The Relationship Between Size at Maturity and Maximum Size in Cichlid Populations Corroborates the Gill-Oxygen Limitation Theory (GOLT)

Abstract

Fish generally mature at a smaller fraction of their maximum sizes than birds and mammals. The farmed tilapia (Family Cichlidae) can tolerate adverse conditions that result in stunting and which also cause the fish to spawn at small size. Such spawning at small size (or ‘early spawning’) is usually perceived as a unique feature of tilapia. The mechanism that explains how stressful environmental conditions tend to reduce the maximum size that fish can reach is very general and should apply to all fish. However, not all fish species are equally hardy, and most fish do not survive in the stunted or dwarf form under stressful environmental conditions. Tilapia, and other cichlids, on the other hand, can handle stressful conditions, if by remaining stunted. The present study shows that tilapia and other cichlids do not spawn ‘earlier’ than other teleosts. Rather, they are exceptionally tolerant of stressful environmental conditions, but with elevated metabolism. By reducing their growth and the maximum size they can reach ‘stunting’, they also reduce the sizes at which their maturity is initiated (‘early spawning’). This corroborates the gill-oxygen limitation theory (GOLT), which identifies spawning as an event rather than a determinant of fish growth.