No description available.

In experimental pools open to emigration, mosquitofish (Gambusia affinis) resided at higher densities than topminnow (Poeciliopsis occidentalis). When Gambusia were introduced to Poeciliopsis pools, all topminnow maintained residency. Only 11% of the Gambusia emigrated from the mixed populations before the plant cover was removed; with no cover, 59% left. A significant number of Gambusia in single species pools also responded to a loss of cover by emigrating. Continuing residency of Poeciliopsis after introduction of Gambusia supports findings that displacement by mosquitofish is not an immediate process. Frayed fins on resident topminnow suggest short-term agonistic interactions. Coexistence of an exotic species (Gambusia) and an endangered desert fish (Poeciliopsis) may depend on the complexity, or spatial variation, of the Southwest's few remaining cienegas. The opportunity to disperse from pools in these habitats may be an important factor in Poeciliopsis survival; such emigratory behavior can be tested in open experimental systems.

No description available.

A study of the feeding and movement patterns of largemouth bass in Alamo Lake, Arizona, using stomach analysis, ultra-sonic telemetry and echo-location revealed that feeding is an important determinant of the behavior of individuals and of their composition. The diurnal foraging activity showed a circadian rhythm associated with dawn, dusk, and mobility of prey. The size and type of prey consumed was largely influenced by the size and hunger state of the predator and by the prevailing environmental conditions. All sizes of bass fed mostly on shad, but the quantities consumed were influenced by the avail- ability of alternate prey and location of shad. Foraging mode and habitat selection were influenced by size and shape of bass and their metabolic requirements, whereby, the juveniles ( ≤ 25.0 cm) and adults ( ≥ 40.7 cm) haunted the littoral zone, but the medium size bass (25.1 - 40.6 cm) occupied the limnetic zone. The diel movements of feeding caused segregation of sizes which in turn enhanced their utilization of the more abundant resources at the most opportune times. For example, during floods, bass moved en masse into the littoral zone to feed on the abundant food caused by allochthonous enrichment, but during draw-down, only the medium size bass migrated into the deeper waters to seek out the limnetic shad leaving the juveniles and larger adults in the littoral zone. Although the movements and feeding of largemouth bass in Alamo Lake may not be thoroughly explained by any single environmental factor, of the variables measured, water temperature, oxygen and turbidity seem to be the most important. Likewise, the high perennial production of bass in the lake may not be attributed solely to (1) the enormous addition of allochthonous nutrients during floods, (2) the concentration of prey during the draw-down, or (3) a high primary production that, in turn, sustains a high secondary production, but to a combination of all these variables interacting in different ways during different seasons.

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No description available.

No description available.