Showing papers in "Fisheries Management and Ecology in 1995"
TL;DR: Carlin tagging data for 1980–1991 were used to examine the influence of smolt size and feeding conditions on the post-smolt survival of ranched River Neva salmon, in the the Gulf of Finland, which is the native feeding area of the stock, and in the Bothnian Sea, where the stock has been introduced.
Abstract: Carlin tagging data for 1980–1991 were used to examine the influence of smolt size and feeding conditions on the post-smolt survival of ranched River Neva salmon, (Salmo salar L.), in the the Gulf of Finland, which is the native feeding area of the stock, and in the Bothnian Sea, where the stock has been introduced. Because of better feeding conditions, the survival rates were higher and less variable in the Gulf of Finland than in the Bothnian Sea. In the Bothnian Sea, the annual variability in survival decreased and the mean value increased with increasing smolt size from the smallest (14–16 cm) to the largest (28–30 cm) size classes. The survival was positively correlated with growth rates, food resources and sea-surface temperatures. This suggests that in the Bothnian Sea the annual variability in survival is mainly because variable marine conditions affect growth rates, and, thus, the vulnerability of the post-smolts to size-dependent predation. In the Gulf of Finland, the survival advantage of large initial size and rapid growth was counteracted by size-selective post-smolt mortality from fishing. The increase in the survival rate with increasing smolt size levelled off at 22 cm, and the correlations between survival, growth and the indices of feeding conditions were mostly insignificant. For large smolts, some negative correlations were recorded, suggesting that the relative significance of mortality from fishing may even exceed that of size-dependent natural mortality. The implications of the results for management are discussed.
102 citations
TL;DR: An adaptive approach to management, involving judicious experimentation with stocking and harvesting regimes, is proposed to gain information on the dynamics of actual fisheries, and to optimize their production.
Abstract: The population dynamics of culture-based fisheries are studied by means of a mathematical model, which incorporates explicit sub-models for density-dependent individual growth and size-dependent mortality The model applies to populations of carps, ie common carp, Chinese carps, and Indian major carps, and coregonids The effects on production of stocking density and size of seed fish, fishing mortality, and size at harvesting are studied in a model population of carp Management implications of the modelling results are emphasized An adaptive approach to management, involving judicious experimentation with stocking and harvesting regimes, is proposed to gain information on the dynamics of actual fisheries, and to optimize their production
78 citations
TL;DR: The relationship between catch and effort was linear at all fishing intensities, suggesting that the qoliqoli were all fished on a sustainable basis.
Abstract: The catch-per-unit-effort (CPUE) and value-of-catch-per-unit-effort (VPUE) of reef-associated fish species from six Fijian native fishing grounds (qoliqoli) subject to different fishing intensities were determined using records of fishing activity from a voluntary logbook scheme. Line and spear fishing techniques were used for more than half the total fishing time (h person-1 d-1) in all qoliqoli, and yet the favoured technique in a given qoliqoli was frequently less efficient (lower CPUE) than other techniques. The popularity of relatively ineffective fishing methods implies that fishermen did not always attempt to maximize their catch rates. To compare fishing effort in different qoliqoli, all effort was rescaled on the basis of its recorded efficiency (measured as multispecies CPUE) and expressed as hours equivalent to boat-based spear fishing over coral by day to catch fish for sale. Total fishing intensity in the six qoliqoli ranged from 72 to 4310 h km-2 reef year-1. The relationship between catch and effort was linear at all fishing intensities, suggesting that the qoliqoli were all fished on a sustainable basis. Furthermore, whilst there was a significant difference in CPUE and VPUE between the one or two qoliqoli with lowest fishing intensity and all the others, there were no significant differences between these other qoliqoli in terms of the VPUE of the saleable multispecies boat catch or the CPUE of species from piscivorous and carnivorous genera (Epinephelus, Lethrinus and Plectropomus) which the fishermen prefer to catch.
