Gillnet Catches Research Articles

In situ target strength of the Baltic Sea herring and sprat

Abstract Using single- and split-beam hydroacoustic equipment (70 and 38 kHz), and gillnet and trawl catches, we derived the relationship TS = 25.5 log10 L − 73.6 (r2=0.95) between acoustic target strength (TS) and fish length (L, cm) for Baltic Sea herring (Clupea harengus membras L.) and sprat (Sprattus sprattus balticus (Schneider)). Fixing the slope to 20, which is a standard practice in hydroacoustics, gave TS = 20 log10 L − 67.8 (r2=0.91). Normally, the fisheries agencies around the Baltic use a TS–length relationship that is based mainly on data from the North Sea and the intercept-value in this equation is 3.4 dB lower than that reported in this paper. This difference corresponds to an approximately twofold difference in assessed stock biomass.

Quantifying the roles of competing static gears: comparative selectivity of longlines and monofilament gill nets in a multi-species fishery of the Algarve (southern Portugal)

Fishing trials with monofilament gill nets and longlines using small hooks were carried out in Algarve waters (southern Portugal) over a one-year period. Four hook sizes of Mustad brand, round bent, flatted sea hooks (Quality 2316 DT, numbers 15, 13, 12 and 11) and four mesh sizes of 25, 30, 35 and 40 mm (bar length) monofilament gill nets were used. Commercially valuable sea breams dominated the longline catches while small pelagics were relatively more important in the gill nets. Significant differences in the catch size frequency distributions of the two gears were found for all the most important species caught by both gears (Boops boops, Diplodus bellottii, Diplodus vulgaris, Pagellus acarne, Pagellus erythrinus, Spondyliosoma cantharus, Scomber japonicus and Scorpaena notata), with longlines catching larger fish and a wider size range than nets. Whereas longline catch size frequency distributions for most species for the different hook sizes were generally highly overlapped, suggesting little or no differences in size selectivity, gill net catch size frequency distributions clearly showed size selection. A variety of models were fitted to the gill net and hook data using the SELECT method, while the parameters of the logistic model were estimated by maximum likelihood for the longline data. The bi-normal model gave the best fits for most of the species caught with gill nets, while the logistic model adequately described hook selectivity. The results of this study show that the two static gears compete for many of the same species and have different impacts in terms of catch composition and size selectivity. This information will be useful for the improved management of these small-scale fisheries in which many different gears compete for scarce resources.

Stomach Contents of Breeding Common Murres Caught in Gillnets off Newfoundland

Abstract We examined the stomach contents of 64 male and 43 female Common Murres (Uria aalge) caught in gillnets during late chick rearing/early chick departure period in August 2002 near breeding colonies in Witless Bay, Newfoundland, Canada. Twenty-six percent of stomachs contained no food. All others contained exclusively fish, and Capelin (Mallotus villosus) was the only fish positively identified. Gravid female Capelin were found in 82% of murre stomachs with contents. The number of fish ingested by murres differed between the sexes, with a mean of 2.7 fish in male stomachs and 1.6 fish in female stomachs. Mean mass of stomach contents did not differ between the sexes (males: 27.0 g; females: 22.6 g). Overall, results were similar to Common Murre diets recorded in the 1980s, despite changes in Capelin distribution and biology. The sample of Common Murres collected as gillnet bycatch was male-biased (59%). Males may have been more vulnerable to being caught in gillnets, perhaps because they were engag...

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...

Comparison of gillnet and trawl in diurnal fish community sampling

The fish community in a shallow, eutrophic lake basin in southern Finland was sampled diurnally with gillnets and trawl. The differences in species number, relative abundances and length distributions were considered. The fish density estimations differed notably depending on the gear and diurnal period. The most abundant species in the trawl catch, smelt, was almost totally missing from the gillnet catch. The proportions of perch, roach, white bream and asp were higher in the gillnet catch. Gillnets regularly underestimated the proportion of small (<10 cm) individuals in size distributions. The trawl probably underestimated the density of piscivores. In the two gears, diurnal changes, in both fish catch and species distribution, were considerable. Both trawl and gillnets are needed to get more reliable figure of fish communities in shallow eutrophic lakes.

