Gillnet Catches Research Articles

Seasonal distribution patterns of fishes within the Bathurst Harbour estuary, southwestern Tasmania

The distribution of fishes within the Bathurst Harbour estuary was surveyed by gillnet on five occasions and by seine on three occasions, between October 1988 and July 1989. Fish distributions did not noticeably change within the estuary in response to seasonal changes in salinity. A number of fishes were, however, possibly prevented from entering the estuary during February 1989 because of low oxygen concentrations then prevailing. The assemblage of Bathurst Harbour fishes collected by gill net was unusual in being dominated by sharks and skates, with all of the common species also being distributed in water depths >50 m along the continental shelf. Thus it has many characteristics typical of a deep water assemblage. Included amongst the Bathurst Harbour fishes was a previously unknown of species of skate which may be restricted to the estuary. By far the most common of the larger fishes was the white-spotted dogfish Squalus acanthias this species comprised 86% of the total gill net catch. Despite these large catches, a tagging study in which a very high proportion (25%) of tagged dogfish were recaptured indicated that the total population in the estuary was not exceptionally large, being in the order of only 2900 animals. Tagged dogfish roamed widely through the estuary.

Sex ratio variation in surf smelt, Hypomesus pretiosus, gillnet catches due to differences in mesh size

Sex ratio variation in surf smelt, <i>Hypomesus pretiosus</i>, gillnet catches due to differences in mesh size

Predictors of Relative Fish Biomass in Lakes and Reservoirs of Argentina

Relationships between total fish biomass and chlorophyll, nutrient levels, and morphometric and climatic factors are shown for a set of 106 Argentinian lakes and reservoirs. The total data base is highly heterogeneous. Relative fish biomass (CPUE) was estimated from gill net catches. A data screening process was applied to the environmental data base to homogenize it. Nutrient, total organic matter content, and mean depth were most important in explaining relative fish biomass variability between lakes and reservoirs. In most heterogeneous sets total organic nitrogen (TON) explained most of CPUE variability. After screening for limnological anomalies (i.e. turbidity derived from inorganic sources, unsuitable conditions for fish, and TN/TP (total nitrogen/total phosphorus) &lt; 37 (molar basis)), TP and TON were highly correlated with CPUE. Mean annual air temperature was significantly related to fish biomass, but the relationship was not significant after nutrients, morphometry, and environmental oxygen conditions had been accounted for. Several possible causes are discussed. The results support the hypothesis that fish/phytoplankton, and fish/zooplankton biomass ratios are inversely related to lake trophy.

Direct and Indirect Estimation of Gillnet Selection Curves of Atlantic Herring (Clupea harengus harengus)

Length-specific selection curves for Atlantic herring (Clupea harengus) were calculated for a series of gillnets ranging in mesh size from 50.8 to 76.2 mm (stretched measure) using Holt's (1963) model (ICNAF Spec. Publ. 5: 106–115). These curves were than compared with direct estimates of length-specific selectivity obtained from a comparison of gillnet catch length frequencies with population length composition data as determined from acoustic surveys. Selection curves calculated indirectly using the Holt model were unimodal and congruent. The empirical selection curves however were multimodal and fishing power varied with mesh size. These differences in selectivities were due to the fact that herring were caught not only by wedging at the maximum girth but also at other body positions such as the gills and snout. Each of these modes of capture have different length-specific selectivity characteristics and, since the relative contributions of the different modes of capture varied both between nets and annually, the selection curve of herring for a particular mesh size is not unique. It can however be reasonably approximated when girth is used as the selection criterion. Direct empirical selectivities are therefore recommended when interpreting population parameters from herring gillnet catch data.

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.

Establishment of Reproducing Populations of Brook Trout after Stocking of Interstrain Hybrids in Precambrian Shield Lakes

Abstract Planted interstrain-hybrid and wild-strain brook trout Salvelinus fontinalis established self-propagating populations in four Precambrian Shield lakes in which earlier plantings of a domestic strain had failed to reproduce successfully. The purpose of this study was to document successful reproduction by interstrain-hybrid and wild brook trout and to suggest reasons why the domestic strain failed to reproduce. Nearly all brook trout were marked before stocking so that planted fish could be distinguished from lake-reared fish in gill-net catches. The tentative conclusion is that during its long period of domestication (more than 20 generations), the domestic strain had lost much of its ability to locate and use suitable spawning areas. A secondary factor was the poor survival to maturity of the domestic strain relative to the hybrid and wild strains.

