Nominal Mesh Size Research Articles

Estimation of selectivity parameters for target and bycatch fishes of the trammel net fisheries in the northern Aegean Sea (eastern Mediterranean Sea)

The size selectivity of trammel nets was investigated in the northern Aegean Sea using 10 different inner-panel mesh sizes ranging from 16 to 70 mm nominal mesh size (bar length). Selectivity estimates were made for the eight most abundant target and bycatch fish species, namely black scorpionfish, Scorpaena porcus Linnaeus, 1758; annular seabream, Diplodus annularis (Linnaeus, 1758); red mullet, Mullus barbatus Linnaeus, 1758; surmullet, Mullus surmuletus Linnaeus, 1758; round sardinella, Sardinella aurita Valenciennes, 1847; European hake, Merluccius merluccius (Linnaeus, 1758); greater weever, Trachinus draco Linnaeus, 1758; and blotched picarel, Spicara flexuosum Rafinesque, 1810, which accounted for 51.5% by number and 42.7% by weight of the fish caught with trammel nets in the sea trials. The SELECT method was used to estimate the selectivity parameters. Five different selectivity functions (i.e., normal scale, normal location, gamma, log-normal, and bi-normal) were applied with the bi-normal function providing the best fit as it had the lowest deviance value for all species and the lowest values for the dispersion parameter ( D /df). The mesh size of 16 mm in most of the cases retained specimens below the size at first maturity ( L m ). The mesh size of 19 mm seems more appropriate for red mullet, surmullet, and blotched picarel, the mesh size of 22 mm for annular seabream and round sardinella, while for European hake and black scorpionfish, the mesh size larger than 26 mm would be more appropriate.

Catch Comparison of Commercial and Experimental Deep Water Cast Nets with Different Mesh Sizes for Whiting (Merlangius merlangus euxinus) Fishery

In the present study, the commercial (16 mm nominal mesh size) and experimental deep water cast nets with different mesh sizes (20 mm, 24 mm, 28 mm and 32 mm nominal mesh size) were tested to compare the captured fish lengths and catch amounts for whiting (Merlangius merlangus euxinus). Experiments were carried out between July, 11 and August, 29, 2018 in Rize province of the south-eastern Black Sea Region. A commercial fishing boat was chartered for 20-day sea trials and in total, 66 set operations were done for all nets. Generalized Linear Mixed Models (GLMM) was used to compare fish sizes caught with experimental and commercial deep water cast nets. Results showed that more than 70 percent of the individuals caught in cast nets with 16 mm, 20 mm and 24 mm size were below the minimum landing size of whiting (13 cm). Among tested nets, only cast net with 32 mm size caught significantly less individuals under the minimum landing size. Finally, the results obtained from this study are discussed in terms of sustainable fisheries of whiting in the Black Sea.

Estimating selectivity of experimental diamond (T0) and turned mesh (T90) codends in multi-species Mediterranean bottom trawl

The effect of shifting from a diamond mesh codend (T0) to a 90° turned mesh codend (T90) on the size selectivity of seven commercially important species in the Mediterranean bottom trawl fishery was evaluated. During sea trials conducted in the north-western Adriatic, two experimental codends made of 54 mm nominal mesh size netting that differed only in mesh configuration were alternately mounted on the same trawl. Overall, the size selectivity was significantly improved for all the species analysed, shifting from T0 to T90. The difference in the average expected L50values between T90 and T0 was particularly marked in European hake (Merluccius merluccius, 21.26 vs 11.26 cm total length), common squid (Loligo vulgaris, 12.06 vs 7.88 cm mantle length) and mantis shrimp (Squilla mantis, 20.78 vs 13.35 mm carapace length). Both codends had an excessive size selectivity especially for red mullet (Mullus barbatus), Mediterranean horse mackerel (Trachurus mediterraneus) and whiting (Merlangius merlangus), thus involving a commercial loss. These findings demonstrate the efficiency of the T90 codend tested at excluding undersized specimens, especially of hake, whose average L50was above the minimum conservation reference size of 20 cm. The adoption of this practical and inexpensive solution could help improve the management of the demersal resources targeted by the Mediterranean bottom trawl fishery.

