Sex Composition of Lamprey Populations

Abstract

SINCE 1947 records have been kept of the size and sex composition of annual spawning populations of Lampetra planeri in the River Yeo in Somerset. In a previous communication¹ it was reported that up to 1954 there had been marked variations in the sex ratio from year to year and that in general these changes were associated with fluctuations in the relative abundance of this small isolated population. Observations made in subsequent years have confirmed the existence of a positive correlation between abundance and the ratio of males to females ; that is, the bigger the population the larger has been the proportion of males. This trend is emphasized in Fig. 1, where the total annual catches have been plotted against the number of males and females separately and the corresponding regression lines fitted to the observations. The lowest observations, which lie close to the intersection of the two regression lines, give sex ratios of 1.2 ♂/♀ for the smallest populations of 50–60, while the highest observed sex ratio of 3.4 corresponds to a relative population of about 200.