Sneaky Sailfish

The Atlantic Sailfish (Istiophorus albcans) is a member of the billfish family, characterized by their elongated and narrow rostrum (bill). Despite being one of the fastest fish in the ocean, recent studies have found that their speed is not the defining factor in a successful hunt, but in fact their bill. Hunting in groups, sailfish herd schools of sardines up towards the surface to feed (Domenici et. al, 2014).

Image: Photographer James Scott Flickr.com

Image: Domenici et. al paper
cited in references

A study published in April 2014, documented the behavior of a sailfish school off the coast of Mexico. Over a week of filming, researchers filmed groups of sailfish up to 40 individuals strong, hunting adult sardines (Sardinella aurita). The sardine school was pushed to the surface by the sailfish, and also targeted above by other predators such as frigate birds (Fregata magnificens) and pelicans (Pelecanus occidentalis). One by one, the sailfish would take turns in chasing the sardine school, pushing their bill into the school, and either individually tapping sardines, or slashing their bill through the entire school. These techniques did not result in the immediate capture of prey, but it was after several damaging attacks on the school that the sailfish were successful in acquiring and ingesting the prey. It was assumed at the time that sailfish only attacked individually, as opposed to as a group, to avoid injury to each other. Previous to this recent study, this technique was only speculated due to stomach contents collected from sailfish that had been caught. Fish in the stomach of the sailfish were sometimes found to have damaging gashes and a significant loss of scales (Domenici et. al, 2014).

Image: Photographer James Scott Flickr.com

As seen in other larger marine vertebrates such as orcas, dolphins and whales, speed and agility can be an issue when prey is significantly smaller than the predator. It is thought that the rostrum on the sailfish (and probably other billfish species) is a morphological adaptation, designed to be narrow as it slices through the water at speeds that are some of the fastest recorded in marine vertebrates ever. The means that the speed of the bill is significantly quicker than the swimming speed of the prey, and it is virtually impossible for the prey, being sardines in this case, to outrun the bill of the sailfish. In no observed instances during the study were sailfish seen to impale or spear sardines (Domenici et. al, 2014). It was also found that the behaviour of the sardines closest to the bill during the attack was the same as those that were out of the bill range, which suggested that the fish had not even noticed the presence of the bill. This may also be another underlying reason as to why the bill is so narrow, because visually it is undetected by prey. 

It is also noticeable in the video (see below!) that the sailfish change colour and raise their dorsal fins as they approach the sardine school. Although not much is known about this behavior, it is possible that the characteristically large dorsal fin is used for herding the school or stability on approach, and their change in colour is potentially linked with excitement, or visual communication with other sailfish in the vicinity (Domenici et. al, 2014). Seeing as this study is only recent and just the beginning of what may be more of an insight into the world of sailfish behavior, it is likely that further research will be undertaken in the future.

Video Credit: Alexander Wilson (youtube.com)

References:

Domenici, P., Wilson, A.D.M, Kurvers, R.H.J.M., Marras, S., Herbert-Read, J.E., Steffensen, J.F., Krause, S., Viblanc, P.E., Couillaud, P., Krause, J. (2014) How sailfish use their bills to capture prey. Proceedings of the Royal Society of Biological Sciences. 281(1784):1471 - 2954. doi: 10.1098/rspb.2014.0444