Gillnet Selectivity Research Articles

Selectivity of multifilament trammel and gillnets for common carp (Cyprinus carpio L., 1758) in Lake Marmara

This study was aimed to estimate the selectivity properties of multifilament trammel and gillnets for common carp ( Cyprinus carpio L., 1758) in Marmara Lake in Manisa, Turkey. Trammel and gillnets of 4, 6, 8, and 10 cm stretched mesh size and 210 d/2 no twine thickness were tested in the Marmara Lake. In addition, selectivity parameter of 11, 12, 13 and 14 cm mesh size of trammel and gillnets were modelled. SELECT method was used to estimate the selectivity parameters. Experiments were carried out monthly in the three different stations identified eastern, middle and western areas which represent longitudinal length of the lake. A total of 36 trials were conducted both nets. A total of 119 common carps were caught ranges between 11.3 - 49.0 cm total lengths. According to the Bi- normal model, which gave the lowest deviance for both trammel and gillnets, selectivity curves were estimated for 4, 6, 8, 10, 11, 12, 13 and 14 cm stretched mesh size modelled lengths as 12.98, 19.47, 25.96, 32.45, 35.70, 38.94, 42.19 and 45.43 cm for trammel nets, 12.40, 18.60, 24.80, 31.00, 34.10, 37.20, 40.30 and 43.40 cm for gillnets, respectively. When considering minimum landing size (40 cm total length), it is recommended that both nets under 13 cm mesh size should be prohibited for common carp fishery in the Lake Marmara. Keywords: Trammel net, gillnets, common carp Cyprinus carpio, size selectivity, Lake Marmara

Seletividade de redes de emalhar para uma espécie de peixe não-nativo em um reservatório neotropical, Paraná, Brasil

Biological invasions are considered the second main cause of loss biodiversity in aquatic environments. Thus, many techniques have been developed with the aim of eradicating undesirable species. Then, this paper had the purpose of estimate the selectivity of the gillnets used to catch Odontesthes bonariensis, introduced specie in Salto Santiago reservoir, Iguassu River, Brazil. Samples were made bimonthly from July 2003 to March 2013. Altogether 6,952 individuals were captured. The mesh sizes of 2.5 cm and 3.0 cm are the most effectively both captured 89.90% of the individuals. The optimal sizes of capture estimates for mesh sizes of 2.5 and 3.0 cm were 13.35 cm and 16.00 cm, respectively. Thus, we suggest the intense use of gillnets with mesh sizes less than 3.0 cm, to capture small sized individuals at first maturation.

Implications of using small meshed gillnets for the sustainability of fish populations: a theoretical exploration based on three case studies

AbstractThe study explores the impacts of varying gillnet mesh size and fishing level on yield per recruit (Y/R), escapement spawning stock (ESS) and mega‐spawners (MS) of three widely distributed freshwater fishery target species. Y/R is maximised when the optimal length of capture (Lcap) is above the size of maturity (L50). However, the unimodal shape of gillnet selectivity results in lower impacts to ESS and MS with both smaller and larger mesh sizes. Under conditions of moderate exploitation, the fraction of MS was significantly larger if small meshed gillnets were used. This is due to the relatively smaller cumulative vulnerability from small mesh sizes through time, as they target a smaller size range of fish, which also grow more quickly through the vulnerable window due to higher growth rates. Therefore, unlike trawls and beach seines, which select all size classes beyond the minimum length of capture (Lc), small meshed gillnets are not necessarily destructive and may rather promote sustained production by allowing a higher proportion of the spawning biomass to remain in the stock. The work also helps to explain the observation of sustained fish production in many developing countries despite the persistent use of gillnets of small mesh size that target small, under‐sized individuals.

