Ultrasonic telemetry was used to follow the movements of two groups of smolts in the estuaries of two small Scottish rivers. Hatchery reared smolts released into a typical wedge flow, partially mixed estuary had movements which were dominated by the influence of tide on the direction of water flow. The net movements of wild native smolts in a two layer flow estuary, in which freshwater flow dominated, was downstream but were intermittent consisting of short steps and numerous long pauses. The hatchery reared smolts escaped from the estuary within a tidal cycle, moving out on an ebb tide. The wild smolts remained in the estuary for periods up to 108 h, none escaping within one tidal cycle.

No abstract

Closed-circuit television was used to provide a visual record ofthe events associated with the counts recorded by a resistivity fish counter. The fish seen were classified as ' large ' fish 2 50 cm length and ' small ' fish of < 50 cm. Some of the ' large ' fish escaped detection by the counter, some ' small ' fish were counted and some counts were registered when nothing was seen. The sensitivity of the counter varied inversely with water conductivity which varied inversely with discharge.Evidence was obtained suggesting that fish ascending a weir tend to swim closer to the bottom than do those descending. It was found that fish tended to move upstream just after sunset and downstream just before sunrise except when the water was turbid and when the fish were very close to spawning. The numbers of fish movements recorded increased during those periods when discharge was decreasing after a spate but when spawning activity was at its peak, increased activity occurred in the complete absence of any change in discharge. Instances of the same fish re-crossing the counting zone several times were recorded and the simultaneous passage of more than one fish through the counting zone was observed.

Histopathological studies were carried out on the pathogenesis of the lesion induced by the insertion of a light plastic identification tag, in salmon parr on the river North Esk, Scotland. The experiments were carried out at mean water temperature of 12" C, 8" C and 4" C. The inflammatory response was similar at each temperature, but the rate of its development was markedly inhibited at 4" C. The qualitative response at the lower temperature was also slightly different in that polymorphonuclear leukocytes appeared to play a slightly more dominant role and the connective tissue response, which in all cases outweighed myofibrillar regeneration was more cellular at that temperature. The tagging lesion provides a very suitable model for the study of short and long term inflammation in salmonids and studies on the modification of these defensive mechanisms by temperature.

The skin of the head of adult Atlantic salmon S. suluv L. was examined by light and electron microscopy. The general structure resembled that of most other vertebrates, but in common with other fish, the melanocytes were restricted to the papillary layer of the dermis. The mucus secreting cells were shown to elaborate a series of separate membrane bound vesicles of secretion. The basement-membrane was complex, with a system of primary, and secondary micro-papillae showing close adherence to the bases of the epithelial cells, which were modified to form a thick, electron dense basal plasma membrane. NO evidence to suggest a reason for the localization of the early lesions of ulcerative dermal necrosis to specific areas of the epithelium of the head was adduced.