Gillnet Catches Research Articles

Gillnet selectivity and size and age structure of an alpine Arctic char (Salvelinus alpinus) population

The aim of the present study was to address possible implications of biased sampling for the commonly adopted uni- and bi-modal size structures and unimodal age structures in Arctic char (Salvelinus alpinus) populations. Multimesh gill nets were used to sample an allopatric population of Arctic char in an alpine lake in central Norway. Direct estimates of gillnet selectivity for different length-classes of Arctic char were obtained by mark-recapture experiments and by successive removal methods. The observed size and age structure in gillnet samples was significantly different from the estimated size and age structure of the Arctic char population when catches were corrected for gillnet selectivity. An observed unimodal size and age structure was a direct result of gillnet selectivity, as smaller and younger fish were underrepresented in gillnet catches. Moreover, an abrupt increase in gillnet selectivity for large Arctic char was related to a niche shift to cannibalism. A model that explains bimodal size distributions in gillnet catches as a result of ontogenetic behavioural change is presented. Complex ontogenetic growth and mortality patterns that have been suggested to be essential in structuring modal Arctic char populations were superfluous in explaining the observed modal population structure in the present study.

Relative selectivity in trawl, longline and gillnet fisheries for cod and haddock

A full-scale fishing experiment on north-east Arctic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), involving a trawler, a longliner and a gillnetter was conducted in order to determine how the length distributions and life history parameters of the catches were affected by gear type. Relative selectivity was analysed for catches taken when the boats fished simultaneously in the same area, and when the skippers were allowed to fish under conditions as close as possible to commercial operation. Trawl and longline-caught cod had similar length distributions, but cod caught by trawl were on average 2.3 cm smaller than longline-caught cod. Cod caught by longline had a lower condition factor and length at age. The longliner caught smaller haddock than the trawler when the boats fished in the same area. When the skippers were allowed to change fishing ground, the mean length of cod increased in the trawl catches and became larger than the mean length in the longline catches. The mean length of haddock in the longline catches increased by 2.8 cm and became larger than that of the trawl catches. Gillnet catches consisted almost solely of large cod. Mean length of gillnet caught fish in 186 mm nets decreased from 86.2 cm to 82.3 cm even if the boat did not change area. Separate selectivity experiments for trawl and gillnet were conducted, and the parameters for the mesh selection are presented. To find the functional form of the longline selectivity we used relative catch proportion in each 5 cm length-group for longline compared to trawl and gillnet, whose selectivities were assumed to be known. When compared to that of the gillnet, the plots indicate that the longline selectivity curve takes the form of a unimodal distribution. When longline catches was compared to trawl catches none of the suggested models seemed to fit, and to conclude anything about the form of the longline selection curve is not possible.

Fish as a resource in a rural river catchment in the Northern Province, South Africa

The rural population in the Mutshindudi River valley, in the north-eastern corner of the Northern Province, accept locally-caught freshwater fish as food (85%), but tinned pilchards are the most commonly eaten fish. Fishing gear was recorded in 70% of the households, but only in 36% was that gear used during that same year. Most fishers were older school-going boys using hook and line as the main fishing gear, but small gill nets, a variety of traps and small-mesh seine nets were also used. Eighteen of the 27 fish species known from the Mutshindudi River were recognised by the fishers and had their own vernacular names. Catches of fishers were dominated by small tilapias, small barb species and catfish. The average catch per fishing trip of just under three hours was 162g and the fish were small (mean 26g), which was however quite acceptable to the fishers. A few older fishers sold their catches but most were taken home to be enjoyed with the main meal. Length distribution data of fishers' catches indicated fish populations with normal size distributions, but experimental gill net catches in two pools revealed populations of fish species not normally caught by fishers, including Marcusenius macrolepidotus, Barbus marequensis and Labeo molybdinus/cylindricus. This, and a low catch per unit effort, may indicate a degree of overharvesting of angling species. Measures proposed to prevent further overexploitation include a ban on draw netting as well as restocking the river with tilapia.

