A regime shift in the North Sea circa 1988 linked to changes in the North Sea horse mackerel fishery

After 1987, Phytoplankton Colour (a visual estimate of chlorophyll) measure d on samples taken by the continuous plankton recorder (CPR) in the North S ea increased substantially, both in level and seasonal extent, compared to earlier years since 1946. Many species of phytoplankton and zooplankton sho wed marked changes in abundance at about the same time. These events coinci ded with a large increase in catches of the western stock of the horse mack erel (Trachurus trachurus L.) in the northern North Sea reflecting a northe rly expansion of the stock along the shelf edge from the Bay of Biscay to t he North Sea after 1987. Using a 3D hydrodynamic model, with input from mea sured wind parameters, monthly transport of oceanic water into the North Se a has been calculated for the period 1976-1994, integrated for a section fr om Orkney to Shetland to Norway. A substantial increase in oceanic inflow o ccurred in the winter months, December to March, from 1988. Higher sea surf ace temperatures were also measured after 1987 especially in spring and sum mer months. These biological and physical events may be a response to obser ved changes in pressure distribution over the North Atlantic. From 1988 onw ards, the North Atlantic Oscillation (NAO) index, the pressure difference b etween Iceland and the Azores, increased to the highest positive level obse rved in this century. Positive NAO anomalies are associated with stronger a nd more southerly tracks of the westerly winds and higher temperatures in w estern Europe. These changing wind distributions may have led to an increas e in the northerly advection of water along the western edge of the Europea n shelf and may have assisted the migration of the horse mackerel. This stu dy is possibly a unique demonstration of a correlation between three differ ent trophic levels of a marine ecosystem and hydrographic and atmospheric e vents at decadal and regional scales. The results emphasise the importance of maintaining into the future long term programmes such as the CPR. (C) 20 01 Elsevier Science B.V. All rights reserved.