Gear selectivity of fishery target resources in Lake Koka, Ethiopia: evaluation and management implications

We evaluated the statistical models that best describe the selectivity of common fishing gears used in Lake Koka for four freshwater species widely distributed in Ethiopia and elsewhere: tilapia Oreochromis niloticus (L.), catfish Clarias gariepinus (B.), common carp Cyprinus carpio (L.), and barb Labeobarbus intermedius (R.). Sampling was conducted from October 2012 to March 2013 using gillnets (60, 80, 100, and 120 mm stretched mesh), longlines (4/0, 6/0, and 8/0 hooks size), and a beach seine with covered codend. Size at maturity (L m) was determined for each species. The SELECT method was used to explore unimodal selectivity models for gillnet and longline gears, while the logistic function was used for beach seine. Results show that a log-normal model best described gillnet selectivity for all species, while a normal scale model best described longline selectivity for C. gariepinus. Gillnets with ≥100 mm mesh and longlines with >4/0 hooks could be safely used, as they allow target resources to attain L m before becoming vulnerable to fishing. The mesh size of the beach seine needs to be enlarged, as the estimated length of capture was much smaller than all L m values. The results have important management implications for protecting juveniles and mega-spawners of the studied species in Lake Koka and beyond.

Monofilament gill net and trammel net selectivity for the silver crucian carp (Carassius gibelio Bloch, 1782) in the Eğirdir Lake, Isparta-Turkey

The aim of this study was determine the selectivity of gillnet and trammel net with different mesh sizes for C. gibelio in Egirdir Lake. Gillnets with mesh sizes of 32, 40, 50, 60, 70, 80 and 90 mm; and trammel nets with of 100, 110, 120, 130 and 140 mm were tested for catching of C. gibelio. Fieldwork was carried out in two different station of Egirdir Lake, in a monthly period from December 2009 to April 2010 and with a total of 10 catching trials. The SELECT method was used to determine the selectivity parameters. Number of 592 C. gibelio species were caught in the experiments which the lengths ranged between 7.3 - 34.8 cm. According to the Bi-modal model which gave the lowest deviation for multifilament gillnets, the optimum lengths were determined as 8.74, 10.92, 13.65, 16.38, 19.11, 21.84 and 24.57 cm for 32, 40, 50, 60, 70, 80 and 90 mm mesh sizes respectively. According to the Normal scale model which gave the lowest deviation for trammel nets, optimum lengths were determined as 27.20, 29.92, 32.64, 35.36 and 38.08 cm for 100, 110, 120, 130 and 140 mm mesh sizes in trammel net respectively.

Gillnet selectivity for freshwater fish species in three lentic systems of Greece

Summary Gillnet size selectivity was studied for freshwater fish species, based on experimental fishing trials carried out with multimesh gillnets in lentic freshwater systems in Northern Greece. Selectivity estimates were based on a large range of mesh sizes, i.e. more than 10 different mesh sizes ranging from 8 to 90 mm bar length. Results showed that the model, in which both mean and standard deviation of the curve were defined as a linear function of the mesh size, revealed the best fit. For seven (i.e. Αlburnus sp. Volvi, Aspius aspius, Carassius gibelio, Lepomis gibbosus, Pachychilon macedonicum, Squalius prespensis and Vimba melanops) of the 11 studied species and the hybrid (Alburnus belvica × Rutilus prespensis), gillnet selectivity parameters were estimated for the first time, contributing to the evaluation of gillnet fisheries' impacts on fish species populations and consequently to fisheries management and species conservation.

The size selectivity of whiting (Merlangius merlangus euxinus) caught by gillnet in the eastern Black Sea of Turkey

The size selectivity of traditional gillnets for whiting, (Merlangius merlangus euxinus) was investigated in the eastern Black Sea between June 2010 and June 2011. Experimental fishing operations were carried out by using gillnets of 16, 17, 18, 20 and 22 mm mesh size. A total of 2038 specimens belonging to 16 different fish species were caught during the experiment. The Share Each Length's Catch Total method was used to fit gillnet selectivity curves. Gillnet selectivity was best described by a bi-modal selectivity curve. The modal catch sizes were estimated as 14.81, 15.74, 16.66, 18.51 and 20.37 cm for 16, 17, 18, 20 and 22 mm mesh sizes, respectively. Modal lengths and spread values increased with increasing mesh size. The majority of fish (71.8%) caught by 16 mm mesh size were less than the length at first sexual maturity.

