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Dive Dry with Dr. Bill

#674: Spawn of Satan

A recent dive had me nearly in tears (but big boys don't cry). The noxious "devil weed," Sargassum horneri, was so thick I couldn't find several of the species I intended to film For 10 years now I've written columns about this alien seaweed that invaded southern California waters in 2003 and made its way to Catalina in the winter of 2005-06. Its ecological impact on our kelp forest ecosystems has been monumental and I've covered many of these in my columns over the years. While editing a video for the Long Beach SCUBA Show's film festival this coming June, I came up with yet another one.

But first I'm going to digress (as I almost always do). In doing research for this video, I was quite amused to find that imitation is the highest form of flattery. Because this Sargassum species is native to Asia and was not present in our waters prior to 2003, it bears no English common name. The Japanese call it akamoku, but I needed a name I could use in my columns. Back in 2010 I coined the name "devil weed." During my research, I encountered a number of official websites that used this name without attribution (credit to me). I just don't get no respect.

Like during other warm water periods in its brief 10 year history here, the "devil weed" has come in thick and tall. It has completely covered almost all of the rocky reef in the dive park. On one recent dive I saw areas where it was at least 12-15 feet tall. I have even filmed it as deep as 100 feet growing on small rocks (and even on critters like wavy top snails) on the sandy bottom.

I have written about how the density of this alien seaweed affects feeding in our native marine life. Filter and suspension feeders such as scallops, tube worms and soft coral certainly are affected since the thick alga prevents good water circulation which they depend on to bring food. Reef predators like male sheephead, octopus and others undoubtedly find it difficult to locate prey. I've heard commercial lobster fishers have had low yields and recreational bug hunters are having trouble finding them at shallower depths.

Indeed, "devil weed" has affected our native critters ability to feed. But my readers know I'm all about mating as well as munching, since those are two of the critical functions of any critter or species. Yes, I only get to experience the latter these days (sniff), but I've done my duty and my son and granddaughters will carry on the species. However, "devil weed" seems to be impacting reproduction in a number of our kelp forest denizens.

One of the ways critters get to offshore islands like Catalina, or even different locations on the mainland coast, is via eggs and larvae that drift in the water currents. Few critters could crawl along the bottom from Palos Verdes to the island in a single lifetime, although some fish can swim the distance. To release eggs and sperm into the water, many species are broadcast spawners. They release a ton (well, maybe just a few grams or ounces) of gametes hoping these will cross paths, fertilize and drift away (in the words of singer Dobie Gray).

Many broadcast spawners will crawl to the top of the reef to get a better chance of releasing their gametes into the water and current. I have filmed brittle stars (Ophioderma panamense) coming out from under rocks at the base of the reef in large numbers and crawling up on top where they are vulnerable to predators, then releasing their gametes and returning to safety. Others like abalone and wavy top snails generally spawn from where they are because they are far to heavy to crawl up on top of the seaweeds to spawn.

With the "devil weed" as thick and dense as it is this year, few if any broadcast spawners can release their sperm and eggs above it. Thus the stream of gametes becomes entangled in the seaweed which acts like a net to capture it. Eggs from females may not reach sperm from males and fertilize. Even if they do, the egg may get trapped and fall close to the parents and not disperse at all. So the "devil weed" has resulted in the spawn of Satan, limiting both reproduction and the dispersal of our native critters.

"Thank you" California Fish and Wildlife for doing nothing to stop this scourge when it first appeared. I applaud the creation of the marine protected areas, but since the "devil weed" is having disastrous effects on them here on Catalina, how successful will they be in ensuring the survival and spread of our native species?

I do like to exhibit some optimism... when I can. I will say I was pleased to see our native giant kelp coming up in spots. The cooling waters have increased the kelp's metabolic efficiency and it is slowly recovering in the few spots where the Sargassum is not thick enough to shade it out entirely. It seems to be doing much better than it was last winter so perhaps there is some hope for next year. We need dense kelp canopy to overshade the Sargassum so it does not grow back as prolific as it did this fall when giant kelp was largely absent.

© 2016 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). 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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Dense growth of "devil weed" and spawning brittle stars, abalone and wavy top snails.

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia