of the Fishery Board for Scotland. 339 



On tlie East Coast of Scotland the period of normal maximmu 

 spawning may be placed about the middle of March. At this time 

 tlie mean tempei-ature of the surface water is a little over 40° F., and 

 the bottom water is almost the same, but a very little colder, in 

 depths of twenty to thii'ty fathoms, where the fishes spawn. In 

 April the mean temperature of the surface water is about 43° F., 

 while the bottom water is about 1"5° colder in the depth stated, 

 approximating to the surface temperature in shallow water.* The 

 egg,i\ of the plaice take about eighteen days to hatch at a temperature 

 of about 42^8, and twelve days at a temperature of 50° F.,t so that 

 the period when the young plaice normally issue in greatest numbers 

 fiom the egg may be placed at about the beginning of the second 

 week in April. 



The larval plaice on hatching is about 7 mm. in length, and it is 

 established that duiing the period of transformation its growth in 

 length is very slow. Holt found specimens in the transformation 

 stage measuring 10 to 13 mm. ; Petersen states that the average size at 

 the end of the post-larval period is 10 to 11 mm. ; Ehrenbaum 

 describes specimens 13 to 17 mm. long, in which the left eye was not 

 over the edge, and the smallest plaice he found on the bottom 

 measiired 13 mm. In the push-net collections referred to, specimens 

 of 12, 13, and 14 mm. were taken in June and July. In rearing 

 experiments Dannevig found that young plaice in which the left eye 

 had passed over to the right side measured about 13'7 mm., and that 

 when somewhat younger and measuring about 12-4 mm., and while the 

 eye was still on the edge, they remained permanently on the bottom ; and 

 he states that the time taken from hatching (May-June) to the com- 

 plete transformation varied from 37 to 48 days. Dr. Kyle, who has 

 made a careful comparative study of the differential characters of 

 port-larval flat-fishes, comes to the conclusion that in the plaice trans- 

 formation is completed when the fish are between thii-teen and sixteen 

 millimetres in length. J 



From the various data it will be seen that the young plaice starts 

 life on the beach when about 12 or 13 mm. (g inch), and that the 

 greater number settle there about the middle of May. Others arrive 

 earlier and later, derived respectively from eggs spawned towards the 

 beginning or the close of the spawning season; but it would be 

 wrong to assume — as is sometimes done in explanation of the con- 

 siderable range of sizes found in fish of the same series — that the 

 duration of the spawning season and of the period of transformation 

 and settlement on the bottom is of equal extent. Eggs spawned at the 

 beginning of February must pass their development, and the larvfe 

 their pelagic life, in water of about 40° F., while those spawned at 

 the beginning of May pass the same stages in water 6"" or 8° higher, 

 and from the data given above it is probable that, while the spawning 

 .season lasts for about four months, the period of transformation and 

 settlement lasts only about three months — namely, from the beginning 

 of April to the end of June or early part of Jvdy, the bulk of the 

 young plaice, as stated, settling about the middle of May. 



From the time of hatching until the end of post-larval life growth 

 in length is slow compared with what it is after transformation is 



* Fulton, rhid., 218; compare Dickson, Qnarl. Joiirn. Rnii. Meteorolimrol Sor., xxv., 

 No. 112 (1899). ./ . . 



t Dannevig, ihid., Thirteenth, 149. 

 ^ X Holt, Scientifie Trans. Roy. Dvblln Soc, iv., v. (1891, 1893) ; Petersen, op. cit., 126 ; 

 Ehrenbaum, Eier und Larmi ron Fixclien der Dent^clien Burht (1896) ; Dannevig, Fifteenth 

 Ann. Rep. Fisherij Board for Scot/and, 175 ; Kyle, Sixteenth ihid., 225 (1898). 



