Seasonal variation in trypsin activity in juvenile Atlantic salmon upper and lower modal groups

Juvenile salmon in their first year of growth showed a bimodal distribution of body lengths by December. For experimental purposes samples of fish from the upper 2% of body lengths were taken as representing the upper modal group (UMG), whilst fish from the bottom 5% of body lengths were taken to represent the lower modal group (LMG). The population of fish from which the samples were taken were fed ad libitum from December to July. During the winter months to March, neither group increased in weight. Growth resumed between March and July. The LMG fish had a very low food intake, as indicated by the relative weight of digesta in the stomach, in the winter months. However, following resumption of feeding, the relative weight of stomach digesta of the LMG fish exceeded that of the UMG fish between May and July. The activity of trypsin in the intestinal digesta followed a similar pattern, the LMG fish showing a higher trypsin activity in the spring months. Starvation of UMG fish for 5 days in winter resulted in accumulation of trypsin in the pancreatic tissues, whilst injection of the trypsin releasing hormone cholecystokinin (CCK) into starving UMG fish resulted in reduction of trypsin in the secretory tissues. CCK also caused reduction of trypsin in the pancreatic tissues of LMG fish, suggesting that the pancreas of this group is potentially fully functional during the winter period. Ultrastructure studies of the pancreatic acinar cells showed evidence for lower secretory activity in the LMG fish, as indicated by smaller numbers of zymogen granules, less well developed Golgi systems and a smaller number of active secreting cells. It appears that trypsin secretion by the pancreas in LMG fish is at a low level during the winter, in response to the reduced amounts of food passing through the gut, which is ultimately controlled by changes in food intake, lowered metabolic level and lowered appetite levels.     

Choice of feeding station in Atlantic salmon, Salmo salar, parr: effects of predation risk, season and life history strategy

The current speed at which underyearling salmon parr held feeding stations was examined from late summer to early winter in laboratory flume tanks that offered a choice between (a) areas with high water flow, high food availability but high predation risk and (b) areas with low flow, little food but shelter from predators. In August, those fish that would become smolts aged 1 + (and which by late winter formed the upper modal group, UMG, of the bimodal size distribution) adopted positions in faster currents than did the fish which would take a further year to reach the smolt stage (the lower modal group, LMG). However, the chosen current speed of UMG fish decreased through the period of study, so that by December all fish were found in areas of low flow, and hence little food. Both date and water temperature had independent effects on the chosen current speed of UMG fish.
The effect of predation risk was investigated using a model trout. A brief sight of this predator caused 47% of fish that had been in the main, exposed currents to move to slacker, sheltered areas; they took 1 h, on average, to return to their previous position. The fish that remained in position upon seeing the predator reduced their rate of tail beating, presumably increasing crypticity. Eventual UMG parr were less likely than were LMG fish to move away upon seeing a predator. The fish moved to faster currents than normal 2–3 h after seeing the predator, possibly compensating for the earlier reduction in feeding rate.     

Effects of light level and growth history on attack distances of visually foraging juvenile salmon in experimental tanks

The effects of light level, developmental pathway, and previous growth history on the foraging attack distances of juvenile Atlantic salmon Salmo salar were examined in circular rearing tanks. Former manipulation of growth rates had no significant influence on distances moved to intercept food items despite the fact that it caused substantial differences in post-treatment growth. Attack distances of fish that were entering a state of overwinter dormancy (lower modal group; LMG) were shorter than those of actively feeding (and growing) fish (upper modal group; UMG). These differences were explained generally by differences in body size between the two groups, suggesting that actual effort per attack was unassociated with growth requirements. Significant differences between growing and non-growing fish in attack distances could contribute to the variation in growth rates through their effect on feeding rates, but were unlikely to have affected energetic costs. This may be due to the fact that attack distances were consistently short throughout the study period as is also evident from the pattern of change between night and daytime. Whereas in the first experiment (daylight v . twilight) fish moved further to reach food during the day, in the second (daylight v . overcast night) nocturnal attack distances matched (LMG fish) or exceeded (UMG fish) diurnal attack distances. Thus diurnal attack distances were probably minimized in the second experiment. These results are interpreted within a framework of overwintering strategies.     

Feeding motivation and response to predation risk in Atlantic salmon parr adopting different life history strategies

Feeding intensity was measured before (baseline level) and after (disturbed level) brief exposure to a potential predator in groups of 0+ Atlantic salmon, Salmo salar. parr destined to follow either a fast-growth, early-smolting life history (upper modal group or UMG fish) or a slow-growth, late-smolting life history (lower modal group or LMG fish). Feeding intensity decreased following exposure to the predator. While the absolute decrease in feeding intensity is constant regardless of baseline level, the proportionate decrease is negatively related to pre-presentation feeding intensity. Parr that are strongly motivated to feed thus maintain a higher food intake and incur greater risks when foraging in the presence of a predator. No differences in baseline or disturbed feeding intensity of UMG and LMG parr were found up to September of their first year. Previous work has shown that baseline feeding intensities decrease in LMG fish after September but increase in UMG fish. In the present study, these differences were reflected in greater disturbed feeding intensities in UMG fish, but fish destined to follow different life history patterns do not differ in risk-taking, once these differences in baseline feeding motivation have been taken into account.     

