Hypoxic refuges, predator-prey interactions and habitat selection by fishes.

Localized hypoxic habitats were created in Delta Marsh, Manitoba, Canada to determine the potential of regions of moderate hypoxia to act as refuges for forage fishes from piscine predators. Minnow traps and giving-up density (GUD) plates (plexiglas plates covered with trout crumble and fine gravel) were used to assess habitat use and perceived habitat quality for forage fishes, respectively, while passive integrated transponder tags provided data on habitat use by predator species to assess the level of predation risk. Data were collected both before and after a hypoxia manipulation (2-3 mg l(-1) dissolved oxygen, DO) to create a before-after control-effect style experiment. Fathead minnows Pimephales promelas were more abundant and consumed more food from GUD plates in hypoxic bays after the DO manipulation, indicating hypoxic locations were perceived as higher quality, lower-risk habitats. The frequency of predator visits was not consistently affected. The duration of visits, and therefore the total time spent in these habitats, however, was significantly shorter. These predator data, combined with the prey information, are consistent with the hypothesis that hypoxic regions function as predator refuges. The refuge effect is not the result of predator exclusion, however; instead predators are rendered less capable of foraging and pose less of a threat in hypoxic locations.

Cohesive social behaviour shortens the stress response: the effects of conspecifics on the stress response in lake sturgeon Acipenser fulvescens.

An examination was made of whether social interactions can have a beneficial effect through the attenuation of the stress response in a social species. In the first experiment, one larger (mean +/-s.e. 194.0 +/- 12.5 g) and seven smaller (32.0 +/- 2.6 g) juvenile lake sturgeon Acipenser fulvescens were placed in tanks to determine whether a classic dominance effect would be established based on body size (n = 6). Large fish did not establish a territory or aggressively interact with smaller fish, as there were no significant differences in nearest-neighbour distances and an absence of aggressive behaviour (biting, chasing and pushing). In the second experiment, it was hypothesized that the presence of conspecifics would have a beneficial effect through an attenuation of the stress response. Fish in groups or isolation were stressed by a brief aerial exposure (30 s), and blood plasma was measured at regular time intervals (0, 20, 40, 60, 120 and 240 min) following the stressor via an implanted cannula (n = 9-11). The presence of conspecifics did not affect the peak cortisol response, however, the overall cortisol response was shorter in duration compared to fish in isolation. Furthermore, secondary stress variables (plasma ions and glucose) showed differences between fish in groups and isolation. The results of these experiments suggest that social interaction plays an important and beneficial role in regulating the stress response in cohesive social species such as A. fulvescens.

Age and longevity in fish, with consideration of the ferox trout.

In the first part of this paper, we review the evolutionary aspects of age and longevity in fish and then summarize the theory of maturity due to Ray Beverton. This theory allows one to predict age at maturity (and thus a putative point for the onset of senescence) from information on growth rate and mortality rate. We illustrate the application of this theory with data on tilapia species and then discuss the limitations of the theory. In the second part of the paper, we develop an individual based model for the ferox trout. This is a morph of brown trout Salmo salar that is an exception to the common notion that caloric restriction extends lifespan, in the sense that ferox trout achieve long life by eating more, not less. The model allows one to identify the role that ecological and biochemical adaptations play in the longevity of the ferox trout.

Sensory compensation and the detection of predators: the interaction between chemical and visual information.

Recent evidence suggests that environmental conditions may affect whether fishes do or do not respond to the presence of chemical alarm cues in water. We present a simple model which suggests that the combination of risk of predation and information from other sources will determine when fishes should react to these chemical cues. We tested this model with a laboratory experiment which manipulated the risk of predation by altering the animals (hungry or well fed), or their environment (presence or absence of cover). We also altered the availability of visual information by manipulating the water clarity. Consistent with our model, fishes were most likely to react to chemical alarm cues in the absence of visual information and when the perceived risk of predation was high. The manipulation of either parameter was able to extinguish this response.