Recently my favorite little channel in the dive park was the site of another lesson in ecology. Unlike some of my favorite lessons, it didn't involve sex (er, reproduction) but the other most important activity of any species... eating. By the way, how many of you noticed that I did not mention S-E-X once in my last column? I think that was a first. I'll try not to make that mistake again!
As I've said before, all species on Earth including Homo sapiens are members of what I call "the mutual eating society." One day we are the eaters, another day the eaten. Fortunately humans, being top or apex predators, don't have many encounters with things that eat them. We have to wait for some virus or bacterium to do its work on us. Killer whales, great white sharks, lions and tigers have similar "immunity." However, most species aren't as fortunate. One day they're munching, the next day they're munched. It is this constant "recycling" of organic matter and transfer of energy that constitutes the ecological drama.
Once again I was looking for male garibaldi enticing their sweeties into their finely manicured nests. I needed to get better footage of this solicitation or "dipping" behavior. I was swimming slowly at 20-25 feet looking down for nests when I sensed something above me. I looked up and the water column was filled with frantic slender silver bodies scattering everywhere. Darting in and out of this chaos were much larger, darker olive-green fish silhouetted against the water surface.
I slowly ascended so as not to interrupt this display. However I doubt any of the fish involved really cared much about my presence. The silvery baitfish were members of a small school of jack mackerel "trapped" between the inner edges of the kelp forest and the rocky breakwater covered with feather boa kelp. They were exhibiting specialized behavior... an avoidance response triggered by their will to survive. The larger fish were kelp or calico bass mercilessly attacking this baitfish smorgasbord. Okay, so "merciless" is a bit too anthropomorphic for my scientist associates. However it truly was carnage and ecology at its finest.
The jack mackerel would swim in rapid circles in the relatively small opening of the channel. The fish were "polarized" with all individuals swimming synchronously, as is expected in well trained schools (why can't human schools these days be as orderly?). The kelp bass would hide in the cover of the feather boa kelp which was similar in color to their bodies. One would dart out to attack the mackerel, often followed by several more. These attacks were unrelenting over the course of at least 10-15 minutes.
Each time the school was attacked, individuals would scatter in all directions. This response certainly must have confused the predatory kelp bass. However it also separated individual mackerel making them easier targets. Quickly the baitfish would collect and become polarized again, swimming in unison. Then with each successive wave of attacks, this would be repeated. I'm sure a number of the kelp bass had very full stomachs, and maybe Thanksgiving-style belly aches later in the day.
As this carnage repeated the cycle, I observed a new variable enter the equation. Splash. Something was attacking the mackerel from above. I had seen several kelp bass leap out of the water and attack as they re-entered the water and swam down through the school. However, this was a sea bird, either a seagull or a grebe, diving into the water to catch dinner. Although the legs appeared to be more grebe-like, the bird's body did not penetrate very far into the water, suggesting a larger bodied non-diving gull.
The poor mackerel were no match for this assault on two fronts. I noticed several of them had their sides flayed, with muscle tissue and bone exposed where the silvery skin used to be. However, many of the baitfish did survive, largely due to the defensive behavior of schooling. In the open waters outside the kelp forest, the kelp bass might have been less willing to leave the cover of the kelp to attack. I have never seen such a group assault as this. The kelp bass seemed to disperse as I swam back to the dive park stairs. I think they were looking for a nice worm for dessert. I know my appetite was whetted by these observations and I went home to gorge myself (but not on mackerel).
© 2004 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.
Trapped school of jack mackerel; kelp bass waiting in
feather boa kelp;
kelp bass striking at mackerel; bird on surface after attempting to dive on mackerel
This document maintained by
Dr. Bill Bushing.
Material © 2004 Star Thrower Educational Multimedia