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.     

Temperature dependent growth and yield of pikeperch, Sander lucioperca, in Finnish lakes

Abstract  A non-linear growth model was used to evaluate the effects of temperature and age on annual length increments of pikeperch, Sander lucioperca (L.), in seven lakes in Finland. Length increments were derived by back-calculation using the Fraser–Lee method. Annual length increments increased from age 1 to age 3 and then decreased, while at the same time length increments and air temperature had positive correlation until age 12. Age- and size-structured yield per recruit models were used in two lakes to evaluate the effects of temperature and gillnet mesh size on pikeperch yield. In these two lakes maximum yield could be obtained with 60 and 70 mm (bar length) gill nets. In the second lake, as typically in Finland, 45–50 mm gill nets are the most frequently used. The use of larger mesh size gill nets would increase pikeperch yield from 685 to 1000 g per recruit based on the present mean temperature. In both lakes increase in temperatures would increase yield if mesh size is simultaneously increased. Higher pikeperch yield can be expected in the future because of climate warming.     

Estimation of gillnet selectivity curve by maximum likelihood method

ABSTRACT: A maximum likelihood method of estimating gillnet selectivity when data are obtained by gillnet fleets consisting of several nets of differing mesh size is presented in this paper. The SELECT model is expanded by application of the relative length (i.e. the ratio of fish length to mesh size) to obtain a master curve of gillnet selectivity. Four kinds of functional model, normal, lognormal, skew-normal and bi-normal are fitted to the data. In addition, two cases in which the relative fishing intensity is either estimated or fixed by catch effort are compared. The bi-normal model has the lower model deviance regardless of whether the relative fishing intensity is estimated or not. The estimation of relative fishing intensity by catch effort is also examined in which the estimates of the parameter of the SELECT model are compared with the catch effort as determined by the number of nets of each mesh size used. For the bi-normal model these quantities compare well. Thus, it is concluded that this method gives reliable estimates even if data for each mesh size is obtained with different catch efforts.     

Effect of thin twine on gill net size-selectivity analyzed with the direct estimation method

ABSTRACT: To analyze the effect of thin twine on gill net size selectivity, a series of fishing experiments using gill nets of two twine thicknesses (no. 0.8 of 0.16 mm diameter and no. 3 of 0.28 mm diameter) and three nominal mesh sizes (41 mm, 46 mm and 51 mm) was conducted in outdoor water tanks containing a rainbow trout ( Oncorhynchus mykiss ) population of known size frequency. For nets of the same nominal mesh size, mesh openings of the no. 0.8 twine nets were approximately 2 mm larger than those of the no. 3 twine nets. The effects of both mesh opening and twine thickness are combined in the conventional analysis with the selectivity curve against fish length. Selectivity against the ratio of fish girth to mesh perimeter was calculated to avoid the direct effect of changes in mesh openings due to twine diameter at the same nominal mesh size. Results demonstrated that the thinner twine gill net was more likely to capture rainbow trout of smaller girth than the mesh perimeter. Fish with girths smaller than the mesh perimeter were often retained by the net with its body tangled by the thinner twine, possibly due to the increased flexibility of the thinner twine.     

Analysis of mesh breaking loads in cotton gill nets: Possible solution to ghost fishing

ABSTRACT:   A small number of fishers in Chiba Prefecture of eastern Japan use cotton gill nets to catch Japanese spiny lobster Panulirus japonicus. To examine the advantages of cotton gill nets, we analyzed changes in mesh breaking load of a new cotton gill net used in a fishing operation. A new cotton gill net was also soaked in a seawater tank to simulate ghost fishing conditions. The average mesh breaking load of new cotton mesh was 50.3 N. This value decreased to 19.0 N after 38 days (∼912 h), and after 82 days (∼1968 h) the mesh could be easily torn (breaking load 0.07 N). Under fishing conditions, the cumulative soak time was only 744.4 h over 19 months. The average breaking load at the end of this period was 43.1 N, a strength 86% that of the presoaked mesh. The mesh breaking load of a cotton gill net continuously soaked for 744.4 h was 26.1 N, as estimated from tank experiment data. Thus, a cotton gill net maintains reasonable strength under typical use conditions, but will degrade if lost at sea.     

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.     

