The short investigation here described was made in March and April, 1922, during the spring mackerel fishing season off the Cornish coast, in order to discover whether the opinion of Skipper Wylie, of Lowestoft, (drifter, Realize) was justified. In his opinion the best catches of mackerel were made wherever his thermometer showed him a sea surface temperature higher than that of the neighbouring water.

About a dozen drifter skippers, who volunteered to undertake the work, were given thermometers, graduated in ⅕° C. (about 20 graduations per cm.) and forms upon which to record their observations, and were shown how to take a surface sample and read its temperature. These thermometers were not very suitable, since the range was too great, and therefore the scale too small. They also suffered from the defect that the mercury thread easily become broken, and if the detached portion ran down into the bulb at the top of the tube, it was difficult to ensure that all the mercury came out again, and that the thread made a proper joint.

The forms used (p. 241) were based upon the log books used during the investigations on pelagic fishes from 1895 to 1911* (Russell, 1915). It was hoped that sufficient data would be collected in the course of a month, but, although the experiment was extended into April, only 32 records are available, covering the period 9th March-19th April.

These records are tabulated in Table 1. In those cases where the temperatures at the times of shooting and hauling differed, both the values are given, but their mean has been used in computing averages.