No abstract

Atlantic salmon (Salmo salar) were subjected to artificial photoperiods to determine the manner and extent of photoperiod control of the parr–smolt transformation. Exposure to continuous light (L24) at first feeding and maintained throughout the rearing period inhibited increases in salinity tolerance and gill Na+,K+-ATPase activity that occurred in spring in fish raised under simulated natural photoperiod (SNP). Fish reared under continuous light and returned to SNP in October (L24OCT) underwent normal increases in salinity tolerance and gill Na+,K+-ATPase activity, whereas those returned in December (L24DEC) underwent delayed and intermediate increases. Plasma thyroxine peaks occurred simultaneously in all groups but were diminished in the L24 and L24DEC groups. Plasma 3,5,3′-triiodo-L-thyronine levels were not affected by any photoperiod treatment. Inhibition of the parr–smolt transformation decreased the potential for growth in seawater. In spite of changes in the timing of the transformation induced by photoperiod treatment, salinity tolerance and gill Na+,K+-ATPase activity were strongly correlated; correlation between changes in salinity tolerance and plasma thyroid hormones were, by comparison, weak. The results demonstrate that continuous light applied early in ontogeny and maintained throughout the rearing period inhibits osmoregulatory changes associated with parr–smolt transformation, whereas increasing day length during winter–spring stimulates transformation.

Two newly developed microdiets for the culture of marine fish larvae were compared to the control ICES Standard Weaning diet and the traditional live foods, rotifers (Brachionus plicatilis) and Artemia nauplii, in a feeding study with larval haddock. Haddock larvae were reared from hatch to 20 days post hatch (dph) in a 5000-l tank and fed algae-enriched rotifers. Between 21 and 24 dph, 1000 larvae were transferred to each of the 20 tanks. From 25 until 45 dph, the larvae were fed five different diets: (1) B. plicatilis enriched with mixed algae species; (2) Artemia nauplii enriched with Algamac 2000 (Aquafauna-BioMarine, California, USA); (3) ICES Standard Weaning Diet; (4) IFREMER-INRA microdiet; and (5) microdiet produced by Louis D'Abramo, Mississippi State University, MS, USA.Survival was significantly higher (P<0.001) in the rotifer and Artemia nauplii treatments (mean±S.E.; 24.9±4.2% and 21.3±6.3%, respectively) than in the microdiet treatments (ICES, 2.2±1.1%; IFREMER-INRA, 4.3±1.8%; and D'Abramo, 4.0±1.2%). Survival was not significantly different (P<0.05) among the microdiet treatments. Mean larval weights were significantly different (P<0.001) between rotifer (1.61±0.12 mg) and Artemia nauplii (2.14±0.14 mg) treatments. The weights of larvae fed microdiets (ICES, 0.75±0.05 mg; IFREMER-INRA, 0.85±0.04 mg; and D'Abramo, 0.86±0.05 mg) were not significantly different from each other; however, all microdiet treatments were significantly smaller than the live feed treatments (P<0.01). There were also significant differences in the fatty acid composition of the larvae fed the different diets although no correlation was found between dietary fatty acid composition and growth or survival of the larvae. The highly unsaturated fatty acid (HUFA) 22:6n-3 showed a dramatic proportional increase in larval tissue relative to its proportional composition in both the live and formulated diets. Dietary proportions of the HUFA 20:4n-6 were similar among the Artemia, ICES and D'Abramo diets (average 1.2-1.3%); however, the proportions of 20:4n-6 in the larvae were significantly higher in larvae fed Artemia (mean±S.E.; 5.4±0.11%) compared to larvae fed either ICES or D'Abramo diets (mean±S.E.; 4.0±0.04% and 4.4±0.08%, respectively). Proportional increases of other HUFA, specifically 20:5n-3 and 22:5n-6 in larvae relative to dietary HUFA, suggest important physiological roles for these fatty acids. It appears that under the current rearing conditions, none of the microdiets examined is a suitable replacement for live feeds in the culture of haddock larvae from 25 to 45 dph and factors other than fatty acid composition must be considered.

Underyearling Atlantic salmon Salmo salar were subjected to extended day length beginning in early August or early September; the photoperiod was 18 h light and 6 h darkness (LD 18:6) during August‐October (A–O), September‐October (S–O), and September–November (S–N). A control group experienced simulated natural photoperiod. The LD 18:6 regimes stimulated growth and, by fate October, treatment fish were significantly larger than the controls. The 3‐month S–N and A–O regimes promoted growth more effectively than the 2‐month S–O regime, Stimulation of growth from August through October was not as effective as that from September through November. Thus, duration (2 versus 3 months) and time of onset of extended day length and return to simulated natural photoperiod appeared to influence photostimulation of growth. Bimodality in length‐frequency distribution in the S–O and control groups was clearly developed by mid‐December. Photostimulation of growth and continued entry offish into the upper modal groups in the A–O and S–N treatments may have prevented development of bimodality by that time, The fish from the upper modal groups under all photoperiod regimes developed high levels of salinity tolerance (to 35‰) during December. This tolerance disappeared during January–March and appeared again, first in the controls and then in the A–O, S–O and S‐N groups, in that order, during April–May. Gill Na+ K+ATPase activity was not elevated in any group during December when high salinity tolerance was observed. This finding is at variance with several previous studies that have shown positive correlations, and suggested causal relationships, between gill Na+K+ATPase activity and salinity tolerance, The rise in gill ATPase activity, corresponding with completion of smolt development in the spring, appeared first in the controls and later in the treatment groups; the S–N fish did not show increased ATPase activity by early May.

We reared Atlantic salmon (Salmo salar) in soft water (hardness 13 mg/L as CaCO3) at two pH levels, 6.4–6.7 and 4.2–4.7, from February to June, to assess the effect of low pH on survival, growth, and the smolting process under rising (4–8.5 °C) or relatively constant (9.5–10.5 °C) temperature. Survival was lower as a result of low pH (4.2–4.7) under both temperature regimes. Neither group exposed to low pH gained weight whereas both control groups gained weight during the experiment. Parr–smolt transformation, as indicated by salinity tolerance and gill Na+, K+ ATPase activity, was impaired as a result of low pH. The large (17–19 cm) parr used in this study were initially salinity tolerant and those at control pH (6.4–6.7) increased tolerance to 35‰ salinity between March and May; those in low pH became intolerant of high salinity. ATPase levels in salmon reared at low pH were significantly lower than those at normal pH levels under both temperature regimes. ATPase activity was significantly greater in fish reared at pH 6.4–6.7 with rising than with constant temperature. Plasma chloride and sodium levels were low in response to low pH, indicating impaired ionic regulation in freshwater. Plasma calcium levels were higher at low pH in both temperature regimes; higher levels were reached under constant temperature. Moisture content rose less sharply under low than under control pH in both temperature regimes. In the rising temperature regime, lipid levels reached similar, low levels under low and control pH conditions. Thyroid hormone (T3 and T4) levels gave no clear indication of effects of low pH on smolting. Smoltification did not proceed normally in our Atlantic salmon subjected to low pH levels.