Habitat utilization by sympatric arctic charr Salvelinus alpinus L. and brown trout Salmo trutta L. in Lake Atnsjø, south‐east Norway

SUMMARY. 1. Habitat utilization, as well as inter‐ and intraspecific relations of different size groups of arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjø, south‐east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June‐October 1985.2. Small individuals (150–230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5–15 m and brown trout at depths of 0–5 m.3. Larger (&gt;230 mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2‐3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0–12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspecific competition for food.4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton, whereas brown trout had a more variable diet, consisting of surface insects, zooplankton. aquatic insects and fish.5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high social position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewarding part of the benthic habitat.

Xenobrama microlepis, a new genus and species of bramid fish, from subantarctic waters of the South Pacific

Xenobrama microlepis gen. et sp. nov. is described on the basis of 17 adult and subadult specimens collected by surface gill nets, bottom trawl and midwater trawl from the subantarctic waters of the South Pacific Ocean. This monotypic new genus is distinguished from other bramid genera by the following characters: inner lower edges of mandible touching each other (except near symphysis); gill rakers short, thick, and stout; subpectoral region very narrow; interpelvic space flat and wide; vertebrae 49–51; and scales in longitudinal series more than 83. The new taxon is widely distributed in the high seas of the South Pacific, 38–54°S, 79–176°W, but is rather rare compared toBrama spp. in catches of drift gill nets.

Relations of Gill-Net Catches of Walleyes and Angler Catch Rates in Ohio Waters of Western Lake Erie

Catches of walleye Stizostedion vitreum in fall gill-net surveys in western Lake Erie were good indicators of angler catch rates the following summer. Experimental gill nets fished near the lake surface and at the bottom efficiently captured enough walleyes to determine relative densities of individual age-groups. Simple linear models with surface gill-net catches as independent variables explained 85–97% of the variability in angler catch rates. Slopes of the relations indicated that walleyes were less vulnerable to angling at ages 2 and 3 than at ages 4 and 5. Relations of bottom gill-net catches to angler catch rates were not as well defined and indicated significant differences only between ages 3 and 5. These findings illustrate the value of fall gill-net surveys in western Lake Erie for predicting walleye sport catches and evaluating potential sport-fishery regulations.

Some Factors Affecting Contribution of Cichlid Species Etroplus suratensis and Tilapia rendalli to the Gill-net Catches of a Man-made Lake in Sri Lanka

In the gill-net fishery of Parakrama Samudra, a man-made lake in Sri Lanka, some factors affecting species composition of the commercial landings were studied from February 1985 to February 1986, with particular reference to the cichlid species, Etroplus suratensis and Tilapia rendalli. The proportions in the catches of a water beating technique, a modified gill-net fishing method, were found to be significantly different from that of normal gill netting. These differences in the catches of E. suratensis and T. rendalli are suggested to be due to their restricted movements, associated with reproductive behavior. The increase of catches of two cichlid species with elevation of water level may have been brought about by their aggregation in recently inundated peripheral areas of the lake to feed on macrophytes. The importance of stratifying data according to fishing method and mesh size of gill net in order to determine species composition is discussed. Using this method 'or analysis, annual fish yield in Parakrama Samudra in 1985 was estimated to be about 46 kg-ha•l.

Effects of Net Length and Set Time on Fish Catches in Gill Nets

Abstract The effects of varying net length and set time on fish catches in gill nets were studied at two sites in the Bay of Quinte, Lake Ontario. A latin square design was used (four locations per site over four nights); either net length was varied, from 25 to 200 yd over 12 h, or set time was varied, from 1.5 to 12 h with 50 yd of net. Treatment, location, and date effects on catches from each site were analyzed by a multiple regression model. Net length effects on catch per unit of effort (CPUE) were negligible for total and some fish species catches; CPUE for walleyes Stizostedion vitreum decreased with increasing net length. Set-time effects on total CPUE were significant; the CPUE of white perch Morone americana increased with set time, but CPUEs for yellow perch Perca flavescens, alewives Alosa pseudoharengus, and walleyes decreased. The results were attributable to specific differences in activity patterns and net-saturation effects. We concluded that use of gill-net catches as indices offish abu...

