56 DECEMBER 2012 • WORLD AQUACULTURE • WWW.WAS.ORG as good had a 56 percent strip-spawn rate (category 4) and an average egg collection of 77 ml per kg of female injected (Fig. 3). The group of fish classified as excellent had an 84 percent stripspawn rate (category 5), and an average egg collection of 350 ml per kg of female injected. The corresponding strip-spawning and egg collection outcomes of fish in the three categories reflect the progression of ovarian maturity visible using ultrasonography. These results indicated that ultrasound imaging of channel catfish ovaries prior to hormone injection was a useful tool for assessing ovarian condition, clearly separating fish that could not be strip-spawned from those that could be strip-spawned. Subsequently a seasonal record of ultrasound images depicting changes in ovarian morphology during the initial period of vitellogenesis (October to November 2008) through the end of the spawning season (June 2009) in Louisiana was compiled. Building on this and other commercial-scale application studies, a classification system is being developed for the channel catfish ovarian cycle to improve selection of broodstock for increased control and efficiency in reproduction. The availability of ultrasonography will help minimize costs of hormone and labor by increasing success in identifying those individuals ready for spawning, with the added potential for sorting among female broodstock that externally would appear similar prior to reproductive conditioning and investment in feeding and maintenance. Image Interpretation and Key Considerations for Use in New Species In anticipation of expanding interest in use of ultrasound imaging in fish reproduction, interpretation of ultrasound images is briefly explained in general (Side Bar 1) and in specific reference to ultrasound images of the channel catfish ovary. In addition, key considerations for ultrasound imaging application in new species are presented (Side Bar 2). Ultrasound imaging requires practice, patience, and persistence to develop the necessary interpretation skills. Tissues have different gray-scale appearances in ultrasound images and organs, because they are composed of various tissues, also vary in appearance. Therefore, an understanding of diagnostic ultrasound principles, mechanisms of image generation (Toal 1996, Krekau 1980, Nyland et al. 2002, Novelo and Tiersch 2011), and basic features (Side Bar 1) of ultrasound images are important for image interpretation. The identification of structures in the general layout of an ultrasound image is achieved by understanding the relationship of the position of the probe against the external anatomy (Fig. 1) to the position of the probe in the ultrasound image in the near-field view (area displaying anatomical structures nearest to the probe at the top of the image) and far-field view (area displaying anatomical structures furthest from the probe towards the bottom of the image). Identification of anatomical structures in the ultrasound image (Fig. 2) requires reliable knowledge of anatomy of the fish, especially in regards to positioning of the probe, location of the organ being assessed, and adjacent structures. After the overall landmarks of the ultrasound image are defined, familiarity with appearance of the target structure in the image is gained through experience. Features such as image appearance, ovary appearance, changes in the area occupied by muscle tissue between the skin and the ovary, the visibility, size and shape of oocytes and ovary, and biological insight of the annual ovarian cycle constitute specific parameters for interpretation of ultrasound images of channel catfish. For instance, images in Figure 2 were obtained during the natural spawning season when ovaries with distinct morphologies were present. During the spawning season (mid-April to late June), the progression from small ovaries (Fig. 2A) to enlarging ovaries due to rapid vitellogenic oocyte growth (Fig. 2 B,C), and atretic Side Box 1: Basic features of B-Mode ultrasound images (Toal 1996) 1. Strong reflectors are white. 2. Poor reflectors are black. 3. Bones and gas appear bright white. 4. Blood and homogeneous fluid appear black. 5. Soft tissue appears greyish, granular, and varies. 6. Connective tissue and fat appear as moderate white echoes. 7. Excessive fat reduces amount of sound that penetrates. Side Box 2: Considerations for use of ultrasonography in unstudied fish species 1. Consider using low cost (US$2,000 — 10,000) ultrasound units. 2. Know the physical location of the internal anatomy of the fish. 3. Work first during the spawning season when gonads are easiest to identify. 4. Collaborate if possible with an experienced ultrasonographer for test trials. 5. Integrate with existing routine procedures used in spawning. 6. Keep the fish submersed and positioned for ease of imaging and minimal fish handling. 7. Ensure that the interpreter’s scanning position and that of the fish are consistent and provide easy access to equipment for systematic imaging and recording. 8. Imaging and recording is most efficient with two operators: (1) the interpreter, who handles the fish and positions the probe, and (2) an assistant, who operates the ultrasound unit, labels files, and records images in coordination with the interpreter.
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