Flotsam Friends

Fish are often found grouped together near debris floating on the surface. This can include floating human debris like lost nets or cargo called flotsam, or natural debris like kelp or seaweed. The question is why do these fish congregate around this debris? The fishes pull towards these floating objects has to do with a few things. They see it as a form of protection and security in the water column. By sticking near such large objects, fish can be better concealed from predators in the open water. They also have protection in numbers as many fish are found near the flotsam and floating plants. By joining the crowd, fish are less likely to be singled out and eaten by a predator if one arrives. Another purpose fish have is food. Floating objects can accumulate algae over time and fish feed on the algae. For floating plants like the sargassum we saw in the Florida Keys, fish will simply feed on the plant itself. This behavior was observed firsthand in the diving spots we visited. Almost every time we stopped the boat and went diving and snorkeling, swarms of Bermuda Chubs and Yellowtail snappers would come to the underside of the vessel. Sargassum patches would drift by and the fish could be seen picking at them for food. Two small fish were even following me around while snorkeling during shark tagging.

This behavior is understandable, but the relationship fish have specifically with ships instead of normal debris is not clearly understood. New research has been performed to see how fish react to ships by sailing out research vessels and allowing them to sit on the surface to see how many fish accumulate and how quickly. Experiments found fish counts grow by orders of magnitude within the first hour of the ship being anchored. This trend occurred day and night and persisted as long as the boat was there. The only times drops in fish counts occurred were when other ships sailed by and fish swam off to them before coming back to the research vessel. This implies that the sound or activity of ships might be what draws them in partially, but as of now this idea is not well understood and further research needs to be done.

If you want to learn more about the reefs, we dove on see this link:

https://floridakeys.noaa.gov/zones/spas/davisreef.html

If you want to read the research article about the fish accumulation to ships, see this link:

https://academic.oup.com/icesjms/article/63/8/1431/712848