Hook selectivity of Giant Trevally (Caranx ignobilis) and Nakedbreast Trevally (Carangoides gymnostethus) (Carangidae) caught in the hook-and-line fishery off Negombo, Sri Lanka

In the present study, an attempt was made to quantify hook selectivity of two carangid species (Caranx ignobilis and Carangoides gymnostethus) which were predominantly caught in the hook-and-line fishery off Negombo, Sri Lanka. As the length frequency data of C. ignobilis and C. gymnostethus recorded by hook size were skewed towards the larger fish, a modification of Baranov-Holt method for gillnet selection was employed. For both species there were positive significant relationships (p<0.001) between the hook size and optimal length while selection range was negatively related to hook size (p<0.001). DOI: http://dx.doi.org/10.4038/sljas.v16i0.6712 Sri Lanka J. Aquat. Sci. 16 (2011): 11-26

Gill net selectivity of yellow perch

Yellow perch Perca flavescens (Mitchill) are typically collected using multi-mesh gill nets; however, gear selectivity information for this species is sparse. Selectivity using experimental gill nets was determined for yellow perch using data collected from southern Lake Michigan from 2007 to 2012 using stretch mesh sizes of 12.5, 16, 20, 25, 32, 38, 46, 51, 64, and 76 mm. Female selectivity was best described by a model with a skewed (log-normal) distribution while males were best described by a normal distribution. When sex data were combined, the skewed (log-normal) distribution provided the best model. Differences in selectivity between sexes were minimal, but likely due to the variant size structure of the local population sampled, with females the larger sex. These findings suggest that bias in individual gill net selectivity can be used to target specific yellow perch size groups or to replace other gear types with unknown selectivity.

Size selectivity of gill-nets and growth of roachRutilus rutilus(Linnaeus, 1758) an alien species in Piediluco lake (Italy)

The analysis of catch selectivity of fishing equipment is a crucial step for the management of alien species in freshwater ecosystems. Roach Rutilus rutilus was introduced in many waterways in Italy often becoming the dominant species of the fish community and it is now considered potential pest. In Piediluco lake the species was first found in 2004 and its abundance has increased at a level that creates concern for the other fish species present in the lake. The growth of R. rutilus and the selectivity of gill-nets on its population were examined in Piediluco lake by analysing the selectivity of gill-nets of different mesh size. Sampling was conducted from January 2011 to October 2012 by means of gill-nets of different-sized mesh (8, 10, 12.5, 15.5, 19.5, 24, 28, 35, 45 and 55 mm). The results showed that R. rutilus in Piediluco lake had a growth faster and higher than in many other lake environments. The results of gill-nets selectivity gave important information to design a plan to control this species and confirmed that the use of suitable fishing techniques could be useful to manage the population of this alien species in Piediluco lake as well as in other similar environments.

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

Summary 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 SSM50% (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 SSM50% with a mesh size of 6.2 cm, while the length of retention of 50% (l50) coincides with the SSM50% with a mesh size of 7.6 cm.

Gillnet selectivity, seasonal, tidal and photoperiod variation in catch in the lower Nun River, Niger Delta, Nigeria

The fish species of the lower Nun River was sampled to determine gillnet selectivity and the effect of season, tide and photoperiod variation on the catch. A total of 11,156 specimens were caught belonging to 14 families consisting of 25 species. Gillnet selectivity was observed with a declining trend from the largest mesh size net of 15 mm (58%) in number and 59% by weight, 12 mm (33%) in number and 33% by weight with the least catch of about 9% by number and 8% by weight from the smallest (8 mm) net. In terms of seasonal variation, the rainy season catches were significantly higher (P< 0.05) than the dry season with relative abundance of 67 and 32% respectively. Tidal variation in the catches showed that catches were higher during the low tidal period (61%) than the high tide (39%). Significant photoperiod differences (P < 0.05) were observed with higher catches by day (58%) than by night. Key words: Gillnet selectivity, seasonal, tidal, photoperiod, variation, Nun River.

Population dynamics and gillnets selectivity of Chrysichthys nigrodigitatus (Lalepede 1803) in lower reaches of the Cross River estuary, Nigeria

