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Foraging behaviour of common mergansers (Mergus merganser) and their dispersion in relation to the availability of juvenile Pacific salmon Wood, Christopher Charles


Common mergansers (Mergus merganser) can inflict serious mortality on salmonid populations when abundant on salmon and trout-rearing waters. I examined the foraging behaviour and dispersion of mergansers to determine when, and where, aggregations of these ducks are likely to occur on salmonid streams on eastern Vancouver Island, B.C. First, I investigated the effect of fish availability on a merganser's hunting performance and its choice of feeding site by stocking enclosed sections of a natural stream with various densities of juvenile coho salmon (Oncorhynchus kisutch). Coho smolt with access to bank cover or with previous exposure to mergansers were less vulnerable to predation. Merganser foraging success was not significantly affected by flock size for flocks ≤ 25. A type II functional response was observed under all experimental conditions. Although 40 g coho smolt were selected over 2 g coho fry, the discrepancy in capture frequency can be explained by differences in conspicuousness due to size without inferring preference. A merganser's daily food requirement (approx. 400 g) can be satisfied at smolt densities of 2 - 30/100 m² depending on the availability of cover and previous exposure to attack. Mergansers spent more time searching in the more profitable enclosures. The frequency of visits to the enclosure site was influenced by previous foraging success and the size of flocks already present. The probability of departure from the site was generally independent of flock size but decreased with increasing profitability. Second, I examined changes in the abundance of mergansers on 5 neighbouring streams during the spring and summer months of 1980 - 1982. Mergansers congregated on hatchery streams following fish releases and reciprocal trends in abundance were evident among streams. At least 9 of 13 mergansers resighted after being marked and released on the Big Qualicum R., visited other nearby streams. Overall, merganser abundance declined steadily from March through June but increased following recruitment of juvenile birds. Flock-size distributions predicted by an equilibrium arrival-departure model, were consistent with those observed during May to mid - June, but not those during late - June. I used a similar model incorporating observed relationships between fish availability and frequencies of arrival and departure, to predict aggregation patterns on hatchery streams; it, too, predicted trends more successfully during March through May than in later months. Social interactions appeared to influence dispersion to a greater extent during late - June to August so that assumptions of the model were violated. Merganser flight activity also declined from May to September. I suggest that the nesting dispersion of mergansers is also influenced by the size of juvenile salmonid migrations early in the spring, because breeding pairs congregate at profitable feeding sites. The number of merganser broods reared on 8 coastal streams was positively correlated (r²=.90) with drainage area and total juvenile salmonid migration including production from hatcheries and spawning channels. Other data on breeding pair densities, time of brood emergence and survivorship of merganser ducklings support the 'committed aggregation' hypothesis, but the evidence is not conclusive. If true, however, breeding pairs are 'deceived' about the natural productivity of enhanced streams; because hatchery fish are unavailable to merganser broods, the intensity of predation on wild salmonids may be unusually severe. Mortality due to mergansers feeding on juvenile Pacific salmon migrating downstream in the Big Qualicum and Little Qualicum rivers was probably < 8%. Despite the aggregative response by mergansers, mortality was depensatory because individual fish were at risk for very short duration. Merganser predation will be minimized if releases are few, but large, and scheduled as late as possible in the spring or early summer. Mortality of stream-resident salmonids due to merganser broods probably exceeded 20% in the Big Qualicum R. However, it is not clear whether this mortality, which occurs before autumn freshet mortality, limits wild smolt production.

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