52 DECEMBER 2012 • WORLD AQUACULTURE • WWW.WAS.ORG Ultrasound Imaging — What It Is, How It Works, and Its Application in Fish Reproduction Ultrasound imaging is a non-invasive technology that makes use of ultrasound frequencies (> 20,000 hertz) and electric energy to create images of internal anatomy. Electrical energy is transmitted to a probe that is placed near the anatomical structure being evaluated. The probe interconverts electric and sound energy, emitting sound waves that travel through a medium (transmission gel or water) until they come in contact with the skin and internal anatomical structures of the target animal. The returning echoes are received by the probe, which transmits corresponding voltages to the ultrasound unit, where the electric information is processed into a visual display on a monitor. The user interface of the ultrasound unit — the monitor display, control keys, knobs, sliders, and ultrasound software — provides the means by which users can control, view, and analyze ultrasound images. Use of ultrasonography in human and veterinary diagnostics is widespread and well known, especially in the fields of reproduction and human pregnancy. Use of this technology in fishes has been applied for the past 30 years in studies of freshwater, marine and anadromous fishes. The majority of the publications on the use of ultrasonography in fish reproduction address studies of wild fish stocks (less so for farmed species) of a broad size range, including Pacific herring, sharks, and endangered species such as the pallid sturgeon and Neosho madtom (Table 1). Studies on cultured species include Atlantic cod, Atlantic halibut, haddock, salmon, and striped bass (Table 1). Currently most use of this technology has been as a research tool. However, ultrasonography has great potential in high-throughput applications such as broodstock reproductive assessment and management. Lack of basic information, such as specific settings and detailed handling procedures, is one potential constraint for expanded use with fishes as compared to human and veterinary applications. In addition, the technology may be perceived as prohibitively expensive, and in fact units designed for human medicine can cost more than US$100,000, although units of around US$10,000 and less are suitable for work with fish. Further, unlike veterinary applications in farm animals where the number of species is small, the application of ultrasound imaging to fish involves highly diverse shapes, sizes, morphologies, life histories, habitats, and reproductive processes across a large number of species (>30,000). Finally, previous use of this technology in fishes has been based on adherence to techniques largely developed for use in farm animals; for example, the use of transmission gels for ultrasound imaging of fish out of the water. Of the known fish species, fewer than 25 have been evaluated thus far using ultrasonography for sex identification and in development of reproductive indices, signaling the potential for applications in additional species. Even as future work with fishes is explored and international interest in using this technology in assisting reproduction in aquaculture and fisheries is growing, a recent review of published literature on the use of ultrasonography in fish reproduction (Novelo and Tiersch 2012) identified the need to: (1) incorporate the use of unrestrained, non-anesthetized, underwater ultrasound imaging, (2) report essential information (Table 2) with specific text descriptions and illustrations of the procedures, and (3) develop standard ultrasound imaging procedures for different taxonomic groups of fish commensurate with their biological diversity and specifically designed to minimize fish handling (Table 3). With these things in place, emphasis can be placed on exploiting the full potential of this non-invasive imaging technology while ensuring animal care and welfare. Ultrasound Imaging of Channel Catfish Channel catfish Ictalurus punctatus is an important farmed product in the US (census.gov/compendia/statab/2012/ tables/12s0898.pdf). However, in the past few years, the hybrid produced by female channel catfish and male blue catfish I. furcatus (channel × blue hybrid) has been increasingly cultured because feed efficiency, growth rate, and processing yields are superior to both parental species (Argue et al. 2003). Genetic improvement programs require high levels of control over reproduction and spawning, especially efforts such as those necessary for the production of the channel × blue hybrid, which requires handling of broodstock, and hormone injection of adult females for collection of eggs capable of fertilization. Furthermore, variable spawning rates (the number of females spawned of the total number of females) obtained in commercial pond reproduction (30-50 percent) (Wolters and Tiersch 2004) and induced spawning research (50-100 percent) (Dunham et al. 2000, Krisanto et al. 2009, Phelps et al. 2011) reveal the critical need to improve procedures for selection of quality female broodstock and efficiency in reproduction. Ultrasonography in fishes has been applied for the past 30 years in studies of freshwater, marine, and anadromous fishes. It has great potential in highthroughput applications such as broodstock reproductive assessment and management. Ultrasound Imaging of Channel Catfish Reproduction Noel Novelo and Terrence Tiersch*1
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