Food consumption of tuna in the Equatorial Atlantic ocean: FAD-associated versus unassociated schools

Since 1991, fishing operations on tuna schools associated with drifting Fish Aggregating Devices (FADs) have become widespread in the purse seine fishery in the Gulf of Guinea. In the offshore South Sherbro area (0–5° N, 10–20° W), FAD-associated catches represent about 75 % of the total catch. This FAD fishery exploits concentrations of skipjack mixed with a smaller amount of bigeye and yellowfin tuna of similar size (46 cm), and some large yellowfin. Catches on unassociated tuna schools are mainly composed of large yellowfin in breeding phase and skipjack. Here we studied tuna diet in relation with the aggregation mode (FAD-associated or unassociated tuna schools), species, and size. The stomach contents of around 800 fish were analysed. Numerous empty stomachs were found, especially in fish caught under FADs. Diets were similar for all small-size tuna sharing the same aggregation type. Small tuna mainly feed on Vinciguerria nimbaria (Photichthyidae), a mesopelagic fish of the micronekton, whereas large tuna mainly feed on Scombridae, mixed with Cubiceps pauciradiatus (Nomeidae) when they were caught in unassociated schools. The feeding habits of tuna are discussed with emphasis on the behavior of V. nimbaria. Estimations of the daily ration of similarly sized tuna with the same aggregation mode were very close. The low estimated rations for small, FAD-associated tuna show that logs do not have a trophic function, but rather are a refuge. In contrast, FADs seem to influence the diet of large tuna because of the Scombridae prey that probably is associated to the FAD.     

Movement and vulnerability of bigeye (Thunnus obesus) and yellowfin tuna (Thunnus albacares) in relation to FADs and natural aggregation points

In Hawaii, a variety of small- and medium-scale pelagic fisheries target fishing effort on a network of coastal moored FADs, natural inshore tuna aggregation points, offshore seamounts and offshore weather monitoring buoys. Large-scale longline vessels also operate in the Hawaii exclusive economic zone (EEZ) and beyond. These circumstances provide an ideal setting for tag-and-release experiments designed to elucidate the movement patterns, residence times, exchange rates and vulnerability of bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) within the Hawaiian EEZ. Preliminary recapture data indicate that FADs, island reef ledges and seamounts exert an overwhelming influence on the catchability of tuna. Recapture rates from these locations vastly outweigh tag returns from open water areas. As of August 31, 1999, a total of l5 387 bigeye and, yellowfin tuna ranging in size from 29 to 133 cm fork length (FL) and from 26 to143 cm FL respectively (mean 59.8 ± 14.1 cm; 58.4 ± 17.3 cm) have been tagged and released throughout the Hawaii EEZ. Recapture rates for both species have been similar with an overall recapture rate of l0.3 %. The location of tag releases reflects the importance of associative behavior and schooling to the vulnerability of tuna; seamounts and FADs accounted for 72.4 % and 23.5 % of all tag releases. Within the main Hawaiian Island group (excluding the offshore seamounts and buoys), 83.1 % of all recaptures have been made on anchored FADs and 11.9 % of recaptures have come from ledges or tuna aggregation areas close to the islands where bigeye and yellowfin tuna become vulnerable to hook and line gear. As these studies continue, additional and longer-term recaptures will provide increasingly detailed information on the movement patterns and vulnerability of bigeye and yellowfin tuna as they grow, move and recruit to different fisheries.     