33 citations
TL;DR: This study examined juvenile salmonid use of a freshwater tidal creek system draining a wetland on the floodplain of the lower Fraser River, British Columbia, Canada, where salmon fry ate dipteran adults, larvae and pupae, cyclopoid and harpacticoid copepods, and Collembola.
Abstract: This study examined juvenile salmonid use of a freshwater tidal creek system draining a wetland on the floodplain of the lower Fraser River, British Columbia, Canada. Chum, Oncorhynchus keta (Walbaum), chinook, O. tshawytscha (Walbaum), and sockeye, O. nerka (Walbaum), salmon fry were abundant in the tidal creeks in spring. The fry were found in non-natal habitat up to 1.5 km from the main channel of the river. The salmon fry ate dipteran adults, larvae and pupae, cyclopoid and harpacticoid copepods, and Collembola. Mysids Neomysis mercedis Holmes (Walbaum), and amphipods, Crangonyx richmondensis occidentalis (Hubricht and Harrison), were also consumed. The upper reaches of an undisturbed creek were the winter rearing habitat for presmolt coho salmon, O. kisutch (Walbaum), where this species ate dipteran pupae and larvae as well as a freshwater isopod, Asellus communis Say (Walbaum).
31 citations
TL;DR: Using catch data, changes in salmon, Salmo salar L., sea-age and seasonal return time were compared between several Scottish east coast rivers over the last 150–200 years, suggesting common influences affecting salmon at sea.
Abstract: Using catch data, changes in salmon, Salmo salar L., sea-age and seasonal return time were compared between several Scottish east coast rivers over the last 150–200 years. Parallel long-term trends were found, suggesting common influences affecting salmon at sea. Long-term associations with marine environmental factors were investigated by literature review and comparisons with an index of Icelandic sea-ice abundance. Apparent associations were shown to exist since about 1900, although the cause was not demonstrated. However, over a longer timescale, the same associations were not apparent. Unique gradual marine changes since 1900 may have resulted in perceptible associations with salmon data which may otherwise be imperceptible when short-term marine environmental variability is greater than long-term variability. Marine factors must affect salmon, but the complexity of the marine environment and controls on age at maturity and time of return mean that a retrospective analysis cannot be expected to identify the causal factors.
31 citations
TL;DR: In this paper, grayling larvae were immersed in tetracycline hydrochloride (TC) and alizarin red S (AL) solutions for 3-4 hours at concentrations of 300 and 70 ppm, respectively.
Abstract: Larvae of grayling, Thymallus thymallus (L.) were immersed in tetracycline hydrochloride (TC) and alizarin red S (AL) solutions for 3–4 h at concentrations of 300 and 70 ppm, respectively. Fluorescent marks on the otoliths from the marked fish were identified using a UV-light microscope. Results obtained with AL were superior to TC in marking otoliths. Fish kept in an aquarium retained good marks for 842 days. Stocking of the Carpathian River Ochotnica with marked grayling larvae revealed good mark retention for 128 days and a survival rate of approximately 12.2%.
27 citations
TL;DR: Current models estimating the impact of red-breasted mergansers, Mergus serrator L., on salmon, Salmo salar L., fisheries in Scotland fail to take account of any annual variation in the proportion of the diet that is smolts.
Abstract: Current models estimating the impact of red-breasted mergansers, Mergus serrator L., on salmon, Salmo salar L., fisheries in Scotland fail to take account of any annual variation in the proportion of the diet that is smolts. During the 1987–1990 smolt runs, the annual variation in the diet of mergansers was estimated from the stomach contents of birds shot on two Scottish rivers. The proportion of salmon in the diet was greatest early in the smolt run (76–91% by weight), and contained proportionately more smolts than later in the run, when coarse fish were more prominent. There was little annual variation in the proportion of the diet that was juvenile salmon. However, the proportion of these fish that were smolts, was twice as great in some years than in others and this appeared to be independent of estimated annual smolt production in the rivers.
