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 plus or minus 0.5 degree C under 12L : 12N). Embryos attacked siblings of equivalent size or slightly larger than themselves, which were incompletely ingested, sucked up to the head and regurgitated (type Ia cannibalism). Two-day-old larvae performed complete ingestion but could not digest the head of their prey, which was regurgitated (type Ib cannibalism). One day later, all cannibals had turned to complete (type II) cannibalism. Type II cannibalism persisted during the larval period (ending c. 144 h after hatching, 26-38 mg fish) and the early juvenile stage (15-30-g fish). The logistics of type II cannibalism (maximum prey to cannibal weight ratio,W sub(p) : W sub(C)in percent) was modelled as 11.9607W -0.3429C (r super(2)=0.974, P< 0.0001), whereW sub(C) is the body weight of the cannibal (g), indicating that cannibals had to turn to increasingly smaller prey during their ontogeny. When being offered prey of different sizes, cannibals of all sizes (0.04-27 g) preferred the smallest prey available almost systematically. A shortage of prey of appropriate size caused them to turn to larger prey, and eventually to exert incomplete cannibalism over siblings exceeding the maximumW sub(p) : W sub(C)ratio. Cannibals could ingest extremely high food rations [R sub(max)(%W sub(C))=47.4242W -0.4002C; r super(2)=0.906, P=0.0126], and showed extremely fast growth (G[% day super(-1)]=-2.5895+0.5194R -0.0007 R super(2); r super(2)=0.974, P< 0.0001). These traits caused cannibalism in dorada to be the earliest and most intense ever reported in fish (95-98% fish cannibalized within the first week, of which c. 40% on the first day). The functional, adaptive and evolutionary implications of early predation and shifts between types of cannibalism in dorada are discussed. Copyright 2000 The Fisheries Society of the British Isles