Asymmetrical Competition between Age Classes as a Factor Causing Population Oscillations in an Obligate Planktivorous Fish Species

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Abstract
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 the 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 vandace 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 vendance populations have life histories with strong inter age class competition and long junvenile versus mature life periods, and are parctically 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 some vendance populations can be explaned 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.