of the Fishery Board for Scotland, 



75 



APPENDIX V. 



EXPERIMENT TO TEST THE LENGTH OF TIME SEA-LICE 

 REMAIN ATTACHED TO SALMON IN FRESH WATER. 



When a salmon is captured in fresh water and is found to have 

 attached to it the marine crustacean ectoparasites, commonly called sea- 

 lice or tide-lice (Lepeoptheirus Stromii (Baird) ), the salmon is, with reason, 

 regarded as having recently left the sea. When, therefore, a salmon is 

 caught in the upper waters of any of our rivers, with sea-lice attached, the 

 ascent from the sea is considered to have been rapid. Salmon with sea- 

 lice have, I understand, been taken in the upper waters of theTay, in the 

 Tummel, and even in the Garry and Lyon. In like manner sea-lice 

 have been noticed on fish taken in the Moriston and Oich, above Loch 

 Ness. It appears, however, that while, as we know, spring fish run 

 slowly in cold water, and summer fish run rapidly in warmer water, 

 some confusion is likely to occur if we apply the sea-lice test to either 

 class of fish indiscriminately when estimating the rate of ascent. And 

 further, it is more than probable that in cold water the sea-lice remain 

 attached to the fish longer than they do in warm water. I have been 

 informed, for instance, on good authority, that in the cold rivers of 

 Lapland sea-lice are found on fish which have ascended certainly several 

 days' journey from the sea. 



It seemed desirable, therefore, to ascertain by actual experiment the 

 time the parasites remained attached to salmon placed under observation 

 in a tank containing water which was gradually changed from salt to 

 fresh. The fact that the fish experimented with were in confinement 

 in comparatively still water no doubt detracts somewhat from the 

 value of the observations, but this drawback was inevitable. Through 

 the kindness of Dr. Fulton, arrangements were made at the Board's 

 Marine Laboratory at the Bay of Nigg, near Aberdeen, for carrying out 

 the experiments, and the actual observations were made by Dr. 

 Williamson. As the experiment was conducted in summer it must be 

 regarded as a guide during summer temperatures only. 



I am indebted to Dr. Fulton for the following notes on the experi- 

 ment : — 



The tank in which the experiment was made had the following 

 dimensions : — Length, 5 feet 6 inches ; breadth, 4 feet 4 inches ; height, 

 3 feet 6 inches. The height of water was about 28 inches ; the 

 bottom of the tank was painted white, and the plate-glass front was 

 covered up with canvas. The top was left uncovered, and a considerable 

 amount of light reached the water. 



Two grilse were obtained from a bag net in the Bay of Nigg on 2nd 

 July; they were both in good condition, one weighing 4 lbs. and the 

 other 5 lbs. approximately. Each fish had attached to it a number of 

 sea-lice. They were put together into the tank described, which con- 

 tained sea water, on the morning of the day mentioned at 10.30. The 

 specific gravity of the water was 27*6° F., and the temperature 52*9° F. 

 (1T6 0 C). The taps supplying sea water and also fresh water were 

 arranged, as previous experiments had determined, so as gradually to 

 reduce the density by a greater and greater proportion of fresh water, 

 and ultimately the tank contained fresh water alone. The particulars 

 in regard to the density and temperature of the water at different 



