Longitudinal and Vertical Spatial Gradients in the Distribution of Fish within a Canyon‐shaped Reservoir

The large‐scale spatial distribution of fish was investigated within a morphometrically simple canyon‐shaped reservoir with a single major tributary and a longitudinal trophic gradient (Římov Reservoir, Czech Republic). Samples of fish were taken by Nordic survey gill nets (several mesh sizes from 8 to 70 mm knot to knot) installed as surface nets at several offshore areas located along the longitudinal axis of the reservoir. Surveys were carried out in late summer during 1999–2003. An obvious distribution gradient of fish was revealed along the longitudinal axis of the Římov Reservoir. The total relative fish abundance and biomass (catch per unit effort) decreased considerably from the upstream end of the reservoir toward the dam. Roach (Rutilus rutilus), bleak (Alburnus alburnus) and bream (Abramis brama) comprised the bulk of catches at all areas. Enhanced dominance of bream was observed in the fish assemblage at the uppermost, more eutrophic area of the reservoir. The highest number of fish species and the highest abundance of young‐of‐the‐year fish were also observed in the tributary area. In the downstream part of the reservoir, gill net surveys along the vertical depth profiles indicated that offshore fish occupied mostly the epilimnion. Extreme flood events affected the Římov Reservoir, however, it seemed they had no significant impact on the gradients described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evaluating catches of salmonids taken by gillnets

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

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

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

ASPECTS OF THE FISHERIES OF LAKE LIAMBEZI,CAPRIVI

SUMMARY

Lake Liambezi, situated in the Linyanti Swamp, Caprivi, is inhabited by 43 fish species. Population estimates with seine nets, rotenone and explosives gave values of 74 to 157 kg ha^?1. The composition of seine net catches differed markedly from gill net catches where non-Cichlids form a majority. Results of a fleet of gill nets show that best catches of 25 kg 50 m^?1 net night^?1 can be made with 60 mm mesh nets, catching predominantly non-Cichlids (Marcusenius macrolepidotus, Schilbe mystus, Synodontis spp.). A small commercial fishery is established on the lake, with 60 active fishermen using large mesh (100–150 mm) gill nets. Their catch consists mainly of cichlids (Sarotherodon andersoni and S. macrochir) and Clarias spp. Production figures dropped from 637 ton in 1973–74 to 115 ton in 1975–76. This drop is ascribed to reduced effort as well as a reduction in catch per unit effort. Recommendations are made to utilise the fishery potential of Lake Liambezi in a more balanced way by introducing small mesh gill nets to harvest non-Cichlid populations.     

Landed and discarded catches from commercial prawn trammel net fishery

This study presents estimates of numbers and biomass of discarded and landed catches from the commercial prawn trammel net fishery in İzmir Bay. A total of 43 finfish and 29 invertebrate species were identified in catches sampled between May and October 2003, with 26 species landed by fishers and 46 species discarded. Melicertus kerathurus, Mullus barbatus and Solea solea dominated the landed catches. Average ratio of landed to discarded fish by number and weight was estimated as 1 : 2.32 and 1 : 0.77, respectively, demonstrating relatively low by‐catch ratios compared to other shrimp by‐catch studies. This study shows that trammel nets have a much lower discard ratio than the shrimp trawls in other parts of the world. The benefits of shifting toward passive trammel nets in coastal areas are discussed.     

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

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

Initial study on catch,species composition and reproductive biology of fishes off the south‐west coast of Sri Lanka,targeted by ring nets while utilizing natural floating objects

