Fish in the Bay – October 2020: Anchovy Spawning & Silverside Tsunami Increase.

Trawl map annotated with Anchovies and Silversides. Anchovy spawning locations are circled in pink.

Two major trends continued in October: 

  1. Anchovy spawn. Anchovies with eggs have now been documented at 18 of the 20 trawling stations in Lower South Bay from August through October.
  2. Silverside Tsunami = Bad News. We caught even more Silversides in October.


Editor’s note from November:  I deliberately waited until after November trawls before publishing this October blog.  I wanted to see how these two events unfolded before speculating any further:  … Both the Anchovy spawn and Silverside Tsunami abruptly ended after late October. Populations of both fishes practically disappeared!  Abrupt temperature drop seemed to be the causative factor.   


Bay-side stations trawling results.


Upstream of Railroad Bridge.

Somewhat Higher Salinity.  Lower South Bay has been relatively salty throughout 2020.  This has helped various marine critters penetrate farther south into Lower South Bay most of the year, for example, Ctenophores in January & February and marine crabs & tunicates through most of the year.  Now, bat rays are showing up at a few of the far upstream stations too.  

Higher than normal salinity has also discouraged a few of our freshwater oriented fish like American Shad, Longfin Smelt, and young Starry Flounder.


1. Anchovy Spawn Update.

Anchovies at Coy2 on 10 October.

2020 has been a very good Anchovy year in Lower South Bay:  6382 were caught over 9 months of 2020 versus a total of 5357 caught the previous three years COMBINED!

Spawning Zone:  Over July, August, September, and October trawls, we documented egg-bearing Anchovies at 18 of 20 Lower South Bay trawling stations.  Stations Coy1 and LSB2 are the only remaining exceptions.


Anchovies continue to concentrate at upstream stations where marsh water is fresher.  They also tend to be larger and darker at upstream stations. 

We measure standard lengths of only the first 30 fish caught of each species from each trawl.  After Anchovy length measurements were compiled for each station from July through October, a few things popped out:

  • The biggest Anchovies were caught upstream in Alviso and Artesian Sloughs.  
  • The greatest numbers of Anchovies concentrated off the main channel of Coyote Creek: at Alv3, Pond A19, and Dmp2.
  • Station Pond A19-4 (the fresher east side of Pond A19) was the Anchovy winner in terms of numbers: more than 30 Anchovies were caught in every trawl

What this means:  The size distribution seems to confirm that bigger, more mature Anchovies were seeking low salinity marshes for spawning.


Three examples of egg-bearing anchovies at station Coy2: two young, one old.

As always, there is much variability:  Both young and old anchovies have been found to be egg-bearing or milt-emitting.  And counterintuitively, there seem to be greater numbers of young ones with eggs. 

… Albeit, our catch methods and/or egg-check procedures could be biased in favor of smaller-younger Anchovies.  Bigger Anchovies tend to be energetic – The big ones more successfully evade hand capture for egg-checks.  They also more frequently jump out of the hand during examination.


Bigger, darker Anchovies are caught at upstream stations.  Particularly colorful specimens continue to be found upstream in Alviso Slough.  


Green Anchovies in Pond A19; Members of a school?  Where did they come from?

Just a hypothesis: 

  • Older-larger Anchovies must deliberately seek far upstream marsh locations for spawning. They may arrive in schools; adults caught together tend to have more uniform darkness and hue. 
  • Yearlings also produce eggs but may not yet be attuned to the migration pattern. Colors of younger ones are always much less developed, and their hues are more random.  We always catch a mix of young and old, blue, green, and gold.
  • These wriggling pelagic fish never sit still for close examination.


Anchovies at Alv3; slightly bluer than usual?

Anchovies were a little bluer this year, particularly at upstream stations, with exception of Artesian Slough. This conforms with our general “Anchovy Color Model:” Blue = high salinity, Green = mid salinity, Gold = low salinity.


Golden and green-gold Anchovies, Art2, 11 October 2020.

Where water is consistently fresher, as in Artesian Slough, Anchovies turn golden-green, golden, then clear.


