WHAT CONTROLS THE ONSET OF ANOREXIA IN MATURING ADULT FEMALE ATLANTICSALMON

1. Atlantic Salmon (Salmo salar) cease feeding before entering their h ome river to undertake upriver migration and spawning. Here we examine whether the onset of this anorexia is related to body condition and/o r gonad development. 2. The food intake and growth rate of 100 individ ually marked one-sea-winter fish (stocked in an outdoor tank) were mon itored regularly over 11 weeks during summer. The relative size of bod y lipid reserves, lean mass and gonad size were estimated in all matur ing females in midsummer and at the end of the experiment. 3. The date on which individual fish stopped feeding varied greatly; after ceasin g to feed all lost weight at a consistent rate of 0.1% per day. 4. The onset of anorexia was significantly correlated with estimates of fat reserves, lean mass and gonad size. However multivariate analysis show ed that lean mass was most important. Female salmon thus appear to con tinue feeding until their lean mass has reached threshold levels. The evolutionary implications of these findings are discussed with respect to inter-population variation in the length and relative difficulty o f upriver migration.