Searching for Equilibrium: Population Parameters and Variable Recruitment in Introduced Blue Catfish Populations in Four Virginia Tidal River Systems
Robert S. Greenlee and Catherine N. Lim
Abstract.—Introduced blue catfish Ictalurus furcatus populations in tidal rivers of the Atlantic slope support important recreational and commercial fisheries, with the James River trophy fishery being nationally recognized. During the period 2001–2008, low-frequency (15 pulses/s) pulsed DC electrofishing was used to sample blue catfish in tidal fresh-oligohaline sections of the James, Mattaponi, Pamunkey, and Rappahannock River systems; 54,174 blue catfish were collected, and 4,660 of these were aged using otoliths. Mean catch per unit effort (CPUE) was generally high (ranging from 223 to 6,106 fish per hour). Trends of increasing CPUE through time occurred in the James (839–4,449 fish per hour) and Rappahannock (1,400–6,106 fish per hour) rivers, and differences in CPUE were detected among rivers. Temporal shifts in growth (mean length at age) were observed, with growth slowing for all ages in the Pamunkey River and slowing for older ages in the Mattaponi (ages 9–13) and Rappahannock (ages 8–12 and age 14) rivers. In the Pamunkey and Rappahannock rivers, a negative relationship existed between growth (mean length at age 10) and density (CPUE). Although density increased dramatically in the James River, growth remained stable. Growth varied among rivers; by the end of the study, mean total length at age 10 ranged from 416 mm in the Rappahannock River to 675 mm in the James River. Growth through age 15 fi t linear models, as opposed to the von Bertalanffy nonlinear curve. In three of the four populations, the maximum age sampled increased in each succeeding survey year, and the maturing of all four populations was reflected in concurrent increases in size distributions. Recruitment was variable, with coincident strong and weak year-classes occurring in all four populations—an implication that landscape-level environmental variables play a role in determining recruitment success. In three of the four populations, patterns in year-class strength persisted, with correlation of catch-curve residuals from surveys separated by time. Approximately 35 years poststocking in the James and Rappahannock rivers and 25 years poststocking in the Mattaponi River, these populations had not yet reached equilibrium. It is unknown what the dynamics of blue catfish abundance, growth, and survival will be in the long-term in these rivers, leaving uncertainty regarding the future of the fisheries the populations support, as well as unanswered questions related to potential effects on other species.