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Predatory functional response of the prickly sculpin (Cottus asper) to density of sockeye salmon (Oncorhynchus nerka) fry Woodsworth, Eric John


The predatory functional response is one of the important components of the interaction between predator and prey populations. This response has not been measured for fish predators and fish prey, in spite of the demonstrated importance of predation in regulating numbers in populations such as the juvenile salmon system used in this thesis. Laboratory exeriments were conducted to determine the form of the functional response of the prickly sculpin (Cottus asper), a common freshwater predator, to density of sockeye salmon fry, Oncorhynchus nerka. An alternative prey, chum salmon eggs (Oncorhynchus keta), were presented to the predator with the salmon fry, in order to facilitate switching at low fry densities and possibly lead to a sigmoid (type III) response. Initial experiments showed that sculpin feeding rate did not substantially differ from day to night; that in the absence of food, hunger increased to a maximum after about seven days' starvation; and that a density of 400 chum salmon eggs in a 2000 1 tank produced maximum consumption level by sculpins of 160 mm total length. The functional response experiments did not indicate a sigmoid rise in consumption over low fry densities. However, the initial decelerated rise in consumption was followed by an anomalous drop in response and a subsequent rise in response. It is suggested that this may result from the summation of separate responses through different sensory modalities, or from interference with predation by fry at intermediate densities. This shape of functional response may imply a stable local equilibrium at intermediate densities of salmon. Sculpins probably have a significant impact on fry numbers only at very localized points in space and time, such as at the outlets of tidal creeks on a falling tide.

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