Effects of a fishing gear on an artisanal multispecific penaeid fishery in a coastal lagoon of Mexico: mesh size, selectivity and management implications

Abstract  The spatiotemporal effects of a bottom net used in an artisanal multispecific penaeid fishery at Celestun Lagoon, Mexico, were evaluated. Selectivity experiments, based on the covered codend method, were carried out with two bottom nets operating simultaneously but using different mesh size codends: 13 mm (MS13: currently used in the fishery) and 25 mm (MS25). Length at 50% retention, L ₅₀, was twice as high with MS25 (15.34 mm carapace length, CL) than with MS13 (6.95 mm CL); this global pattern was recurrent for all species, seasons and hydrological zones. These differences in selectivity determined that MS13 mainly retained recruits and juveniles, whereas MS25 mainly retained sub-adults. However, smaller shrimps were caught by both mesh sizes during the 'nortes' season (November to February) and at the outer zones of the lagoon. Thus, an increase in mesh size from 13 to 25 mm, together with fishery closures within specific area–season windows, is suggested to reduce risks of overfishing of these shrimp populations.     

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.     

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。     

Size selectivity of the Spisula solida dredge in relation to tooth spacing and mesh size

A study was undertaken on the northwestern coast of Portugal to evaluate the selectivity effect of both tooth spacing and mesh size on the catch of the white clam (Spisula solida). Two typical clam dredges, equipped with different tooth spacing and mesh size, were towed side by side. Three tooth spacing (20, 40 and 60 mm) and three mesh sizes (35, 40 and 50 mm) were investigated. The dredge selectivity experiments were performed by attaching a cover bag with a 20 mm mesh to the gear. Individual hauls were analysed by the SELECT model. The Fryer’s mixed model of between-haul variation was also applied to data. The results showed that the space between teeth does not have an effect on selectivity. Tooth length was directly related to the dredge’s capture efficiency. The only factor that contributed to dredge size selection was mesh size. The 50% retention lengths estimated for mesh sizes 30, 40 and 50 mm were 25.68, 26.35 and 31.94 mm, respectively. For all mesh sizes, very narrow selection ranges were observed. The most appropriate mesh size to be enforced within the Portuguese northwestern S. solida fishery should be 40 mm.     

Selectivity estimates for Amblygaster sirm (Clupeidae) in the small-meshed gill net fishery on the west coast of Sri Lanka

Selectivity of Amblygaster sirm by gill nets of seven mesh sizes ranging from 2.3 to 3.8 cm stretched mesh was studied for a period of 12 months in the coastal waters off Negombo on the west coast of Sri Lanka. The size range of fish caught was 9.0–22.0 cm. Selection by mesh sizes smaller than 3.0 cm was towards the lower end of the selection curve.

Estimated values for selection factors ranged from 5.11 to 6.03 and those for optimum selection lengths varied from 12.9 to 19.7 cm. The highest selection factor was observed for 2.9 cm mesh, probably due to wedging of larger individuals in this mesh.     

蓝点马鲛(Scomberomorus niphonius)大网目流刺网参数优化研究

在对黄渤海区蓝点马鲛(Scomberomorus niphonius)流刺网进行10种网型主要参数分析和筛选的基础上,2013年4月10日—5月15日,使用鲁昌渔64068渔船在黄海中南部对蓝点马鲛大网目流刺网网片做了4种规格网目尺寸的优化对比实验。结果显示,120 mm 网目尺寸的网片渔获尾数和重量最大,单位捕捞努力量渔获量(Catch per unit effort, CPUE)为0.1696尾/片·日和373.78 g/片·日,渔获效果优于110、115、125 mm 的网目。2015年4月12日—5月27日,使用鲁昌渔64068和鲁莱州渔66287渔船分别在黄海中南部、烟威外海、莱州湾海域对蓝点马鲛大网目流刺网主要参数做了网型优化对比实验。结果显示,鲁昌渔64068渔船使用120 mm 网目尺寸网型捕获的蓝点马鲛平均叉长为764.18 mm,最大个体叉长为1000 mm,最小个体叉长为490 mm;平均重量为3623.27 g,最大重量为7500 g,最小重量为850 g;CPUE 分别为0.1379尾/片·日和517.65 g/片·日。鲁莱州渔66287渔船使用122 mm网目尺寸网型捕获的蓝点马鲛平均叉长为780.47 mm,最大个体叉长为910 mm,最小个体叉长为600 mm;平均重量为4065.17 g,最大重量为7400 g,最小重量为2000 g;CPUE分别为0.0778尾/片·日和283.35 g/片·日。120 mm 网目尺寸网型的渔获效果优于122 mm 网目尺寸的网型,春季捕捞大个体蓝点马鲛的流刺网网目尺寸以120 mm 左右适合当前的蓝点马鲛群体组成。120 mm 网目优化网型主要参数为：渔具主尺度33.00 m×18.54 m,网衣规格500目×175目,网目尺寸为120 mm,网线材料为直径0.48 mm 的尼龙单丝(PAM),横向缩结系数为0.55,静浮力配备为267.9 g/m,浮沉比为1.768∶1。120 mm 网目优化网型的另一特点为通过缩小上缘网网目尺寸,用上缘网替代上缘纲和浮子纲2根纲索,既保持了渔具主要结构和性能不变,又节约了渔具材料。设计方法为：上缘网的网目尺寸=主网衣网目尺寸×主网衣横向缩结系数。     

