Fish in the Bay – April 2024, Traditional Baby Fish Month.

Once again, it is that time of year: Baby Fish Month!  Hundreds to thousands of tiny baby “unidentified” gobies traditionally show up in April. 


Water remains very fresh after the extended rainy season.  Low salinity is keeping marine fishes away:  Anchovies were absent from all upstream stations.  Freshwater oddballs continue to make an appearance: Common Carp, Sacramento Sucker, and Prickly Sculpin.


Late season low salinity appears to be pushing Anchovies, Halibut, and Bay Pipefish farther downstream than we are accustomed to seeing. 

  • Some fishes, like Gobies, Starry Flounder, and Striped Bass appear to be unphased by the freshness.


1. Baby Fishes.

Traditional Baby Fish Month was established in 2017.  By that point, data showed that Unidentified and/or Yellowfin Goby counts tended to surge around April each year – give or take a month or so.

High Baby Goby years: numbers were literally “off the chart” in 2017, 2021, and 2022.

Low Baby Goby years:  2015, 2019, and 2020.  (NOTE: Trawls were NOT performed in April 2020 Due to Covid safety shutdown.   We may have entirely missed the Baby Goby surge that year.)   

Other Goby Baby Booms:  Arrow Goby babies tend to hatch around May or June. If they show up early, they sometimes augment Traditional Baby Fish Month counts.  On the other hand, Cheekspot and Shimofuri Gobies spawn through the warm season hence their baby surges show up later in summer or fall. 


Other fish babies: 17 Herring, 14 Halibut, 5 Prickly Sculpin, a couple of Staghorns, and most of the nearly 600 Anchovies were also very young “babies.”  However, these other babies are easily distinguishable from gobies, and the other species generally hatch at slightly different places or times. 

  • Strictly speaking, ‘Traditional Baby Fish Month’ is defined by the hordes of baby Unidentified/Yellowfin Gobies we see in April and sometimes May.
  • Gobies are described as “babies” when they are around 25 mm or less in length. Gobies are usually distinguishable from sculpins and Clupeiforms even at this small size.


Sami sorts and counts some of the Yellowfin fry from Pond A19.

Total Baby Fish count = 485 unidentified + 62 Yellowfin Gobies.  (over 240 from Pond A19 alone!)


Pacific Herring count = 17.  All Herring caught in April were also “babies” measuring around 30 to 40 mm. 

These Herring hatched in upstream marshes over the last few months. They head out to sea as they gain energy and orientation for the journey.  They must leave Lower South Bay by June at latest.  With luck they will first return around December as 70 to 80 mm yearlings. 



Anchovy young-of-year and babies, LSB2, 7 Apr 2024.

Northern Anchovy count = 595.  There were very few full-sized Anchovy adults in this bunch. Roughly half of them measured at or below 50 mm – which is “baby-size” for an Anchovy. 

  • Almost all Anchovies in April were caught at LSB2 at the extreme salty edge of our trawling range.


2. Bugs!

Top panel:  Swarms of Ghost Midges march across the landscape near Zanker Landfill.
Bottom panel: Equally dense swarms of Mysids pack the upstream ends of sloughs underwater.

By now, one may ask: “Why do baby fishes (and birds) hatch around April.  Answer: Bugs!!!  In a healthy marsh, bugs bloom in the springtime warmth after abundant rain. 


Bug close-up #1.  Aquatic bugs are unfamiliar to most people.  However, they are abundant here and serve as staple foods for most of the small fishes and shore birds. 


Bug close-up #2.  Amphipod photos from May came out better, plus a couple of Gammarids (aka “Scuds”) were also spotted.


Shrimp, the bigger bugs.  Shrimp also feed on Mysids and Corophium, in addition to larvae of polychaete worms, clams, ostracods, etc.  It’s a bug-eat-bug world at the slough and Bay bottom.

Crangon.  At this time of year, Crangon shrimp hatchlings are the same size and almost indistinguishable from mysids at the upstream end of sloughs.  They grow quickly and migrate downstream as their size increases.   

Exopalaemon (aka Palaemon modestus).  Exopalaemon shrimp persist mainly in Dump Slough.  In April, these shrimp were observed only in a few other places where water was fresh enough.  It was surprising that Exopalaemon were so scarce considering this year’s freshness.  


Palaemon (aka Palaemon macrodactylus).  Non-native Palaemon shrimp were present at almost all stations but abundant only in the main stem of Coyote Creek (stations Coy1 thru Coy4 and Alv3) where they likely compete head-to-head with native Crangon.


3. Ducks.

Ducks are wary of our trawling boat.  We only see them from great distance before they scatter.


4. Harbor Seals.

Seal pupping season.  We cruised past the Calaveras Point Seal Rookery at high tide.  Only a few pups were spotted amongst the tightly packed adults. 

Comments are closed.