Reports from anglers and divers indicated that in late August and September 1978 a substantial mortality of the bottom fauna and fish had occurred in the bays along the south coast of Cornwall, particularly in the St Austell area. It was therefore decided to carry out an investigation by diving and at the same time collect water samples from affected areas for further analysis. The following notes record the observations made during the period 19–28 September at various localities but mainly in St Austell Bay (see Fig. 1).

Since May 1977 the Penzance Branch of the British Sub-Aqua Club has carried out a routine survey, as a contribution towards the Underwater Conservation Year Programme, on the fauna and flora of a sublittoral reef situated about 300 m to the south west of the Gear Reef in Penzance Bay. The reef is situated in water of about 17 m depth and is generally surrounded by firm sand with patches of Zostera. The reef itself is about 30 m long and 17 m wide at its widest point and 8 m high with vertical faces and overhangs at its northern end. The sides slope by stages to the sand at the southern end and contain many deep fissures with a large one at the northern end widening to a cave that passes through from east to west. The reef is aligned with its longitudinal axis running north-south and provides a diverse environment for a rich fauna and flora, a considerable background knowledge of which has been gained by the survey: by late August 1978 a total of over 30 dives, each of approximately one hour's duration, had been made in this area.

There have been several reports in recent years of the mass occurrence or blooms of the dinoflagellate Gyrodinium aureolum Hulburt in northern European Waters (Ballantine & Smith, 1973; Helm et al. 1974; Pingree et al. 1975; Pingree, Holligan & Head, 1977; Tangen, 1977). Gyrodinium aureolum is probably one of the most common ‘red-tide’ dinoflagellate species in these waters and some of the blooms of G. aureolum (ranging from 100 to 20000 cells ml-1) have been followed by the mass mortality of various fish and invertebrate species (Tangen, 1977). It has been suggested that the adverse effects of G. aureolum on marine organisms are caused by (a) the production of toxins and/or (b) the oxygen depletion during darkness due to dinoflagellate respiration and decomposition of cells.

Dinoflagellate blooms occurred repeatedly along the coasts of France in 1978, and the question was raised, both in newspapers and in the report of the Inquiry Committee of the French Senate on the ‘Amoco-Cadiz’ disaster, of a possible relationship between these occurrences and the oil spilled from the tanker. This hypothesis does not, however, appear to be supported by the data regarding bloom occurrences. These are listed in Table 2, compiled from various sources. Those which concern Brittany and nearby areas, as well as the extent of coast fouled by the ‘Amoco-Cadiz’ oil, are shown in Fig. 2. It is clear from this map that some of the ‘red tide’ occurrences were quite remote from the site of the wreck at Portsall; those outside the area shown in Fig. 2 (e.g. Seine Estuary) were, of course, even more so.