Proliferative gill disease (PGD) in catfish is caused by the myxozoan parasite Henneguya ictaluri. The complex life cycle requires Dero digitata as the oligochaete host. Endemic in most catfish ponds, efforts to control PGD by eradicating the oligochaete host have been unsuccessful. Smallmouth buffalo (Ictiobus bubalus; SMB) are opportunistic bottom feeders and a putative option for the biological control of D. digitata. The study was conducted in 14 ponds that were stocked with 8,000 channel catfish/pond. Catfish in one-half of the ponds in were polycultured with SMB stocked at a rate of 300 SMB/acre. In the first year of the study, pond muds were sampled regularly monthly for the presence of benthic invertebrates including D. digitata. After the first year ponds stocked with SMB had lower oligochaete numbers compared to ponds without SMB. Ponds were harvested at the end of the production season and re-stocked similarily.in a similar manner the following year. There were no differences in percent survival, total weight, total feed fed or feed conversion between the mono- and polyculture ponds. In the second and third year of the study, benthic populations were again sampled, pond water was collected for qPCR estimation of H. ictaluri actinospore concentrations and sentinel fish were used to estimate disease severity., in addition to benthic samplin There were no differences in numbers of benthic organisms, actinospore concentrations, or disease severity in sentinel fish among treatments. Under these study conditions, the presence of SMB does not have a measureable effect on PGD incidence, severity of disease or overall catfish production. As a result SMB are no longer recommended as a biological control for PGD, saving farmers between $200-300/acre on an ineffective treatment.
The commercial catfish industry is the largest aquaculture industry in the United States and is a vital component to the economy of several southern states Disease related losses account for nearly half of the losses experienced by commercial producers The three most common diseases in catfish aquaculture are bacterial septicemias caused by the gram negative bacterium Edwardsiella ictaluri and Flavobacterium columnare, and proliferative gill disease (PGD) caused by the myxozoan parasite Henneguya ictaluri. Proliferative gill disease is the most prevalent parasitic disease associated with the commercial production of channel catfish. The complex parasitic life cycle involves a myxospore stage in the channel catfish and an actinospore stage in the ubiquitous benthic oligochaete Dero digitata. Prolonged exposure of catfish to this actinospore stage results in a severe inflammatory response at thesevere inflammation of the gills, dramatically impairing the fish’s ability to carry out key physiological functions, namely osmoregulation, acid/base balance and gas exchange end result isresulting in decreased production and , with mortality rates approaching 100% in some severe outbreaks. The exact economic impact is unknown but is much greater than what can be attributed to direct losses associated with infection. Producers report significant losses in production due to poor feed consumption resulting from sublethal infections. On a 1000 acre farm, a conservative 50% reduction in feed consumption, typically lasting for 21 days between April-May, would result in a loss of fish growth of approximately 840,000 ponds, representing resulting in a potential reduction in gross sales of $1.1 million. Currently there are no effective treatments for the disease and management practices designed to curb disease related losses are associated with reducing additional losses of re-stocked fish following catastrophic losses.
A potential strategy for disease control is through disrupting the parasite life cycle by reducing the oligochaete host in the pond environment. Several producers in the Mississippi Delta who raise smallmouth buffalo in polyculture with channel catfish have reported a lower incidence of PGD, although these claims are anecdotal and have not been substantiated. Smallmouth buffalo (SMB), a native species in Mississippi, are omnivorous opportunistic forage feeders with a subterminal mouth, feeding primarily on benthic invertebrates, zooplankton and algae. As such, they are a prime candidate for the biological control of the benthic oligochaete D. digitata. Previous research has shown polyculture of smallmouth buffalo with channel catfish does not have any measurable effects on water quality, nutrients or phytoplankton communities. Preliminary data collected from initial experimental trials conducted at our research facility demonstrated a significant reduction in benthic oligochaetes in pond sediments. This suggests smallmouth buffalo have the potential to dramatically reduce oligochaete populations in catfish ponds, indirectly reducing PGD incidence within the system by reducing the oligochaete host. Since low infestation levels have no measureable effect on fish health and production, SMB do not need to completely eradicate D. digitata from the pond environment to be effective. As long as oligochaete populations are kept below concentrations that allow for the accumulation of actinospore levels resulting in morbidity, severe outbreaks can be avoided, and significant losses associated with PGD can be deterred.
