The proportion of rainbow trout (Oncorhynchus mykiss) concealing themselves in simulated interstitial spaces was examined in the presence of surface ice, in turbid water, and in clear water. Tests were conducted in enclosures in a small Idaho stream with structures that provided five rectangular spaces varying in width and height, one circular space, and one triangular space. Space use was assessed each morning by trapping test fish inside the structures. Significantly more fish concealed themselves under clear water conditions than under either surface ice or turbid water conditions. Spaces narrower than the width of a test fish with extended pectoral fins and spaces taller than the height of a test fish with dorsal fin extended were used less than would be expected if space use was random. The frequency with which two or more fish occurred together in the same space was similar to that expected if fish occurred together at random. Fish rarely returned to the same space on consecutive nights.
1. The emergence time of Pteronarcys californica in streams in the Henry's Fork catchment, Idaho, U.S.A. was negatively correlated with mean April water temperature. Emergence was in mid‐ to late May at sites influenced by groundwater, where April water temperature averaged 7.9 °C. Adults emerged in mid‐June in streams receiving run‐off from snowmelt (mean April water temperature 5.4 °C). Intermediate emergence times were observed in a regulated section of river where water temperature was influenced, on one bank, by dam release (mean April water temperature 4.5 °C) and, on the other, by a spring‐fed tributary stream (mean April water temperature 6.3 °C). 2. During each of the three study years, emergence was earlier on the bank of the regulated section that was warmer during April and May. The mean body length of P. californica exuviae, collected from the warm side of the river, averaged 1.2 mm longer than those collected from the cold side. 3. We tested the effect on emergence of altering springtime water temperature by translocating P. californica in cages from one location to another during April. Individuals moved to sites with higher April water temperature emerged earlier than individuals that remained at the site from which they were collected.
– There has been little investigation of the winter ecology of adult trout during winter, especially in regard to concealment behavior. We compared day vs night underwater counts of adult rainbow trout and brook trout from four streams. At water temperatures between 1°C and 9°C, daytime counts accounted for 44% and 16% of nighttime snorkeling counts for rainbow trout and brook trout adults, respectively. As winter progressed, nighttime counts declined more so for brook trout than rainbow trout, but the decline was not significant for either species. Nocturnalism of both species was higher in streams with colder water temperatures. We observed few fish within concealment structure; however, by electrofishing concealment habitat during the day, we captured 10 times more adult trout than we counted immediately beforehand by snorkeling. Adult trout were concealed in cobble‐boulder substrate and woody debris during the day.Note
The behaviour of wild underyearling rainbow trout Oncorhynchus mykiss in concealment habitat in a laboratory stream aquarium at 2 and 6 C was recorded daily with an infrared video camera for 90 min over dawn. Aggressive behaviours (threat nips, nips and chases) were frequent during this time as fish entered concealment habitat. Aggressiveness varied widely among fish groups, with a range of 1 to 45 aggressive acts being initiated during a 90-min filming period. Larger fish initiated most of the encounters and removed a higher proportion of fish from concealment than did smaller aggressors. Thirteen per cent of the aggressive acts resulted in the recipient being completely removed from concealment habitat. The highest combined frequency of aggressive acts was 0·64 per visible fish per 10 min period and occurred at relatively low light levels (300 lx). The data suggest that when fish density is high, such as when habitat is limited, aggressive behaviour in winter may cause some underyearling salmonids to be excluded from concealment. 1996 The Fisheries Society of the British Isles
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