A positive correlation between number of lateral shields and total length is shown to exist in Gasterosteus aculeatus L. populations (gymnura-form), consisting of fishes of exactly the same age. These populations were reared in the laboratory from artificially fertilized eggs. (Heuts, 1947 b). The same correlation, found in natural populations, is thus caused by faster growth rates in more plated individuals. This possibility was considered previously (Heuts, 1944). Also in artifïcial populations, the correlation is shown to decrease with increasing average length of the population, being + 0,64, +0.56 and + 0,11 for populations having reached respectively 21,81± 0,90 mm, 32,39±0,59 mm and 35,14± 0,608 mm average size. The rapid breaking down of the association between the two characters considered, was equally found to happen at exactly the same growth level in natural populations (Heuts, 1947 a). It means evidently that after having reached the size of 33 to 35 mm, growth is slowing down first in more plated individuals, allowing the less plated ones to recover their initial lag. |
