Yolk utilization phase of teleost development, interrelationships of weight, water content, and specific gravity: relationships with reproductive strategy, and usefulness in culture situations
The water content was the main variable determining egg specific gravity of a series of 13 species studied in our laboratory, and can be described by the equation s.g. = 1.2 - 1.93 x 10−3%H20 (R2 = 0.97). The species included benthic, marine, and freshwater spawners, and marine and estuarine pelagic spawners. Benthic spawners tend to have more dense yolks, which are important in post-hatching growth. With many pelagic spawners, somatic growth between hatching and first feeding is relatively unimportant, the development of swimming and feeding capability being of higher priority. In culturing early life stages, two aspects of the yolk utlization phase are potentially important – timing of first-feeding and providing optimal environmental conditions during yolk utilization. We hypothesize that the larva passes through a developmental maximum toward the end of yolk utilization which coincides with the optimum time for first-feeding. For some species (e.g., salmon), wet weight is a parameter which is maximal near the time of first-feeding. In general, the gain in wet weight during yolk-sac utilization is related to initial yolk present at hatch by the equation: wet weight = (dry yolk weight) (Yfd) (E) efd where wet weight is the gain in wet weight, yfd is the fractional dry weight of yolk, efd is the fractional dry weight of embryonic tissue, and E is the gross conversion efficiency. On a calorific basis, the portion of the yolk not used for weight increase can be accounted for by metabolic rate. For species which have a sedentary yolk utilization phase, and consequently a high gross conversion efficiency, the gain in wet weight may be considerable. The relationship is typical of salmonids, and possibly also lumpfish (Cyclopterus lumpus), and wolffish (Anarhichas lupus). For pelagic larvae which swim constantly and have higher metabolic rates, and lower gross conversion efficiencies, larval wet weight may decrease during the yolk utilization phase. Larval length is typically at a maximum near the end of yolk utilization. This developmental pattern is typical of striped bass (Morone saxatilis).