Mesh selection in a sea trout, Salmo trutta L., commercial seine net fishery

Abstract The length distribution of adult sea trout, Salmo trutta L., from a commercial seine net fishery using a nominal minimum mesh size of 50 mm or 102 mm is described. The smaller mesh size captured sea trout larger than 210 mm whilst the larger mesh size caught sea trout mainly above 400 mm, with no fish below 320 mm. A sea trout length/ girth relationship is described, and coupled with biological data allowed the calculation of mesh sizes for the appropriate escapement of different sea ages of sea trout.     

闽东渔场底层三重流刺网作业调查分析

底层三重流刺网作业是福建省闽东地区近几年发展起来具有一定代表性的作业。笔者通过调查霞浦县三沙镇189艘流刺网作业船,占全镇总数的89%,分析其经济效益及对渔业资源的利用情况。结果表明,目前闽东的底层三重流刺网已经完全替代了传统的流刺网。该网具的渔获物选择性较强,渔场适应性广,生产水平高,效益好,但由于网目规格偏小,对经济鱼类的幼鱼损害较大,要适度发展该作业则应限制网目使用最小规格,以减少渔业资源损害,保障渔业资源的可持续利用。     

Size selectivity of a trammel net for oval squid Sepioteuthis lessoniana

ABSTRACT:   The size selectivity of a trammel net for herded oval squid Sepioteuthis lessoniana in Tateyama Bay, Chiba Prefecture, was estimated by comparison between the mantle length frequency distributions of oval squid caught by a trammel net and by a set net. The measured mesh sizes of the inner net of the trammel net and of the final section of the set net were 85.3 and 11.3 mm, respectively. In the trammel net fishery where oval squid are herded into the net, most of the oval squid are caught in the bag-shaped inner net. Hence, the logistic function was employed for the size selectivity curve of the trammel net. The 'share each length's catch total' (SELECT) model was implemented for the estimation of the selectivity curve. The size selectivity r ( l ) of the trammel net for the oval squid was expressed as a logistic function of the mantle length l : r ( l ) = exp(−18.57 + 0.88  l )/[1 + exp(−18.57 + 0.88  l )]. From these logistic parameter estimates, the 50% selection mantle length and selection range ( L ₇₅– L ₂₅) were calculated as 21.07 and 2.49 cm, respectively. The selection probability of oval squid whose mantle girth was equivalent to the mesh perimeter of the inner net was 0.09. Accordingly, oval squid of a girth smaller than the mesh perimeter were likely to pass through the mesh to escape from the net.     

Influences of twine diameter and mesh area on the efficiency and durability of baited hoop nets targeting portunids

The utility of varying twine diameter (0.5–0.8 mm Ø) and/or mesh area (0.5:1) in south‐eastern Australian recreational hoop nets was assessed in response to concerns over (1) impacts to discarded, undersize Scylla serrata L. and Portunus pelagicus Forskål and (2) marine debris (lost meshes). There was a negative relationship between Ø and breakage, with more in the narrowest (16.16–18.42 meshes deployment^?1) than thickest (8.79–4.68) twines. Mesh damage was also affected by interactions between netting area and (1) soak time (only S. serrata) and (2) catches, with all nets similarly damaged during <3.5‐h soaks, or when only one crab was caught, beyond which the large‐area nets incurred more. Irrespective of net, there were consistent positive relationships between mesh damage and water temperature, and more damage overnight. Large crabs took longer to remove, but more so from thicker‐twined and large‐area nets, and some also sustained the most damage. Twine diameter did not affect S. serrata catches, but was negatively associated with catches of P. pelagicus, while the small‐area nets caught proportionally (to area) fewer portunids. The results reflect species‐ and size‐specific behavioural variability, but support using thick twine and limiting deployment durations for hoop nets. Alternatively, other less controversial traps might offer a more holistic solution to the stated concerns.     

