Net Mesh Size Research Articles

The influence of gear selectivity and spawning behavior on a data-poor assessment of a spawning aggregation fishery

We applied several data-poor techniques to perform an assessment of the Gulf corvina (Cynoscion othonopterus) fishery in the Gulf of California from 1997 to 2012 and to investigate the effects of gear selectivity and age-dependent variation in spawning frequency on estimates of sustainability in spawning aggregation fisheries. The length composition of the catch varied significantly among years but showed no clear directional pattern. However, the average length was above the long term average after the implementation of a regulation that standardized mesh size of nets, and the average length of fish captured after the implementation was significantly higher than during previous periods. Results using three simple metrics based on catch length compositions indicated that fishing activities were sustainable due to the exclusion of juveniles from the fishery and the targeted harvest of adults at the optimal length. However, the low proportion of older, fecund fish in the fishery is a serious cause for concern. Modeled estimates of spawning potential ratios (SPR) were consistently higher when spawning frequency was assumed to be age invariant and were significantly higher after the implementation of gear regulations. However, SPR values only reached levels above 35%, a common reference point for sciaenid fishes, during the current fishing period (2010–2012) under conditions of age invariant spawning frequency. Results of this study support previous claims that suggest estimates of reproductive potential are highly sensitive to age-dependent variation in spawning frequency and imply that such details related to spawning behavior require more attention, particularly for fisheries that target spawning aggregations. Our results also suggest that spawning aggregations can be harvested sustainably through conventional regulations if juveniles are excluded, fish are harvested at optimal length, and older, fecund individuals are protected from harvest. Given the uncertainty of the status of the corvina fishery based on the discordant results of this study and the inherent vulnerability of the species to overfishing, We recommend the precautionary approach be applied to management decisions until more robust information is acquired on stock abundance, the relationship between spawning frequency and age or length, and reproductive output.

Age-growth models for tilapia Oreochromis aureus (Perciformes, Cichlidae) of the Infiernillo reservoir, Mexico and reproductive behaviour

In presence of a decrease in the catch from Infiernillo reservoir, Michoacán-Guerrero, Mexico, age and growth of tilapia Oreochromis aureus (Steindachner 1864) were estimated, as indispensable attributes for fishery management. Evaluations included scales and opercular bones readings and length frequency analysis. The population structure was determined too. Due to the great variation of tilapia individual growth and with the objective to have reliable growth estimations, linearized, non-parametric and non-linear methods were used and their applicability discussed. Comparison of ages determined from these methods revealed that opercular bones readings had advantages over scales. Reproduction and competition for space and food were considered as responsible factors for ring-formation on tilapia hard structures. The non-linear method was considered the best for modelling populations highly variable, which is characteristic of tilapia and tropical fish with multiple spawns. The growth parameters estimated were Linfinity = 479 mm, K = 0.46/yr, t0 = -0.055 yr. The tilapia population in Infiernillo reservoir is dominated by juveniles less than 1.5 years old. The fishery activity is affecting the tilapia population growth because are caught when they have not yet spawned. Therefore, this fishery is in urgent need of regulation. Increasing the mesh size of the gill nets might contribute to protect the reproductive potential of the stock.

