Evaluation of a by-catch reduction device for glass eel fishing traps

Abstract  The effectiveness of a new design of glass eel, Anguilla anguilla (L.), fishing trap was tested in the Ebro River delta fishery (Eastern Mediterranean, Spain). The modified fishing trap incorporated a by-catch reduction device (BRD) to increase trap selectivity and minimise by-catch of non-targeted fish, without affecting glass eel captures. The BRD is inexpensive and easy to implement, consisting of a cylinder covered with a rigid square-mesh of 2.0–2.4 mm knot-to-knot, which is easily fitted to the fishing trap entrance. Testing of the BRD was made by paired comparisons between captures of targeted and non-targeted fish species from both conventional and BRD-modified traps. The use of a BRD in the glass eel traps resulted in a 68.9% reduction in the number of captured non-targeted fish specimens, while it did not reduce glass eel yields. The BRD implementation was fully effective for all species and individuals with sizes >40 mm total length, with the exception of the black-striped pipefish, Syngnathus abaster Risso. These results support the implementation of the BRD in glass eel fisheries along the Eastern coast of the Iberian Peninsula where these fishing traps are used.     

Temporal changes in European eel, Anguilla anguilla, stocks in a small catchment after installation of fish passes

Abstract  Changes in the abundance of European eel, Anguilla anguilla L., in the River Frémur, France, were examined over an 8-year period. Natural connectivity of the river was disturbed by three high dams that inhibited eel upstream migration and reduced recruitment by elvers and yellow eels. After eel passes were installed, fish became more abundant upstream (mean density 0.5 eel m⁻²). Moreover, except in the more upstream areas, no decline in eel numbers and biomass was found, in contrast to the general decline of eel throughout its distribution range. It was concluded that eel passes are important to conserve and/or to recover eel stocks.     

Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment

Abstract –  Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla , in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996.     

Changes of eel exploitation in Polish lakes

The paper discusses briefly eel exploitation and traditional methods of catching eel. Attention is drawn to changes taking place in eel exploitation in Polish lakes, and to their causes. Most effective methods of eel catching are reviewed. It is pointed out that utilization of active fishing gear, mainly seine nets and electric aggregates, creates significant possibilities of increasing eel catches. Eel yields up to 40 kg/ha are not unusual with this type of gear. Furthermore, active fishing gear is more effective and can be used also in lakes in which it is difficult to apply traditional eel traps. The latter fact is of special significance in view of the increasing number of lakes stocked with eel, and improvement of eel stocking effectiveness.     

Dynamics of upstream migration of the European eel, Anguilla anguilla (L.), in the Rivers Severn and Avon, England, with special reference to the effects of man-made barriers

The upstream migration of European eels, Anguilla anguilla (L.), was studied during 1991–1993 in the Rivers Severn and Avon using traps mounted on weir or sluice barriers. Only pigmented elvers and juvenile eels were trapped at the tidal limits, catches being equivalent to about 0.8% of the commercial glass eel catch in the lower estuary. First catches were made as temperatures rose above 10–11 °C. Pigmentation-stage analyses and body size data indicated that estuarine migration was slow and that natural mortality was probably very high. Relatively more eels were trapped in the Severn compared with the Avon, but in both rivers the number of immigrants decreased rapidly upstream of the tidal limits, whilst the average size and age increased. The number and severity of weir and sluice barriers to be surmounted exerted a greater effect than distance alone. Recapture rates of marked eels were low (1–2%), implying variable migratory tendencies and/or high mortality. Mean migration rate in the non-tidal rivers was 0.64 ± 0.6 km day^–1 and some eels were not recaptured until one or two years after release. Speed of migration increased with temperatures above 15–16 °C. Relationships between migration dynamics, barriers and the scarcity of upriver stocks of eels and distorted population structures in the two rivers are discussed. Recommendations are made for the provision of passes and/or stocking to enhance migration and recruitment.     

