474 REPORT ON TRAWLING AND OTHER INVESTIGATIONS 



larger fish of these two species than of the smaller fish. It will be 

 shown below that, for the plaice, the explanation of this difference is to 

 be found in the i)eculiarities of their migrations. 



3. In Start Bay the seasonal fluctuations of plaice are only appreci- 

 able in the case of mature fish. Practically the whole of the plaice 

 below 12 inches in length appear to reside in the bay throughout the 

 year, and do not usually emigrate on the approach of winter until they 

 have attained the size mentioned. 



In Torbay and Teignmouth Bay the seasonal fluctuations of plaice 

 extend also to the immature medium-sized fish. (Experiments with 

 marked fish, especially in Teignmouth Bay, show that the plaice in 

 these bays emigrate at a smaller size than in the case of Start Bay.) 



4. The annual fluctuations in the abundance of plaice are traceable 

 to changes which take place from year to year in the numbers of the 

 smallest fish present in the bays. A decrease in the number of the 

 smallest group one year is followed by a decrease in the number of 

 fish one year older in the following year. An increase in the smallest 

 is similarly followed by an increase in the numbers of larger fish in the 

 years immediately following. 



5. The observed changes cannot be explained as due to the prohibition 

 of trawling in the bays, since fluctuations in both directions have taken 

 place during the period of closure. 



6. The facts are most fully explained on the hypothesis that changes 

 from year to year in the physical conditions which influence the distri- 

 bution of floating eggs and larvte cause sometimes a larger and some- 

 times a smaller proportion of the eggs and fry to set into the bays 

 and undergo a successful metamorphosis on the rearing-grounds there. 



The chief agency capable of inducing these changes appears to be 

 the direction of the winds during the spawning season. The records 

 tend to show that a prevalence of south-easterly winds during the 

 spawning season of the plaice in any year in the South Devon district 

 is followed by the survival of a greater percentage of the fry of that 

 year, and an unusual amount of north-westerly winds by an increased 

 mortality. 



7. The closure of the bays to trawlers does not appear to have 

 appreciably favoured the dab at the expense of the plaice, since, 

 during the year ending June, 1902, the proportion of dabs to plaice 

 was less in Start liay and Teignmouth Bay than during the previous 

 investigations from 1895-98, and was higher only in the case of Tor- 

 bay, where conditions peculiarly favourable to the dab are found. In 

 the summer of 1902 an unprecedented increase in the number of 

 small dabs was shown by the trawling records for each bay, and 

 exceeded a simultaneous increase in the number of small plaice. 



