234 THE SEA FISHERIES 



feeds on the surrounding plankton. An estimation of the number of 



fish eggs and larvse present in a given area of sea water is possible 



by Hensen's method, and other interesting deductions maybe made. 



The first calculations of this kind were made in the West Baltic 



for cod and fiat -fish (plaice). Hensen concluded that for the Ecken- 



forde waters, where the fishery for cod and flat-fish is carried on 



in an area of about i6 sq. miles, there are in January an average 



of 30, in February from 45 to 50, in March at least 60, and in April 



50 floating eggs of those species per square metre of surface, with 



an average depth of 20 metres. These eggs take on the average 



fifteen days to develop under the conditions obtaining in the West 



Baltic, so that the numbers given above must be doubled in order 



to give the number occurring per month under a square metre of 



surface. This gives from January to April 370 eggs. The ratio of 



adult males to females in these species is known, as is the average 



number of eggs produced by a female fish, and the statistics of the 



commercial fishing are also on record. From these data Hensen 



calculated that the number of cod and plaice annually caught by 



the Eckenforde fishermen would, if allowed to remain in the sea, 



have produced 23,400 million cod and 73,895 million plaice eggs 



yearly. These numbers, which are calculated from a nine -year 



average, give for every square metre of the 16 sq. miles of sea fished 



over 266 cod and 84 plaice eggs, a total of iio-6. This added to 



the 370, calculated above, gives a total of 480-6, which represents 



the number of eggs that would have been produced from all cod 



and plaice captured and free, annually for each square metre of 



surface. Consequently iio-6/48o-6 or 1/4-4 gives the fraction of 



cod and plaice actually captured per annum, or, in other words, 



man captures for his own use every year about one-fourth of the 



total number of adult fish in this particular area in the West Bahic, 



a result which is strikingly confirmed for other areas by the fish 



marking experiments described below. If these data are correct, 



it is obvious that considerable depletion of the fishing grounds can 



take place in certain restricted areas. The estimation of the number 



of floating fish eggs according to this method has important practical 



bearings. It becomes possible to compare the fertility of other 



regions with this area of the West Baltic, and so obtain some idea 



of the probable annual catch of fish. The results for the North Sea 



invariably gave a greater number of eggs, and the open ocean 



invariably a less number than the West Baltic. 



During recent years successful investigations have been made 

 into the migratory habits of marine fish.^ The method employed 

 is to mark certain species of fish with numbered labels, and up to 



1 For a general account see The Migrations of Fish, by A. Meek, London, 1916. 