This study evaluates the catch rates, species composition and reproductive biology of flotsam‐associated fishes targeted by ring nets off the southwest coast of Sri Lanka. Catch and number of multiday boats operating with ring nets were collected at Beruwala fishery harbor on the southwest coast of Sri Lanka, March to October 2013, by making fortnightly field visits. Fish samples were collected randomly from the unloaded ring net catches to analyze reproductive biology. Twelve fish species belonging to four major families: Scombridae, Carangidae, Coryphaenidae and Balistidae were identified in the ring net catches and five species, Decapterus russelli, Katsuwonus pelamis, Thunnus albacares, Elagatis bipinnulata and Canthidermis maculatus were predominant. An average of 25 ± 12% multiday boats landing at the Beruwala fishery harbor operated with ring nets each month, with an average monthly catch rate fluctuating from 730 ± 101 to 3924 ± 1094 kg per boat per trip. Catch rates of tunas and carangids were significantly higher than the other fish groups (anova ; d.f. = 3, P < 0.05). Total fish landed by ring nets at the Beruwala fishery harbor during the study period was 1456 tonnes. Species belonging to the family carangidae had the highest percentage contribution (46.6%) to the ring net landings followed by tuna (34%). Similar‐size individuals ranging from 24 to 31 cm were landed by ring nets, with all landed individuals belonging to K. pelamis, T. albacares and E. bipinnulata having immature gonads. The findings of this study will be useful locally as well as regionally to manage the ring net fishery and to aid in implementing measures to manage the highly migratory tuna species.     

The variability in the catches of charr, Salvelinus alpinus L., and perch, Perca fluviatilis L., from multi-mesh gill nets

Multi-mesh gill nets set at Rawlinson's Nab in the south basin of Windermere for 24 months from 1979 to 1981 caught predominantly charr and perch. Nets set parallel to the shore caught significantly fewer fish than those set perpendicular to the shore. Variability in the mean catches of perch increased from winter to summer when the size of the catches also increased. Charr were mostly caught from November until February. Estimates of variance associated with the geometric mean catch of a single net (set perpendicular to the shore) were calculated for the periods winter for the charr and winter, summer, spring and autumn for the perch. These variances were used to calculate 95 and 99% C.L. of the geometric mean. For limits to be below half or twice the geometric mean, 12 nets would have to be used for charr during winter. For perch the number of nets needed would be six in winter, 20 in spring and autumn and 40 in summer. It is concluded that the nets are of little value in assessing changes in stock abundance.     

插网网目尺寸及采样时间对潮沟游泳动物采样的影响

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

Estimation of the efficiency of the biconical trap for Glossina fuscipes fuscipes along the lake Victoria shore, Kenya

Prior to estimating the efficiency of the unbaited biconical trap for Glossina fuscipes fuscipes Newstead (Diptera:Glossinidae) the flying height of the insects was estimated using 1 m2 electrified nets placed at 0 and 4 m above the ground level. The degree of avoidance of these nets by the flies was determined by comparing catches in traps surrounded and those not surrounded by incomplete rings of nets. On the basis of the catches in traps surrounded by nets, the expected catches on both sides of the nets were computed and compared with the observed catches, to further estimate this degree of avoidance. About 48% of males and 35% of females were captured above one metre. An average of 61% of males and 40% of females appeared to avoid the nets. Between 18% and 40% fewer tsetse were caught in traps surrounded by an incomplete ring of nets of respectively 1 m and 2 m radius than in traps not surrounded. After corrections for net avoidance and flying height, it appeared important to determine the optimum radius for the incomplete ring of nets for a reliable efficiency estimate.     

Catches of invertebrate drift by pump and net

Catches of invertebrate drift by pump and surface net were compared. Faunal composition was similar in the two methods but pump catches of drifting benthos, micro-crustaceans and suspended solids were significantly greater than those by nets.     

Comparing fish catches taken with gill nets of different combinations of mesh sizes

Based on net selectivity curves and the pooled relative efficiency of combinations of mesh sizes, models for comparing gill-net catches taken with different combinations of mesh sizes are presented for perch, burbot and, in more detail, for salmonids. The models enable comparison of length frequencies and the numbers of fish within any length range caught with an acceptable net efficiency. A standard for comparing salmonids is proposed.     

Sampling estuarine fish using multi-mesh gill nets: Effects of panel length and soak and setting times

