Fish in the Bay – Traditional Baby Fish Month!

Once again, it is April.  Time for another “Traditional Baby Fish Month.”  – Yes, we invented this April tradition and gave it a silly name.  But there is a fundamental truth behind it, as explained below.

Expanded Report.  This “Fish in the Bay” report covers the three regions we now survey in “South SF Bay – south of the San Mateo Bridge.

• Our expanded area now includes Lower South Bay (LSB – south of the Dumbarton Bridge) …  PLUS, restored areas in Bair Island and Eden Landing Ecological Reserves.  We extended our fish and bug surveys in the two new regions in mid-2025.   
  • Good news: The expanded area now covers a broader geographical portion of SF Bay and, in particular, a wider range of ambient salinities from consistently salty, at Bair and Eden, to considerably fresher in the upstream reaches of LSB. 
  • Bad news: New regions add new creatures.  New creatures add new stories and more confusion!

1. Baby Fish Month = Gobies.

Baby Fishes!  Based on fish monitoring surveys in LSB, by 2017 we recognized that ‘jillions’ of tiny fishes suddenly showed up almost every April.  UC Davis researchers could tell from the short-chubby body shapes that these babies were overwhelmingly gobies.  

• After 2017, we began to anticipate arrival of “Traditional Baby Fish Month” in April.    It occurred a month or two late in 2018 & 2019, we missed it entirely in 2015 and 2020 – (***we did not trawl at all during COVID pause in April 2020).  We also documented a few years of “double” or “extra” baby fish months in 2019, 2022, 2024, and 2025. 
• Most of this seasonal fish surge corresponds with annual recruitment of Yellowfin Gobies and, to a much lesser extent, Arrow Gobies, in LSB.    
• Our improving fish ID skills both clarify and complicate the picture a bit.  In the old days, baby fishes shown in the panels above would have all been recorded as “Unidentified Gobies/Baby Gobies.” In more recent years, with increasing confidence, we identify these babies as “Yellowfin Gobies” or whatever other type of tiny/baby goby we happen to see. 
  • The data provides improved clarity, but now we have to drill down a bit in the data base to determine if the fishes were adults or babies on any given day.
  • Life is complicated.   

Baby Fish Month (Yellowfin & Arrow Goby babies) also coincides with arrival of large adults.  The biggest gravid females and their long broad-jawed male Yellowfin partners also appear this time of year.  We have learned much about Goby spawning cycles from watching these big ones.  Other Goby species display remarkably similar habits and appearances as their respective spawning times approach at different times of year.

• San Francisco Bay is home to several goby species.  Natives include Arrow, Bay, Cheekspot, and Longjaw Mudsuckers.  Non-native gobies are comprised of Chameleon, Shimofuri, Shokihaze, and Yellowfin. 
• With time and experience, the UC Davis team has learned to read the signs of sexual dimorphism, spawning readiness, and baby fish arrivals for most of these species.     

New regions add more complexity to the picture!

• Chameleon Gobies and Cheekspots are far more abundant at Bair Island and Eden, where waters are consistently saltier.   
• Alas!  Yellowfin Gobies are not nearly so prevalent in the saltier regions immediately to the north.  As a result, “Traditional Baby Fish Month” isn’t so much of a thing in the new regions. 
• Similarly, Shimofuri and Shokihaze Gobies are rare to almost absent at Bair and Eden locations. 

Note:  Raw data is shown in the table above.  Always keep in mind that figures must be adjusted to reflect “Catch Per Unit Effort.”

• In LSB, Trawl numbers are derived from 10-minute trawls at nearly twice as many stations.
• In contrast, only 5-minute trawls are performed at Bair and Eden at roughly half as many stations. 
• Rule of thumb: Multiply Bair and Eden numbers by roughly four (4) to estimate CPUE comparison with LSB. 

2. Pelagic Fishes. 

Anchovies!  Contrary to all expectations, we caught over 2000 baby Anchovies at the mouth of Redwood Creek next to the Bair Island Ecological Reserve.

• Anchovies have not contributed to “Traditional Baby Fish” tallies in the past.  In fact, April is nearly always a low lull month for Anchovies in LSB. 
• Bay Anchovies spawn in LSB from early summer through fall.  Occasionally, we see a “baby anchovy” surge in late winter.  But for the most part, we have long wondered – where do baby Anchovies go to recruit?
• Now we know!  At least some of our baby Bay Anchovies recruit in Redwood Creek in the deep channel under the Port of Redwood City!
• Bair Island was the clear “Anchovy Winner” this April.  Baby Fish Month here is about Anchovies!!!

We caught fewer baby Anchovies farther south in LSB over the same week. 

