278 DISCOVERY REPORTS 



It was shown by Pearcey (1885), who made a study of the dense phytoplankton 

 concentrations in the region of the Shetland Islands in relation to the herring fisheries, 

 that whenever his fishing vessel shot her nets in dense phytoplankton {Rhizosolenia 

 shrubsolei and Thalassiosira nordenskioldii) the catch of herring was very small. He 

 also noted that animal plankton was scarce in the regions of dense phytoplankton. 

 In the same year Shrubsole (1885) records gelatinous organisms, from his description 

 almost certainly Phaeocystis, reducing the catches of fish. Later Bullen (1908), for the 

 Plymouth area, showed that zooplankton was usually scarce in regions of heavy phyto- 

 plankton, and that the heaviest catches of mackerel were taken in regions of least 

 phytoplankton, although he appeared to attribute this to the relative scarcity of zoo- 

 plankton in the phytoplankton regions rather than to the effect of the phytoplankton 

 itself. 



In 1 92 1 I suggested that the poor herring fishery of that autumn might be due to a 

 dense concentration of the diatom Rhizosolenia which I had observed on the fishing 

 grounds, 1 and the following year I began experiments with the plankton indicator (Hardy, 

 1926) designed to obtain further evidence on the influence of phytoplankton on the 

 herring. Only twelve records were obtained in the autumn of 1922, but they showed a 

 zone of the colonial flagellate Phaeocystis stretching across the herring grounds. Six 

 of the observations were made in regions of Phaeocystis, and here all six of the catches 

 of herring were low, between 0-5 and 6 crans, and six observations were made in water 

 free of Phaeocystis where the catches of herring were between 15 and 45 crans. 2 



Savage (1930), by some striking surveys of Phaeocystis patches at the time of the spring 

 fisheries of 1924 and 1926, and the autumn fishery of 1927, gave valuable evidence in 

 support of the influence of Phaeocystis on the herring, but in a recent paper (1932) 

 expressed the view that there was little evidence that concentrations of diatoms had 

 a similar effect. Mr Savage and I have now made (1935) a review of the possible 

 influence of phytoplankton upon the fluctuations in the herring fisheries over a 

 period of 12 years and we believe that there are good grounds for considering that 

 the fluctuations of the autumn fishery are at least in part bound up with the 

 phytoplankton, whether Phaeocystis or diatoms. Whilst Bullen seemed to imply that 

 the mackerel were scarce in regions of phytoplankton because of the shortage in these 

 areas of their food, the zooplankton, Pearcey considered that both fish and zoo- 

 plankton were directly affected together. "In every case" he writes "where the nets 

 were lying among Rhizosolenia not a single herring was caught, while outside of the 

 diatomaceous zones they were found in abundance. Many experiments were made to 

 test the exclusive power of the diatoms, all giving the same results." Farther on he 

 says, "There can be no doubt, then, that this vegetable matter exercises a very great 



1 Hardy (1923). 



- Experiments with the plankton indicator used on herring drifters have been continued recently by 

 Hardy, Henderson and Lucas (report in preparation); out of 1256 plankton samples obtained with 

 corresponding records of catches of herring it was found that in 55 phytoplankton predominated and 

 coloured the disc green. The average catch of herring for these 55 occasions was 1-8 crans ; the average catch 

 for the remaining 1201 occasions when the discs were not coloured green by phytoplankton was 9-9 crans. 