ОЦЕНКА УРОВНЯ ЭКОНОМИЧЕСКОЙ БЕЗОПАСНОСТИ МИНЕРАЛЬНО-СЫРЬЕВОГО КОМПЛЕКСА БРЯНСКОЙ ОБЛАСТИ

Silicon carbide black grinding powders 54CF180-F36, mass-produced at Volzhsky Abrasive Plant are studied. Powders are screened into fractions according to GOST R 52381. Measurement of a grain width of each fraction was carried out by means of a photome-tric method using specially developed software. For the vast majority of fractions the grain width follows the normal distribution law. The deviation of the grain width from the normal law can be observed at the frac-tions settled on the second sieve and the pallet. With decreasing the nominal mesh size of a pass sieve in each grit size the arithmetic mean of grain width of the main fraction decreases, dispersion reduces from the fraction settling on the second sieve to the fourth and increases at the fraction settling on the pallet. In case of the identical mesh size of a pass sieve the mean and dispersion of grain width fraction increase with grinding powder grit increase. For each grit size the functional dependences of the mean and dispersion of grain width fraction on a mesh size of a pass sieve are established.

ЗАВИСИМОСТЬ СРЕДНЕГО И ДИСПЕРСИИ ШИРИНЫ ЗЕРЕН ШЛИФОВАЛЬНЫХ ПОРОШКОВ КАРБИДА КРЕМНИЯ ЧЕРНОГО ОТ РАЗМЕРА ЯЧЕЙКИ ПРОХОДНОГО СИТА

Silicon carbide black grinding powders 54CF180-F36, mass-produced at Volzhsky Abrasive Plant are studied. Powders are screened into fractions according to GOST R 52381. Measurement of a grain width of each fraction was carried out by means of a photometric method using specially developed soft-ware. For the vast majority of fractions the grain width follows the normal distribution law. The deviation of the grain width from the normal law can be observed at the fractions settled on the second sieve and the pallet. With decreasing the nominal mesh size of a pass sieve in each grit size the arithmetic mean of grain width of the main fraction decreases, dispersion reduces from the fraction settling on the second sieve to the fourth and increases at the fraction settling on the pallet. In case of the identical mesh size of a pass sieve the mean and dispersion of grain width fraction increase with grinding powder grit increase. For each grit size the functional dependences of the mean and dispersion of grain width fraction on a mesh size of a pass sieve are established.

MORPHOMETRIC CHARACTERIZATION OF THE SAND FRACTION IN A SAND GRAIN IMAGE CAPTURE SYSTEM1

ABSTRACT Morphology studies assume significant importance in analysis of phenomena of granular systems packaging, in particular with a view to the use of the technique of soil stabilization named particle size correction in forest roads. In this context, this study aimed to develop and operationalize a Sand Grain Image Capture System and, hereby, determine the morphological indices of the sand fractions of two sandy soils called João Pinheiro (JP) and Cachoeira da Prata (CP). Soil samples, air-dried, were sieved (2.0 mm nominal mesh size) for removal of gravels. The materials that passed through the sieve were subjected to dispersion, washing in 0.053 mm nominal mesh size sieve, removal of organic matter and iron oxides to obtain the clean sand fractions. Subsequently, each soil sample was sieved for separation into twelve classes, between the diameters of 0.149 mm and 1.190 mm, using a Rotap shaker. Next, tests were carried out to characterize the morphometric attributes of the twelve classes of sand fractions of the soils studied. For validation of the performance of the Sand Grain Image Capture System, the results were compared to those obtained using a standard procedure for image analysis. The analysis of the results led to the following conclusions: (i) the sand fraction of the JP soil presented higher values for the morphometric indices roundness, elongation and compactness compared to sand fraction of the CP soil; and (ii) the Sand Grain Image Capture System worked properly, with practicality.