Responses of plankton and fish communities to mass removal of planktivorous fish in a two-basin lake in southern Finland

Responses of plankton and fish communities to mass removal of cyprinid fishes were studied in a two-basin lake, Aimajarvi, in southern Finland during 1997–2001. Fish were removed from the lake with fyke nets and seining. By spring 2001, the cumulative catches were 276 kg ha−1 in the northern and 226 kg ha−1 in the southern basin. Roach dominated the catches of both basins. From summer 1997 to 2001 the total biomass of phytoplankton decreased from 10–30 to 5–13 mg l−1 in the more eutrophic northern basin and from 5–19 to 2-4 mg l−1 in the southern basin, respectively. The occurrence of maximum biomass of blue-green algae moved from July to August towards autumn during the study period. The mean biomass of cladoceran crustaceans in July to August showed a slightly increasing trend from 100–150 to 150–200 μg C l−1. The biomasses of Daphnia spp. and Bosmina coregoni were higher in 2000 and 2001 compared to earlier years. The annual mean total catches of multimesh gillnets, dominated by roach, were 3.9–5.1 kg net−1 in the northern and 1.8–2.6 kg net−1 in the southern basin. A slight decrease took place in the gillnet catches in the southern basin but not in the northern. The increased growth of young perch and roach in the last years of the study period was attributed to the thermal conditions rather than to the fish removal.

Gill net and long-line catch comparisons in a hake fishery: the case of southern Portugal

SUMMARY: During 1998, a study of the European hake (Merluccius merluccius) static gear fishery was undertaken in southern Portugal. This study aimed to compare long-line and gill net catches within the same area in terms of catch composition, fishing yield, by-catch and discards, and specifically for hake, catch size frequency distribution and quality of the fished product. At least 35 species of fish and invertebrates were caught, with hake dominating the catches in terms of both weight and number. The by-catch represented 23% and 15% of the catch in weight for long-lines and gill nets respectively. No illegal sized hake (under 27 cm total length) were caught in either gear during the study period. Discards of non-commercial species were similar for both gears and considered to be negligible (<3% by weight). Hake discards were significantly different between gears (7% for long-lines and 42% for gill nets, by weight), due to fish deterioration related to soaking time. Higher daily yields were obtained for long-lines (258.37 kg / 9,000 hooks) compared with those for gill nets (127.12 kg / 10 km net). These findings will help to improve the management and conservation of this valuable hake fishery.

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.

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.

Gillnet selectivity of small coastal sharks off the southeastern United States

Gillnet selectivity parameters for the Atlantic sharpnose, Rhizoprionodon terraenovae, blacknose, Carcharhinus acronotus, finetooth, Carcharhinus isodon, and bonnethead, Sphyrna tiburo, sharks were estimated from fishery-independent catches in multi-panel gillnets with stretched mesh sizes ranging from 8.9 to 14.0 cm in steps of 1.3 cm, with an additional size of 20.3 cm. Mesh selectivities were estimated using a maximum-likelihood model, which fits a gamma distribution to length data for each mesh size using the log-likelihood function. The Atlantic sharpnose and finetooth shark exhibited the broadest selection curves. Peak selectivities for the Atlantic sharpnose were reached from 750 mm FL for the 8.9 cm mesh to 1150 mm FL for the 14.0 cm mesh in 50 mm FL increments per mesh. Peak selectivity for the finetooth shark was reached at 550 mm FL for the 8.9 and 10.2 cm meshes, increased to 650 mm FL for the 11.4 mesh, and 750 mm FL for the 12.7 and 14.0 cm meshes. Selectivity was highest at 1150 mm FL for the 20.3 cm mesh. The bonnethead and blacknose shark exhibited narrower selection curves, with peak selectivity occurring at 450 mm FL for the 8.9 cm mesh, 750 mm for the 12.7 cm mesh in 100 mm FL increments per mesh. Maximum selectivity for the 20.3 cm mesh was 950 and 1050 mm FL for bonnethead and blacknose shark, respectively. The θ 1 values for blacknose and finetooth shark were most similar (140.58 and 141.25), whereas the value calculated for Atlantic sharpnose was the highest (211.95) and that for the bonnethead (131.77) was the lowest. Values calculated for θ 2, a parameter that describes the variance of sizes by mesh, ranged from 27,259 for the bonnethead to 189,873 for the finetooth shark. Although gillnets used in this study were not directly constructed for use in estimation of gillnet selectivities, information on mesh selectivities estimated herein has direct applicability to commercial gillnets with meshes of similar sizes.