Experiments on the effects of soak (1, 3 and 6 h) and setting times (18:00, 22:00 and 3:00 h), and panel length (20, 50, 120 m) on catches of fish in multi-mesh (38, 54 and 89 mm) gill nets were done in a south-east Australian estuary to develop an optimal, representative and standardized sampling methodology for future fishery-independent surveys of estuarine icthyofauna. Univariate analysis revealed that while the longest soak times and panels often caught significantly more numbers and species of fish, there were no differences between short and intermediate soak times. Further, where differences between soak time and panel length treatments were detected, fish were often being caught in the same relative proportions. Standardized catch-per-unit-of-effort (CPUE) (numbers of fish caught 20 m^− 1 of net h^− 1) decreased significantly with increasing soak time, but there were no differences in CPUE between different panel lengths for the total numbers of fish and key species caught. Multivariate analyses failed to detect any differences in fish assemblages between soak times and panel lengths for 38 and 89 mm mesh. However, inconsistent and species-specific differences in the abundances of common species accounted for differences in assemblage structure for the 54 mm mesh. Similarly, while the size ranges of most species of economic importance were comparable between different panel lengths and soak times, there were inconsistent differences in the proportions of fish captured across size classes for some species. Setting time had no effect on the mean numbers of fish or species caught, structure of assemblages or size-ranges of most species investigated. Based on these results, the use of 20-m panels soaked for 1 h at any time during the night was considered optimal for future surveys. The benefits of this uniform sampling methodology are discussed in terms of increased replication, reduced costs and the potential for lower fish mortality.     

Un modéle mathématique de l'évitement d'un filet à plancton,son application pratique,et sa vérification indirecte en recourant au parasitisme de l'amphipode hypéride Vibilia armata Bovallius

An index of avoidance of a net by a pelagic organism, ranging from 0 to 1, is defined and an equation relating it to the diameter and the speed of the net is found. A set of derived equations allow calculation of the proportion of individuals of a given organism avoiding a net, based on comparison of pairs of tows made with nets of two different diameters, or at two different speeds.The model is tested in an indirect way using the peculiarities of the parasitism of the pelagic amphipod Vibilia armata Bovallius. During the night, females of this species hang on their weakly swimming hosts (salps) which cannot avoid the net, while males of the species are free-living and thus capable of avoidance. The proportion of sexes in catches varies with the net diameter and allows one to estimate avoidance values. Data from plankton catches show a good fit with the model predictions.Data selected from the literature are used to show the application of the equations. High indexes of avoidance for the smaller nets were found for the copepod species tested.     

Gear type,species composition and economic value of fisheries in the mangroves of Pak Phanang,Thailand

Fishing gear and methods, catch composition, annual catch numbers and the monetary value of fisheries based in the mangroves of Pak Phanang, Nakhon Si Thamarat province, Thailand, were studied from September 2004 to February 2005. The fishing gear and methods were surveyed by visual observation and interviews. Sample catches were taken for each type of fishing gear and method. Semi-closed questionnaire surveys were conducted among fishers, local traders, local administrative officers and related organizations to derive information on utilized species, gear and methods, fisher and fishery status, and trading and catch price. The Pak Phanang mangrove fishery has three types of multi-species capture gear: channel traps, gill nets and lift nets. It also has three kinds of single-species (group) gear/methods: crab traps, catfish hooks and hand capture using a long tail fishing boat. A total of 57 fish species in 27 families, and 23 shell fish species in 8 families were recorded from the catch samples. Penaeid shrimp (25%), and ambassid (31%) and mugilid (24%) fishes were the abundant groups in the channel traps, while ariid (36%) and mugilid (19%) fishes were abundant in the lift net catches. Ambassid (42%) fish dominated the gill net catches. Species richness (number of species) varied depending on sampling month and type of fishing gear. The total annual catch and its monetary value were estimated to be 442–551 tons and US $368,038–733,973, respectively. The estimated annual catch per unit area of mangrove was 63–79 kg ha⁻¹, which generated a market value of US $368,038–733,973, respectively. The estimated annual catch per unit area of mangrove was 63–79 kg ha⁻¹, which generated a market value of US 52–105.     

The variability of the catch from gill nets set for pike Esox lucius L.

The variability of the catch of pike Esox lucius L. from gill nets in Windermere over the period 1961–71 has been analysed. Altogether 2952 catches have been treated by analysis of variance after an appropriate transformation. A representative value of the residual between catch variance has been used to estimate the reliability of the means of catches based on different numbers of net settings. It is shown that many settings of gill nets are required to get a reliable mean catch.     

