Dive Dry with Dr. Bill

006: Sea Urchins

When I began diving Catalina waters in the late 1960's, sea urchins were considered a "problem" in southern California ecosystems. Formerly luxuriant kelp beds were greatly reduced and the bottom dominated by urchin "barrens" along some areas of the mainland coast. Urchins were thought to have peaked because of the local extinction of sea otters, a major predator of theirs, by hunting in the 1800's and increased fishing for sheephead, ocean whitefish and other fish predators. Scientists were using lime to kill the urchins and divers were asked to crush them. However, Catalina waters were generally not affected by this problem.

Research revealed that the problem was not really the urchins, but the continual dumping of raw or partially treated sewage into the ocean. This sewage provided an alternate food source, allowing urchins to remain after kelp beds had been decimated by them. Young kelp plants were eaten on the spot and new forests could not become established. Human pollution rather than the sea urchin was a major part of the problem. In 1972 federal legislation for clean water restricted the dumping of such sewage and the "urchin problem" diminished in most areas over time.

However, even today divers (including some instructors) carry on the "tradition" of killing sea urchins... in part to attract sheephead and other fish to show their classes since the fish love to eat the eggs and flesh inside the protective hard shell or test. Legally, this practice is now forbidden... doubly so within the marine reserve of Catalina's Dive Park.

The urchins are an important part of healthy kelp forest ecosystems. They feed largely on drift material that breaks off older kelp. Rarely in our waters do urchins come out of their hiding places during the day to crawl over living kelp. If you "knew" there were several large sheephead just daring you to come out, would you? I have also seen garibaldi trying to feed on dislodged sea urchins, attacking the many waving tube feet they use to move around. Although urchins and starfish are relatives (both are "spiny-skinned" animals or Echinoderms), starfish are another major predator on them.

Catalina waters are home to several species of sea urchin. These include the black urchin, a southern form; the closely-related red and purple cooler water urchins; and the white urchin. Color alone can often be used to tell them apart, although there are more significant differences that biologists employ.

During the 1970's there was a major commercial fishery for red sea urchins. Their eggs were in demand as a caviar-like delicacy in Asian markets. In 1974 nearly eight million pounds of red urchins were harvested and shipped to meet this demand. In addition, purple sea urchins are often harvested for research since scientists use their eggs when studying developmental biology.

© 2003 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.

Image caption: Red sea urchin with spines for protection

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia