Diet of juvenile invasive blue catfish (Ictalurus furcatus) in a southeastern US Atlantic coast estuary

Freshwater fishes have been introduced outside their native range to establish recreational fisheries, but management conflicts arise when such introductions also result in potentially harmful effects on native species. In this case study, we focus on Blue Catfish Ictalurus furcatus, which were introduced in the Chesapeake Bay region and are now considered invasive. In many tidal tributaries, Blue Catfish have increased dramatically in abundance, expanded into high-salinity habitats (up to 21.8 psu), and negatively affected native species, prompting calls for the development of an effective management plan. However, management of this conflict species is complicated by multiple competing objectives, including control of population size, maintenance of trophy fisheries, and expansion of commercial fisheries for Blue Catfish. Seven management recommendations were advanced by the Invasive Catfishes Work Group to control the spread and limit the ecological impacts of Blue Catfish on native species. We highlight opportunities for addressing these complex management issues and guide our suggestions using results from research on invasive Blue Catfish ecology and population dynamics, as well as management of invasive species in general. A formal approach, such as structured decision analysis, is required to resolve conflicts among user groups and to address the wicked problem of Blue Catfish in the Chesapeake Bay region.

Genotype-environment interactions for growth and survival of channel catfish (Ictalurus punctatus), blue catfish (Ictalurus furcatus), and channel catfish, I. punctatus, ♀ × blue catfish, I. furcatus, ♂ hybrid fry at varying levels of sodium chloride

We compared the swimming performance of juvenile blue catfish Ictalurus furcatus and hybrid catfish (female channel catfish I. punctatus × male blue catfish) using a protocol in which fish were forced to swim at a fixed velocity and their time to fatigue recorded at speeds of 30–120 cm/s. Hybrid catfish swam significantly longer than blue catfish at all of the speeds tested except 30 and 40 cm/s. The maximum sustained swimming speed (no fatigue in times ≥200 min) was 30 and 40 cm/s for blue catfish and hybrid catfish, respectively. The maximum prolonged swimming speed (fatigue in times >20 s but <200 min) was 100 cm/s for blue catfish and 120 cm/s for hybrid catfish. The burst swimming speed (fatigue in times <20 s) for blue catfish was 110 cm/s. The burst swimming speed for hybrid catfish was not determined; however, it is higher than its maximum prolonged swimming speed of 120 cm/s (highest speed tested). Hybrid catfish were able to swim longer before fatiguing than were blue catfish at 50 cm/s (156 versus 97 min) and at 60 cm/s (33 versus 6 min). Both blue catfish and hybrid catfish fatigued in less than 5 min at 70 cm/s, less than 3 min at 80 cm/s, and less than 60 s at 110 cm/s. No blue catfish swam at 120 cm/s while hybrids swam for 26 s at that speed. The results of this study will help fish producers understand the relationship between the speed of water currents produced by various aeration equipment and fatigue in pond-raised blue and hybrid catfish fingerlings.

