Distribution of Plaice Eggs — Dornoch Firth. 



II 



were spawned earlier in the same locality since there is a fall in the 

 absolute density of the eggs even as early as February. Such facts 

 show how difficult it is to map out the spawning areas of a species from 

 the occurrence of eggs in all stages of development in the later half of 

 the spawning season without an intimate knowledge of the influence- 

 of the currents on the earlier spawned eggs. 



In the spawning season 1904 1905 similar observations were made 

 in all four months, and the series gives very consistent results. Hori- 

 zontal hauls of one half-hour's duration were taken with one metre 

 cheese-cloth nets at the following depths : surface, 5 metres, 10 metres, 

 mid-water, and bottom. In addition, horizontal hauls of one half- 

 hour's duration were made with the Petersen Young Fish Trawl at 

 the surface, mid-water, and bottom. The total numbers of eggs at 

 the different stages of development obtained in these hauls for each 

 month make excellent index numbers for a comparison of the relative 

 densities from month to month. 



Dornoch Firth — Spawning Season 1904-1905. 

 combined results op horizontal hauls. 



The results for this single spawning season agree very closely with 

 the averaged results of the area over the ten years' period. The fre- 

 quency is highest in January and diminishes towards March, and this 

 diminution is accompanied by a decline, both relative and absolute, 

 in the number of eggs in the initial stages of development. This 

 decline was very marked in the spawning season 1904-1905, thus showing 

 that spawning had not only begun early that season but was also to 

 close earlier than on an average season. If the later developmental 

 stages be considered separately it is seen that the maximum frequencies 

 of the " |3 " and " y " stages occurred in February, whilst the maximum 

 frequency of those eggs in the " 5 " stage did not occm- until March or 

 afterwards. Although there are maxima for eggs in the later stages of 

 development, the total numbers which are indices of their density are 

 very far from those of the newly spawned eggs at maximum frequency. 

 The spawning area of a species may be somewhat restricted for obvious 

 reasons, but as pelagic eggs advance in development there is a progressive 

 expansion in their area of distribution. Within the Dornoch Firth, 

 however, a certain percentage of the eggs remain within the area during 

 the whole com-se of their development. 



