Patrick W. Colgan

Predator foraging success and habitat complexity: quantitative test of the threshold hypothesis

SummaryNumerous 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.

The role of learning in fish behaviour

SummaryThe behavioural patterns of fish are the result of innate (‘built-in’) patterns of maturation (developmental changes) and of learning processes (imprinting and trial-and-error learning). Innate behavioural patterns are considered to be ‘hard-wired’ and inflexible. However, through learning, fish can adapt to environmental change. For instance, the homing behaviour of fish may be partly the result of the development of specific parts of the brain and partly because of changes in behaviour with experience. Similarly, one can assume that the feeding mode of fish involving snap-responses is innate, but learning enables fish to modify their foraging behaviour in response to a fluctuating environment. By reviewing these and other examples, such as the role of recognition learning and socially transmitted behaviour, one can illustrate the importance of learning in the everyday life of fishes. Although learning plays a large role in the behaviour of fishes, the learning capacity of fishes may also be useful to fisheries research and hatchery operations.

Selection between Densities of Artificial Vegetation by Young Bluegills Avoiding Predation

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 predators 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 p...

Risk of predation, hydrodynamic efficiency and their influence on school structure

SynopsisLaboratory studies were conducted on 15 schools of blackchin shiners, Notropis heterodon, to determine if they altered their structure in response to changing environmental demands. The hypothesis tested was that fish schools should sacrifice a flat, hydrodynamically efficient structure in favour of an unobstructed visual field in the presence of a predator by staggering in the vertical plane. Ten schools were exposed for two weeks to a simple environment with only a current. For the next two-week period a predator was added. Five control schools were exposed to the simple environment for both two-week periods. Six of the ten treated schools increased their staggering in the vertical dimension as predicted while none of the control schools changed. This result was suggestive that hydrodynamic advantages were sacrificed. Respirometer experiments indicated these fish were capable of achieving some hydrodynamic benefits from schooling but these benefits may be a function of fish size.

Eggs in the nests of males and their effect on mate choice in the three-spined stickleback

Abstract It is thought that female three-spined sticklebacks, Gasterosteus aculeatus , prefer to mate with males whose nests already contain eggs. This is based on the results from a study where gravid females were tested sequentially, first with a male without eggs, and then with a male with eggs. However, that experimental design fails to detect important effects due to male-male competition. Simultaneous and sequential choice experiments were conducted to determine the relative importance of eggs in a nest with respect to female mate choice. In the simultaneous choice tests, males with eggs in their nest were more likely to obtain a second spawning than males without eggs. However, there was little evidence of females actively ‘choosing’ mates on the basis of presence or absence of eggs. Rather, males that had recently spawned courted more intensely and were more successful at leading females to their nests. In only a few cases could a males failure to spawn be attributed to a female aborting entry into a nest without eggs. In the sequential choice tests, significantly more females entered and spawned than aborted entry into nests without eggs. Furthermore, aborted entries were not seen exclusively in nests without eggs. The results indicated that the presence of eggs had a general priming effect on courtship behaviour of males, but evidence for active female choice was lacking. Overall, females did not exhibit the behavioural patterns predicted from the hypothesis that females should spawn preferentially in nests with eggs.

Prey choice by marten during a decline in prey abundance

SummaryWe examined variation in diet choice by marten (Martes americana) among seasons and between sexes and ages from 1980–1985. During this period prey populations crashed simultaneously, except for ruffed grouse (Bonasa umbellus) which was common at the beginning and end of the study, and masked shrews (Sorex cinereus) which were abundant in 1983. Marten were catholic in selection of prey and made use of most available mammalian prey, ruffed grouse, passerine birds, berries, and insects. Diet niche was widest during the latter three years when prey was scare, particularly in late winter. Diet niche breadth was negatively correlated with abundance of all common prey species. Proportion of small prey species in the diet was correlated with absolute abundance of those species, but proportion of some large prey was related to their relative abundance. Diet choice varied among years and among seasons. Berries and insects were common in summer diets while large prey, particularly varying hare (Lepus americanus), were more frequent in winter diet than in summer diet. We found little evidence that any small mammal species was a preferred prey. Sexual size dimorphism between the sexes did not affect prey choice, nor did age. Reduced foraging effort in winter resulted in a wider diet niche only when prey was scarce. The only prediction of optimal foraging models fully supported by our data was a wider diet niche with reduced prey abundance. However, among the three most profitable prey species choice was dependent on the absolute abundance of the most profitable type (varying hare). We suggest that marten primarily forage for large prey but employ a strategy which results in encounters with small prey as well. These small prey are eaten as they provide energy at minimal cost, between captures of large prey.