Three feeding experiments were conducted to evaluate growth and body composition in blue caffish Ictalurus furcatus or channel caffish I. punctatus when fed diets containing 22% protein with or without supplemented methionine and/or lysine. All experiments were conducted in 110‐L aquaria that were part of a recirculating system. In Experiment 1,15 juvenile blue catfish (2.7 g) were randomly stocked into aquaria and fed one of three diets containing different (22%, 27%, and 32%) percentages of protein. Fish were fed twice daily to excess for 10 wk. In Experiment 2, juvenile blue catfish (5.4 g) were randomly stocked into aquaria and fed one of six diets containing either 22% (diet 1) or 32% (diet 2) protein. The diet containing 22% protein (diet 1) had either 0.3% crystalline L‐methionine (diet 3), 0.4% crystalline L‐lysine (diet 4), or 0.3% L‐methionine + 0.4% L‐lysine (diet 5) added. A sixth diet was formulated to contain 32% protein and 0.2% crystalline L‐methionine. Fish were fed in excess for 12 wk. In Experiment 3, juvenile channel caffish (10.3 g) were stocked and fed diets 14 from Experiment 2, twice daily in excess for 8 wk.In Experiment 1, after 10 wk, final individual weight, weight gain (%), and specific growth rate (SGR) of blue catfish fed diets containing three protein levels were not significantly different (P > 0.05) and averaged 12.9 g, 378%, and 2.2%/d, respectively. Fish fed the diet containing 27% protein had higher (P < 0.05) whole‐body protein (65.4%) compared to fish fed diets containing either 22% or 32% protein. In Experiment 2, final weight, weight gain (%) and SGR of blue catfish were not significantly (P > 0.05) different among diets and averaged 24.7 g, 355%, and 1.8%/d, respectively. Percentage whole‐body protein and lipid were not significantly (P > 0.05) different between fish fed diets containing 22% or 32% protein. In Experiment 3, channel caffish fed a diet containing 32% protein had significantly (P > 0.05) higher final individual weight, weight gain (%), and SGR compared to fish fed diets containing 22% protein, with and without supplemental methionine or lysine.Results indicate that blue catfish may be able to utilize a diet with 22% protein, and that addition of crystalline methionine and/or lysine did not improve growth. However, channel catfish grown in aquaria did not appear to have similar growth when fed a diet containing 22% protein compared to fish fed 32% protein, even when supplemental methionine or lysine was added. Further research on blue catfish and the use of a low‐protein diet (22% protein) needs to be conducted in ponds.

Blue catfish (Ictalurus furcatus) juveniles (59 g) and channel cafish (I. punctatus) juveniles (84 g) were stocked at 24,700 fish/ha into twelve 0.04-ha earthen ponds and fed diets containing either 25 or 35% protein according to a winter feeding schedule. After 189 days, final biomass in ponds stocked with channel catfish was significantly greater (P < 0.05) than in ponds stocked with blue catfish, largely reflecting differences in stocking weight. Both species lost weight during the winter period, whether fed 25 or 35% protein diets. At harvest, fat levels were significantly higher and protein levels significantly lower in blue catfish than in channel catfish, despite the blue catfish's smaller size. Channel catfish appear to rely more on fat stores during the winter period than blue catfish. Blue catfish demonstrated no growth advantage over channel catfish during the winter period.

Blue catfish alloherpesvirus (BCAHV) is a novel virus isolated from the blue catfish (Ictalurus furcatus). To date, the ultrastructure, virulence and immunogenicity of BCAHV have not been reported. Given the importance of blue catfish in producing channel ♀ (I. punctatus)×♂ blue (I.furcatus) catfish hybrids and the increasing demand for hybrid catfish in the US catfish industry, the susceptibility of blue, channel and hybrid catfish to BCAHV was assessed. Further, the cross-protective potential of BCAHV against Ictalurid herpesvirus 1 (IcHV1) was investigated in channel and hybrid catfish that survive BCAHV exposure. Neutralization assays revealed BCAHV is refractive (neutralization index [NI]=0) to anti-IcHV1 monoclonal antibody Mab 95, compared to IcHV1 (NI=1.8). Exposure of blue catfish fingerling to 1.3×105 TCID50 /L BCAHV produced cumulative mortality of 51.67±0.70% and pathologic changes similar to those of channel catfish virus disease. No mortality was observed in channel or hybrid catfish. Twenty-eight days post-challenge, surviving channel and hybrid catfish were exposed to 9.4×104 TCID50 /L IcHV1 (LC50 dose), resulting in 100% relative per cent survival compared to naïve cohorts. These data provide baseline information for BCAHV and lay the groundwork for future studies. Data also identify BCAHV as a potential vaccine candidate against IcHV1. Based on host range and immunogenicity evaluations, in addition to genome sequence data from previous studies, BCAHV should be given consideration as a new species of Ictalurivirus.

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.

