Fish in the Bay – April 2025, Part 2. Corbula Attack, Gobies, Bottom Fishes, & Tides!

Part 2 of the April report.

1. Corbula Clams.

The main stem of Coyote Creek between UCoy2 and Coy1 has always been a Corbula hotspot.

April Corbula Clam count = 17,792.  This is a record-breaker for us!  Our highest count until now was long ago: 13,522 in September 2012.  Since then, annual Corbula counts usually varied between 2,500 to 8,500.  This month’s count is insane!  

• We were stunned by the Corbula numbers as we trawled Coyote Creek upstream stations on April 9^th: 5,650 at Coy1, 3640 at UCoy2.
• Very few Corbula were picked up at other upstream stations. This is not surprising: Corbula need a flowing river environment.  Dead-end sloughs and restored salt ponds are not agreeable to them.     

Alviso Slough is the home of jumbo-sized Corbula.  They look older and crustier here.

As we trawled the downstream stations the next day, Sami suggested we try measuring these clams.

• This was not an easy or accurate task using only a measuring board. (A measuring caliper would be a better tool –  https://taytools.com/collections/tools-measuring-calipers?srsltid=AfmBOooWeFfuxXr07PHvNJ644W86f7U0I5IS5ykIaSqcbz-P-dt9k1Er )  Nonetheless, we worked with what we had. 
• Consistent with previous observations, the largest, and presumably most reproductive clams appear to concentrate within a fairly narrow range of bottom salinity: around 4 to 10 ppt.  At stations where bottom salinity was higher or lower, Corbula were less numerous, smaller in size (as measured at the widest diameter), and shells appeared younger and shinier. 

Bear in mind, these Corbula size measurements were crude, and bottom salinity readings were instantaneous figures, not averages.  It was just the best we could do given the urgency of the situation.

We found fewer Corbula downstream from the Coyote Creek and Alviso Slough breeding zones.

No apparent trend in clam size was seen in the main stem of Coyote Creek out to Lower South Bay (LSB) stations.

Corbula appear to be perpetually smaller and youngish where salinity is above around 10 ppt.

• These clams may get outcompeted or consumed by other organisms in higher salinity water.
• Or perhaps they simply need steady river flow and a constant food supply to mature to crusty old age.  

The seeds of this population boom may have begun as early as November 2023 or sometime in 2024.  Monthly counts began edging upward about that time.  The annual count in 2024 was 14,615 – more than double for a typical year, but the first alarming jump was just last month.  … and now, another big jump.

2. Goby World.

Shimofuri Goby count = 8.  Shimofuri numbers steadily increased in LSB over the last decade or so.   

• Huge increase: Only three Shimos were caught in 2012, for example. Since then, the average annual Shimo count climbed to over 2,600 from 2021 to 2023.   
• Then a sudden crash: Shimo counts rapidly dropped to only 371 in 2024.  They remain low so far in 2025.

Shimos remain present in LSB.  The recent decline is likely only temporary. 

One indication that Shimos may rebound soon is that we caught 23 of them at Station UCoy1.5 on April 9th.

• UCoy 1.5 is not one of our regular monitoring locations. This trawl was performed as a “reference site” for the new Wetlands Regional Monitoring Program (WRMP). 
• Technically, these Shimos are “off the books” with respect to the regular monitoring program. But here we found them hiding in plain sight, so to speak.
• Also, notice the variation in body colors and patterns: tan, dark brown, and watermelon striped. Chameleon Gobies at Eden Landing showed the same patterns the previous day. https://www.ogfishlab.com/2025/04/08/fish-in-the-bay-april-2025-wrmp-eden-landing-trawls/  
• As mentioned many times, Shimos and Chameleons are very close cousins.

Shokihaze Goby count = 82.  This is the highest monthly tally since May 2021.  

• Shokihazes became rare and Shimos boomed for three years after 2021.
• The Shimo-Shoki ratio has now flipped again after mid-2024. Shokis are up.  Shimos are down.  These two species clearly compete head-to-head for territory. 
• There have been no clear impacts from this Shimo-Shoki battle on three out of four other goby species: Yellowfin, Cheekspot, or Arrow. … But one goby species has been hit hard.

Bay Goby – 25 mm standard length, LSB2, 10 April 2025.

