producing fewer but larger eggs (3, 17). Development continues while the 
young larvae drift in the water column, absorbing their yolk and developing 
the mouth parts and swimming ability to capture food and avoid predators. 
Although the methods used to determine the extent of first year mortality in 
natural populations are at best imprecise (25), it is clear that among high 
fecundity species, losses early in life are extremely high, with the highest 
mortality rates among the early larval stages. 
As early as the end of the last century, Fabre-Domergue and Bietrix (9) 
encountered heavy mortality among laboratory reared marine Fish larvae which 
had exhausted their yolk reserves. Hjort (12) concluded, based on his studies of 
year to year fluctuations in Norwegian cod and herring abundance, that 
year-class strength was probably determined early in the larval development of 
these species. The term “critical phase” or “critical period” has been used, in a 
general sense, to refer to that span of time in the early development of the 
individuals comprising a particular year-class during which the ultimate number 
of recruits is determined (11). In a narrower usage “critical period” may be 
used to refer to that point in development of the larval fish at which all sources 
of endogenous (yolk) nutrition have been consumed, and active feeding must 
commence if death by starvation is to be avoided. Hjort (12) proposed death 
following yolk exhaustion as only one of several possible mechanisms by which 
events early in development might affect the subsequent size of a given 
year-class. In 1956, Marr (23) reviewed the available evidence in support of the 
existence of a “critical period”. He concluded that there was little evidence to 
suggest that mass starvation occurred in the sea among larvae that had recently 
absorbed their yolk, or that survival curves for natural populations revealed any 
noticeable inflection at the point of yolk absorption. 18 years later, May (25) 
noted that little new data has been gathered since Marr’s review that could 
contribute meaningfully toward the resolution of the problem of whether or 
not a “critical period” at yolk absorption exists as a widespread phenomenon 
among fish species. He suggested that while among high fecundity species, 
year-class strength is certainly determined during early development as Hjort 
maintained, the physiological mechanisms that have evolved to meet 
environmental challenges that confront the developing larva must be addressed 
on a species by species basis. 
The prolarva, from the time it is hatched until it captures its first meal, is 
reliant on its yolk reserves to provide the structural materials for continued 
ontogenetic development, as well as to provide energy to fuel its maintenance, 
activity, and growth needs. Unless sufficient satisfactory food is taken after the 
exhaustion of yolk reserves, structural tissue already laid down is metabolized 
to support the continued costs of swimming in search of prey, until the larvae 
is so debilitated by the effects of starvation that it is unable to capture and 
utilize suitable prey when it does become available. Blaxter and Hempel (4) 
235 