Do drifting and anchored Fish Aggregating Devices (FADs) similarly influence tuna feeding habits? A case study from the western Indian Ocean

Anchored and drifting Fish Aggregating Devices (FADs) are intensively used in tropical tuna fisheries. In both small-scale and industrial fisheries, skipjack (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) are the main targets. The increasing development of this fishing practice by industrial purse seiners has raised the question of the impact of FADs on tuna communities, as they might act as an ecological trap. This study investigated the feeding habits of skipjack and yellowfin tuna associated with anchored and drifting FADs in the western Indian Ocean. The diet of 352 tunas was analysed taking into account the type of FAD, ontogenetic variations, and the resources richness of the area. Poor-food and rich-food areas were defined according to the abundance of stomatopod Natosquilla investigatoris, the main prey of tunas, on the fishing sites. Diet composition was expressed through functional groups of prey. Significant dietary differences were found between both FAD types, as well as an effect of individual size. Around anchored FADs tuna preyed on diverse assemblages of coastal fish and crustacean larvae and juveniles, whereas a low diversity of epipelagic prey dominated the tuna diet associated with drifting FAD. Compared to anchored FADs, the frequency of empty stomachs was significantly higher and the stomach content mass significantly lower among skipjack and small yellowfin tunas caught around drifting FADs. This was magnified in poor-food areas, where drifting FADs often evolved, suggesting that these FADs could negatively impact the growth of skipjack and small yellowfin tuna. Larger yellowfin tuna exhibited differences in their dietary habits between anchored and drifting FADs, and between poor-food and rich-food areas. However, drifting FADs did not impact them as strongly as juveniles of yellowfin or skipjack tunas. Our study gives new highlights on possible detrimental effects of FAD on tunas, and this has to be considered in future sustainable management strategies of tuna fisheries.     

Habitat analysis of the commercial tuna of the Eastern Tropical Pacific Ocean

We have extracted information on the habitats of bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) in the Eastern Tropical Pacific Ocean by matching the spatial‐temporal distribution of catch and effort of purse seine and longline fleets collected by the Inter‐American Tropical Tuna Commission with oceanographic conditions and subjecting the matched data to Quotient Analysis and General Additive Models (GAMs). These analyses yielded the following results. The habitats defined by the GAM analysis of young fish differ significantly between two periods, one before and one after the introduction of fish aggregation devices (FADs). This was not true for the older fish caught by longline. We speculate that these changes were caused by the extensive use of FADs. Younger bigeye and yellowfin caught by the purse seine fleet have a different preference of environmental variables compared to older fish caught by longline. This is to be expected since tuna of different age groups have different sizes, metabolic capabilities and swimming skills. Moreover, as revealed by GAMs, the habitats of young fish differ between species to a much larger degree than those of older fish. Our results indicate the fundamental differences between fishing methods, targeted species, and operating region of the two fisheries. Specifically, young bigeye occupy equatorial waters farther from the coast and where the hypoxic layer is deeper, young skipjack occupy more productive waters associated with equatorial and coastal upwelling, and young yellowfin occupy broad areas where waters are underlain by a shallow hypoxic layer.     

中西太平洋金枪鱼围网渔获物组成分析

根据 2 0 0 4年 7月 2 8日至 9月 1日在中西太平洋海域的金枪鱼围网生产调查结果 ,以及“金汇 2号”2 0 0 3年全年的生产数据 ,对中西太平洋金枪鱼围网渔获物组成进行了初步分析。结果显示 ,渔获物种类有鲣鱼 (Katsuwonuspelamis)、黄鳍金枪鱼 (Thunnusalbacares)和大眼金枪鱼 (Thunnusobesus)等 19种 ;渔获物重量组成中鲣鱼占 70 .5 1% ,黄鳍金枪鱼占 2 6 .92 % ,其它鱼类占 2 .5 6 % ;鲣鱼的叉长范围为 2 7～ 81cm ,优势叉长组为 4 0～ 5 0cm ,占 4 1% ;黄鳍金枪鱼叉长范围为 32～ 16 5cm ,优势叉长组为 5 0～ 70cm ,占 33% ,另一优势叉长组为 110～ 130cm ,占 2 0 % ;渔获物重量组成存在海域差异 ,在 16 2°E以东海域鲣鱼比例高于以西海域 ,黄鳍金枪鱼则是在 16 2°E以西海域的比例较高。     

