Fish in the Bay – Springtime is Bugtime!

March 4, 2026 Jim Ervin
0 Comment
Fieldwork, Fish in Lower South Bay, News

March Madness!  We are back in Lower South Bay (LSB). Actually, we continue to return to LSB every month, but I fell off the monthly blog reports after June: https://www.ogfishlab.com/2025/06/12/fish-in-the-bay-june-2025-return-to-lower-south-bay-lsb/  .

The local picture: Corbula Surge.  For most of last year, LSB experienced the largest Corbula Clam surge we had ever seen.  

  • Bad News: the Corbula Attack persisted throughout 2025.  By the end of the year, we had tallied over 150,000 nasty non-native Corbula clams!  These hyper-efficient filter-feeders are well-documented ecosystem destroyers. 
  • Good News?  So far, we are seeing fewer Corbula in 2026:  The total for Jan-March is just shy of 7,700.  (This is still high by historical standards.  We only saw higher numbers in 2012 near the start of our modern trawling record.)  
  • See additional discussion about the Corbula menace below.   

In other news: The fishes seem to be telling us that SF Bay is feeling extended El Nino-like conditions.  Pelagic fish numbers (Anchovies, Herring, and Longfin Smelt) have not yet rebounded from the strong El Nino of 2023-24.  In addition, numbers of other La Nina-fishes, like Staghorn Sculpin and English Sole have again dropped very low. In contrast, Halibut counts (an El Nino fish) remain high.

  • Fishes can’t lie!  The NOAA “Oceanic Nino Index (ONI) as measured near the equatorial mid-Pacific says that the Pacific flipped from El Nino to La Nada around mid-2024, but our fishes say they still feel El Nino!   https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php 
  • Peruvian fishermen originally defined “El Nino” as the time near Christmas, every few to several years, when sea surface temperatures warmed and fish catches (mainly Anchovies) crashed.  
  • Coastal cool ocean upwelling feeds phytoplankton blooms.  Phytoplankton feed the fishes. Along the western coasts of both North and South America, cool currents flow southward from Alaska and/or northward from Antarctica.
    • During cool La Nina, upwelling years pelagic fishes flourish. 
    • Warmer El Nino years suppress cool upwelling currents, and fishing sucks!.
  • News Flash:  “Extraordinary and prolonged March [2026] heatwave to break records and decimate mountain snowpack across U.S. Southwest, including much of California” https://weatherwest.com/archives/43745 

1.  March Bug Bloom in Artesian Slough.

Art2 on 3 March.

Every spring, we witness this phenomenon.  Clean but relatively sterile treated wastewater from the largest wastewater treatment plant in LSB rapidly mixes with Bay water to become a turbid-soupy living stew of microbes, marsh plants, birds, and fishes. 

  • Magic happens within the first ½ mile stretch of Artesian Slough.
  • Bugs are the bridge to bigger fishes! 

The muddy bottom of Artesian Slough looks and feels different. The bottom mud is lighter and fluffier. Instead of dense clay, or black sulfurous material we see in most places elsewhere, this stuff feels slippery on the fingers; like good gardening soil that has been processed through the guts of earthworms … or, in this case, Amphipods!

  • By station Art1, only a few hundred feet downstream, we find blooms of Mysids and Corophium, plus tiny fishes!  
  • This is the exact point where water chemistry first meets biology.  

By station Art2, just another several hundred feet downstream, bugs become dense and diverse. 

Amphipods, Mysids, and smaller unseen Ostracods (“Seed Shrimp”) are crustaceans.  They are analogous to terrestrial insects.  Like insects, small crustaceans consume microscopic prey and condition the soil. More importantly, they convey nutrients up the food-web into consumable protein for fishes and birds. 

These bugs are tiny.  The largest of them measure roughly one centimeter from tip-to-tail, the length of the nail on your pinky finger.  