As a prolific invasive species, Blue Catfish Ictalurus furcatus threaten native organisms in numerous estuarine and tidal freshwaters along the Atlantic coast of the United States. However, no published estimates of consumption rates are available for Blue Catfish in the scientific literature. This information is critical for development of bioenergetics models or estimation of population‐level impacts on native species. Using a combination of field and laboratory studies, we provide the first estimates of daily ration, maximum daily ration, and consumption to biomass ratios for Blue Catfish populations. Ad libitum feeding trials conducted in our laboratory reveal that maximum daily ration in Blue Catfish varies by prey type, temperature, and fish size, with maximal feeding occurring in medium‐sized Blue Catfish (500–600 mm total length) and at temperatures ≥15°C. Furthermore, estimates of daily ration were higher for fish prey (Gizzard Shad Dorosoma cepedianum) than for crustacean prey (blue crab Callinectes sapidus). Diel feeding chronologies based on field‐collected diet samples from 1,226 Blue Catfish demonstrated river‐specific variability in daily ration and maximum daily ration. Blue Catfish daily ration ranged between 2.27% and 5.22% bodyweight per 24 h, while maximum daily ration ranges between 8.56% and 9.37% bodyweight per 24 h. Estimates of consumption to biomass ratios varied by river and Blue Catfish size groupings but range between 2.42 and 3.39, which is similar to other benthic omnivores. This research will inform the assessment of predatory impacts of invasive Blue Catfish in the Chesapeake Bay and beyond as it will enable researchers to estimate predatory impacts through the coupling of population models, food habit information, and consumption rate information (current study).

The hybrid between female channel catfish (Ictalurus punctatus) and male blue catfish (Ictalurus furcatus) is superior in feed conversion, disease resistance, carcass yield, and harvestability compared to both parental species. However, heterosis and heterobeltiosis only occur in pond culture, and channel catfish grow much faster than the other genetic types in small culture units. This environment-dependent heterosis is intriguing, but the underlying genetic mechanisms are not well understood. In this study, phenotypic characterization and transcriptomic analyses were performed in the channel catfish, blue catfish, and their reciprocal F1s reared in tanks. The results showed that the channel catfish is superior in growth-related morphometrics, presumably due to significantly lower innate immune function, as investigated by reduced lysozyme activity and alternative complement activity. RNA-seq analysis revealed that genes involved in fatty acid metabolism/transport are significantly upregulated in channel catfish compared to blue catfish and hybrids, which also contributes to the growth phenotype. Interestingly, hybrids have a 40-80% elevation in blood glucose than the parental species, which can be explained by a phenomenon called transgressive expression (overexpression/underexpression in F1s than the parental species). A total of 1140 transgressive genes were identified in F1 hybrids, indicating that 8.5% of the transcriptome displayed transgressive expression. Transgressive genes upregulated in F1s are enriched for glycan degradation function, directly related to the increase in blood glucose level. This study is the first to explore molecular mechanisms of environment-dependent heterosis/heterobeltiosis in a vertebrate species and sheds light on the regulation and evolution of heterosis vs. hybrid incompatibility.

The effects of winter feeding on growth, body composition, and processing yield were compared for co-cultured blue catfish Ictalurus furcatus, channel catfish I. punctatus, and channel catfish×blue catfish hybrids. Fish (∼0.4–0.7 kg) from each genetic group were stocked communally at 5,625 fish/ha in ten 0.04-ha ponds during mid-November. Fish in five ponds were fed at 2% of initial body weight twice per week; fish in the other five ponds were not fed. The study was terminated after 14 weeks, and fish were weighed and processed. Analyzed traits were weight change (%), survival, and yields (%) of carcass, shank fillet, nugget, head, viscera, skin, intraperitoneal fat, liver, and ovary. Among fed fish, hybrids gained the most weight, channel catfish had intermediate weight gain, and blue catfish gained the least weight. Unfed blue catfish lost more weight than unfed channel catfish and hybrids. Survival was not different among genetic groups or between feeding regimes. Interactions among main effects for processing and body composition traits made generalizations difficult, but carcass yield was consistently higher for blue catfish and hybrids than for channel catfish, higher for females than for males, and higher for fed fish than for unfed fish. Shank fillet yield was higher for hybrids than for blue catfish and channel catfish, higher for females than for males, and higher for fed fish than for unfed fish. Nugget yield was higher for blue catfish than for channel catfish and hybrids and was higher for fed fish than for unfed fish. Blue catfish and hybrids had higher intraperitoneal fat yield and lower liver and ovary yield than channel catfish. Fed fish had higher intraperitoneal fat and liver yield than unfed fish. Winter feeding improved growth and fillet yield in all groups, but the benefits of winter feeding were lower for blue catfish than for channel catfish and hybrid catfish.