Vertical behavior of bigeye tuna (Thunnus obesus) in the northwestern Pacific Ocean based on archival tag data

The behavior of bigeye tuna (Thunnus obesus) in the northwestern Pacific Ocean was investigated using archival tag data for 28 fish [49–72 cm fork length (FL) at release, 3–503 days] released in Japanese waters around the Nansei Islands (24–29°N, 122–132°E) and east of central Honshu (Offshore central Honshu, 32–36°N, 142–148°E). Vertical behavior was classified into three types based on past studies: ‘characteristic’ (non‐associative), ‘associative’ (associated with floating objects) and ‘other’ (behavior not fitting into these two categories). The proportion of fish showing associative behavior decreased and that of characteristic behavior increased as fish grew, and this shift was pronounced at 60–70 cm FL. The fish usually stayed above the 20°C isotherm during the daytime and nighttime when showing associative behavior and below the 20°C isotherm during daytime for characteristic behavior. A higher proportion of characteristic behavior was seen between December and April around the Nansei Islands, and between September and December for offshore central Honshu. Seasonal changes in vertical position were also observed in conjunction with changes in water temperature. In this study, ‘other’ behavior was further classified into five types, of which ‘afternoon dive’ behavior, characterized by deep dives between around noon and evening, was the most frequent. The present study indicated that in the northwestern Pacific Ocean, the vertical behavior of bigeye tuna changes with size, as well as between seasons and regions.     

Association of early juvenile yellowfin tuna <Emphasis Type="Italic">Thunnus albacares</Emphasis> with a network of payaos in the Philippines

To understand how early juvenile yellowfin tuna use the habitat and environment provided by fish aggregating devices (FADs), fish (19–31 cm FL) implanted with ultrasonic transmitters into their abdominal cavities were released in a network of payaos in Panay Gulf, the Philippines. Self-recording receivers were attached to the anchor ropes of the payaos to detect the presence of the fish. Some aspects of the behavior of juveniles were similar to those reported in adults. One juvenile showed a diurnal vertical swimming pattern, swam within a limited shallow range during the nighttime, and dived to deeper waters during the daytime. Two juveniles performed deep dives over 100 m during payao-to-payao excursion. Three juveniles showed a diurnal horizontal swimming pattern that was synchronized. In contrast, juveniles stayed <6 days in the network, shorter than adults. No juveniles returned to the same payao after an interruption of over 24 h. It is suggested that juveniles in this area are just starting to migrate and are temporarily staying around a payao for a few days to forage before continuing their migration.     

Metabolic biochemistry of cardiac muscle in three tuna species (bigeye, Thunnus obesus ; yellowfin, T. albacares ; and skipjack, Katsuwonus pelamis ) with divergent ambient temperature and oxygen tolerances

Bigeye tuna (Thunnus obesus) have much greater vertical mobility than yellowfin (T. albacares) and skipjack (Katsuwonus pelamis) tunas, due to an apparent greater tolerance of the changes in ambient temperature and oxygen occurring with depth. In an attempt to identify physiological processes (e.g., effects of temperature on cardiac function) responsible for these behavioral differences, we examined enzyme activities (at 12 °C, 17 °C, and 25 °C) of cardiac muscle in all three species. Contrary to our expectations, we found few differences and no clear explanatory patterns in maximum enzyme activities (V_max) or enzyme activity ratios. For example, citrate synthase (CS) activity was the same in bigeye and skipjack tunas, but 40% lower in yellowfin tuna, whereas carnitine palmotoyltransferase (CPT) activity in skipjack tuna was approximately double that in the other two species. The ratio of CPT to pyruvate kinase (PK) activity, a measure of the tissues preference for fatty acids as metabolic substrates, was the same in bigeye and yellowfin tunas, but elevated skipjack tuna. The ratios of lactate dehydrogenase (LDH) to CS activity and of PK to CS activity (anaerobic–aerobic enzyme activity ratios – taken as measures of the tissues ability to tolerate hypoxia) were both elevated in yellowfin tuna cardiac tissue relative to the other two species. We also found no differences in temperature sensitivity (Q₁₀ values) when comparing cardiac enzyme activities across species, nor effects of temperature on the substrate affinity (K_m) of LDH. In sum, our results do not suggest any clear metabolic difference in the cardiac muscle that would explain the apparent greater tolerance of bigeye tuna to acute hypoxia and ambient temperature changes or their substantially greater vertical mobility.     