  • Amphipods are comprised of Corophium and slightly larger Gammarid types.  They are bottom-dwelling burrowing bugs similar to crickets, termites, or ants.
  • Mysids are shrimp-like crustaceans that swim a little higher in the water column.  I liken them to aquatic insect larvae, like midges or mosquitoes.
  • All these bugs are excellent fish food!

People who raise fish as a hobby understand the importance of live bugs in a complete fish diet.

Microscopic phytoplankton explode during a very short period each spring. 

  • Amphipod and Mysid reproductive cycles are timed to exploit this rich source of Omega-3 laden food. Their belly pouches swell with eggs and/or baby young around this time of year.
  • We watch for ‘termite mounds.’

During our March trawls, we encountered a big Corophium (amphipod) mound at Art3 near the end of Artesian Slough.  We see similar Corophium explosions farther upstream in Artesian Slough and at various places along Alviso Slough from time to time.

Know your bugs!  Love your bugs!

We welcome springtime bug blooms! 

  • BTW:  The male-female pairing of gammarid amphipods (“Scuds”) shown above is called “amplexus” or “precopulatory guarding.”   Many different types of crustaceans do this.  During this pairing, the male uses his front legs (gnathopods) to hold the female for several days until she molts and is ready to mate. 

Small fishes eat tiny bugs.  Not surprisingly, bigger fishes loiter immediately downstream in Lower Coyote Creek.     

2.  Spring Bug Bloom is Everywhere!

Mysids bloom best in restored salt ponds and adjacent sloughs.  The biggest blooms happen every spring just after the heaviest rains.  

Baby Halibut, young Anchovies, and a Longfin Smelt in a swarm of Mysids at station Pond A21-1.

Mysids are an essential fish food for Longfin Smelt, young Anchovies, and Baby Halibut.  They are soft and squishy, highly nutritious, and packed with intense shrimp flavor.   

More baby fishes & Mysids at Pond A21 in March.

Bigger shrimps like Exopalaemon and Crangon also start showing up in spring. Both types flourish at the fresher end of brackish marshes. 

  • Baby boy Crangon recruit in low-salinity water and move progressively downstream as they age.  They return as adult females in the coldest months of their second year to release broods of the next generation.
    • Crangon are protandrous hermaphrodites: they usually hatch as males and then transition to female as they grow. Many crustaceans have similar sex-switching life-cycles.  
  • Exopalaemon are brackish water shrimp that spend their entire life-cycle in low-salinity marshes.  Unlike Crangon, Exopalaemon shrimp reproduce during the warm season.

Isopods probably have less influence on fish diet.  These heavily armored critters have hard shells with very little meat on their bodies. Some larger fishes, like Striped Bass and Sturgeon, are documented to eat them, however; it is unknown whether Isopods are preferred, or “good,” food for fishes.  Small fishes, smaller than perhaps 70 or 80 mm, cannot consume these armored beasts at all.

Isopods are analogous to terrestrial “pill bugs” or “sow bugs.”  Most isopods in SF Bay are non-native species that were introduced over a century ago.

  • Sphaeronoma, (pill bug / “seapill”) types generally live in upstream fresher marshes. (At least that’s the rule in LSB. Curiously, we have been finding Sphaeronoma varieties in saltier waters at some of our newer trawling locations in San Pablo Bay.) https://en.wikipedia.org/wiki/Sphaeromatidae
  • Synidotea (sow bug types) usually inhabit slightly saltier waters. One variety, Synidotea laticauda shown above, is extremely common all over LSB and most other parts of SFB.  https://academic.oup.com/jcb/article-abstract/14/4/700/2328226 

Are Isopods good or bad?  Personally, I don’t see great value in them, but many isopod advocates strongly disagree:

3. Invertebrate trends – 2012 to Present. 

The UC Davis team began documenting numbers of clams, mussels, tunicates, & jellies in LSB well over a decade ago.  More recently, we added smaller bugs to the list: Mysids, Amphipods, & Isopods.    

