Fish in the Bay – October 2023, Part 2: Musculista Explosion, Green Water, & Big Brown Birds.

This report covers additional observations from October.

Musculista senhousia, aka “Asian Date Mussels,” or Arcuatula senhousia, have been showing up in LSB in drastically increasing numbers since last year (2022). 

Musculista originate in Asia, from Siberia to Singapore.  They have invaded many estuaries around the world over the last century or so.  They were first identified in San Francisco Bay in 1946.  They probably arrived as ballast-water invaders. 

  • Smithsonian Marine Invasions Lab: 
  • Cheng and Hovel (2010): senhousia is a highly successful invader that is found in estuaries throughout the west coast of North America, Australia, New Zealand, the eastern Mediterranean, and the south of France (reviewed in Crooks 1996). Asian mussels are ecosystem engineers that form mats of byssal threads that can alter community composition in soft-sediment systems that typically lack habitat structure (Crooks 1998; Crooks and Khim 1999; Mistri 2002). They are approximately 100-fold more abundant than native bivalves in southern California, reaching densities of up to 10,000 m−2 (Dexter and Crooks 2000; Crooks 2001).”
  • Watson et al (2021): “As an autogenic ecosystem engineer (conspecifics can bind together using byssal threads to form dense mats in its native and non-native ranges22,38,39) high numbers not only alter the space and type of available substrate but also sediment conditions40,41.”

We never counted this mussel in Lower South Bay (LSB) trawl data until 2016.  (Musculista could have been present prior to that year, but, if so, the numbers were too small to note.)  From 2016 through 2021, we counted only a few dozen organisms per year.  

In 2022, the annual Musculista count jumped to 428.  As a result of higher-than-normal salinity during the dry drought year, Musculista spread upstream as far as stations Alv1 and UCoy2.  But then, in early 2023, massive rainwater flushing killed off most of the upstream invaders.  At that point, I thought this current Musculista invasion had been abated.  I was wrong.


Musculista Invasion.  October count = 1,626.  Musculista were only picked up at the two LSB stations in October, but previous monthly catches and one aborted trawl this month suggest that the current invasion is big and widespread.    

Musculista surged bigger than ever this summer and fall. The year-to-date count is almost ten times bigger than last year: 3,713. There may have been a massive shift in benthic bottom ecology near the downstream stations in LSB.   

The aborted trawl.  The first attempted trawl at LSB2 caught a Musculista-studded mud ball that was too big to wrestle onto the deck.  That catch was aborted.  The second trawl came up cleaner, and it still contained 300 Musculista – more than were caught in all years prior to 2022. 


1. Other Invertebrates seen in October.

Encrusting Bryozoan.  This encrusting type of Bryozoan is common at the higher-salinity stations in LSB.  The “bryoliths” have long been known to form slow-growing rocky reefs along the edges of the deep channel.  (I suspect that a massive Musculista invasion may spell trouble for Encrusting Bryozoan.  Or, perhaps and hopefully, vice-versa?  An invert war is coming!)   

Japanese Littleneck Clam count = 9.  Slightly larger versions of this highly edible clam can be purchased at most local seafood markets.  As the name suggests, JLC is also non-native.  

Tunicate count = 957.  Tunicates (aka Sea Grapes or Molgula manhattensis) are rapid-blooming animals that seem to exploit late-summer phytoplankton blooms.  Our catch numbers vary from a few hundred to many thousands per year.  So far, the year-to-date count is an unexceptional 1,426.

Philine count = 5.  We continue to see very few non-native Philine.  This mollusk preys on small bivalves, but whether it is capable of eating Musculista is unknown. 


A small selection of upstream invertebrates in Alviso Slough.

Nereid Polychaete.  I do not yet know the identity of this worm.  I still call it “the Long-Skinny Worm.”  Over the past three years, we have usually seen this worm in higher salinity. 

Mossy Bryozoan.  Alv1, Alv2, and sometimes UCoy2, support small gardens of Mossy Bryozoan colonies.  The colonies bloom at the foot of creeks. They must be a good thing. 

