Variability in muscle growth characteristics during the spawning season in a natural population of Atlantic herring Clupea harengus

Abstract

Muscle growth characteristics were investigated in a herring Clupea harengus L. pop- ulation in the Blackwater estuary, Essex, England, between May and July 1998. Larval thermal his- tories were reconstructed using internally logging temperature recorders deployed within the estu- ary 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 (6.4 to 9.8°C), those developing mid-season (8.3 to 12.5°C), and those devel- oping 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 early- season 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 cellu- larity therefore varied for cohorts of larvae hatching at different times during the spawning season.