EARLY-LIFE HISTORY OF HERRING LARVAE IN CONTRASTING FEEDING ENVIRONMENTS DETERMINED BY OTOLITH MICROSTRUCTNRE ANALYSIS

Newly hatched autumn-spawned herring larvae Clupea harengus were relea sed in two 2500-m(3) outdoor mesocosms and reared over a 2-month perio d. Hydrographic conditions were similar in the two mesocosms, but the average plankton density was initially more than 10 times higher in me socosm B compared to mesocosm A (>11(-1) v. <0.11(-1)). Half-way throu gh the experiment the feeding conditions reversed with three times hig her average densities in mesocosm A than in mesocosm B (>31(-1) v. sim ilar to 11(-1)). Herring larvae were sampled with a 0.3-m(2) two-chamb ered net twice weekly, and survivors were harvested by draining the me socosms at the end of the experiment. Otolith growth trajectories of i ndividual larvae were determined by relating radial otolith size with number of increments from the outer edge of the otolith (days before c apture). The increment widths during the first 3 weeks after hatching, including the first-check size, were generally wider among larvae fro m mesocosm B (relatively good initial feeding conditions) than among t hose from mesocosm A (poor initial feeding conditions). The otolith gr owth pattern also confirmed that the surviving herring in mesocosm A b elonged to the upper size range of larvae in the mesocosm after only 2 -3 weeks from hatching, no such trend was found in mesocosm B. In both mesocosms the otolith size-al-age indicated that with the present sam pling gear, herring larvae larger than 20-25 mm were underrepresented in the net samples. The information obtained from otolith-size-at-age is compared with other morphometric and biochemical measures of size a nd condition of larvae obtained throughout the experiment. (C) 1997 Th e Fisheries Society of the British Isles.