Comparative analysis of ichthyoplankton sampling gear for early life stages of walleye pollock (Theragra chalcogramma)

Comparative gear tows were conducted during the summers of 1987–1990 in the Gulf of Alaska. Efficiencies of four types of ichthyoplankton sampling gear were compared with respect to total catch and size distribution of larval fish. Fish were sampled using the bongo net (333 and 505 μm mesh), 1 m Tucker net, 3 m Tucker net and the Methot frame net. Results indicate that the bongo and the 1 m Tucker nets caught similar numbers and size distribution of fish. The larger 3 m Tucker and Method frame nets caught a larger size class of fish but a smaller total catch. Although there was a slightly significant diel difference in total catch detected in the smaller nets, no diel differences in size distribution of fish caught were detected for any of the nets.     

拖网囊网网目及其对东海带鱼选择性的研究

本文介绍了东、黄海拖网囊网网目的套网系列试验结果，指出当带鱼的刺鱼体周相当于网目内周时，其体长一般大于这种网目对带鱼的50％选择体长。文中比较了按照刺鱼体周求算网目内径同用50％选择体长决定网目内径的不同之处。根据网目内径与刺鱼体长的相关式，提出保护东海带鱼资源的拖网囊网最小网目内径为56毫米，并依据网目的选择性及带鱼的生物学资料，讨论了采用这一网目尺寸的可行性。     

A method for calculating numerical estimates of gear selectivity curve

The present paper proposes a method for estimating the numerical curve of gear selectivity without the assumption of function of selectivity curve and estimating confidence intervals, using data of catch per unit effort at length for several nets of different mesh sizes obtained from fishing experiments. The paper assumes such geometric similarity as the selectivity is the same if the rate of fish length to mesh size is the same. The basic idea is due to a technique of numerical integration that uses the derivatives at various points. The natural logarithm of selectivity is expressed by an approximation equation using its cumulated derivatives. The values of derivatives at various lengths are calculated from the data. The confidence intervals are made from the bootstrap samples. The method was applied to the data of catch at length by mesh size of gill net for pink salmon Oncorhynchus gorbuscha . Data collection, modification and issues of the method are discussed.     

HDPE圆形重力式网箱受力变形特性的数值模拟

该研究旨在综合探讨不同网箱周长、浮管管径、网衣高度及网目大小对整体网箱受力变形的影响,为网箱的科学合理选型提供数据参考。设定的网箱周长40—80m,浮管管径250～630mm,网衣高度6～20m,网目大小45～115mm。通过数值模拟方法对4种规格高密度聚乙烯圆形网箱在不同组合条件下网箱锚绳受力、波流力以及容积损失率进行了数值计算。结果表明,大规格网箱的锚绳受力、波流力更大,容积损失率更小,锚绳数量的增加可以大大降低锚绳受力。相比浮管管径,网衣高度和网目大小对网箱受力变形的影响更显著。整体网箱的受力变形随着网衣高度的增加而增大,随网目的增大而减小。     

Blood parameters and electrocardiogram in squeezed fish simulating the effect of net damage and recovery

ABSTRACT:   In the present study, rubber bands were tied around the girth of rainbow trout Oncorhynchus mykiss to simulate and assess the physiological damage and survival rates of fish that encounter gill nets. Physiological condition was assessed by analysis of blood lactate, pH, oxygen partial pressure and plasma potassium concentration before and after release from binding. Twenty-four hours after release, half of the test fish had died. Although the binding forces used to hamper fish did not differ significantly, blood lactate levels in the dead fish rose to 10-fold higher than those in the survivors. Consequently, the pH level fell in the group that died, whereas in the surviving group it fell only slightly and soon recovered. Potassium concentration increased after release from binding in fish that died, and the P o ₂ levels in these fish continued to drop until just before death. It was concluded that when fish get entangled in fishing nets, blood lactate accumulates because of physical fatigue and the elevated lactate levels lead to metabolic acidosis. Finally, fatal metabolic conditions could cause higher mortality rates, even in escapees.     

