Distribution of Plaice Eggs — Dornoch Firth. 11 



period of spawning. Thus the date at which any locality has been 

 visited is important, though the results of a visit at an unfavourable 

 time are no true index of the actual density of spawning within the 

 area. Again, the most favoured spawning area one year may not be 

 the most favoured in other years. As a first approximation to the 

 density of spawning within an area the highest fre(|uency got at any 

 time in the locality has been taken as the index. Only those eggs in 

 the initial stages of development need be considered since, as has been 

 indicated, eggs in the later stages of development may not have had 

 their origin within the area in which they are ultimately found. 



Some standard of measurement must be adopted to compare the 

 frequencies of one area with those in another area. In the present 

 investigations the standard vertical Hensen egg-net has not been 

 regularly used, but vertical hauls with a cheese-cloth net with a mouth 

 of one metre in diameter have been taken at all the observation stations. 

 This net, although not a standard one like the Hensen net, is nevertheless 

 a most efficient instrument for the capture of pelagic fish eggs. 



The greatest number of plaice eggs taken in a single vertical haul 

 was seventeen, obtained off Lossiemouth on the 22nd March 1906. 

 Although, comparatively speaking, the density of plaice eggs was very 

 great on this occasion, the number seventeen compares very un- 

 favourably with the numbers obtained from the quantitative egg-net 

 collections in the Southern North Sea. The Dutch and German in- 

 vestigations show that in this region at- numerous points between 

 51° 39' N. and 53° N. over one hundred eggs per square meter were 

 captured at different times between the middle of January and the 

 middle of February. Indeed, the South-west corner of the North Sea 

 has been proved to be the most important spawning area in the whole 

 Southern North Sea for plaice, and the maximum number obtained 

 at one time was 570 plaice eggs per square metre of sm'face water. In 

 the Northern North Sea only three vertical hauls contained more than 

 ten plaice eggs, and the majority of vertical hauls had either one plaice 

 egg or none. As a spawning area for the plaice the Northern North 

 Sea is, therefore, of quite minor importance as compared with the 

 Southern portion. But although this is so it is still highly desirable 

 to know which areas on the East of Scotland are the more favoured 

 ones at the spawning period. 



Another standard of comparison has therefore been adopted. The 

 number of plaice eggs in a horizontal haul of one half-hour's duration 

 with a one metre cheese-cloth net has been chosen as the easiest and 

 apparently most consistent standard. Several anomalies arise through 

 adopting this method, for it is difficult to so regulate the speed of a 

 vessel when towing a small and fragile cheese-cloth net that, in a drag 

 of given duration, a fixed volume of water may pass through the net. 

 Again, the eggs may, or may not, be uniformly distributed both hori- 

 zontally and vertically within an area. This point we shall return to 

 later. The records of the surface nets, with all their imperfections, 

 are, however, the only ones available. A table (II.) has been drawn 

 up for January, February, and March for the ten years, which shows 

 the gi'eatest number of eggs taken at one time, the average number of 

 eggs captured, and the number of hauls made at each locality. The 

 greatest frequency was obtained in the Dornoch Firth on the 27th 

 February 1906 at Station V., when 1319 plaice eggs were taken in a 



