Cardiovascular responses of eggs, embryos and alevins of Atlantic salmon and rainbow trout to nitric oxide donors, sodium nitroprusside and isosorbide dinitrate, and inhibitors of nitric oxide synthase, N-nitro-l-arginine methyl ester and aminoguanidine, following short- and long-term exposure

Atlantic salmon embryos and alevins Salmo salar that had been exposed to isosorbide dinitrate (ISDN) for 4 weeks, on transfer to fresh water, showed an increase in heart rate. Unexposed embryos and alevins showed a decrease in heart rate following transfer to 100 μmol l⁻¹ ISDN for 4 h. This is in contrast to adult rainbow trout and higher vertebrates where tachycardia occurred in response to nitric oxide (NO) donors. The decreased heart rate in response to ISDN was inhibited by 2 mg 1⁻¹ methylene blue, indicating that NO activates cardiovascular events via guanylyl cyclase and cyclic guanidine monophosphate. Heart rate of rainbow trout alevins Oncorhynchus mykiss exposed to 100 μmol l⁻¹ aminoguanidine responded with a slowly developed but significant bradycardia over 10 min as did those reared in aminoguanidine for 4 weeks then transferred to fresh water. A potentiated increase in heart rate on exposure to the NO donor sodium nitroprusside (SNP), occurred within 1 min in salmon alevins reared in l -nitro-arginine methyl ester ( l -NAME) for 4 weeks, indicating up-regulation of NO receptors. The evidence for down-regulation of SNP-reared alevins exposed to l -NAME was less well defined. The results suggest that both salmonid embryos and alevins have a functional l -arginine-NO pathway and that NO has a physiological role in control of cardiovascular events.     

Predicting fluctuations in the size of newly emerged sea-trout fry in a Lake District stream

The objective was to predict interannual fluctuations in the size of sea-trout fry when they emerged from the redd, using models developed from field data for 70 excavated redds (≥three per year), and from experimental data on egg and alevin development at 30 constant temperatures in the laboratory (range 1·5—10·5) C with 100 naturally fertilized eggs at each temperature). Egg weight increased with female length and also with the number of eggs laid in a redd, both relationships being well described by a power function. Early spawners were the largest females laying the largest and most numerous eggs, whilst late spawners were the smallest females laying the smallest and least numerous eggs, with middle spawners being intermediate between these two extremes. Mean values for egg weight and number of eggs per redd were obtained for these three groups. The numbers of early, middle and late spawners for each year of a 30-year study and the mean values from the excavated redds were used to estimate weighted means for the number of eggs per unit area and egg weight. Mean values varied considerably between years (30-year ranges: 518–7964 eggs per 60 m2; 112–138 mg wet weight). In the laboratory, mean weights of newly hatched alevins and newly emerged fry were both related positively to mean egg weights. Alevin and fry mean weights were independent of the number of days required for 50% of the eggs to hatch or fry to emerge. Models described in a previous paper formed the basis of those used to predict fry weights over the emergence period. Model predictions were validated by field data for the whole emergence period in 8 years (1967–1971, 1974, 1975, 1980), and by pre-fry weights on single dates in 21 years (1967–1987). As pre-fry densities on these single dates were very similar to egg densities for the same year class, mortality in the egg and alevin stages was very low. The chief objective was therefore fulfilled, and the extent of interannual fluctuations for the 30-year study showed some variation in mean fry weight (30-year ranges: 153–193 mg for both the whole emergence period and the date on which 50% of fry emerged) but a progressive decrease in fry weight through the emergence period. Possible reasons for this variation are discussed, and it is concluded that the size of the female spawners is the dominant factor.     

Initial feeding in migratory brown trout (Salmo trutta L.) alevins

Initial feeding of brown trout was investigated under laboratory conditions. Fifty per cent feeding occurred when yolk constituted approximately 31% of total alevin dry weight, and feeding rate was positively correlated to developmental stage. The possible ecological implications of initial feeding are discussed.     

First access to territorial space and exposure to strong predation pressure: A conflict in early emerging Atlantic salmon (Salmo salar L.) fry

Summary This work aimed to elucidate conflicting factors that may explain the narrow and synchronous emergence in salmonids. Fry are highly vulnerable to predation and stand a better chance of surviving if they emerge synchronously. On the other hand, fry that leave the gravel first should increase their chance of obtaining one of the limited number of feeding territories. The risk involved in early emergence for Atlantic salmon fry was evaluated by exposing them to predatorySalmo trutta. Yolk sac alevins were incubated in an artificial redd in order to catch them by their time of emergence. Early-, peak- and late-emerging fry were then successively marked and transferred to flume tanks in which the predators were either present from the start or not introduced until all fry had been added. When the predators were initially present, the predation pressure differed depending on the time of fry emergence, resulting in survival rates of 11.6, 44.9 and 51% in early-, peak- and late-emerging fry, respectively. By assuming that the predation rate of the three emergence groups was dependent both on time of emergence and fry density the survival rates were calculated to be 7.5, 41.1 and 53.5% in groups I, II and III, respectively. These figures corresponded well to the observed rates. When the predators were added after completed emergence the resulting survival rates were 56.9, 39.7 and 25.2% in early-, peak- and late-emerging fry, respectively. Thus, predation after complete emergence gave a survival probability that varied across the three emergence-date groups, despite being exposed to a predator during the same number of days. The presence of fish predators in combination with a limited territorial space seems to make both early and late emergence hazardous and favours synchronous swimming movements.     

An individual-based model for predicting the emergence period of sea trout fry in a Lake District stream

The objective of this study was to predict interannual fluctuations in the emergence period of sea trout fry, using models developed from field data for 70 excavated redds, and laboratory data on egg and alevin development at 30 constant temperatures (range 1·5–10·5° C with 100 naturally fertilized eggs at each temperature). Egg weight and numbers per redd both increased with female length; a power function described the relationship. Early spawners were the largest females laying the largest and most numerous eggs, whilst late spawners were the smallest females laying the smallest and least numerous eggs; middle spawners being intermediate between these two extremes. Mean values for egg weight and numbers of eggs per redd were obtained for these three groups. Hatching and emergence times in the laboratory decreased with increasing temperature. Of five models tested for hatching time, the best fit was provided by a three-parameter hyperbolic model which formed the asis of the individual-based model used to predict egg hatching and fry emergence. Model development was described in detail and the final equations predicted the times taken for 5, 50 and 95% of the fry to emerge, and hence the period over which 90% of the fry emerged. Analogous models were obtained for egg hatching. All models were excellent fits to the laboratory data. Hatching times for eggs kept in perforated boxes in the stream were almost identical to those kept at similar mean temperatures in the laboratory. Model predictions of fry emergence times were validated by field data for 8 years (1967–1971, 1974, 1975, 1980). The chief objective was therefore fulfilled, and predictions for the 30-year study (1967–1996) revealed a large variation in the timing of emergence (extremes: 11 March–4 April 1989, 15–20 May 1979). Most of the variation in median emergence date was due to variations in water temperature, with spawning dates as a secondary factor; the latter, however, had a greater effect on the length of the emergence period.