Gillnet Catch Rates Research Articles

Abundance, spatial distribution, and diet of endangered Hovsgol grayling (Thymallus nigrescens)

The Hovsgol grayling, endemic to Lake Hovsgol, Mongolia, is considered endangered, but published descriptions of the species abundance, distribution, or behavior do not exist. We used hydroacoustics, vertical and horizontal gillnetting, zooplankton sampling, and stomach content analysis to characterize abundance, distribution, prey avail- ability, and diet of Hovsgol grayling. Pelagic Hovsgol grayling densities averaged 34.2±6.8 individuals·ha �1 (biomass of 4.4±0.9 kg·ha �1 ) and were concentrated along the western and northern areas of the lake. Gillnet catch rates were 7.5 times higher for littoral sets than pelagic sets. Pelagic vertical distributions of zooplankton and Hovsgol grayling were concentrated above 50 m, with grayling exhibiting diel vertical migrations from 15 m at night to 30 m during the day. Smaller Hovsgol grayling fed primarily on zooplankton while larger individuals fed more heavily on benthic prey in littoral and pelagic areas. The results from this study may be used to guide conservation management and monitoring strategies for Hovsgol grayling, and provide a conservation reference point as human population growth and environmental change continues in the Lake Hovsgol catchment.

The influence of abiotic factors on gillnet catch rates of yellow perch in southern Lake Michigan, 1989–2006

Abstract The difference in yellow perch, Perca flavescens (Mitchill), catch rates were evaluated for gillnets set on the bottom at 10‐ and 15‐m depths in southern Lake Michigan during June, July and August from 1989 to 2006. More yellow perch were captured in gillnets set at 10 m than at 15 m. Differences in water temperature were significant in explaining variation in catch rates, but differences in Secchi depth and wave height were not significant. These results suggest that yellow perch may associate with habitat based on water temperature during the summer and move in response to thermal changes. In addition, this portion of the lake lacks noticeable heterogeneous structural habitat features that normally influence yellow perch behaviour. Fisheries managers should consider sampling based on temperature in addition to depth or other habitat features when designing sampling protocols.

Diel variation in gillnet catches and vertical distribution of pelagic fishes in a stratified European reservoir

The diel changes in gillnet catch rates and vertical distributions of pelagic fishes were determined in a deep, moderately eutrophic European reservoir. In late summer, when steep vertical gradients in temperature and dissolved oxygen developed in the reservoir, monofilament gillnets (12.5–70 mm bar mesh) covering the upper 7.5 m of the water column were set at two open-water locations over 44- and 46-h periods, respectively. Gillnets divided the sampled strata (i.e., the epilimnion and metalimnion) into five horizontal layers each 1.5 m thick. Captured fish were removed from the nets at regular 2-h intervals. The catches were numerically dominated by bleak Alburnus alburnus, roach Rutilus rutilus, perch Perca fluviatilis, bream Abramis brama and asp Aspius aspius. Catch rates varied considerably with the time of day. The total gillnet catch per 2-h was on average 3–4 times higher at twilight than during the day or at night. Decreased net visibility and increased fish activity were likely responsible for the high gillnet catches at dusk and dawn periods. The catches of individual species were not distributed uniformly across the five sampled depth layers. The majority of bleak and asp were captured in the surface layer (0–1.5 m), irrespective of day, twilight or night. During the daytime, roach were most abundant in the layer just above the thermocline (3–4.5 m), but at night they occupied the upper part of the epilimnion (0–3 m). Perch inhabited the lower epilimnion and the upper metalimnion (1.5–6 m), and were caught only during daytime and twilight. For bleak and roach, greater proportions of larger individuals were caught during the day, while proportions of smaller fish increased in twilight and night catches. This implies that individuals of the two species made diel migrations between pelagic and littoral habitats.

Variation in Gill-Net and Angling Catchability with Changing Density of Northern Pike in a Small Minnesota Lake

Abstract The variation in the catchability coefficients (q) of gill netting and angling was monitored while manipulating the density of a population of northern pike Esox lucius during a series of nettings. The nettings removed nearly one-half of the population of northern pike age 2 and older from a small Minnesota lake. Estimates of q differed among age-classes of fish, thereby corroborating the size selectivity of the multimesh experimental gill nets. The results showed that gill-net catch rates tracked the declining population size of northern pike relatively closely. However, daily estimates of q declined during the removals, indicating hyperdepletion in catch rates. The decline in q contrasts with the results of several previous studies, which showed inverse relationships between q and density, and was most likely a result of gear size selectivity and fish behavior. Changes in age and size structure stemming from our intensive nettings helped cause the declining catchability. Angling catch rates wer...

