of the Fishery Board for Scotland. 



51 



' and Alloa the difference in salinity between bottom and surface water is 

 1 about equal to that at the same level between stations from 4 to 6 miles 

 ' apart. For instance, bottom water at Grangemouth is about as salt as 

 ' surface water at Blackness, and bottom water at Blackness as salt as 

 ' surface water midway between Queensferry and the Oxcar Rock. Seaward 

 ( of Inchkeith the difference between bottom and surface water is very slight 

 ' indeed.' 



The influence of the tide on the salinity of the Firth of Forth may be 

 described as follows : — 



* From the mouth of the Firth to Inchkeith the result is simply a 

 ' to-and-fro movement of the water without any very apparent consequences, 

 1 as the salinity differs little from point to point. Above Inchkeith the 



* to-and-fro motion gradually changes into one of shearing. The tidal 

 1 water, from its superior specific gravity, tends to pass under the lighter 

 ' brackish water of the estuary, and to push its way below the downward 

 ' moving stream of the river. After some time the river current slackens, 

 ' then stops, and finally turns, mixture with the salt water becoming more 

 1 complete.' 



Although, when selecting the trawling stations, we considered chiefly 

 which portion of the Firth would show best the results of limited protec- 

 tion, they all happen to lie in water having practically the same salinity, 

 with the exception of No. IV. (Plate I.), over which the water has an 

 average salinity of 3 "300 per cent, while at the other stations the salinity 

 does not vary more than 0'050 from 3 '400 per cent. 



However sensitive ordinary fish may be to changes in salinity, they are 

 apparently still more sensitive to changes of temperature. The tempera- 

 ture of the Forth varies considerably. The southern margin is very much 

 warmer than the northern in summer, and very much colder in winter. 

 This variation of temperature is more marked at Station IV. than any of 

 the others, the cold being more intense in winter and the heat greater in 

 summer. During spring the temperature of both the river and firth from 

 side to side and from surface to bottom is nearly the same, varying in the 

 years already noted from 40° to 43° F. As the season advances the water 

 gets rapidly warmer, 'the surface heating more rapidly than the lower strata, 

 ' and the river more rapidly than the sea. Consequently, a typical summer 

 ' distribution is arrived at, in which there is a continuous gradient of 

 ' temperature from river to sea, and from surface to bottom. The tempera- 

 ' ture at Alloa in August may be about 60°, that at the Isle of May 55°, 

 s and the bottom water from 3° to 2° colder. After the autumnal equinox 

 ' cooling sets in, and this is most rapid in the landward reaches and on the 

 ' surface, slowest at sea and on the bottom. As a result, a state of 

 ' uniformity is reached in October or November, with a temperature about 



* 50° from river to sea and from surface to bottom ; then, as cooling goes on, 

 1 the river becomes coldest, and in January or February the minimum is 

 ' reached, and there is a uniform rise of temperature from river to sea and 

 ' from surface to bottom. As spring advances heating ensues, and once 

 ' again a uniform temperature is attained.' 



' Considerations of salinity and also of temperature lead to a distinction 



* being drawn between various parts of the Firth. Between Inchkeith and 

 ' the sea there is a constancy of salinity and of temperature from point to 

 1 point, and from surface to bottom, and from high to low tide, that stands 

 ' sharply in contrast with the region between Inchkeith and the river, 

 ' where there is rapid change in all conditions from many causes.' These 

 two very different parts of the Forth Dr Mill proposes to call the Firth 

 proper and the Estuary. Inchkeith may be taken as the boundary between 

 estuary and firth, while the line dividing firth from sea lies somewhere 



