These round-faced Batfish with the characteristic long
flowing fins, are usually graceful inhabitants
of the coral reefs. Here we found them far out
to sea, committed to sheltering beneath a log
they had probably joined while it was floating close to the
Photo: Genevieve Johnson
May 10, 2001
The Dispersal of Reef Fishes
Since leaving Madang a week ago, the Odyssey has been with sperm whales nearly every day.
While in the Solomon, as in the Bismarck Sea to the North, we encountered several widely dispersed groups of whales, with tight clusters of up to eleven animals. We have been able to identify many as being adult females as well as several mother/calf pairs. We have seen several very tiny calves, tiny by whale standards of course, perhaps only a couple of days old judging by their length of 10-12 ft. and the fetal folds, a sign of their recent births.
As we proceed in collecting data from sperm whales, following them mostly upwind into a sizable 2-meter swell, we continue to weave and dodge a virtual minefield of forest debris. Heavy logging in much of Papua New Guinea has meant branches, logs, even entire trees, some the length of the Odyssey have washed into the sea. During daylight hours, the crew person on observation is able to spot any obstacles allowing us to safely avoid them, but at night we have to rely on radar to detect the larger logs.
Apart from their role at the surface as a hazard to boats and a convenient rest stop for passing sea birds, there is a whole other dimension to these floating denizens, below the surface. As we drifted by a particularly large tree trunk the other day, we took the opportunity to peer into the water beneath.
Batfish, triggerfish, damselfish, juvenile dorado and rainbow runners had established their own micro community, or ecosystem around this enormous piece of floating debris. A dense school of rainbow runners swam away from the log and surrounded us, following us down as we dove and then turned with us as we headed back toward the surface.
Some of the larger pelagics are quite capable of leaving the protection of the log. The other species however, the reef fishes such as the bat, trigger and damselfish probably hitched a ride when the log was close to shore. Their commitment to adhering to this natural floating vessel, means they may eventually perish without the protection of the reef, perhaps being eaten by larger predators as they drift together, farther out to sea. Or they may continue to thrive under the protection of the log until they chance upon another reef.
What we were looking at was the possible dispersal of marine fishes on natural rafts. Maybe some of these reef fishes will be carried by the ocean currents resulting in resettlement upon a reef as new recruits, or on a reef that their species does not currently inhabit. They may die, or find a niche and thrive, establishing a new population. We witnessed the results of dispersal on natural rafts by terrestrial animals in the Galapagos Islands. It is believed that the tortoises and iguanas arrived on natural rafts, evolving into 14 species of giant tortoise and the marine and land iguanas we see today.
This is Genevieve Johnson speaking to you from the Bismarck Sea, surrounded by the sounds of sperm whales.
Log by Genevieve Johnson