23 citations
TL;DR: This empirical model indicates that catches of pikeperch depend on both juvenile abundance and water temperature, and that one year prior to recruitment into the commercial fishery explained 86% of the variation in catches.
Abstract: The abundance of 0+ pikeperch, Stizostedion lucioperca (L.), in trawls was compared with water temperature, water level and commercial catches 6–9 years later. The abundance of juveniles in autumn was positively correlated with the water temperature but not with water level in May or in June. The sum of a year abundance of juveniles showed a strong correlation with the catch years later. This abundance and the cumulative water temperature between August and October inclusive in five consecutive years one year prior to recruitment into the commercial fishery explained 86% of the variation in catches. This empirical model indicates that catches of pikeperch depend on both juvenile abundance and water temperature.
22 citations
TL;DR: In this paper, target spawning requirements for a number of rivers in several management areas were derived in terms of the contribution of both riverine and lacustrine habitats, and derivations of associated parameter values were reviewed, and limitations and research needs were discussed.
Abstract: In recent years, there has been increasing pressure from user groups in Newfoundland and Labrador, Canada, to manage Atlantic salmon, Salmo salar L., on a river-specific basis. The requirement to maximize benefits to users while ensuring that conservation goals are met has resulted in an initiative to develop river-specific target spawning requirements. In Newfoundland, anadromous juvenile Atlantic salmon make widespread use of lacustrine habitat for rearing. Depending on the river, lacustrine habitat can account for a substantial proportion of total production. Therefore, target spawning requirements for a number of rivers in several management areas were derived in terms of the contribution of both riverine and lacustrine habitats. The approach adopted, and derivations of associated parameter values, are reviewed, and limitations and research needs are discussed.
22 citations
TL;DR: The conflict between the c Cormorant, Phalacrocorax carbo sinensis Shaw, and commercial and recreational fisheries was investigated in the foraging area of a cormorant colony of 5000 breeding pairs in Horsens Fjord, Denmark.
Abstract: The conflict between the cormorant, Phalacrocorax carbo sinensis Shaw, and commercial and recreational fisheries was investigated in the foraging area of a cormorant colony of 5000 breeding pairs in Horsens Fjord, Denmark. Depredation of commercial pound nets was studied by stocking a large net pen with hatchery-reared rainbow trout, Oncorhynchus mykiss (Walbaum). When avian predation was precluded with a cover net, background mortality was around 15% day-1. When the cover net was removed, mortality increased to 98% day-1. Direct observation revealed that a flock of cormorants emptied the pound net in about 30 min, consuming 110 fish weighing a total of approximately 50 kg. The cormorants were able to catch trout weighing more than 1 kg, i.e. almost 50% of their own body weight. The impact of the cormorant colony on recreational fishing in the area was studied by stocking Carlin-tagged sea trout, Salmo trutta L., smolts into the two rivers that drain into the fjord. The cormorant colony lies between the river outlets and the open sea. Tag recovery from a 3-year programme was only about 2%, compared with about 10% for smolts stocked in areas of low cormorant density.
19 citations
TL;DR: Brown trout yields from stocking were higher in lakes with proportionally high yields of vendace or vendace and whitefish and proportionally low yields of pike, and three major determinants of brown trout yields in these lakes were found in both analyses: fish community, stocking rate and fishing effort.
Abstract: Lake-to-lake variation in brown trout, Salmo trutta m. lacustris L., yield from stocking was examined in 34 lakes in northern Finland. The trout were mainly stocked as 2–3-year-old fish. Catch statistics were compiled with information on water quality, water level fluctuations, fishing effort and lake geomorphology. Absolute brown trout yields (kgha-1) increased with increasing stocking rate, but there was an indication of non-linearity at higher stocking densities. Relative yields (kg per thousand trout released) were highest at low stocking rates. Stepwise multiple regression analysis was used to determine the best predictive model for lake-to-lake variability in brown trout yields. Seventeen measured regressands were used initially, and then replaced with scores obtained in a principal component analysis of highly correlated water quality variables and species-specific fish yields. Three major determinants of brown trout yields in these lakes were found in both analyses: fish community, stocking rate and fishing effort. Brown trout yields from stocking were higher in lakes with proportionally high yields of vendace or vendace and whitefish and proportionally low yields of pike.
