No abstract

During the flood season of 1992–1993, 139 species of fishes were collected from a floodplain lake system in the central Amazon Basin. Fish species distribution was examined relative to abiotic variables in seven vegetation strata on Marchantaria Island, Solimões River. Both environmental variables and species distributions were influenced by a river channel to floodplain‐interior gradient. Species diversity was significantly higher in vegetated areas than in unvegetated areas, with deeper water Paspalum repens stands harbouring the highest diversity. As a result, species richness and catches were positively related to habitat complexity, while catch was also negatively related to dissolved oxygen (DO) and water depth. Low DO and shallow waters appeared to act as a refuge from predation. Fish assemblages were related to water chemistry, but species richness was not. Canonical correspondence analysis provided evidence that floodplain fish assemblages formed by the 76 most common species were influenced by physical variables, macrophyte coverage and habitat complexity, which jointly accounted for 67% of the variance of fish species assemblages. Omnivores showed no pattern relative to the river channel to floodplain‐interior gradient while detritivores were more likely to be found at interior floodplain sites and piscivores closer to the river. Piscivores could be further separated into three groups, one with seven species associated with free‐floating macrophytes in deep water, a second with five species found in shallow waters with rooted grasses and a third with six open water orientated species. The results suggest that fish assemblages in the Amazon floodplain are not random associations of species.

The 'flood pulse advantage' is the amount by which fish yield per unit mean water area is increased by a natural, predictable flood pulse. Evidence for this increase is presented from tropical and temperate fisheries. It is argued that increasing multispecies fish yield by restoring the natural hydrological regime is consistent with increasing production of other trophic levels and with restoration from ecological and aesthetic viewpoints. When applied to a river-floodplain system, this restoration would provide a large, self-sustaining potential for recreation, commercial exploitation, and flood control. An interim 'natural flood pulse' restoration approach is proposed for systems modified for navigation. This approach approximates the natural hydrological regime in a river reach and is intended as a first step in the long process of restoring the watershed.

Models for fish capture efficiency (catchability) using a boat‐mounted AC electrofishing sampling protocol were estimated for warmwater fishes in Illinois lakes through a calibration process. Catchabilities were determined for 37 inshore zones in blocked enclosures or ponds and 5 large water bodies during fall or spring. The abundances of vulnerable fish populations were determined by census following draining or by treatment using rotenone or primacord of known catchability. Inshore catchabilities, based on a zone 0‐13 m from shore, were strongly dependent on fish length (as a unimodal function), fish taxa, mean depth, and surface macrophyte cover. Under average environmental conditions, maximum catchabilities by taxon ranged from 0.0018 to 0.14 and ranked (highest to lowest) as follows: largemouth bass ?Micropterus salmoides, common carp Cyprinus carpio, crappies Pomoxis spp. in spring, shad Dorosoma spp., bluegill Lepomis macrochirus, green sunfish L. cyanellus, crappies in fall, freshwater drum Aplodinotus grunniens, suckers (Catostomidae), and catfish (Ictaluridae). Catchabilities for common carp and shad were significantly lower in large water bodies, indicating that significant numbers were outside the inshore zone. However, the results for other species, including large samples of largemouth bass and bluegills, indicated that populations in similar whole lakes could be estimated from the predicted inshore electrofishing catchabilities in fall or spring. Strong biases in relative population density when inferred by catch per unit effort were demonstrated under differing environmental conditions. Also, estimates of ratios related to fish size, such as mortality rates, were seriously biased when based on the size structure of uncorrected catches. Therefore, catchability models are considered essential for assessing the absolute and relative attributes of fish populations across water bodies.