How to capture fish in a school? Effect of successive predator attacks on seabird feeding success

Abstract

Prey aggregations, such as fish schools, attract numerous predators. This typically leads to the formation of multispecific 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. Using underwater film footage 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. 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. 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. Our study suggests that, in multispecies predator assemblages, the cumulative effect (through disorganization 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 multispecific assemblages.

Positive Interactions among Foraging Seabirds, Marine Mammals and Fishes and Implications for Their Conservation

“…Their results suggest that both surface-feeding species and diving species should have greater success when foraging together. Furthermore, a higher frequency of attacks, particularly if varied in direction in three-dimensional space, would prevent schooling prey from organizing themselves, and result in higher success rate among all predators (Lett et al, 2014;Thiebault et al, 2016).…”

“…; Hunt et al, 1988;Harrison et al, 1991;Veit et al, 1993), prey capture probably increases over some range of flock sizes. If this is true, then certainly population growth rates of seabirds that depend on finding feeding flocks to find sufficient food need to be linked to the presence, frequency and size of those flocks (Thiebault et al, 2016). Irons (1998) found that breeding black-legged kittiwakes returned to the same feeding areas, and selectively joined flocks in preferred feeding areas-with preference shown for large flocks, which were typically associations with diving auks (Uria).…”

“…This process is often called "local enhancement" (Thorpe, 1956); the same term may refer to birds cooperating in the herding of prey in addition to passively providing cues to the presence of prey. The term "facilitation" means cooperation among species in which only one of the species receives direct benefit (Stachowicz, 2001;Thiebault et al, 2016). It is unclear for most seabird aggregations whether one or more species participating in the interaction derive benefit, however there are important implications; detection of prey may be positively density dependent through local enhancement (Grünbaum and Veit, 2003).…”

“…It is unclear for most seabird aggregations whether one or more species participating in the interaction derive benefit, however there are important implications; detection of prey may be positively density dependent through local enhancement (Grünbaum and Veit, 2003). In addition to serving as cues to the presence of prey, some marine predators such as dolphins, seals, tunas and other fishes, drive prey to the surface so that these prey become more readily available to birds ("facilitation"; Harrison et al, 1991; International Council for the Exploration of the Seas, 2010; Thiebault et al, 2016). Hence marine predators modify local resource distributions, with implications for the evolution of coexisting species and their interdependence (Laland et al, 1999;Laland and Boogert, 2008).…”

A Balanced Mixture of Antagonistic Pressures Promotes the Evolution of Parallel Movement

“…attack single peripheral prey individuals, and density attacks) are not uncommon (e.g. multi-species feedings)464748. Exposure to such conflicting, antagonistic predation pressures might have played a pivotal role in the evolution of collective behaviour.…”