Mediterranean juvenile bluefin tuna: life history patterns

As there is a lack of information on the growth and migrations of bluefin tuna, information about them was gathered using the structural and chemical characteristics of their otoliths and mercury levels in body tissues as indicators of physiological and habitat characteristics. The otoliths of juvenile tuna caught in the Spanish Mediterranean littoral were studied. Otolith increments, assumed to be formed daily, were enumerated. Measurements by wavelength dispersive electron microprobe confirmed the presence of strontium in otolith tissue, and an inverse relationship between strontium/calcium (Sr/Ca) concentration ratio and temperature is suggested. Electron microprobe analyses combined with daily increment analyses of otoliths provided life history profiles for individual fish. Additional Sr/Ca concentration ratio data on fish supported the idea that Sr/Ca ratios can provide information on the environmental history of individual fish. Body concentrations of mercury were related to otolith analyses to suggest age structure, critical life history periods, growth environment, stock structure, food web position, and migration history. The techniques applied present an innovative approach to management-related problems, and the combination of chemical analyses with structural analyses promises to expand our knowledge of the life history of migratory fishes.     

High‐throughput species identification: from DNA isolation to bioinformatics

Ichthyoplankton collections provide a valuable means to study fish life histories. However, these collections are greatly underutilized, as larval fishes are frequently not identified to species due to their small size and limited morphological development. Currently, there is an effort underway to make species identification more readily available across a broad range of taxa through the sequencing of a standard gene. This effort requires the development of new methodologies to both rapidly produce and analyse large numbers of sequences. The methodology presented in this paper addresses these issues with a focus on the larvae of large pelagic fish species. All steps of the methodology are targeted towards high‐throughput identification using small amounts of tissue. To accomplish this, DNA isolation was automated on a liquid‐handling robot using magnetic bead technologies. Polymerase chain reaction and a unidirectional sequencing reaction followed standard protocols with all template cleanup and transferring also automated. Manual pipetting was thus reduced to a minimum. A character‐based bioinformatics program was developed to handle the large sequence output. This program incorporates base‐call quality scores in two types of sample to voucher sequence comparisons and provides suggested identifications and sequence information in an easily interpreted spreadsheet format. This technique when applied to tuna and billfish larvae collected in the Straits of Florida had an 89% success rate. A single species (Thunnus atlanticus) was found to dominate the catch of tuna larvae, while billfish larvae were more evenly divided between two species (Makaira nigricans and Istiophorus platypterus).     

Cardicola opisthorchis n. sp. (Trematoda: Aporocotylidae) from the Pacific bluefin tuna, Thunnus orientalis (Temminck & Schlegel, 1844), cultured in Japan

A new aporocotylid blood fluke is described, based on specimens from the ventricle of the Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel), cultured in Wakayama and Nagasaki Prefectures, Japan. The new species is morphologically similar to the members of the genus Cardicola Short, 1953, but shows distinct differences in the body form, location of the testis and the orientation of the ootype. The body of the new species is long and slender, whereas other Cardicola species are small and generally lanceolate. The testis is mostly located posterior to the caeca and anterior to the ovary, occupying 31–45% of body length, in contrast to the known Cardicola species, whose testis is typically intercaecal. The ootype is oriented anteriorly, while in most congeners, it is directed posteriorly or horizontally. Phylogenetic analyses of this aporocotylid, together with Cardicola orientalis Ogawa, Tanaka, Sugihara et Takami, 2010 from the same host, were conducted based on DNA sequences of the ITS2 rDNA and the 28S region of ribosomal RNA. The analyses revealed that the new blood fluke belongs to the genus Cardicola despite the marked morphological differences. Thus, this aporocotylid is named Cardicola opisthorchis n. sp. and the generic diagnosis is emended in this paper. In addition, 100% identity among the ITS2 sequences from the present species, Cardicola sp. from T. orientalis in Mexico and Cardicola sp. from the northern bluefin tuna, Thunnus thynnus (Linnaeus) in Spain suggests that C. opisthorchis n. sp. has a broad geographical distribution and that it infects both the Pacific and northern bluefin tuna.     

印度洋中西部和大西洋西部水域大眼金枪鱼的食性比较

根据2004年8月至2005年3月大西洋西部水域及2003年12月份至2004年5月份在印度洋中西部水域金枪鱼延绳钓渔业所获取的大眼金枪鱼数据，对两个诃查区域内的大眼金枪鱼食性进行了研究。结果表明，印度洋中西部水域大眼金枪鱼的食物组成包括沙丁鱼、鱿鱼、乌贼等13个饵料类群，其中主要摄食鱿鱼和沙丁鱼；大西洋西部水域大眼金枪鱼主要摄食沙丁鱼、鱿鱼、虾类等13种饵料类群，主要以沙丁鱼为饵，其次为鱿鱼。印度洋中西部水域大眼金枪鱼空胃率非常高，基本上维持在60％以上；大西洋西部水域大眼金枪鱼空胃率相对较低，基本上都在30％以下。印度洋中西部水域大眼金枪鱼平均饱满指数变化不大，基本上维持在0．40～0．55之间。大西洋西部水域大眼金枪鱼平均饱满指数变化也不太大。印度洋中西部水域大眼金枪鱼各月平均饱满指数高于大西洋西部水域，且各月空胃率高于后者。印度洋中西部和大西洋西部水域大眼金枪鱼Shannon-Weiner多样性指数H’基本上都1．50～2．00之间变化。相同调查月份内，印度洋中西部水域大眼金枪鱼食物Pielou均匀度指数J’均高于大西洋西部水域。     

Serological evidence of an antibody response in farmed southern bluefin tuna naturally infected with the blood fluke Cardicola forsteri

