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.     

Effects of different baits on monofilament gillnet effectiveness in a freshwater reservoir fishery (Keban Dam Lake,Turkey)

Aim of this study was to test the gillnet efficiency in Keban Dam Lake using different baits as attractants. Gillnets of 50 m length (stretched mesh sizes: 40, 50, 55 mm) were employed between January 2009 and December 2010 in the study. Two types of grain (corn and wheat) as well as processed plant material (beet cake) and animal livers were tested as bait to attract both carnivorous and omnivorous species. The catching efficiency of baited and non‐baited gill nets was compared. Overall, a total of 358 fish from eight different species were caught in baited gillnets, whereas only 189 fish were caught with non‐baited gillnets. Most fish (55.90%) were caught with corn, wheat or beet cake as bait; the fewest (59 fish, 9.96%) were captured with animal liver. About 34.1% were caught with non‐baited gillnets. By species, Capoeta trutta (Cyprinidae) was most frequently captured. In conclusion, the gillnets bated with corn, wheat, and beet cake yielded higher catches (P < 0.05) than did the non‐baited monofilament gillnets.     

单层刺网不同网目渔获组成——以上海青草沙水库为例

2010—2012年采用刺网(多网目单层刺网和单网目三层刺网)、电捕、底拖网和地笼对上海青草沙水库的鱼类多样性进行了调查.调查共采集到鱼类34种,隶属8目、12科,其中鲤形目鱼类物种数最多(19种)、鲈形目次之(6种).多网目单层刺网仅采集到鱼类19种,其中刀鲚为优势种.聚类分析表明,单层刺网网目规格对渔获物物种组成影响较为显著,网目为2 cm的网片的渔获物优势种为刀鲚和贝氏〖FK（W*8/9。1〗〖PS5号F体.eps,BP#〗〖FK)〗
,其他网目网片优势种为刀鲚.渔获物物种数和单位努力捕获数量均有随着网目规格增大而下降的趋势.网目规格对优势种刀鲚的体长分布影响显著,且随着网目规格的增加而增大.为更加科学合理地评估某水域的鱼类群落多样性,需要结合多种采样方法进行鱼类调查.     

Release method and anatomical hook location: effects on short-term mortality of angler-caught Acanthopagrus australis and Argyrosomus japonicus

One field and 3 aquaria experiments were done to quantify the short-term mortality of yellowfin bream Acanthopagrus australis and mulloway Argyrosomus japonicus after being angled and subjected to 3 general handling treatments. Anglers were supplied with identical J-type hooks and asked to handle hooked fish by either (1) physically removing the hook or (2) cutting the line (5 cm from the mouth of the fish) and leaving the hook in. Some hooked A. japonicus were subjected to a third handling treatment where the line was cut underwater without exposing the fish to air. Technical and biological data were collected before all fish were released into sea cages and monitored for 5 d. Control fish were seined and similarly caged and monitored. Concentrations of plasma glucose and cortisol were collected from a sample of fish on the first and last day of the experiments. Significant predictors of mortality for both species involved the presence of blood at the mouth and an interaction between anatomical hook location and hook removal. A. australis and A. japonicus that had their ingested hooks removed experienced the greatest mortalities (87.5 and 72.7%, respectively). Typically, these fish suffered damage to their oesophagus, stomach wall and vital organs. Mortality rates of A. australis and A. japonicus were significantly decreased to 1.7 and 16%, respectively, when they were released with their lines cut, with some of these fish free of hooks after 5 d. In contrast, few mortalities occurred in either species when the hooks were removed or the lines cut on mouth-hooked fish or in A. japonicus when it was released with no air exposure. For A. australis, the field- and aquaria-based experiments provided comparable results in terms of identifying treatment-specific effects, but there were potential biases in rates of hook ingestion. Irrespective of the treatment of fish, all experiments caused physiological changes measured as elevations in either plasma cortisol or glucose. We concluded that anglers should cut the line from hook-ingested A. australis and A. japonicus, but remove the hook from mouth-hooked individuals to prevent subsequent ingestion. Further research is required to examine the longer-term consequences of these handling practices on fish health.     