Selectivity and retention of pollock Pollachius virens in a Gulf of Maine trawl fishery

Measuring the size selectivity of pollock, or saithe, Pollachius virens, has been challenging due to the patchy, elusive nature of the species. By using data collected opportunistically during a redfish Sebastes fasciatus selectivity study, selectivity of three sizes of mesh opening (114, 140 and 165mm double 5mm twine diamond) for pollock was determined using a commercial fishing vessel fishing off Provincetown, Massachusetts, USA. Fifty-six tows were completed in March and April 2013; 21 included sufficient catches of pollock to estimate size selectivity. Robust, simple models for the mean L50s (50% selection length) and SR (selection ranges), and confidence intervals, were developed for all three tested codends, incorporating both within and between haul variability. Selection parameters and selection ranges were determined for codends with nominal mesh sizes of 114mm (L50: 34.8cm; SR: 2.4cm), 139mm (L50: 45.6cm; SR: 3.1cm), and 165mm (L50: 52.4cm; SR: 3.6cm). All measures of model validity were positive, indicating robust and reliable findings that can be used to provide guidance to fishery managers, stock assessment scientists, and fishermen on size-dependent retention of pollock by codend mesh size.

The effect of monofilament thickness on the catches of gillnets for common sole in the Mediterranean small-scale fishery

Experimental fishing trials were carried out on an Italian commercial vessel to test gillnets of different monofilament diameters targeting common sole (Solea solea). The gillnets only differed in the nominal monofilament diameter: 0.18, 0.20, 0.22, 0.25, and 0.30mm. The nominal mesh size (stretched) was 72mm. A total of 47 taxa were caught. The mean number of species caught with 0.30mm monofilament was significantly lower than those obtained with the three thinnest diameters. Total number, number of commercial and discard of non-commercial species significantly decreased with the increase of monofilament diameter. The number of common sole and tub gurnard caught was not statistically different amongst diameters. No statistical differences were observed for the length–frequency distributions of common sole and tub gurnard caught with the different monofilament diameters. The analysis of the mechanism of capture for the common sole showed that for all diameters the main type of retention was snagging, and the percentage of individuals retained in such way decreased with the increase of diameters, while an opposite trend was observed for the common sole that were gilled. Monofilament thickness had a substantial effect upon the catch rates of the gillnets.

Assessment of the Selective Properties and Optimization of Mesh Size of Pelagic Trawl Codends, Used for Fishing Mackerel (Trachurus Spp) in the Gulf of Guinea

Over the past few decades, it has become widely recognized that the management strategies of world fisheries must ensure sustainability of target species. The intervening years have seen many improvements to the concept of gear selectivity and methods for measuring the selectivity of fishing towed gears. Improved understanding of the principles of the selection of fish by gears has changed the list of parameters which are known to have a significant effect upon selection. The recent development of new mathematical models and the increased availability of powerful computers have resulted in improvements in the analysis procedures for the data produced to measure a gear’s selectivity. The catch of mackerel in the gulfofGuineahas steeply declined during the last two decades, and resource management is clearly required. Therefore, the need for evaluation of trawl codends used in this fishery and the potential improvements to their selectivity are of prime importance. In this paper, we use semi-empirical models to define selective properties of pelagic trawl codends targeting black mackerel (Trachurus spp) in theGulfofGuinea. These properties are determined using the experimental and theoretical methods of assessing the parameters of the selectivity curve, and by plotting the curve. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Trawl codend selectivity is estimated for 17 internal diamond mesh sizes in the range 47 - 79 mm. Using the basic selectivity equations, we determine the needed mesh size A = 58 mm for fishing mackerel in the gulf of Guinea. This nominal mesh size gives room for nj = 0.1 catch of juveniles, which not exceeds the allowable proportion [nj] = 0.1. To provide resource conservation, there is the need to make amendments in the fishery regulations for more rational exploitation of mackerel stoks, because the currently use nominal mesh size A = 56 mm is rather unselective.

