Sargassum @ Catalina

[EricJ]

I dove Catalina on January 16th and 17th and saw lots of sargassum filicinum. It seemed especially abundant at Ship Rock and Isthmus Reef.

I recall seeing this invasive seaweed before but never in such large amounts.

What kind of threat does this pose to our local kelp forests?

 

[drbill]

I have written about this invasive species and its various threats to our native ecosystems for several years now. You can read several of my columns on my web site (and here on SB if you look in the marine life and ecosystems section).

I had hoped to get permission from CDF&G to begin removing this invasive species about 4 years ago, but was told each diver had to possess a CA fishing license and was limited to taking just 10 lbs (wet weight) of Sargassum filicinum per diver per day. A permit to establish a larger scale removal operation would have taken about four months to get.

I haven't been diving our waters since mid-November. In part my decision was due to the effect of S. filicinum on our native species which I dive to film. I have heard from others that it is especially prolific this year, but can't verify that scientifically.

Control is felt to be beyond possibility. Even if we removed EVERY adult specimen (which itself entails risks of spreading the spores), the spores currently on the reefs after 5 years of reproduction would cause the species to return again.

I'm considering moving to the tropics during the winter months and focusing my efforts there while the S. filicinum dominates our waters. It will die back in late spring and summers we will be free of it, but it does take some time for the reefs to reacquire their natural species composition and it only lasts until about late October. Following the die-off of this annual kelp, the reefs are pretty barren... which easily illustrates the kelp's impact on native plant and animal species that would normally encrust or be vagrant on the reef.

This is (unfortunately) an excellent example of how quickly a non-native invasive species can come to dominate local ecosystems.

 

[mselenaous]

EricJ,
I also dove the park on Jan 9th & again on 14th and was very surprised how overgrown the sargassum has become. I don't recall it being this thick last winter. Although the water vis was clear, you lose the ability to see inside the majestic kelp forrest that Catalina is so famous for. I found it much harder to find the various landmarks one gets accustomed to when diving a known area. But the diving was still fun!

 

[kelpmermaid]

It does seem to be undergoing a season resurgence at the moment, comparing what I saw in November to what I just saw this weekend. I thought its home waters were warmer than ours, so I'm surprised that it dies off in summer.

 

[fnfalman]

The grass tends to die out around April time, or at least that's what seemed to me the last couple of years.

 

[drbill]

It does seem to be undergoing a season resurgence at the moment, comparing what I saw in November to what I just saw this weekend. I thought its home waters were warmer than ours, so I'm surprised that it dies off in summer.

Sargassum filicinum is an annual kelp rather than a perennial like many of the native kelps we are familiar with. It completes its life cycle in less than a year.

It is native to Japan and Korea where water temperatures are comparable to ours or even colder. I do find it surprising that it is doing better than previous years with water temperatures slightly elevated this year. However, that may be a result of increasing spore production over the years it has been invading Catalina waters.

 

[SPKelpDiver]

Dr. Bill, I have a reports of Sargassum at San Clemente as well. I think I remember you telling me that you haven't heard any reports of it at Clemente. Based on the report I got, it is abundant on the frontside of the island.

 

[drbill]

Wow, that's bad news then. It is also reportedly moving down the mainland coast. This stuff is really bad news. One of its close relatives, Sargassum muticum (wireweed) invaded our waters decades ago from Asia. It has not proved to be near as invasive and there seems to be a balance between it and the native species.

 

[Teamcasa]

Too bad there is not a way to eradicate that stuff. The park was so overloaded with the stuff it was really disappointing.