Résumé
1. Prey aggregations, such as fish schools, attract numerous predators.
This typically leads to the formation of multi-specific groups of
predators. These aggregations can be seen both as a place of increased
competition and as a place of possible facilitation between predators.
Consequently, the functional role of such predator-prey aggregation is
uncertain, and its effect on individual feeding success is virtually
unknown. 2. Using underwater film footages of different predators feeding
on fish schools during the sardine run in South Africa, we directly
measured the in-situ feeding success of individual Cape gannets Morus
capensis in different foraging situations. 3. We determined the types of
Cape gannet attacks (direct plunge dive or plunge dive followed by
underwater pursuit) and we measured the occurrences and timing of attacks
from the different species (mostly Cape gannets and long-beaked common
dolphins Delphinus capensis). We also estimated the size of the targeted
fish schools. These observations were complemented with a simulation model
to evaluate the cumulative effect of successive predator attacks on the
prey aggregation structure. 4. The probability to capture a fish in one
feeding attempt by Cape gannets averaged 0.28. It was lower when gannets
engaged in underwater prey pursuit after the plunge compared to direct
plunge (0.13 vs. 0.36). We found no effect of the number of prey on
gannets’ feeding success. However, the timing and frequency of attacks
influenced strongly and positively the feeding success of individuals. The
probability to capture a fish was the lowest (0.16) when no attack
occurred in the few seconds (1-15 s) prior to a dive, and the highest
(~0.4, i.e. more than twice) when one or two attacks occurred during this
time window. The simulation model showed that a prey aggregation
disorganized just after an attack, and that the maximum of disturbance was
obtained a few seconds after the initiation of the successive attacks. 5.
Our study suggests that, in multi-species predator assemblages, the
cumulative effect (through disorganisation of school cohesiveness) of the
multiple species attacking a prey aggregation may increase the feeding
success of each individual. Therefore, facilitation between predators is
likely to overcome competition in these multi-specific assemblages.