Comparison of a rigid grid to a square‐mesh panel in size selection for commercial fishing in the Persian Gulf shrimp trawl fishery

Two bycatch reduction devices (BRDs): a juvenile and trash fish excluder device (JTED) and a square‐mesh panel (SMP), were tested in the Persian Gulf shrimp fishery from a small‐scale shrimp trawler in October–November 2012. Each device was alternatively tested by using a small‐mesh cover net to retain individuals that escaped the BRDs. Three valuable fish species—tigertooth croaker (Otolithes ruber), silver pomfret (Pampus argenteus) and silver sillago (Sillago sihama)—were selected for detailed analyses. Average escapement rates by number for commercial species in the trawl nets with either JTED or SMP were calculated as 24% and 33%, respectively. There were significant differences in escapement rates for Otolithes ruber and Sillago sihama between JTED and SMP (p < .05). In both BRDs, Pampus argenteus had the smallest escapement rate. A two‐sample Kolmogorov–Smirnov test detected a significant difference (p < .05) in the length‐frequency of Sillago sihama, and Otolithes ruber captured in the codend and cover net equipped with JTED. Selectivity curves of both BRDs indicated that for each species, the length of 50% retention probability (L50) was substantially smaller than the length at maturity (Lm), except for Otolithes ruber. From an ecological perspective, the codend equipped with SMP was superior to the codend equipped with JTED because it retained substantially fewer under‐sized fish and had the smallest selection range. It therefore seems that replacing the standard codend with a codend equipped with SMP provides the best outcome for the fishery whereby fishermen will increase their catches of larger fish while simultaneously reducing their under‐sized fish catches.     

鱼类早期资源调查中不同网具采集效率的研究

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

A comparison of four types of sampling gear used to collect shovelnose sturgeon in the Lower Missouri River

We compared variability in catch per unit effort (CPUE) and size structure of shovelnose sturgeon Scaphirhynchus platorynchus collected in 2003 and 2004 with stationary winter gill nets, drifted trammel nets, hoop nets, and otter trawls in the Lower Missouri River, USA to determine the most precise types of gear to collect all sizes of sturgeon so that refinements of long‐term monitoring protocols can be made. A total of 1947 net sets or trawls collected 8743 shovelnose sturgeon, with 67% of the fish collected during winter gill netting (16% of total samples). Mean coefficient of variation (CV) among all months for juvenile (age 3 and younger; <250 mm fork length) sturgeon was highest for gill nets and lowest for otter trawls (P = 0.0008). Mean CV of subadult and adult shovelnose sturgeon (≥250 mm) was highest in hoop nets compared to other gear types (P = 0.0002). All gear and mesh sizes collected the most common sizes of shovelnose sturgeon (500–600 mm), but only otter trawls and trammel nets collected fish <150 mm. The higher precision of winter gill nets and summer otter trawls led to fewer samples needed to detect changes in CPUE as compared to hoop and trammel nets. Sampling only with types of gear that do not collect younger shovelnose sturgeon may hinder management decisions that rely on recruitment trends to determine the effects of management actions (e.g. channel modifications).     

A fourteen-year overview of the fish assemblages and yield of the two oldest Algarve artificial reefs (southern Portugal)

Artificial reefs have been deployed worldwide for the last three decades in response to problems concerning coastal resources, ecosystems and fisheries. In many countries they have became important elements of integrated fisheries management plans. In Portugal two artificial reef systems (ARSs) were deployed by the Portuguese Institute of Marine Research (IPIMAR) in 1990, in the southern coast (Algarve). They were located off Faro and Olhão, over different sea-bottom types and located at different distances from the coastline. To analyse the effect of ARSs deployment on local fish assemblages and to evaluate their effectiveness in terms of mean fishing yields and mean number of species caught, fishing surveys have been conducted over 14 years (256 net sets) using a gillnet to sample the ARS of Faro and Olhão and respective control sites. The fishing yields from the ARs continually exceeded those from the control sites, in both the mean number of species caught and the mean CPUE in weight (1.8–2.6 times); both were higher at Faro. Moreover, the comparison between fish assemblages from the ARS and respective control sites showed that the deployment of the man-made structures did not change the composition of the fish assemblages caught by the gill nets, or the equilibrium of the community, since the relative proportion of the different functional groups of fish remained stable. The ARSs are a useful management tool on the Algarve coast, enhancing and diversifying the catches, thus contributing to improved local artisanal fisheries, which play a major role in this region.