中国金枪鱼围网船队大眼金枪鱼渔获物的特征变化与人工集鱼装置禁渔期的关系

研究了2012-2015年中国金枪鱼围网船队大眼金枪鱼(Thunnus obesus)渔获物的特征变化与人工集鱼装置(fish aggregation devices,FAD)禁渔期的关系,文章收集了2012-2015年中国大陆金枪鱼围网船队在中西太平洋的渔捞日志数据,对随附鱼群捕捞努力量与小体大眼金枪鱼和大体大眼金枪鱼的船均产量进行分析比较。结果显示:1)从2013年开始,对随附鱼群投网的次数占总投网次数的比例有所降低,均不超过50%;2)K-S检验显示研究期内禁渔期前后的船均随附鱼群网次存在显著差异(P0.05);3)2013-2015年大眼金枪鱼渔获量的平均水平明显低于2012年;4)从2013年开始,禁渔期结束后的第一个月(即11月)的船均产量都发生猛增;5)从捕捞努力量与渔获量的相关性结果看,不论是小体大眼金枪鱼还是全部大眼金枪鱼,2013年和2014年两者都呈现出显著的强正相关关系(P0.05)。这些结果表明2012年以后中国船队对大眼金枪鱼幼鱼的兼捕水平有所下降,延长FAD禁渔期的管理措施对于保护大眼金枪鱼幼鱼在某些年份可能具有一定的效果。     

越南开发南海金枪鱼资源的历史和趋势

近十年来,越南将南海的金枪鱼资源作为其＂外向型＂渔业的重要支撑,不断增加捕捞强度,产量逐年升高。本文总结了越南发展南海金枪鱼渔业的过程,分析了南海金枪鱼资源的开发趋势。越南现代化的金枪鱼捕捞技术主要来自日本,使用的渔具主要有金枪鱼延绳钓、手钓、刺网和小型围网,捕捞的种类主要为鲣鱼、黄鳍金枪鱼和大眼金枪鱼,主要作业区域在西沙群岛南部海域和南沙群岛海域。越南2009年金枪鱼的产量已达到5.9×104t,计划2015年达到30×104t。根据越南海洋渔业研究所（RIMF）的评估,南海中西部的金枪鱼资源量为66~67×104t,可捕量23.3×104t,其中鲣鱼的可捕量21.6×104t,黄鳍金枪鱼和大眼金枪鱼的可捕量1.7×104t。随着全球金枪鱼捕捞配额的缩减和越南＂外向型＂渔业经济的发展,越南将继续加强对南海金枪鱼资源的开发。     

金枪鱼延绳钓钓具的最适浸泡时间

根据2010年10月—2011年1月金枪鱼延绳钓海上调查数据,分两种起绳方式,建立每次作业每一根支绳的浸泡时间计算模型。将钓具的浸泡时间以1 h为间隔分别统计每个区间的支绳数量及大眼金枪鱼(Thunnus obesus)、黄鳍金枪鱼(Thunnus albacores)的渔获尾数,并计算其钓获率(CPUE)。结果表明:1)大眼金枪鱼和黄鳍金枪鱼的CPUE都随浸泡时间的增加呈现先增后减的趋势,这是由于饵料的诱引效果变化及渔获的丢失引起的;2)二次曲线可拟合浸泡时间与大眼金枪鱼和黄鳍金枪鱼CPUE的关系;3)大眼金枪鱼和黄鳍金枪鱼CPUE最高的浸泡时间分别为9.9 h和10.1 h。建议:1)今后在金枪鱼延绳钓作业中,保证每一根支绳在水中的浸泡时间为9.5~10.5 h,以提高捕捞效率并减少副渔获物;2)可把延绳钓钓具的浸泡时间作为有效捕捞努力量,并用于CPUE的标准化。研究结果可用于提高捕捞效率并减少副渔获物的技术方案制订,并为渔业生产和CPUE的标准化提供科学参考。     