Sampling performance and operational quality of a frame trawl used to catch juvenile fish

SUMMARY: A sampling trawl with a rigid mouth; that is, the frame trawl, was designed so as to catch the large larval and juvenile fish more efficiently. The net consisted of four 13 m panels (nylon multifilament and twine diameter φ 0.75 mm, mesh size 8 mm). The frame of the net's mouth was composed of four stainless steel pipes (length 4 m, diameter φ 76 mm, thickness 5 mm). Several operations were conducted to investigate the capability of catching juveniles. A lot of Sardinops melanostictus juveniles, which are generally difficult to catch, were caught during every operation. Underwater observations proved that juveniles could not elude the capture. The frame trawl requires no special handling and is possible to operate in a rough sea.     

Catching invasive Chinese mitten crabs while releasing the endangered European eel and other fish by‐catch: The implications of fyke net design

1. Modifications of a traditional fyke net design were trialled in 42 hauls over 5 months at 42 locations in the Thames estuary, London. These trials were to determine whether the modified nets could be used to catch invasive mitten crabs while at the same time releasing endangered eels, back into the river.
2. The modifications included rings of different diameters fixed into the netting to provide escape apertures and also a variation in mesh size. A standard, unmodified net was included as a control.
3. Captured mitten crabs, eels and other fish by‐catch were measured and recorded for all deployed nets. Mitten crabs and eels were caught in all nets except those of the largest mesh size (70 mm) which caught no eels. This may have been the combined effect of the mesh size and it being set on the square, versus the normal diagonal netting which may become increasingly constricted in one axis, under tension. Such a square mesh net could be used to trap crabs of carapace width > 65 mm, while releasing all eels.
4. The smallest rings, 22 mm internal diameter, inserted into the mesh may have allowed the escape of eels < 35 cm length, but retained larger, market legal, individuals. This suggests that a slightly smaller escape ring could potentially be used to release eels of ≤ 30 cm in length, in line with current regulations.

     

海州湾秋季坛子网网目选择性研究

2018年秋季,利用囊网内径网目为25、35、45、55 mm的坛子网在黄海海州湾渔场N 35°02.335′,E 119°24.842′附近海域进行了11网次试验。从Logistic选择性模型、坛子网不同网目对主要渔获物小黄鱼的选择性曲线、渔获物组成、幼鱼比例及选择性分析等方面,研究坛子网网囊的选择性。试验结果显示,坛子网九月主要渔获物为小黄鱼、带鱼和三疣梭子蟹;随网囊网目尺寸的增加,渔获物质量逃逸率及尾数逃逸率均迅速增加,网目尺寸为55 mm时其值均超过80%,且质量逃逸率及尾数逃逸率的值比较接近;渔获物组成中,小黄鱼、银鲳和三疣梭子蟹幼体比例非常高,基本全为幼体,带鱼幼鱼比例也超过70%;根据小黄鱼的可捕体长,获得坛子网针对小黄鱼的最适网囊网目尺寸,为(40.59±3.19)mm。研究结果可为我国海州湾渔具管理以及渔业资源的可持续发展与管理提供科技支撑。     

Gear-based management of a tropical artisanal fishery based on species selectivity and capture size

The population density, species composition and lengths of fish landed by artisanal fishermen using six types of gear: large and small traps, gill nets, hand lines, spears and beach seines were studied in the multi‐species fishery of southern Kenya. Selectivity and catch composition among gears were determined by studying the species richness, diversity, size and mean trophic level of the catches for each gear type, to develop gear‐based management recommendations for this artisanal reef lagoon fishery. One hundred and sixty‐three reef and reef‐associated species from 37 families were recorded in the catch. Beach seines and small traps accounted for the highest number of fish landed (34–35 individuals per fisherman per day). These gears also caught smaller fish than big traps, spears and gill nets. Beach seines caught the highest number of species (14 ± 7 species per day) while most other gears caught four to five species per day with no differences between gears. Predatory species with a mean trophic level of 3.6 dominated catches from hand lines, while the mean trophic level of the other gears was low and ranged from 2.6 to 2.9 with no differences between the gears. The high diversity and small size of fish caught in beach seines indicates that its selectivity overlapped most with large traps and gill nets. Spears and small traps also showed high similarity in species selectivity and small traps captured the smaller mean size of fish, indicating that they are likely to pre‐empt the resource of spears. Large traps, hand lines and spears catch the largest individuals and the species composition of the catch differed sufficiently such that their selectivity should overlap the least and may, therefore, be the preferred mix of gears. The elimination or reduction of beach seines and small traps should reduce the catch of small fish and overlap in selectivity among the existing gears.     

