摘要:

Abstract–Stock dynamics of the introduced tilapiaOreochromis mossambicusand co‐occurring indigenous cyprinids were studied in a typical lowland reservoir in Sri Lanka. All species exhibited peaks in reproductive activity during the rainy seasons in November and May.O. mossambicuswas estimated to grow to 14 cm in its first year and to 21 cm in its second. A high natural mortality (Mwas found, both for the cyprinids (M=1.3 yr−1to 4.7 yr−1over a range of species) and forO. mossambicus(M= 1.9 yr−land 1.7 yr−1for females and males). The fishing mortality (F) ofO. mossumbicus, estimated for the length range from 15.0 to 18.5 cm, was 5.5 yr−1for females and 4.7 yr−1for males. The total fish biomass was estimated at 1 829 kg/ha, with 128 kg/haO. mossambicusand 1 098 kg/ha small pelagic cyprinids, mainlyAmblypharyngodon melettinus. The total biological fish production was around 5 400 kg/ha/yr, withO. mossambicusand the small pelagic cyprinids accounting for 400 kg/ha/yr and 3 600 kg/ha/yr respectively. The commercial fishery yielded 242 kg/ha/yr (including 70% or 169 kg/ha/yrO. mossambicus), indicating an incomplete utilization of the total biological fish production. It was concluded that there is scope for an approximately 100% increase of the total yield. Reimplementation of a 76 mm minimum mesh size, to prevent overfishing ofO. mossambicus, is recommended as a management measure. Additional options are direct exploitation of the untapped resource of small pelagic cyprinids, or indirect exploitation by means of a culture‐based fishery onLates calcarifer, a cont

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00032.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract–Various sizes of roach, perch, pike, zander (pike‐perch) and crucian carp were collected from lakes of different trophic levels in order to study the variation of lipids and fatty acids (FA) within and between species. Freeze‐dried samples of the dorsal muscle were analysed quantitatively for total lipid content and FA content. The results indicate that total lipid and FA contents can vary considerably, both within and between species. In contrast to herbivorous fish, carnivorous‐piscivorous fish FA patterns were more constant and independent of size ‐ fry excepted. Lipid and FA contents of roach from two oligotrophic lakes were significantly higher than in roach from a eutrophic lake. Differences in basic food webs may be responsible for these results. In the oligotrophic lakes, the algal flora was dominated by species classified as high quality food for grazers, e. g. flagellates and diatoms. It was shown in earlier papers that these algal groups contain two long‐chained FA of φ 3 type (eicosapentaenoic acid and docosahexaenoic acid) which are used as criteria indicating high nutritional value. Conversely, the eutrophic lake was dominated by blue‐greens, a group of autotrophs lacking these long‐chained φ 3 FA. Blue‐greens have generally been classified as poor food for grazers. (The fatty acids are described by three numbers, x:yφz, where x=number of carbon atoms, y=number of double bonds, and z=position of the first double bond counted from the methyl

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00033.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract–The utilization of lakes, and inlet and outlet streams by juvenile Atlantic salmon (Salmo salarL.), brown trout (Salmo truttaL.) and Arctic charr (Salvelinus alpinus(L.), were investigated in 16 watercourses northern Norway, all known to inhabit salmon stocks. In lakes, fish were caught by small mesh size gill nets, while in rivers fish were caught electrofishing. In the shallow littoral (0‐3 m depth) there were juvenile salmon in 15 of 19 investigated lakes, juvenile trout in 17 and juvenile charr in seven. Trout dominated significantly in numbers in the shallow littoral of seven lakes, while salmon and charr dominated in three lakes each. When trout and salmon were frequent in the shallow littoral, charr was usually not present in this habitat, but were found in the profundal zone in most of the lakes. Atlantic salmon parr utilized both shallow and deep lakes, and used both stones and macrophytic vegetation as shelter. The utilization of lakes by salmon parr seemed to be closely related to utilization of small inlet streams for spawning. In most inlet and outlet streams salmon dominated over trout in numbers, while charr were absent. This is the first documentation of lake‐use by naturally occurring salmon parr in Scandi

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00034.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract–A bioenergetics model was used to estimate the food consumption of northern pike (Esox luciusL.) in two lakes in northeastern Finland, and the results compared with those obtained by a method based on stomach contents and gastric evacuation rates. The annual specific food consumption was 7–5 g per one gram pike for one‐ and two‐year‐old pike, and 4–3 g g−1in age groups 3–6 years old, according to the bioenergetics model. The proportion of ingested food used for growth was 10–30%, and was highest in the youngest age groups. The food consumption rates estimated by the stomach contents method were considerably lower, only about half those produced by the bioenergetics model. The method based on stomach contents tends to underestimate the food consumption. The bioenergetics model was considered more valid for estimating food consumption, but some of the parameter values for northern pike may differ in populations from different latitudes. According to a sensitivity analysis, the food consumption estimate of the bioenergetics model was most sensitive to parameters in the allometric functio

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00035.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY