Osmotic condition, buoyancy change and mortality in larval cod Gadus morhua. A bioassay for assessing near-term mortality

Abstract

In larvae ot marine fish, density differences are known to be associated with nutritional state and osmotic failure We hypothesized that these density measures can be used to predict the nearterm mortal~ty of larvae. Specifically, we predicted increased larval mortality to be positively and continuously associated with increased larval density. Under controlled conditions, we measured (1) the average density (g cm'3) and daily percent mortality of fed and starved larval cod Cadus morhua L, in culture tanks and (2) the relationship between larval density and percent mortality of both fed and starved larvae whose survival was monitored for a 24 h period following density determination. We observed a strong, positive linear relationship between average larval density and average percent mortality in both experiments (r2 = 0.79 and r2 = 0.78 respect~vely). The regression slopes did not differ significantly between expenments. A linear regression on the pooled data yielded a highly significant relationship (rZ = 0.77, p < 0.0001). A second-order polynomial regression was also signlf~cant and improved the fit (R2 = 0.80, p < 0.0001). While the second-order polynomial regression better described the functional relationship between average density and average mortality, the simpler first-order model was judged preferable for predicting near-term mortality. The consistency and strength of the Linear regressions suggests that larval density can be a reliable predictor of larval mortahty. We determined the density of larvae associ.ated with 50% mortality (potential 'critical' density value) using linear, polynomial and logistic regressions. All 3 produced similar 'critical' denslties and we used the linear model to estimate this value. Larval cod having densities >1.0338 g cm-3 have a 50% probability of dying within 24 h. The potential for using larval density to predict the probability of near-term mortality in laboratory and field studies is discussed.

Effects of Light Intensity and Salinity on Growth, Survival, and Whole‐Body Osmolality of Larval Southern Flounder Paralichthys lethostigma

“…In cod, buoyancy is a good indicator of larval condition. Sclafani et al (2000) found a positive linear relationship between larval density and mortality in this species. Some proposed strategies employed by larvae to maintain optimum vertical positioning within the water column are density changes via osmoregulation (Sclafani et al 1997) and synthesis or breakdown of lipids (Guisande et al 1998), which appear to enhance buoyancy in marine fish (Hagen et al 2000;Nursall 1989).…”

Metabolic and behavioural adaptations during early development of the Antarctic silverfish, Pleuragramma antarcticum

“…Moribund eggs and larvae will thus sink, as has been reported for a number of fish species (Blaxter and Ehrlich 1974;Coombs 1981;Yin and Blaxter 1987). As noted by Sclafani et al (2000) in larval cod, increases in larval density resulting from osmotic compromise overwhelm any changes resulting from nutritional effects. These authors determined a critical density for larval cod of 1.0338 g cm -3 , above which there is a [50% probability of dying within 24 h. We observed sinking of Pleuragramma embryonated eggs at temperatures between -1 and 6.5°C under which conditions hatching still took place and the larvae actively swam away from the surface.…”

Paternal effects on early life history traits in Northwest Atlantic cod, Gadus morhua

“…Other studies have also demonstrated paternal effects on larval and juvenile survival/traits in marine fish (Butts and Litvak, ,b) Because there were equal numbers of larvae in each beaker, density‐dependent mortality rates as mentioned in Sclafani et al. (), can be excluded.…”