OSMOREGULATION, NUTRITIONAL EFFECTS AND BUOYANCY OF MARINE LARVAL FISH - A BIOASSAY FOR ASSESSING DENSITY CHANGES DURING THE EARLIEST LIFE-HISTORY STAGES

It has been hypothesized that marine fish larvae in the advanced stage s of starvation would show increased density (p = mass volume(-1)) fro m water loss due to osmoregulation failure. Changes in larval buoyancy are currently attributed to swim bladder regulation and protein synth esis or catabolism. Osmoregulation-related changes in density is an al ternative mechanism, the importance of which remains untested in the l aboratory and the influence of which on vertical distributions Is unkn own. We provide evidence that loss of osmotic control is a plausible m echanism for increased density of larval cod (Gadus Morhua L.). Furthe rmore, our results show that this mechanism is not restricted to larva e in the advanced stages of starvation. ''Relative'' larval densities are estimated using a modified density gradient. We use a gravimetric method to separate the effects of nutrition from osmoregulation failur e. We assessed the importance of sampling strata on estimates of larva l density. Proportional sampling within three depth strata (stratified sample) produced the least biased method for determining the ''averag e'' density of a population of larvae in laboratory culture. Larvae sa mpled from the bottom third of the culture tank were significantly mor e dense then those sampled from the surface. This was true for larvae of all ages. The average change in density from hatching till death fr om starvation for larvae sampled in the surface stratum was nominal(De lta p = 5.0 x 10(-4) g cm(-3)), while the change for those sampled fro m the bottom stratum was large Delta p = 3.8 x 10(-3) g cm(-3)). These large density differences suggest that larvae sampled from the bottom stratum were either osmotically stressed or were facultatively changi ng their density via regulatory pathways. Preliminary observations sug gest that vitality is lower amongst those larvae which are sampled nea r the bottom. The small change in average density of larvae sampled fr om the surface stratum was due to starvation. The density differences we observed between ''osmotically stressed'' and ''starving'' larvae c ould readily have been misconstrued as differences in feeding and grow th experienced by individual larvae. The potential bias of increased d ensity from osmoregulation failure must be considered as a factor in e xperimental designs developed to assess the effect of fed and starved treatments on buoyancy for larvae of all ages. The simple bioassay we describe may prove useful both as a means of assessing larval conditio n and as a mechanism for evaluating factors affecting larval vertical distributions in the field.