TL;DR: A genetically marked strain of Atlantic salmon, Salmo salar L., was established, with a low frequency of a glucose-6-phosphate isomerase genotype, but there was a disparity between markers in estimated contributions to stocked and control sections.
Abstract: At the River Bush salmon station in Northern Ireland, a genetically marked strain of Atlantic salmon, Salmo salar L., was established, with a low frequency (0.194) of a glucose-6-phosphate isomerase genotype (GPI-3* 100/93). As part of a salmon enhancement programme, 43500 of these fish were stocked as swim-up fry into a tributary of the nearby Margy River in Spring 1990, following a baseline genetic survey which indicated an absence of the GPI–3*93 allele in the wild population. This survey also indicated a significant frequency difference of a malate dehydrogenase allele (MDH-B1,2*85) present both in the stocked and wild fish. A post-stocking electrofishing survey in summer 1990 indicated higher summerling densities in stocked sections compared with unstocked (control) sections, with an estimated survival of stocked fry to summerling ranging from 24% to 29%. Genetic analysis based on GPI-3*100/93 and MDH-B1,2*100/85 genotypes provided similar estimates of the overall contribution of stocked fish at 59.8% to 61.9%, respectively, although there was a disparity between markers in estimated contributions to stocked and control sections. A further genetic survey in 1991 estimated that 32.2% (MDH) to 69.1% (GPI) of the 1 + parr population consisted of stocked fish, the MDH-based estimate having declined greatly between summerling and 1 + parr stages. Possible reasons for the disparity of marker results are outlined, and the implications for use of low-frequency genetic marking discussed.
TL;DR: On stretches where angling activity was moderate, there was a tendency towards increase, whereas on stretches where activity had been high for several years, the activity seemed to have stagnated, indicating that a saturation had been reached.
Abstract: Postal questionnaires, interviews and accurate reports from anglers were used to study angling activity, yield and catch per effort of grayling, Thymallus thymallus (L), and brown trout, Salmo trutta L, on three stretches of the regulated river Glomma and two stretches of its tributary, the river Rena in southeastern Norway Angling activity was expressed as hours angling per kilometre of river and varied from 364 (CL 245–598) to 1337 (CL 912–1991) h km-1 Grayling and brown trout dominated the catches The catches were estimated between 476 (CL 334–659) and 753 (CL 517–1155) grayling and brown trout per kilometre river stretch The mean weights were estimated between 216 and 353 g for grayling, and 260 and 395 g for brown trout The yield of grayling and brown trout was estimated between 87 (CL 61–124) and 464 (CL 349–623) kg ha-1 yr-1 The quotient grayling/brown trout in the catches varied between 041 and 51, and was found to be significantly negatively correlated with angling effort (r = - 0838, P <005) Catch per effort was estimated between 132 (CL 82–218) and 382 (CL 287–513) g h-1, and was also negatively correlated to angling effort, according to: Catch per effort (g h-1) = - 0210 × effort (h km-1) + 405 (r = and 0899, P <002)
Angling activity in earlier years was determined from number of permits sold where data were available On stretches where angling activity was moderate, there was a tendency towards increase, whereas on stretches where activity had been high for several years, the activity seemed to have stagnated, indicating that a saturation had been reached
TL;DR: The simple method described predicts the ecological consequences of an introduction through potential competition between species based on elementary niche classifications and can assist pre-introduction assessments and is particularly useful where limited data exist.
