Juhani Pirhonen

Feed intake, growth rate and body composition of juvenile Baltic salmon exposed to different constant temperatures

Feed intake, specific growth rate and changes in body composition of age 1+ and 2+ Baltic salmon, Salmo salar L, were studied for fish held under constant temperature conditions. The 1+ fish (60 g) were reared for 6 weeks at 11, 15, 17, 19 or 23 °C and 2+ fish (250 g) were held at 15 °C. Feed intake of 1+ salmon increased from 176 kJ kg-1 day-1 at 11 °C to 275 kJ kg-1 day-1 at 17 °C and decreased to 229 kJ kg-1 day-1 at 23 °C. Specific growth rate increased from 1.18% day-1 at 11 °C to 1.59% day-1 at 15 °C and decreased to 0.56% day-1 at 23 °C. Optimum temperatures for feed intake and growth were estimated at 17.8 °C and 15.6 °C, respectively, and estimated upper thermal limits for feeding and growth were 29.0 °C and 24.1 °C, respectively. Models for feed intake and growth rate in relation to temperature and fish size are presented. The utilization efficiency of ingested energy decreased from 57% at 11 °C to 22% at 23 °C. For all groups of 1+ fish, most (approximately 86%) of the weight gain consisted of water. Lipid deposition accounted for about 52% of the body energy gain irrespective of rearing temperature.

Effect of low temperature on feed intake, growth rate and body composition of juvenile Baltic salmon

Feed intake, specific growth rate and changes in body composition were studied in juvenile (140-170 g) Baltic salmon, Salmo salar, reared at three temperatures (2, 4 and 6 °C) under continuous light conditions. Feed intake increased from 20.4 kJ kg-1 day-1 at 2 °C to 63.8 kJ kg-1 day-1 at 6 °C, and growth rate increased from 0.10% day-1 to 0.37% day-1 over the same temperature interval. The estimated lower temperature limits for feeding and growth were approximately 0.35 °C and 0.6 °C, respectively. Amongst fish reared at 2 °C the majority (86%) of the weight gain consisted of water and protein, and these fish deposited very little lipid. Lipid deposition increased amongst the groups of fish held at the higher temperatures, and amongst the salmon reared at 6 °C lipid accounted for 43% of the body energy gain.

Effect of prolonged feed restriction on size variation, feed consumption, body composition, growth and smolting of brown trout, Salmo trutta

Abstract 1+ brown trout ( Salmo trutta ) were reared under three different feeding frequencies from the end of May until the beginning of November. Those groups fed only twice a week for 4 h (TW) or once a day for 30 min (OD) were significantly smaller than the control fish (fed 4 h in the morning and 4 h in the evening) at the end of the feed restriction period. All the groups were fed in excess. After 2 months an increase in growth rate was observed in the TW groups. Decreased feeding frequency did not affect the size variation within a group, even though the feed was divided more evenly between the individuals in feed restricted than in control groups. At the beginning of November all the groups were switched to 4 h day −1 ad libitum feeding. That switch did not cause any compensatory growth during the winter. The decrease of the growth opportunity between the end of May and the beginning of November did not markedly affect smolting behaviour or hypo-osmoregulatory capacity during the following spring.

Movements of hatchery reared Salmo trutta during the smolting period, under experimental conditions

Abstract In an experimental flume tank, two-year-old, 20 cm Salmo trutta from both anadromous and freshwater resident stocks showed increased downstream movement, mostly diurnally, as the water temperature rose, from early May to early June, most intensely from 23 May–9 June, after which movement ceased. Water velocity was 20 cm s−1: at first the downstream speed of the fish was ca. 7 cm s−1, suggesting that the fish were resisting displacement. At the most intense movement, the fish headed downstream, at only ca. 4 cm s−1 (0.2 body lengths s−1) faster than the water. Maturing males moved significantly less than did immature males and females. Individual fish that ate more, and that grew faster, moved less. The more that condition factors increased between February and mid-May, the less the fish moved. Taken together, these findings suggest that spring smolt emigration of trout, whether or not they belong to anadromous or freshwater resident stock, is principally by passive displacement.