Marten activity in uncut and logged boreal forests in Ontario

Marten (Martes americana) experience greater survival and production rates in old growth forests than in postclear-cut forests. To determine whether total time active and foraging success contribute to these differences, we compared marten movements and hunting activities in their preferred old-forest habitats to those in logged forests. Marten in uncut forests captured up to 119 % (P = 0.003) more prey biomass than those in logged forests, depending on prey populations. We found few marten activity differences by forest type attributable to differences in density and prey species,or habitat structure. Distances between core areas in individual home ranges were greater (P < 0.001) in logged forests than in uncut forests

Individual and species recognition in centrarchid fishes: evidence and hypotheses

SummaryWe conducted laboratory experiments to determine if juveniles of three species of centrarchid fishes displayed abilities for species or individual recognition. In one experiment we reared rock bass (Ambloplites rupestris) in social isolation and in social groups. Both the isolated and group-reared fish spent significantly more time close to conspecifics than to heterospecifics. These results suggest that species recognition in rock bass is controlled by a closed genetic program and that social experience is not necessary. A second set of experiments was conducted to determine if year-old blue-gill (Lepomis macrochirus), pumpkinseed (L. gibbosus), and rock bass juveniles could discriminate between familiar and unfamiliar conspecifics. Bluegill spent significantly more time with familiar conspecifics than with unfamiliar conspecifics, while the other two species displayed no such trend. Bluegill spent significantly more time with familiar conspecifics than did either of the other two species. We postulate that these interspecific differences in recognition abilities may be related to differences in habitat and group parameters among the species.

The Effects of Prey Availability and Predation Risk on Habitat Selection by Juvenile Bluegill Sunfish

We examined the effects of variation in prey abundance on foraging site selection by juvenile bluegill sunfish, in the absence and presence of a predator. Groups of bluegills chose between two plots of artificial vegetation differing in plant stem density (i.e., 100 and 500 stems/m2). Two prey distribution treatments were tested. In the first treatment, the 500 stem plot had sufficient prey to satiate all group members, whereas the 100 stem plot had one-fifth this amount. In the second treatment, we reversed this prey distribution. We tested bluegills before, during, and after exposure to a potential predator (largemouth bass, Micropterus salmoides). In all trials the number of bluegills choosing, and time spent foraging in, each plot of vegetation was recorded. We also recorded bluegill foraging rate in, and the proportion of prey consumed from, each plot. With no predator present (i.e., before and 24 h after exposure to a predator) bluegills preferred to forage, and achieved a higher foraging rate, in the vegetation plot with the greatest initial number of prey. The presence of a predator had no significant effect on patch choice or foraging rate when the most rewarding plot of vegetation also offered a refuge from predation (i.e., 500 stems/m2). In contrast, when the 100 stem plot contained the greatest number of prey, bluegills reacted to the presence of a predator by increasing time spent in the 500 stem plot while simultaneously increasing their foraging rate when in the 100 stem patch. Therefore, bluegills altered their foraging activities to reduce time spent exposed to predation.

The effects of predator presence and shoal size on foraging in bluntnose minnows,Pimephales notatus

SynopsisShoals of 3, 5, 7, 10, 15, and 20 bluntnose minnows,Pimephales notatus, were allowed to forage in the absence and presence of a fish predator, which was separated from the shoal by a clear plexiglass partition. A typical dilution effect was observed in that individual fish in larger shoals were approached less frequently by the predator. In the absence of a predator, foraging latency decreased significantly and the rate of foraging increased with increasing shoal size. Foraging latency for each shoal size tended to increase in the presence of a predator and foraging rate decreased, significantly for shoals of 7, 15, and 20 fish. Members of larger shoals were safer and enjoyed a greater level of food consumption, perhaps due to decreased individual vigilance for predators and social facilitation. However, foraging effort decreased when a predator was present, as more time was allocated to predator avoidance.