During WW2 the British came up with a contingency plan in case the German U-boats tried to starve them. A study was done from 1941-43 that involved the intended use of plankton taken from the lochs of Scotland using 100 fleets of 10 nets to gather 26 tons a day to feed 35,000 people! The idea sounded good as it should have been easier to catch the little plankton than ordinary fish. For every 1 kg of fish, 10 kg of plankton would have to be gathered. But there would be shortages b/c plankton would only be available from April to October, only 7 months out of the year. So, the plan was scrapped. The German U-boat threat was serious but Doenitz never got anywhere near 300+ boats at one time into the N Atlantic! So, Britain did not face starvation.