Gillnet Selectivity for Bluefish (Pomatamus saltatrix, L. 1766) in Çanakkale Strait, Turkey

To determine the selectivity of gill nets with 22, 23, 25 and 28 mm nominal mesh size (bar length) for bluefish (Pomatamus saltatrix, L. 1766), the study was conducted at three different stations between 20 and 30 m water depths at Canakkale Strait, Turkey, between February 2007 and April 2012. Gill net with 22, 23, 25 and 28 mm mesh size (bar length) each having same twine thickness (210 denier / 3) and 100 mesh depth vertically were rigged for this purpose. The fish samples were obtained by drive-in fishery method during winter season. The SELECT method was used in determining selectivity parameters. By comparing the deviances of five different models of SELECT method, lognormal model gave the best fit. The modal lengths of the gill nets with 22, 23, 25, and 28 mm mesh size were estimated 22.24, 23.25, 25.27 and 28.30 cm, respectively. The results of the trials indicated that minimum gill net mesh size for this fish should use larger than 25 mm

Effect of twine thickness on selectivity of gillnets for bogue, Boops boops,in Turkish waters

To investigate the effect of twine thickness on the selectivity of multifilament gillnet targeting bogue,Boops boops L., four different stations were sampled between March and November 2008 in the northern Aegean Sea. Gillnets with 22, 23, and 25 mm nominal mesh size (bar length) each having two different twine thicknesses (approximately 0.45 mm and 0.54 mm ∅) were applied for this purpose. The deviances from the SELECT method revealed that lognormal models provided the best fits for both of the twine thicknesses. Results from the two-way ANOVA analyses revealed that the mean total lengths increased with the mesh size (F = 87.36; df = 2; P < 0.0001) and decreased with the twine thickness (F = 46.12; df = 1; P < 0.0001). The 22 mm mesh size net (0.45 mm ∅) captured significantly larger fish than the 23 mm mesh size net (0.54 mm ∅) probably due to the higher elasticity and flexibility of the thinner twine. Thus, fisheries managers should take into consideration the twine thickness while advising mesh size regulations in gillnet fisheries.

Effect of mesh size and codend circumference on selectivity in the Mediterranean demersal trawl fisheries

The effects of mesh size and codend circumference on selectivity of some commercially important species were evaluated. Four different codends were used in the sea trials. The nominal mesh sizes tested were 48 mm and 56 mm and two different nominal circumferences were associated with each mesh size. Selectivity experiments were carried out in the Central Adriatic Sea, in the course of two different periods of the year (May and September 2005) on two different fishing grounds normally exploited by local fishermen. Reliable selectivity results were obtained for red mullet ( Mullus barbatus), little squid ( Alloteuthis media) and European hake ( Merluccius merluccius). For all the species, the codend circumference plays a role which is as important as mesh size. The positive effect of increasing mesh size, from 48 mm to 56 mm, could be made fruitless by an increase in codend circumference. According to species, the change only in the mesh size from 48 mm to 56 mm significantly increases L50 by +15–42%, while by increasing only the codend circumference of around 13–17%, L50 decreases by 9–41%, thus being effective as by increasing mesh size. As a consequence the selection factor (i.e. SF is the ratio of L50 and the codend mesh size) depends on the codend circumference and does not result mesh-size-independent. We confirm that it is not sufficient to regulate selectivity in multispecies fisheries just by setting a minimum mesh size and that there is a need in mixed fisheries to improve selectivity by using additionally mechanisms. Possible solution would be to design policies and management programmes that seek to minimise discards, which include active rather than static management, obligations to land all catches, and no MLS and reduced benefits from landings of juvenile fish.

