Feed preferences in juvenile cod estimated by inert lanthanid markers -: effects of moisture content in the feed

There are indications that low ingestion rates of formulated feed may limit the growth of early juvenile fish, including cod. This has particularly been observed during the weaning period, when the fish must change from a presumably palatable live prey to usually a dry diet. The study of the physical properties of the diet seems to be an underestimated area of research, compared to studies on the nutritional properties of the diet. In this study we used lanthanide markers as a tool in estimating diet preferences for examining the effect of dietary moisture content on ingestion rates in juvenile cod ( 0.8 g). Fishmeal-based diets were produced with five levels of water, giving diets with 35, 42, 54, 65 or 95% dry weight. Each diet was marked with a small amount of a lanthanide or yttrium oxide. The fish were hand fed on a mixture of these five diets. Ingestion rates where then estimated by killing 1/2 of the fish after one day and the rest after four days of feeding. The whole fish with gut content was homogenized and marker content was measured by ICP-MS. The amount ingested of each of the five diets could then be calculated from these figures. Despite the high variation in diet selection between individuals the results clearly indicate a higher intake of moist feed. This could be caused by a better palatability or to a higher availability caused by a lower sinking rate of the moist feed.