Catfishes (Ictaluridae) are among North America’s most important ichthyofauna for human consumption and recreation. As such, research on Blue Catfish Ictalurus furcatus, Channel Catfish I. punctatus, and Flathead Catfish Pylodictis olivaris has been abundant in historic and contemporary literature. A common technique for studying catfish involves marking or tagging fish to understand behavior, sampling efficiency, and population dynamics. Marking and tagging methods used in these studies may be divided into eight primary types, including fin clips, strap tags, anchor tags, transbody tags, injectable tags, skin alterations, origin markers, and telemetry. Each of these methods is appropriate for specific objectives and characterized by varying retention rates and data resolution (i.e., batch or individual identification) that warrant consideration when designing projects. For example, fin clips may be most appropriate as a short-term mark requiring batch identification (e.g., stocking evaluation), whereas telemetry might be most appropriate as a long-term tag requiring individual identification (e.g., habitat selection). This review summarizes marking and tagging methods used for evaluation of Blue, Channel, and Flathead catfish and synthesizes results using examples from previously conducted catfish research. Further discussion is included on information gaps, emerging trends, and guidance for scientists planning and conducting studies that require marking of catfish.

Globally, invasive species cause extensive economic damage and are a major threat to biodiversity. Generalist species are particularly dangerous invaders, as they can thrive in degraded habitats and endure environmental stochasticity, often outcompeting more specialized native taxa. Blue catfish Ictalurus furcatus were first introduced into the Chesapeake Bay during the 1970s, and now form dense populations in several tidal rivers. Despite being labeled as a dangerous invasive, the feeding ecology of this species is largely unknown. We used a stratified random design to collect stomachs from 16,110 blue catfish in tidal freshwater, oligohaline, and mesohaline segments of the James, Pamunkey, Mattaponi, and Rappahannock Rivers. Indices of diet breadth and omnivory reveal that blue catfish are generalist omnivores with some of the highest diet breadths ever observed in an estuarine fish species, while trophic level calculations demonstrate that blue catfish are a mesopredator occupying lower trophic levels than previously claimed. Cumulative prey curves revealed that large numbers of stomachs are necessary to adequately characterize the diet of blue catfish, thus previous diet descriptions of this species should be considered with caution. Blue catfish feed primarily on invasive aquatic vegetation and Asian clams, though the economically-valuable blue crab Callinectes sapidus is also consumed regularly. While the per capita impact of blue catfish on imperiled native species appears to be low, this impact could still be substantial due to high population densities.

