Changes in the productivity of the southeastern North Sea as reflected in the growth of plaice and sole
Rijnsdorp, A.D.; Van Keeken, O.A.; Bolle, L.J. (2004). Changes in the productivity of the southeastern North Sea as reflected in the growth of plaice and sole. CM Documents - ICES, CM 2004(K:13). ICES: Copenhagen. 15 pp.
Part of: ICES CM Documents - ICES. ICES: Copenhagen. ISSN 1015-4744
Flatfish landings in the North Sea have decreased considerably since the late 1980s, mainly due to a decrease in plaice. In this paper changes in growth of plaice and sole are analyzed to explore changes in the productivity of the North Sea. Under the assumption that variations in the productivity of the ecosystem will be reflected in variations in the growth rate, the time trends in mean length at age and condition are determined using market sampling information and research vessel data. Both lengths at age and condition factor increased since the mid 1960s to reach a highest level in the mid 1970s. Since the mid 1980s, length at age and condition decreased to a level intermediate between the low level around 1960 and the high level around 1975. Growth rate of the juvenile age groups was negatively affected by intraspecific competition. Length of 0-group fish attained in autumn showed a positive relationship with the temperature in the 2nd and 3rd quarter, but for the older fish no temperature effect could be detected. Also, no correlation could be detected with the NAO-index. The overall pattern of the increase in growth and the later decline correlated with the temporal patterns in eutrophication (discharge of dissolved phosphates by the Rhine) and bottom disturbance (horse power days of beam trawling). It is concluded that the productivity of the southeastern North Sea for flatfish has decreased over the last two decades, possibly in relation to a decrease in the inflow of nutrients and an overall change in the North Sea ecosystem.
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