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Pad III. — Tiventidh Annual Report 



for the month,* but there is considerable variation in different years, 

 especially in the temperature in shallow water, and during each month, 

 according to Avhether there is prolonged frost or low air temperature in 

 January, February, and March, or prolonged high or low air tempera- 

 tures in summer. The minimum temperature on the beach, while 

 usually occurring in the early part of February, may not occur until 

 the end of February, or even the middle of March, and a corresponding 

 variation takes place with the maximum towards the end of July or 

 beginning of August. 



These facts throw light not merely on the normal growth of the 

 young plaice, but on the variation in gi'owth in different years, since 

 the habitat selected by this species at this stage of its life, viz. the 

 margin of the sea, is the one most exposed to variation in temperature 

 caused by change in the temperature of the air.t They, moreover, 

 explain the retardation or acceleration of the spawning season in different 

 years. After a hard winter the temperature of the water in twenty or 

 thirty fathoms is lower than after a mild winter, and spawning is later or 

 earlier accordingly. This may, perhaps, be partly due to the slower or 

 cpiicker development of the reproductive elements under the direct 

 influence of temperature, but that it is also due to voluntary inhibition 

 of spawning is shown by what takes place in the tanks at the hatchery. 

 When plaice actually spawning are transferred from the sea to the tanks 

 the spawning process is 'inhibited for days or weeks from fright, and the 

 fish may viltimately become egg-bound and die with the ovaries 

 enormously distended; and when the temperature \\\ the tanks falls 

 suddenly from the onset of frost or snow, spawning which has begun 

 with quite healthy fishes may be quite arrested until the temperature 

 begins to rise, as occurred last year.J 

 * They are as follows :— 



t The selection of this habitat is clearly of advantage to the species in summer, since 

 it is the region of maximum warmth, but it is curiou:-; that the beach should still be 

 frequented in winter when it is the coldest region ; for although the larger individuals 

 and some of the others appear to withdraw to slightly deeper water, they may still be 

 procured on the beach in considerable numbers. There is probably a greater advantage 

 to the species by the comparative immunity from enemies which the situntion confers, 

 since few piscivorous fishes venture into shallow water unless in exceptional circum- 

 stances. In the experiments on board trawlers, I have found that after a gale, when the 

 shallow water near the coast is rendered less transparent by mud or sand in suspension, 

 cod and codling may be found there in great numbers, prej'ing chiefly upon herrings 

 or young whitings, which drop from their mouths as they are brought to deck ; while at 

 the same time most other fishes, as haddocks, plaice, and dabs, may be almost absent. 



X On 24th January the temperature of the water in the pond was 5'7° C, and 60,000 

 eggs were collected. From this time it gradually sank to 3"7° on 26th January, and the 

 number of eggs collected fell daily to 30,000 on that date. From 26th January the 

 temperature declined daily to 1-6° on the 3bth, after which it rose slightly to 3'6° on 4th 

 February, and during this period of eight days no eggs could be found in the pond. 

 After reaching 4'6° on 9th February it fell again to 1'4° on 14th February, and spawning 

 was again inhibited for three or four days. The temperature in the pond rose to 6'0° 

 and 7"0° in the middle of March, and on an average more than 1,000,000 eggs were col- 

 lected daily at this time ; it again declined to between 3° and 4°, and 2° and 3° from 24th 

 March to 1st April, but at this period, although the daily quantity of eggs diminished to 

 740,000, spawning as a whole was not interrupted. The temperature rose again rapidly 

 to 6°, and the number of eggs collected daily increased to between one and two millions. 



