Fitness is generally considered to increase along body size, but this has rarely been investigated in dept in fish. This question was addressed by Perez and Munch who reviewed 40 studies reporting early mortality rates and sizes of fish, and discovered that in 77% of the studies selection favored large size in early life and that the selection differentials were much stronger than those typically found in terrestrial species. A natural explanation for the observed strength of selection can be the extremely high mortality fish typically experience early in their life. However, the observations also raise the question of how fitness effects of body size change across an individual’s life history and which are the possible trade-offs. For instance one might ask that if body size were strongly correlated over ages, would the selective pressure arising from fishing targeting large body size later in life be strong enough to overrule the advantage of large size early in life, leading to evolutionary shifts towards smaller size?
Perez KO, Munch SB (2010) Extreme selection on size in early lives of fish. Evolution 64: 2450-2457