Population Dynamics and gillnet selectivity of Chrysichthys nigrodigitatus (Lalepede 1803) in the lower reaches of the Cross River estuary of Nigeria was studied for two years (January 2008 – December 2009), with respect to selectivity, gillnet sizes and population of the species based on length frequency analysis. Estimates of growth parameters, asymptotic length (L∞) = 98.25cm and growth constant (K/year) = 0.96yr -1 . Total mortality rate (Z) was 6.27 yr -1 , natural mortality (M) = 1.22 yr-1, Fishing mortality (F) = 5.05 yr -1 and Exploitation rate (E) = 0.81. Significant correlation (r = 0.9140 – 0.9770 and r = 0.976 – 0.977). (P < Z0.05) was observed between mesh sizes of gillnets and mean values of the morphometric characteristics (such as half-circumference, total length and total weight) of the fish. There is low percentage frequency range of the number of fish caught in different mesh sizes, possibly with low hanging coefficient range (0.4-0.5). Gilling, snagging, wedging, entangling were observed in the selection of the fishes. In conclusion, selectivity in Chrysichthys nigrodigitatus gillnet fishery in the Cross River estuary may be moderately adequate despite the influence of several other factors. The selection distribution curve using 17.5cm estimated modal total length recommends the usage of 50mm mesh size and above for exploitation of the species. Keywords: Gillnet, Selectivity, Mortality, Length, Population

Comparative analysis of two sampling gillnets of rocky reef area in Gouqi Islands,Shengsi,Zhejiang

The survey experiment of two different type of gillnets was carried out at rocky reef area to find the suitable survey nets for rocky habitat during July and August in 2009.Indices such as Sorensen similarity,Margalef's species richness index,Shannon-Wiener diversity,Peilou's species evenness index were calculated,and the constitution and difference of catch were analyzed.Results showed that(1)Catch and species of European gillnet were lower than multi-mesh trammelnet.20 species was caught by European monolayer gillnet,and total weight was 8 572.6 g while 29speceies and 20 793 g catch by multi-mesh trammelnet.(2)Dominant species of European gillnets were:Sebastiscus marmoratus,Hexagrammos agrammus,Collichthys lucidus and multi-mesh trammelnets' dominant species were:Sebastiscus marmoratus,Hexagrammos agrammus,Collichthys lucidus,Scomber japonicas.The selectivity of European gillnets was much higher than multi-mesh trammelnets.(3)European gillnet and multi-mesh trammelnets had no significant difference in community structure index,but index of Sorensen similarity was only 0.41,which showed that the species of two gillnets is different.(4)It is suggested that the selectivity of two gillnets was different,the average length of catch was 106.1 mm for European gillnet and 119.1 mm for trammelnets.(5)Different species are distributed in different mesh gillnets,the dominant species such as Sebasticus marmoratus,Hexagrammos agrammus distributed mainly in five mesh gillnets.Our research indicates that the indices of fish community have no significant difference,however,multi-mesh trammelnet has a high catch efficiency,and is more durable than European monolayer gillnets for survey in rock reef area.

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

Simulation-based evaluation of the accuracy in indirectly estimating gillnet selectivity

We used stochastic simulations to evaluate the accuracy in indirectly estimating selectivity for experimental gillnets and to gage how estimator performance was influenced by factors such as sample size and gillnet configuration. Included in our evaluations were cases where both selection curve model parameters and relative fishing intensities of gillnet panels were estimated simultaneously. We additionally explored how estimation accuracy was affected by assuming incorrect relative fishing intensities when fitting selectivity models. For binormal and lognormal selection curve models, parameter estimates were generally accurate and precise when intensities were fixed at their true values, although some biases could result in estimates of model spread under small sample sizes. Conversely, when relative fishing intensities were estimated simultaneously with binormal selection curve model parameters, substantial bias and imprecision could result for both selection curve parameters and intensities depending on sample sizes. Assuming equal relative fishing intensities for gillnet panels when intensities actually varied resulted in some estimator bias of selection curve model parameters depending on the underlying intensity pattern and assumed age composition and growth patterns of the population. In the case of binormal selection curve models, overall performance of parameter estimates was often as good as or better than when relative fishing intensities were estimated. Given the potential problems in estimating relative fishing intensities in gillnet selectivity evaluations, including the potential of parameter confounding for some selection curve models and high bias and imprecision with small sample sizes for other selection curve models, we do not recommend this as a standard approach when indirectly estimating gillnet selectivity.