Abstract: Practical models for predicting the impacts of introduced biota are urgently required to assess the benefits and risks of introductions. The simple method described predicts the ecological consequences of an introduction through potential competition between species based on elementary niche classifications. The model's predictions are tested against field data for common carp, Cyprinus carpio L., which escaped from fish farms during a study of the feasibility of fish stock enhancement in a large river basin. Recorded effects of carp are based on pre- and post-carp gillnet catches, observations of local villagers, socio-economic censuses and other sources of data. The predicted ecological interactions of common carp are in broad agreement with those recorded. The method can assist pre-introduction assessments and is particularly useful where limited data exist. In this particular study, common carp have contributed significantly to an improved capture fishery by fortuitous accident; production from aquaculture was negligible. High niche overlap between carp and certain resident fish species is predicted and this may be of more consequence in highland than lowland regions. Organisms introduced/transferred for aquaculture should be assessed within a much wider forum. The intended impacts of an introduction/ transfer, which in the case study were entirely erroneous, is an area in need of considerable attention.
TL;DR: K. sinensis was recovered from many naturally infected common carp, Cyprinus carpio L., and one tench, Tinca tinca (L.) in 18 localities in England.
Abstract: Khawia sinensis Hsu is recorded for the first time from the British Isles. K. sinensis was recovered from many naturally infected common carp, Cyprinus carpio L., and one tench, Tinca tinca (L.) in 18 localities in England. Prevalence ranged from 4.5 to 100% and intensity of infection from one to 51 worms per fish. The worms were identified as Lytocestidae by the medullary arrangement of the vitelline glands and the cortical position of the testes, and as K. sinensis on scolex characteristics and detailed arrangement and measurements of genital organs. No evidence of pathogenicity was observed, although K. sinensis has been reported as a serious pathogen of cyprinids in overseas fisheries. The potential threat of this tapeworm to carp fisheries in England is discussed.
TL;DR: Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.
Abstract: Rate of recapture (gill netting), habitat use, and diet of three strains of stocked brown trout, Salmo trutta L., were compared with resident brown trout in a Norwegian lake. The strains originated from an alpine lake, from a boreal lake, and from the native brown trout population in the lake. Overall recapture rate was 5–8% for all strains. The low recapture rate could be due to the relatively small size at stocking; mean fish length varied between 13.1 and 14.5 cm with strain and stocking method. Two years after release, the frequency of the different strains decreased from about 12% in the first year to stabilize at about 1%. The alpine strain showed the highest overall recapture rate, whereas the native strain was recaptured at an intermediate rate. The overall recapture rate of scatter-planted brown trout was higher than that of spot-planted brown trout. Immediately after being stocked, introduced fish ate less and had a less-varied diet than resident trout; however, stocked fish adopted a natural diet within the first summer. The distribution of trout between the pelagic and the upper epibenthic habitat was similar for both the resident and the stocked brown trout. Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.
TL;DR: During the biomanipulation studies, aquatic invertebrate standing stocks greatly increased after the removal of fish and decreased following restocking, and support findings of a contemporaneous large bio-manipulation experiment carried out in two adjacent lakes.
Abstract: The effects of perch, Perca fluviatilis L., and bronze bream, Abramis brama (L.), predation on the production of insects emerging at the water surface and the autumn biomass of benthic macro-invertebrates were assessed in a series of experimental ponds. The study attempted to discover whether fish affect food availability for diving and dabbling ducks in freshwater habitats. Ten ponds (bisected by fine netting) were used for the study: one was left totally fish-free, whilst one half of each of three groups of three ponds was stocked with either perch, bream or perch plus bream. The relative abundance of emerging insects (mainly Chironomidae) was significantly reduced in two perch ponds, two perch + bream ponds and two bream ponds. The autumn standing crops of benthic macroinvertebrates were significantly reduced in one perch pond, three bream ponds and one perch plus bream pond. These results support findings of a contemporaneous large bio-manipulation experiment carried out in two adjacent lakes. During the biomanipulation studies, aquatic invertebrate standing stocks greatly increased after the removal of fish and decreased following restocking.
TL;DR: In this paper, the factors important in determining fishing power in the Lake Kariba sardine, Limnothrissa miodon (Boulenger, 1906), fishery were identified using data from the major fishing area of the lake, the Kariba basin, for the periods 1980-1982 and 1988-1990.
