This report attempts to establish guide-lines for electrofishing in population studies and is the result of literature studies and experience from electrofishing in Denmark, Finland, Norway and Sweden. Equipment, safety and training, sampling design and precision requirements for various types of investigations, population estimation and fishing practice are discussed. The results are put forward in the form of recommendations.Special attention is paid to the sampling design of surveys in streams of different types and for different purposes. Examples of the computation procedures are also included.

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Species biomass and size composition of fish faunas along a productivity gradient were studied in south Swedish lakes. Generally, with increasing productivity (measured as chlorophyll content), Salmoniformes were replaced by percids, which in turn were replaced by cyprinids, as suggested in previous studies. However, percids showed two peaks in biomass, one in medium productive lakes due to perch and one in highly productive lakes due to zander. Benthic piscivores were present in all lakes, whereas pelagic piscivores were absent in the least productive lakes. The proportion of piscivores in the total fish biomass showed a peak in medium productive lakes, largely reflecting the importance of piscivorous perch. The median size of the dominant cyprinids (roach) in the systems studied increased as the importance of piscivores increased, which was interpreted as a size refuge response to increased predation pressure. The same pattern was present for the dominant planktivore, vendace, in low productive systems. Although a predictable pattern ofchange in the fish fauna was found along the productivity gradient, other environmental factors such as structural complexity may be the ultimate cause of the observed succession pattern.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Wiley-Blackwell and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos. Asymmetrical competition between age classes as a factor causing population oscillations in an obligate planktivorous fish species Stellan F. Hamrin and Lennart PerssonHamrin, S. F. and Persson, L. 1986. Asymmetrical competition between age classes as a factor causing population oscillations in an obligate planktivorous fish species. -Oikos 47: 223-232.A population of the obligate planktivorous vendace, Coregonus albula, exhibited an oscillation cycle with a periodicity of two years. A hypothesis based on asymmetrical competition between age classes favouring small vendace is put forward to explain the oscillations. The hypothesis is based on one central argument, namely that small vendace will have a competitive advantage during summer due to their lower metabolic requirements and the low abundance and mean size of available resources. This argument is supported by data on seasonal changes in growth relationships and resource availability. The oscillation cycle is suggested to be due to the fact that the individual size of 1-yr-old vendace belonging to a strong year class will be relatively small at the beginning of summer due to high within age class competition. At the same time they have, as an effect of their high population number during their first summer, competed heavily with that year's one-year-old vendace with a resulting low reproductive output of the latter. One year old vendace of a strong year class will therefore grow rapidly during summer for two reasons: (1) their own small size at the beginning of the summer and (2) low competition intensity from the few young of the same year. This will result in a high reproductive output and a strong year class will thus give rise to another strong year class. Oscillating vendace populations have life histories with strong inter age class competition and long juvenile versus mature life periods, and are practically semelparous. These factors have been found to be the prerequisite for the occurrence of oscillations in theoretical models on age structured populations. The absence of oscillations in some vendace populations can be explained by high interspecific competition, reduced intraspecific competition and/or prolongation of the mature life period, factors that will violate the conditions necessary for the appearance of oscillations. nonyjiH5I 06IraTHbX ruIaHKTOHOcaOB -eBponefICKoR pnyuiKH Coregonus aZbula 06HapymJKa Kone6aTeJrbHbrf u LIKI c nepHOIqHocTTSm B 2 ro,;a. BbaBHHyTa mnoTe3a, ocHOBaHHaq Ha acH4MeVTpHqHofi KHKypeHHH MeWy Bo3paCTHbH KaccaMH c rIpeHMyuecTBom M p5]6, I o&06bHCHeHH Konle6aHHI4. r,...