해수에 잔류하는 미세플라스틱의 정성정량 분석법 확립

미세플라스틱의 분포, 거동, 생태계 영향 등은 해양생태계 보전을 위한 중요한 연구관심사가 되고 있다. 그러나 국내외적으로 통일된 조사 및 정성 정량 분석법이 제시되어 있지 않아 자료들의 비교에 한계가 있다. 본 연구는 해수에 잔류하는 미세플라스틱에 대한 국내외 조사방법들을 고찰하고, 불확실성을 증가시킬 수 있는 과정들에 대해 재현성여부를 평가하여 적용가능한 조사 및 정성 정량 분석법을 확립하였다. 또, 확립된 방법을 적용해 인천경기연안 해수에 잔류하는 미세플라스틱의 분포특성에 대한 사전조사 결과를 제시하였다. 해수는 표면해수층(surface microlayer; SML)과 해수층(subsurface water; SSW)로 나누어 SML층은 2 mm mesh screen를 이용해서 채취하였고 SSW층은 동물플랑크톤채집용 trawl net(mesh size=<TEX>$330{\mu}m$</TEX>)과 식물플랑크톤채집용 hand net(mesh size=<TEX>$20{\mu}m$</TEX>)을 이용해서 채취하였다. 채취된 양에 기초해 계산된 SML층은 대략 <TEX>$100{\mu}m$</TEX>로써 SML층을 잘 반영하는 것으로 판단된다. 미세플라스틱의 우점적인 크기는 < <TEX>$300{\mu}m$</TEX>으로 관측되어 SSW층에 대한 채취는 trawl-net보다는 hand-net이 더 적절한 것으로 판단된다. 적외선 분광분석(IR)을 통해 플라스틱 여부와 polymer성분을 결정하였는데, SML에 대해서는 IR분석결과에 기초해 계수한 농도와 육안계수법에 기초한 농도가 잘 일치하였으나 SSW에 대해서는 그렇지 못해서 기존 연구들의 육안계수에 의한 결과 값들이 불확실할 수 있음이 관측되었다. 본 연구에서 확립된 방법을 적용해서 사전조사한 결과 인천경기연안 지역 해수 중 잔류하는 미세플라스틱 농도는 낙동강 하구연안에서 관측된 값과 유사한 범위에 있었다. 본 연구에서 확립된 분석방법은 미세플라스틱에 대한 향후 연구를 위해 중요한 기초자료로 활용되리라 사료된다. Despite of emerging and increasing concerns to microplastics, no standard methodology has not been proposed for determination of microplastics. This study aims to develop the analysis method for microplastics in seawater by overviewing methodologies proposed by previous studies and by assessing some processes in those methodologies which possibly cause uncertainties in microplastic determination. Furthermore, we present preliminary results of distribution characteristics of microplastics in seawater of Incheon/Kyeonggi coastal region which is based on our new methodology. Microplastics in surface microlayer (SML) and subsurface water (SSW) were collected using mesh screen and planktonic nets (trawl net with <TEX>$330{\mu}m$</TEX> mesh size and hand net with <TEX>$20{\mu}m$</TEX> mesh size), respectively. Microplastics with < <TEX>$300{\mu}m$</TEX> was predominant, indicating hand net as the better collection tool for SSW. As for SML, FT-IR based microplastic concentration was well matched with naked-eye based concentration which has been used in most of previous studies. However, a poor relationship was observed for SSW, indicating that concentration data of previous SSW studies should be corrected. Incheon/Kyeonggi bay seawater contained the similar concentration range with those in coastal region of the Nakdong River. Our methodology can be used as a basic tool for further microplatic studies.

Comparison of catches and species composition for flounders caught using gillnets, gillnets with supporting lines, and trammel nets

To compare the catches made using gillnets, trammel nets, and gillnets with supporting lines, several experiments were conducted with commercial vessels near Uljin and Pohang in Eastern Korea between July 2010 and May 2011. Two sets of 13 different nets were used, including 5 panels of gillnets and trammel nets each with stretched mesh sizes of 7.6, 9.1, 10.6, 12.1, and 13.6 cm and 3 panels of gillnets with a mesh size of 9.1 cm with supporting lines with different line spacing. The outer (stretched) mesh size of the trammel nets measured 51.5 cm. The target fishes of the fishing nets were various types of flounders. The catch rate of flounders was 50.7% of the total catch in weight. The total catch for all nets was 443.8 kg. The predominant species was pointhead flounder (Cleisthenes pinetorum). The total catch by trammel nets was 1.4 times that of the comparable gillnets. But more pointhead flounder were caught by gillnets than by trammel nets, though there was no significant difference. Fishermen catching the pointhead flounder in Korea said that there was no need to use trammel net to catch it; this was an unexpected finding compared to the findings of other flounder fisheries. The amounts of roughscale sole, brown sole, and blackfin flounder caught by trammel nets were greater than those caught by gillnets. The mean lengths (standard deviation) of blackfin flounder, pointhead flounder, brown sole, and roughscale sole were 21.0 (4.57), 22.9 (3.40), 24.7 (4.90), and 28.3 (5.43) cm, respectively; there were significant differences in mean length (p < 0.00001). Therefore, in order to catch flounder efficiently, the fishing nets and mesh size should be chosen according to the target species. One advantage of using supporting lines is that it prevents breakage by strengthening the material especially when utilized on a rough bottom. Catch by using gillnet with supporting lines was not greater than that by using trammel net for the conservation of fisheries resources.

