摘要:

ABSTRACTThis paper reports on a fundamental study of the fish fauna already present within the irrigation canals. The fauna was investigated with a view to determining its use in the control of nuisance biota. Twenty‐seven species of fish were collected, comprising 22 genera and 12 families. This fauna is closely allied to the fauna of the source water, the Blue Nile. Several Blue Nile species are absent and some of those collected are new recordings for the area. Each species of fish collected had neither a suitable feeding habit nor a wide enough distribution to have any significant effect on the levels of nuisance biota. This includes the mosquito fish.Gambusia affinis, which was specifically introduced for mosquito control. Improvement in the ecological conditions within the canals involves both the management of the already‐existing fish stocks and the introduction and management of additional species. A reduction in the levels of aquatic weeds using phytophagous fishes should be a high priority with malacopliagous and larvivorous species also being introduced and their stocks managed. Species suitable for introduction, which are generally endemic to the Nile system, are suggested. The fish species diversity falls as the canals become smaller and the water within them subject to much greater variations in flow and quality. The lowest order canals (field canals) are probably unsuitable for permanent fish stocks. The nuisance biota within field canals can be controlled by simple improvements in crop‐watering procedures. The widely distributed and diverse fauna found within minor canals suggests that problems due to fish‐kills brought about by heavy pesticide and herbicide usage are not so severe as previously thought. The management of the ecological problems is a complex task due to the interrelationships of many

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00839.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe Carlin‐Larsson population model of Baltic salmon was used to simulate reduced effort in an offshore fishery for feeding salmon and its effect on home water fisheries. Some simplifying assumptions had to be made; no changes in seasonal mean weight or exploitation rate in the home water fisheries were assumed. With a simulated termination of the offshore fishery, seasonal mean weights from the time before the excessive offshore fishery started were also applied to give a maximum effect on the catches. The total non‐catch fishing mortality in the offshore fishery was estimated at 5% per season (total M = 10% per season).A total ban on the offshore fishery would mean a sixfold increase in catch‐value for home water fisheries and a total value twice that in the present situation with the present seasonal mean weights. With the higher mean weights the simulated catch would be 87% and its value 182% higher than at pr

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00840.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTPhysical factors and brown trout densities were studied in a small Danish lowland stream. The densities of brown trout larger than 15 cm were significantly correlated with gradient, mean depth, coefficient of variation in current velocity 7 cm above the bottom, the ratio between wetted perimeter and width, amount of overhanging banks and degree of macrophyte cover. Coefficient of variation in current velocity 7 cm above the bottom was the most important factor for brown trout density (rs= 0.8364, 24 df, (P<0.001)), which supports the idea of this value as a measure of stream complexity. A rather small relation between trout density and amount of overhanging bank cover (rs= 0.4179, 24 df, (P<0.050)), contrary to the closer relationships found in previous studies, is discussed as an effect of the self‐shading capacity of this rather narrow and deep strea

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00841.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe relative proportions of mature wild and stocked brown trout in four Irish lakes were compared with their subsequent occurrence in spawning runs. Data indicate that stocked trout, of two strains, did not run the streams to spawn in significant numbers. Considerable stocks of introduced fish, which had shed their milt or eggs, were present in these lakes in springtime. The failure of brown trout stocked in lakes to run spawning streams subsequently has not been recorded previously.

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00842.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTAnalysis of the sex ratio of angling Atlantic salmon catches from March to mid‐June on the Elorn river in 1979 and 1980 showed a majority of females among the spring salmon population. Comparison between serodiagnostic technique and visual observations of gonads indicated that the former method provided a very low error margin for sex ratio determination on all females in the early stages of maturation and entering rivers. Study of sexual characteristics of spring salmon bearing copepod parasites or not indicated that fish entering freshwater would initiate a new stage of the ovogenesis process characterized by a failure of about 30% of the oocytes and a correlation between fecundity and size that did not exist at the time of the entry into freshwate

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00843.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTExperiments to determine effects of skin colour on growth in different water temperatures were conducted with albino and normally pigmented channel catfish (Ictalurus punctatus). In Experiment 1, fish were reared in 29°C, 27°C, or 22°C water from hatching to 40 weeks of age. Two additional groups were initially reared in 27°C water for either 12 or 16 weeks and in 22°C water for either the remaining 28 or 24 weeks, respectively. Results indicated that: (1) albino fish reared in 29°C water were superior in growth to normal fish; (2) albino fish did not differ from normal fish in growth when reared in either 27°C or 22°C water; (3) albino fish were inferior in growth to normal fish when reared in 27°C water for either 12 or 16 weeks of the 40‐week growth period; (4) genetic differences between families and the age of fish strongly influenced relative growth of both colour types; and (5) both albino and normal catfish were able to compensate in growth at a later age.Experiment 2 began when fish were 48 weeks old and included both albino and normal fish which were either full‐siblings or not genetically related. Results indicated that: (1) albinos and normal catfish did not respond differently to a 5°C water temperature change; (2) genetically related and randomly selected albino and normal fish produced consistent results concerning water temperature, skin pigmentation, and sex effects on growth; (3) increasing water temperature from 22°C to 27°C for 19 weeks resulted in increased weight and length of the fish by 144 and 80%, respectively; and (4) growth increase during the first 19‐week period resulting from the higher water temperature was followed by a significant increase in the subsequent 10‐week growth when the fish were in the lowe

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00844.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00845.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Book Reviewed in this article.Fibre Ropes for Fishing Gearby GERHART KLUST.

ISSN:1355-557X    

DOI:10.1111/j.1365-2109.1984.tb00846.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY