. Associated schools produced more variable tuna catches and higher bycatch rates. Fishing trips in the south had significantly lower bycatch rates than those in the north. This study is the first to quantify bycatch rates in the Maldives pole-and-line tuna fishery and the influence of school association on catch composition. Ratio estimator methods suggest roughly 552.6 t of bycatch and 27.9 t of discards are caught annually in the fishery (based on 2015 national catch), much less than other Indian Ocean tuna fisheries, e.g. gillnet, purse-seine, and longline.

Convergence in Thunniform Anatomy in Lamnid Sharks and Jurassic Ichthyosaurs.

Among extinct ichthyosaurs the Jurassic forms Ichthyosaurus and Stenopterygius share a number of anatomical specializations with lamnid sharks, characterized in the white shark, Carcharodon carcharias These features allow their inclusion within the mode of high-speed thunniform swimming to which only two other equally distinctive phylogenetic groups belong, tuna and dolphins-a striking testaments to evolutionary convergence. Jurassic ichthyosaurs evolved from reptiles that had returned to the sea (secondarily adapted) about 250 million years ago (MYA) while lamnid sharks evolved about 50 MYA from early cartilaginous fishes (originating ca. 400 MYA). Their shared independently evolved anatomical characteristics are discussed. These include a deep tear-drop body shape that helped initially define members as thunniform swimmers. Later, other critical structural characteristics were discovered such as the crossed-fiber architecture of the skin, high-speed adapted dorsal and caudal fins, a caudal peduncle and series of ligaments to enable transmission of power from the musculature located anteriorly to the caudal fin. Both groups also share a similar chemistry of the dermal fibers, i.e., the scleroprotein collagen.

Atlantic Bluefin Tuna (Thunnus thynnus) Biometrics and Condition.

The compiled data for this study represents the first Atlantic and Mediterranean-wide effort to pool all available biometric data for Atlantic bluefin tuna (Thunnus thynnus) with the collaboration of many countries and scientific groups. Biometric relationships were based on an extensive sampling (over 140,000 fish sampled), covering most of the fishing areas for this species in the North Atlantic Ocean and Mediterranean Sea. Sensitivity analyses were carried out to evaluate the representativeness of sampling and explore the most adequate procedure to fit the weight-length relationship (WLR). The selected model for the WLRs by stock included standardized data series (common measurement types) weighted by the inverse variability. There was little difference between annual stock-specific round weight-straight fork length relationships, with an overall difference of 6% in weight. The predicted weight by month was estimated as an additional component in the exponent of the weight-length function. The analyses of monthly variations of fish condition by stock, maturity state and geographic area reflect annual cycles of spawning and feeding behavior. We update and improve upon the biometric relationships for bluefin currently used by the International Commission for the Conservation of Atlantic Tunas, by incorporating substantially larger datasets than ever previously compiled, providing complete documentation of sources and employing robust statistical fitting. WLRs and other conversion factors estimated in this study differ from the ones used in previous bluefin stock assessments.

Gonadogenesis and slow proliferation of germ cells in juveniles of cultured yellowfin tuna, Thunnus albacares.

To develop techniques for seedling production of yellowfin tuna, the behavior of primordial germ cells (PGCs) and gonadogenesis were examined at 1-30 days post hatching (dph) using morphometric analysis, histological examination, and in situ hybridization. Immediately after hatching, PGCs were located on the dorsal side of the posterior end of the rectum under the peritoneum of the larvae, and at 3 dph they came into contact with stromal cells. PGCs and stromal cells gradually moved forward from the anus prior to 5 dph. At 7-10 dph, germ cells were surrounded by stromal cells and the gonadal primordia were formed. In individuals collected at 12 dph, PGCs were detected by in situ hybridization using a vasa mRNA probe that is a germ-cell-specific detection marker. The proliferation of germ cells in the gonadal primordia began at 7-10 dph. We observed double the number of germ cells at 30 dph (22 ± 3.2 cells), compared to that at 1 dph (11 ± 2.1 cells). Therefore, based on our data and previous reports, the initial germ cell proliferation of yellowfin tuna is relatively slower than that of other fish species.

Spawning Dynamics and Size Related Trends in Reproductive Parameters of Southern Bluefin Tuna, Thunnus maccoyii.

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.

Direct ageing of Thunnus thynnus from the eastern Atlantic Ocean and western Mediterranean Sea using dorsal fin spines.

