The flathead catfish Pylodictis olivaris is a highly piscivorous ictalurid native to central North America whose range has been extended throughout much of the United States. With this range expansion, many populations of native fishes have experienced declines in the number of individuals due to direct predation by flathead catfish. Previous evidence suggests that flathead catfish are opportunistic feeders and may be the least gape limited of North American freshwater piscivores. To better understand the size of prey vulnerable to flathead catfish, we measured gape dimensions for individuals of various sizes to determine the maximum size prey a flathead catfish can kill based on its gape limitations. Our results show the relationship of total length to horizontal and vertical gape and the relationship of flathead catfish total length to the total lengths of ingestible-sized prey of different body shapes. Furthermore, comparisons of the body depth of three common fish species to the gape dimensions showed that no size of largemouth bass Micropterus salmoides, bluegill Lepomis macrochirus, or gizzard shad Dorosoma cepedianum would preclude predation by flathead catfish. Our results support the assumption that the flathead catfish is one of the least gapelimited piscivores.

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Information regarding the life history and biological requirements and preferences for the razorback sucker Xyrauchen texanus is limited. Current recovery efforts involve rearing fish to juvenile or adult stages in hatcheries before they are released into areas of critical habitat that increase their survival. Our objectives were to develop a model based on habitat use by hatchery‐reared razorback suckers in the lower Colorado River of Arizona and California for predicting preferred habitat and to determine whether fish would use reconnected off‐channel habitats. To this end, we monitored the use of existing and reconnected habitats by hatchery‐reared razorback suckers implanted with sonic transmitters. Off‐channel (backwater) habitats were preferred over main‐channel or side‐channel habitats. We constructed a model using logistic regression to predict which types of backwaters (based on morphology and water quality) were most suitable for use by razorback suckers. The model works well for predicting sites that are less suited for use and can be used to prioritize habitats for reconnection purposes because razorback suckers were found to utilize reconnected habitats.

We examined the effects of latitude of origin on the first-year growth and survival of largemouth bass Micropterus salmoides. We brought largemouth bass from three latitudinally disparate populations (northern largemouth bass M. s. salmoides from south-central Wisconsin; Florida largemouth bass M. s. floridanus, originally from central Florida but currently stocked throughout Alabama; and native northern largemouth bass from southern Alabama) to a common rearing environment to compare their age-0 growth over two replicate growing seasons. During 1999, the northern subspecies from Wisconsin grew faster than the northern or Florida subspecies from Alabama. However, in 2000 the Florida subspecies grew faster than the age-0 fish from either northern subspecies population. Our results indicate that the effects of prey availability and age-0 density can overwhelm any predisposition for growth based on latitudinal or genetic origin. Given concerns about the genetic consequences of largemouth bass stockings, the lack of any consistent growth advantage in the first year represents an important piece of information to be taken into account when deciding about stocking nonnative subspecies of largemouth bass.

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