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 