• Baby Anchovies seek extremely small phytoplankton or zooplankton food that must be only available in consistently higher salinity waters either in the northern most parts of LSB, or farther north. 
• Interestingly, we see larger juvenile and young adult Anchovies, and practically no babies, as we move upstream into fresher waters along Coyote Creek.  We surmise that these bigger young Anchovies seek larger food organisms that inhabit fresher waters.  (We have noticed this Anchovy-size gradient phenomenon for years now. The pattern becomes clearer year.)      
• Summer Anchovy spawning season is approaching.  Older Anchovies are just beginning to stage for it!

Pacific Herring count = 68. (55 at Eden, 7 in LSB, 6 at Bair)  Herring spawn in late winter in central and northern parts of SF Bay.  Their eggs and fry disperse widely.  We occasionally see a few young recruits and a few clumps of eggs in LSB in late winter/early spring. 

• Not surprisingly, we catch more young Herring just a little farther north at Bair and Eden.

3. Elasmobranchs. 

Leopard Shark count = 8.  Adult female Leopard Sharks migrate into LSB to give live birth to pups in warm summer months.  April is still a little early for them.  Nonetheless, some Sharks have arrived! 

• Four mid-to-large adult females were picked up at Bair Island. (Shark Winner!)
• Three small babies at LSB1
• One more baby at Eden Landing, station Oal2.

Bat Ray count = 7.  All Rays caught in April were young.  Big pregnant females should start showing up soon as the weather warms.  LSB was “Bat Ray Winner” for April.

• More Rays are found in LSB compared to either Bair or Eden Landing.  This is counterintuitive.  Water salinity throughout LSB is generally fresher.  Elasmobranchs, like Bat Rays, are usually thought to avoid low salinity if they can.  But, here they don’t.
• Our Rays must be attracted to LSB by food availability, possibly warmth(?), and also as a nursery refuge from fierce predators, like Sevengill Sharks.

4. Flatfishes & Drums … sentinels of warm waters? 

Halibut count = 65.  We have long considered Halibut to be our best El Nino-indicator fish.   

• Halibut counts increased in LSB with the last El Nino of 2023/24 and remain high even though El Nino ended almost two years ago. Nonetheless, the Halibut year-to-date count in LSB is 152.  That is the highest January-to-April count since 2016, shortly after the big El Nino of 2014/15.
• Truth be told, Halibut simply prefer warmer water, and the water is usually a degree or two warmer in LSB.  The population of young recruits increases dramatically during El Ninos … or any other time the water is relatively warm.

Tonguefish count = 14.  This month we caught 11 Tonguefish in LSB plus another 3 at Eden Landing.

California Tonguefish is our other main “El Nino fish.”  This month’s count of 11 is the highest monthly total in LSB since June of 2016!   Like Halibut, Tonguefish seem to be indicating that it feels like El Nino – even though we are NOT currently in El Nino. 

• La Nina fishes are easy to understand. Most of them spend a portion of their life cycles off the coast where cool nutrient-rich upwelling increases food availability.  Thus, numbers of fishes like Anchovies, Staghorn Sculpin, and English Sole increase during La Nina years.
• For El Nino fishes, it seems to be warmth, not food availability, that boosts their numbers.  Hence, Halibut and Tonguefish numbers are simply a reflection of warm Sea Surface Temperature (SST) in the local area.

White Croaker and Seabass.  Seven Croakers and one baby White Seabass were caught in LSB in April.  A few more of each were caught last month.  These members of the Drum family are still “rare oddballs” for us.  The year-to-date count of Croakers for 2026 now matches our record year in 2021, but usually we only ever see a few specimens every few years, at most.

• Of note, Both White Croakers and White Sea Bass are known to expand their ranges along the West Coast when waters warm up.  However, the influence of El Nino over these fishes is complex: warmer water increases growth rate, but simultaneously, the limited food availability during El Nino can also reduce their populations. In any case, we have not caught enough Croakers and Seabass in LSB over the last decade to make any conclusions from our own limited data.

• Young White Seabass grow faster in warmer waters …
  • Williams et al. (2007) https://www.researchgate.net/publication/226571967_El_Nino_periods_increase_growth_of_juvenile_white_seabass_Atractoscion_nobilis_in_the_Southern_California_Bight
  • “… white seabass that hatched in 1996 and 1997 grew significantly faster than those that hatched in 1998, 1999, and 2001. These differences in growth rates of cohorts appeared to be driven by variation in sea-surface temperature (SST). Growth rates averaged over the first three or 4 years of life were significantly positively correlated to average daily SST during the first 1–4 years of life. Increased growth of juvenile white seabass during the warm El Niño period likely provided a number of benefits to this warm-temperate species. This study demonstrated that some species will benefit from these warm-water periods despite reduced system-wide primary production.”

Shiner Surfperch count = 10.  Shiner Surfperch, aka “Shiner Perch,” are a common live-bearing fish at coastal piers.  Some reside in the Bay as well.  We caught 5 adults and 3 babies in Corkscrew Slough at Bair Island, plus two more at Eden Landing this month. 