欧洲鳗免疫球蛋白单克隆抗体的制备与特性

应用杂交瘤单克隆抗体技术制备了13个分泌抗欧洲鳗免疫球蛋白的单克隆抗体细胞株，并对这些单克隆抗体的特性进行了分析。经抗体级份和亚级份测定，其中IgM有3株，IgG1有5株，IgG2a有3株，IgG2b有2株；所有的单克隆抗体与欧鳗免疫球蛋白均有ELISA反应特性，抗体滴度10^4-10^7，有七株单抗具有WESTERN BLOT反应特性，能在变性条件下识别欧鳗免疫球蛋白的重链。进一步实验证实这些单抗能特异地识别欧洲鳗、日本鳗的免疫球蛋白，而与鲫、淡水白鲳、罗非鱼、胡子鲶、美洲斑点叉尾Hui血清免疫球蛋白以及水产动物常见病原菌如气单胞菌、爱德华氏菌、弧菌、柱状屈桡杆菌、沙门氏菌及大肠杆菌等无任何交叉反应。单克隆抗体9D7，对纯化的欧洲鳗免疫球蛋白的检测灵敏度为16ng。实验结果证明这些单抗具有高度特异、高度灵敏等特点，可用于鳗鲡免疫球蛋白的结构分析、免疫应答水平监测和病原诊断，具有广阔的应用前景。     

Estimation of production and long‐term changes in the population of European eel Anguilla anguilla (L.) in an Irish lake

Monitoring of yellow‐phase and silver‐phase Anguilla anguilla during their continental life history is necessary for evaluation of stock recovery measures. Eel population data for an Irish lake (Lough Sheelin) were compiled for the period 1993–2014. Catch data from 2009 to 2014 provided minimum estimates of recent silver eel production ranging annually from 0.79 to 1.84 kg/ha. Long‐term changes in yellow eel abundance and silver eel size structure were assessed as part of a fishery monitoring programme. Yellow eel catch per unit effort (CPUE) in the lake (from longline surveys) was considerably higher in the 1990s (52.2–62.1 eels/100 hooks) than 2002–2008 (1.9–15.8 eels/100 hooks). Conversely, during 1993–2014, the mean size of silver eels migrating from the lake increased significantly (p < .001), from 659 mm to 838 mm. The results suggest that in the absence of direct yellow eel abundance data, interannual variation in silver eel size structure may be a useful monitoring tool for local eel stocks as part of Eel Management Plans (EMP's).     

Estimation of glass eel (Anguilla anguilla) exploitation in the Severn Estuary,England

Mark-release-recapture trials were conducted to determine the exploitation rate of glass eel Anguilla anguilla (L.) by handheld dip nets in the Severn Estuary in the spring of 2020. The glass eel were marked with Rhodamine B at a concentration of 0.1 g/L, 200 glass eel/L for 4.5 hr and then placed in fresh water for 36–48 hr to monitor mortality. In trial 1, 891 ± 100 of the 20,455 glass eel were recaptured while in trial 2,373 ± 172 of the 27,923 glass eel released were recaptured. The exploitation rate was estimated to be 4.36% (±0.49) in trial 1 and 1.33% (±0.62) in trial 2. The size of the glass eel population from trial 1 was estimated to be 24.69 t (22.46 – 28.81), and the overall exploitation rate of the fishery for the season was 7.8% (6.7% – 8.6%). Comparisons are made with studies in other estuaries and with conservation targets set by the EU Eel Regulation and the Eel Management Plan for the River Severn. The study suggests the fishery is not the main cause of the Severn RBD failing to meet escapement targets.     

Reproductive migration of fish and movement in a series of reservoirs in the Upper Parana River basin,Brazil

This study evaluates the effectiveness of two fish passes at two hydropower dams (Canoas I and II) in the Upper Parana basin, which form part of a cascade of three reservoirs. Fish from 12 migratory species (3089 specimens) were captured during their ascending, reproductive migration and were tagged with hydrostatic tags. The recapture data (294 specimens over two consecutive years) showed that there is a strong tendency for the maintenance of ascending migration through reservoirs with fish passes but with differences in migratory activity within the same species. No eggs, larvae or juveniles of these species were found in samples collected over 5 years in the reservoirs above the fish passes. These data suggest that fish passes have contributed to the restoration of the migratory routes of adult fish but that in the absence of suitable spawning or nursery habitats for these species; they probably act as ecological traps and do not contribute to the recruitment of the species.     

