Sibling cannibalism in dorada under experimental conditions. I. Ontogeny, dynamics, bioenergetics of cannibalism and prey size selectivity

Cannibalism among sibling dorada Brycon moorei started before the yolk sac was completely absorbed, as soon as oral teeth were completely developed (1 .2 mg fish, 21 h after hatching, 39 h after fertilization at 27.0 +/- 0.5 d egrees C under 12L: 12N). Embryos attacked siblings of equivalent size or s lightly larger than themselves, which were incompletely ingested, sucked up to the head and regurgitated (type Ia cannibalism). Two-day-old larvae per formed complete ingestion but could not digest the head of their prey, whic h was regurgitated (type Ib cannibalism). One day later, all cannibals had turned to complete (type II) cannibalism. Type II cannibalism persisted dur ing the larval period (ending c. 144 h after hatching, 26-38 mg fish) and t he early juvenile stage (15-30-g fish). The logistics of type II cannibalis m (maximum prey to cannibal weight ratio, W-p: W-C in percent) was modelled as 11.9607 W-C(-0.3429) (r(2) = 0.974, P < 0.0001), where W-C is the body weight of the cannibal (g), indicating that cannibals had to turn to increa singly smaller prey during their ontogeny. When being offered prey of diffe rent sizes, cannibals of all sizes (0.04-27 g) preferred the smallest prey available almost systematically. A shortage of prey of appropriate size cau sed them to turn to larger prey, and eventually to exert incomplete canniba lism over siblings exceeding the maximum W-p: W-C ratio. Cannibals could in gest extremely high food rations [R-max (% W-C) = 47.4242 W-C(-0.4002), r(2 ) = 0.906, P = 0.0126], and showed extremely fast growth (G [% day(-1)]= -2 .5895+0.5194 R-0.0007 R-2; r(2) = 0.974, P < 0.0001). These traits caused c annibalism in dorada to be the earliest and most intense ever reported in f ish (95-98% fish cannibalized within the first week, of which c. 40% on the first day). The functional, adaptive and evolutionary implications of earl y predation and shifts between types of cannibalism in dorada are discussed . (C) 2000 The Fisheries Society of the British Isles.