Benthic macroinvertebrate communities as aquatic bioindicators of contamination by Giardia and Cryptosporidium

Benthic macroinvertebrates (community composed mostly by aquatic forms of insects, such as stonefly nymphs, dragonfly nymphs, water bugs or beetle larvae) are often used in biological monitoring programmes to evaluate the ecological status of rivers and thus to indicate the repercussions of anthropogenic activities. The aim of the present study was to evaluate the use of this indicator community to detect human enteroprotozoan parasites that are transmitted via water. In total, 32 samples of macroinvertebrates were collected, with the aid of surber nets of mesh size 500 μm, from nine rivers in Galicia (NW Spain), on different occasions between 2005 and 2009. The samples were homogenised (0.04 M phosphate buffered saline, pH 7.2), sieved (150 and 45 μm mesh), and concentrated (by a diphasic method). Aliquots of the sediments were then analysed by a direct immunofluorescence technique with monoclonal antibodies against Giardia and Cryptosporidium. Giardia cysts were detected in one (3.1%) of the samples and Cryptosporidium oocysts were detected in four (12.5%) of the samples. This work is the first study carried out to investigate the presence of Giardia and Cryptosporidium in this benthic community. The results demonstrate that benthic invertebrates could be used as bioindicators of contamination by these waterborne protozoans. Moreover, as this aquatic organisms act as intermittent accumulators and its monitoring enables chronological analysis of perturbations, in both the short- and mid-term, this may represent a suitable alternative or complementary method to the usual techniques of detecting human and animal enteropathogens in water samples.

Lagoons in the East Coast; are They Potential Sites to Develop Fisheries in Sri Lanka

Lagoons are highly productive ecosystems which provide habitats and nursery grounds formany fish species and contribute to fisheries productions of a country. The status of fisheryin lagoons in the East coast has not been assessed in the past due to civil unrest in thecountry. This study was conducted in two lagoons in Ampara Distrcict; Panama and Pottuvil.The status of fishery was assesed over a six month period using data obtained from the dailycatch brought to the landing centres. Thirty five species of food fish, four species of shrimpsand the mangrove crab, Scylla serrata are harvested from the lagoons using different fishingmethods. The bulk of the food fish caught (63%) in the Panama lagoon were represented bySiganas lineatus, Oreochromis niloticus, Mugil cephalus, Gerres argyreus and Mystus guiliowhile only Oreochromis niloticus and Mugil cephalus contributed to a high percentage (59%)in the Pottuvil lagoon. The total productions of the Panama and Pottuvil lagoons wereestimated to be 74.05 kg/ha/yr and 67.48 kg/ha/yr respectively. The productivity is high inboth the lagoons compared to the average annual productivity of a Sri Lankan lagoon whichis about 22 kg/ha/yr. However, in comparison to the extent of the two lagoons (Panama 73haand Pottuvil 270ha), productivity of the Panama lagoon is significantly higher than that of thePottuvil lagoon (P=0.01) which makes Panama a potential site to develop lagoon fisheries inthe east coast. The extent of the mangrove vegetation (Panama 83ha and Pottuvil 46.6ha)with respect to the total surface area of the lagoons is a possible reason for the productivitydifference of the two lagoons. Both the lagoons show seasonal fluctuations due to rainfall andsalinity changes according to the Canonical Correspondence analysis that was performed andfish migration patterns have been observed in these lagoons. The life styles of the localpeople are highly dependent on lagoon fisheries in these areas thus; the environmental stressis higher on the ecosystem. Non regulation of net mesh sizes, destruction of mangroves,illegal land use and use of motor boats have caused certain environmental issues hence,suitable management practices should be introduced to ensure the sustainability of the lagoonecosystem.