Effects of improving longitudinal connectivity on colonisation and distribution of European eel in the Loire catchment, France

Abstract –  Major migration barriers have been identified as an important factor in the drastic reduction of freshwater eel stocks at the catchment scale. In this study, we developed a way to assist decision making in the management of migration barriers so as to increase eel presence throughout the Loire catchment (France). Models were developed according to eel size-class and various different management strategies to improve accessibility, taking into account: (i) all the migration barriers from the tidal limit to different upstream limits (downstream–upstream strategy), (ii) all the migration barriers with a given degree of passability (migration barrier passability class strategy), (iii) all the migration barriers in a given sub-catchment area (sub-catchment strategy) and (iv) all the migration barriers in the entire Loire catchment (global strategy). Eel were actually found to occur in the Loire at about 50% of the sites sampled. The improvement of habitat accessibility by removing all obstacles to colonisation or installing fish passes, for example (global strategy), would lead to eels being found throughout the whole river. If this management strategy is impossible because the high cost, two others, also producing significant eel continental colonisation must be implemented. Either all barriers from the tidal limit with medium passability and/or migration barriers in the sub-catchment area which is closest to the sea and least accessible must be managed. In both cases, the area significantly exploited by eels and the probability that this species will be present will be doubled. However, these results can only be counted on if the fluvial recruitment increases significantly.     

Reducing the mortality of freshwater turtles in commercial fish traps

• 1. Mortality of air‐breathing vertebrates has been recognized for some time as a significant risk in Australian inland fisheries. There has often been conflict between the desires of fishers to maximize catches of their target species and the implementation of effective methods to reduce nontarget bycatch.
• 2. Two trials were conducted to evaluate the effectiveness of modifications to fish traps designed to prevent the capture of freshwater turtles (Emydura macquarii) and to facilitate their release. The first study evaluated the use of 100 mm exclusion rings to prevent turtles entering eel traps. The second study assessed two configurations of a carp trap designed to release the accidental catch of turtles.
• 3. Eel traps fitted with 100 mm exclusion rings significantly reduced the turtle bycatch, with no significant difference being observed between mean size of eels captured in traps fitted with exclusion rings and traps without rings.
• 4. The trials on the modified carp trap confirmed that they effectively retained carp and released a majority (77%) of turtles over a 4 h period. Turtles retained in the carp traps were significantly longer than those that found their way out of the trap.
• 5. This study demonstrates the different approaches that can be taken to achieve a reduction in non‐target bycatch associated with traps, and illustrates the importance of exploiting both the physical and behavioural differences of the target and non‐target species in order to develop appropriate gear designs that effectively restrict the entry, or facilitate the release, of bycatch species.

Copyright © 2004 John Wiley & Sons, Ltd.     

Otter guards in river fyke-net fisheries: effects on catches of eels and salmonids

The effects of otter guards on the fishing efficiency of eel fyke-nets were investigated in two Danish rivers. The otter guards were made of 75-mm mesh netting and were placed across the fyke entrance. The fishing efficiency was measured as catch-per-unit effort and hauling time. Guard nets were mounted in half the fyke-nets, while the other half served as controls. After every haul, the fyke-nets with guard nets were returned to controls and vice versa . For legal-sized eels and salmonids, the guard-net significantly ( P < 0.05) reduced the catches, causing a 30% reduction of the eel catch and a 53–55% reduction of the salmonid catch. The use of otter guards significantly increased the hauling time. The average time used for hauling a guarded fyke-net was increased by 15–25% for both upstream- and downstream-oriented fyke-nets, respectively. Blockages caused by water-borne debris appear to be the main cause of problems. Anguilla anguilla , by-catch, eel, fyke-net, otter guards, salmonid.     

