Trade-off between number and intraovarian growth rate of offspring in Zoarces viviparus

Trade-off between number and intraovarian growth rate of offspring was studied using 15 samples of females caught at 7 sites in the Baltic Sea and the Skagerrak. The growth of normal embryos appeared to be synchronous within broods, and there was no correlation between the females' size and relative fecundity. Females with higher relative fecundity had significantly smaller average larval length which may suggest existence of offspring competition for maternal energy supplies. Our results also show that reduced offspring size in females with high relative fecundity did not fully compensate for the increase in offspring number: the ratio of embryo mass to female somatic weight was positively correlated with relative fecundity. The negative relationship between female condition and relative fecundity indicates that increased investment in offspring affects the energetic state of the female. However, the last effect was rather moderate.