Effectiveness of Restricted Areas in Reducing Incidental Catches of Game Fish in a Gill-Net Fishery

Abstract A small-scale gill-net fishery for suckers Catostomidae has existed in specified areas of Lake Seminole, Georgia, but has been a source of conflict between the netters, who have continually asked for expansion of the legal areas and season, and sport fishermen, who have opposed netting because they view it as a threat to game-fish populations. Test netting was conducted to (1) assess the gill-net catch in the legal netting areas, (2) determine the effects of the recently expanded fishing season and legal fishing area, and (3) evaluate other areas that could be opened to gillnetting without harming game-fish populations. The catch composition of game fish, suckers, rough fish, and commercial catfish Ictaluridae varied significantly (P < 0.05) among areas. The legal areas were appropriate for netting due to a low catch of game fish, abundance of suckers, and moderate numbers of commercial catfish. Restriction of gillnetting to carefully selected areas may be a valuable technique for limiting the in...

Recruitment of Yellow Perch ( Perca flavescens) Affected by Stock Size and Water Temperature in Lakes Erie and St. Clair, 1965–85

Two influences on recruitment of yellow perch ( Perca flavescens) were considered: parental stock size and spring water temperatures. The data for this analysis were obtained from two sources. First, from samples of trawl catches (near bottom) from April to October (1965–85) in the western, central, and eastern basins of Lake Erie, and from Lake St. Clair and second, from gillnet catches from April to July in the three basins of Lake Erie (1978–86). The relative abundances (CPUE) of stocks in these study areas were as follows: western > St. Clair > eastern > central basin. Degree-days and mean water temperatures of the basins were ranked as follows: St. Clair > western > central > eastern basin. Rates of warming of water were greatest in the western basin, followed by St. Clair, central basin, and eastern basin. The rates of increase in water temperature (April to June) were positively related to average water temperatures and total degree-days (above 0°C. Only in the western basin of Lake Erie could the variations in recruitment to age 1+ be apportioned to the effect of parental stock size (12%) and water warming in the spring (51%). Recruitment at age 0+ (young-of-year(YOY)) was explained also by both warming rate (17%) and parental stock size (12%) for the samples from the western basin. The pooled data obtained from samples of catches in the gillnets indicated that only parental stock size (19%) determined the abundance of perch at age 1+ yrs-old. The relative abundance of yearlings was related to the abundance of YOY perch in the previous year in only the central and western basins (trawl sample). The dependence of recruitment on the density of the parental stock was evident from the regression of the ratio of recruits (age 1 +) to spawners on parental stock size in the combined trawl samples from Lakes Erie and St. Clair, and in the separate samples from the eastern and central basins of Lake Erie and Lake St. Clair. Similarly, recruitment at age 0+ (YOY) was also shown to be density-dependent in all but the western basin of Lake Erie. The coefficients obtained by regressing recruitment (0 +, 1 + years) on stock size were significant and less than 1 in the pooled sample from the western basin and St. Clair, indicating density-dependent recruitment.

Age and growth of two commercially imported sharks (Carcharhinus tilstoni and C. sorrah) from Northern Australia

The age and growth of Carcharhinus tilstoni and C. sorrah, the two most abundant shark species in commercial gill-net catches off northern Australia, were investigated by the examination of vertebral rings. Corroborating evidence for age and growth estimates was obtained from length-frequency distributions and tag-recapture data. To aid validation of these estimates, tetracycline was injected into sharks at the time of tagging. Growth is relatively rapid in the first year of life: vertebral ageing indicated 17 cm growth in total length (TL) for C. tilstoni and about 20 cm for C. sorrah during the first year after birth. By the time the sharks are 5 years old, growth has declined to 8-10 cm per year in C. tilsoni and 5 cm per year or less in C. sorrah. The von Bertalanffy growth parameters for C. tilstoni are L∞ = 194.2, K = 0.14, t0 = -2.8 for females, and L∞ = 165.4, K = 0.19, t0 = -2.6 for males; for C. sorrah the parameters are L∞ = 123.9, K = 0.34, t0 = -1.9 for females, and L∞ = 98.4, K = 1.17, t0 = -0.6 for males. The greatest recorded ages for C. tilstoni were 12 years for females and 8 years for males, and for C. sorrah, 7 years for females and 5 years for males. Sexual maturity is reached early: at 3 to 4 years in C. tilstoni and 2 to 3 years in female C. sorrah.

