Fish in the Bay – April 2023, Traditional Baby Fish Month

April is “Traditional Baby Fish Month.”  The tradition is fairly new in Lower South San Francisco Bay.  New for us, at any rate.  

  • It started in 2014 when the current monitoring system of 20 stations was fully established. That April, we noticed a springtime surge of hundreds of “unidentified gobies.”  
  • The rest is history: Baby gobies have bloomed in all but a few Aprils since then.


However, Baby Fish Month has become more complicated with each passing year.  Owing to a combination of a restoring ecosystem and our own improving baby fish identification skills, we now routinely detect more species of baby fishes. … Lots more!


Gobies are still the most reliable component of Baby Fish Month.  But, we now routinely track springtime surges of young Sculpin and English Sole.  Baby Longfin Smelt, Pacific Herring, Starry Flounder, and Crangon Shrimp have also become a big part of the springtime story.


1. Restored “Island Ponds” are baby fish nurseries.

Restoration of former Salt Ponds A19 and A21 adds greatly to baby fish populations.  Both ponds were breached in March 2006.  Over the next decade, they filled up with sediment.  Marsh plants slowly colonized and eventually grew big.


Pond A21, being a little farther downstream, filled up and matured much faster.  As reflected in these April 2023 numbers, the pond is now a major contributor to our ‘Baby Fish Month’ totals.

One example …

Unidentified Gobies = Baby Gobies.  929 “unidentified gobies” were caught this month; close to half of them from Pond A21 alone.  The vast majority of these tiny babies are Yellowfin Gobies that are too young and small to identify.


Baby Herring & seven young Staghorn Sculpin from Pond A19 on 2 April 2023.

A second example …

Pacific Herring.  We caught 2,010 baby Herring this April.  All were larval or very young juvenile at oldest. Over 1,300 of them were from Pond A21.   

Baby Herring typically show up in March thru May.  Herring are a “La Nina fish.”  Cool ocean upwelling increases their overall populations.  But, the Herring story is a little more complicated: 

  • Adult Herring spawn mainly in Central and North Bay, far north of here.
  • Larval Herring drift considerable distances soon after hatch. Their early survival strongly depends on robust rainwater flushing. 
  • The combination of La Nina plus rain made 2023 a very good Herring year for Alviso Marsh!


2. Artesian Slough is another baby fish & bug nursery.

Micah probes for Secchi Depth in the clear black water of upstream Artesian Slough.  (The discharge weir from the San Jose/Santa Clara Regional Wastewater Facility can be seen at far upper left.)


Modern miracle:  Treated wastewater almost immediately begins feeding this productive ecosystem.  The baby fishes shown above were caught just a few thousand feet downstream of the discharge point.


Shiner Surfperch at Art2 on 2 April 2023.

Shiner Surfperch.  8 Shiners were netted in April.  They were all adult-sized: 80 to 133mm.  Most of them had full bellies indicating pregnant females.  Unlike most fish, Shiners don’t lay eggs.  They are livebearers.  In springtime, females seek low-salinity brackish sloughs to deliver their broods of 2 to 36 young. 


Bluegill.  We caught a rare non-native Bluegill at Art2.  This was the first Bluegill we have seen since the big freshwater flush of 2017.  No doubt, many more live farther upstream in Coyote Creek.   Bluegills are very common freshwater invaders in California and many other parts of the world. 

  • According to Wikipedia: “The fish usually displays 5–9 vertical bars on the sides of its body immediately after being caught as part of its threat display.”


Corophium Amphipod.  These bugs are too small and too numerous to count.  We only record estimates.  The bug shown here was a female full of eggs.


3. Fish and Shrimp grow bigger in the main stem of Coyote Creek.

Longfin Smelt.  184 Longfin Smelt were counted this month.  That is a new record for an April.  The big rains in December through April stimulated robust Longfin spawning and recruitment!  

Most Longfin babies were caught in Pond A21 and the lower portions of Coyote Creek.  This is consistent with literature and our previous observations:  Larval Longfins migrate downstream to suitable feeding areas soon after hatch.  This also matches with the Longfin overall seasonal migration.  Longfins do not tolerate higher summer temperatures.  All of them will soon relocate to cooler deeper waters as temperatures rise.


Staghorn Sculpin.  We caught 513 Staghorns and two tiny Prickly Sculpin.  This was the third-best April Staghorn count after 2018 and 2021. 

Staghorn Sculpin is another La Nina fish: coastal sculpin populations surge during La Ninas. But, our local Sculpin spawning and recruitment is also heavily influenced by rainwater flushing.  Good flushing = lots of baby sculpins.  So far, 2023 is a good Sculpin year, albeit, not the best we have ever seen.


Adult Staghorns, like Longfins and Anchovies, spawn as far upstream as they can tolerate.   Marshes, where salinity fluctuates around 10 ppt (or lower for Longfins), seem to be preferred spawning places for all three species.

  • This month, many larval and young Staghorns were found upstream in Alviso Slough, Artesian Slough, and Coyote Creek.
  • Staghorn young migrate downstream quickly after hatching. Larger juveniles and young adults were found at downstream Coy3, Coy4, and LSB stations. 


Striped Bass.  13 Striped Bass were counted.  They were all young Bass between one and two years old; 100 to 240 mm, or 4 to 9 inches, standard length.  We generally catch smaller Bass at upstream stations and larger ones closer to the Bay. 

