REFERRING to Prof. Herdman's interesting observations upon the above (NATURE, July 17), I may perhaps mention that the mackerel-drifters, when fishing upon the usual grounds around Scilly and in the Bristol Channel, are largely influenced in their selection of a suitable position by the finding of so-called “yellow water”. This condition of the sea in the area under consideration arises from the presence of vast shoals of Calanoids—e.g. Calanus finmarchicus, Pseudocalanus elongatus, c the fishermen state that it is possible at times to observe the entire extent of a “splat” of “yellow water”.