Weight-length relationships of 22 fish species from Paraíba do Sul River in Rio de Janeiro State, southeastern Brazil

The present study reports the weight–length relationships of 22 fish species from middle stretches of the Paraíba do Sul River, a riverine system located in the most important economic regions of Brazil and which is under great pressure due to damming and others ventures. Data were collected quarterly in 2010 and 2011 from three reservoirs and the adjoining downstream by using three different mesh sizes of gill nets. Of the 22 species, eight had no records in the FishBase WLR database; new maximum lengths are given for eight species.

Mesh size and bird capture rates in shasha forest reserve, ile-ife, Nigeria

Few studies have been conducted to verify how mist net mesh size affects the capture rates of birds in Nigeria. This research was undertaken from 06.30 to 18.30 hours between February 2010 and January 2011 at Shasha Forest Reserve (4o20' to 4o40' E, 7o00' to 7o 10' N) near Ile-Ife in southwestern Nigeria, to verify how mesh size affects the capture rates of birds and the efficiency or otherwise of the mist nets involved. Bird species occurrence was recorded using eight nylon mist nets each of 36mm and 61mm mesh sizes in linear transects. The linear transects cuts across different vegetational types with varying degrees of human disturbance. A total of 970 birds were captured and made up of 584 (60.2%) with 36mm mesh and 386 (39.8%) with 61mm mesh. Of the 106 species mist-netted 93(87.7%) were caught by 36mm while 61mm caught 73(68.9%). The 36mm mesh net was more effective for birds with less than 200mm body length while 61mm mesh was more effective for birds with more than 250mm body length. However, both meshes were suitable for birds between 151 and 200mm long. Thirty six millimeter mesh net captured most of the birds below 20g while 61mm mesh captured was more efficient for the larger species. Both mesh sizes were effective for birds with body weights between 41.00 and 80.00g. Deployment of both nets will provide an improvement of abundance estimates for some larger species. By using only 36mm mesh nets, there may be an improvement in the capture rate of small birds and similarly 61mm mesh would substantially improve the capture rate of bigger and heavier birds.

Development, validation and use of a dynamic model for simulate the climate conditions in a large scale greenhouse equipped with insect-proof nets

Easily simulating inside climate conditions in the large insect screened Canary type greenhouses used intensively along the Moroccan coast is particularly important in enhancing this production system. We have therefore developed a global greenhouse climate model based on combined sensible and latent heat balances in which the ventilation flux has been simulated combining buoyancy forces and wind effect.In the region of Agadir in South Morocco, a big commercial greenhouse, farming tomatoes and equipped with insect-proof nets provided the measurements which validated this model. The precise estimation of sensible and latent heat fluxes exchanged among the different components of the greenhouse balanced the energy and the mass in the system so that inside air temperature and water vapour content were deduced accurately.A study on the effects of various types of insect-proof nets was carried out later to analyse the impact on the climate of varying the net mesh size in the greenhouse. It was shown that with an unchanged open vent surface, the use of an anti-Thrips net was incompatible most of time with a satisfactory climate for plant growth in the coastal areas of southern Morocco.

