Gillnet selectivity and the efficiency of alternative combinations of mesh sizes for some freshwater fish

The selectivity curves of nylon gillnets for burbot and two groups of Arctic charr of different fatness were found by the Gulland-Harding method. They are compared to the selectivity curves of whitefish, brown trout, and perch. Based upon these curves and the relationships between most efficient mesh size and fish length, the pooled relative efficiency of a geometrical and an arithmetical series of mesh sizes was calculated for salmonids, burbot, and perch. This efficiency of the geometrical series is different for the three groups of fish, but equal for each group over the interval between the modal lengths of the smallest and largest mesh sizes. The efficiency of the arithmetical series is also different for the three groups, increases linearly with fish length, and is valid over the interval between the modal lengths of the second smallest and the second largest mesh sizes. Each series has its special advantages, and both are easily adjusted to the fish length interval of interest. They are especially adapted for the catch of salmonids. For these, the variations of condition factor is more important than any characteristics of the species, and its influence upon the most efficient mesh size to fish length relationship and the pooled relative efficiency of the two series of nets is evaluated.     

Direct estimates of the selectivity of a multimesh and a series of single gillnets for brown trout

Hatchery brown trout Salmo trutta (1406) of length 5–20 cm were released into a 7.5 ha shallow lake. They were recaptured with 28 multimesh gillnets of a type proposed for use in the Nordic countries and 42 single gillnets of mesh size 8–22 mm (knot to knot). Corrected for the effect of different twine thicknesses, the single nets showed the highest selectivity for 16.5 mm and smaller meshes and the multimesh nets for the larger meshes. The height of the selectivity curves increased exponentially with mesh size, and most distinctly for the multimesh nets. Over a limited range of mesh sizes the selectivity of the two types of net did not differ much. Extended over mesh sizes 10.0–55.0 mm, the catching abilities of the two types of net became considerably different.     

Anadromous trout threatened by whitefish gill‐net fisheries in the northern Baltic Sea

We estimated the effect of the gill‐net fisheries targeted at whitefish (Coregonus sp.) on anadromous sea trout, Salmo trutta, in the Gulf of Bothnia, Baltic Sea using separate data for fish species. The analysis of sea trout captures was based on tagging and recapture data collected in 1998–2011, while whitefish data were derived from individual samples of commercial fisheries from the same period. The mesh sizes used in gill‐net fishing and the seasonal and temporal distributions of recaptured sea trout and sampled whitefish were compared in the northern and southern Gulf of Bothnia. The trout had typically spent 1–2 years at sea, and they were mainly immature with a median body length of 40–43 cm at the time of recapture in gill nets. Despite the increase in the minimum permitted landing size from 40 to 50 cm in 2008, the median length of recaptured trout remained unchanged during the study period. Most (59%) of the gillnetted trout were caught in the southern Gulf of Bothnia in gill nets with mesh sizes of 40–45 mm, which were also used in the whitefish fishery (72%). In the northern Gulf of Bothnia, nets with a smaller mesh size of 25–39 mm took 83% of the whitefish catch and 39% from recaptured trout. In both areas, the overlap in mesh sizes used to gill‐net catch whitefish and sea trout increased during the study period. There were clear seasonal and areal differences in the relative probability of sea trout being captured in gill nets, suggesting that carefully tailored spatial and temporal restrictions on gill‐net fisheries could provide a tool to protect young sea trout without causing intolerable difficulties for the fisheries targeting other species.     

Evaluating catches of salmonids taken by gillnets

Data from 10 185 gillnetted brown trout ( Salmo trutta L.) and 5777 Arctic charr [ Salvelinus alpinus (L.)] were analysed. Relative to mesh size, the most efficiently caught fish length (the modal length) was somewhat larger with floated nets than with smaller nets placed on the bottom. There were no such consistent differences between the two species. Fish that exceeded the modal lengths were caught more efficiently than smaller fish. The relationship of most efficient mesh size to fish length and condition factor fitted excellently to a linear equation. Based on this equation, the selectivity curve for Arctic charr gillnetted in the Nesjø reservoir, the different efficiency of nets of 0.1 and 0.17mm twine and an equation expressing the increasing height of the selectivity curve with increasing mesh size, a model is presented to compare length frequencies and total numbers of gillnet catches taken with any combination of mesh sizes.     