Changes in water quality within anglers' keepnets during the confinement of fish

The influence of mesh size and loading density on conditions within keepnets during the confinement of fish was examined. The performance of a mixed-mesh net was also evaluated. Confinement of fish within nets of a standard diameter and length and of three different mesh sizes for 5 h resulted in no deleterious changes in levels of dissolved oxygen, un-ionized ammonia, carbon dioxide, or pH at any of three sample points within each net. Transfer of fish to similar nets at a similar, and a higher, loading for 5 h resulted in a depletion of oxygen, and accumulation of ammonia, in the high-load nets. No changes in carbon dioxide levels or pH were observed in nets containing either moderate or high loads. Confinement of fish in mixed-mesh nets for 5 h resulted in a depletion of dissolved oxygen in the lower section of the net and an accumulation of un-ionized ammonia. There were no changes in carbon dioxide or pH. These results are discussed with reference to the water quality requirements of freshwater fish. It is concluded that changes in water quality within anglers' keepnets during the confinement of fish are not of sufficient magnitude to adversely affect the confined fish.     

Estimation of basket trap selectivity for changeable nassa (Nassarius mutabilis) in the Adriatic Sea

Size selectivity of basket traps for changeable nassa (Nassarius mutabilis) was estimated through comparative fishing trials with traps varying in mesh size and net colour. Selectivity curves of two commercial (19 mm white twine and 19 mm black twine) and three experimental (23, 26 and 28 mm white twine) traps were estimated using SELECT method. In addition, theoretical size-selectivity curves were calculated from morphometric data using a simplified version of Sechin method.Differing from the opinion of fishermen, N. mutabilis did not show any preference for dark coloured twine. Retention curves estimated with SELECT method had a very sharp selectivity and well fitted catch data in all cases. Theoretical size-selectivity curves were very close to size-selectivity curves obtained from experimental data. The two types of curve were very similar in shape, although the theoretical ones were shifted towards larger size classes and showed a slightly steeper slope than SELECT curves. The mesh size commonly used by fishermen is not adequate for rationally exploiting the resource, as catches mainly included undersized specimens, whilst the 23 and 26 mm mesh sizes would represent a good compromise between reduced occurrence of undersized individuals and satisfactory yields of commercial product.     

Mesh selectivity of encircling gill net for gizzard shad Konosirus punctatus in the coastal sea of Korea

ABSTRACT:   Fishing experiments were carried out in Yeongil Bay, Korea using an encircling gill net with four different mesh sizes and two different hanging ratios to measure the mesh selectivity for gizzard shad Konosirus punctatus . Twenty-six trials were conducted giving a total catch of 485 gizzard shad. The 'share each length's catch total' (SELECT) analysis with maximum likelihood method was used to fit the different functional models, the normal, lognormal, and bi-normal model, for a selectivity curve to the catch data. In addition, two cases in which the relative fishing intensity was either estimated or fixed were compared. The bi-normal model with the fixed relative fishing intensity was found to fit the data best. For the selectivity curve on the bi-normal model, the relative length (the ratio of fish length to mesh size) with the maximum efficiency was obtained as 3.70. From this, the focal fish lengths in the commercial encircling gill net, with mesh size ranging from 5.0 to 6.0 cm, were inferred to be 18.5–22.2 cm.     