The Blue Catfish Ictalurus furcatus is commonly raised in warmwater aquaculture in the United States to produce Channel Catfish I. punctatus × Blue Catfish hybrids. It is also a prominent aquatic invasive species of concern in the mid-Atlantic region of the United States. Here, Yersina ruckeri was isolated from moribund Blue Catfish and hybrid catfish from disparate regions of the USA. The goal of the research here was to compare these Y. ruckeri strains to each other and other known strains for which adequate sequence data was available. In addition, we sought to determine if the strain from Blue Catfish was pathogenic to Rainbow Trout Oncorhynchus mykiss. Moribund hybrid catfish from culture ponds in Mississippi were processed for diagnostic evaluation in March 2016. In April 2022, a moribund Blue Catfish specimen was collected from a tributary of the Nanticoke River in Maryland. Bacterial isolates were identified and characterized using biochemical tests, antimicrobial sensitivity screening, serotyping, and complete or partial genome sequencing. Disease pathology was described via histology. The isolate from Blue Catfish was used in challenge experiments to determine if it was pathogenic to Rainbow Trout. Multilocus sequencing typing was conducted using the PubMLST database. Biochemical testing was consistent with Y. ruckeri. A draft genome of the Y. ruckeri isolate was assembled based on Oxford Nanopore Technology sequencing and identified a single genomic replicon (3,791,418 bp) consistent in size to other Y. ruckeri genomes and a pLT plasmid (60, 933 bp). The challenge study demonstrated no significant virulence of this isolate for Rainbow Trout (Y. ruckeri). This isolate was most similar to other strains isolated from ictalurids. Notably, the gyrase B gene from this isolate was identical to that of archived strains isolated from moribund Mississippi hybrid catfish aquaculture during 2016 and these isolates share identical PubMLST sequence type profiles. Similarly, they shared a pLT plasmid that differed by only 6 bp. This plasmid has never been reported from trout isolates and appears to be unique to ictalurids. Analyses here provide preliminary genetic evidence that geographically distant (Maryland and Mississippi, USA) isolates of Y. ruckeri from ictalurids are genetically similar to each other and Y. ruckeri (strain SC09) that infects ictalurids in China. This strain is not a biothreat to Rainbow Trout at typical culture temperatures.

In estuaries, salinity is believed to limit the colonization of brackish water habitats by freshwater species. Blue catfish Ictalurus furcatus, recognized as a freshwater species, is an invasive species in tidal rivers of the Chesapeake Bay. Salinity tolerance of this species, though likely to determine its potential range expansion and dispersal in estuarine habitats, is not well-known. To address this issue, we subjected blue catfish to a short-term salinity tolerance experiment and found that this species tolerates salinities higher than most freshwater fishes and that larger blue catfish tolerate elevated salinities for longer periods compared with smaller individuals. Our results are supported by spatially extensive, long-term fisheries surveys in the Chesapeake Bay region, which revealed a gradual (1975–2017) down-estuary range expansion of blue catfish from tidal freshwater areas to habitats exceeding 10 psu [practical salinity units] and that large blue catfish (> 200 mm fork length) occur in salinities greater than 10 psu in Chesapeake Bay tributaries. Habitat suitability predictions based on our laboratory results indicate that blue catfish can use brackish habitats to colonize new river systems, particularly during wet months when salinity decreases throughout the tidal rivers of the Chesapeake Bay.

Objective Predatory invasive fishes may consume species of management interest and alter food webs. Blue Catfish Ictalurus furcatus is a large-bodied, salinity-tolerant species that exhibits broad diet breadth and preys on species of both conservation concern and fisheries management interest. To better understand the ecological consequences of the establishment of Blue Catfish fisheries, estimates of predatory impacts are needed. Methods Using a Monte Carlo simulation, we integrated abundance estimates, diet information, and consumption-to-biomass ratios to estimate population-level Blue Catfish predation for a large Chesapeake Bay tributary along the mid-Atlantic coast of the United States, the James River. Result Population-level annual predation estimates by Blue Catfish exceeded 100 metric tons for several species or taxa of interest, including an estimated 400.7 metric tons (95% CI = 272.6–613.2) of blue crab Callinectes sapidus. Prey species abundances were unknown and thus limited opportunities to evaluate prey population responses. For instance, effects of Blue Catfish on blue crab populations remain unknown without tributary-specific estimates of blue crab abundance, but comparisons to landings data suggests that Blue Catfish predation on blue crab in the James River may be low compared with harvest. Conclusion Estimation of Blue Catfish predatory effects may inform development of management goals and objectives that balance diverse stakeholder interests. This work provides beneficial information to assess trade-offs of Blue Catfish fisheries and their effects on coastal aquatic resources.