On the phenology of North Sea ichthyoplankton

The timing of fish larvae abundance was investigated at Helgoland Roads in the southern North Sea for the years 1990, 1993e1996, 1998, and 1999. From the 44 species identified, 27 were sufficiently abundant in more than 3 years and had well defined seasonal changes to calculate their phenology. The passage of the 15%, 50%, and 85% annual cumulative abun- dance thresholds was used to define the ''start of season'', ''middle of season'', and ''end of season'', respectively. The timing of a significant number of these events was correlated with the mean annual winter sea surface temperatures (SSTs) in weeks 1e10 with higher temperatures leading to earlier appearance. The timing of the end of season was also neg- atively correlated with the SSTs. Sole (Solea solea) is presented as an example of the gen- eral functional relationship.