Bay Goby count = 2.  Bay Gobies were once the common native mid-sized goby here until the 1980s.  But in all modern trawling surveys since 2010, we have seen only a few adults and a smattering of babies (7 baby Bay Gobies per year on average). 

• More likely than not, the expansion of both Shokihazes and Shimofuris pushed Bay Gobies into deeper, saltier water and effectively out of LSB altogether.
• The arrival of Yellowfin Gobies by the early 1960s also likely stressed the local Bay Goby population. But the near extirpation we see today didn’t happen until after the arrival of Shokis and Shimos a few decades later.

3. Other Brown Bottom Fishes.

Halibut count = 48.  California Halibut are sticking around!  The strong but short El Nino of 2024 stimulated a small baby boom.  We should continue to see more young Halibut as the seasons warm.

English Sole count = 90.  It is hard to understand how tiny baby English Sole, which hatch off the coast, can swim and surf the tides to South San Francisco Bay and beyond.  However, seeing is believing!  

• English Sole are “La Nina Fishes.” In LSB, we see more proof:
  • The Pacific Ocean was in a warm El Nino phase for much of 2024 = English Sole count in LSB was only 34.
  • Ocean conditions flipped to La Nina in January 2025 = English Sole count year-to-date has already jumped to 118. https://www.climate.gov/news-features/blogs/enso/january-2025-update-la-nina-here
  • Granted, this is a small jump. During the strong and persistent La Nina years of 2021 and 2022, we counted several hundred to over two thousand baby English Sole.

Staghorn Sculpin count = 484.  Staghorns were already discussed in Part 1 of this report.  This is just another reminder that Staghorn Sculpin are our other documented “La Nina fish.” 

• Somehow, La Nina conditions also stimulate Staghorns.
  • During the El Nino year of 2024, we saw only 194 Staghorns.
  • In 2025, the Staghorn count has already jumped to 532. More will show up as the year progresses.   
• The El Nino Southern Oscillation (ENSO) drives weather conditions across the Earth.
  Here in LSB, we keep our fingers on the Pulse of the Planet.

Starry Flounder count = 2.  2023 was the year of the Starry: We caught 2,030 baby Starries before that year was over.  Now, we are seeing bigger adults but many fewer of them.  The year-to-date count is 13. 

4. Big Fishes.

Upstream: Sturgeon on Sonar count = 7.   –  Downstream:  Bat Ray count = 4.  Leopard Shark count = 1

Big Fish populations transition around April to May as water temperatures warm up. 

• Upstream creeks and sloughs are Sturgeon feeding grounds during cool winter months. But Sturgeon do not tolerate summer warmth so much.  Most of them will retreat to deeper, cooler waters soon.
• Bat Rays and Leopard Sharks mate and give birth in LSB during the warm season. April is still a little early for them.  Maybe this will be a good elasmobranch year!

5. Water Color Analyses.

An incident on April 8^th gave us new insight into ever-changing water colors.

• Water color as we approached station LSB2 was conspicuously “Mélange Green.” The Moon phase was “Waxing Gibbous,” almost exactly between neap and spring tides. 
• Then a fortuitous disaster struck: the net got stuck in some underwater obstruction.
• During the roughly 90 minutes it took the UC Davis team to recover the net, ebb tide flows stirred up a large amount of sediment. Water color changed from “Melange Green” to brownish “Antiquity” as far as the eye could see in all directions.

Sediment resuspension usually controls water color in LSB.  …

We are constantly on watch for exceptional orange, red, or dark brown stains that could signify toxic fish-killing red tide blooms.

Lunar phases versus water color in LSB: a conceptual model.

Water colors in LSB normally vary from greenish to brownish.  But what exactly is normal color here?  Green originates from phytoplankton, primarily photosynthetic diatoms and dinoflagellates.  Brown signifies turbidity from resuspended sediments in a shallow muddy bay. 

• Moon phases have a huge impact.
  • Higher tidal amplitudes and velocities during Spring Tides (full and new moons) resuspend sediments. This turns water in LSB brown.
  • Smaller amplitudes from Neap Tides (quarter moons) allow sediments to settle – water looks greener.
• Observations on April 8^th highlight the shifting tidal dynamics that occur during the transition between neap and spring tide phases.

Oingo Boingo – Weird Science.  https://www.youtube.com/watch?v=f9DremX8RDk