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.

What Does Mt. Pinatubo Have to Do with Walleyes?

The eruption of Mt. Pinatubo in the Philippine Islands during June 1991 was followed by a global temperature decline of 1°F through 1993. May to July temperatures in Minnesota during 1992 and 1993 were 3°F below the 1979–1998 average. The 1992 and 1993 year-classes of walleyes Stizostedion vitreum from Minnesota's nine largest walleye lakes, which are gillnetted annually, were the two weakest since 1979. Mean June temperature explained more than 40% of the variation in year-class strength. Mean statewide walleye gill-net catch per unit effort (CPUE) from lake surveys increased at a rate of 1.3% annually after 1979 and was positively related to rising June temperatures, which explains 57% of the variation from lakes in glacial drift and 28% of the variation from Precambrian shield lakes. Walleye fry stocking was also positively related to increased walleye gill-net CPUE, but fingerling stocking was not. Chaotic events, such as large volcanic eruptions, may negatively affect walleye recruitment, and these effects may be evident to anglers several years later.

Comparative fixed gear studies in the Cyclades (Aegean Sea): size selectivity of small-hook longlines and monofilament gill nets

Fishing trials with monofilament gill nets and longlines using small hooks were carried out at the same fishing grounds in Cyclades (Aegean Sea) over 1 year. Four sizes of MUSTAD brand, round bent, flatted sea hooks (Quality 2316 DT, numbers 15, 13, 12 and 11) and four mesh sizes of 22, 24, 26 and 28 mm nominal bar length monofilament gill nets were used. Significant differences in the catch size frequency distributions of the two gears were found for four out of five of the most important species caught by both the gears ( Diplodus annularis, Diplodus vulgaris, Pagellus erythrinus, Scorpaena porcus and Serranus cabrilla), with longlines catching larger fish and a wider size range than gill nets. Whereas longline catch size frequency distributions for most species for the different hook sizes were generally highly overlapped, suggesting little or no differences in size selectivity, gill net catch size frequency distributions clearly showed size selection, with larger mesh sizes catching larger fish. A variety of models were fitted to the gill net data, with the lognormal providing the best fit in most cases. A maximum likelihood method was also used to estimate the parameters of the logistic model for the longline data. Because of the highly overlapped longline catch size frequency distributions parameters could only be estimated for two species. This study shows that the two static gears have different impacts in terms of size selection. This information will be useful for the more effective management of these small-scale, multi-species and multi-gear fisheries.

Bycatch in the gillnet and beach-seine fisheries in the Western Cape, South Africa, with implications for management

Interview questionnaires and access point surveys were conducted in order to describe and quantify the catch composition of the inshore net-fisheries in the Western Cape, South Africa. A total of 138 562 fish, representing 29 species from 20 families, was recorded in 141 monitored commercial gillnet fishing operations between February 1998 and October 1999. Numerically, the legal target species, harders Liza richardsonii, dominated the catches, contributing 94.87% of the total gillnet catch. Elf, Pomatomus saltatrix, horse mackerel Trachurustrachurus capensis, gurnard Chelidonichthys capensis and barbel Galeichthys feliceps were the most common bycatch species, and contributed 4.2% to the total catch numerically and occurred in 12–47% of the marine 44–64-mm gillnet catches that were monitored. Five species most frequently targeted by shore-anglers on the West Coast: galjoen Dichistius capensis, white stumpnose Rhabdosargus globiceps, hottentot Pachymetopon blochii, silver kob Argyrosomus inodorus and whit...

