Numerous studies have demonstrated a negative relationship between increasing habitat complexity and predator foraging success. Results from many of these studies suggest a non-linear relationship, and it has been hypothesised that some "threshold level" of complexity is required before foraging success is reduced significantly. We examined this hypothesis using largemouth bass (Micropterus salmoides) foraging on juvenile bluegill sunfish (Lepomis macrochirus) in various densities of artificial vegetation. Largemouth foraging success differed significantly among the densities of vegetation tested. Regression analysis revealed a non-linear relationship between increasing plant stem density and predator foraging success. Logistic analysis demonstrated a significant fit of our data to a logistic model, from which was calculated the threshold level of plant stem desity necessary to reduce predator foraging success. Studies with various prey species have shown selection by prey for more complex habitats as a refuge from predation. In this stydy, we also examined the effects of increasing habitat complexity (i.e. plant stem density) on choice of habitat by juvenile bluegills while avoiding predation. Plant stem density significantly effected choice of habitat as a refuge. The relationship between increasing habitat complexity and prey choice of habitat was found to be positive and non-linear. As with predator foraging success, logistic analysis demonstrated a significant fit of our data to a logistic model. Using this model we calculated the "threshold" level of habitat complexity required before prey select a habitat as a refuge. This density of vegetation proved to be considerably higher than that necessary to significantly reduce predator foraging success, indicating that bluegill select habitats safe from predation.Implications of these results and various factors which may affect the relationships described are discussed.

Following exposure to a predator, socially dominant individuals may reduce their risk of predation by waiting until subordinates have resumed foraging before doing so themselves. Although such status-related ordering in the resumption of foraging activity has been observed in several bird species, the underlying mechanism(s) facilitating such a delay remains unknown. Social status per se and status-related foraging benefits prior to a threat of predation (i.e., individual hunger level) have both been suggested as possible mechanisms. We tested between these two alternative suggestions using pairs of stream-dwelling juvenile Atlantic salmon, for which the dominant-subordinate relationship was known. Fish were tested at equal and unequal hunger levels. Fish were presented with drifting prey, followed by a predation threat in the form of an aerial predator model. Which fish (i.e., dominant or subordinate) initially resumed foraging activity after exposure to the predator model was recorded. When both fish were at an equal hunger level, the dominant fish was more likely to resume foraging first. When the dominant and subordinate fish differed in their hunger level, the hungrier fish was the first to resume foraging regardless of social status. These results support the conclusion that hunger level, rather than social status per se, determines the order in which juvenile Atlantic salmon resume foraging after exposure to a predator.

Although predator avoidance has been proposed as one possible factor influencing the distribution of fish among substrate types, no study has addressed this question directly. Groups of juvenile Atlantic cod were offered a choice between pairs of the following three substrates: sand, gravel-pebble and cobble. Their distribution on these substrates was compared prior to, during and following exposure to a predator (i.e. a larger conspecific). With no apparent risk of predation, juvenile cod preferred sand or gravel-pebble. When cobble was present, juveniles hid in the interstitial spaces of this substrate in the presence of a predator. With no cobble present, juveniles showed no preference between sand and gravel-pebble, and did not seek refuge from predation in association with these substrates. Following exposure to a predator (i.e. 2.5 h later) larger juvenile cod again showed a preference for the finer-grained substrates, but smaller individuals continued to associated with the cobble. The presence of cobble resulted in fewer juveniles being captured and a significant increase in the latency until the first juvenile was captured by the predator. Results are discussed with respect to the effects of predation on the distribution and survival of fishes among substrate types.

No abstract

Young (35–50 mm, standard length) bluegills Lepomis macrochirus are restricted to vegetated habitats by predation pressure. Vegetation provides refuge by hindering predator foraging success. In this study, we tested the ability of bluegills to actively perceive and select densities of vegetation where they are safe from predation. Bluegills were presented with two plots of artificial vegetation (cover plots) of different densities (1,000, 250, 100, and 50 stems/m2) in an experimental arena and then confronted with a predator, a largemouth bass Micropterus salmoides. Prey distribution in the arena before, during, and after exposure to the predator, as well as the predatorˈs foraging activities, were recorded for each trial. Before predator introduction, bluegills in all combinations of cover plots spent most of their time in or around the thicker plot of vegetation in the arena. With the predator present in combinations with cover plot 1,000, prey chose this plot as a refuge when attacked. Cover plot 250 was selected as often as positions at the water‐air interface, and plots 100 and 50 were ignored as refuges. After removal of the predator, only in combinations with cover plot 1,000 did prey remain around the thicker cover plot present. Predator success (number of prey caught/total number of attacks) was lowest in trials with cover plot 1,000 present. When confronted with a predator, young bluegills appear capable of perceiving and selecting plots of vegetation offering safety from predation.