Why does egg size increase with maternal size? Effects of egg size and egg density on offspring phenotypes in Atlantic salmon (Salmo salar)

Question: Why does per-offspring investment (e.g. egg size) increase with maternal body size? Theory: Fecundity also increases with maternal body size, and when egg/larva/juvenile dispersal is limited, offspring developing at higher densities (e.g. in larger clutches of eggs) experience more stressful conditions during development. Hence, an increase in per-offspring investment with maternal body size may be a form of maternal compensation for the negative effect of fecundity on offspring performance. Hypothesis: In fishes with demersal eggs and larvae, offspring in larger clutches use more energy during development, resulting in a smaller subsequent juvenile size. This may occur because larvae in larger clutches are prone to mutual physical disturbance, and larvae may expend more energy seeking or creating oxygenated areas within the nest. Organisms: Atlantic salmon (Salmo salar). Location: Aquatron experimental research facility, Halifax, Nova Scotia, Canada. Methods: We buried 12 full-sib families in 24 gravel egg pockets at high density (200 eggs per egg pocket, or 1.27 eggs per cm³, n = 12) and low density (25 eggs per egg pocket, or 0.318 eggs per cm³, n = 12) and monitored subsequent juvenile size, survival, time of emergence, and developmental stage at emergence. Results: We found no evidence that density per se affects offspring phenotypes. However, offspring from larger eggs emerged later and at an earlier developmental stage than offspring from smaller eggs. We show that the fitness trade-off between size-at-emergence and emergence time can result in a lack of correlation between egg size and offspring fitness. Our results do not provide evidence that the positive correlation between egg size and maternal size is adaptive, but we do provide new evidence that bigger is not always better.