Determining the sex of individuals is often crucial to effective fishery management. Growth rates, for example, can vary by sex, as female walleye grow faster and attain larger sizes than males (Henderson et al. 2003). In impoundment management, stocking female only largemouth bass can prevent overpopulation and maximize growth potential (Bonvechio and Rydell 2016). In aquaculture, manipulating catfish sex ratios can influence egg and young production (SRAC 2014), requiring culturists know the sex of fish. As catfish are one of the most popular groups for aquaculture, sex-related aspects of production and spawning behavior have been fairly well-described (at least in captivity for some species).
The North American catfish (family Ictaluridae) are cavity spawners. They spawn in the cover of undercut banks, woody debris and man-made materials, including even aluminum cans for smaller species such as the madtoms (genus Noturus). As a result, fisheries managers sometimes sink spawning boxes to improve spawning habitat for catfish in impoundments. The boxes provide cover to enhance survival of young, reducing predation by sunfishes and largemouth bass. Spawning begins in late spring or summer when water temperatures exceed 70°F for blue and channel catfish, whereas flathead catfish spawn between 66-75°C (Hubert 1999, Jackson et al. 1999, Hewitt et al 2009).
Catfish sex can be determined by examining secondary sex characteristics. In the genus Ictalurus (i.e. blue Ictalurus furcatus and channel catfish Ictalurus punctatus), males develop wide, muscular heads with thick lips, but this is particularly evident in large, mature individuals (Figure 1). The pigmentation of males is often much darker than females, becoming dark blue or grey in color. This often leads to misidentification of channel catfish for blue catfish by anglers within the species’ native ranges where blue catfish are uncommon. These differences are most drastic during the spawning season and in larger individuals. In juvenile fish and smaller adults, these differences may not be as evident and other methods aid biologists in determination of sex. We find that most large (> 50 lbs) blue catfish we encounter are males, which is the opposite of many other gamefish, where females are larger (walleye, yellow perch, musky).
Figure 1. female blue catfish (left) versus male blue catfish (right). Note the wider head and heavier musculature of the male.
In my experience working with catfish, I have used three different methods to determine sex. Two are non-lethal techniques and the third involves observation of the gonads during dissection. A female full of eggs is easy to identify, but a mature male may not be so obvious (Figure 3). Most people without fisheries training probably identify a male due to the lack of eggs. However, post-spawn females and young females without large egg-filled ovaries could lead to inaccurate assumptions on sex. The testes of blue and channel catfish are not large, distinct white organs like you see in many other fish species. They are located close to the “roof” of the gastric cavity and consist of numerous small finger-like projections.
Figure 2: Differences in chin pigmentation of female (left) and male (right) blue catfish from the James River, Virginia. The female remains lightly colored during the spawning season, whereas the male becomes a dark blue or grey color, often with mottling.
While dissection provides definitive identification of gonads, management objectives may deem sacrificing fish undesirable. In these cases, non-lethal sex determination techniques are generally effective. During a study of a West Virginia channel catfish population, we correctly identified the sex of over 99% of fish examined when validating non-lethal methods with dissection. Catfish sex can be determined by examining the urogenital region of the fish. Just behind the anus is the opening(s) of the urogenital systems. Biologists can identify the sex of an individual by looking for the urogenital papilla, which is a nipple-like projection that surrounds the urogenital opening of males. The female urogenital region lacks the distinct urogenital papilla and looks more flat or even recessed slightly. Females have a urogenital septum that separates the urinary and genital openings, giving the opening a more slit-like appearance. Fish can be sexed by looking for the urogenital septum using a blunt dissection probe (or a mechanical pencil if you’re an unprepared fisheries student). Next time you handle a catfish, be sure to take a gander at its underside and try your hand at catfish sexing!
Figure 3. Pictured are female (left) and male (right) gonads of dissected blue catfish from the James River.
Figure 4. Urogenital regions of female (left) and male (center and right) blue catfish from the James River, Virginia. The center photograph shows the urogenital papilla of a large male (~40 lbs.), whereas the photograph on the right shows a more typical urogenital papilla of smaller males. Note the anus is shown at the top of these photographs and the urogenital opening(s) are directly below.
Bonvechio, T.F. and J.J. Rydell. 2016. Use of a female-only stocking strategy to establish a trophy largemouth bass fishery in a Georgia small impoundment. Journal of the Southeastern Associated Fish and Wildlife Agencies 3:136–143.
Henderson, B.A., N. Collins, G.E. Morgan and A. Vaillancourt. 2003. Sexual size dimorphism of walleye (Stizostedion vitreum vitreum). Canadian Journal of Fisheries and Aquatic Sciences 60: 1345–1352.
Hewitt, A., J. Ellis and M. C. Fabrizio. 2009. Fisheries of the York River System. Journal of Coastal Research 57:99–110.
Hubert, W.A. 1999. Biology and management of channel catfish. Pages 3–22 in E.R. Irwin, W.A. Hubert, C.F. Rabeni, H.L. Schramm, Jr., and T. Coon, editors. Catfish 2000: proceedings of the international ictalurid symposium. American Fisheries Society, Symposium 24, Bethesda, Maryland.
Jackson, D.C.. 1999. Flathead catfish:biology, fisheries and management. Pages 23–35 in E.R. Irwin, W.A. Hubert, C.F. Rabeni, H.L. Schramm, Jr., and T. Coon, editors. Catfish 2000: proceedings of the international ictalurid symposium. American Fisheries Society, Symposium 24, Bethesda, Maryland.