Striped Bass count = 4.  Only four bass were caught in LSB.  None were seen at Bair Island or Eden Landing in April.   For LSB, this is the lowest Striped Bass count for an April since 2013 at least.  Similarly, the 2026 YTD count is our lowest since 2012!

We noticed tell-tale lesions on three of our four Bass in April. 

• Two Bass had fairly round and distinct gouges consistent with Lamprey bites.  This is actually a good sign.  Young Lampreys migrate from the creeks to the ocean this time of year.  Lampreys are both indicators of good creek health and beneficial filter-feeders in the creek bottoms in their larval stage.  Lampreys are good!
• One Bass had a more vague-looking wound on the right side that could be from a tapeworm.  Tapeworm-induced lesions have been reported in SF Bay Striped Bass for many years.
  • Lesions in Striped Bass from San Francisco Bay  https://wildlife.ca.gov/Conservation/Marine/Parasites  “… when this larval [tapeworm] stage is eaten by striped bass from San Francisco Bay, the larvae are killed and encapsulated with fibrous tissue. If there are lots of larvae the fibrous tissue will form a “raft” in the viscera. … The reddening or “strawberry” mark seen by the fisherman is the inflammation caused when these “rafts” touch the inside of the body wall.”

• Lampreys and Tapeworms might be thought of as just a couple of ways that Mother Nature strikes back at these non-native predators!

6. Invertebrates.

Shrimp counts – see table above.  LSB supports robust populations of shrimp and mysids: Shrimp are present year-round.  Mysids bloom shortly after rains in late fall through spring.

• Shrimp in LSB concentrate in deep channels at the mouths of Coyote Creek and Guadalupe River.  We call this place “Shrimp Alley.”  Conversely, in areas like Bair or Eden with limited creek or river flows, we find vastly fewer shrimp and mysids. 
• San Francisco Bay needs more “Shrimp Alleys!

Corbula Clam count = 13,780 in LSB, 93 at Eden Landing, 0 at Bair.  Non-native Corbula clams continue to surge in LSB.  

This environmental crisis keeps rolling along.  The good news is that Corbula numbers are a bit lower than 2025 so far. 

• 2026 year-to-date count is 21,469 clams. 
• 2025 January-April count was 26,991.
• The bad news is that Corbula numbers remain the highest we have ever seen. 

These bad clams are ecosystem killers.  We have not yet discerned which fishes or bugs are impacted by these filter-feeding monsters, but microscopic food is being depleted to the detriment of other critters.   

Japanese Bubble Snail count = 285.  All Bubble Snails were collected at Bair Island and Eden Landing reserves.

Japanese Bubble Snails (Haminoea japonica) are invasive pests that do not exist in LSB.  Until we sampled at Bair and Eden, we had never seen them before.  Apparently, Bubble Snails cannot support the low and variable salinity anywhere in LSB. 

• They are “simultaneous hermaphrodites” that proliferate rapidly in warm shallow sloughs and ponds where salinity is consistently above, say 21 ppt(?).
• Their environmental impact is unknown – other than hosting a flatworm parasite that can cause “swimmer’s itch” in humans: https://wwwnc.cdc.gov/eid/article/16/9/09-1664_article
  • “In June 2005, cercarial dermatitis appeared again in San Francisco Bay. The Alameda County Department of Environmental Health received ≈90 reports of skin irritation that occurred after water contact at Robert Crown Memorial Beach … preliminary investigations suggested that the intermediate host involved was another exotic species, Haminoea japonica, from Asia. This snail was observed in San Francisco Bay in 1999 (22) and on the eastern shore of the bay, near Robert Crown Memorial Beach, in the summer of 2003, where it has become extremely abundant (A.N. Cohen, unpub. data).”
• Parasites notwithstanding, Bubble Snails are slimy and somewhat revolting.

Polychaete count = 2 in LSB, 32 at Bair Island, 137 at Eden Landing.

New Lands = New Worms!  One of the many complications from our expanded monitoring efforts is how to deal with all the new kinds of Polychaete (segmented) worms we find.

• In LSB, life is relatively simple.  We only ever see a few “Pile Worms” (Alitta Succinea), Scale worms, or a small ‘long-skinny worm” that is yet to be identified.
• As you may deduce from this month’s numbers alone, Polychaete-life becomes vastly more abundant and complex as we motor north from our LSB homeland.  We cannot yet identify, much less record all the wormy creatures we find: Spaghetti Worms, Long-Skinny Worms, Tiny Scarlet Worms, Feather Duster Worms, Tube worms(?), and many others.
• Polychaetes are good.  Polychaetes are important fish and bird food. They condition the soil and regulate the microbial community.  We like Polychaetes.    
• But … What is their specific impact on the environment?  How important is it to distinguish one type of Polychaete from another?

New creatures add confusion.   If we don’t make sense of this world, who will?

Santana – Aqua Marine  https://www.youtube.com/watch?v=oK9wql4yRfI&list=RDoK9wql4yRfI&start_radio=1