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Dive Dry with Dr. Bill

#394: Urchin Barrens

Nearly 41 years ago when I first arrived on Catalina aboard the dive boat "Golden Doubloon," I was a real "newbie" when it came to the critters and environmental issues of southern California. My academic training at Harvard included some of the finest biological minds around, and I was well prepared in terms of biological theory. Why we had even dredged Boston Harbor to obtain marine critters (but mostly came up with bottles, cans and an old fisherman's boot). But to keep ahead of my students at the old Toyon school, many of whom had been diving in SoCal waters for several years, I had to put my learning skills into overdrive to become familiar with the local species and identify the important ecological issues in my new surroundings. In the late 1960s there were very few good field guides to our marine life, and my college classmate Al Gore had not yet "invented" the Internet that I rely on so much today.

One of the big issues I quickly became aware of was the Santa Barbara oil spill caused by a blowout on Union Oil's Platform A in January of 1969. I'll focus on that next week. Today I want to address the second issue that dominated discussion of the marine environment during the late 60s and 70s. I'm referring to the decline of our giant kelp forests during the 1900s with the finger being pointed at the poor sea urchin. In those days SCUBA divers were told to crush the "offending" urchins with hammers and knives, and lye was sprinkled on them to destroy their exoskeletons. The assumption seemed to be that the over abundance of sea urchins was entirely due to their greed and voracious appetite for our beautiful giant kelp. I don't remember succumbing to that folly, perhaps in part because my college research focused on the echinoderms which included the sea urchins and starfish. To this day it still bothers me to see divers kill sea urchins in the dive park to feed to fish. That should end soon with the designation of the dive park as fully protected marine reserve.

Over the next few years we discovered that it wasn't really the poor urchin that was responsible, but the species Homo sapiens (which my readers may have some personal knowledge of). The decline of our giant kelp beds began early in the 1900s. However, it accelerated after World War II as the human population grew exponentially along our State's mainland coast. Back in those days, increasing amounts of untreated sewage were being dumped into the nearshore waters across the Channel. Although disgusting to us, the sewage contained large amounts of yummy and nutritious organic matter that sea urchins could feed on. Instead of dying out due to self-imposed starvation after the urchins had decimated the kelp forests, they had an alternate food source that kept their numbers high and prevented the young kelp from re-establishing as in the past. Those were the days when strange brown "sea cucumbers" (real ones are also a type of echinoderm) could be seen in our harbor... a species I was vaguely familiar with even in Boston (especially since I worked my way through Harvard by cleaning dormitory bathrooms!). The Federal Water Pollution Control Act, passed in 1972, required treatment of sewage before it was released into the ocean.

Despite Occam's Razor, the simplest solution to a problem is not always the complete answer in complex ecological systems. Other factors involved in the persistence of sea urchins, and their continuing damage to our kelp forests, were related to a second set of impacts by human beings. Over the past 60 years or more, we have over harvested two of the major predators that formerly kept urchins in check. Large sheephead love to crush these spiny skinned critters in their strong jaws and munch on the gonads, and surprising to some, lobster also eat them to a fair degree. As these species were depleted, controls on sea urchins were released allowing their populations to grow unnaturally high, and the kelp beds continued to suffer declines.

I was recently reminded of this issue recently while diving Quail Rock off the northwest coast of Santa Cruz Island as the guest of Rod Roddenberry and the Roddenberry Dive Team. West Grotto and Fern Grotto, the other two sites we dove that weekend, had plenty of urchins but not in the quantities I observed at Quail Rock. At that site, purple and red sea urchins were massing on the bottom. Several giant kelp holdfasts were being attacked by them and the smaller purple urchins were climbing up the lower end of the stipes like Jack and the bean stalk. When large numbers of urchins are present, decimating the giant kelp and other algae, we refer to these regions as "urchin barrens."

Fortunately such barrens are rare around Catalina, but they still persist along the SoCal mainland. Why would they also be present in the waters off the northern Channel Islands where sewage effluent was not an issue? I can only speculate, but I do have some thoughts. Point Conception is a major geographic boundary for both sheephead and lobster. Neither normally extends far north of it, so these species are in sub-optimal waters relative to their environmental preferences. Populations of a given species often exhibit low densities in the extremes of their geographic ranges. I proved this to my students back in the early 1970's with my infamous "Idiot's Delight" lab exercise. Students counted the number of barnacles in 3" or 9" squares in the lower, middle and upper intertidal ranges of the barnacle's distribution. I think some of those students may still be down there counting away after 40 years!

In the waters off Santa Cruz Island I don't remember seeing a single lobster, although I often overlook them since I stopped hunting bugs back in the mid-1970s. I don't know if over fishing was the primary factor, unless coupled with possible low densities near their northern geographic limit. I did see several large male sheephead and observed just a single female. Here in the dive park on Catalina females outnumber male sheephead by a large margin. All sheephead begin life as young ladies, some later turning into males. My hypothesis to explain the reversed sex ratio on Santa Cruz is that during prior El Niño years, warm water currents carried sheephead larvae north to the island and beyond. These larvae grew into adult females, then turned into the older males I was observing. The poor males may have to wait until the next El Niño pulse before suitable mates appear! I can empathize with them.

So the situation on Santa Cruz Island poses an interesting question. If the urchin barrens are not caused by pollution from the sewage of the island's tiny human population, and if sheephead and lobster were not the "usual suspects" in controlling them, what IS the dynamic there that allows urchin barrens to form? In this ecosystem there are other urchin predators that ecologically replace the lobster and sheephead. These include the starfish or sea stars mentioned in a previous column, especially the sunflower and leather stars. However, they just aren't scarfing up enough of those purple urchins at least at Quail Rock. Guess I'll just have to ask the biologists from the Channel Islands National Park who have been monitoring these areas since the 1980s. Interesting how different things can be less than 100 miles away as the brown pelican flies.

© 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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Urchins feeding on giant kelp holdfast and moving up towards the stipes; lobster finishing off sea urchin
and female sheephead looking to take another bite of a black urchin.

This document maintained by Dr. Bill Bushing.
Material and images © 2010 Star Thrower Educational Multimedia