Fish health depends on a varied bug diet.  But, which bugs?  How many? And, how do invertebrate trends affect fish ecology? We still have much to learn:  

  • 2020-2021.  Comb Jelly explosions in early 2021 & 2022 suggested that zooplankton abundance may have been high because Comb Jellies (aka Ctenophores) are carnivores that feast on microscopic zooplankton, that in turn, feed on phytoplankton.
  • 2012-2022. Eastern Mud Snail numbers surged in 2021 and even higher in 2022.  Snails are detritivores that feed on benthic or dying phytoplankton.  Were their increasing numbers in early 2022 a harbinger of the next disturbance?
  • July-August 2022.  Red Tide!  Microscopic raphidophyte H. akashiwo exploded in Central and South SF Bay.  Ironically, these tiniest of creatures exploit a small window of opportunity after large diatoms have salvaged most available nitrogen and phosphorus nutrient near the surface.  (Unlike diatoms, H. akashiwo is motile and supplements its diet by feeding near the bottom at night.)  Coincidentally (?), Mud snail numbers temporarily dropped in LSB during the H. akashiwo surge.
  • November-December 2022.  Both Tunicate and Mud Snail numbers rebounded immediately after the H. akashiwo explosion.  Were filter-feeding Tunicates and surface-deposit-feeding Snails feasting on the dead and dying remains from the H. akashiwo explosion?  Given otter trawl uncertainties, we can never be sure.  We only have clues. 
  • August & October 2023. Musculista (Asian Bag Mussels) increased to numbers never seen before in the 10+ year LSB record. The Musculista surge was accompanied by an almost proportionate increase in Bat Rays. Bat Rays could have been at least one element of control of this particular bottom-up productivity explosion. https://www.ogfishlab.com/2023/11/14/fish-in-the-bay-november-2023-part-3-bat-rays-to-the-rescue/
  • Late 2023-to-present. Corbula clams increasingly dominated the benthic bottom.  Filter-feeding Corbula populations could be responding to increases in microscopic food, but water flow, salinity, and temperature are equal or greater influences.
    • Eastern Mud Snails and Tunicates may be the biggest losers from this latest Corbula tsunami.  Populations of both all but disappeared as Corbula took over.  Again, this could reflect either food availability or water quality parameters of flow, salinity, and temperature.    

4.  Corbula Explosion in 2025 & Beyond. 

Jumbo-sized Corbula Clams at Art3 on 3 March 2026.

Corbula Clams are the most recent disruption.  Seemingly out of nowhere, Corbula numbers increased exponentially after late 2024. They suddenly jumped from dozens-to-hundreds of clams per month to many thousands!

  • Could surges of snails, mussels, and clams represent symptoms of problems deeper in the microbiota?  Or perhaps, these reflect swings in the multi-decadal ocean cycles?  … Or, is it climate change?
  • We must understand this chaos!   

Example: Corbula observations in July 2025.

We intensified monthly Corbula Clam studies throughout 2025 to try to understand this population explosion.

  • Biggest Corbula.  These clams grow to largest sizes (over 20 mm in width) in slightly fresher, nutrient-rich locations near stations Alv1 and Art3.
  • Most Corbula. The greatest numbers of Corbula are consistently found at stations Coy1, UCoy2, and Alv1 in the main stems of Coyote Creek and Alviso Slough (Guadalupe River) where freshwater meets the salt.
  • Fewest Corbula.  Corbula are rare-to-absent in restored salt ponds and side-channels. They do not proliferate outside of the main creek channels.   
  • Limited range of salinity.
    • Corbula do not tolerate fresh water.  Their numbers drop sharply as we travel upstream on Coyote Creek and Artesian Slough.  We also find many dead and dying clams immediately after flushing rainstorms. 
    • Corbula are equally limited by salty water.  Both clam numbers and sizes decrease rapidly as we travel downstream of Coy3 to Coy4 and ultimately to LSB stations.  We rarely find dead shells and almost never find adults downstream.  Here, the limitation is probably biological rather than physiological. At higher salinities, Corbula clams are relentlessly attacked by sessile organisms like Barnacles, Tunicates, and Musculista.  These other critters latch onto Corbula shells and impede clam growth.  Other saltwater predators of mollusks, like Philine snails, Atlantic Oyster Drills, English Sole, Bat Rays, and diving ducks, also have considerable impact.         