Corbula Clam count = 332.  The year-to-date Corbula count is 1,839.  This is good news.  Annual counts have been as high as 8,500 in recent years.  A little over a decade ago, annual counts reached into the tens of thousands.  Some type of natural control is keeping Corbula at bay.  Some combination of Diving Ducks, Bat Rays, Sturgeon, predatory mollusks, epiphytic organisms, and/or crabs must be inhibiting their spread here.  


The Shrimp Scene.

Crangon count = 5,129.  Crangon are up.  The 2023 Crangon count has almost reached 40,000.  This is a good year.  However, we counted almost 70,000 in 2018 and again in 2020.  We can do better!

Palaemon count = 2,000.  Palaemon are down.  The wetter year helped force the annual Palaemon number down to 22,000 so far.  This is less than half the number we saw during the two previous drought years.

Exopalaemon count = 377.  Exopalaemon were hit hard by flushing rain early in the year, but they are bouncing back quickly.  “Exos” are fresher water cousins of Palaemon shrimp, so lower salinity actually helps them propagate.


Red Algae Watch.  Summertime blooms of Ceramium have become common, but both Cryptopleura and Gracilaria were scarce through the drought.  We continue to pick up sprigs of the later two.  Rain must be good for them.     


2. Water Color Assessments (WCAs).

Water Color Chart from the Kudela Ocean Sciences Lab.  We had been looking for a helpful color guide.  This seems to be a good one.  The color names were adopted from Sherwin Williams paints. 

Janai showed us how to use this Color Chart in October.  In theory, greenish, brownish, or bluish water should indicate some aspect of prevailing phytoplankton populations.  In reality, this analysis is loaded with interfering factors, such as brightness of sunlight, sediment loads, current tides, water clarity, and so on. 

  • Can this chart be useful as a Water Color Assessment tool?


WCA – early morning test.  Sunlight is an essential element for performing WCAs. 

  • Blue sky reflection interferes with results if the viewing angle too shallow or the Sun is too low.
  • An overcast sky will likely exert a strong gray bias.


WCA – Light angle test.  Equally unsurprising, viewing angle with respect to the Sun can also confuse results. 

  • Water Color Assessments must take light angle into account.


October WCA – Overall Assessment. 

  • Water was “Luau Green” #27 at all stations.
  • Green is good. However, I was a bit surprised by this finding.  We usually see some color variation between upstream and downstream locations. 
  • This science is still in its infancy!


Janai demonstrating use of the Kudela Lab color chart at LSB1.

Color and water clarity work in tandem.  The color we perceive looking down into the water is influenced by water clarity / turbidity.  Color intensity varies by the depth of light penetration.  As shown in the above examples, water color was the same “Luau Green” at all stations. 

  • Color at upstream stations was muted and grayish where Secchi readings were around 50 cm. 
  • Color at LSB stations was more intense. Secchi readings were 75 and 80 cm.

Water clarity was relatively high at all stations in October, (i.e. turbidity was low.)

  • All October Secchi readings were the highest recorded since September-October 2020.  For example, Secchi readings at LSB1 and LSB2 were 75 and 80 cm respectively. In January, they were 25 and 18.
  • Secchi readings (water clarity) in LSB usually peak in September-October and then bottom out (highest turbidity) from around January or March through August because of stormwater flushing and diatom blooms.


Historical analysis:  Photos from the past year were reviewed in the table above to assess the range of colors that LSB waters typically exhibit.

Conclusion:  This Kudela Lab color chart seems to be useful.


Alviso Slough (Alv1) at 8:31 AM, Sunday 8 October.  The Sun was still too low for WCA.


3. Brown Pelicans.

Brown Pelicans hovered over and around us as we pulled our first (aborted) trawl at LSB2.  These birds were completely undisturbed by our presence.


As the trawl progressed each of the eight to ten birds plunged into the water.  They were feeding on some type of small pelagic fishes. 

  • “Brown pelicans are the only species of pelican that hunts with such dramatic plunging dives. Other pelican species fish by swimming on the surface of the water, which is also a method sometimes used by juvenile brown pelicans.”


Trawl results suggest three possibilities:  Inland Silverside, Longfin Smelt, or most likely, Anchovies.


Brown Pelicans were an endangered species when I was growing up.

Pelicans are an auspicious sign!

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