Predator-prey functional responses and predation by staghorn sculpins (Leptocottus armatus) on chum salmon fry (Oncorhynchus keta)

Mathematical models describing the components of predator-prey interactions are reviewed and evaluated, and new equations representing selected aspects of the relationship are proposed. A model of prey selection that distinguishes between predator performance and prey vulnerability is devised and shown to lead to conclusions that may be qualitatively different from those produced using previous methods. The feeding habits of staghorn sculpins (Leptocottus armatus), the extent to which they utilize estuarine habitats and their predatory response to chum salmon fry (Oncorhynchus keta) are examined for the purposes of (i) ascertaining the factors shaping sculpin foraging behaviour and (ii) assessing their potential for limiting survival of juvenile salmon. During periods of fry migration, sculpin populations in the estuaries of Big Qualicum River, Salmon Creek and Rosewall Creek (on Vancouver Island, B. C.) were composed predominantly of small juveniles less than 80 mm in length. Tolerance to waters of low salinity, which decreased with sculpin size, was found to be the major variable governing residence in these areas. There was little evidence that the migration of fry was important in attracting sculpins to estuaries. Sculpins preyed on a wide diversity of fauna concentrating on benthic crustaceans, particularly the amphipod Eogammarus confervicolus. Juveniles were active throughout the day, but feeding became progressively more restricted to periods of low light intensity as they grew. The smallest that captured fry were 40-45 mm in length. When chum fry were offered to starved sculpins in field enclosures, the response of those less than 80 mm in length was type 2 (Holling 1965) whereas that of 80-99 mm sculpins was type 3 (sigmoid). Predation on fry was inversely related to light intensity from dawn to dusk, and positively correlated with' light levels during the night. When benthic invertebrates were added, sculpins exhibited an overall preference for fry, which were 4-5 times more profitable in terms of net energy intake. However, preference for fry declined markedly as their abundance relative to other prey increased, indicating a divergence from the usual predictions of optimal foraging theory. Capture rates by sculpins initially naive to salmon fry increased up to three-fold over 3-5 two hour trials. It is suggested that the foraging strategy of sculpins given a choice between salmonid fry and benthic invertebrates represents a balance between the requirement of minimizing risk of starvation and the need to evade their own predators (particularly birds). The schooling behaviour of fry requires that sculpins, even when experienced, must devote considerable attention to the attack process and in so doing, run the risk of being eaten themselves. The combined effects of the schooling response, which reduces the incentive to attack fry, and a profusion of alternative prey, which decreases average hunger levels, were thought to be responsible for low fry consumption in natural situations. In Big Qualicum River, an estimated 240,000 and 40,500 chum were captured by sculpins in 1979 and 1980, respectively. This represents corresponding percentages of only 0.51% and 0.06% of the fry populations, and was calculated to be less than one-tenth of the potential that could have been realized. Predation rates on coho fry (0. kisutch) were considerably greater, despite a smaller population size. Estimated consumption was 817,700 (42.97%) and 144,000 (9.09%) in 1979 and 1980. Systems where sculpins could consume higher proportions of chum fry populations were identified as small, shallow, warm estuaries of intermediate to high salinity with relatively few suitable benthic invertebrates and small numbers of fry. Recommendations for reducing sculpin predation in such cases are proposed. Birds, particularly Bonaparte's gulls (Larus Philadelphia), were found to be even more avid predators than sculpins on juvenile salmon in Big Qualicum River. In contrast to sculpins, they exhibited pronounced numerical responses to the appearance of fry in the estuary. An estimated 10-25% of the hatchery-reared chinook salmon (O. tshawytscha) and 2-4% of the coho were removed by birds in the years 1979-81.

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