Comparison of Schindler-Patalas Traps and Wisconsin Nets for Monitoring Zooplankton in a Large, Shallow Reservoir

Zooplankton collection methods can differ substantially in the information produced; therefore, determining the best method or methods for a particular ecosystem is essential in understanding limnological processes. We compared results of two sampling methods, Wisconsin net vertical tows and Schindler-Patalas traps, that have been used in a long-term monitoring program in a large, shallow reservoir, Kentucky Lake, USA. Although there were differences in net mesh size and volume of water sampled, statistically similar cladoceran and copepod communities were captured by both methods. Population densities and the number of taxa collected did differ between methods, with many smaller rotifer taxa being found only in the vertical tows, but there were higher densities of larger taxa in the traps. Annual patterns were similar for most larger taxa, except that Wisconsin net tows revealed an autumn density peak for Bosmina longirostris not well-detected in the trap samples. Given the biases of each method, the Schindler-Patalas trap appears to be more efficient overall in long-term monitoring studies, particularly in shallow systems where multiple samples are taken frequently at a number of sites.

서해 꽃게 자망의 망목 선택성 연구

A series of fishing experiments were carried out in the western coastal waters of Korea from May, 2006 to September, 2007, using gill nets of different mesh sizes (m=101, 121.8, 152.5, 176.8, and 191 mm) to determine the mesh selectivity of gill net for swimming crab, Potunus trituberculatus. The catch species was composed of swimming crab (87%), Thomas's rapa whelk Rapana venosa (5%), arthritic neptune Neptunea cumingi (2%), Japanese swimming crab Charybdis japonica (2%), marbled sole Pleuronectes yokohamae (1%), pen shell Atrina pectinata, ocellate spot skate Okamejei kenojei, and so on. The carapace length (CL) of each swimming crab caught in the experimental nets was measured. Sponge crab (egg-bearing female crab) was appeared from the end of June to August in the coastal waters. The master selection curve was estimated by applying the extended Kitahara's method. The selection curve showed that the gill nets of larger mesh size allowed more crabs of small carapace size to escape. The optimum value (CL/m) was 0.645 and the CL/m was estimated to be 0.301, 0.354, 0.391, 0.422, and 0.450 when the retention probabilities were 0.1, 0.2, 0.3, 0.4 and 0.5, respectively.

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

Summary 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.

Fishers' knowledge identifies environmental changes and fish abundance trends in impounded tropical rivers

The long-term impacts of large hydroelectric dams on small-scale fisheries in tropical rivers are poorly known. A promising way to investigate such impacts is to compare and integrate the local ecological knowledge (LEK) of resource users with biological data for the same region. We analyzed the accuracy of fishers' LEK to investigate fisheries dynamics and environmental changes in the Lower Tocantins River (Brazilian Amazon) downstream from a large dam. We estimated fishers' LEK through interviews with 300 fishers in nine villages and collected data on 601 fish landings in five of these villages, 22 years after the dam's establishment (2006-2008). We compared these two databases with each other and with data on fish landings from before the dam's establishment (1981) gathered from the literature. The data obtained based on the fishers' LEK (interviews) and from fisheries agreed regarding the primary fish species caught, the most commonly used type of fishing gear (gill nets) and even the most often used gill net mesh sizes but disagreed regarding seasonal fish abundance. According to the interviewed fishers, the primary environmental changes that occurred after the impoundment were an overall decrease in fish abundance, an increase in the abundance of some fish species and, possibly, the local extinction of a commercial fish species (Semaprochilodus brama). These changes were corroborated by comparing fish landings sampled before and 22 years after the impoundment, which indicated changes in the composition of fish landings and a decrease in the total annual fish production. Our results reinforce the hypothesis that large dams may adversely affect small-scale fisheries downstream and establish a feasible approach for applying fishers' LEK to fisheries management, especially in regions with a low research capacity.

