472 REPORT OX TRAWLING AND OTHER INVESTIGATIONS 



The Causes of the Observed Fluctuations. 



It has been shown in the preceding pages that during the year 1901-2 

 there was a remarkable scarcity of immature medium - sized plaice 

 (8 to 11 inches) in all three bays, as compared with the period 1895-8, 

 previously investigated. This scarcity was limited to the year 1901 and 

 the first half of 1902, and was followed in the summer of 1902 by an 

 increased abundance of plaice of this size, as well as of small dabs. 



We have seen that there are distinct indications in the trawling 

 records that these fluctuations were generally preceded in the previous 

 year by corresponding fluctuations in the abundance of fish one year 

 younger. The available evidence does not amount to a conclusive 

 demonstration ; but the indications of this correlation are sufficiently 

 definite to form the basis of a working hypothesis. The existence of 

 such a correlation would lead at once to the conclusion that the causes 

 of the observed fluctuations were quite independent of the restrictions 

 placed upon trawling in the bays, and were to be sought in the con- 

 ditions which naturally influence the reproduction of the fish from year 

 to year. The fluctuations in the numbers of small flat-fish in the bays 

 are therefore probably attributable to changes in the physical conditions 

 in previous years, which caused sometimes a larger and sometimes a 

 smaller proportion of the floating eggs and larvae to set into the bays 

 and undergo a successful metamorphosis there. 



These remarks apply to Teignmouth Bay and Torbay, but only in- 

 directly to Start Bay, which apparently possesses no natural rearing- 

 ground (or " nursery ") for the smallest plaice, and seems to derive the 

 bulk of its population of medium-sized plaice by migration from the 

 other bays to the northward. 



Of the various physical factors capable of producing the changes 

 above mentioned, none would appear to possess so great an importance 

 as the direction of the winds during the spawning season of the fish, 

 owing to their dominant influence upon the set of the currents.* The 

 problem deserves fuller treatment than it is possible to give to it on 

 the present occasion, but an analysis of the meteorological records of 

 the Eousdon observatory (near Lyme Eegis) for the years 1891 to 1901 

 lends distuict support to the view expressed. 



For the region under discussion we may assume that south-easterly 

 winds (i.e. K., S.E., and S.) would be favourable, and north-westerly 

 winds {i.e. N., N.W., and W.) would be unfavourable, — the spawning 

 season of the plaice in this district being January, February, and March. 



(1) We have already seen that small plaice of 4 and 5 inches were 



* Cf. Garstaug, "Report on the Surface Drift of tlie English Channel and Neighbouring 

 Seas during 1897,'" Journ. M. B. A., vol. v., pp. 199-231. (Other literature cited.) 



