SEA- FISHERIES LABORATORY. 



207 



The results of Table 16 are studied by making shortest 

 half -ranges from the various yearly distributions. 































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23- 























22- 



















81- 

 20. 

 19- 















IQ + 











.flArt 





17 - 













Ib- 





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Fig. 17. Variation in length (shortest half -ranges) of plaice caught 

 on the Mersey nursery grounds, during September and October, 1908-1920, 

 by a trawl-net of 6 -inch mesh. 



The figure represents the prevalent sizes of plaice on the 

 above grounds. The lengths of the columns, read off on the 

 scale on the left-hand side, give these prevalent sizes. Now 

 the latter certainly increased regularly from 1908 to 1911 

 (which latter year was about the time of a maximum in plaice 

 abundance), but after 1915, when there should be a minimum, 

 the prevalent size of the fish does not increase towards another 

 maximum about 1920, as one might expect. Perhaps, to expect 

 BO much would be foolish for we may have to reckon with 

 the fact that the migration periods that we noted above may 

 be variable — and here we have studied two mouths only. 

 In fact, this question of variation in length is very complex, 

 and we are still without the knowledge that would enable us to 

 deal with it satisfactorily. 



Fluctuations in the Size of Plaice Liverpool Bay. 



Next, we take another region, Liverpool Bar to Blackpool 

 (mostly within territorial waters). All experimental fishing 



