Do you remember in Finding Nemo, when Dory and Marlin are trying to escape to the ocean and land on a dock? Then hundreds of Pelicans flay after them and try to scoop them up with their beaks saying “Mine, Mine, Mine”? As viewers of the scene it is quite stressful to watch because there are moments when the birds are very close to getting the fish. Surviving predatory attacks is a very common occurrence for creatures, namely fish who could be another creature’s dinner at any moment if they can’t defend themselves.

One study focused on how a specific kind of fish: sulphur mollies use their school of fish to escape bird predation (Doran et al. 2022). These fish are known to swim up to the surface of the water frequently due to the low oxygen concentration in the water at increasing depths. When they swim to the surface is also when hungry birds who are looking for a fish dinner tend to attack. How they defend themselves from a bird attack is very interesting. The school of thousands of fish works together to create conspicuous, repeated waves. To create the wave, there are a few different tasks to be done by fish in specific locations. For example, the fish that is closest to the predator will dive down first using their tail-kick to disturb the water’s surface. Fish that are nearby the first fish will repeat the same behavior causing the disturbance at the water’s surface to move away from the predator in an L-shaped wave.

Many times the behavior of the bird can warn the school of fish that they are going to attack the water and attempt to capture some of the fish. The bigger birds especially trigger pre-attack waving when nearing schools of fish.

Scientists hypothesized that the waves produced by the fish would slow the more times they occurred due to the inability of fish to get necessary oxygen if they are repeatedly diving deeper when tail-kicking. However, the prediction was not true as the intervals of waves were overall equal in length the entire time that the schools of fish produced the waves. It was also found that waves were regular, conspicuous, and repeated. These findings indicate that the fish could be sending a signal but could also just be an escape behavior that also produces confusion in predators.

To gain an understanding of how the antipredation wave-producing of schools of fish affected the predation behavior of birds, scientists recorded videos of the fish-bird interaction in the water. They counted the number of times that the birds would swoop down into the water in an attempt to get a fish, the number of times that the bird flew away with a fish, the number of waves produced by the schools of fish, and most significantly the time between intervals of attack by the birds on the fish (“waiting bout”). Scientists also attempted to reproduce the wave-like behavior by the schools of fish in order to determine if it has any effect on the time in which the birds attack or the waiting bout. They found that overall the experimentally-produced waves nearly doubled the waiting time of birds before attacking fish. This is significant because it provides evidence for the researcher’s hypothesis that the production of waves will decrease the time of attacks by birds. Additionally, it was found that the success of attacks or the birds leaving an attack with a caught fish decreased as the number of waves produced increased. Finally, evidence was found to support the idea that the production of waves also reduced the probability of future attacks by birds.

Many hypotheses about animal behavior can be made following these results. Firstly, it can be assumed that the fish understand the tail-kick behavior of nearby fish as a cue indicating possible predator presence nearby. This would likely cause them to quickly dive deeper as well and aid in their fitness advantage as they did not have to detect the predator themselves. This could have led to the evolution of synchronized diving behaviors in schools of fish (kind of like the synchronized divers that compete in the Olympics!). Additionally, birds that attacked fish that were producing waves probably learned to wait for waves to resolve before making their second attack on a school of fish.

Hopefully, this answers your questions about how fish in the real world (outside of Nemo’s cartoon universe) defend themselves when they are being hunted for dinner by birds! Additionally, it might help you better understand the behavior of animals or encourage you to keep asking questions about animal behavior!

 

Citations: 

Doran, C., Bierbach, D., Lukas, J., Klamser, P., Landgraf, T., Klenz, H., … & Krause, J. (2022). Fish waves as emergent collective antipredator behavior. Current Biology, 32(3), 708-714. https://doi.org/10.1016/j.cub.2021.11.068

 

 

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