El canibalismo como mecanismo regulador denso-dependiente de mortalidad natural en la anchoíta argentina Engraulis anchoita. Su relación con las estrategias reproductivas de la especie.
Cannibalism is not an abnormal behaviour and it can be observed in a wide number of animals. In Pisces, cannibalism may be the result of endogenous intraspecific factors (nutritional state, size, age, state of development) or exogenous intraspecific factors (food, density, light). Nevertheless, external fertilization and the high fecundity of teleost fish tend to increase egg cannibalism, in particular among filter feeding fishes such as anchovies. In the present work, two important Argentine anchovy Engraulis anchoita stocks were studied. The first is to the north of 41°S, and spawns during spring on the coastal and shelf waters of Buenos Aires Province,Argentina, and Uruguay. The second one inhabits the Patagonian region, from 41°S to 47°S. It spawns during late spring and summer when a seasonal stratification is induced by thermocline formation. In both regions, food was scarce during the spawning season and the principal prey were copepods (calanoid, harpacticoid and cyclopoid). In the northern region greater spawning occurred in coastal waters (less than 50m) and the highest density of copepods was registered on the shelf waters (more than 50m). By contrast, in Patagonia larger densities of copepods and anchovy eggs were closer to the frontal areas.Comparing both regions, higher densities of copepods were calculated in the Patagonian area. To estimate the percentage of eggs mortality caused by cannibalism, it is necessary to determine the number of hours per day that anchovy spend on feeding during spawning season. A total of approximately 12. hours per day, from dawn until late afternoon were calculated. Egg cannibalism estimated reflected significant differences between northern and Patagonian populations during the four years studied: an estimated mean value of 3.9-10.Eggs for the northern and 0.8-2.6 eggs for the southern population. Comparing day and night egg cannibalism, during day light hours the mean value was 8.10 while after sunset the value diminished to 2.96 eggs. To estimate the percentage of eggs mortality due to cannibalism the methodology developed by Hunter and Kimbrell (1980) and MacCall (1980), and a modification of each, was utilized. Under the Hunter and Kimbrell method, the egg mortality of the Argentine anchovy northern stock caused by cannibalism for the period 1993-96 was about 45-84. ,while in the Patagonian stock the values ranged between 2-15. . Applying the MacCall method, the percentages were lower than Hunter and Kimbrell: 15.33. in the northern population and 1-5. in Patagonia. The functional relationship between egg density and egg consumption rate by individual anchovies in both areas was a power function. In other words, egg cannibalism operates in a compensatory manner when both plankton eggs densities and consumed eggs are high. But in the Patagonian area cannibalism would increase with large densities of copepods. This observation indicates that most of eggs are eaten accidentally when schools are feeding on the other more common organisms. Analyzing the contribution of egg cannibalism to the total energy necessary for spawning, it. is estimated that this behaviour can support satisfy 3-8. of the daily energy requirements. However, in some places where the egg cannibalism is very intense, anchovy can meet their energy requirements without any alternative food. Finally, it is concluded that cannibalism can affect the reproductive strategies of both anchovy populations in different ways. In spite of some strategies that help to avoid it, other ones tend to support that behaviour. It is probable that the reproductive strategy of the northern population, in which eggs are spread in a larger area over an extended reproductive season, may be the principal cause of the high cannibalism on eggs estimated.