Hydrographic Effects on the Vertical Distribution of Haddock (Melanogrammus aeglefinus) Eggs and Larvae on the Southwestern Scotian Shelf

Abstract

Relationships between the density (sigma-t) structure of the water column and the depth distribution of haddock (Melanogrammus aeglefinus) eggs and larvae in May of 1985 and 1986 were investigated. During the early embryonic period the egg concentration decreased exponentially with depth; this contrasted sharply with stage IV eggs which showed a subsurface maximum, the location of which was determined by their specific gravity. Depth of the centre-of-mass (Zcrn) of stage IV eggs ranged from 13 to 47 m over the survey area whereas the sigma-t associated with the Zcm depth varied only slightly (CV = 0.4%) about a mean of 25.58. In stratified offshore waters, haddock eggs were aggregated at middepth but exhibited a progressively more uniform vertical distribution towards the weakly stratified nearshore. Where low-density water (sigma-t < 25.5) occurred near-shore eggs were concentrated at the deepest sampling stratum, suggesting that peak egg concentrations were below this level or possibly on the bottom. The depth distribution of haddock larvae was identical to that of late-stage eggs and was therefore established by buoyancy changes occurring during the embryonic period. Day and night Zcm was similar, averaging 28 and 31 m, respectively, and stability of the depth distribution of larvae appears to persist well into the juvenile period.