Smaller eggs can generate larger fish

The eggs of the same female may vary in size and it has been hypothesised that this may be related to environmental conditions or adaptive responses.

There is evidence that the variation in the size of the ova may subsequently influence growth rates, behaviours and phenotypes.

Eight-month study

To corroborate this hypothesis in rainbow trout, researchers from the University of Oregon, Brigham Young-Idaho University and the Oregon Hatchery Research Center conducted an eight-month study of fish from eggs of different sizes divided into three groups.

The first group consisted of fish born from small eggs raised with other fish born from small eggs. The second comprised fish from large eggs raised along with other fish born from large eggs. And the third was a mix of fish from large and small eggs.

At the end of the experiment, the scientists observed that the growth of the fish varies inversely with the size of the ova from which the fish originated. Thus, in the first eight months after hatching, fish that originated in smaller eggs grew faster and had a longer average length than fish from larger eggs.

Consistently higher growth

In addition, they found that this effect was repeated regardless of whether the fish were bred in mixed or uniform groups.

“The smaller ova fish hatched earlier and smaller in size than the larger ova fish, however, the former showed consistently higher growth rates than the hatched fish of larger eggs,” the study authors noted.

As an explanation they explained that “given that many attributes of the life cycle seem to be determined by the size or growth rate during the first year, the size of the ova could be a significant predictor of important changes in the life history of fish”.

Review the abstract of the study entitled Egg size and growth in steelhead Oncorhynchus mykiss here.