Measurements of condition and growth of cod larvae reared in mesocosms: individual variability as a function of environmental condition or genetic inheritance

Size, dry weight and RNA/DNA measurements of 3876 cod larvae from 26 different families of recruit and repeat spawners reared in two mesocosms ( 2500m³ and 4400m³) under natural conditions were analysed. To be able to relate individual data to parental background, DNA microsatellite analysis was performed. The larvae from the two groups (recruit, repeat) already differed significantly in size and weight at hatching with the larvae from the recruit spawners being larger and heavier at the start of the experiment. Growth curves fitted for the larvae from the recruit spawning groups showed a trend of greater sizes at given ages. For all sampling dates offspring of recruit spawners had significantly higher sizes and dry weights than the repeat spawners. RNA/DNA ratios from recruit spawners showed a trend to higher ratios compared to the repeat spawners. The 2500m³ mesocosm was characterized by low plankton density during the transition from exogenous to endogenous feeding followed by a higher density during the metamorphosis period, while the 4400m³ mesocosm showed the opposite situation. The change in the food density occurring in the mesocosm was reflected in the growth rates. Survival was slightly higher in the mesocosm with the higher food density in the beginning, survival between recruit and repeat is assumed to be the same. Estimates of non-parametric probability distributions of the RNA/DNA ratios differed in the amount of scatter (variability) between mesocosms indicating that the higher food density lead to more better conditioned and fewer badly growing extremes of individual larvae in the first three weeks. When the feeding conditions changed the larvae from the low food environment could compensate and reach similar conditions, but were lacking some of the bad conditioned extremes. RNA/DNA analysis within each mesocosm showed that the individual fish exhibited very different growth and condition responses under the same environmental conditions. These different growth responses in both mesocosms were not related to being offspring of first or recruit spawners but seem to be caused by the environmental conditions.