Larval growth and drift pattern and the separation of herring spawning groups in the North Sea

Data from two internationally co-ordinated survey programmes are used in an attempt to display the events in the early life that took place during the 'recovery period' of the North Sea herring stock, 1979-1986. During the period, an increasing importance of the spawning grounds off Buchan and off Yorkshire is evident from distributions of newly hatched herring larvae.

The dramatic changes in relative importance of spawning grounds are traced in the data on both late larvae (6 months old) and juveniles (18 months old). The onset of extended spawning off Buchan and off Yorkshire was followed by increase in distinct groups of late larvae of large mean length, and by enlargement of a group of medium-sized juveniles.

It is hypothesized that the identified groups are interconnected. Thus, larvae spawned off Buchan and off Yorkshire are found to have growth rates superior to those attained by larvae from west of Scotland and around the Orkney/Shetland Isles, whereas the relationship is reversed in the juveniles. In the investigated period, larval drift routes exhibited the same trends from year to year, the drift of the northerly spawned larvae being the most variable.

Thus, changes in the relative contributions from spawning grounds apparently influence the overall spatial distribution as well as size composition of larval and juvenile North Sea herring. The findings indicate that groups of larvae retain, to a large extent, separate distributions until metamorphosis, and point to larval drift pattern as a determinant of spawning group distinctness.