Tailoring codend mesh size to improve the size selectivity of undifferentiated trawl species

Tiger flathead, Neoplatycephalus richardsoni (Castelnau), and sand flathead, Platycephalus bassensis Cuvier, are undifferentiated and managed with a common legal minimum length (LML). The Commonwealth Trawl Sector (CTS) and the Tasmanian Danish‐seine fishery (TDSF) use a minimum codend mesh sizes of 90 and 70 mm, respectively. The codend mesh size should be tailored to the LML, which is based on the length of first maturity of females (M₅₀). This study found the length–girth relationship of N. richardsoni and P. bassensis was not significantly different. Using the covered codend method, these two species had 50% retention lengths (L₅₀) of 294 ± 2 and 307 ± 3 mm, in 70‐mm and 90‐mm codends, respectively. L₅₀ estimates for mesh sizes from this study and others produced a curvilinear relationship: y = 120 ln(x) ? 214, with an r² of 0.8504. The size at maturity (M₅₀) for female N. richardsoni was 337 mm, which is larger than the estimate for female P. bassensis (247 mm). There is a mismatch between the estimates of L_50, the estimates of M₅₀ and the LMLs in each fishery, leading to suboptimal exploitation of female Platycephalus. The model produced in this paper recommends a codend mesh size of 98 mm for both fisheries to exploit Platycephalus species sustainably.     

Size selectivity of Korean flounder (Glyptocephalus stelleri) by gillnets and trammel nets using an extension of SELECT for experiments with differing mesh sizes

In 2008, Korean flounder (Glyptocephalus stelleri) was designated by the Korean Ministry for Food, Agriculture, Forestry and Fisheries as a species for restoration in the coastal waters of Korea. However, there are currently no regulations regarding the minimum landed size of flounder, or the minimum legal mesh size used by the coastal gillnet and trammel net fisheries. Selectivity experiments for gillnets and trammel nets were conducted using commercial vessels in several areas off the eastern coast of Korea between October 2006 and June 2009. The selectivity analysis required an adaption of the SELECT method because trammel net mesh sizes were altered part-way through the experiment. Gillnet and trammel net selectivity was best described using a bi-modal selection curve. The two gears were found to have different selection curves for a given mesh size. The length of maximum retention was similar for the two gears, but the selection curve of the trammel net was substantially wider than that of the gillnet. For the largest size of mesh used in the commercial fishery (8.5 cm stretched), 50% relative retention was estimated to occur at a length of 20.4 cm for the trammel net, and 22.5 cm for the gillnet. In comparison, the length of 50% maturity for this species is approximately 26 cm. As there are currently no size restrictions on landings of Korean flounder, these results suggest that use of these gears may pose a serious threat to local stocks of this species.     

莱州湾海域春季不同网目尺寸地笼网渔获性能分析比较

为了让地笼网在人工鱼礁区进行捕捞许可提供科学依据,2018年4月在渤海莱州湾海域采用套网法进行地笼网不同网目尺寸(20、30、40、50、60 mm)的渔获物调查,探讨不同网目尺寸地笼网的渔获性能。对试验结果从相对重要性指数(IRI)、渔获量、主要渔获物优势体长、尾数和重量逃逸率、经济品种渔获量和口虾蛄体长分布进行分析。结果显示,试验共捕获渔获物37种,其中优势种8种;渔获物的数量与种类逃逸率随着网目尺寸增加而增加,40~60 mm网目种类逃逸率达80%左右,尾数逃逸率超过90%,质量逃逸达50%以上,表明40~60 mm网目具有明显的释放效果;经济品种渔获量占比与网目尺寸呈正相关;以主要渔获种类口虾蛄分析,使用50 mm网目捕捞未达到可捕体长的口虾蛄占比最少,为10.3%;经综合分析,50 mm网目尺寸地笼网的渔获性能较好,符合生产和生态的要求,因此推荐在海洋牧场海域使用50 mm网目地笼网。     

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.