Abstract: The factors important in determining fishing power in the Lake Kariba sardine, Limnothrissa miodon (Boulenger, 1906), fishery were identified using data from the major fishing area of the lake, the Kariba basin, for the periods 1980–1982 and 1988–1990. Comparisons were carried out using fishing power calculated from the mean catch of some standard vessels. Three methods were used for identification of the important attributes. Analysis of variance (ANOVA) was carried out for each component to compare the mean fishing power at all levels within each variable. Multiple regression analysis was used to build predictive models and to determine the factors which best predict fishing power. Factor analysis was used to ordinate vessel types and examine any vessel groupings related to fishing power. Factor analysis showed that vessel and net size were the most important variables. Vessels without engines for propulsion, of low value and without radios had less fishing power than the other vessels in the fleet. Vessels from the same company had similar fishing power, suggesting that some unmeasured variable such as quality of management linked to the fishing companies had a significant effect on catch. The factors which were important in determining catch were length of vessel, depth of the net, presence and absence of echo sounder, mobility, the type of light, number and wattage of the underwater lights in the 1988–1990 fishing period. Vessel length and net category were most important in the period 1980–1982. The models accounted for between 37.6 (1981) and 61.2% (1988) of the variation in fishing power. During the development of the fishery, there has been a change from purse seine to lift nets, and increases in the use of echo sounders, engines for the propulsion of vessels and radios, all of which could have increased effective fishing effort.
TL;DR: In this article, Rod catch and flow data from the River Derwent were analyzed for each month between June and October and the mean value determined for each 2-cumec flow band.
Abstract: Twenty-seven years of daily salmon, Salmo salar L., rod catch and flow data from the River Derwent were analysed. Daily catch was standardized for each month between June and October and the mean value determined for each 2-cumec flow band. In June and July the mean standardized catch remained relatively low and stable for flows up to approximately 9 cumecs. Thereafter it increased steadily with increasing flow, reaching a peak at 21 cumecs and 41 cumecs for each month respectively. A linear model accounted for the greatest proportion of the variability in August and September with mean standardized catch increasing steadily up to a maximum value of 43 and 39 cumecs respectively. However, in October, a quadratic model was more appropriate, with mean standardized catch decreasing as flow increased up to 9 cumecs and thereafter increasing steadily with flow, reaching a maximum value at 21 cumecs. The pattern of the relationship at higher flows was not clear because of the paucity of data. These findings are discussed in relation to the management of the water resource for the maintenance of the fishery.
TL;DR: The major food items of adult largemouth bass, Micropterus salmoides (Lacepede), in Lake Naivasha, Kenya are not markedly different from those in its native range, with feeding intensity correlated with water temperature.
Abstract: The major food items of adult largemouth bass, Micropterus salmoides (Lacepede), in Lake Naivasha, Kenya are not markedly different from those in its native range. Although insects and their larvae are major components of the diet for both temperate and equatorial populations of juvenile bass, the equatorial population also eats Procambarus clarkii (Girard), juvenile fish and aquatic weeds. In temperate populations they also eat crustaceans, rotifers and oligochaetes. The frequency of occurrence of the major prey organisms in the stomachs of M. salmoides varies seasonally with population peaks in these organisms. Similar to North American populations, the Naivasha population feeds during the day between dawn and dusk. In contrast to temperate populations, the population of M. salmoides in this equatorial lake feeds throughout the year, with feeding intensity correlated with water temperature.
TL;DR: In this article, the length distribution of adult sea trout, Salmo trutta L., from a commercial seine net fishery using a nominal minimum mesh size of 50 mm or 102 mm is described.
Abstract: The length distribution of adult sea trout, Salmo trutta L., from a commercial seine net fishery using a nominal minimum mesh size of 50 mm or 102 mm is described. The smaller mesh size captured sea trout larger than 210 mm whilst the larger mesh size caught sea trout mainly above 400 mm, with no fish below 320 mm. A sea trout length/ girth relationship is described, and coupled with biological data allowed the calculation of mesh sizes for the appropriate escapement of different sea ages of sea trout.