Editor’s note from November #2:  The abrupt end of Anchovy spawning by early November, before any flushing rains and associated chlorophyte, dinoflagellate, or copepod nauplii blooms, could adversely impact survival of Anchovy larvae.  Abundant literature notes that larval Anchovies must find appropriate tiny food immediately after yolk sac absorption, several days after hatch.  Unfortunately, we do not collect phytoplankton data to associate with our Anchovy spawn and recruitment.


2. Silverside Tsunami Continues.

Silverside Tsunami continues!  October was another record-breaking month for Silversides.  Silverside numbers climbed even further off the chart! …  Over a thousand were caught at Alv1 and Pond A21 each.  Another 3000 were netted in Pond A19. 


Anchovy and Silversides trend lines:  The Silverside Tsunami increased in October!

A Silverside is a Silverside.  I mentioned before that there is no functional difference between an “Inland Silverside” (Menidia beryllina) versus a “Mississippi Silverside” (M. audens).  But, since we are catching these fish in fairly saline brackish water, they are more likely descended from the “Inland Silverside” variety.  This USGS Fact Sheet backs that up:

  • Smith (1985); Hubbs et al. (1991); Page and Burr (1991); Etnier and Starnes (1993). Chernoff et al. (1981) concluded that Mississippi Silverside  audensis conspecific with M. beryllina.  
  • Suttkus et al. (2005) determined that  audensis found in freshwater, while M. beryllina is found in brackish water.”
  • The Fact Sheet notes that Silversides tend to displace native California fishes pretty much wherever they take hold.
  • Even more Silversides: other references mention the ocean-going “Atlantic Silverside” (M. menidia) and the relatively fragile freshwater “Brook Silverside” (Labidesthes sicculus).   As near as I can tell, all four Silverside species are conspecific in one place or another.


Editor’s note from November #3:  The cold snap in early November decimated Silverside populations:  numbers fell from 6,796 in October to 67 in November

Silversides are warm-season breeders, but their lifespan is said to be around 16 months to two years.  They generally do NOT completely die off in winter.  I only found a few sources mentioning winter-time Silverside die-offs: 

  1. “… the Stockton Reservoir population in Missouri died out during the severe winter of 1976 to 1977.”
  2. “Overwintering mortality in the 80%-90% range has been reported for M. beryllina in Rhode Island waters (Bengtson 1982). In the warmer waters of Lake Texoma [Oklahoma], Hubbs (1982) did not find significant winter mortality (minimum water temperature ~10 degrees C).

Where did the Silversides go?  Our mild early-November cold snap should not have killed them.  They may have fled upstream toward fresher water.  We are confident that they will return regardless!


3. Continued Signs of High Salinity in Lower South Bay.

Bryozoan hash at LSB1.

Encrusting Bryozoan continues to grow as a coral-like reef over the bottom of the deep channel in the deep Bay.  This location has long harbored an expanse of ancient oyster shells from the days when oysters were farmed here over a century ago. 

Bryozoan encrustation is not new, but it grew much more extensive in 2020.  High salinity may have stimulated this ongoing growth. This expands and enhances a hard-substrate bottom that encourages spread of several marine-oriented species farther southward into Lower South Bay.  


Tiny orange-striped green anemones on ancient oyster shells at LSB1.

Different kinds of critters live on hard substrate versus the more typical LSB mud bottom.  In 2020, we are seeing more Decorator Crabs, Chameleon Gobies, and Orange-striped Green Anemones.  


Decorator Crab.  These spindly-legged crabs are built for hard or rocky substrate.  In addition to higher-than-normal salinity, expansion of Encrusting Bryozoan hard bottom in the deep Bay might be helping Decorators colonize further south this year. 


Tunicates growing on Corbula Clams at Coy3.  They were also collected at Station Coy4.

Tunicates (aka Sea Squirts or Sea Grapes).  These little brown lumps usually show up at LSB stations around December through March.  We are seeing them a bit earlier and farther upstream than normal, again owing to higher salinity. 

Like anemones and Decorator Crabs, Tunicates need something hard or firm to cling to.  Interestingly, as they colonize at far upstream locations, Tunicates are using Corbula Clams as anchors.  It makes me wonder if Tunicates may aid in curbing Corbula colonization into higher-salinity areas.  


