In recent years there seems to be a shift in maturity of sole (market samples). In 2018 during the BTS spawning plaice (August) was caught and the plaice was exceptionally skinny while the length-weight relationship at first sight did not seem to differ from previous years. This leads to the assumption that maturation cycles in sole and/or plaice are changing. On top of that, part of the sole stock in the North Sea is indeterminate spawner, and part is determinate.
The question arises if sole in the southern North Sea are switching to an indeterminate spawning type. Maturity can only be reliably studied macroscopically in the period directly prior to spawning and during spawning. Microscopic maturity staging is the only reliable method for year-round observations.