Spring rotifer community structure in the Alcantara River (Sicily, Italy), using different mesh size nets: relation to environmental factors

The present study focus on some aspects of zooplankton structure in the Alcantara River (Sicily, Italy), in relation to environmental factors. Zooplankton samplings were performed in spring in four sites, located from up- to downstream along the river course. Four low-flow velocity station points were chosen along a transversal river section in each site. Samples were taken from all station points in the four sites by two different mesh sizes (55 and 100 μm) rectangular nets. Rotifer abundances were an order of magnitude higher in 55 μm mesh size samples than in 100 μm mesh size ones. The two communities also resulted significantly different (ANOSIM test, ρ=0.212; P=0.1%). Generally, low abundances (from 3470±5133 to 422±474 ind.m−3) were explained by low chlorophyll a concentration and the high-flow regime of this river. Rotifer dominated the zooplankton community. Cladocerans, copepods and nauplii occurred with considerably lower abundances than rotifers. However, the relative contributions of these taxa to total abundances depended on the mesh sizes used. Euchlanis and Adineta genera exhibited the highest abundances in the rotifer assemblage. Conductivity alone or in association with temperature and dissolved oxygen was the most important environmental factor affecting rotifer community distribution. Cephalodella sp., Lepadella sp. and Trichotria pocillum showed a high positive relation to pico-plankton, showing this fraction as a possible rotifer food item. This paper demonstrated a higher efficiency of the finest net to characterize riverine zooplankton community that increases from up to downstream in terms of abundances and diversity.

Effect of a size-selective biomanipulation on nutrient release by gizzard shad in Florida (USA) lakes

Key-words: Dorosoma cepedianum, biomanipulation, size-structure, nutrient excretion Although fish removal for biomanipulation is often highly size-selective, our understanding of the nutrient cycling effects of this size-selection is poor. To better understand these effects, we measured nutrient excretion by gizzard shad (Dorosoma cepedianum )o f differing sizes from four central Florida (USA) lakes and combined these measures with gillnet biomass and size-structure data to compare lake-wide effects among lakes and years. Direct removal of P in fish tissue ranged from 0.16−1.00 kg·P·ha −1 ·yr −1 . The estimated reduction in P excretion due to harvest ranged from 30.8−202.5 g·P·ha −1 ·month −1 , with effects strongly tied to the biomass and size structure harvested. The amount of P release prevented per kg of fish removed was lower in previously unharvested lakes, due to the initial removal of larger fish with lower mass-specific excretion rates. Gill net mesh size impacted the size distribution of harvested fish, with smaller fish that excrete more P per gram being more vulnerable to smaller mesh sizes. In Lake Apopka, decreasing the mesh size by 1.3 cm yielded P excretion reductions that were 10.7−15.1% larger. Fish harvesting to reduce internal nutrient cycling can be most effective by increasing total harvest and by harvesting smaller size classes over multiple years.

The decline of Alestes baremose Boulenger, 1901 and Hydrocynus forskahlii (Cuvier, 1819) stocks in Lake Albert: Implications for sustainable management of their fisheries

The fish stocks of Lake Albert face immense exploitation pressure which has led to “fishingdown” of their fisheries, with some larger species having been driven to near-extinction, while others such as Citharinus citharus have almost disappeared. Both A. baremose (Angara) and H. forskahlii (Ngassia) historically formed the most important commercial species in Lake Albert until the early 2000s but recent Catch Assessment Surveys (2007-2013) revealed a sweeping decline in their contribution to the commercial catch from 72.7% in 1971 to less than 6% in 2013. The catch per unit effort also registered a two-fold decline from 45.6 and 36.1 kg/boat/day to 22.6 and 18.1 kg/boat/day for A. baremose and H. forskahlii respective between 1971 and 2007. Over 50% of illegal gillnets, below the legal minimum limit of four inches (101.6 mm) used on Lake Albert target the two species. Gillnet experiments found the three inch (76.2 mm) gill net mesh size suitable for sustained harvest of the two species. The study concludes that optimal utilization of the two species and probably other non target fish species is achievable through species specific management strategies, coupling species specific licensing, and controlling harvest of juvenile individuals, overall fishing effort and fish catch on Lake Albert and protecting the vulnerable fish habitats. Key words : Albert Nile, ecosystem approach to fisheries, native species, recruitment overfishing, stock collapse