The usefulness of SMB serving as a biological control measure for limiting the severity of PGD will bewas evaluated in commercially raised channel catfish in a three year study.. A three year production study was conducted in ponds containing commercially raised channel catfish. Parasite levels in pond water, oligochaete populations in pond sediments, and fish production between paired treatment and control ponds served as a measure of efficacy. Data was used to establish treatment recommendations for the biological control of this disease.
A three year production study was conducted in 14 ponds containing commercially raised channel catfish stocked yearly at a rate of 8,000 fish per acre. Catfish in one-half of the ponds were raised in polyculture with SMB stocked at a rate of 300 SMB/acre. Fish were harvested yearly at the end of the production cycle and re-stocked with channel catfish and/oror channel catfish with SMB. During the first year, sediments samples were monthly for determining the presence of oligochaetes. In the second and third year of the study, pond water was collected for qPCR estimation of H. ictaluri actinospore concentrations and sentinel fish were used to estimate disease severity, in addition to benthic sampling. Fish production, disease severity in sentinel fish, parasite levels and oligochaete populations were analyzed by analysis of variance and used as a measure of treatment efficacy.
Pond sediments were sampled monthly for the presence of benthic invertebrates. In the first year, ponds stocked with SMB (30 g/fish) had statistically lower oligochaete populations than pond containing just channel catfish but no differences in production variables (survival, feed efficiency, and total weight gain) were noted between ponds with and without SMB. After harvest, starting in the second year of production, ponds were restocked with catfish and additional SMB to account for losses of SMB during harvest. Resulting from aDue to a limited supply, larger SMB (approximately 200 g) were used to re-stocked SMB treatment ponds in years 2 and 3. In contrast to year one, there were no differences in oligochaete numbers were noted between treatment ponds after the first year. Results from years 2 and 3 reveal no differences in benthic populations, actinospore concentrations as determined by quantitative PCR nor disease severity in sentinel fish. In the second and third year of the study, pond water was collected for qPCR estimation of H. ictaluri actinospore concentrations and sentinel fish were used to estimate disease severity in addition to benthic sampling. There were no differences in numbers of benthic organisms, actinospore concentrations, or disease severity in sentinel fish. Under these study conditions, the presence of SMB does not have a measureable effect on PGD incidence, severity or overall catfish production. It is not know why oligochaete numbers were lower in ponds stocked with SMB in the first year but were similar in years 2 and 3. Size dependent prey preference may have been responsible for treatment differences observed in year 1, where oligochaetes and other small invertebrates maybe a preferred diet of smaller SMB. Although it is not known if the treatment effects observed in year-1 where biological in nature or the result of a type-1 statistical error, the data does not support the use of SMB as a biological control for proliferative gill disease.
Educational & Outreach Activities
Finds were presented at two extension based workshops (NWAC Fall Seminar Series in Stoneville and Macon, MS) and at the Eastern Fish Health Workshop, (April 2014, Shepards Town WV). Findings were published in a technical publication for distribution to stakeholders and are being prepared for publication in the scientific literature.
Smallmouth buffalo were not shown effective in controlling proliferative gill disease in commercially raised channel catfish. As a result, SMB are no longer recommended as control measure for this parasitic disease.
Smallmouth buffalo were not shown to impact disease prevalence or fish production and as a result SMB are no longer recommended as a biological control for this disease. The discontinued use of SMB will save producers between $200-300 per acre by not using an ineffective treatment.
As a a a result of this research farmers have discontinued the practice of raising catfish in polyculture with SMB.
Areas needing additional study
Proliferative gill disease is a continual reoccurring problem in catfish aquaculture affecting all populations ages/sizes of commercially raised channel catfish. Attempts at breaking the life cycle of this parasitic disease by biological control with smallmouth buffalo appears ineffective and is no longer a recommended culture practice. Research is now focused on using hybrid catfish and single crop intensive production systems as a means of controlling the infectious life stages of this parasite. Our current research has demonstrated poor life stage development in hybrid catfish which in turn should lead to lower parasite loading rates and infection levels within closed production systems. Utilizing the same approach as outline in this project, the prevalence of H. ictaluri in channel and hybrid catfish production systems is being evaluated in experimental and commercial catfish production systems. If effective, hybrid catfish will provide a means of breaking the parasites life cycleand reducing the impact of proliferative gill disease in catfish commercially raised in earthen impoundments.