Muscle growth characteristics were investigated in a herring Clupea harengus L. population in the Blackwater estuary, Essex, England, between May and July 1998. Larval thermal histories were reconstructed using internally logging temperature recorders deployed within the estuary over the spawning season. The hatch dates of larvae were estimated using otolith microincrement analysis. Larvae were split into 3 groups; those developing from eggs laid early in the season when temperatures were low

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... .4 to 9.8°C), those developing mid-season (8.3 to 12.5°C), and those developing late (10.9 to 15.6°C). The number and size distribution of red and white myotomal muscle fibres varied between larvae from early-and mid-spawners in relation to estimated age. At approximately 60 d, the cross-sectional area of white muscle in mid-season larvae was 145% greater than in earlyseason larvae of an equivalent age because of 60% more muscle fibres and a 22% greater mean fibre diameter. The number and average diameter of red muscle fibres were proportional to body length, with no differences between the groups of larvae. However, for a given length, the average diameter of the white muscle fibres was significantly greater in mid-than early-season larvae. Muscle cellularity therefore varied for cohorts of larvae hatching at different times during the spawning season.