Hybrid seine for full fish community collections

Seines are simple and effective fish collection gears, but the net mesh size influences how well the catch represents the fish communities. We designed and tested a hybrid seine with a dual-mesh bag (1/4″ and 1/8″) and compared the fish assemblage collected by each mesh. The fine-mesh net retained three times as many fish and collected more species (as many as eight), including representatives of several rare species, than did the coarser mesh. The dual-mesh bag permitted us to compare both sizes and species retained by each layer and to develop species-specific abundance correction factors, which allowed comparison of catches with the coarse-mesh seine used for earlier collections. The results indicate that a hybrid seine with coarse-mesh wings and a fine-mesh bag would enhance future studies of fish communities, especially when small-bodied fishes or early life stages are the research focus.

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

연평어장에서 생분해성 꽃게 삼중자망의 어획성능

To study the catching efficiency of biodegradable trammel net for swimming crab (Portunus trituberculatus), three types (biodegradable, monofilament and multifilament) of trammel nets were used in the field test, and the tests were carried out 16 times with two different mesh sizes (105mm and 160mm) in the Yeonpyeong fishing ground of Korea, 2009~2011. The catching efficiency of three type nets was analyzed by catch in number, catch in weight and average weight per individual of small and large size swimming crab by net types and mesh sizes. Statistical T-test was also carried out to verify the efficiency between the three types of nets. The results are as follows. The catch in number of swimming crab was 24,667 and formed about 81.0% of total catch. Of all swimming crab catch, small swimming crabs with less than 64mm in carapace length which is a prohibited landing size by law formed 48.1%, larger swimming crabs with more than 64mm in carapace length which is a landing size formed 51.9%. In 105mm mesh size trammel net test, the catch share in number of small size swimming crab by biodegradable trammel net was 47.5% in comparison with multifilament trammel net and 74.2% in comparison with monofilament trammel net, so biodegradable trammel net has more protective effects on small size swimming crab than other types of trammel nets. The protective effects for small size swimming crab by biodegradable trammel net was 25.8~52.5% in comparison with other types of trammel nets. The catch share in weight of large size swimming crab by biodegradable trammel net was 98.3% in comparison with multifilament trammel net and 92.3% in comparison with monofilament trammel net, so biodegradable trammel net has a similar catch efficiency to multifilament trammel net. The results of 160mm mesh size trammel net test have shown similar results of 105mm mesh size trammel net test. This study shows that biodegradable trammel net is a more useful fishing gear than multifilament and monofilament trammel net because biodegradable trammel net has lower catch rate than other types of nets in small size swimming crab and similar catch rate than multifilament trammel net which is a well used net by fishermen.

A comparative study of accumulated total mercury among white muscle, red muscle and liver tissues of common carp and silver carp from the Sanandaj Gheshlagh Reservoir in Iran

The Sanandaj Gheshlagh Reservoir (SGR) is a mercury polluted lake that is located in the West of Iran. Common carp (Cyprinus carpio) and silver carp (Hypophthalmichthys molitrix) are the most abundant fishes in the SGR. A total of 48 common and silver carps (24 each) were captured randomly, using 50×6m gill net (mesh size: 5×5cm) during July to December 2009. Each month, the levels of accumulated total mercury (T-Hg) in white muscle, red muscle and liver tissues of these fishes were measured using an Advanced Mercury Analyzer (Model; Leco 254 AMA, USA) on the dry weight basis. There were no statistically significant differences between T-Hg concentrations in white muscle, red muscle and liver in common carp in comparison with similar tissues in silver carp (P>0.05). The content of T-Hg in liver tissue of both species was lower than of white and red muscle tissues. Higher levels of accumulated T-Hg were observed during summer. Results showed that T-Hg concentrations in common and silver carps target tissues were strongly dependent on age, length and weight (P<0.05). The results indicated that the levels of accumulated T-Hg in tissues of all samples with weights of over 850g were greater than those limits established by WHO and FAO (500ngg−1).