Fish in the Bay – September 2022, Red Tide Emergency Trawls.

As we are all now aware, single-celled phytoplankton called “Heterosigma akashiwo” bloomed into a dark reddish-brown stain across much of Central and South San Francisco Bay from mid-July through early September.  This was a rare red tide event that killed many fishes.  Thousands of small gobies and Anchovies washed up on the shores near Oakland and Alameda along with many dozens of larger fishes including Striped Bass and Sturgeon. 

Due to the extreme ecological hazard, regular monthly fish monitoring in Lower South San Francisco Bay (LSB) was conducted a week ahead of schedule, on 4 thru 5 September, to hopefully document any possible red tide intrusion or biological disturbance. 

(* What is the H. akashiwo danger level?  At least one published study detected toxicity to fishes when H. akashiwo cell density was as low as 2,000 cells/ml. However, other research papers found no toxic effect until cell densities rose to around 50,000 cells/ml or higher.  The discrepancy in results may be caused by difficulty in preserving and counting small and fragile H. akashiwo cells.)   


Good News!  No fish kill was detected in Lower South San Francisco Bay!   Fish and bug counts were typical for a September.


Only one dead fish was observed: the remains of a White Sturgeon were examined in Pond A19 – details below.  Plus, we caught several large Striped Bass, 18 Bat Rays, over 1,100 Anchovies, and even one live Sturgeon at Coy1.


1. Know your Phyto: H. akashiwo!

Heterosigma akashiwo is just one of thousands of varieties of single-celled phytoplankton that live in salty ocean and estuarine water.  It is commonly found in temperate coastal sea waters around the world.  (The name “akashiwo” in Japanese means “red tide.”)  H. akashiwo red tide blooms occasionally, and sometimes frequently, kill fishes in Canada, Chile, China, Japan, Scotland, Spain, Tasmania, New Zealand, and on both coasts of the United States at the very least.

Red tide madness!

The “flying potato” and many similar Dinoflagellates & Raphidophytes are photosynthetic but also well-known to be predators of other microbes.  This is their key adaptive strategy: they can find nutrients when Diatoms and other purely photosynthetic (autotrophic) phytos can’t.  Several recent papers describe a typical seasonal succession pattern: 

  • Diatoms bloom and cover the top of the water column in early spring or summer as irradiance increases. 
  • As the Diatom population scavenges all available N and P … & also shades itself out, it crashes and sinks.  
  • One or more Dinoflagellate/Raphidophyte species, like Heterocapsa sp., or Prorocentrum sp., (or H. akashiwo), etc. explode & take over.  


Predatory Dinos and Raphidophytes hunt for microbial food after dying and dead Diatoms have sucked up all available dissolved nutrients.  Studies indicate that temporarily low concentrations of Phosphorus (P) after a preceding Diatom bloom is one critical factor that Dinos/Raphidophytes are able to exploit.  

Our question: 

What prevented H. akashiwo from blooming into a red tide frenzy in LSB?  … Water temperature, nitrogen concentrations, salinity, and sunlight in LSB were all within ranges that should have supported an explosive bloom.  We could also see a dense rich green broth of diatoms and other phytos at most stations throughout this past summer.  But, for some reason, the H. akashiwo bloom seems to have dissipated as it passed south of Dumbarton Bridge. 

  • Literature suggests that vast microscopic armies of bacteria, copepods, and even other types of phytoplankton could have provided some protection. References will be discussed in a separate blog post. 


2. Results from September monthly trawls

Striped Bass, Bat Rays, and Anchovies at Coy3

Striped Bass.  We caught 9 Striped Bass in September.  This was a bit better than average for a September. 

Bat Rays.  The 18 Bat Rays were mostly small new recruits.  Like the Bass, this catch was better than average.

White Sturgeon.  One White Sturgeon was caught at Coy1.  Unfortunately, no photo is available.  This was the second sturgeon caught in 2022.  We usually net one or two Sturgeon in any year.  2016 was the only exception when three were caught.  The big fishes usually outswim our slow-moving net.


Baby Shovelnose Guitarfish and four more large Striped Bass at Coy4.

Shovelnose Guitarfish.  We caught one Guitarfish in April, then this one in September.  We later caught four more in October.  2022 is our record year by far!


Northern Anchovy.  1,103 Anchovies were counted.  This number is higher than average for Anchovies in a September.  As usual, the largest Anchovies were seen at far upstream stations in Coyote Creek and adjacent sloughs.

Longjaw Mudsucker.  Five Mudsuckers were caught which is also fairly typical for a September.  They tend to get scarce (dive into burrows) as summer ends.          


One of four Nudibranchs at LSB2 on 4 Sep 2022.

Nudibranchs!  Four Nudibranchs were caught in September.  They appear to be yet more Enosima Aeloids (Sakuraeolis enosimensis).  We first spotted one or two of these invaders from East Asia a few years ago.  Then we caught several of them in August thru October 2021.


Seven More Nudibranchs were caught in October 2022!  Consistently slightly higher salinity during the past two La Nina drought years is likely facilitating the spread of these attractive sea slugs.  Enosima Aeloids were first reported in the northern parts of SF Bay in the 1970s. They are very small and fragile.  It is possible that we just did not notice them until the last few years. 

For comparison: these are some similar-looking nudibranchs that are native to the California coast.


3. Dead Critter Observations in LSB: 1 Sturgeon, 2 Birds.

In addition to conducting regular trawls, the UC Davis crew motored out to far corners of LSB to visually scout for any signs of dark, reddish, or brownish water color in addition to any dead or dying fish and birds.   


Dead Sturgeon in Pond A19.  Micah and Alec checked it for any tags.

Dead White Sturgeon.  The crew spotted a dead White Sturgeon on a mud bank in Pond A19 (see photo above).  This fish appears to have been dead for at least a week or more.  This was the only dead fish or bug that was observed during the September weekend.    


Dead Birds.  Due to the Red Tide emergency, the UC Davis crew was particularly attuned to spotting any signs of dead or dying creatures.  Only two dead birds were observed, each at widely separated locations.  It is unlikely that either of these two were victims of an H. akashiwo attack.   


4. Greenish Water!

On both days of the September weekend, water color ranged from greenish-browns to brownish-greens which is pretty typical here.  Conspicuous dark brown or reddish-brown streaks or stains characteristic of an H. akashiwo red tide event were not observed on either day.   

Hooray!  The red tide emergency of late summer 2022 inflicted no noticeable casualties in Lower South San Francisco Bay!


Enjoy this slow-motion video of various phytos and other microbes under high magnification:

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