Mortality of Baltic sea trout (Salmo trutta) after release from gillnets

We explore the mortality rate of disentangled sea trout in whitefish fishery using gillnets with a 35–43 mm bar length. The study was conducted during the main fishing seasons in the Gulf of Bothnia in the northern Baltic Sea. Overall 59.5% of the sea trout were alive at the end of a 2–7 day observation period following release from the gillnets. Altogether, 12.1% of the captured fish were found dead in gillnets and 28.4% died due to injuries during an extended observation period. The average length of the captured sea trout was 435 mm, indicating that the majority were spending their first or second year at sea. The proportion of the survived and not injured (no observed damage) fish was highest in larger fish, >450 mm. The injured and not injured fish died equally frequently. Post capture survival was not connected to the removal time from gillnets or type of observed injuries but to the type of entanglement. Most of the fish were entangled by a mesh around the body, which caused extensive scale loss and open sores on the skin. The smallest fish may have had internal wounds that were not registered in this study. These results can be used in fisheries management to estimate the mortality of multi‐species gillnet fishing to sea trout populations in relation to management actions.     

Length–weight relationships for seven fish species of the Napo River,Upper Amazon,Peru

Length–weight relationships (LWR) were estimated for seven fish species from the Napo River and its main tributaries, Arabela and Curaray rivers, Amazon basin, Peru. A total of 652 specimens were sampled between 2012 and 2014 using seines and gillnets. This study reports new maximum total length for four species and new LWR and maximum total weight records for three and five species, respectively.     

Damage and partitioned mortality of teleosts discarded from two Australian penaeid fishing gears

Six field experiments were perfromed to (1) quantify the scale loss and partitioned (immediate and short-term) mortality of key teleosts discarded from a penaeid seiner and trawler operating in 2 estuaries in southeastern Australia, and (2) assess the utility of modified operational and/or onboard handling procedures for maximising survival. For both gears, several non-target species were caught, handled and discarded according to 2 general categories of treatment ('mild' and 'extreme') representing the plausible limits of severity of commercial operations. The mild treatments involved the shortest conventional deployments of the gears followed by the immediate sorting of catches in water-filled trays, while the extreme treatments comprised the longest conventional deployments and sorting in dry trays. Discards were examined for immediate mortalities and scale loss before sorting onboard, while live individuals of key species, along with appropriate numbers of controls, were released into sea cages and monitored for mortalities up to 5 d. For both gears, there was a trend of higher percentages of immediate mortalities in the extreme treatments, and more scale loss from dead than live seined and trawled silver biddy Gerres subfasciatus, seined tarwhine Rhabdosargus sarba and trawled yellowfin bream Acanthopagrus australis. Despite considerable interspecific variabilities, few intraspecific differences were detected between treatments for the short-term mortalities of live discards; however, for all species, these deaths were greater than those incurred by the controls (most of which survived). The partitioned mortality estimates were combined to provide a range of total mortalities for seined (95.97 and 99.07%, respectively) and trawled (71.74 and 97.64%) G. subfasciatus, seined R. sarba (23.95 and 100%) and trawled southern herring Herklotsichthys castelnaui (100%) and A. australis (3.40 and 35.01%). Because most deaths occurred irrespective of onboard handling procedures, we conclude that simply reducing the duration of gear deployments would provide a first step toward mitigating discard mortality in these fisheries.     

Changes in experimental gillnet catches from the Kafue Floodplain, Zambia, since construction of the Kafue Gorge Dam

The Kafue Floodplain fishery produces an annual fish yield of about 5000 tonnes. The 1971 closure of the Kafue Gorge Dam downstream from the floodplain modified flooding patterns. Pre-impoundment studies predicted this modification would increase fish stocks. Experimental gillnets set in 1975 and 1976 caught 18 species on a regular basis, nine of which were abundant enough for comparison with catches in similar gillnet samples taken prior to completion of the Kafue Gorge Dam. Of these nine species, experimental gillnet catches of three predator species were smaller in 1975–76 than in 1969–70. Catches of only one species increased significantly. Whether this change is due to the modified flooding pattern is unclear; unusually dry flood conditions in 1973 and increased fishing effort may have contributed.     