This study deals with important methodology issues that affect age estimates of eastern Atlantic bluefin tuna Thunnus thynnus using dorsal fin spines. Nearly 3800 spine sections were used from fish caught in the north-east Atlantic Ocean and western Mediterranean Sea over a 21 year period. Edge type and marginal increment analyses indicated a yearly periodicity of annulus formation with the translucent bands (50% of occurrence) appearing from October to May. Nucleus vascularization seriously affected specimens older than 6 years, with the disappearance of 40-50% of the presumed annuli by that age. An alternate sectioning location was a clear improvement and this finding is an important contribution to the methodology of using this structure for ageing the full-length range of eastern T. thynnus. Finally, there were no significant differences between the coefficients of von Bertalanffy growth model estimated from mean length at age data (L∞ = 327.4; k = 0.097; t0 = -0.838) and those estimated from the growth curves accepted for the eastern and western T. thynnus management units.

Demographic structure, sex ratio and growth rates of southern bluefin tuna (Thunnus maccoyii) on the spawning ground.

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.

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).

Muscular system in the pacific bluefin tuna Thunnus orientalis (Teleostei: Scombridae).

The muscular system in the Pacific bluefin tuna Thunnus orientalis is studied in detail. For the first time, a complete description of the muscular anatomy of a thunnid is provided here. Eighty-two elements including subdivisions of components of the muscular system are identified. This is less than found in a basal perciform and two other investigated scombrid species, owing mainly to the absence or fusion of pectoral, pelvic and caudal fin muscles. The absence of elements of the basal perciform pattern was most prominent in the caudal fin, which includes only the flexor dorsalis, flexor ventralis, hypochordal longitudinalis, and interradialis. In the caudal fin, the medial fan-shaped ray was identified as the first dorsal ray, judging from myological and neuroanatomical characters. The highly developed gill filament muscles in Thunnus orientalis and sheet-like rectus communis control gill ventilation. Long body muscle tendons reduce the metabolic energy needed during rapid and continuous swimming. These characters are interpreted as adaptations in the context of the oceanic life style of the species.

Trade-based estimation of bluefin tuna catches in the Eastern Atlantic and Mediterranean, 2005-2011.

The Eastern Atlantic and Mediterranean stock of Bluefin tuna Thunnus thynnus (BFTE) has long been considered overfished and at risk of collapse. Although ICCAT quotas for this stock have decreased considerably over the past years, uncertainty exists about the degree of catch beyond this quota. The extent of such catch is an important piece of information in stock assessment models as well as being an indicator of the effectiveness of fisheries management. We present a model using Bluefin tuna trade data to infer actual catches. Basing our calculations on 25 countries involved in BFTE trade, we estimate that between 2005 and 2011, allowable quotas were exceeded by 44 percent. This gap between catch and quotas has slightly increased over past years, leading to estimated excess catches of 57 percent for the period between 2008 and 2011. To improve assessments, preparation and design of BFTE management, we suggest that the estimated total removals reported in this paper be included in stock assessment models for BFTE. An implication of our findings is that ICCAT member states should take stronger measures to monitor and enforce compliance with quotas.

Reproductive biology of female bigeye tuna Thunnus obesus in the western Pacific Ocean.

The reproductive biology of female bigeye tuna Thunnus obesus was assessed by examining 888 fish (ranging from 84·9 to 174·4 cm fork length, LF ) caught by Taiwanese offshore longliners in the western Pacific Ocean from November 1997 to November 1998 and November to December 1999 and 258 gonad samples from these fish. The overall sex ratio of the catch during the sampling differed significantly from 0·5, but males were predominant in sizes >140 cm LF . Reproductive activity (assessed by histology), a gonado-somatic index, and the size-frequency distributions of whole oocytes indicated that spawning occurred throughout the year and the major spawning season appeared to be from February to September. The estimated sizes at 50% maturity (LF50 ) of females was 102·85 cm (95% c.i.: 90·79-110·21 cm) and the smallest mature female was 99·7 cm LF . They are multiple spawners and oocytes develop asynchronously. The proportion of mature (0·63) and reproductively active (0·70) females with ovaries containing postovulatory follicles indicated that they spawn almost daily. Batch fecundity for 15 females with the most advanced oocytes (>730 µm) ranged from 0·84 to 8·56 million eggs (mean ± s.d. = 3·06 ± 2·09). The relationships between batch fecundity (FB , in millions of eggs) and LF (cm) and round mass (MR , kg) were FB=9·91×10-14LF6·38 (r(2) = 0·84) and FB=8·89×10-4MR2·05 (r(2) = 0·80), respectively. The parameters estimated in this study are key information for stock assessments of T. obesus in the western Pacific Ocean and will contribute to the conservation and sustainable yield of this species.