TL;DR: The overall high survival rate for these species was the result of a small proportion of fish being hooked in the gills or deep in the mouth, and the size limit could be increased to allow smaller fish to grow to sizes preferred by sport fishermen.
Abstract: The survival of small-sized (<50.8 cm) chinook salmon, Oncorhynchus tshawytscha (Walbaum), and coho salmon, Oncorhynchus kitsutch (Walbaum), caught by sport fishing was determined to assess the potential for increasing the size limit for these fish. Fishermen were recently catching smaller salmon than in the 1970s, but salmon growth rates had not changed. To be an effective management option, the survival rate of hooked and returned fish must be high. The overall survival rates were high: 76% for chinook salmon and 70% for coho salmon. There was no significant difference in survival of the coho salmon with size of hook used (P = 0.31). Any mortality among fish was generally acute; fish hooked deep in the mouth or gills generally bled and died shortly after capture. Fish hooked in the gills had a significantly greater mortality (P = 0.0002). The overall high survival rate for these species was the result of a small proportion of fish being hooked in the gills or deep in the mouth. Since the survival rate of the salmon was high, the size limit could be increased to allow smaller fish to grow to sizes preferred by sport fishermen.
TL;DR: The ratio of total weight of forage fish to that of carnivorous fish indicated an unbalanced population in the lake, while the proportion of the fish population at harvestable size suggests a balanced population, but this index may have been overestimated because of a minimum harvestablesize.
Abstract: Studies were conducted to determine the composition and relative abundance of commercially desirable and less desirable fish populations of Rawal Dam reservoir. Fifteen fish species belonging to 11 genera were identified. The less desirable species were abundant, constituting 62.3%, 84.8% and 78% of the total number of fish sampled on three occasions. Carnivores were represented by two species with very low percentage composition by number or weight. The ratio of total weight of forage fish to that of carnivorous fish indicated an unbalanced population in the lake. The proportion of the fish population at harvestable size suggests a balanced population, but this index may have been overestimated because of a minimum harvestable size.
TL;DR: The ability of consumers and fishermen to distinguish-between wild striped bass, Morone saxatilis (Walbaum), and its hybrids may be important to marketing farm-raised fish and managing wild stocks.
Abstract: The ability of consumers and fishermen to distinguish-between wild striped bass, Morone saxatilis (Walbaum), and its hybrids may be important to marketing farm-raised fish and managing wild stocks. The ability of randomly selected persons to distinguish between wild and hybrid specimens was examined and it was found that a significant improvement in recognition occurred with only minimal training. Average correct response of 70% without training rose to nearly 90% after training. Moreover, the respondents' sex and age were important factors in determining successful identification.
TL;DR: In this paper, the average residence time of trout in Lake Trawsfynydd was found to be 6 days for rainbow trout and 10 days for brown trout, and less than 7% of the recaptured trout of either species had a residence time more than 20 days.
Abstract: Small amounts of radioactivity in liquid effluent are discharged under authorization into Lake Trawsfynydd in north Wales. MAFF inspectors from the Directorate of Fisheries Research (DFR) advise the Welsh Office on the terms of authorization and on the power station operators' compliance with them. DFR also has the responsibility for environmental monitoring, including fish caught for consumption. Trout angling is particularly popular in Lake Trawsfynydd, and because of angling pressure, additional brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), need to be introduced. An important factor in determining the concentration of radionuclides in these fish is the length of time that they spend at liberty. During the fishing season, samples of these stocked trout were tagged and released to assess the average residence time. This was found to be 6 days for rainbow trout and 10 days for brown trout. Less than 7% of the recaptured stocked trout of either species had a residence time of more than 20 days. Radiocaesium concentrations in recaptured trout were very low. Trout which avoided recapture and overwintered in the lake were found to have radiocaesium concentrations similar to those of indigenous trout sampled at the same time.