网目尺寸对金枪鱼围网沉降性能及网具形态的影响

根据以往的研究结果和海上网具性能实测,并在听取生产第一线船长意见的基础上,把上海开创远洋渔业有限公司现行金枪鱼围网主网衣下部约三分之一网衣的网目尺寸放大50%,即从260 mm增大到390 mm,以探讨改进围网渔具的沉降性能。实验应用田内准则换算并制作成模型网,在日本下关西日本日网公司专业围网动水槽进行模型试验,比较了侧流、背流、顺流(流速8 cm/s,相当实物网0.20 m/s)3种相对流向的放网模式下放网以及无流(0 cm/s)海况下放网,网目尺寸改变前后围网的沉降性能、网具形态和闭合效果。结果显示,主网衣下部网目尺寸从30 mm (对照网,相当于实物网网目尺寸260 mm)放大到45 mm (试验网,相当于实物网网目尺寸390 mm),网具的沉降深度和沉降速度分别提高1.6%和1.1%;试验网的收绞时间比对照网缩短14.3%,且能保持较好的形态;有流时的3种放网模式下,试验网和对照网浮子纲在放网结束时刻的初始包围面积基本相同;在收绞结束时,试验网浮子纲包围面积为对照网的1.33倍,绞纲结束前6 s,试验网沉子纲包围面积为对照网的1.79倍,说明增大网目尺寸有利于网具包围面积的增大,改善了被围鱼群的活动空间。     

Relationships between Stocking and Anglers' Catches in Draycote Water Trout Fishery

The trout fishery at Draycote Water was investigated during the 1980 fishing season. Fish stocked were batch-marked according to the date of introduction by freeze branding, and catch data were obtained by the cooperation of anglers. Population estimates were made at the end of the fishing season using gill nets and mark-recapture techniques. Of the 32,960 marked brown and rainbow trout stocked, 69.8% were caught and declared by anglers. Returns of rainbow trout were better (78.1%) than those of brown trout (44.2%). Over 90% of all fish caught were taken within 45 days of stocking. Catch per unit effort fluctuated widely but was closely associated with stockings of fish. Catchability (Q) of stocked fish was found to diminish rapidly with time after stocking. At the end of the fishing season the estimated population of marked trout was 4045 (2 × SE = 250), compared with a theoretical number (stock less total catch) of 10,802, giving a mean daily apparent natural mortality, made up of the true natural mortality plus the undeclared catch, of 1.36%.     

Identification of histones as endogenous antibiotics in fish and quantification in rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) skin and gill

Antimicrobial polypeptides (AMPPs) are increasingly recognized as a critical component of innate host defense. Among the AMPPs, polypeptides related to histones have been identified from many animals. Using peptide mapping, we further confirm the identity of two histone-like proteins from fish as members of the H2B (sunshine bass) and H1 (rainbow trout) histone groups. We optimized the conditions for measuring rainbow trout HLP-1/H2B via sandwich ELISA. We used two antibodies, one to the amino terminus and one to the carboxyl terminus, of trout histone H2B, as the capture antibodies, and we used peroxidase-labeled antibody raised to calf histone H2B as the secondary antibody. Specificity of the detecting antibody was confirmed by specific reactivity with histone H2B in tissue extracts via western blotting. The test was reproducible and capable of detecting as little as 5 ng of histone H2B (0.05 μg/ml). Histone H2B levels expressed in gill tissue of juvenile, healthy rainbow trout were well within concentrations that are lethal to important fish pathogens. However, there was a significant, age (size)-dependent decline in histone H2B concentrations as fish matured, until levels became virtually undetectable in market-size fish. In contrast, levels in skin appeared to remain high and unchanged in small versus large fish. Antibacterial activity in skin and gill tissues was closely correlated with histone H2B concentration measured via ELISA, which supports our previous finding that histones are the major AMPPs in rainbow trout skin and gill.     

Theoretical study of the influence of twine thickness on haddock selectivity in diamond mesh cod-ends

Using the cod-end simulation model PRESEMO, the influence of twine thickness on cod-end selectivity is investigated. The reduction of lateral mesh opening that arises as a result of both twine bending stiffness and the physical presence of the twine is considered. While it is shown that this leads to a reduction in cod-end selectivity with an increase of twine thickness, it does not fully explain the relationship found in the available experimental data.

The effect twine thickness may have on the ability of a fish to deform a mesh during the early part of a haul and how netting made of thicker twine may discourage a fish from making escape attempts is investigated. The influence that these factors may have is examined and when included in PRESEMO the resulting simulations are a much better representation of the experimental data.