Effects of open- and closed-system temperature changes on blood O2-binding characteristics of Atlantic bluefin tuna (Thunnus thynnus)

We investigated the effects of open- and closed-system temperature changes on the O₂ affinity of Atlantic bluefin tuna (Thunnus thynnus) blood using in vitro methods essentially identical to those previously employed on tropical tuna species. Bluefin tuna blood has a general O₂ affinity (P ₅₀ = 2.6–3.1 kPa or 19–23 mm Hg at 0.5% CO₂) similar to that of skipjack tuna, yellowfin tuna, and kawakawa blood (P ₅₀ = 2.8–3.1 kPa at 0.5% CO₂) but significantly above that of bigeye tuna blood (P ₅₀ = 1.6–2.0 kPa at 0.5% CO₂). We therefore hypothesize that bluefin tuna are less tolerant of hypoxia than bigeye tuna. Further, we found the P ₅₀ of bluefin tuna blood to be slightly reduced by a 10°C open-system temperature increase (e.g., from 4.83 kPa at 15°C to 3.95 kPa at 25°C) and to be completely unaffected by a 10°C closed-system temperature change. Bluefin tuna blood, therefore, had a significantly reduced Bohr effect when subjected to the inevitable changes in P CO ₂ and plasma pH that accompany closed-system temperature shifts (0.04–0.09 Δlog P₅₀ΔpH⁻¹) compared with the effects of changes in plasma pH accomplished by changing P CO ₂ alone (0.81–0.94 Δlog P₅₀ Δ pH⁻¹). This response is similar to that of skipjack tuna blood, but different from yellowfin or bigeye tuna blood. During closed-system temperature changes at oxygen levels above P ₅₀, however, bluefin tuna blood showed a reversed temperature effect (i.e., P O ₂ decreased in response to an increase in temperature). Unlike in other tuna species, temperature effects on O₂ affinity of bluefin tuna whole blood were similar to those previously reported for hemoglobin solutions, suggesting that red cell-mediated ligand changes are not involved.     

Primary structure and thermostability of bigeye tuna myoglobin in relation to those of other scombridae fish

ABSTRACT:   In the present study, the cDNA encoding myoglobin (Mb) of bigeye tuna Thunnus obesus was cloned and its amino acid sequence deduced in order to investigate the relationship between the primary structure and thermostability of scombridae fish Mb. An open reading frame of bigeye tuna Mb cDNA contained 444 nucleotides encoding 147 amino acids. The primary structure of bigeye tuna Mb was highly conserved when compared with those of bluefin tuna and yellowfin tuna Mb, the sequence identity being 95.2–100.0%. It also showed relatively high identity (82.3–89.1%) with the counterparts of scombridae fish. Myoglobin was then isolated from the dark muscle of four scombridae fish including bigeye tuna. Differential scanning calorimetry and circular dichroism measurements on these Mb revealed that the thermostability of bigeye tuna Mb was lowest and that of skipjack Katsuwonus pelamis Mb highest among the scombridae fish Mb examined. The α-helical contents of scombridae fish Mb at 10°C were in the range of 39.8–44.8%, clearly lower than that of horse Mb (55.3%), suggesting instability of fish Mb. The melting temperatures of these Mb fell in the range of 75.7–79.9°C, lower than that of horse Mb (84.2°C). These results strongly suggest the instability of fish Mb.     

