Effect of Photoperiod on Feeding, Intraperitoneal Fat, and Insulin-like Growth Factor-I in Sunshine Bass*

The length of the photoperiod has been implicated as affecting growth of sunshine bass. This study was done to determine if photoperiod length might affect insulin-like growth factor-I (IGF-I), which is a major hormonal regulator of growth in fish. Growth, feed conversion, peritoneal fat content, and plasma concentrations of IGF-I of sunshine bass were compared on fish held on a short (8 h) or a long (16 h) photoperiod, or after the fish were switched from one photoperiod to the other. Fish were fed daily to apparent satiation for up to 5 wk and were not fed for the last 2 wk of the experiment. Body weight and intraperitoneal fat content increased for the first 2 wk of the experiment and then remained steady for the remainder of the experiment, and feed consumption decreased from 3.5% during the first 2 wk to about 1.5% for the second 2 wk and finally to only 1% during the last week of feeding. Plasma IGF-I concentrations fell steadily during the entire experiment and was at the lowest level during the last 2 wk when the fish were not fed. There was no consistent effect of the photoperiod in any of the conditions; however, feeding and IGF-I concentrations were affected by fish density. Environmental conditions required for the successful culture of hybrid striped bass are fairly well understood (Tomasso 1997), as are the lethal concentrations of toxic by-products of fish metabolism such as ammonia (Openborn and Goudie 1993; Weirich et al. 1993). The impact of environmental conditions on physiological growth regulators is less well understood. If physiological measurements reflect the growth of fish, evaluation of culture condition could be accomplished much quicker and cheaper than lengthy growth studies. In outdoor culture situations, management for optimum conditions is limited by the changing of the seasons and cost constraints. In recirculating or flow-through culture, environmental conditions, such as temperature and photoperiod, can be more easily regulated. Palmetto bass (striped bass female 3 white bass male) had higher survival in shaded aquariums and ponds than in nonshaded aquariums and ponds (Rees and Cook 1985). Palmetto bass have also been shown to grow faster during increasing rather than decreasing photoperiods in fiberglass tanks (Woiwode and Adelman 1991) and in ponds (Kerby 1993). The length of the photoperiod can easily be manipulated in intensive culture and might be an important environmental condition that could be easily manipulated with little additional cost to producers. Fish growth is most directly affected by feed nutrients and the amount of feed consumed. The major endocrine axis controlling growth is regulated by the pituitary/liver/somatic axis. Growth hormone from the pituitary stimulates the liver to synthesize and release insulin-like growth factors (IGF-I and IGF-II) from the liver. IGF-I is a powerful mitogenic factor that helps maintain cell number by reducing apoptosis and is considered the most important hormonal growth regulator in mammals (Lupu et al. 2001). Extensive reviews of this axis in fish have been published (Duan et al. 1999; Moriyama et al. 2000; Kelley et al. 2001). Plasma IGF-I concentrations have been correlated with 1 Corresponding author. 2 Present address: Catfish Genetics Research Unit, United States Department of Agriculture, Agricultural Research Service, 141 Experiment Station Road, Stoneville, MS 38732, USA. * Mention of a trade name, proprietary product, or specific equipment in the article is solely for the purpose of providing information and does not imply recommendation or endorsement by the US Department of Agriculture. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 37, No. 4 December, 2006 Copyright by the World Aquaculture Society 2006