Relative selectivity of Greenland halibut ( Reinhardtius hippoglossoides, Walbaum) by trawls, longlines and gillnets

Selectivity parameters for Greenland halibut ( Reinhardtius hippoglossoides, Walbaum) were calculated from trawl and gillnet catches in the Norwegian scientific fisheries for greenland halibut. Within the chosen individual areas and seasons, our data show no trend in length distribution with depth. A trouser-trawl selectivity experiment gave an L 50 at 42 cm in a 135 mm codends. A selectivity analysis of the gillnets using a log normal model showed maximum probability of retention for lengths of 46.8–73.5 cm for 140–220 mm stretched meshes. By using expected catch proportions and life history parameters, the selectivity in longline and trawl was shown to be bell-shaped, both gears losing both the smallest and largest fish. Calculated estimated life-history parameters were biased by sampling gears, giving slower growth and larger L∞ for females caught in gillnets. The maturation length ( M 50) of female Greenland halibut was largest in trawl catches, smallest for gillnet catches. The sex ratio in the gillnet catches was shown to be a direct function of mesh size. These analyses will provide a better understanding of possible sampling bias when sampling a stock by means of only one type of gear.

Estimation of roach ( Rutilus rutilus (L.)) and smelt ( Osmerus eperlanus (L.)) stocks with virtual population analysis, hydroacoustics and gillnet CPUE

Dynamics of roach ( Rutilus rutilus (L.)) and smelt ( Osmerus eperlanus (L.)) stocks were analysed with virtual population analysis (VPA), hydroacoustics and by analysing gillnet catches per unit effort (CPUE). The research area was the Enonselkä basin of Lake Vesijärvi in southern Finland. In 1989 intensive trawl fishing on previously unexploited roach and smelt stocks was started. Heavy exploitation continued until 1993. Drastic reductions in fish stock densities were observed. The VPA indicated a decline in the roach stock from c. 3900 ind. ha −1 in 1989 to c. 520 ind. ha −1 in 1993. In the smelt stock the VPA indicated a respective decline from c. 18000 to c. 1900 ind. ha −1. In 1994–1996 fishing was sustained on a lower level. Anyhow, hardly any increase in stock density of roach and smelt was observed in 1994–1996. The estimates with hydroacoustics were in concordance with the VPA estimates in both the roach and the smelt stock. The gillnet CPUE-estimates were in agreement with hydroacoustics in roach stock assessment. However, gillnet CPUE was not suitable for analysing the dynamics of the smelt stock.

Lake Selection by Madagascar Fish-Eagles

We investigated fish abundance and habitat characteristics at 32 lakes occupied by Madagascar Fish-Eagles (Haliaeetus vociferoides) and 32 randomly selected unoccupied lakes between Morondava and Boriziny in western Madagascar from May to November 1995. We measured lake and shoreline habitat characteristics and used gill nets to sample fish populations. Compared with unoccupied lakes, lakes occupied by fish-eagles were deeper and clearer, had more shoreline perch trees, and yielded gill-net catches with more fish, a higher total fish mass, and more fish species. A logistic regression model with number of shoreline perch trees and number of fish species caught in gill nets as independent variables correctly classified fish-eagle use for 76.6% of the lakes in the study. A model with number of shoreline perch trees alone was 71.9% accurate, suggesting that perch-tree availability is the most important factor limiting populations of Madagascar Fish-Eagles. The results indicate that Madagascar Fish-Eagles require bodies of water with large shoreline trees and ample populations of fish.

The length–weight relationship of Albacore, Thunnus alalunga, from the Indian Ocean

To comply with the compilation of catch databases of Taiwanese gillnet fishery and stock assessment, a new length–weight relationship of albacore, Thunnus alalunga, from the Indian Ocean was determined using data from gillnet catches. Four different fitting algorithms were used. The four resultant formulae were not significantly different. However, the formula W=0.056907 FL 2.7514, where W is the body weight in g and FL is the fork length in cm, estimated by least square of residuals excluding suspected outliers which were diagnosed by least median square of residuals, is considered as an acceptable and useful one. Consequently, the equation can be used to compile catch databases mainly from the surface gears and for other assessment purposes.

