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Results of pond studies indicated that the number of young-of-the-year (age-O) largemouth bassMicropter• salmoides that grow to 15 cm or longer depends on the presence of suitable prey such as bluegills Lepomis macrochirus and gizzard shad Dorosoma cepedianum. Prey availability, which influences growth and length distributions of young largemouth bass, may be a major determinant of recruitment to adult stock. On the premise that large age-0 fish have lower mortality rates than small members of the same cohort, we applied a model of size structure to the problem of forecasting eventual recruitment from cohorts of young largemouth bass. The model confirmed that length distributions of age-0 fish can have important influences on subsequent recruitment; good firstsummer growth may mitigate, in part, such detrimental events as water-level fluctuations. The model may be a more refined predictor of recruitment than density of age-0 fish. (1975), is the addition of new fish to the stock (exploited) portion of the population. Classical stock-recruitment theory considers recruitment as particular functions of parent stock density, modified by random environmental influences. Beverton and Holt (1957) derived a well-known recruitment function from the premise that young-of-the-year (age-O) fish initially are lost to predation but reach some critical size at which 1 Based in part on a Doctoral dissertation by S. J. Gutreuter entitled Structure and Dynamics of Exploited Fish Populations: A Modeling Synthesis. 2 Present address: Inland Fisheries Branch, Texas Parks and Wildlife Department, 4200 Smith School Road, Austin, Texas 78744. 3 A cooperative program of the U.S. Fish and Wildlife Service, the University of Missouri, and the Missouri Department of Conservation. Recruitment, as defined by Rickerthey are invulnerable to predation. In this derivation, the food supply for prerecruits, through density-dependent growth that is proportional to food consumption, and a size-dependent source of mortality causes either an asymptotic or a domed relation between recruitment and parent stock. Despite this early indication of the importance of food supply and size-related mortality to the recruitment process, relatively few investigators have probed these topics. Work on largemouthbass Micropterus salmoides provides insights into the importance of food supply to recruitment. Aggus and Elliott (1975) documented differential growth of age-0 largemouth bass in Bull Shoals Reservoir, Missouri-Arkansas; fish that made an early dietary transition from invertebrates to fish grew faster than those that did not. Their data suggested a positive relation between the density of young of year longer than 11 cm in August 317

Fourteen species of freshwater fish were trained to execute a simple conditioned response in a shuttle box – to move in response to light to avoid an electrical shock. There was no relation between learning ability and phylogenetic position. Better learners included striped bass (Morone saxatilis), bigmouth buffalo (Ictiohus cyprinellus), common carp (Cyprinus carpio), and channel catfish (Ictalurus punctatus). Bluegill (Lepomis macrochirus) and northern pike (Esoxlucius) were poor learners. Yellow perch (Perca flavescens) and redbelly tilapia (Tilapia zilli) could not be trained. Some fish retained their learned behavior for months, although performance deteriorated with time. Older channel catfish learned better than juveniles, but there was no difference between juvenile and older largemouth bass (Micropterus salmoides). Temperature (18–28 °C) and feeding level (ranging from starvation for 25 d to ad libitum) did not affect learning of channel catfish, but the protozoan disease, ichthyophthiriasis, and perhaps our treatment of fish for the disease retarded it.

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