鱼类早期资源调查中不同网具采集效率的研究

用网目为0.500和0.776mm的筛绢分别制成长度为1.5、2.0和2.5m的弶网、圆锥网和纺锤形网,采用网箱式、滤过式集苗桶和非滤过式集苗桶3种集苗器,于1998年5-6月在长江九江江段进行了采集仔鱼效果的比较研究。结果显示,弶网采集的数量最少,纺锤形网采集的数量最多。不同长度网具采集的数量没有显著差异,但长度为1.5m的圆锥网所采集仔鱼的规格明显小于长度为2.0和2.5m圆锥网所采集的。用网口面积分别为0.38m~2和0.20m~2的圆锥网组与不同类型集苗器进行比较,所采集仔鱼的数量和规格均没有差异。不同网目网具所采集仔鱼的数量没有显著差异,但网目为0.500mm圆锥网所采集仔鱼的平均规格显著小于网目为0.776mm圆锥网的。综合比较,以网目为0.500mm的纺锤形网的采样误差较小。     

Sampling and filtration efficiency of two commonly used plankton nets. A comparative study of the Nansen net and the Unesco WP 2 net

The paper presents the results from a study in which two commonlyused plankton nets, the Nansen net and the WP 2 net, were compared.The study was performed at six stations in the Baltic Sea andthe samples were collected during seasons of both high and lowparticle abundance. The comparison included both qualitativeand quantitative filtration capacity of the nets under differentenvironmental conditions. Different mesh sizes were also tested.The results show that the Nansen net has an efficiency of 50–70%compared to the WP 2 net under favourable conditions. When conditionsare unfavourable, i.e. during periods of high particle abundanceor during long hauls, the efficiency may be as low as 25–30%.The mesh size of the nets proved to be of decisive importancefor the qualitative composition of the samples. Most of thesmaller size fractions such as rotifers, copepod nauplii andyoung copepodites passed through the 160 and 200 µm meshes.The 90 µm nets sampled these fractions well but with thedrawback of retaining vast amounts of phytoplankton at certaintimes of the year.     

Length–weight relationships of seven fish species in the Yangtze River Estuary and its adjacent waters,Eastern China

The length–weight relationships (LWRs) for seven fish species belonging to five families in the Yangtze River Estuary and its adjacent waters are presented. Cynoglossus gracilis, Cynoglossus macrolepidotus, Ctenotrypauchen chinensis, Collichthys niveatus, Ophichthus apicalis and Erisphex potti were collected monthly in 2009 using trammel nets (the size of smaller mesh net was 1.5 m × 15 m × 4 panels with mesh sizes of 2.5, 3.4, 4.3 and 5.8 cm; the size of larger-mesh net was 2.4 m × 30 m × 4 panels with mesh sizes of 5.0, 6.0, 7.0 and 8.0 cm; soaking overnight). Lophiogobius ocellicauda were collected December in 2020 using shrimp trawl nets (mesh size 2.0 cm). The precision of measurement for the fish specimens is 0.1 cm total length and 0.1 g total weight. The estimated ranges of the parameters a and b for the seven fish species were from 0.0001 to 0.0289 and 2.718 to 3.541, respectively. Two new maximum total length were recorded for Ctenotrypauchen chinensis and Ophichthus apicalis.     

Assessment of two mesh sizes for interpreting life cycles, standing crop, and percentage composition of stream insects

Using Surber-type samplers and dip-net samplers, we assessed the efficiency of nets having pore sizes of 720 μm and 320 μm for determining standing crop and percentage composition of the stream fauna, and for collecting representative size-class specimens of Ephemeroptera and Plecoptera to be used in life-cycle studies. Except for one species, samples collected with either the 720-μm or 320-μm dip-net led to the same general inferences about the species' life cycle. Of fifty possible sample comparisons, there were twelve samples where the size-class frequencies of particular species collected in the 720-μm dip-net were significantly different from the size-class frequencies of the 320-μm dip-net; for five of these samples a deficit of large nymphs (> 5.0 mm) in the 320-μm net mainly contributed to the significant χ² values. On one date, we used double-bag samplers with both th e 720-μm and 320-μm nets attached to either the Surber or dip-net sampler. Approximately 50% of the insects by numbers passed through the 720 fxm mesh ofeach sampler, but only 5% by volume-biomass. Shape of the insect as well as body length was important in assessing mesh-size efficiencies. The 720-μm mesh of the double-bag dip-net sampler retained most of the Nemoura dnctipes (having stout appendages) and Epeorus longimanus (flattened) nymphs 2.0 mm in body length and larger; whereas most Baetis (streamlined) nymphs smaller than 3.0 mm and all Paraleuctra (needle-like shape) nymphs passed through the 720-μm mesh.     