Gill-Net Saturation by Lake Trout in Michigan Waters of Lake Superior

Abstract We conducted experimental fishing for lake trout Salvelinus namaycush in Michigan waters of Lake Superior to determine the importance of soak time on catch per effort (CPE) in numbers per kilometer of standard gill net. We modeled CPE as a nonlinear function of the number of nights between setting and lifting (soak time), in which the nets fill at a certain rate toward some maximum after which the nets cannot hold more fish. We found that lake trout CPE increased with soak time at a rate that varied with lake trout density toward a saturation level that was independent of lake trout density. The CPE values of nets soaked 2–5 nights divided by the CPE of nets soaked 1 night were significantly lower than would be expected had CPE increased as a linear function of the number of nights soaked. We derived a means for correcting gill-net CPE values for differing soak times to a common base of 1 night soaked. We concluded that it is inappropriate to assume lake trout catches in gill nets will increase i...

Gear-independent patterns of variation in catch of juvenile Atlantic cod (Gadus morhua) in coastal habitats

Habitat and size of juvenile Atlantic cod (Gadus morhua) change substantially during the first 3 years after settlement, and hence, cohort size cannot be followed using a single gear. We investigated whether catch could be calibrated across gear types by deploying pairs of gears repeatedly in the same habitat. As expected, size selectivity differed substantially among gears. Trawls and seines generally collected individuals <200 mm. Gillnets and jiggers collected individuals >150 mm. Size modes, corresponding to age-classes, were common to catches of most gears. Highest catches were taken by trawls and seines. Gillnet catches were orders of magnitude lower. Standardized catches could not be calibrated across pairs of gears deployed in the same habitat at approximately the same time. However, it was possible to identify spatial depth gradients and diel changes in catch that were independent of gear. Consistent spatial and temporal patterns across gears were interpreted as characteristic of fish populations, not just of gears. Density in coastal habitats was higher at night and was higher at 4-7 m than at greater depths. These results, in conjunction with other studies, establish that coastal depths of 4-7 m represent the centre, and not the edge, of the distribution of age 0 cod in Newfoundland during autumn. Hence nursery areas during the early 1990s, a time of historically low spawning stock biomass, must be identified as the coastal zone, not offshore.

Estimating gillnet selectivity using nonlinear response surface regression

We present a method of estimating the selectivity of experimental gill nets in which type A and type B curves are solved for simultaneously as a response surface using nonlinear regression. The modeling approach provides a general statistical framework for estimating selectivity parameters, evaluating different functional forms of the selectivity model, and testing for differences between models. We applied this approach to the gillnet catches of Louisiana spotted seatrout (Cynoscion nebulosus) in a five-panel experimental net from data collected from 1988 to 1995. The selectivity of the experimental gill nets for female and male spotted seatrout could be described by a common response surface that was based on a four-parameter normal probability density function (r2 = 0.95). By estimating type A and type B curves simultaneously the model was able to explain 74% more variation in the data than when compared with methods that estimate type B curves for each size-class individually. Statistical comparison of annual response surfaces was not significantly different (p > 0.05) and suggests that the estimation approach was relatively insensitive to interannual variation in population size composition.

Maturation and gill-net selectivity of two small sea breams (genus Diplodus) from the Algarve coast (south Portugal)

Diplodus annularis and Diplodus bellottii, two species of small sea breams from the Algarve coast (south Portugal), were studied to obtain information on the spawning period, first maturity, weight–length relationship and gill-net selectivity curves. These species are important in local gill-net catches, and there is no information on their biology and selectivity in the Algarve waters. Spawning lasts for 6 months. Total length (TL) at first maturity for females was 13.4 cm in case of D. annularis and 11.4 cm in case of D. bellottii. The relationships between weight and length were W=0.0132 TL 3.096 for D. annularis and W=0.0093 TL 3.193 for D. bellottii. The Sechin method was used to estimate gill-net selectivity curves. The estimated selectivity curves correspond closely to the catch curves of the legal mesh size (60 mm), but were narrower for the other meshes. D. annularis and D. bellottii will be caught using the legal mesh size in the following length ranges: 13–19 cm and 11–19 cm.