     

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 diamond and square mesh codends for hairfin anchovy Setipinna taty in Chinese stow net fisheries

ABSTRACT:   Selectivity experiments of diamond and square mesh codends with different mesh size (30, 35, 40 and 45 mm) for hairfin anchovy Setipinna taty were carried out using the covered codend method in Chinese stow net fisheries. In the present study, 5151 specimens were subsampled and asymmetric and symmetric selectivity curves (Richards and Logistic curves) were fitted to the catch data, using the maximum likelihood method according to the SELECT model. The results show that the Richards curve gives a better fit to the selectivity curve for diamond mesh codends with 30, 35 and 40-mm mesh sizes, whereas the logistic curve gives a better fit for the diamond mesh codend with 45 mm mesh size and square mesh codends with 30, 35, 40 and 45-mm mesh size. With the exception of 45-mm mesh size codends, the L ₅₀ of diamond mesh codends were insignificantly larger than those of square mesh codends. However, the selectivity curves of square mesh codends were steeper than those of diamond mesh codends, except the 30-mm mesh size codends. As for diamond mesh codends, according to the linear relationship between the L ₅₀ and mesh size, 36.5-mm mesh size codends could release 50% juvenile hairfin anchovy (below 1-year old). However, diamond mesh codends with such a mesh size are not selective for other species.     

近海张网对黄鲫和小黄鱼的年龄选择性研究

渔具对捕捞对象的年龄选择性对于渔业资源评估等工作是至关重要的,但相关研究较少。使用套网法对网囊网目尺寸分别为35 mm和45 mm的近海张网对黄鲫(Setipinna taty)和小黄鱼(Larimichthys polyactis)的年龄选择性进行分析。数据分析采用2种年龄选择性曲线模型:即估算网囊对个体体长选择性后根据个体生长方程换算和直接使用Logistic曲线代表年龄选择性曲线;通过对渔获体长分布和年龄分布数据的拟合,结果显示,2种年龄选择性曲线均能较好地拟合数据,网囊对黄鲫和小黄鱼的50%选择年龄t50和年龄选择范围SRt均随着网目尺寸的增加而增加,35 mm和45 mm网目尺寸的网囊对黄鲫的t50分别约为4月龄和8月龄,对小黄鱼的t50分别约为0.6龄和1.5龄,明显小于黄鲫和小黄鱼的推荐开捕年龄,反映出目前近海张网渔具网目过小,可能会对幼鱼资源产生不利的影响。     

Evaluation of a simple means to reduce discard in the Kattegat-Skagerrak Nephrops (Nephrops norvegicus) fishery: Commercial testing of different codends and square-mesh panels

Discarding of fish species in the Kattegat-Skagerrak Nephrops directed fisheries remains at high levels. In this work we have tested four sets of codends pair-wise with the catch comparison technique under full commercial conditions to assess their potential in reducing the catch of undersized commercial species, in particular cod. We investigate the position of a 90 mm square-mesh panel (SMP), the effect of increasing the mesh size from 90 to 120 mm in the SMP, the effect of inserting a 90 mm SMP in an 80 mm codend and the effect of increasing the codend mesh size from 90 to 120 mm in a total of 89 hauls. Inserting the SMP 3–6 m from the codline compared to 6–9 m from the codline, reduced catches of cod above the minimum landing size (MLS) and Nephrops above and below the MLS. Increasing the mesh size in the SMP from 90 to 120 mm increased the catch of cod above the MLS in weight with an overall increase of 12% in the revenue. The effect of installing a 90 mm SMP 6–9 m from the codline in a nominal 80 mm codend had no effect on the catch of cod. Increasing the nominal codend mesh size from 90 to 120 mm reduced the catch of all species below the MLS, except monkfish. Catches of Nephrops above the MLS were, however, reduced by approximately one third and the total loss in revenue was 21%.