Comparison of Concurrent Trap-Net and Gill-Net Samples for Black Bullheads

ABSTRACT We compared concurrent trap-net (i.e., modified fyke net) and gill-net catches of black bullhead (Ameiurus melas) from 28 South Dakota water bodies sampled in 2000 or 2001. The length-frequency distribution of sampled black bullheads was significantly different (P = 0.0001) between gears, primarily because the trap nets commonly collected bullheads less than 150 mm total length, while such fish were collected in low numbers by the experimental gill nets. Size structure of stock-length (i.e., ≥ 150 mm) black bullheads was similar between gears, as evidenced by a correlation of proportional stock density values (P = 0.0001) with the y-intercept not different from 0 and the slope not different from 1. However, mean catch per unit effort (CPUE) of stock-length black bullheads in the two gears was not correlated (P = 0.10). Mean trap-net CPUE had a broad range (from near 0 to over 2,300 per net-night), while mean gill-net CPUE did not exceed 250/net-night, perhaps because of net saturation. Thus, both trap nets and experimental gill nets provided similar indices of population size structure, but the trap nets may provide a better index of relative abundance for black bullhead population monitoring.

Plastic debris collars on juvenile carcharhinid sharks ( Rhizoprionodon lalandii) in southwest Atlantic

Three juvenile Brazilian sharpnose sharks ( Rhizoprionodon lalandii) caught in gillnets in southeast Brazil, southwest Atlantic, were found with plastic debris rings around their gill or mouth region. The rings caused severe abrasion on the sharks’ tissues as the animal grew, the collars probably hampering normal feeding and/or ventilation since two of the collared individuals were emaciated. The rings were identified as detachable lid parts from plastic bottles, likely thrown overboard by fishery and/or recreation boats. As several carcharhinid shark species dwells and reproduce in shallow waters, the impact of discarded plastic debris likely is greater on this shark type.

Geographical, seasonal occurrence and incidental fishing captures of basking shark Cetorhinus maximus (Chondricthyes: Cetorhinidae)

This is the first report on the presence of basking sharks Cetorhinus maximus in Galician waters (north-west Iberian Peninsula) from sightings, strandings and incidental catches by fishing gear. Morphometric, biological, geographic and temporal data on 19 basking sharks recorded from 1988 to 1998 are presented. Average total length is 401·4 cm. Male:female ratio is 0·6. Seventy per cent of records are from incidental fishing catches in bottom gill nets. Approximately 74% of sharks were recorded during February, March and April, which may suggest that the species occurs seasonally in this area.

Environmental Influences on Landlocked Fall Chinook Salmon Reproductive Characteristics

Abstract From 1988 to 1998, we evaluated relationships among several environmental and biological variables and the fecundity, egg size, and embryo survival to the eyed stage of landlocked fall chinook salmon Oncorhynchus tshawytscha from Lake Oahe, South Dakota. The environmental variables recorded in Lake Oahe included monthly and total yearly inflows and outflows, end-of-month elevations, yearly mean end-of-month elevations, and 13°C and 15°C habitat volumes. Biological variables included gill-net catch per unit effort (CPUE) of lake herring Coregonus artedi, rainbow smelt Osmerus mordax, and chinook salmon (for age-0, age-1 and older, and all fish); chinook salmon age-1 mean relative weight; and rainbow smelt abundance in both the 13°C and 15°C habitat volumes. Chinook salmon fecundity was significantly related to rainbow smelt CPUE (r = 0.841) and chinook salmon CPUE (r = 0.853). Fecundity was also significantly related to mean relative weight of age-1 and older salmon collected in gill nets the year...

A 27‐year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown troutSalmo trutta, population in a 70‐ha oligotrophic lake in central Norway was exploited using larger mesh gill‐nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller‐mesh sizes gill‐nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish &gt;3 years old for period II. The gill‐net catchability was correlated negatively with spawner biomass and number of trout &gt;3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density‐dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density‐dependent recruitment of 1‐year‐old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.

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.