Philine, aka Tortellini snails, at Coy4.

Philine Snails. (aka Philine auriformis, Tortellini Snail, New Zealand Sea Slug, Bubble Snail, Snotball Snail).  This marine gastropod is another predatory nuisance invader we frequently see at LSB stations. 

Like all the other marine-oriented species, the Philines pushed farther south into the mouth of Coyote Creek at Coy4 in October.  We collected 178 of them there.  They really do look like a plate full of tortellini pasta, except Philines are slimy and gross!    


4. Sharks and Rays.

Bat Rays.  October 2020 was one of our better Bat Ray months on record: 32 were caught.  This includes two baby Bat Rays netted at Coy1 and one at Art3.  This is far upstream of their typical range, again no doubt owing to higher-than-normal salinity.


Baby Bat Ray at Art3. 


Leopard Shark.  We caught two small Leopard Sharks in October.  A photo of the underside shows the rows of electrical receptors along the shark’s jaw.  These are the “Ampullae of Lorenzini” that allow sharks and rays to sense the electrical field emitted by prey hiding on the bottom.


Leopard Shark under a Sun Halo at Alv2.

Micah pointed out the Sun Halo as he held up our number 2 Leopard Shark of the day for my photo.  Micah sees many things that I miss.


Brown Smoothhound, Coy4, 10 Oct 2020.

Brown Smoothhound.  We caught another Brown Smoothhound this month.  This one also had a few hitchhikers.


Creepy copepod parasites on the Smoothhound dorsal fin!

Copepod parasites on the Brown Smoothhound.  Our SF Bay sharks are often afflicted with copepod parasites.  Some people call these “Sea Lice.”  I think the best candidate species is Achtheinus oblongus:  But, perhaps some copepod expert may know better? 

Compared to normal copepods, the parasitic varieties are big!  Most copepods are about the size of grains of sand at best.  And, they cause damage.  This shark’s fin looks eaten along the outer edge.  The damage is not life-threatening, but doesn’t help the shark either. 

I removed the bugs for closer inspection.  The little blood-suckers came off easily with the nudge of a thumbnail.  Mainly, I just didn’t want to leave them on the shark.


5. Another Tire!

Another tire at Station Alv2.  This was a big truck tire.


6. Other Amazing Sights.

Golden Eagle perched on Pond A19 northern levee, 11 October 2020.

Golden Eagle.  Micah spotted this over-sized bird as we trawled the north side of Pond A19.  We did not realize we had an eagle until telephoto shots resolved its identity.  Golden Eagles are very common in the high hills to the east, and they occasionally forage in Santa Clara Valley to the south.  But, seeing one on a salt pond levee is a very surprising first for us.    


Bay Pipefish.  This pipefish encountered late on Sunday had a red and swollen belly.  On close examination, we realized that this was a recently impregnated male with two rows of freshly-laid eggs sealed in his brood pouch.

Interesting gender-bender!  This Pipefish is the sperm-donor.  Biologically, that makes him a male.  But, in every other respect of mate selection and incubation of eggs, he behaves like a she. The female courts him.  He selects the largest, strongest female to impregnate him with eggs!  Most Sea Horses and Sea Dragons do it this way too.


Harbor Seals and shorebirds at Calaveras Point.

Once a month we trawl past muddy Calaveras Point at the mouth of Lower Coyote Creek and Guadalupe River where they meet the Bay.  And, every month I do my best to train my shaky telephoto lens on lounging Harbor Seals.

This colony of red, black, and gray coated seals has been here since at least 1900.  They have probably been here long before that.  They say, that San Francisco Bay has the only Harbor Seals in the world that develop red coats. The red coats may be a result of iron or something else in the water.  No one knows for certain.


Harbor Seal “Head Alert.”

Some researchers count “head alerts” as a sign of seal stress or disturbance.  A “head alert” is when the seal turns its head and looks at you.  It is not possible to pass Calaveras Point without seals looking at you.  As near as I can tell, they look at every boat that passes. 

But, recognizing that concern, I occasionally shoot a string of photos showing a full “head alert” sequence to check the number of seconds it takes the seal to check us out, then return to relaxation.  It took 6 seconds this time.   – We always give them space.

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