摘要:

SummaryPerch, Percafluviatilis L. were studied in Slapton Ley which is a eutrophic freshwater lagoon in south Devon, England. The perch were caught in traps from February 1970 to June 1971. The traps selected males during the spawning season. The water temperature did not affect the numbers of trapped fish directly, but the sight of perch in the traps appeared to attract others. Sex ratios were found to be equal in the first 2 years of life but older year classes were predominantly female. The population consisted of young individuals. Males were found to mature in their first year and most females matured in their second. The fecundity varied from an average of 1000 eggs in perch n i the 9.75 cm length group to 30,500 eggs in fish in the 27.75 cm group. A few shoals of perch moved throughout the Ley although the majority of fish stayed in the same area. Population estimates showed a range from 10,000 to 95,000 for fish 9.75 cm and above. The large variations in population estimates and survival rates were attributed to the necessity of using traps which had variable catches and to the harmful effects of handling the perch. Chironomids were important in the diet of perch of all sizes. Plankton and Asellus were other important foods, the latter especially during the winter months. Perch 11.5 cm to 13.9 cm selected the widest range of food organisms. Only perch 14.0 cm and above ate fish.

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1974.tb00106.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryThe age of perch. Perca fluviatilis L. was determined from the annual bands laid down on the opercular bone. Year class monthly mean lengths were based on this ageing. The opercular bones have also been used for back‐calculating growth. It was found that back‐calculated mean lengths per age were similar to those calculated by direct measurement. There was little difference in growth between the sexes for the first 2 or 3 years of life. The investigation was too short for a correlation to be found between growth increments and degree days above 14°C although Slapton Ley was found to be warm in comparison to other British waters and the perch grew fast. Ford‐Walford plots gave L∞= 22.0 cm for males and L∞= 25.0 cm for females. In length‐weight relationships, fry and immature females were homogeneous between their monthly subgroups, and fry obeyed the cube law throughout. Mature males and females were heterogeneous between monthly subgroups. Growth has been expressed in weight form and instantaneous growth rates are given. These rates are higher for fry than older fish. Condition was closely related to gonad development in mature fish. Condition fell sharply after spawning and then rose rapidly in May and June. A sudden drop in condition was noted in males during August which cannot be ful

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1974.tb00107.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryThe population dynamics and production of Daphnia hyaiina^ the dominant cladoceran i n Eglwys Nynydd, a shallow eutrophic reservoir in South Wales, were studied for 2 years against a background of limnological measurements. The appearance and development of successive generations from egg to adult could be followed from changing numbers in arbitrarily defined size classes. Seasonal variations in mean length, mean brood‐size and proportion of gravid adults were recorded and mean brood‐size was related to changing food and temperature conditions. Egg‐development times for D. hyaiina were determined in culture and the population parameters finite birth (S), instantaneous birth (b′), instantaneous population change (r′), instantaneous death (d′) and finite death rates (D) were estimated from field data. Turnover and production estimates were calculated from finite death rates and biomass. The calculated potential rate of increase (b′) was nearly always greater than the observed rate of increase (r′): seasonal changes in death rate (d′) generally parallel changes in birth rate (b′) but remain somewhat out of phase. Population oscillations are probably due t o a delay in the expression of the effects of population density upon birth and death rates.The mean biomass of Daphnia in 1970 was 0‐57 mg dry wt/l (0‐88 g C/m2) and in 1971 0‐32 mg dry wt/l (0.49 g C/m2). Annual production for Daphnia was 11‐8 mg dry wt/l (18‐2 g C/m2) in 1970 and 8‐30 mg dry wt/l (12 8 g C/m2) in 1971. Information available on primary production in the reservoir suggests that the production of Daphnia accounts for less than 2% of gross primary production. However, the pattern of population growth of Daphnia in Eglwys Nynydd almost certainly reflects a food limited system. In summer, blue‐green algae may be abundant but serve as a poor food source: throughout the blue‐green bloom egg production remains low, at t

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1974.tb00108.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryThe development and use of an improved automatic system for the accurate measurement of dissolved organic carbon (D.O.C.) in fresh waters is described. The method, which covers the range 0‐25 mgl−1D.O.C., is based on the oxidation of organic carbon to carbon dioxide by U.V. radiation. The carbon dioxide produced is measured by a non dispersive infra‐red gas analyser (I.R.G.A.). The I.R.G.A. is also used in the semi‐automatic measurement of particulate organic carbon (P.O.C.) by a combustion method which allows very low concentrations to be measured using samples of 200 ml or less.Results of surveys of two river systems indicate that D.O.C. concentrations of less than 2.0 mg l−1are typical in unpolluted chalk streams. D.O.C. concentrations of streams draining acid heathland were found to be significantly higher (3‐5.5 mg l−1).D.O.C. concentration was found to increase rapidly during a spate, in a river draining areas of mixed underlying geology. The throughput of D.O.C. during the twenty‐five to fifty spates which occur annually in the river could amount to 20‐30% of the annual flux of D.O.C.P.O.C. concentration in chalk spring waters (0.03–0.04 mg l−1) were found to be higher than expected from preliminary estimates using membrane filtration techniques.The relevance of the measurement of D.O.C. and P.O.C. flux to estimates of the energy budgets of stream ecosystems is discussed and published methods for the automatic measurement o

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1974.tb00109.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

Summary(1) Chaoborus anomalus and Chaoborus ceratopogones were found together throughout the year in Lake George, Uganda, the larvae forming 65‐85% by number of the macro‐benthos of the central lake area.(2) First and second instar larvae were wholly planktonic and third and fourth instar larvae mainly benthic during the day, migrating into the water column at night. A proportion of the third and fourth instar larvae were found in the water column during the day in the centre of the lake where light attenuation was greatest. C. anomalus showed a greater tendency to be benthic than C. ceratopogones.(3) Planktonic larvae were distributed in dense patches thought to be cohorts resulting from oviposition by swarms of adult females. The maximum density of planktonic larvae recorded was 31.7 × 102/m2column. Third and fourth instar larvae were more evenly distributed in the benthos of the central lake area, the maximum density recorded was 79.9×102/m2.(4) First and second instar larvae occurred mainly in the inshore regions, very few were found in the centre of the lake. Oviposition is likely to take place inshore. The patches of planktonic larvae were not stationary. Dispersal of larvae away from the shore may be active or may result from complex oscillations of the water mass produced by storms. In C. ceratopogones the floating egg batches could he carried away from the north shore by currents produced by inflows and prevailing winds.(5) Adults of C. ceratopogones were more numerous during the rainy seasons than during the dry seasons. A similar, though less marked seasonality was found in the occurrence of C. anomalus adults. The biomass of benthic larvae of C. ceratopo‐gones was estimated at 387.0 mg C/m2in the dry season, June 1970, and 130 0 mg C/m2in the rainy season, October 1970. That of C. anomalus was constant at 241.7 and 254.3 mg C/m2in June and October, 1970 respectively.(6) No difference was found in the horizontal distribution of the larvae, or in the size and weight attained by the various stages.(7) Interspecific differences were found in the mean number of eggs per adult female (309, C. anomalus and 224, C. ceratopogones); in the nature ofthe egg batches; n i the morphology of the larval mouthparts and pharyngeal sphincters, as well as i n the vertical distribution of the larvae and seasonal occurrence of the adults. On the basis of these differences the co‐existing species are thought not to be competing for the same

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1974.tb00110.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY