By Peter Auster
Associate Professor, University of Connecticut
Today was a bad day to be a small fish. Well, most days are not good days to be small in the ocean as the diversity of predators that may call you “lunch” is large. Size is a refuge from predation in the sea.
Jeff Godfrey videotapes predation events for Team Piscivore. (Photo: Peter Auster)
The first dive this morning of Team Piscivore witnessed the greatest rate of predation so far on this cruise. Large schools of Atlantic bumper and redear herring were attacked by barracuda, greater amberjack, blue runner and other mid-water predators. As these beasts drove prey to the seafloor species such as scamp and black sea bass swam under the fleeing schools of fish and attacked from below.
Our second dive at another location was much like the first in that we saw many predation events in one dive. However, rather than attacking large schools of prey in the water column these events were focused on tiny young-of-the-year fish at the face of the ledge. Each dive so far we have observed tens of thousands of small tomtates in very dense schools located just in front of the shallow crevices at ledges. These fish were uniformly about 1 cm (a little less than ½ inch) long from the tip of the snout to the end of the tail fin. Today we observed these same size classes of fish but a larger size class was present in some numbers and were about 2 + cm long (or about 1 inch). Up to nine blue runner attacked these larger fishes along the bottom of ledges near the margin with the sandy seafloor. Lookdown, black seabass, scamp, and gag joined the melee that lasted generally for no more than 20 seconds. This same type of predation event was observed multiple times.
Amberjack chasing prey near the seafloor. (Photo: Peter Auster)
Between dives we observed the movements of fishes on the Didson sonar, ably run by Laura Kracker and Victoria Price, and gave us another whole perspective illustrating how large animals aggregate off reef faces and then searched long distances along the edges of reefs for prey … far beyond our ability as divers to see from one location.
Witnessing predation like this, so many times in one dive as well as dive after dive, is a rare occurrence from a global perspective. We know this stuff happens day after day and night after night. This is how populations are ultimately limited by the environment. Even a lack of food in the sea results in slow growth, starvation and ultimate consumption by some predator. Otherwise we would be up to our proverbial butts in fish!
I have conducted research at many locations around the world and there are a handful of locations I might choose to study such things. That we can come to Gray’s Reef National Marine Sanctuary and predictably see these types of species interactions makes this a special place. On the philosophical side of my work I often muse that Mother Nature hands out gifts to us humans very sparingly. Seeing a rare or spectacular species like sea turtles or whales underwater, or seeing some tantalizing bit of the secretive lives of fishes on a dive is one of those gifts that elicits my silent thanks.