Biomass and Abundance Biases in European Standard Gillnet Sampling

The European Standard EN 14757 recommends gillnet mesh sizes that range from 5 to 55mm (knot-to-knot) for the standard monitoring of fish assemblages and suggests adding gillnets with larger mesh sizes if necessary. Our research showed that the recommended range of mesh sizes did not provide a representative picture of fish sizes for larger species that commonly occur in continental Europe. We developed a novel, large mesh gillnet which consists of mesh sizes 70, 90, 110 and 135mm (knot to knot, 10m panels) and assessed its added value for monitoring purposes. From selectivity curves obtained by sampling with single mesh size gillnets (11 mesh sizes 6 – 55mm) and large mesh gillnets, we identified the threshold length of bream (Abramis brama) above which this widespread large species was underestimated by European standard gillnet catches. We tested the European Standard gillnet by comparing its size composition with that obtained during concurrent pelagic trawling and purse seining in a cyprinid-dominated reservoir and found that the European Standard underestimated fish larger than 292mm by 26 times. The inclusion of large mesh gillnets in the sampling design removed this underestimation. We analysed the length-age relationship of bream in the Římov Reservoir, and concluded that catches of bream larger than 292mm and older than five years were seriously underrepresented in European Standard gillnet catches. The Římov Reservoir is a typical cyprinid-dominated water body where the biomass of bream > 292mm formed 70% of the pelagic trawl and purse seine catch. The species-specific relationships between the large mesh gillnet catch and European Standard catch suggested that the presence of carp (Cyprinus carpio), European catfish (Silurus glanis), tench (Tinca tinca) or bream warrants the use of both gillnet types. We suggest extending the gillnet series in the European Standard to avoid misinterpretation of fish community biomass estimates.     

Ultrastructure and Histochemistry of the Food Trapping Mucous Film in Benthic Filter-Feeders (Ascidians)

Transmission and scanning electron microscopy of food trapping filters or “mucous films” of several Ascidians reveal the same type of extensive net with elongate rectangular meshes. The ultrastructural appearance and mesh size varies only slightly within the group. Pore sizes from 0.2 to 0.5 μm in width and from 0.5 to 2.2 μm in length with a thickness of the filaments from 10 to 40 nm are probably representative for the nets under in vivo conditions. These observations corroborate earlier experimental results which indicate a remarkably high and constant efficiency of filtration in all species kept undisturbed. Great quantities of water are evidently filtered through these nets at relatively low resistance and particles even below bacterial size (0.5 μm) are effectively trapped. Histochemical reactions indicate the presence of both mucoproteins and acidic polysaccharide components and that the filaments are probably made up of a peptide core surrounded by polysaccharides.     

Mathematical modelling of implant in an operated hernia for estimation of the repair persistence

This paper presents mathematical modelling of an implanted surgical mesh used in the repair process of the abdominal hernia. The synthetic implant is simulated by a membrane structure. The author provides a material modelling of the implant based on the dense net model appropriate for technical fabrics. The accuracy of the proposed solution is evaluated by comparing the simulations of the dynamic behaviour of the system with the experiments carried out on physical models of implanted mesh. The model can be used to estimate the repair persistence for different mesh materials, fixing systems and different numbers of tacks to be provided during the surgery in order to resist the cough pressure and required action to avoid hernia recurrence. The persistence of the repaired hernia is assessed on the basis of the values of the forces in the tissue–implant joints because the usual form of the repair failure is due to as the joint disconnection or tissue failure.     

A morphological and geometric method for estimating the selectivity of gill nets

We propose a new method for estimating gill net selectivity which estimates the probabilities leading to retention by analyzing both the fish morphology and the mesh geometry. This method estimates the number of fish approaching and contacting gill nets of different mesh sizes as an intermediate step towards computing the selectivity. Instead of assuming an underlying probability distribution as in indirect methods, we split the entire interaction between a fish and the gill net into several stages, each with its own probability. All the necessary parameters to compute these probabilities can be obtained from measurements of the fish, knowledge of the mesh geometry, and catch data from different mesh sizes. The framework offers three pathways for computing the total number of fish contacting the gill nets and has the capability to use both wedged and entangled fish in the analysis. As a proof of concept, the method is applied to catch data for cod (G. morhua) and Dolly Varden (S. malma) to estimate the number of fish contacting the gill nets in both cases. By estimating the number of fish contacting the gill net in addition to the selectivity, this method provides an important step towards deriving estimates of fish density in a particular fishery from gill net measurement.     

