This study quantified the use of cool water sources by wild 0þ, 1þ and 2þ year Atlantic salmon Salmo salar during high water temperatures (i.e. >23°C) in summer 1995 and 2004. During these events, 0þ year Atlantic salmon did not aggregate or increase in abundance in cool water sites. Interestingly, 1þ and 2þ year Atlantic salmon numbers increased in cool water sites. In addition, these older juveniles formed numerous, discrete aggregations along the plume created by a tributary with aggregation locations being similar between years. Aged 2þ year fish aggregations were at the coolest sources whereas 1þ year aggregations were in locations cooler than the main river. Fish in aggregations on average used deeper sections (average depth: 380 mm) compared with the coolest available habitat in the thermal plume (average depth: 230 mm). Hence, during high temperature events, older juvenile Atlantic salmon moved to cooler water sites and then aggregated in deeper microhabitats.

The body composition of protein and fat in Atlantic salmon Salmo salar and brown trout Salmo trutta before and after winter was investigated in a temperate, small river, normally ice covered from the middle of November until the end of March. Fat, protein and specific energy declined greatly in winter but were replenished rapidly in spring. Rates of decline were slower for the smallest fish, which also had the lowest specific content of fat, protein, and energy, while the differences in absolute amounts were greatest for the largest fish. The mean specific fat content was reduced by 45-70% during winter, relative to the pre-winter period (September). Mean daily reductions in specific enegy of the larger size groups of brown trout (3·7 10 3 kJ g 1 day 1 ) were almost half of the corresponding values for the largest Atlantic salmon (6·3 10 3 kJ g 1 day 1 ) during winter. A minor reduction in protein content was found during winter, with mean reductions of 6-10% in comparison to those in September. During spring the fat content was replenished rapidly, particularly for the smallest salmon fry (a threefold increase from April to June). Fat content in the larger salmon and trout increased by about 1·8 times. Based on estimated metabolic rates, digested energy during wintertime may contribute about two-thirds of the brown trout fry's energy demand. For Atlantic salmon, the corresponding value is about 50%. The winter period put considerable stress on the young salmonids living in lotic environments, in particular for the smallest fry with the lowest energy content before winter and the largest losses during winter. This should make the fry more vulnerable to adverse abiotic and biotic factors. 1998 The Fisheries Society of the British Isles

No abstract

Smaller virgin spawners of a stunted population of brown trout (Salmo trutta) had higher specific energy content than repeat spawners, indicating that once maturation has started, the trout does not completely recover from its first spawning. Males had higher amounts of total and specific energy after spawning compared with females (adjusted for length differences). The greatest energy expenditures were recorded among spawning females (specific total energy losses of about 20%, which were twice the losses of male spawners). Repeat females lost 40% of their fat content. The gonads of the females accounted for 20% of the total protein content. The loss in carcass protein and fat for the spawners was greater than the corresponding loss in visceral fat and protein. Survival rates paralleled the amounts of energy expended in reproduction. Overall survival rate of first-time spawning males (females) was about 0.90 (1.00), while the corresponding value for repeat spawners was about 0.65 (0.50). It is predicted that brown trout from a stunted population should make a relatively small investment in reproduction and thus represent a case of low investment among iteroparous fish species.

– Seasonal changes in protein and fat were investigated in brown trout (Salmo trutta L.) of a stunted population from a small, temperate zone lake. The lake was ice‐covered during winter (about 200 days), and hypolimnic water temperature during winter was about 4°C. During winter there was an increase in specific fat, in particular among the larger sized fish, while there was a general decrease in specific protein content among both small (<5 winters) and large (age 5 or older) trout. During winter (end of October to mid‐May) an average trout gained 32 kJ of fat, but protein energy content decreased by 14 kJ, yielding a net energy increase of 18 kJ or a daily energy gain of 0.09 kJ  ·  day−1. During the ice‐free season (mid‐May to the end of October) the trout increased mainly in protein content with daily energy gains of about 1.4 kJ  ·  day−1, a value about 14 times higher than the corresponding winter value. Trout living in lakes may store considerable amounts of fat during wintertime in contrast to the depletion of fat reserves found among stream‐living trout in the same area, which face lower water temperatures in the winter period.Note