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.     

Otolith and somatic growth rates in Atlantic salmon parr, Salmo salar L: evidence against coupling

It is often assumed that otolith growth is in some way dependent on somatic growth (i.e. that the two processes are coupled). We examined the relationships between sagitta radius and fork length in 0+ Atlantic salmon parr that would subsequently smolt aged 1 + (UMG fish) or 2+ (LMG fish). Repeated measurements of fork lengths of individually marked parr, taken over a 211-day period from first feeding, were compared to sagitta radii on the same measuring dates (obtained by analysis of daily increments). The results showed that there was a linear relationship between fork length and otolith radius in UMG parr. However, this was not true for LMG parr. These fish enter a state of natural anorexia in their first autumn (despite excess food), but their otoliths continued to grow at the same rate despite the virtual cessation of somatic growth; they had therefore developed disproportionately large otoliths by the end of the study period. The relative growth rates of soma and otoliths first changed in LMG fish in late July/early August; this is the most precise estimate yet obtained of the timing of divergence in the developmental pathways of UMG and LMG parr. The rate of sagitta accretion was consistently lower in LMG parr, possibly indicating a lower metabolic rate in these fish. The results are discussed in relation to previous theories of the relationship between otolith and somatic growth.     

A note on growth and mortality of trout, Salmo trutta L., in afferent streams of Windermere

Trout were sampled in six afferent streams of Windermere during the spawning season. The main migration of adult trout into the streams took place during early November. Length-weight relationships and scale-fish length relationships are given. There was no significant difference in length for age or sex between cohorts within streams. Three groups of fish could be identified depending on whether the fish entered the lake at 1+, 2+, or 3+ years. The percentage number of the population entering the lake at each of these ages was 16, 70 and 14%, respectively. After the fish had migrated from the streams their growth in length fitted the von Bertalanffy growth model. Fish entering the lake at 1+ and 2+ years had similar A.95 values (6·31 and 6·97 years) as the latter grew slightly faster in the lake. Those entering the lake at 3+ years grew at a slower rate to the other two groups but achieved a longer ultimate length. Faster growing fish had higher instantaneous mortality rates. Twenty-eight to 29% of the males remaining in the stream at 1+ years became sexually mature. Faster growing females became sexually mature earlier. Only those fish which entered the lake at 1+ years grew close, during their early years before sexual maturity, to the theoretical maximum given by Elliott's model.     

Microsatellite allelic heterogeneity among hatchery rainbow trout maturing in different seasons

Allele frequencies were determined at 14 microsatellite loci in 284 female and 50 male rainbow trout that were sampled throughout the spawning season from a commercial trout farm. Phenotypic selection has expanded the spawning season of the broodstock from 2 weeks to 8 months. Females maturing in different seasons showed significantly different allelic distributions (P<0·001) at all loci. The spawning time for the majority of females sampled could be predicted based on their genotypic information [chromosome segment sharing coefficient (CSSC) values]. CSSC analyses assigned 100, 56, 76 and 68% of summer, fall, winter, and spring spawning females, respectively to the season from which their gametes were actually collected. Alternatively, only 38 and 14% of summer and spring spawning XY males respectively, were assigned to the correct season. Loci linked to thermal tolerance and spawning time quantitative trait loci (QTL) showed significantly greater heterogeneity (higher average D_s values) in allele frequencies than those not known to be linked to QTL based on previous work. Thus, phenotypic selection for spawning time has led to concomitant changes in allele frequencies at markers of QTL. This suggests that the QTL detected in our previous work have detectable effects in fish from other genetic backgrounds.     

Allozyme heterozygosity, date of first feeding and life history strategy in Atlantic salmon

The relations between allozyme heterozygosity, relative date of first feeding and life history strategy in juvenile Atlantic salmon Salmo salar were examined using eggs obtained from a 400 family cross (20 male × 20 female adult Atlantic salmon). Multilocus heterozygosity, through its positive associations with the timing of first feeding and growth rate, was correlated with life history strategy in juvenile Atlantic salmon, albeit under genotype × environmental (temperature, food availability) regulation. Under hatchery conditions, a 10 day difference was observed in the relative date of first feeding between early and late first feeding Atlantic salmon. Early first feeding Atlantic salmon exhibited a significantly higher mean heterozygosity, grew faster at ambient water temperature (April to November) and a significantly higher proportion adopted the early freshwater maturation (age 0+ years, male fish) or early migrant (age 1+ years, mainly female fish) strategies compared to late first feeding Atlantic salmon. Elevated water temperatures over the winter (December to April, >10·5° C) provided additional growth opportunity allowing previously mature male parr (mainly early first feeders) and lower modal group parr (mainly late first feeders) to adopt the early migrant strategy by the following spring.