Dietary Studies on the Predatory Fishes of the Norman River Estuary, with Particular Reference to Penaeid Prawns

The predatory fish community and their prey in the Norman Rivere estuary, Gulf of Carpentaria Australia, are compared with the communities of other tropical inshore areas, to investigate patterns of predation in tropical inshore areas particularly in relation to penaeid prawns. Abiotic factors (turbidity, freshwater input) and diversity of habitat types affect the composition of both prey and predator communities, resulting in large differences in the communities of tropical inshore waters. The stomach contents of 2059 predatory fish from the Norman River estuary were analysed over four sampling trips in the wet and dry seasons. The stomachs of 61% (1255 fish) of 54 species contained a total of 676·2g (dry weight) of food, while 39% (804 fish) were empty. Teleosts were the main component of the diet (by dry weight) of 13 of the 22 species analysed, followed by annelids for two species. Five species had only teleosts in their stomachs. Most predator species ate benthic or bentho-pelagic prey, while three species—Rhizoprionodon taylori, Scomberoides commersonianusandLeptobrama mulleri—ate mainly pelagic prey. Although 19 species ate some penaeids, onlyPolydactylus sheridaniate little else. This species,Lates calcariferandEleutheronema tetradactylumate 94·5% of all the penaeid prey and 97·9% of all the commercially important penaeid prey recorded in the study. Penaeid predation indices (calculated from gillnet catch rates, proportion of penaeids in the diet and a consumption rate of 3% body weight per day) were 0·23g of penaeid per net-metre per day forP. sheridani, 0·15 forL. calcariferand 0·03 forE. tetradactylum. Commercially important penaeid predation indices were 0·11, 0·13 and 0·01, respectively. These values are intermediate between those previously recorded for the main penaeid predators in other inshore areas of the Gulf of Carpentaria (Embley River estuary and Groote Eylandt).

Ecology and Status of Piscivores in Guri, an Oligotrophic Tropical Reservoir

Guri Reservoir in Venezuela supports sport fisheries for two cichlid species (pavón venado or speckled pavon Cichla temensis and pavón mariposa C. orinocensis) and payara Hydrolycus scomberoides. The lake receives acidic, hyperoligotrophic “black waters” from the Caroní River and experiences water level fluctuations associated with seasonal precipitation and hydroelectric operations. During the early 1980s, piscivore populations expanded in the new reservoir, but the quality of sportfishing declined during the 1990s. During 1993–94, we studied the ecology of the two Cichla species, H. scomberoides, and a fourth piscivore, Plagioscion squamosissimus (curvinata or silver croaker) and compared our data with those from similar surveys performed 7–9 years earlier (0–2 years after attainment of the reservoir's current crest height). Seine and gill-net samples from the lake's northern region produced 50 fish species from 18 families. Seine samples were dominated by a small characid, Hemigrammus micropterus, whereas gill-net samples were dominated by a large detritivorous characiform, Prochilodus rubrotaeniatus. Gill-net catch rates were low in all three surveys. All four piscivores from our recent survey had better body condition than in a 1985–1986 study, an indication of possible growth compensation. Diet breadths of all piscivores were low, and diet overlap was low for H. scomberoides with the two Cichla species and for P. squamosissimus with C. orinocensis. Hydrolycus scomberoides and C. temensis consumed significantly larger prey than P. squamosissimus, and differences in the relative proportions of specific characid and cichlid fishes consumed by the two Cichla species indicate that they forage at different depths within the littoral zone. The combined effects of hyperoligotrophic waters entering the reservoir, reduced inputs of dissolved nutrients from submerged terrestrial vegetation, low retention time of the reservoir, and harvest probably influence the low abundance of the two Cichla species in Guri Reservoir.

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 (r2) 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.

Tandem-Set Fyke Nets for Sampling Benthic Fishes in Lakes

Abstract We evaluated the effectiveness of two types of tandem-set fyke nets (round and D-shaped) for sampling fish in five lakes and compared fyke-net catches with contemporaneous gill-net catches. Fyke nets captured more benthic and cover-oriented species than gill nets, whereas gill nets captured more pelagic species. Seasonal variation in species composition and catch per unit effort (CPUE) was observed with fyke nets as well as gill nets. When sampling with fyke nets was limited to one season, large sample sizes were needed to detect changes in CPUE. Efficiency, ease of use, and low mortality of captured fish make tandem-set fyke nets a viable alternative to gill nets for assessing changes in community structure of benthic fishes in individual lakes. However, the large sampling effort required to detect changes in CPUE (unless target species are very abundant) limits the utility of fyke nets for monitoring the relative abundance of benthic species over time.