Gill net selectivity in the White Nile fisheries, Khartoum State, Sudan

Gill-net selectivity is a size and type of fish that caught depending upon specific mesh size of a used gill-net. In this study, four different mesh sizes (4, 6, 8 and 15 cm) were used in al-Kalakla Fishery (KF) and Jabel Awlia Dam Fishery (JADF) in the White Nile for studying their selectivity. The results of a catch per unit effort (CPUE) study showed that meshes 4 and 6 cm had higher productivity in fishing in KF than that in JADF; whereas, meshes 8 and 15 cm caught more fishes in JADF than in KF for the entire year. In KF, autumn season was the best for fishing with meshes 6 and 8 cm. In JADF, summer and autumn were the best for fishing by meshes 4 and 6 cm. The gill-nets of the study were highly selective for fish species according to their body size. Five fishes of small sizes (body depth at around 4 cm) as Tilapias (Oreochromis niloticus: Linnaeus, 1758; Tilapia zilli: Gervais, 1848;Sarotherodon galilaeus: Linnaeus, 1758), Kas (Hydrocynus forskali: Cuvier, 1819;Hydrocynus vittatus: Castelnau, 1861; Hydrocynus brevis: Gunther, 1864), Nile Perch (Lates niloticus: Linnaeus, 1758), Dabis (Labeo vulgaris: Heckel, 1847) and Bayad (Bagrus bayad: Forsskal, 1775) were caught intensively by mesh of 4 cm during all seasons in both KF and JADF; whereas, meshes of 8 and 15 cm caught occasionally bigger fishes ranged between 8 to15 cm as Nile Perch and Bayad. The selectivity of these nets reflected over-fishing caused in the White Nile. Key words: Fisheries, freshwater fish, River Nile, fishing gear, fishing nets.

Gillnet selectivity in the Ebro Delta coastal lagoons and its implication for the fishery management of the sand smelt, Atherina boyeri (Actinopterygii: Atherinidae)

Multimesh nylon gillnets were set in three Ebro Delta (North-East of Spain) lagoons to determine mesh selectivity for the inhabiting fish community. Each gillnet consisted on a series of twelve panels of different mesh size (ranging from 5.0 to 55.0 mm bar length) randomly distributed. The SELECT method (Share Each Length’s Catch Total) was used to estimate retention curves through five models: normal location, normal scale, gamma, lognormal and inverse Gaussian. Each model was fitted twice, under the assumptions of equal and proportional to mesh size fishing effort, but no differences were found between approaches. A possible situation of overfishing in the lagoons, where artisanal fisheries are carried out with a low surveillance effort, was assessed using a vulnerable species inhabiting these brackish waters as case study: the sand smelt, Atherina boyeri. The minimum size for its fishery has not been established, thus remaining under an uncontrolled exploitation situation. Therefore, a Minimum Landing Size (MLS) is proposed based on sexual maturity data. The importance of establishing an adequate MLS and regulate mesh sizes in order to respect natural maturation length is discussed, as well as, the proposal of other measures to improve A. boyeri fishery management.

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 &lt; 0.0001) and decreased with the twine thickness (F = 46.12; df = 1; P &lt; 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.

Assessment of the impact of gillnets on the population structure of the shovelnose guitarfish Rhinobatos productus from the Gulf of California, Mexico

The shovelnose guitarfish Rhinobatos productus is the most abundant species in the artisanal ray fishery of the Gulf of California. This ray is captured when it approaches the coast for reproductive purposes and is highly vulnerable to gillnets used in the fishery. Despite the high production of elasmobranchs in the Gulf of California, studies on reproductive and technological aspects are scarce. In the present study, we analyzed the interaction between size-at-maturity and size-selectivity, which are represented by a logistic function and gamma distribution, respectively. Interaction of such vectors is applied to the size-frequency distribution of the population monitored in Sonora (Mexico) during 1998–2004. The effect of maturity ogives revealed a proportion of mature individuals of 68% and 83% for males and females, respectively. Gillnet selectivities with different mesh sizes had a varied effect on the size structure given that males reach smaller sizes than females. For males, selectivity curves of nets with mesh sizes of 3.5, 5.0, 6.0, 8.0, and 8.5 inches reduced the original length-frequency distribution by 59%, 9%, 8%, 85%, and 94%, respectively. The same nets reduced the original length-frequency distribution of females by 79%, 41%, 16%, 48%, and 63%. Gillnets with mesh sizes of 5.0 and 6.0 inches had higher selectivity for both sexes coinciding in the size interval of maximum abundance. The results of the present study revealed a negative impact of the fishery on the adult structure of the population and to a lesser extent on juveniles. A demographic analysis of this population incorporating both maturity and selectivity vectors will allow determining the resilience to the current exploitation pattern by elasticity analysis.