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Dive Dry with Dr. Bill

#399: The "Shadow" Knows

I'm primarily a solo diver because buddies often unintentionally interfere with my filming, unless they are my tried and true buddies who know how to dive "same day, same ocean" with me. I just don't like to be shadowed. Keeping a buddy in sight often causes me to overlook potential video subjects, and of course my primary reason for being "down under" is to bring back footage and stills to share with you, my loyal readers and viewers of my cable TV show! Heck, I'd never just dive for the fun of it, or to experience the Zen-like qualities of floating through the water column weightless (quite an accomplishment given my current girth). However, there are fish that specialize in shadowing other fish... and on occasion even human divers.

One of these "shadowy" critters appeared in my video viewfinder on a dive a few Sundays ago with my friends Nadja and Matt. I was filming a rubberlip seaperch when I noticed a female rock wrasse following it, looking for table scraps to munch on. Rubberlips are bottom feeders that suck up chunks of the algal turf, weed out the critters they find palatable, and spit out the inedible debris. This feeding pattern is referred to as "winnowing" and is common to a number of fish in the surfperch family. It also tends to be very messy. I know Mom would not approve when I spit out my peas at the family dinner table! The rock wrasse knows a potentially good thing when it sees it. One fish's trash is another fish's treasure... maybe. Why put too much effort into obtaining food when you can just follow behind some messy eater and pick at their scraps.

One goal of most life forms is to minimize the energy expended and maximize the nutritive value in feeding behavior. Of course some critters such as filter feeders and bottom-dwelling ambush feeders have fine tuned this. They just sit on the bottom and wait for food to come near enough to them. Then BAM... gotcha! Just a quick burst of energy to grab something good to chow down on! Others like marlin and yellowtail may swim long distances following their food source as it migrates up and down the coast. Of course the California gray whale undertakes a round trip journey of 10,000 miles between its winter calving grounds off Baja and its summer feeding areas "way up north."

I mentioned that the rock wrasse, a relative of the sheephead, will also "shadow" divers. Many buoyancy challenged "newbies" on SCUBA have a tendency to kick up clouds of sand from the bottom. In doing so, they mimic the behavior of bottom feeding fish like the rubberlip. Of course I'd never do such a thing... but I do like solo diving so no one can check on me to verify that! I often see rock wrasse, sheephead and other fish species following these divers and searching the disturbed sandy bottom for "munchies." I guess you could refer to them as "shadowed divers" (not to be confused with Robert Kurson's book Shadow Divers).

Another place I see this is when I submerge off Descanso Beach where bathers are constantly walking on the bottom and stirring up the sand. I found it difficult to swallow that one local environmentalist complained about the proposed Sea Trek operation on the basis that it would stir up the bottom. Open your eyes underwater at that location and you'll see a much bigger effect by the swimmers there, and no one is going to suggest they should be eliminated (are they?). Assuming the recommendations I made to the Coastal Commission are followed (as I expect they will be), the Sea Trek participants will be walking on raised plastic platforms and will not be stirring up the bottom. I see no reason to "stir up" a stink about that! Heck, bat rays and surge create more of a "problem" at that site. Sandy bottoms are often high energy habitats and highly disturbed ones so the critters are adapted to such conditions.

When I am fortunate (and I do mean fortune-ate) enough to travel to tropical destinations, I often observe other species that shadow their co-residents around coral reefs. Often times the species that does the shadowing is a carnivore and the fish that it follows is an herbivore. There is logic to this. An algae eating fish is usually not seen as a threat to invertebrates and small fish. Therefore they do not flee when one approaches. If the shadowy one follows behind or on the outer side of the herbivore, the prey may not see it... until it is too late! Certainly an interesting strategy. Unfortunately I'm too big myself to "shadow" behind anything other than an elephant, hippo or whale!

I was able to film several examples of shadowing by fish down in Belize a few years ago. The species doing the actual shadowing was the bar jack, a relative of our local jack mackerel. The two species it shadowed there were both commonly called hogfish. The larger, common hogfish is the ecological equivalent of our local sheephead, filling the same basic "niche" (ecological occupation) there as ours does here. The Spanish hogfish is primarily a cleaner species, picking parasites and diseased tissue off larger fish. It is the ecological equivalent of our local senorita, and also our rock wrasse which occasionally cleans larger fish such as giant sea bass and halfmoon. Beyond "a shadow of a doubt," I find this behavior quite interesting to observe.

While I was filming an ocean whitefish exhibiting interesting behavior just inside the kelp forest on that same dive, Nadja and Matt had an interesting encounter. I had my back to them as I intently focused on the whitefish. When I finished and turned towards them, Matt signaled that two BIG fish had just swam by. I knew he was referring to giant sea bass. I missed seeing them. In fact, I have yet to see one this season despite the fact many others have done so right in our dive park. This past weekend I missed seeing yet another one. I've seen giant sea bass just three times... all while standing on the Pleasure Pier as they were being hand fed scraps from cleaned fish caught by anglers. One of these days I'm going to have to look up from my viewfinder as I film. I may finally see one this season... or even a baby gray whale this fall (I missed one at Church Rock a few years ago).

© 2010 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Our local rock wrasse (female left and male right) "shadowing" a rubberlip seaperch;
bar jack shadowing a Spanish hogfish and a common hogfish down in the tropical waters off Belize

This document maintained by Dr. Bill Bushing.
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