世界主要金枪鱼捕捞产量分析

根据FA0 1950 ～ 2011年世界主要金枪鱼类渔业生产数据统计,将长鳍金枪鱼、黄鳍金枪鱼、大眼金枪鱼和鲣鱼等8种世界主要金枪鱼类每10年的产量总和按不同鱼种和海域进行了总结.结果显示,鲣鱼的累计总产量最高,其平均年产量涨幅最快;除马苏金枪鱼年平均产量有所下降,北方蓝鳍金枪鱼保持稳定外,其他主要金枪鱼类均有增长,但平均增长率最高的是青干金枪鱼.各主要渔区中以中西太平洋海域累计总产量最高,平均年产量有上升趋势,大西洋海域以中东大西洋为产量最高,印度洋海域以西印度洋为产量最高,平均增长率以印度洋海域为最高,其他海域相对持平.我国（包括台湾省）捕获累计总产量最高的是鲣鱼,为418×104 t,占世界总产量比例最高的是长鳍金枪鱼,为22.9％.我国（包括台湾省）主要金枪鱼类捕获总产量占世界总产量比例最高为东南大西洋海域,最低为东南太平洋海域.论文结合世界主要金枪鱼类以及主要捕捞海域的开发现状和我国国情,提出我国目前面临的几点困难以及发展壮大我国金枪鱼渔业的建议.     

Just a FAD? Ecosystem impacts of tuna purse‐seine fishing associated with fish aggregating devices in the western Pacific Warm Pool Province

The western and central Pacific Ocean supports the world's largest tuna fisheries. Since the 1990s, the purse‐seine fishery has increasingly fished in association with fish aggregating devices (FADs), which has increased catches of juvenile bigeye and yellowfin tunas and vulnerable bycatch species (e.g., sharks). This has raised concerns regarding the sustainability of these species’ populations and the supporting ecosystem, but may provide improved food security of Pacific Island nations through utilisation of FAD‐associated byproduct species (e.g., wahoo). An ecosystem model of the western Pacific Warm Pool Province was used to explore the potential ecological impacts of varying FAD fishing effort (±50% or 100%) over 30 years. The ecosystem has undergone a significant change in structure since 1980 from heavy exploitation of top predators (e.g., tunas) and “fishing up the food web” of high‐trophic‐level non‐target species. The ecosystem appeared resistant to simulated fishing perturbations, with only modest changes (<10%) in the biomass of most groups, although some less productive shark bycatch species decreased by up to 43%, which had a subsequent positive effect on several byproduct species, the prey of sharks. Reduction of FAD effort by at least 50% was predicted to increase the biomass of tuna species and sharks and return the ecosystem structure to a pre‐industrial‐fishing state within 10 years. Spatial disaggregation of the model and integration of economic information are recommended to better capture ecological and economic changes that may result from fishing and/or climate impacts and to develop appropriate management measures in response.     

Using historical fisheries data to predict tuna distribution within the British Indian Ocean Territory Marine Protected Area,and implications for its management

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Bigeye tuna (Thunnus obesus) vertical movements in the Azores Islands determined with pop-up satellite archival tags

Movement patterns of 17 bigeye tuna (Thunnus obesus) near the Azores Islands were analyzed between April and May 2001 and 2002 using pop‐up satellite archival tags. Despite short attachment durations (1 to 21 days, 8.2 days on average), their vertical movements revealed much shallower distribution of bigeye tuna in comparison with previous studies in the tropical Pacific and tropical Atlantic. Depth and temperature histograms were unimodal, although overall depth distribution during the day was deeper than during the night due to daily incursions in deeper waters. Although generalized additive models showed significant non‐linear relationships with weight of the fish and sea level anomaly (as a proxy for variability of thermocline depth), the effect of these variables on bigeye depth appeared minor, suggesting that vertical movements of bigeye in the Azores during the spring migration may be influenced by food availability in upper water layers.     