Feeding time, feed intake and growth of baltic salmon, Salmo salar, and brown trout, Salmo trutta, reared in monoculture and duoculture at constant low temperature

Abstract It has previously been reported that the growth of salmonids is improved when they are reared in duoculture, possibly due to reductions in the levels of intraspecific aggression. These claims were examined in more detail by studying the time of feeding, feed intake and growth in Baltic salmon, Salmo salar , and brown trout, Salmo trutta , held at constant low temperature (2.7–3°C) and reared in monoculture or duoculture for 3 months. Rates of feed intake and growth were initially low, but increased during the course of the experiment, this being particularly evident for the salmon. At the same time, interindividual variations in feed intake and growth tended to decrease, and group feed intakes were negatively correlated with interindividual variations in feed intake. Rates of growth of brown trout did not differ significantly between groups reared in monoculture and duoculture. By contrast, rates of feed intake and growth of Baltic salmon were depressed when they were held together with brown trout. Baltic salmon, held together with brown trout, had a significantly greater incidence of fin damage than those held in monoculture, possibly indicating that they were subject to aggression from the trout. Fish of both species tended to feed during the hours of daylight, and there was no evidence of an increase in nocturnal feeding activity amongst fish of either species when held in duoculture.

Influence of the length of the daily feeding period on feed intake and growth of whitefish, Coregonus lavaretus

Abstract A study was carried out to examine the influence of the length of the daily feeding period on feed intake and growth of whitefish (initial weight 60–90 g), with the aim of providing guidelines as to the feeding regimes to be used when cultivating this species. Fish were held at 14.5 °C under a 24L:0D photoperiod. Following a period during which fish were fed throughout the day, feeding regimes of 6, 12, 18 or 24 h feeding per day were imposed on 3 tanks of fish per treatment. These feeding regimes were maintained for 6 weeks during which feed intake (3 times by X-radiography) and growth were monitored. When feeding was restricted to 6 h each day daily intake tended to be initially reduced, but by the end of the experiment daily feed intake did not differ between groups of fish held on the different feeding regimes. This indicates that the fish fed according to the time-restricted regimes were able to compensate for the reduced length of the feeding period, but some weeks were required before the compensatory mechanisms took full effect. Time-restricted feeding also resulted in fish on the 6 h regime having significantly lower rates of growth (0.98 ± 0.08% d − ) than those on the 24 h regime (1.42 ± 0.14% d −1 ) early in the experiment. During the latter stages of the experiment, there were no significant differences in growth rates amongst the fish fed according to the different regimes. The differences established early in the trial were still apparent after 6 weeks, with overall rates of growth for the entire trial period seeming to be directly related to the length of the feeding period (6 h: 1.27 ± 0.09% d −1 ; 12 h: 1.39 ± 0.06% d −1 ; 18 h: 1.46 ± 0.19% · d −1 ; 24 h: 1.65 ± 0.05% d −1 ). However, the only significant differences recorded were between the 6 and 24 h feeding regime groups. Length of the feeding period was not found to have any significant effect upon feed conversion, although there was a tendency for the feed:gain ratio to be lowest in the groups of fish fed according to the time-restricted feeding regimes (6 h: 0.57 ± 0.05; 12 h: 0.58 ± 0.06; 18 h: 0.65 ± 0.07; 24 h: 0.64 ± 0.08). The results indicate that whitefish are flexible in their feeding behaviour, and adapt readily to imposed regimes over a period of a few weeks. These modifications to feeding responses result in good growth performance and good feed conversion.