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brownSalmo truttawas assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1−1), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l−1). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1−1, and inorganic aluminium levels as low as l·0 μg 1−1. The cpue was inversely correlated with lake altitude (r2=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid‐neutralizing capacity, had the highest predictive power of the variability in cpue (r2=0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue (r2=0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1−1), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l−1). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1−1, and inorganic aluminium levels as low as l·0 μg 1−1. The cpue was inversely correlated with lake altitude (r2=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue (r2= 0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue (r2= 0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.

Responses of Ruffe ( Gymnocephalus cernuus (L.)) Abundance to Eutrophication

The responses of ruffe ( Gymnocephalus cernuus (L.)) abundance to eutrophication were studied in the brackish water of the Baltic Sea off Helsinki. The productivity gradients in the area permitted an analysis of the dependence of ruffe catches taken with gill nets in 1969 to 1972 on spatial differences in eutrophication level. There appeared to be a relationship between the level of primary productivity and the catch per unit effort of ruffe. The gill net catches showed that ruffe were more abundant in eutrophic than oligotrophic areas but less abundant in the most eutrophic areas. Due to the better purification of municipal sewage and the relocation of effluent discharge sites, primary production has declined in the most eutrophicated inner bays during recent decades. The responses of ruffe reproduction to these changes were studied on the basis of catches taken with small beach seines in 1982 to 1996. The number of zero-catches of 0+ ruffe increased significantly after oligotrophication, but decreased again in 1996 after the accidental reintroduction of waste waters.

Habitat, benthic prey, and distribution of juvenile lake sturgeon (Acipenser fulvescens) in northern Ontario rivers

The spatial and temporal distribution and relative abundance of juvenile lake sturgeon (Acipenser fulvescens) and benthic macroinvertebrates were examined in the Mattagami and Groundhog rivers, tributaries of the Moose River in northern Ontario, Canada. Relative density and diversity of the benthic fauna were low, the most common taxa being Ablabesmyia sp. (Diptera) and Hexagenia sp. (Ephemeroptera). Benthic organisms were most abundant in substrate dominated by sand and clay. An appreciable portion of the riverbed was strewn with wood chips and was particularly depauperate of macrobenthos. Lake sturgeon foraged exclusively on the benthos, especially taxa of Ephemeroptera, Diptera, and Trichoptera. Hexagenia was the only prey species in greater density in sturgeon stomachs than in the benthos. The largest concentration of juvenile lake sturgeon was found adjacent to substrate dominated by sand and clay. Gillnet catches of lake sturgeon suggest that they are more active at night, which may enhance their foraging success. In these northern rivers, the trophic status of juvenile lake sturgeon is marginalized by an impoverished food supply, in part the consequence of habitat loss resulting from anthropogenic activities.

Patterns of Fish Abundance Associated with a Dredge Disposal Island: Implications for Fish Habitat Enhancement in a Large Reservoir

Abstract We evaluated trends in fish abundance associated with in-water disposal of dredged material in Lower Granite Reservoir, Idaho–Washington. Fish assemblages were sampled before (1985) and after (1993) construction of a 0.37-ha disposal island to assess local changes in fish community structure, In addition, resident fish abundance was monitored for 5 years (1989–1993) near the disposal island and compared with patterns of fish abundance at other shallow-water sampling stations in the reservoir. Ranked abundance of fishes collected in gill-net samples differed before and after construction of the disposal island. Of 12 common resident fishes in Lower Granite Reservoir, only 6 species were represented in gill-net catches before construction of the island, whereas 11 species were represented in catches after construction. Trophic structure and trends in feeding-guild abundance were not significantly different between disposal and reference stations from 1989 to 1993. However, herbivores and insectivor...