Swarming divers and turtles in competition for food
Strength in numbers is not a phrase that applies to the recent settlement of estimated millions of Blacklip Butterflyfish (Chaetodon kleinii) at Molokini and off the south coast of Maui. This many fish compete mightily for the limited amount of available food. We didn’t realize how serious the competition was until we had the opportunity to observe these masses of fish in two completely different habitats separated by 3 miles.
Molokini is a little volcanic cone that sits 3 miles off the coast of Maui in about 300 feet of water. Because it is so far offshore it is often exposed to currents and sometimes upwellings, both of which bathe the island in food for plankton-eating fish and invertebrates. Thus, the estimated millions of Blacklip Butterflyfish that settled at Molokini in July and August and September of this year (see Sept. 2 blog post) have been well-provided for by virtue of Molokini’s geographical position. They have distributed themselves roughly evenly around the crater, finding habitat all around it, and feed by picking the abundant plankton out of the water a few feet from the protection of cracks and crevices in the rock and reef.
The Blacklip Butterflyfish that settled along the coastline of Maui, however, have not been able to distribute themselves evenly because much of the bottom is flat, open sand. The sand provides no shelter so they have been forced to congregate over the isolated rocks and reefs. In addition, these Maui areas do not receive as much current, and therefore planktonic food items, as does Molokini. Such high concentrations of fish mean that competition for the limited plankton around these rocks is fierce. Yes, they are plucking tiny planktonic animals out of the water, but in addition they are performing a behavior that is very unusual for fish – juvenile or adult. They are swarming over us, picking who-knows-what off any exposed skin, dive gear and camera equipment.
If you remember the old (and effective) Johnson’s Wax TV commercial for OFF mosquito repellent you will be able to picture what diving during this time has been like. In the commercial when the man puts his untreated arm into the terrarium of mosquitos they immediately swarm his arm. This has been the scenario at certain rocks off of Maui. As soon as we come close enough to the rock these youngsters swarm us, looking for any part of us that might be edible.
Of course, we are not the only visitors to these rocks. Turtles, which are used to receiving some attention from several species of adult fish who clean their shell of algae and organic matter, are swarmed as well.
Competition for food drives them to us. The ones that are bolder and more willing to approach large animals such as turtles and divers might be rewarded with more food. But they could also be exposing themselves to greater danger by moving farther away from their shelter. The ones that stay near the rock – perhaps they miss out on a good food source, but they may also be safer from predators. Interestingly, on days when there was current on these rocks, the fish were up in the water feeding and not at all interested in us. The current was presumably bringing them all the food they needed.
It has been six weeks since they began swarming us and as their numbers decreased, so did the frenzy of unusual behavior. They stopped swarming divers and turtles, perhaps because there was enough food in the water for their reduced numbers. Or, perhaps as they get older, like many animals, they lose the fearless behavior and become more cautious.
While it lasted it was a wonderful experience out of the norm – to have fish coming to us instead of fleeing from us! In 35 years of diving in Hawaii we have never seen swarming behavior like this.
And if you’re wondering what it looked like from the turtle’s perspective….
Text and photos by Pauline Fiene
We would be so interested to hear if anyone else has observed behavior like this from a species of reef fish.