Density‐Dependence Mediates the Effects of Temperature on Growth of Juvenile Blue Catfish in Nonnative Habitats

The combined effects of conspecific density and climate warming on the vital rates of invasive fish species have not been well studied, but may be important in predicting how successful they will be in the future. We evaluated the effects of temperature and population density on monthly time series of sizes of age-0 Blue Catfish Ictalurus furcatus in the James, York, and Rappahannock River subestuaries (defined here as tidally influenced bodies of water that feed into the Chesapeake Bay) from 1996 to 2017, using growing degree-days (GDDs, °C day) as a measure of thermal time. Our predictive linear mixed-effects model explained 86% of the variation in the length of age-0 Blue Catfish. In addition, it indicated a strong positive effect of temperature on the growth rate of age-0 Blue Catfish, with individual fish biomass during warm years up to 63% higher than during cool years. Growth rate was influenced negatively by the abundance of age-0 and older fish, resulting in at least fourfold differences in the predicted biomass of Blue Catfish by the end of the first year of life depending on conspecific density. We also observed regional differences in the growth rates of Blue Catfish in the three subestuaries we examined; although growth occurred in all subestuaries, growth was highest for the Rappahannock River population even though this river accumulated the fewest GDDs. Rising water temperatures due to global climate change will likely increase the growth rate of age-0 Blue Catfish in the Chesapeake Bay region, potentially intensifying the negative impacts of this invasive species on the ecology of Chesapeake Bay. However, individual populations respond differently to warming temperatures, and thus, potential increases in the growth rate of age-0 Blue Catfish may be partially offset by local conditions that may serve to limit growth. The Blue Catfish Ictalurus furcatus is an invasive species of significant management concern in the Chesapeake Bay region. Originally introduced to the James River (southernmost), York and Rappahannock (northernmost) rivers, this species has undergone considerable population increases as well as range expansions into many subestuaries in the Chesapeake Bay region (Schloesser et al. 2011; Fabrizio et al. 2018), though the populations in the James, York, and Rappahannock rivers remain largely distinct stocks with little mixing among the subestuaries (Higgins 2006). Through competition and predation, Blue Catfish may negatively affect the abundance of local fauna like American Shad Alosa sapidissima, river herring Alosa spp. , Atlantic Menhaden Brevoortia tyrannus, and blue crab Callinectes sapidus, many of which are of economic or conservation concern (MacAvoy et al. 2009; Schloesser et al. 2011; Schmitt et al. 2019). Therefore, there is considerable interest in limiting range expansion and minimizing the negative impacts of Blue Catfish on native resources, particularly those that are the targets of restoration efforts (ICTF 2014). The management of invasive Blue Catfish can be informed by characterization of its population dynamics in the Chesapeake Bay region. The growth of individuals is an essential component of the dynamics of fish populations, and contributes to the variation in mature biomass and production of fish stocks (Kwak and Waters 1997; Stawitz and Essington 2019). Growth during the first few months after hatching is particularly critical in determining the recruitment potential of fishes (Oele et al. 2019), *Corresponding author: vaskarnepal@gmail.com Received August 21, 2019; accepted November 12, 2019 Transactions of the American Fisheries Society 149:108–120, 2020 © 2020 American Fisheries Society ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1002/tafs.10217