LIFE HISTORY OF LAKE HERRING OF LAKE HURON 401 



Petersen reports some interesting facts on tlie relation between these fisheries and 

 the stock of plaice. I can do no better than quote him verbatim. On page 9, he 

 writes : 



In 1S99, I introduced fishing by Snurrevaad in Great Belt and thi.s Idnd of fishing soon had a 

 rapid growtli. (See Report XXVII, 1920.) 



The old dense stocli, consisting chiefly of small, old plaice, was fished up so that the stock became 

 much less dense; but gradually the plaice got bigger, on the average, though much younger. 



Now, the fishing is pursued chiefly on younger plaice but formerly mostly on older plaice. 

 The density of the stock is now very sparse; where in former years we took 200 to 300 plaice in one 

 haul, with Snurrevaad we can now take but one plaice or two or none at all. Here, I have virtually 

 seen an accumulated stock fished up and replaced by a new one of younger but bigger fishes; to 

 be sure not nearl}' as numerous pr. ha., but it is obvious that the stock is quickly renewed, so that 

 the statistics have been able to note good progress since 1899 in total production in kg., and the 

 production has been kept up until this day in spite of great fluctuations from one year to another. 



Again, page 18: 



It is thus beyond doubt that the intensity of fishing, while diminishing the density of the stock 

 in most of the Danish seas, has increased the growth rate of the individuals, so that the produc- 

 tivity as a whole has been kept up, and the average size of the plaice in our southern waters is there- 

 fore larger than formerly. 



Heincke has already called attention to the fact that in the age of the p'aice we have a kind of 

 measurement of the fishing intensity, the age will always decrease with a growing fishing intensity; 

 and I shall add that in the growth rate we have another means of measuring the fishing intensity, 

 about which I shall give further particulars in the following. [See, also, Garstang, 1926.] 



Is the acceleration in the growth of the Saginaw Bay herring due to intensive 

 fishing perhaps? It is true that the adult plaice differ greatly in their mode of life 

 from the lake herring. Adult plaice are bottom forms and bottom feeders. They 

 migrate, but very slowly. As the amoimt of food found on the bottom is strictly 

 limited the rate of growth of the plaice depends directly on the amount of space and 

 food that is available for each mdividual. The lake herring, however, are pelagic 

 plankton feeders. Intense fishing would reduce the number of spawners, the number 

 of eggs laid in the fall, and the number of fry hatched in the following spring. The 

 fingerUngs are known to occur in immense schools (Hanldnson, 1914) and the fry may 

 have the same habits. The amount of food that each obtains may depend on their 

 number. In any season in which their number had been reduced greatly by over- 

 fishing there would then be less crowding of the growing fry and more food available 

 for each individual, a lessened competition. This might increase the growth rate. 



It seems tmlikely, however, with the abimdance of plankton organisms and their 

 rapid rate of reproduction, that plankton-feeding forms can so far reduce their food 

 supply as to affect their own growth rate. However, if we are to explain the increased 

 growth rate of the Saginaw Bay herring of the 1919 hatch on the basis of lessened 

 competition due to heavy fishing we must suppose that the number of spawners in 

 the fall of 1918 was so reduced by fishing that the number of fry hatched in the spring 

 of 1919 fell below the usual number so far that an increase in their growth rate above 

 the normal took place. We are not concerned here as to whether the lessened number 

 of spawners in the fall of 1918 may have residted from extremely heavy fishing in 

 1918 or gradually due to continued heavy fishing in the several years preceding 1918. 

 All we need to assume is that intense fishing had reduced the number of spawners 

 in the fall of 1918. 



