RNA DNA RATIOS AND GROWTH OF HERRING (CLUPEA-HARENGUS) LARVAE REARED IN MESOCOSMS/

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were rele ased in two outdoor mesocosms of 2500 m(3) (A) and 4000 m(3) (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyl l a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A ( <0.1 l(-1)) compared to in Mesocosm B( >11(-1)). H alf-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31(-1)) as compared to Mesocosm B(s imilar to 11(-1)). The average temperature declined steadily in both m esocosms from 18 degrees C at release to 11-12 degrees C by the end of the experiment 60 d later. The RNA:DNA values of individual herring l arvae were related to protein growth rates and temperature adjusted ac cording to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth in dices were significantly correlated with other somatic growth estimate s. The average estimated protein growth rate in the two mesocosms foll owed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of In RNA vers us In DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocos m A had RNA:DNA values similar to the starvation control kept in the l aboratory the first days after release, confirming that larvae in Meso cosm A initially were in poor nutritional condition. On the other hand , the majority of the herring from Mesocosm B were characterised as st arving or in poor nutritional condition towards the end of the experim ent. The assessment of growth and nutritional condition were in accord ance with independent survival estimates which suggested that the majo rity of the total mortality occurred during the first 15 d in Mesocosm A and thereafter in Mesocosm B.