ISSN:0020-9309    

DOI:10.1002/iroh.19940790102

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

ISSN:0020-9309    

DOI:10.1002/iroh.19940790103

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractModels for the prediction of fishery production and/or harvest based on primary production, algal biomass, or nutrients and morphometry have been effective in many lakes and reservoirs. Lake Chapala, México's largest, is located on the Río Lerma, one of México's principal rivers. It was made a reservoir in 1903 by the construction of a hydroelectric dam on the out‐flowing Rio Santiago. For the first half of this century Lake Chapala was famous for its native white‐fish (Chirostoma lucius) fishery. This fishery has collapsed. The present fishery consists of smallChirostomaspecies and the introducedOreochromis.During the past 17 years the water level has fallen by over 3 meters. Now very shallow, clay resuspension creates low visibility (Secchi<1 m). Nutrients are abundant with total phosphorus exceeding 1 mg 1−1and inorganic nitrogen exceeding 0.5 mg 1−1. Photosynthesis is limited by light and especially the unfavorable mixing depth to photic depth ratio. Models based on phytoplankton production or biomass underestimate the fishery by about one order of magnitude while a morphoedaphic model overestimates the fishery to the same extent. We sought to explain alternate pathways to support the realized fishery. Experiments suggest that bacterial prodution, with a bypass of the microbial loop, may offer a partial explanation. Management practices to increase the fishery based on an increased autotrophic base to food chain would fail without consideration of bacterial processes in this highly turbid

ISSN:0020-9309    

DOI:10.1002/iroh.19940790104

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractThe purpose of this study was to evaluate the present limnological conditions of Lagartijo reservoir (North central Venezuela) and to compare them with conditions reported in early studies. Lagartijo reservoir supplies an important part of the water demand for Caracas city and wide fluctuations on water volume occur annually.Relevant physical, chemical and biological data were obtained from three sampling stations between July 1990 and April 1992. The reservoir was permanently stratified with a well defined thermocline. The main tributary (Lagartijo river), with a temperature about 6° C colder than the surface temperature of the reservoir, flows through the hypolimnion producing distinct chemical and biological properties. The average of the daily integral phytoplankton photosynthesis was 1550 mg C m−2d−1. The present annual average of the net photosynthetic activity is about 40% lower than the reported for 1970 and corresponds to an apparent decrease in phytoplankton biomass and to the reduction of the euphotic zone. Photosynthetic activity was higher for the wet season than for the dry season. Actual chlorophyll a concentration in the euphotic zone averages 4.9 μg l−1. Abundance of zooplankton was significantly decreased in comparison to 1976. Mean zooplankton biomass (dry weight) for the upper 10 m layer was 596 μg l−1. Copepods accounted for most of this biomass, followed by cladocerans and rotifers. For the first time, a freshwater medusa (probablyCraspedacusta sowerbyi) was collected from Venezue

ISSN:0020-9309    

DOI:10.1002/iroh.19940790105

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractFour Cuban reservoirs (Higuanojo, Tuinicu, Lebrije and Zaza) in the Sancti Spiritus province were studied in 1988–1989. The study was based on monthly samples of physical, chemical and biological components of the pelagic ecosystem.The reservoirs were warm‐monomictic with more or less complete water mixing during the dry season and stable stratification during the wet season when inflow of water was highest. The mixing event was very important in regulating the biology and chemistry of the reservoirs. During the stratified period the water was anoxic below 10 m depth and reduced compounds, especially ammonium, accumulated. In the reservoirs with less stable stratification, ammonium was occasionally oxidized and the nitrogen lost to the atmosphere through denitrification. Phytoplankton biomass varied considerably among reservoirs and was not only correlated to total‐P, but also to light, Zooplankton biomass and composition was related to availability of good quality food and to the intensity of fish pred

ISSN:0020-9309    

DOI:10.1002/iroh.19940790106

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractThe Caroni River, a tributary of the Orinoco River, in south‐eastern Venezuela, and originating in the Guayana Shield, has been impounded by the construction of two dams. The largest, Guri, is 90km upriver from the confluence with the Orinoco. The final stage of the dam and hydroelectric power plant of the Guri complex, was completed in 1986. The Guri Reservoir (62° 32′–63° 18′ long. W and 6° 55′–7° 52′ lat. N) is a blackwater lake, due to the high concentration of humic substances in the water. The surface of the reservoir is 4250km2at normal maximum operating water level of 270 m a.s.l., has a maximum depth of 150 m and a storage capacity of 135.7 km3. The waters of the Caroni, flowing through an old geological basement (Pre‐Cambrian), highly weathered and eroded, are poor in mineral nutrients, making Guri's waters oligotrophic, that is biologically manifested in low organic production (Lewis and Weibezahn, 1976; Gonzalezet al., 1991). Its hydrological regime is closely related to the regional dry‐wet climatic pattern. Chemically, the waters are characterized by low pH and alkalinity, relatively high content of Si, Al und Fe, low anion concentrations with sodium dominating the series, low content of nitrogen, and very low concentrations of phosphorus. Phytoplankton is represented predominantly by Chlorophyta and Chrysophyta, with low biomass and depressed production (Gonzalezet al., 1991). Zooplankton (only Copepoda were studied) is represented by 8 spp. and densities are low (Gonzalez, 1992). Suspended particulates concentrations present descending gradient from the tail of the reservoir towards the dam, but in absolute ter