Net mesh size affects production of giant freshwater prawn Macrobrachium rosenbergii cultured in lake‐based cages

Cage culture of freshwater prawns in open waters is prone to the entry of predators and competitors that particularly hamper production. This study was conducted to determine how smaller net mesh sizes to reduce entry of unwanted species inside the cages affects the production of Macrobrachium rosenbergii in lake‐based cages. Juvenile prawns were stocked in cages (7 × 7 × 1.5 m) of two net mesh sizes at 10 individuals m^?2 and cultured for 10 months in a shallow eutrophic lake in the Philippines. The two net mesh sizes were either 5 mm‐mesh B‐nets or and 1 mm‐mesh Hapa nets. Each treatment had four replicates each and was fed based on biomass with commercially formulated feed. Monitoring of various production parameters was done during the two phases of culture: batch phase on days 63 and 127 and the selective harvest phase on days 187, 219, 253, 281 and 313, when the experiment was terminated. For the first 127 days of culture, the weight, percent weight increase, daily growth rate (DGR), specific growth rate (SGR), yield and feed conversion ratio (FCR) were significantly better in prawns reared in the Hapa compared to the B‐nets. During the selective harvest phase the blue claw, orange claw and berried females were selectively harvested and the remaining prawns returned to the cages. After changes in stocking density through culling, ancova was used to compare the effect of mesh size with the total number of prawns returned to the cages as a covariate. Yield was significantly higher in the Hapa nets. Weight, DGR, SGR and FCR were also consistently higher in the Hapa nets, although not always significantly different. The overall better performance of prawns reared in the Hapa net cages was due to: (i) the reduction in the entry of predator and competitor species in the finer‐meshed Hapa compared to the larger mesh B‐net, (ii) more natural food trapped inside the Hapa cages, and (iii) a higher number of selectively harvested prawns, which decreased stocking density in the cages and improved growth. Use of small mesh size nets is recommended in the cage culture of M. rosenbergii in inland natural water bodies.     

Gillnet selectivity for Chilean hake (Merluccius gayi gayi Guichenot, 1848) in the bay of Valparaíso

To determine the selectivity of gillnets in the Chilean artisanal fishery for the Chilean hake Merluccius gayi gayi, an experimental net was used with three mesh sizes (5.2, 6.8 and 7.6 cm). A total of 2279 specimens of Chilean hake were caught (24–56 cm total length, TL), mainly via gilling and snagging. The catch proportion below the size of sexual maturity SSM_50% (37 cm TL) for each mesh size tested was estimated. Models were fitted separately according to the sex to compensate for differences in size compositions, with the males fitted to a binormal model and females fitted to the normal location model. Analysing both sexes combined, the model with the lowest deviance was lognormal, with estimated modal lengths of 30.9, 40.2 and 43.9 cm TL for meshes of 5.2, 6.8 and 7.6 cm, respectively. The estimated selectivity factor (SF) indicates that the modal size matches the SSM_50% with a mesh size of 6.2 cm, while the length of retention of 50% (l₅₀) coincides with the SSM_50% with a mesh size of 7.6 cm.     

Length–weight relationships for 13 fish species in the Tangwang River,a tributary of Songhua River,China

The present work provides length–weight relationships (LWRs) for 13 fish species belonging to six families. The fishes were collected with gill nets (mesh sizes 10, 20, 40, 60, 90, 120 mm) and set nets (5, 8, 10 mm mesh sizes) from April 2012 to September 2015.     

Length–weight and length–length relationships of seven fish species from Irtysh River and its tributaries,northwest China

Length–weight and length–length relationships are reported for seven fish species belonging to four families and seven genera from the Irtysh River and its tributaries, northwest China. A total of 1,351 individuals were collected using three layers of drift gill nets (inner mesh size 13 cm, outer mesh size 26 cm), set gillnets (mesh size 2.5 cm), and purse net (mesh size 1 cm) on a monthly basis from April to October 2013. One length–weight relationship, five length–length relationships, and two maximum lengths are presented for the first time in FishBase. This biometric data and their relationships are relevant to the management and conservation of local fishes and fisheries.     