Variability in growth rates of freshwater eels (Anguilla spp.) in New Zealand

Abstract— Both species of New Zealand freshwater eels (the shortfinned eel Anguilla australis and the longfinned eel A. dieffenbachii ) are widespread and abundant, supporting important commercial and recreational fisheries. This article reviews growth studies from 35 widely distributed sites throughout New Zealand and discusses factors that influence growth rates. Length at given age is characterized by high intra- and inter-population variability; growth rates for eels >30 cm are typically slow (2–3 cm per year) and linear, with females generally growing faster than males. Water temperature affects the length of the growing season, although growth rates are not correlated with latitude. Other factors suggested as affecting growth rates are eel density, quantity and quality of food, and interactions between both eel species. Al though growth of some New Zealand eel populations is the slowest recorded for any species of Anguilla , growth in culture can be rapid, similar to that of other temperate eel species.     

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

Due to a recruitment decline of more than 90% in 30 years, the European eel (Anguilla anguilla L.) has been classified by IUCN as critically endangered. Although the species has been studied intensively to obtain knowledge to improve management, studies about the resident yellow stage are relatively scarce. In this study, 52 large female yellow eels were tagged with acoustic transmitters in a Belgian polder system and tracked by a network of 23 automatic listening stations. We studied both circadian and seasonal movement patterns and the effect of environmental variables on these patterns. Large female yellow eels were most active at night in late summer and early autumn. A generalised linear mixed model showed that their movement is only slightly influenced by environmental variables. Moreover, as yellow eels show high site fidelity (i.e., the majority was detected only in the habitat type of their catch‐release location), they do not encounter many human‐induced connectivity problems in polder systems, which makes these systems highly suitable as eel growth habitat. These results can contribute to an effective eel management regarding habitat protection and restoration.     

Evaluating light trap efficiency by application of mesh to prevent inter- and intra-specific in situ predation on fish larvae and juveniles

Amongst passive collecting gear types, light traps have been used extensively to sample the larvae and juveniles of fish in both marine and freshwater environments, and especially so in structurally complex habitats. Although a number of modifications have been proposed to increase efficiency, no account has hitherto been made of the possible loss of trapped larvae and juveniles as a result of within-trap predation by piscivorous fish. To address this issue, we evaluated the efficiency of modified light traps by the addition of mesh (3 mm knot-to-knot wrapped around the entrance chambers), by combining results from a long-term monitoring study with those from a designed field experiment. Abundance of larvae and juveniles collected during the monitoring program was higher in the years following mesh application (hence, beyond monthly variation), and this was supported by the experiment. Based on the latter, light/mesh traps consistently caught only larvae/juveniles, hence excluding adults; whereas the opposite was true of the light/no-mesh trap combination. Also, mesh/no-light traps did not capture any fish, unlike no-mesh/no-light traps, which sampled only adults. Our findings suggest that in situ predation on larvae and juveniles within light traps should not be overlooked. The occurrence of piscivorous and cannibalistic behaviour in Hypseleotris spp. is also discussed.     

Safeguarding the welfare of farmed fish at harvest

Fish welfare at harvest is easily compromised by poor choice of handling and slaughter methods, lack of attention to detail and by unnecessary adherence to fish farming traditions. The harvest process comprises fasting the fish to empty the gut, crowding the fish, gathering and moving the fish using brails, fish pumps, and sometimes also road or boat transport and finally stunning and killing the fish. The harvesting processes commonly used for bass, bream, carp, catfish, cod, eel, halibut, pangasius, salmon, tilapia, trout, tuna and turbot are outlined. These harvesting processes are discussed; the consequences for fish welfare identified and practical tests which can be made at the harvest site highlighted. Welfare at harvest for the majority of farmed fish species can be improved by adopting and adapting existing procedures already known to be beneficial for fish welfare through their use in other fish farming systems or with other species. It is seldom necessary to develop completely new concepts or methods.     

黄鳝胃肠的生长抑素分泌细胞及超微结构

在肠卷石蜡法切片上，用ABC免疫组织化学方法，显示消化管全长的生长抑素分泌细胞（又称D细胞），结果只在胃体部显现，主要散在分布于胃腺中，个别出现在粘膜上皮。消化道内分泌细胞根据基部有否胞质突起，可分为4种类型，在黄鳝胃中均有，其中Ⅲ型数量最多，I、Ⅳ型次之，而Ⅱ型数量最少。由于产生生长抑素的D细胞在消化道内分泌细胞中作用多并且类型复杂，故黄鳝胃还是一个十分重要且复杂的内分泌器官。应用透射电镜，对黄鳝的胃肠作超薄切片观察显示在胃腺上皮细胞内含大量的微管泡系和一定量的酶原颗粒，表明其兼有泌酸及产生酶原的功能，属泌酸胃酶细胞。在肠上皮细胞游离面，含有大量微绒毛，但幼鳝比成鳝的长、密且整齐，提示幼鳝的肠上皮比成鳝有更强的吸收能力。     

日本鳗鲡仔鱼摄食机理及其营养策略

鳗鲡属(Ａｎｇｕｉｌｌａ)鱼类共有19种和亚种,其中以欧洲鳗鲡(Ａ.ａｎｇｕｉｌｌａ)和日本鳗鲡(Ａ.ｊａｐｏｎｉｃａ)最为重要。鳗鲡属鱼类发育需经历奇特的柳叶状仔鱼(ｌｅｐｔｏｃｅｐｈａｌｕｓｌａｒｖａ)阶段,其人工育苗技术是一道世界难题。柳叶状仔鱼现象仅见于真骨鱼类最原始的一个类群海鲢总目Ｅｌｏｐｏｍｏｒｐｈａ(鳗鲡目Ａｎｇｕｉｌｌｉｆｏｒｍｅｓ,海鲢目Ｅｌｏｐｉｆｏｒｍｅｓ,背棘鱼目Ｎｏｔａｃａｎｔｈｉｆｏｒｍｅｓ)。有关柳叶状仔鱼的发育策略,已从形态学、生理学及生物化学等方面进行了不少研究,但柳叶状仔鱼…     

Comparison of glass eel stages of American eel and speckled worm eel in a northeast Florida estuary

Since 2001, a glass American eel, Anguilla rostrata (Lesueur), survey has been conducted at Guana River Dam, Florida, USA. Although present in earlier samples, glass speckled worm eel, Myrophis punctatus Lütken, were not identified until a large proportion appeared in samples in 2006. To determine whether the species could be separated in historical samples, weight–length relationships were compared between the two species. Then, with data from the full survey period, relationships between catch rate and annual percent contribution of glass speckled worm eel were explored in relation to temperature, water flow and salinity. Glass speckled worm eel were found to weigh significantly less and had distinguishing physical characteristics that could be used for identification. Glass speckled worm eel constituted between 0 and 34% of the catch, with no detectable pattern in timing or abundance among years. In 2006 and 2012, which had the two largest catches of glass speckled worm eel, relationship between nightly catch rate and measured environmental variables was inconsistent, even when measured variables had a similar range in values. It is recommended that south‐eastern, including Gulf of Mexico, USA, states restrict harvest of glass American eel to prevent the incidental capture and exportation of glass speckled worm eel. Efforts should be made to ensure proper species identification in glass eel surveys.     

Some remarks on eel management in lakes

The paper presents basic factors determining eel production in Polish lakes. Analysis of data from 582 lakes over 29 years (1950–1978) showed that ell yield in lakes is basically determined by the artificial stocking of eels. The relationship between eel catch and eel stocking is very close and to a certain extent independent of the fishing effort. Prospects of further development of eel management in Europe are presented, as well as basic problems facing this management on a general scale. A need is identified for further studies in this field.