ISSN:0020-9309    

DOI:10.1002/iroh.19940790107

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractThe recently established large reservoirs Manantali on the river Bafing, tributary to the Senegal River and Sélingué, located on the river Sankarani, tributary to the Niger River, have followed similar changes in limnology and fish populations as those established in West Africa 20 to 30 years ago. Basic water parameters, such as oxygen content, pH, electrical conductivity and temperature show seasonal variations largely determined by the changes in wet and dry seasons. Both waterbodies are monomictic, with water mixing taking place in January, the coldest period of the year. Pre‐impoundment forest clearence of the future reservoir bottom in the shallow Sélingué reservoir was carried out only in the proximity of the dam site. In the deeper reservoir Manantali the savanna forest was cleared from an area of about 25 percent, and the tree gallery was cleared along the river. The initial filling of Manantali reservoir was followed by a nearly complete drainage of the reservoir. After refilling some months later, the dissolved oxygen content stayed reasonably high and therefore there was no fish mortality. Also no hydrogen sulphide or a large‐scale spread of noxious aquatic plants developed. At present the fish fauna is dominated by cichlids and pelagic species, i.e. at Sélingué by clupeids. and at Manantali by cyprinids (Leptocypris niloticus). In both reservoirs was the genusCitharinusconsiderably reduced. At Sélingué reservoir disappearedProtopterus annectens.The fish yield estimate is 27 kg/ha/year for Manantali, and 40 kg/ha/year for Sélingué. Both rerservoirs have recorded a slight increase in electrical conductivity. They both serve as a valuable water resource and fish supply for the

ISSN:0020-9309    

DOI:10.1002/iroh.19940790108

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractThe reservoir fisheries in the semi‐arid Northeast Brazil have been studied based on the data collected by the Department of Action Against Droughts and placed against the background of what is known of similar fisheries in the tropics elsewhere. Of about 60,000 reservoirs in the region, DNOCS controls about 100 reservoirs since 1970. Physical characteristics are provided together with total areas of the reservoirs. Unfortunately very little basic limnological data are available.In all 42 species of fish and crustaceans were introduced into these reservoirs of which only fourteen fish species and one shrimp were successful in establishing self‐propagating populations, others being stocked on a regular basis. The recent stocking ofColossoma macropomumandCyprinus carpioare however unsuccessful in reservoirs.The fish yield of DNOCS controlled reservoirs for 1950–1990 was 112 kg.ha−1(Davies, 1972; Gurgel and Oliveira, 1987). It amounts now to about 20,000 tons annually taking into account incomplete and scaled down reporting of catches. This indicates a total fish catch from 60,000 reservoirs in the Northeast of Brazil of very substantial size.The CPUE and catches of all species and that ofTilapia rendalliandOreochromis niloticusare given together with a discussion of the fisheries and recommendations for management. The total CPUE varied from 18–1,100 kg.ha−1. However, most fishermen are only part‐timers.It appears likely that very substantial yields of fish are being harvested in Northeast Brazil but are under‐reported. Management strategies now in operation in reservoir fisheries are still very pragmatic. With the accumulation of data and generalizations, more viable strategies c

ISSN:0020-9309    

DOI:10.1002/iroh.19940790109

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

摘要:

AbstractThe freshwater sardineClupeichthys aesarnensisis a zooplanktivore like many clupeids. In Ubolratana reservoir, Thailand, it feeds selectively on larger zooplankters, especially Cladocera and to a lesser extent on Copepoda. Other zooplankters are also consumed together with both living and dead insects and occasionally its own young. It feeds during the day. Our results indicate that the slight seasonal differences in food consumed probably reflects differences in availability at different seasons and sites. The fish grows in weight, as expected, at a higher rate than the cube of its length. The condition factor is the same for males and females and shows some seasonal variation. Data on fecundity and gonadosomatic index mark this fish as highly fecund and a year‐round spawner. Its short life span, favourable growth and high fecundity make it a valuable source of cheap protein. In 1990, 17% (about 300 tons) of the fish catch from Ubolratana came from this fis

ISSN:0020-9309    

DOI:10.1002/iroh.19940790110

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY

ISSN:0020-9309    

DOI:10.1002/iroh.19940790111

出版商:Akademie Verlag, Berlin    

年代:1994

数据来源: WILEY