The Channel Catfish Fishery at Pony Express Lake, Missouri, 1963-1984

Abstract Pony Express Lake in Missouri was initially stocked in 1964 with fingerling largemouth bass Micropterus salmoides, bluegills Lepomis macrochirus, redear sunfish Lepomis microlophus, and 5-8-in channel catfish Ictalurus punctatus. The lake was opened to fishing in January 1966. Since the opening, annual supplemental stockings of 8-10-in channel catfish have been made at rates of 13-50 fish/acre. No negative effects on other species were documented that could be related to channel catfish stocking. The size distribution of channel catfish in gill-net catches, all years combined, corresponded to the size composition of angler catches except for fish less than 12.0 in. Experimental gill nets (1/2-2-in meshes) were most effective for sampling the smaller fish in the population. Channel catfish averaged 15.4 in at age 4 after completing two growing seasons in the lake. Annual daytime fishing pressure averaged 396 h/acre from 1966 through 1984, 33% of which was directed at channel catfish. Mean total an...

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.

Short-term effects of liming on perch Perca fluviatilis populations in acidified lakes in South-West Sweden

Gill net catches of perch Perca fluviatilis in eight acidified lakes immediately before liming are compared] with catches in the same lakes two years after liming. Simultaneously, six control lakes, not limed, were investigated with the same procedures. A significant increase in the average catches of perch was found in the limed lakes, but not in the control lakes. This result confirms earlier reports, although seldom supported with data from controls, on successful recovery after liming.

Fish community structure response to major habitat changes within the littoral zone of an estuarine coastal lake

The littoral environment and fish fauna of Swartvlei, an estuarine lake, was monitored for four years during which major habitat changes occurred. Initially (1979) the zone was dominated by the submerged macrophytes Potamogeton pectinatus, Chara globularis and Lamprothamnium papulosum. This plant community was replaced by filamentous algal mats during 1980 and with the disappearance of these mats in 1981 the littoral zone was transformed into a sandy habitat. There was a highly significant decline in the numbers of fishes in the littoral zone between the macrophyte and sand phases but no significant decrease in fish biomass between the two phases. Analysis of gill net catches revealed an increase in the CPUE of the family Mugilidae between the macrophyte and sand phases but a decline in the CPUE of vegetation associated species such as Monodactylus falciformis and Rhabdosargus holubi over the same period. The increase in mullet stocks during the sand phase was attributed to epipsammic micro-algal production and the input of allochthonous detritus during the 1981 floods. The three fish species diversity indices used in this study showed minor fluctuations between the habitat phases and these variations were related to changes in the equitability of distribution between the individual species within each habitat type. The numbers of fish species recorded during the macrophyte, algal mat and sand phases varied by less than 20%. The resilience of estuarine fishes to major alterations in their environment was illustrated by the fact that all fish species recorded at the beginning of 1979 were present at the end of 1982, despite major habitat and food resource changes.

The variability in catches from multi‐mesh gillnets fished in three Canadian lakes

Three types of multi‐mesh gillnet were fished in three lakes in Alberta during spring and summer of 1983. Eight species offish were caught in a total of 89 lifts. The variability of the catches was analysed after a transformation into y= log (x+1). For the major species (apart from yellow perch) there were remarkable similarities between species for error variances associated with catches from nets and from meshes. The total error variance associated with a catch from any net was 0‐0575 (C.V. = 59‐7%). Total error variance associated with a catch from a single mesh was 0‐0852 (C.V. = 75‐6%). These values are compared to those from other studies and an error variance estimate of 0‐0868 (C.V. = 76‐5%) was considered to be an appropriate value for gillnet catches in fresh waters. In sampling programmes more than six nets would need to be used for confidence limits to be below half or more than twice the geometric mean.

Intervention Analysis of Power Plant Impact on Fish Populations

Alewife (Alosa pseudoharengus) and yellow perch (Perca flavescens) abundances, estimated from monthly gillnet and trawl catches at two transects, were monitored before (1973–74) and during (1975–82) operation of the D. C. Cook Nuclear Power Plant, southeastern Lake Michigan. Intervention analysis, a technique which accounts for autocorrelated observations, and analysis of variance (ANOVA) were applied to the monitoring data to assess any plant impact beginning in 1975. Both analyses disclosed no significant power plant impacts except for gillnetted yellow perch adults. The ANOVA indicated a significant decrease in abundance at the plant-discharge transect relative to the reference transect as plant operation began, which established a plant effect; intervention analysis showed no change. When April and May catches (months of low abundance) were deleted, this plant effect was insignificant. Monte Carlo simulation showed that as the first-order autoregressive coefficient increased positively, type I error of the ANOVA F-test increased. However, ANOVA was more powerful than intervention analysis when a first-order autoregressive component was included. Impact assessment based only on ANOVA can result in detection of impact when actually there was no effect (type I error) when observations are serially correlated (lack independence).