Fingerling stocking size has no influence on proliferative gill disease severity in farm-raised channel catfish (Ictalurus punctatus)

Authors: Richardson, Bradley; UL-HUDA, NOOR - Mississippi State University; WARE, CYNTHIA - Mississippi State University; CAMUS, ALVIN - University Of Georgia; Older, Caitlin; YAMAMOTO, FERNANDO - Mississippi State University; GOODMAN, PENELOPE - Mississippi State University; REIFERS, J - Mississippi State University; WALKER, CHARLES - Mississippi State University; STILWELL, JUSTIN - Mississippi State University; MARANCIK, DAVID - St George'S University; WISE, DAVID - Mississippi State University; GRIFFIN, MATT - Mississippi State University

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2024
Publication Date: 4/4/2025
Citation: Richardson, B.M., Ul-Huda, N., Ware, C., Camus, A.C., Older, C.E., Yamamoto, F.Y., Goodman, P.M., Reifers, J.G., Walker, C.M., Stilwell, J.M., Marancik, D.P., Wise, D.J., Griffin, M.J. 2025. Fingerling stocking size has no influence on proliferative gill disease severity in farm-raised channel catfish (Ictalurus punctatus). Journal of Aquatic Animal Health. 37(1)1-10. https://doi.org/10.1093/jahafs/vsae002.
DOI: https://doi.org/10.1093/jahafs/vsae002

Interpretive Summary: Proliferative Gill Disease (PGD) is a leading parasitic disease in the US catfish aquaculture industry. There are currently no approved chemical therapeutics for the control of this disease, so management primarily consists of molecular assays to test for the parasite in pond water prior to stocking fish. Due to the labor- and time-intensive assays, farmers asked whether stocking larger fingerlings could help reduce mortality associated with the disease. In this study, USDA scientists in Stoneville, MS collaborated with Mississippi State University, University of Georgia, and St. George's University (Grenada, West Indies) to evaluate the efficacy of stocking larger catfish fingerlings to mitigate PGD risk. Two commercially-relevant sizes of fingerling catfish were stocked into net pens within commercial ponds actively experiencing PGD outbreaks. After 7 days, net pens were retrieved, mortalities were counted. Gill clips were collected from surviving fish to assess parasite load and gill damage. Fish size showed no significant influence on mortality, gill damage, or parasite load. The results of this study show no appreciable benefits of stocking larger catfish to reduce PGD risk. Stocking size should thus be predicated on other production and management goals within a given farm. This research supports the Secretary of Agriculture’s priorities to increase profitability of farmers.

Technical Abstract: The myxozoan Henneguya ictaluri is the causative agent of proliferative gill disease (PGD) in channel and channel × blue hybrid catfish, which is a significant disease concern within the commercial catfish industry of the southeastern United States. Incidence of PGD occurs most frequently in fingerling sized catfish, often during spring-time, when fish are being transferred from nursery ponds to growout ponds. As a result, PGD can have devastating impacts on the industry at multiple stages of production. Management strategies for PGD primarily involve the avoidance of stocking fish into ponds with existent lethal concentrations of the parasite, determined through sentinel fish exposures, or, in some instances, employing H. ictaluri-specific quantitative PCR to estimate parasite concentrations before stocking. Objective: This study aimed to evaluate the potential of stocking larger fingerlings to improve survival during stocking and investigate the influence of pond water parasite burden and fish size on three metrics of gill condition. Methods: Two sizes of fingerlings (~12 cm and ~20 cm), which correlate to industry-relevant fingerling weights of 40lbs/1000 and 120lb/1000, were stocked into nylon-mesh net pens located in 19 commercial ponds with varying levels of H. ictaluri activity. Water samples were collected from each pond on the day of stocking to estimate parasite loads. After one week, the fish were removed from the ponds. Mortality was recorded and all survivors were euthanized for gross, histologic, and molecular assessment. Additional water samples were collected on the day of sampling to evaluate temporal trends of H. ictaluri actinospores in each pond. Gill biopsies from surviving fish were evaluated with a previously established lesion scoring system to estimate gill damage based on the presence of chondrolytic lesions in gill clip wet mounts. Histologically, the number of characteristic PGD lesions was assessed as well as the number of presporogonic stages present. Results: There was a significant difference in survival odds between the small and large fingerlings, with small fish odds of survival being approximately 10% higher for a given parasite load. Generalized linear regression showed no interaction between parasite burden in the pond water or gill tissues and fingerling size. In all regressions, only parasite concentrations in pond water or gill tissues were significant predictors of any gill condition metrics. Conclusion: Results from this study suggest that stocking of larger fingerlings provides no appreciable protection from PGD mortality or sub-lethal gill damage. Though smaller fingerlings regularly showed slightly better average gill condition compared to larger fingerlings, these findings were minute and primarily only found in highest parasite concentrations which were likely influenced heavily by survival bias.