From an examination of over 20 yr of data from the Northwest Miramichi River and some additional data from small tributaries to the Nashwaak River, highest densities of 100 underyearling and 80 yearling or older Atlantic salmon (Salmo salar) per 100 m2 were found at sites where water velocities averaged 50–65 cm/s. At sites with lower or higher water velocities maximum observed densities decreased. Experiments in laboratory streams demonstrated that underyearling Atlantic salmon < 7 cm (total length) occurred in shallow (10–15 cm) pebbly (1.6–6.4 cm diam) riffles of natural streams by choice. As they grew they began to prefer deeper (> 30 cm) riffles with boulders (> 25.6 cm diam). Yearlings > 10 cm reduced the numbers of underyearlings < 6 cm in these deeper habitats by chasing them, and occasionally by catching and eating them. Social interactions, such as displays used in territorial defence, did not occur between yearlings and underyearlings until the latter exceeded 6.5 cm, the size at which they began to move to deeper riffles. Planting densities for hatchery-reared salmon recommended in the literature were refined, taking the space and habitat requirements of different-sized juvenile salmon into account. Key words: spatial relations, rearing capacity, stocking density, stream ecology

Aggressive behaviour of Atlantic salmon (Salmo salar) parr, tested in groups of eight, was greater during 3 days of deprivation of food than during 3 days of feeding. Frontal and lateral displaying appeared to increase more than charging and nipping. I.rcreased agression was only partly a result of increased locomotion, and therefore was partly a direct effect of hunger.Strong social hierarchies developed, larger fish nipping smaller ones more than the reverse. The correlation between size and hierarchical status was usually sufficient to account for the strength of hierarchy observed.Upon deprivation of food the dominant of any pair of socially unequal fish on the average increased its nipping of the subordinate. The nipping rate of dominanis by subordinates did not change consistently; many low ranking fish decreased their nipping rate upon deprivation. These results would strengthen hierarchies during deprivation. Initiation of attacks by small fish, as opposed to nipping, increased consistently during deprivation.Nipping was more frequent between fish of equal status and size than between fish of unequal status or size. This could have accounted for the strength of hierarchies in groups in which the correlation between size and position was insufficient to do so.The increase in aggression upon food deprivation could function to increase the size of feeding territories when food is scarce. This and the strengthening of hierarchies would cause emigration of some fish from the area. Concentration of aggression between fish of equal size and status could permit parr of different ages to coexist.

Smolt production at different egg deposition densities is estimated from data on survival rates and space requirements of juvenile Atlantic salmon (Salmo salar) reported in the literature. Average maximum production of smolts is estimated to be approximately 5/100 m2 for 2+ smolts, 2/100 m2 for 3+ smolts, and 1/100 m2 for 4+ smolts. Minimum egg depositions recommended for production of these numbers of smolts are 220/100 m2, 165–220/100 m2, and 80/100 m2 for each age of smolts, respectively. The escapement of adults required to produce these depositions must be estimated from observed average weights of returning females and a reported fecundity of Atlantic salmon between 1650 and 1760 eggs/kg of female. With the exception of Ungava rivers, average smolt age in any particular river can be estimated from the number of days per year on which water temperature reaches or exceeds 7 °C. Key words: fishery resources, fishery management, production (biological), escapement, survival, game fish, freshwater fish, rivers

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Brook trout (Salvelinus fontinalis) ate 26% of 58 juvenile (7–11 cm) Atlantic salmon (Salmo salar) introduced into an artificial stream 24 h before the trout, but they ate only 8% of 60 salmon which had been present long enough to establish territories. Man-made environmental alterations may decrease territorial behavior of young salmon thereby increasing their vulnerability to predation.

When wild juvenile (parr) Atlantic salmon (Salmo salar L.) caught by electrofishing, and an equal number of hatchery-reared parr, matched for size with the wild ones, were released at three sites in unfamiliar streams containing resident parr, more hatchery-reared than wild parr could be observed by skin-diving in the areas 1 and 2 weeks later.Observed mortalities of wild parr were not sufficiently higher than those of hatchery parr to explain this result. Nor could a higher proportion of wild parr be found hiding compared with hatchery parr when one release site was electrofished a week after the release. Wild parr were found in greater numbers at points farther from one of the release sites than were hatchery-reared parr, and also more wild fish passed through a salmon counting fence approximately 100 m from a fourth release site than did hatchery-reared parr.This greater dispersal of wild parr from the release site compared with hatchery parr has important consequences to estimates of comparative survival between the two stocks.

Exposure of young Atlantic salmon (Salmo salar) to 1.0 ppm fenitrothion for 15–16 hr caused a 50% decrease in the number holding territories 6 days following treatment. Some severely affected fish also swam stiffly and ceased feeding, but these effects disappeared within 48 hr following return to clean water. Territories were not reclaimed for approximately 2–3 weeks. Exposure to 0.1 ppm fenitrothion for 15–16 hr caused a lesser (20%) reduction in numbers of fish holding territories.When mealworms (Tenebrio sp.) injected with 2–5-μliters pure (100%) fenitrothion were force-fed to young salmon, 50% were regurgitated 8–12 hr afterwards. Almost all mealworms containing 10–20-μliter fenitrothion were regurgitated. The proportion of worms regurgitated remained constant during a week of daily force-feedings but, 24 hr after the third or fourth feeding, all fish except controls could be made to flex tetanically by rapping on the aquarium, and they made little attempt to escape a dipnet.