The critical period of high mortality of larvae fish —A discussion based on current research

Most fishes suffer high mortality during early development stages. The existence of a “critical period” was hypothesized by Hjort (1914). However, several decades of research have resulted in considerable controversy over the hypothesis. This paper attempts to clarify the critical period concept, demonstrate its usefulness, and resolve some of the controversy surrounding it. Based on our own studies and the literature, we find that within the “aquaculture school” (those concerned with culturing fishes, particulary larval stages), there is a great deal of evidence for the critical period concept. But we redefine the concept of “critical period” of mortality of fish larvae in this way. The deepest, sharpest, and fastest physiological and ecological changes during the development of the larvae are produced with the shift from endogenous nutrition to exogenous. At this stage, the old functions are quickly being replaced by new functions inside the body. Because of this shifting of function and imperfection of organs, the larvae are most sensitive at this time to environmental factors, particularly food supply. A higher specific mortality rate often occurs immediately folowing the period of strictly endogenous yolk feeding, and during the period of first exogenous feeding. Then it may be followed by a lower specific mortality rate. The significance of a “critical period” for fish populations is discussed. A good understanding of the “critical period” would provide a means for predicting the strength of year classes in natural fish populations, and also suggests approaches for increasing survival in aquaculture systems.