Selectivity of conventional and alternative codend design for five fish species in the Aegean Sea

Summary Currently, traditional Mediterranean trawls are generally made with non-selective netting and the fishing boats are involved in multi-species fisheries. As a result, most near-shore stocks are over-exploited. Weather permitting, the demersal trawl fleet tends to fish in relatively deeper, international waters of the Aegean Sea, where the catch is usually higher. Therefore, the need for evaluation of the codends used in this fishery and the potential improvements to their selectivity are of prime importance. In the present study, selectivity data were collected for hake (Merluccius merluccius), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and fourspotted megrim (Lepidorhombus boscii) in commercial (300 MC) and square mesh top panel (SMTPC) codends. Trawling was carried out at depths of 274–426 m onboard a commercial vessel chartered for a 15-day sea trial in August 2004. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Results of the selectivity analysis indicated that the commercially used 40 mm nominal mesh size PE codend was rather unselective for the species investigated in this study. In general, the square mesh top panel codend has relatively higher L50 values than the commercial codend. However, except for blue whiting, even this codend is rather unselective when 50% maturity lengths (LM50) are considered.

Aspects of the fishery, maturation and net selectivity of the wedge sole (Dicologoglossa cuneata) off south-western Iberia: implications for fishery management

The wedge sole (Dicologoglossa cuneata, Moreau 1881) is a commercially important species for the artisanal fleet operating off the south-western Iberian Peninsula. During 2004 and 2005 a study was undertaken aiming to provide a scientific basis for management. Data collection included information on aspects of the fishery, spawning season, size at first maturity, tangle and gill-net selectivity. Seven nominal mesh sizes were used (40, 45 and 50 mm tangle nets and 40, 50, 60 and 70 mm gill-nets). Spawning lasts from December to June, with peaks in March and May. Length at first maturity for females was estimated at 18.5 cm. Catch rates decreased sharply with increasing mesh size, with tangle nets providing the highest yields. The log-normal selectivity model provided the best fit for specimens that were wedged. The higher catch-per-unit-effort of the smaller mesh sizes and the modal length of the fitted selectivity curve being below the size-at-maturity for wedge sole, suggests that the 50 mm nominal mesh size tangle net is the most appropriate for ensuring the fishery sustainability. Nevertheless, the minimum legal size should increase to at least 18 cm and a time–area closure should be implemented off the Guadiana River mouth.

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%.

Selectivity experiments to estimate the effect of escape windows in the Skagerak roundfish fishery

The objectives were to measure roundfish selectivity and to test if square-mesh windows inserted in the codend could improve the selectivity. Sea trials were conducted with a commercial trawler in the Skagerak area. Three codend types were tested: (1) a standard codend with 104 mm meshes; (2) a standard 104 mm codend with two 85 mm square-mesh side windows; (3) a standard 104 mm codend with an 85-mm square-mesh top window. The twin-trawl method was used where one side of the rig had a 35-mm (nominal mesh size) control codend. Hauls of each codend were fitted simultaneously in a fixed- and random-effects model. In total, 24 and 30 hauls of haddock and cod, respectively, were included in the final selectivity analysis. This indicated that insertion of a top window in a standard codend improved the selectivity resulting in a lower selectivity ratio (SR/L50) and a higher L50 for cod and haddock, whereas insertion of side windows gave a lower selectivity ratio for both species, but a higher L50 for cod only. The results suggest that the recent mesh size increase to 120 mm in a standard codend is not sufficient to reduce the probability of discards to a negligible level with the existing minimum landing sizes.

