A noninvasive method to determine fat content in small fish based on swim bladder size estimation

Abstract

The presence of fat stores in fish is widely used as a correlate of fish health and fitness. Techniques ABSTRACT to measure fat content with some accuracy are available for mediumsized and large fish, but apart from morphometric indices, a noninvasive method to determine fat content in small fish has hitherto been lacking. In this study, we introduce a novel method to measure the fat content in live fish that can be applied also to small fish of less than 0.5 g of body mass. This approach relies on a precise measurement of the swim bladder volume, from which fat content can subsequently be deduced. As fat is positively buoyant, fish with larger fat stores require a smaller swim bladder to attain neutral buoyancy. To determine swim bladder volume, we developed a measuring device, which makes use of the differential compressibility of air and water. A fish is placed in a pressure-tight chamber to which a standardized amount of water is added. The resulting change in pressure Δp is inversely proportional to the volume of the swim bladder. Using juveniles and adults of Simochromis pleurospilus (Nelissen, '78; Pisces: Tropheini) a small cichlid fish, we show that Δp is tightly related to structural size, mass, and body condition. Most importantly, this approach allows to predict the visceral fat content of small fish more precisely than the six most commonly used morphometric body indices.The presence of fat stores in fish indicates the possession of surplus energy, as fat is the primary energy storage substrate in fish (Love, '70; Adams, '99;Tocher, 2003). Since fat content is a good index of future survival in some species (Simpkins et al., 2003) and a strong indicator of reproductive potential in some fish stocks (Marshall et al., '99), the amount of fat deposited by an individual has been used as a correlate of health or, more generally, of fitness (Adams, '99). Interests to quantify fat deposits of fish have thus emerged in different disciplines. A commercial fish breeder will use it as an indicator of health and animal welfare in his live stock (Novotny and Beeman, '90), ecologists may use it to judge the well being of wild populations (Liney et al., 2006;Stevenson and Woods, 2006) and in evolutionary studies the existence of energy reserves are an important determinant of optimal allocation decisions within trade-offs between growth, storage, and reproduction (Glazier, '99 Many research fields have adopted small fish as model species, often requiring to obtain repeated measurements of the study animals. For example, marine biologists study dispersal of small reef fish (Mora et al., 2003), behavioral biologists investigate mate choice in sticklebacks (Bakker et al., '99;Milinski, 2003), evolutionary biologists study species radiation and adaptation in cichlids (Barlow, '98;Seehausen, 2006), guppies (Carvalho et al., '96) or sticklebacks (Schluter, '93), and toxicologists test the effects of chemical compounds on zebra fish (Bresch et al., '86). Existing techniques to measure the fat content in fish...

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