Monthly variability of hydroacoustic fish stock estimates in a deep lake and its correlation to gillnet catches

Monthly (April to November) hydroacoustic surveys and parallel gillnet catches were used to determine vendace Coregonus albula abundance, biomass and population dynamics in a deep oligotrophic lake. By hydroacoustic surveys, recruitment of 0+ year vendace could clearly be detected. In contrast, gillnet catches resulted in low numbers of 0+ year fish, and similarly under-represented proportions of the oldest vendace. Consequently, the correlation between hydroacoustics and gillnets with respect to fish numbers and geometric mean fish total length was high only for the age groups 1 + to 4+ years. Annual variability in hydroacoustic estimates offish abundance and biomass was high (CV=26–29%) which reflects the seasonal population dynamics of vendace.     

Predicting variability in catch-per-effort in Atlantic cod, Gadus morhua, trap and gillnet fisheries

The relationships between catch rates and fish abundance, hydrographic conditions and fishing effort, for Atlantic cod caught by trapnets (fixed gear) and gillnets (non-fixed gear) in the northern Gulf of St Lawrence have been quantified. Daily changes in trap catch rates were accounted for by changes in salinity, currents and mean local cod densities in 1985 ( R ²= 0.78), and predicted 1986 trap catch rate changes (by 1985 model) were correlated significantly with those observed ( r = 0.60, P < 0.05). In contrast, the daily changes in 1985 gillnet catch rates were determined by currents, maximum (not mean) local cod densities, and fishing effort (negative) ( R ²= 0.68), while predicted 1986 gillnet catch rates (by 1985 model) were significantly correlated with those observed ( r = 0.35, P < 0.05). Seasonal catchability coefficients of the traps were similar in 1985 and 1986, but for gillnets this index was an order of magnitude higher in 1986 than in 1985.     

An indirect estimate of bimodal gillnet selectivity of smelt

This study showed that it is possible to model a bimodal gillnet selectivity curve without knowing how individual fish have been caught. A bimodal model based on a modified normal density distribution was fitted over the length ranger of smelt Osmerus eperlanus from 8·9 to 17·0 cm. The model had eight parameters, and the height of the second peak was allowed to change with fish length. The coefficient of determination ( r ²) was 0·915 and the residuals were distributed symmetrically against variable axis. Corrected relative length distribution did not differ statistically from the relative length distribution of the trawl catch. In earlier studies of other species, pooled relative efficiency of multimesh gillnets composed by a geometric series mesh-size combination has been constant on the average. Because the model increased along fish length, the pooled relative efficiency was increasing also with fish length. Therefore, it is suggested that the gillnet catches of smelt always have to be corrected for gillnet selectivity.     

Fine-scale behaviour of bottlenose dolphins around gillnets

We studied the fine-scale behaviour of bottlenose dolphins Tursiops truncatus around gillnets in North Carolina, USA, during May and June 2002. We made observations from an overhead digital video camera, suspended from a helium-filled aerostat, tethered 70 m above a fishing vessel. We positioned the camera above a gillnet set for Spanish mackerel Scomberomorus maculatus. We observed frequent encounters (n = 36) and interactions (n = 27) between dolphins and the net, but no dolphins became entangled. Most dolphins diverted their course around the net, but on nine occasions we observed dolphins engaging in depredation. We conclude that interactions between dolphins and these gillnets are common, but that entanglement is rare.     