Changes to vertical thermoregulatory movements of juvenile bigeye tuna (Thunnus obesus) in the northwestern Pacific Ocean with time of day,seasonal ocean vertical thermal structure,and body size

Vertical movements related to the thermoregulation were investigated in 12 juvenile bigeye tuna (Thunnus obesus) in Japanese waters using archival tag data. Movements changed with time of day, season, and body size. During daytime, bigeye tuna descended to greater depths, presumably to feed in the deep scattering layer (DSL). Thereafter, they repeatedly ascended to shallower layers, suggesting attempts at behavioral thermoregulation, although the beginning of vertical thermoregulatory ascents might reflect a shift in DSL depth. By the end of such movement, the whole‐body heat‐transfer coefficient might decrease because, although the depth and ambient temperature of the upper layers did not change, the body temperature gradually decreased significantly just after ascent for thermoregulation. Seasonal patterns indicated that the vertical thermal structure of the ocean might influence this ascent behavior. For example, from January to May, bigeye tuna made fewer ascents to less shallow waters, suggesting that they respond to increasing depths of the mixed surface layer by reducing energy expenditure during vertical migration. In addition, as body size increased, fewer thermoregulatory ascents were required to maintain body temperature, and fish remained deeper for longer periods. Thus, vertical thermoregulatory movements might change with body size as bigeye tuna develop better endothermic and thermoregulatory abilities. We hypothesize that bigeye might also increase cold tolerance as they grow, possibly due to ontogenetic shifts in cardiac function.     

Tuna and swordfish catch in the U.S. northwest Atlantic longline fishery in relation to mesoscale eddies

To analyze the effects of mesoscale eddies, sea surface temperature (SST), and gear configuration on the catch of Atlantic bluefin (Thunnus thynnus), yellowfin (Thunnus albacares), and bigeye tuna (Thunnus obesus) and swordfish (Xiphias gladius) in the U.S. northwest Atlantic longline fishery, we constructed multivariate statistical models relating these variables to the catch of the four species in 62 121 longline hauls made between 1993 and 2005. During the same 13‐year period, 103 anticyclonic eddies and 269 cyclonic eddies were detected by our algorithm in the region 30–55°N, 30–80°W. Our results show that tuna and swordfish catches were associated with different eddy structures. Bluefin tuna catch was highest in anticyclonic eddies whereas yellowfin and bigeye tuna catches were highest in cyclonic eddies. Swordfish catch was found preferentially in regions outside of eddies. Our study confirms that the common practice of targeting tuna with day sets and swordfish with night sets is effective. In addition, bluefin tuna and swordfish catches responded to most of the variables we tested in the opposite directions. Bluefin tuna catch was negatively correlated with longitude and the number of light sticks used whereas swordfish catch was positively correlated with these two variables. We argue that overfishing of bluefin tuna can be alleviated and that swordfish can be targeted more efficiently by avoiding fishing in anticyclonic eddies and in near‐shore waters and using more light sticks and fishing at night in our study area, although further studies are needed to propose a solid oceanography‐based management plan for catch selection.     

Swimming depths of the giant jellyfish <Emphasis Type="Italic">Nemopilema nomurai</Emphasis> investigated using pop-up archival transmitting tags and ultrasonic pingers

The swimming depths of 12 individual Nemopilema nomurai with bell diameters of 0.8–1.6 m were investigated using pop-up archival transmitting tags and ultrasonic pingers, and the validity of the research method was evaluated. The N. nomurai studied frequently showed vertical movement, with the swimming depth ranging from 0 to 176 m, The mean swimming depths of most individuals were less than 40 m. The swimming depths of N. nomurai in the northern Japan Sea in the winter were mostly deeper than those of this species in the southern Japan Sea in the autumn. This result suggests that the range of the depths almost depends on the vertical structure of the ocean. Swimming depths during the nighttime were significantly deeper than those during the daytime. More specifically, during the daytime, the swimming depths in the afternoon tended to be shallower than those in the morning, while during the nighttime, the swimming depths after midnight were deeper than those before midnight.