Spatio-temporal variations in gillnet catch rates in the vicinity of North Sea oil platforms

Use of decommissioned North Sea oil platforms as artificial reefs may provide an alternative to their being removed. Three fishing experiments with gillnet fleets lasting 3–5 days and using different mesh sizes were conducted in the vicinity of two platforms to study their potential as sites for commercial fishing. In each case, five fleets were set in a star-shaped pattern centred around the platform and running up to a distance 1400 m. Catches consisted mainly of cod (Gadus morhua) and saithe (Pollachius virens), and in one case also of ling (Molva molva). In two experiments, catch rates of nets set within a distance of 110–165 m of the platform were three to four times higher than those of nets set at greater distances. In the third experiment, the highest catches were taken 150–300 m from the platform, also by a factor of four compared to catches taken at greater distances. However, patterns differed among species, areas, and seasons. Catch rates varied considerably between fishing days, and in only one experiment did catch rates (specifically of ling) decline consistently throughout the fishing period, suggesting gradual depletion. Although the study provided evidence of pronounced aggregations of fish close to the platforms, responses are complex and results are inconclusive regarding species-specific temporal and spatial patterns.

A Comparison of Fish Distributions in Simple and Complex Lake Basins

ABSTRACT Gill nets were used to determine the spatial and seasonal distributions of fishes in Lake Kampeska (simple morphometry) and at Lake Enemy Swim (complex morphometry), South Dakota. Each season (spring, summer, and autumn) gill nets were randomly placed at littoral and limnetic sites and fished for approximately 24 h. Gill nets were allowed to fish for approximately 24 h. At Lake Kampeska, 811 fishes were collected representing 16 species; statistical analyses were completed for five species. A total of 2,881 fishes was collected at Lake Enemy Swim with analyses completed for seven species. Significant (P <0.05) differences between littoral and limnetic gill net catch rates were identified during the spring at Lake Kampeska for walleye (Stizostedion vitreum), yellow perch (Perca flavescens), and white sucker (Catostomus commersoni). At Lake Enemy Swim, significant differences between littoral and limnetic sites occurred for yellow perch (spring and autumn) and rock bass (Ambloplites rupestris) (summer). Differences between median lengths of littoral and limnetic caught fishes were identified for white sucker at Lake Kampeska and yellow perch and smallmouth bass (Micropterus dolomieu) at Lake Enemy Swim. Significant seasonal differences in gill net catch rates and median lengths were identified at both lakes.

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.

Relationship between Walleye Fingerling Stocking Density and Recruitment in Lower Lake Oahe, South Dakota

Abstract This study evaluated four methods ofexpressing fingerling stocking density for walleye Stizostedion vitreum: number per surface area, number per littoral-zone area, number per shoreline development value, and number per shoreline distance, Walleye fingerlings were stocked in embayments of lower Lake Oahe in 1988 and 1989 at varying densities. Stocking success was assessed by means of fall gillnetting for young-of-year walleyes. Mean gill-net catch rates varied significantly within each method of expressing stocking density. Effects of stocking density on gill-net catch rate for walleye young of the year were analyzed with linear regression. Fingerling densities expressed as number per shoreline development value had the highest coefficient of determination. Shoreline development value may provide a better measure of walleye fingerling habitat than traditional expressions of stocking density such as number per surface area. Based on this assessment, a minimum stocking density of 17,006 fingerlings...

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 (R2= 0.78), and predicted 1986 trap catch rate changes (by 1985 model) were correlated significantly with those observed (r= 0.60, P &lt; 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) (R2= 0.68), while predicted 1986 gillnet catch rates (by 1985 model) were significantly correlated with those observed (r= 0.35, P &lt; 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.

Use of Gill-Net Data to Provide a Recruitment Index for Walleyes

Abstract Fall catch rates of young-of-the-year (age-0) walleyes Stizostedion vitreum in 25-mm (bar measure) gill nets were highly correlated with catches of yearlings in 38-mm-mesh nets the following fall in six Kansas reservoirs (r = 0.835; P < 0.001), indicating that fall gill-net catch rates of age-0 fish are useful indicators of walleye recruitment.