Catch composition of drift gillnets in a multi-species fishery with an emphasis on kingfish (Scomberomorus commerson) in Northern Persian Gulf

Gillnets are the primary fishing gear used in tropical multi-species fisheries along the Iranian southern coastal waters. Therefore, it is necessary to investigate the catch composition and performance of fishing gear for the possible negative effects on the stocks. Catch data and length frequency distribution (just for Kingfish, S. commerson) were examined for three kinds of drift gillnets: 1) small-meshed nets, 2) medium-meshed nets, and 3) nets with a single mesh size of 165 mm. This study was conducted in the northern part of the Persian Gulf from 2014 to 2015. Overall, 60 species of 32 families from four fish groups (i.e., Pelagic, Demersal, Benthopelagic, and Reef-associated) were recorded. Each net-type was used to target more than one species, with most species being considered as bycatch. Commercial fish species were caught the most, accounting for around 83% of the total catches. S. commerson is targeted by all kinds of nets, with medium-meshed nets being most efficient. The majority of S. commerson caught by the nets were immature. Overall, the total ban of gillnets with small mesh sizes is recommended to protect young S. commerson and prevent the possible risk of growth overfishing. Studies on gillnet selectivity are needed to set minimum mesh regulations for S. commerson and other commercial species if the potential impacts of gillnet are known.     

Developing ultraviolet illumination of gillnets as a method to reduce sea turtle bycatch

Fisheries bycatch of marine animals has been linked to population declines of multiple species, including many sea turtles. Altering the visual cues associated with fishing gear may reduce sea turtle bycatch. We examined the effectiveness of illuminating gillnets with ultraviolet (UV) light-emitting diodes for reducing green sea turtle (Chelonia mydas) interactions. We found that the mean sea turtle capture rate was reduced by 39.7% in UV-illuminated nets compared with nets without illumination. In collaboration with commercial fishermen, we tested UV net illumination in a bottom-set gillnet fishery in Baja California, Mexico. We did not find any difference in overall target fish catch rate or market value between net types. These findings suggest that UV net illumination may have applications in coastal and pelagic gillnet fisheries to reduce sea turtle bycatch.     

The consequences of habitat occupation and habitat overlap of the introduced tilapia Oreochromis mossambicus and indigenous fish species for fishery management in a Sri Lankan reservoir

Habitat occupation and habitat overlap of the introduced tilapia Oreochromis mossambicus and co-occurring indigenous fish species were studied in a shallow lowland reservoir in Sri Lanka. Results were used to discuss the possible existence of empty habitats in reservoirs and the position of the introduced O. mossambicus in the fish community. Predictions were made of the effects of a subsidiary small-meshed gillnet fishery for indigenous minor cyprinids on the yield of tilapia. The zooplanktivorous halfbeak Hyporamphus gaimardi, an indigenous invader from brackish water estuaries and lagoons, has successfully colonized the pelagic habitat in this reservoir. The habitat of the introduced O. mossambicus overlapped significantly with those of the indigenous minor cyprinids. Only the smallest size classes of O. mossambicus (below 45 mm) arc spatially segregated from these cyprinids. Spatial distribution patterns are influenced by the rainy seasons which trigger an inshore movement of O. mossambicus and several other species around November and May. Barbus spp, (Cyprinidae) need riverine habitats for spawning, but for all other species the available habitats within the reservoir are suitable to complete their full life cycle. Exploitation of Barbus spp. with gillnets with a mesh size of 30-mm stretched mesh is predicted to have considerable detrimental effects on the existing fishery for O. mossambicus. However, a gillnet fishery with 15-mm stretched mesh to exploit Amblypharyngodon melettinus, Rasbora daniconius and H. gaimardi in the open water zone is expected to be successful without harming the existing fishery for O, mossambicus or the populations of Barbus spp.     

