Noel Novelo and Terrence Tiersch

Aquaculture is the farming of aquatic animals and plants for local and international commerce. Channel catfish (Ictalurus punctatus) is the most important food fish for U.S. aquaculture – especially in Mississippi, Arkansas, Alabama, Texas and Louisiana – where its consumption is part of the local cuisine. Channel catfish farming – which grew out of local resources and the ingenuity of farmers in Southern states and out of research by universities and federal and state agencies – has become the most valuable U.S. aquaculture industry.

Channel catfish aquaculture reached production levels exceeding an average of $450 million dollars per year from 1998 to 2008. It contributed nearly half of the total U.S. aquaculture sales from 2004 to 2006 (Figure 1). Louisiana, one of the top four channel catfish producing states during peak production from 1999 to 2008, generated $109 million in total aquaculture sales in 2007. The catfish industry achieved national prominence as an economic, geographic and cultural symbol of the Southern states in large part because of the natural biological advantages for farming of this fish. These include the straightforward pond spawning methods for providing seed stock (fry and fingerlings for growing), ease of feeding, water quality tolerance and cooking versatility.

These biological advantages provided the basis of the extensive catfish farming and mass production (300 to 700 million pounds per year) in the southern United States (Figure 1). Quantity and sales value fluctuated, with the widest gap between production and sales in 2002. After 2006, catfish production began to decline steadily because of major economic factors such as cheaper imports competing for the well-established U.S. catfish market, national recession, high fuel prices, and high prices for corn, soybeans and other feed ingredients. From 2000 to 2010, farmed water surface for catfish declined from 180,000 to 95,000 acres in the United States and from 14,000 to 1,334 acres in Louisiana. During the same period, the number of catfish operations declined from 895 to 454 in the United States and from 80 to 20 in Louisiana. According to the Ag Summary published by the LSU AgCenter, Louisiana catfish acreage decreased to 609 acres in 2012, with six Louisiana producers selling 1.7 million pounds of catfish valued at $1.8 million.

Economic challenges and declining production meant that catfish farms had to find new, resourceful ways of cutting costs and improving production efficiency, while maintaining and increasing profit. One effective method for improving desirable production traits in aquaculture, as in other fields of agriculture, is through genetic improvement. In the case of the U.S. catfish industry, one rapid means of generating genetic improvement was to produce hybrid catfish obtained from artificial spawning of the female channel catfish with the male of another species – the blue catfish (Ictalurus furcatus). Based on research this hybrid seed stock has demonstrated higher returns for feed invested, higher fillet yields and higher disease resistance than either the channel catfish or blue catfish parent.

Incorporating this hybridization technique into the channel catfish industry required major changes in the method of spawning in hatcheries and presented a suite of new challenges. Most important of the changes in hatchery methods was that female channel catfish and male blue catfish could not be mated in ponds during the natural spawning season.

Additional and more specialized steps in the fry-fingerling production process required:

• Handling thousands of individual broodstock fish, including distinguishing between males and females.
• Selecting females ready for artificial spawning.
• Purchasing of hormones and anesthetics.
• Injecting females to stimulate egg production.
• Checking for readiness to release eggs.
• Manually collecting eggs.
• Obtaining blue catfish males and harvesting sperm.
• Developing commercial-scale procedures for collecting and fertilizing eggs.
• Investing in labor and training for hybrid catfish production.

A fundamental step in hybrid catfish production is egg collection. This requires individually handling thousands of fish during the spawning season to select females to receive a hormone injection, and then identifying those that are ready for collecting mature eggs.

Farmers cull females presumed to have small, undeveloped, immature or degraded eggs. As a result, they discard females with flat bellies and keep only females with external features that indicate readiness for egg collection. Visually assessing females has been used in research and continues to be a method of selecting females. But it is highly subjective, varies from farm to farm and relies heavily on workers whose skill levels vary among hatcheries. Even within natural pond spawning, only 30-50 percent of the females in a pond will produce seed stock. Obtaining highly desired hybrid seed stock, therefore, intensifies the challenge of selecting females with eggs capable of fertilization.

Ultrasonography is commonly used as an assisted reproductive technology in humans and farm animals. Ultrasound imaging uses sound waves and electricity to display images of internal anatomy. The outstanding feature of this technology is that it can provide quick, noninvasive insight into internal biological processes. This technology has the potential to assist catfish farmers in artificial spawning, particularly in selecting female channel catfish for egg collection.

Of the more than 30,000 fish species, ultrasound has been used for reproduction studies in fewer than 30 species. A review of the use of ultrasonography in fish reproduction revealed the need for improving handling and ultrasound imaging to develop standardized methods of evaluating channel catfish females in hatcheries.

To test this technology in channel catfish, initial trials were conducted in 2004 at the AgCenter Aquaculture Research Station during the natural spawning season when the ovaries were large and easily visible. Keeping the fish submerged and using the water as a transmission medium produced clear images of the ovary. Subsequently, standard handling and imaging techniques to rapidly view the ovaries of unrestrained, submersed, unanesthetized channel catfish were developed and have been used for the past eight years.

The instrument used for viewing channel catfish ovaries consists of a laptop ultrasound unit with a linear probe initially designed for veterinary diagnostic imaging of farm livestock. It can be taken to ponds and raceways by using a cart that includes power for the unit, a cooler for holding 10 to 15 gallons of water for positioning the female in an upright swimming position, and baskets and dip nets for moving fish from holding systems. Rapid high-throughput imaging and record ing were accomplished by two operators, with one person positioning the probe and fish and the other person labeling and storing the images. At one seven-hour session a team of two can process 650 catfish with an average imaging duration of 30 seconds for each.

This combination of equipment and procedures meant that the additional handling previously used for fishes – which involved applying ultrasound gel, positioning the fish in a holding apparatus, using anesthesia and exposing the fish to air – were not necessary. Possible injury to valuable fish was avoided with minimized handling, thus providing the full potential of rapid, noninvasive imaging assessment in fish reproduction. This provided unprecedented access and recording of biological data on the internal form, structure and reproductive condition of the ovary. This has practical applications in catfish egg production in genetic improvement efforts such as the channel-blue hybrid catfish.

Although ovarian and egg development is a complicated biological process that can be studied at microscopic levels, real-time imaging of the ovary provides an immediate assessment of reproductive conditions, as well as new biological insights for its use in research and commercial settings.

Presently, the U.S. catfish industry is using hybridization technology for improving production. In commercial hatcheries producing hybrid catfish, ultrasound imaging for assessing ovarian development in channel catfish assists in cutting expenses by culling poor-quality females. This saves on expenses and focuses efforts on the females most likely to give quality eggs. In the future, these same capabilities developed for hybrid production at a commercial scale can be applied for genetic improvement to produce superior channel catfish to avoid the extra effort in production and use of hybrids.

Noel Novelo is a doctoral student and Terrence R. Tiersch is a professor at the Aquaculture Research Station, Baton Rouge, La.

(This article was published in the winter 2013 issue of Louisiana Agriculture magazine.)