  • Always keep in mind that adult Bass are fast. Even larger Bass were probably present but too fast for us to catch. 


More Striped Bass at station Coy3.


Crangon Shrimp.  1,164 native Crangon were caught in April.  Again, the size-gradient pattern was familiar: tiny baby Crangon were found at upstream stations in Artesian Slough and Coyote Creek.  They tended to be larger the farther downstream we trawled.  Like fish species discussed above, Crangon also migrate to deeper, saltier water as they age.    

Palaemon Shrimp.  541 were caught.  Unlike Crangon and other natives, Palaemon Shrimp (Palaemon macrodactylus) and their related cousin, the Exopalaemon (Palaemon modestus) are year-round residents.  Females brood eggs and release young at least twice each warm season.

Non-native Palaemon have a significant advantage in the shrimp-eat-shrimp world of the brackish marshes.  Young Crangon must pass through an army of adult-sized Palaemon on their way out to the deeper bay.  The photo above illustrates the size difference we observed at station Coy1. In a one-on-one shrimp fight, young Crangon don’t stand a chance!  


4. Interesting Invertebrates.

Musculista (Asian Date Mussels).  We counted only 19 live Musculista along with at least several dead shells.  Low salinity from rainwater flushing continues to impede their progress, but even this low April count is still much higher than what we had seen prior to 2022.

Musculista mussels are famous for consolidating estuarine bottom material with their byssal threads.  This can be good for promoting growth of other invertebrate populations. But on the other hand, this bottom consolidation can also wipe out desirable eel grass beds.

  • Our own ad hoc field investigations suggest that Musculista may provide some control over nonnative Corbula Clams by ensnaring them in the byssal threads.

Corbula Clam.  The Corbula count remains low.  There were only 11 in April.  The year-to-date count is 176. This is very good news since Corbula are well-documented ecosystem killers. 

California Horn Snail (a dead shell).  California Horn Snails no longer live here.  They were once the most common brackish water snail, but they were outcompeted by the Eastern Mud Snail many decades ago. Small enclaves of Horn Snails still exist in one or two isolated high-saline marshes nearby, but in practical terms, Horn Snails have been extirpated.


Two more Musculista at Coy3.  One of the two has trapped four live Corbula in a byssal thread stranglehold!


Ctenophores (Comb Jellies).  Only 11 Comb Jellies were seen in April.  The total count for 2023 is only 14.  This season’s big freshwater flush and low salinity is keeping the Ctenophores away. 

Ctenophores are well documented as providing important protection against zooplankton population explosions in bays and estuaries.  

  • At some point, an overabundance of zooplankton will draw down phytoplankton levels to the point of upsetting the ecosystem.
  • But, on the other hand, zooplankton also serve as critical food for baby fishes.
  • For these reasons, we keep an eye on our Ctenophores.   


5. Fun Fact: Many marsh plants are edible.

Spartina stems (aka Cordgrass) at Dump Slough, 2 April 2023.

Spartina stems.  As an experiment, we plucked some fresh Spartina at the water’s edge when we paused to untangle the net in Dump Slough.  Spartina is a tall thick grass that grows in brackish and saltwater. 

Spartina leaf blades are thin and tough.  They do not look edible at all.  But, if you grasp the base of the stem and pull firmly, the fresh core of the stalk pulls out fairly easily. Surprisingly, the core tastes as good as it looks.  

Micah, Luca, and I taste-tested this plant for science.   The Spartina was absolutely delicious.  We all agreed that it combines the crunchiness of celery with the meatiness of ‘heart of palm.’  I would definitely eat this at home.  However, all three of us promptly spat out our samples after each test.

  • We were curious, not stupid.
  • One never knows what unseen toxins or microscopic parasites might lurk in wild spartina stems.
  • Better safe than sorry. When in doubt, cook your food.


6. The Biggest Critters.

White Sturgeon.  Three Sturgeon were spotted on sonar but not caught in the net:  two in Alviso Slough and one at station Coy2.  These are some that survived the fish-killing Red Tide of August 2022.  They have returned to feed and grow here. 

Harbor Seals.  Two seals were foraging as we trawled Alv2.  The Seals at Calaveras Point seem to range farther upstream from their home base in springtime.  I am guessing they may do that because fishing is so good this time of year.  Or maybe it has more to do with the Seal lifecycle: Seal moms birth their pups around April.  These moms may have babies to feed.


Red Harbor Seals at Calaveras Point, 1 April 2023.

Freshly-washed Red Seals.  The Seals are reddest in springtime but often covered in mud.  April showers make them shine like new pennies.  Some of these females are probably due to deliver soon. 

  • Luca and I scanned the colony from the distance for any signs of newborn pups. We did not spot any on April 1st


Update:  Western Sea Kayakers paddling in Alviso Slough recently spotted a sturgeon jumping from the water.  It inspired their club secretary, Eric Larkin, to send me this piece from Longfellow’s “Song of Hiawatha.”

“…Forth upon the Gitche Gumee,
On the shining Big-Sea-Water,
With his fishing-line of cedar,
Of the twisted bark of cedar,
Forth to catch the sturgeon Nahma,
Mishe-Nahma, King of Fishes,
In his birch canoe exulting
All alone went Hiawatha.”

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