In this study, adaptive immune response was investigated in farmed southern bluefin tuna, Thunnus maccoyii, infected with a sanguinicolid Cardicola forsteri. A cohort (Cohort₂₀₀₅) of southern bluefin tuna was sampled between March 2005 and August 2006. Samples were taken at the transfer of wild caught tuna to sea cages and then at regular intervals. Parasite intensity, abundance and prevalence data were recorded. An ELISA was developed to detect and quantify an antibody response against the blood fluke in southern bluefin tuna serum. Intensity and prevalence of the blood fluke were shown to peak in May 2005 at 10.9 flukes per infected fish (SE = 1.72) and 97.5% prevalence and then decreased to low prevalence (10%) and intensity (1.0). There were no significant changes in prevalence or intensity in 2006. Antibody titres and seroprevalence increased from 1.37 U μl⁻¹ and 10% at transfer in March 2005 to reach a peak in December 2005 of 25.86 U μl⁻¹ (SE = 6.26 U μl⁻¹) and 66.66%. No significant changes were observed in antibody titres for the same cohort of fish during 2006. Parasitological and serological values from Cohort₂₀₀₅ were compared to a 2006 cohort (Cohort₂₀₀₆) in March 2006 and August 2006 to determine if prior infection in Cohort₂₀₀₅ elicited any protection against infection in 2006. Although significant differences were not observed in intensities between cohorts it was shown that Cohort₂₀₀₅ had significantly lower abundances and prevalences of blood fluke infection than Cohort₂₀₀₆. Although there was no significant difference in mean antibody titres between cohorts in March 2006, the mean antibody titre of Cohort₂₀₀₆ was significantly greater than that of Cohort₂₀₀₅ in August 2006. No significant differences were observed in seroprevalence. This is one of the few studies to demonstrate the development of acquired resistance in fish against a parasite in an aquaculture environment under natural infection conditions.     

First record of female germ cells in the testis of a wild Mediterranean northern bluefin tuna Thunnus thynnus L. 1758

The presence of female germ cells was observed in the central portion of the testis of a Mediterranean northern bluefin tuna Thunnus thynnus , 2250 mm fork length and 217 kg. This is the first report of intersex in this species.     

Histological investigation on the ovarian cycle of the bluefin tuna in the western and central Mediterranean

The histological analysis of eastern Atlantic bluefin tuna Thunnus thynnus ovaries caught from February to September 1999–2000, made it possible to distinguish the presence of seven oocyte developmental stages and allowed the characterization of six time-dependent ovary maturity stages. The ovaries of mature (fork length, L _F ≥ 110 cm) bluefin tuna were non-active from August (spent period) to March (quiescent period) when they contained only perinucleolar-stage oocytes. Ovary development started in April to early May (recrudescent period) with the appearance of oocytes at the lipid stage. Vitellogenesis appeared in mid-May (ripening period) and post-vitellogenesis occurred in late May to mid-June (pre-spawning period). In late June to early July, hydrated oocytes, a sign of imminent spawning, were found only in specimens caught in Balearic waters. Females ranging between 100 and 110 cm L _F, captured during the recrudescent and ripening periods, had the largest oocytes at the lipid stage, most of which were degenerating. An extensive vitellogenic atresia was observed in the ovaries of five females caught during the spawning period in non-spawning areas.     

DIEL PATTERNS IN AGGREGATIONS OF PELAGIC DOLPHINS AND TUNAS IN THE EASTERN PACIFIC

Data collected by scientific technicians aboard tuna purse seiners in the eastern Pacific Ocean since the early 1970s have allowed us to study the biology and herd dynamics of pelagic dolphins. A pattern of increasing group size in the morning and subsequent decline in the late afternoon or night was evident for spotted, spinner, and common dolphins, as well as for large yellowfin tuna that associate with dolphins. Diel patterns were also apparent in the formation of mixed-species herds of spotted and spinner dolphins and tuna-dolphin aggregations. Different patterns, however, were displayed by bottlenose dolphins and by yellowfin and skipjack tunas that did not associate with dolphins. It appears that these diel patterns are produced by an interaction of predation pressure and prey distribution.     

Ultrastructure of oogenesis in the bluefin tuna, Thunnus thynnus

Ovarian ultrastructure of the Atlantic bluefin tuna (Thunnus thynnus) was investigated during the reproductive season with the aim of improving our understanding of the reproductive biology in this species. The bluefin, like the other tunas, has an asynchronous mode of ovarian development; therefore, all developmental stages of the oocyte can be found in mature ovaries. The process of oocyte development can be divided into five distinct stages (formation of oocytes from oogonia, primary growth, lipid stage, vitellogenesis, and maturation). Although histological and ultrastructural features of most these stages are similar among all studied teleosts, the transitional period between primary growth and vitellogenesis exhibits interspecific morphological differences that depend on the egg physiology. Although the most remarkable feature of this stage in many teleosts is the occurrence of cortical alveoli, in the bluefin tuna, as is common in marine fishes, the predominant cytoplasmic inclusions are lipid droplets. Nests of early meiotic oocytes derive from the germinal epithelium that borders the ovarian lumen. Each oocyte in the nest becomes surrounded by extensions of prefollicle cells derived from somatic epithelial cells and these form the follicle that is located in the stromal tissue. The primary growth stage is characterized by intense RNA synthesis and the differentiation of the vitelline envelope. Secondary growth commences with the accumulation of lipid droplets in the oocyte cytoplasm (lipid stage), which is then followed by massive uptake and processing of proteins into yolk platelets (vitellogenic stage). During the maturation stage the lipid inclusions coalesce into a single oil droplet, and hydrolysis of the yolk platelets leads to the formation of a homogeneous mass of fluid yolk in mature eggs.