Corbula Summary.  The above table summarizes CPUE Corbula numbers (top section) and largest sizes (bottom row, in tan).

  • Blue and brown arrows indicate where Artesian Slough (blue arrow) and Alviso Slough (brown arrow) flow into the main stem of Coyote Creek.
  • These nuisance clams densely colonize at only a few locations in LSB.  They could be controlled, albeit never eradicated, via physical removal at relatively low cost … but, would this be worthwhile???

Another thought – These nasty clams could be used to our advantage: metals detection in Guadalupe River and Coyote Creek.

 5.  LSB: The fish nursery.

Hungry Hippos at UCoy2

Young fishes tend to show up where bugs are most numerous. 

Striped Bass count = 2.  Year-to-date count = 4.  We caught low numbers of bass in 2022 and 2019.  But over a roughly 14-year record, this is one of the lowest years for Striped Bass.

Gobies are particularly adapted for exploiting bug hotspots.

Yellowfin Goby count = 7.  Year-to-date count = 33.  Historically, Yellowfins became our most numerous goby sometime after their introduction in the late 1950s to 1960s.  Numbers have dropped in recent years, albeit we saw lower numbers in 2020, 2018, and 2017.

Arrow Goby count = 7.  Year-to-date count = 25.  So far, this is a relatively good year for native Arrows.  Admittedly, catch numbers for these tiny fishes are a little fuzzy.  Young Arrows are hard to distinguish from baby Yellowfins and Cheekspot Gobies. But even considering that uncertainty, we saw far fewer Arrow gobies in 2024, 2020, 2019, 2018, and 2014 at the very least. 

Shimofuri Goby count = 39.  Year-to-date count = 116. In 2021, and again in 2023, Shimos overtook Yellowfin Gobies as the most numerous goby in LSB. But, least for the moment their numbers have dropped a bit.   

Shokihaze Goby count = 97.  Ytd count = 182.  Shokihazes are up.  These close cousins of the Shimofuris compete with them for space and resources. 

  • Shokihazes tend to predominate in saltier water slightly downstream.  Shimofuris rule in fresher upstream sloughs.  Nonetheless, the two gobies occupy much of the same range.  When numbers of one species increase, it’s usually at the expense of the other.

Longfin Smelt at Pond A21.

Longfin Smelt count = 8.  Year-to-date count = 173.  Only 8 Longfins were caught in LSB during March trawls, and only one of those was from the upstream reaches (a young-of-year at Dmp2).  Five large females were picked up at downstream stations: Alv3, Coy4, and one in Pond A21.

  • As is typical with warming weather this time of year: a few large spawning-ready or spent females continued to loiter downstream where food is abundant. Even fewer juvenile stragglers remained scattered in the marshes.  Most of the winter 2025/26 migration cohort has already vacated these brackish environs for cooler waters closer to the ocean.
  • Overall, this has been our lowest Longfin season since 2017/18.  Hopefully, Longfins will return in greater numbers next fall.       

Northern Anchovy count = 91.  Ytd = 335.  Similar to Longfin Smelt, Anchovy numbers are down. 

  • January-thru-March Anchovy numbers were lower in 2020, 2018, 2017, and 2015, so this is not terribly alarming.  But, counts have been on the low side over the past two years. 
  • We are seeing fewer pelagic fishes overall: Anchovies, Shad, Herring, and Longfin Smelt are all relatively scarce so far in 2026.    

6.  Flatfishes.

California Halibut count = 40.  Year-to-date count = 94.  All Halibut in March were tiny babies or yearlings at most.  The big adults generally stay far out in the deeper Bay.  