Relationship between Na+,K+-ATPase activity and migration behaviour of brown trout and sea trout (Salmo trutta L.) during the smolting period

Abstract Gill Na + ,K + -ATPase activity was determined on three occasions (11 May, 30 May, 11 July) from PIT-tagged 2-year-old brown and sea trout ( Salmo trutta ), whose smolt migration was monitored in an annular flume tank. Of these three occasions, gill Na + ,K + -ATPase activity was highest at the end of May coincidentally with the highest activity of downstream migration. Na + ,K + -ATPase activity was significantly higher in brown trout than in sea trout in the end of May, but on the other two dates, there were no differences. There were no statistically significant correlations between the Na + ,K + -ATPase activity and downstream migration at an individual level. The present result suggests that even if there is a general tendency for the elevation in gill Na + ,K + -ATPase activity of two forms of S. trutta during the smolting period, this does not necessarily occur simultaneously with the changes in downstream migration behaviour at an individual level.

Do highly ornamented and less parasitized males have high quality sperm? – an experimental test for parasite‐induced reproductive trade‐offs in European minnow (Phoxinus phoxinus)

Parasites take their resources from hosts and thus directly reduce available resources for hosts’ own body functions, such as growth and reproduction. Furthermore, parasite infections cause significant indirect costs to their hosts in terms of increased investments on immune defense. In this study, we investigated the impact of parasite infection on the sperm quality and expression of secondary sexual ornamentation (saturation of the red abdominal colouration and number of breeding tubercles) in the Eurasian minnow (Phoxinus phoxinus). We exposed minnows to a high and low dose of common nonspecific fish ectoparasite, the glochidia larvae of duck mussel (Anodonta anatina) and tested whether parasite infection leads to trade-off in sperm quality and/or ornamental expression. We found that glochidia infection reduces the curvature of the sperm swimming trajectory, number of breeding tubercles, and possibly male competitive ability, but does not affect expression of male color ornamentation. Furthermore, glochidia infection was found to reduce sperm motility, but only when all the noninfected individuals were excluded from the model. Supporting one of the predictions by phenotype-linked fertility hypothesis both in high-infection and low-infection group male breeding colouration was positively associated with sperm quality. Our results suggest that although glochidia infection may have negative impact on male reproductive success, parasite-induced costs may not create strong trade-off between breeding colouration and sperm quality or that such trade-off become detectable only in resource-limited conditions.

Effect of stocking time on yield and location of recapture in two forms of brown trout (Salmo trutta) when stocked in respect to migration activity

Abstract Tagged 2-year-old trout smolts, Salmo trutta , were stocked into River Isojoki in 1996, 1997 and 1998 (a sea trout strain), and Lake Konnevesi in 1997 and 1998 (a brown trout strain) in April, May or June–July. Stocking dates were determined in respect to the migration activity of 100 PIT-tagged fish held in the laboratory. Migration activity was relatively similar in both trout forms each year. In April, movement activity was low; in May, movement activity of the PIT-tagged fish increased and in June–July, movement had ceased. There was a general tendency for lower migration activity in maturing males than for immature fish, but differences were not statistically significant every year. In sea trout, yield varied widely between years within a stocking time and between stocking times, being least in June–July (54.3 kg/1000 fish) and 77.3 and 86.3 kg/1000 fish in April and May, respectively. Brown trout yield was higher from stockings in June–July (181 and 312 kg/1000 fish) than from those in April or May (between 87 and 117 kg/1000 fish). Brown trout stocked in April or May tended to migrate downstream from the Lake Konnevesi more than did trout stocked in June–July.

What do male tench, Tinca tinca, advertise with morphological ornaments?

In contrast to females, tench Tinca tinca (L.) males have large pelvic fins with a thickened and bent second fin ray. Males also produce notable ventral protuberances during breeding, but the function of these male ornaments is not known. Using wild-caught fish, we found that both the size of pelvic fins and ventral protuberance were dependent on body mass/length ratio but not necessarily on condition. Plasma testosterone concentration and relative gonad mass were positively correlated with condition factor. Plasma testosterone concentration was not related to measures of non-specific immune function, but correlated positively with the size of pelvic fins corrected for body length. However, the studied male ornaments were not related to the measures of immune defence or to the load of Diplostomum spp. We did not find evidence for male–male dominance or female preference for large male characteristics within the natural variation in these traits. Thus, our study suggests that also other mechanisms than sexual selection on good genes might contribute to the maintenance of sexual dimorphism in tench.