Evidence for the secondary sexual development of the anal fin in female kokanee salmon Oncorhynchus nerka

This study examines whether the anal fin undergoes secondary sexual development similar to other reproductive traits in salmonids. This hypothesis was tested by comparing the anal‐fin size of female kokanee salmon Oncorhynchus nerka that were in the early and late stages of sexual development. Females in an advanced stage of maturation had significantly larger anal fins relative to females in an early state of maturation (+4–7%), indicating that the anal fin undergoes secondary sexual development. The magnitude of this secondary growth was comparable with snout length (+9–10%), which is known to undergo secondary sexual development in female salmonids. When morphological trait dimensions were compared between the sexes, the anal fin was the only morphological trait found to have a female‐biased sexual size dimorphism. This is the first study to show that the anal fin of female salmonids undergoes secondary sexual development.     

插网网目尺寸及采样时间对潮沟游泳动物采样的影响

在长江河口九段沙盐沼湿地的一条潮沟中,我们比较了两种不同网目尺寸（4mm和8mm）的插网（fyke net）及日夜潮对游泳动物群落采样效率的影响。共捕获鱼类16种5476尾、虾2种4982只。采用方差分析、柯尔莫诺夫-斯米尔诺夫非参数检验法和无度量多维标定分析对两种网目插网的捕获效率,样品的体长分布,群落结构及日夜潮间的差异进行了分析。结果表明,网目尺寸对样品中游泳动物的物种数和鱼类的多度没有影响,但是小网目的插网对虾类有较高的捕获率。两种网目尺寸的插网均在夜潮中捕获较多的鱼类个体,但虾类在日潮捕获较多。在日潮中,网目尺寸对捕获物中五种优势游泳动物的体长分布无显著影响。在夜间,小网目的插网对斑尾复缎虎和脊尾白虾的较大个体具有较高的捕获率,但对较大个体鲅的捕获率不如大网目的插网。样品中游泳动物的群落组成在日夜潮间存在明显差异,大网目捕获物的样本间差异比小网目小。     

Length–weight relationships of four indigenous fish species from Brahmaputra River,Assam, India

The present study was undertaken with the objective to estimate the length‐weight Relationships (LWRs) for four species collected from the Brahmaputra River, Assam, India. The fish samples were collected quarterly from different sampling sites using gill net (mesh size ranges 1–1.5 cm, locally called Mola lungi jal), cast net (mesh size ranges 1.0–2.0 cm, locally called Kevalijal) and triangular dip nets (mesh size ranges 0.75–1.25 cm locally called khorajal) during the time period between May, 2016 to April, 2018 (2 years). The b value of the LWR ranged from 3.028 (for Parambasis lala) to 3.196 (for Psilorynchus balitora). In this study the relationship between length‐weight were highly correlated (p < 0.05).     

Mesh-size effects on drift sample composition as determined with a triple net sampler

Nested nets of three different mesh apertures were used to study mesh-size effects on drift collected in a small mountain stream. The innermost, middle, and outermost nets had, respectively, 425 µm, 209 µm and 106 µm openings, a design that reduced clogging while partitioning collections into three size groups. The open area of mesh in each net, from largest to smallest mesh opening, was 3.7, 5.7 and 8.0 times the area of the net mouth. Volumes of filtered water were determined with a flowmeter. The results are expressed as (1) drift retained by each net, (2) drift that would have been collected by a single net of given mesh size, and (3) the percentage of total drift (the sum of the catches from all three nets) that passed through the 425 µm and 209 µm nets. During a two day period in August 1986, Chironomidae larvae were dominant numerically in all 209 µm and 106 µm samples and midday 425 µm samples. Large drifters (Ephemerellidae) occurred only in 425 µm or 209 µm nets, but the general pattern was an increase in abundance and number of taxa with decreasing mesh size. Relatively more individuals occurred in the larger mesh nets at night than during the day. The two larger mesh sizes retained 70% of the total sediment/detritus in the drift collections, and this decreased the rate of clogging of the 106 µm net. If an objective of a sampling program is to compare drift density or drift rate between areas or sampling dates, the same mesh size should be used for all sample collection and processing. The mesh aperture used for drift collection should retain all species and life stages of significance in a study. The nested net design enables an investigator to test the adequacy of drift samples.