  • Halibut numbers have been very good in recent years, 2024, 2025, and now 2026.  The count remains high this year – even though the last ‘strong but short duration’ El Nino ended in May 2024. 

More Flatfishes.  San Francisco Bay is always a robust flatfish nursery.  At least three of the local flatfishes are sentinel indicators of coastal ocean upwelling.

English Sole count = 1.  Ytd = 5.  English Sole numbers have plummeted.  Sole are our ‘La Nina’ flatfish.  We see hundreds to thousands of them per month in La Nina years like 2021 and 2022.  They bottom out in, or just after, El Nino years.  2026 is a bad year for English Sole; roughly as bad as 2020 and 2015, but not as bad as 2017.  Are English Sole also signaling El Nino-like conditions in our local area???

Starry Flounder count = 2.  Ytd = 6.  Starry numbers have been steadily declining since the peak Starry year of 2023.  Starries aren’t as much affected by ENSO or other ocean cycles.  Abundant rain helps them, but we haven’t completely figured out all the reasons they boom or crash in any given year.

7. Oddballs.

Oddball fishes. 

White Croaker count = 2.  Ytd = 3.  White Croaker used to be more common here.  At least, that is the story I have been told.  Now, Coakers are quite rare and almost always tiny babies.  Three so far in 2026 is a lot.  We caught only one (1) over the four years from 2022 through 2025. 

  • In Southern California, some people nickname Croakers “Sewer Trout” because they tend to hang out near effluent channels of wastewater treatment plants.  https://www.pierfishing.com/white-croaker/  
  • Don’t pay attention to ‘those people.’  Adult White Croakers are gorgeous shiny silver-gold members of the “Drum” family.  These types of fishes vocalize by croaking or drumming, hence the name.  https://en.wikipedia.org/wiki/Sciaenidae

White Seabass count = 3.  Ytd = 3.  White Seabass are even rarer in LSB.  This is the first time we have ever seen more than one at any given time.  Only small baby Seabass ever make it this far south in SF Bay.  Big adults live in deep salty water near the coast.

  • The name is misleading.  This fish is not actually a “bass” by strict definition.  Instead, White Seabass is another member of the Drum family, like the above-mentioned Croakers.  https://en.wikipedia.org/wiki/White_seabass
  • The four or five dark bars across the bodies of White Seabass babies fade away as they age.
  • BTW:  I once ate a Gulf Coast “Redfish” or Red Drum.  It was one of the most delicious fishes I ever tasted.  For that reason alone, I hold Drums and Croakers in very high esteem.  https://tpwd.texas.gov/fishboat/fish/didyouknow/coastal/reddrum.phtml        

California Tonguefish count = 1.  Ytd = 4.  Tonguefish and Halibut are our two main “El Nino” flatfishes.  Tonguefish have been extremely rare in LSB after the last big El Nino in 2015-16.  (We only saw one tonguefish during the strong but short El Nino of 2023/4.)

  • Four Tonguefish in LSB is unusual. Interestingly, we also caught a baby Tonguefish at Eden Landing in February and another one in Petaluma in January.   

8. Trash Talk.

Blue M&M at UCoy1.5

Sami found this unconscious Blue M&M stuck in the marsh at UCoy 1.5.  We are still piecing together the story.  A bottle of booze, a spray paint can, and a discarded butane bottle were found nearby.

  • Per Google AI:  Blue is the “cool,” suave, and self-assured Almond M&M’s character introduced in 1995 after a consumer contest.  He is known for wearing shades, playing the saxophone, and maintaining a calm, confident demeanor. 
  • Evidence suggests that Blue might have ended a wild night here! 

Another clue:  A spare tire with rim was recovered nearby.  Blue M&M has much to explain when he finally revives. 

Information Society – What’s On Your Mind  “Pure Energy”  https://www.youtube.com/watch?v=zFJK4QRzIhM&list=RD1t-gK-9EIq4&index=7 

Fieldwork, Fish in Lower South Bay, News