THE WESTERN MACKEREL FISHERY. 289 



mackerel shoaling densely, the fishermen state that there is no mis- 

 taking it. An oily appearance at the surface of the water generally 

 occurring in " splats," i.e. patches, is also said to be a sure indication of 

 drift fish. 



A milky appearance of the surface generally occurring in inshore 

 waters, where there is no addition of china clay to the water, is asso- 

 ciated by the fishermen with shoals of small mackerel, the milky 

 appearance being due, it is stated, to excrement. 



" Signs " of shoaling fish offered by the presence of sea-birds preying 

 upon them occur more frequently, according to the fishermen's state- 

 ment, in inshore waters, and the point is one which has already been 

 described by previous writers, and need not therefore be discussed here. 



COMPARISON OF " SIGNS " OFFERED BY DIFFERENT TYPES OF WATER 

 WITH THE CONDITION OF PLANKTON OCCURRING IN SUCH WATER. 



In order to endeavour to ascertain to what extent these colour 

 " signs " are produced by plankton conditions, a number of plankton 

 samples were taken for me in 1906-7 by fishermen, and labelled with 

 reference to the particular type of water from which they were 

 derived. 



" Stinking Water." — Sample No. 39, Plankton Tables, was taken by 

 myself on April 10th, 1907, in an area of water termed by the fisher- 

 men " stinking," which, it was stated, extended from the Lizard to 

 Land's End in a zone of varying width about ten miles or more from 

 the shore. At the particular position at which it was taken, 6 miles 

 N.W, X W. of Lizard, the fishermen were agreed in stating that the 

 water was a fair sample of the " stinking " type, and that it would be 

 useless to shoot in it. The colour and characteristics generally were in 

 accordance with the description already given ; but although the fisher- 

 men were agreed in saying that there was an obnoxious smell, I was 

 unable myself to detect it. The analysis of the sample showed (by the 

 method of comparison described in a former section) a moderate pre- 

 ponderance of phytoplankton over zooplankton, but the total bulk of 

 the sample was comparatively small. As will be seen on reference 

 to the Plankton Tables, the phytoplankton was mainly composed of 

 diatoms, of which Chaetoceras boreale and C. densum were both common. 

 Fhaeocystis glohosa appeared to be rare. The zooplankton comprised 

 three Copepods, Acartia claiisi, Oithona similis, and Pseudocalanus 

 elongatics, moderately common. Calanus jinmarchicus was rare. 



" Green Water." — It will be convenient at this point to compare the 

 foregoing with another sample (No. 40, Plankton Tables) taken on the 

 same date outside the " stinking water." This was at a position 16 miles 



