of the Fishery Board for Scotland. 



109 



On the Number of Pelagic Eggs in Loch Fyne. 



In this part of the research Upper Loch Fyne only will be considered. 

 Upper Loch Fyne is cut off sharply from the rest of the Clyde by its 

 narrow entrance at Otter, 15 fathoms in depth. It is thus possible to 

 consider it as, to some extent, an independent piece of water, and the 

 data given by observations and experiments confined to this restricted 

 area are perhaps more likely to lead to definite results on certain points 

 than comparatively few observations scattered over a larger region. 



In order to determine the number of eggs in Loch Fyne two methods 

 were employed, viz. : — By means of nets towed horizontally, and by 

 vertical hauls. Neither of these are other than approximately accurate. 

 On each station seven nets were towed horizontally for a distance usually of 

 one quarter-mile, at various depths below the surface. The upper edge 

 of the ring of the top net was as far as possible on a level with the 

 surface. The other six nets were at depths of 1, 2, 4, 6, 10, and 15 

 fathoms respectively. Each tow-net had a mouth 2 feet in diameter 

 (radius | yard), and it may be here taken for granted that it filtered, in 

 travelling a quarter-mile, a column of water equal to the superficial area 

 of its mouth multiplied by \ mile, i.e., = f 2 X J- X 506§ cubic 

 yards. 



The mile here referred to is a sea mile, which equals 2026f yards. 



i. The volume of water filtered by 7 nets = 7 x v 2 x 1 x 506:} 

 cubic yards. 



There are four stations (II., III., IV., and V.) in Upper Loch Fyne. 

 Therefore the total volume of water filtered by the fleet of seven nets in 

 working the four stations once = 4 x 2 7 2 x ^ x 506f cubic yards — 

 4954-^ cubic yards. 



ii. The water area of Loch Fyne at low water = 28*5 square sea 

 miles. The experiments were carried out in the upper stratum of water 

 15 fathoms in thickness. The volume of water sampled at the four 

 stations is equal to the area x depth. 28 - 5 miles square = 117.060,266f 

 square yards. The volume of water sampled = 1 17,060,266^ x 30 

 cubic yards. 



In order to obtain the number of eggs in the loch, the total number of 

 eggs obtained on the four stations is to be multiplied by a coefficient 

 which is found by dividing the volume of water in the upper stratum of 

 15 fathoms by the volume of water filtered in obtaining these eggs. 



ul The Coefficient = 117 f 6 t X 30 = 708,87272 



4954/ T 



Two errors are introduced into the above calculation — (1) It is assumed 

 that all the water which the net meets passes through it, but that is not 

 the case. The cloth of the net in this case is of fairly open material 

 (see p. 80), and the quantity of water which flows over the edge of the 

 mouth is probably comparatively small. This error, however, has the 

 effect of giving too low an estimate. (2) The second error is incurred by 

 taking the whole surface area of the loch instead of only that portion of 

 the area beneath which there is a depth of at least 15 fathoms. But as 

 Loch Fyne is very deep, even close up to the edge, and since the eggs 

 were for the most part obtained from depths of six fathoms and less, a 

 depth which is found very near tlie shore, the error is probably a small 

 one. This error has the effect of magnifying the result. The errors 

 numbered 1 and 2 will therefore to a large extent neutralise one another. 

 A few of the tow-net collections have been lost, and allowance will be 

 made for these in the calculations. 