浅水湖泊鱼类群落结构评估: 复合网目刺网与网簖的比较

准确评估鱼类群落结构是渔业管理和鱼类资源养护的必要前提, 而不同渔具采样结果往往会产生较大差异。研究同时采用欧洲标准采样网具-复合网目刺网(Multi-mesh gillnet)和中国传统网具-网簖对长江中游典型浅水湖泊扁担塘的鱼类群落结构进行了评估。2种网具共采集到27种鱼类, 并发现黄尾鲴(Xenocypris daviai)和湖鲚(Coilia nasus taihuensis)2种新记录种, 扁担塘的鱼类群落结构较1999年和2003年均发生了较明显的改变。2种渔具捕获的鱼类组成、相对丰度和生物量以及鱼类体长分布频率均存在显著性差异。基于鱼类的相对生物量和相对丰度的NMDS排序表明2种网具捕获到的鱼类群落结构也存在显著差异。另外还比较了复合网目刺网与其他定量采样网具间的差异, 作者认为复合网目刺网比较适合长江中下游浅水湖泊鱼类群落研究的定量取样, 但仅凭单一网具评估鱼类群落结构具有局限性。     

Distribution of Micropogonias furnieri (Pisces: Sciaenidae) in Sepetiba Bay,Rio de Janeiro,Brazil

Distribution of Micropogonias furnieri in the Sepetiba Bay, an ecosystem highly important as rearing ground for this species that represents 5.8% of the numerical catches by otter trawling, was analyzed based in monthly and bi-monthly samplings from two programmes (beach seines = continental margin; and otter trawl = inside of the Bay), over three annual cycles, between 1993 and 1997. Spatial comparisons were assessed by dividing the Bay in two zones for the beach seines (inner and outer Bay) and three zones for the otter trawl (inner, central and outer Bay), following depth, salinity and transparency gradient, as well as for the influence of the sea. Beach seines (30 m extension, a 10 m length x 2.5 m of height x 7 mm mesh) and otter trawls were used. Trawls covered 1.5 km and the boat (net mouth 8 m, mesh size 12 mm between opposites knots in the cod). Temperature (degree C) and salinity (p.s.u.) were taken in each sampling; depth (m) and transparency (m) were measured during the trawl. Fish from beach seine were basically young-of-the-year; in the trawl they were individuals of larger size with total length varying from 70 mm to 300 mm. Spatially, highest CPUEs were found for the continental margin in the inner Bay, and for the inside Bay, in the inner and central zones. Highly significant correlations were detected among M. furnieri abundance and low salinity, transparency and depth in the inner Bay, with no defined pattern for the continental margin. Temporally, differences in fish abundance were shown only for the second annual cycle (1996) for the continental margin, with peaks in September/October.     

Quality and status of fish stocks in lakes: gillnetting,seining, trawling and hydroacoustics as sampling methods

All sampling methods give selective or biased estimates of fish species abundance, distribution and size structure. This creates problems, e.g. in regard to the Water Framework Directive of the European Union, which demands evaluation of the quality and status of fish stocks in lakes. We compared fish sampling by means of Nordic multimesh gillnets, seining, trawling and hydroacoustics in two Finnish lakes in summer 2007 and 2008. Sampling methods were used ‘as such’, i.e. no special design was implemented for method comparison. In the shallow eutrophicated lake the species’ composition of gillnet sampling and seining were very different. The biomass-% of percids dropped from gillnet (61%) to seining (9%) and that of cyprinids grew from 39 to 90%, respectively. In the deep pelagic area of the oligotrophic lake, vendace and smelt predominated in trawl catches. The number of fish caught by gillnetting in that area was too small to make any conclusions about the species composition. In the eutrophicated lake, the combined length distribution for all fish species differed significantly between gillnetting and seining. In the oligotrophic lake, the gillnet catches were too small for any comparison of fish size. The difference in the length distribution of fish between trawl and echosounding was significant in most analysed depth layers. In upper depth layers acoustics sampled larger fish than trawling, and in deeper layers smaller fish. Using a combined acoustic-trawl method, the pelagic fish biomass was estimated to be approx. 17 kg ha⁻¹ in the deep and oligotrophic lake. We conclude that in large and deep-water areas, the use of active gear is enough in fish sampling to evaluate the quality and status of fish stocks. Gillnetting together with seining is an appropriate method to work out the quality and status of fish stocks in shallow and littoral areas of large lakes. Variation in the catch selectivity of fish sampling gear requires a discrete ecological classification for each type of gear.     

Comparing two fish sampling standards over time: largely congruent results but with caveats

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