Gill net selectivity for European hake Merluccius merluccius from southern Portugal: implications for fishery management

: A study of European hake Merluccius merluccius gill net selectivity was undertaken off the Algarve (Southern Portugal), between 1999 and 2001. Four nominal mesh sizes (70, 80, 90 and 100 mm) were used in fishing trials and the ‘share each length class catch total’ (SELECT) method was used to fit gill net selectivity curves. Hake were caught in the same size range by all mesh sizes, between 17 and 65 cm total length. While most fishes were wedged, significant and similar proportions were entangled in all mesh sizes, contributing to the wide size range, and in some cases, bimodal shape of catch size frequency distributions. Insignificant numbers of undersized hake were caught, with most catches consisting of mature female fish. Catch rates decreased sharply with increasing mesh size. The bimodal model gave the best fit for hake that were wedged, with estimated modal lengths of 40.1, 46.7 and 51.0 cm for the 70, 80 and 90 mm nominal mesh sizes, respectively. The high catch per unit effort of the smallest mesh size, with most fish caught being female, together with the fact that the modal length of the fitted selectivity curve is well below the size at maturity for hake in Portuguese waters, suggests that the 80 mm nominal mesh size is more appropriate for ensuring resource sustainability.

Ghost fishing by lost cod gill nets in the Baltic Sea

An experiment was set up to study the catching efficiency of lost bottom gill nets. In September 1998 and May 1999, 24 fleets of normal commercial gill nets were deployed in a typical gill net fishing ground in the Hanö Bay in the southern Baltic Sea (ICES sub-division 25). At each of the nine fixed exposure times up to 27 months, three randomly chosen fleets were retrieved. The catch comprised mainly of cod ( Gadus morhua) and flounder ( Pleuronectes flesus). The experimental catch rate of cod was compared to that of commercial nets used in the same area at the same time. During the first 3 months, the relative catching efficiency of “lost” nets was found to decrease rapidly by around 80%, thereafter stabilising around 5–6% of the initial level. There was strong evidence that the experimental nets would have continued to catch fish beyond the observed 27 months. It was observed that nets with a long exposure time seemed to have a tendency of catching smaller individuals than would be implied by their nominal mesh size.

Escape windows to improve the size selectivity in the Baltic cod trawl fishery

A rapid decrease of the stock of Baltic cod ( Gadus morhua) has provided the incentive to improve the size selectivity in the trawl fishery. Use of escape windows is permitted in the legislation to give means of improving the size selectivity of cod as an alternative to a traditional standard codend. The history of the use of escape windows in the Baltic Sea cod fishery is reviewed. The present escape windows do not function optimally. The objective of this new experiment was to compare an improved design of escape window, which is placed in the upper panel, with that of standard codend. Three standard codends, with 105, 120 and 140 mm nominal mesh size, and three window codends, with 110, 125 and 135 mm window nominal mesh size, were tested during sea trials conducted in June and July from a commercial trawler using a twin trawl rig and the covered codend technique. A fixed and random effects model of the codend selectivity was formulated to analyse the results and determine the effects of codend type, mesh size and other recorded variables. L50 and SR increased significantly with the mesh size. L50 was significantly increased and SR significantly reduced for a window codend with the same window mesh size (WMS) as that of a standard codend. The L50 of the standard codends decreased with catch weight. An unexplained port–starboard effect was found for the window codends where the L50 depended on which of the two trawls the codend was attached to. The potential of these escape windows as a management tool to improve the size selectivity in the Baltic cod fishery is discussed.

The effect of twine thickness on the size selectivity and fishing power of Baltic cod gill nets

Sea trials were carried out on a Danish commercial vessel measuring the size selectivity and fishing power of gill nets used to catch Baltic cod ( Gadus morhua). A comparison was made of two different twine thicknesses at two different times of the year. Nominal mesh sizes of 70–130 mm were used. Method of capture, condition factor and girths were measured for sub-samples of the cod caught. A model of the size selectivity of the gill nets was adapted to the experimental conditions where two gears were fished on the same population. This model was fitted to the catch data for each set. Subsequently a model was fitted for the mean selectivity taking between-set variation into account. The selectivity curve that fitted the data best was given by the sum of two normal distributions. It was found that twine thickness and trials period had relatively little effect upon the shape of the selectivity curve. Twine thickness had a substantial effect upon the fishing power of the nets.