Fish in the Bay – August 2023 Highlights: Shimos, Anchovies, & Blackbirds in the Bulrush marsh.

In August, the fish populations finally transitioned to a more typical summertime assemblage of Anchovies and Gobies.   

However, a few distinct differences remain. These will be discussed in a subsequent post: 

  • A huge Starry Flounder baby boom continues.
  • A few odd-ball fresher water fishes continued to show up.
  • Leopard Shark and Bat Ray counts remain very low.

This post describes 1) The Shimofuri Goby population explosion, 2) Summertime spawning Anchovies, and 3) Possible sightings of tricolored blackbirds in the tall bulrush along Dump Slough.


1. Shimofuri Goby Explosion

Shimofuri Goby count = 898.  For better or worse, Shimofuri Gobies have displaced Shokihazes as the number two non-native goby in Lower South Bay since 2020.  (Yellowfin Gobies are still #1.) 

  • We observed a bigger Shimo explosion in October 2021 (997) along with several smaller booms in each of the past three years. Each population boom appears to emanate from the fresher water ends of Alviso and Artesian Sloughs. 
  • Recent beach seine data suggests that adjacent muted tidal ponds (Pond A8 complex and Pond A16) could be serving as spawning and/or recruitment habitats.
  • Shimos tolerate a wide range of salinity which likely explains some of their success.
  • It is not known whether non-native Shimos are any better or worse for the local ecosystem than Yellowfin or Shokihaze Gobies.


2. Anchovy Color & Size Gradient.

Anchovy count = 675.  This month’s Anchovy count is still somewhat depressed due to freshwater flushing earlier in the year.  Nonetheless, Anchovies continue to assemble into their typical color and size gradient seen in previous years:  larger and more golden Anchovies upstream; smaller and more bluish Anchovies downstream – with some exceptions as noted below.

The observed size gradient strongly suggests that mature adults migrate farther upstream for spawning during summer months.  Younger and smaller Anchovies tend to loiter at downstream stations.  

  • Egg and milt-bearing Anchovies were observed at all but three Coyote Creek stations in August.  


Summer spawning Anchovies are generally largest and most golden at upstream, fresher water stations.


Like other fishes in the Clupeiform family, Anchovy dorsums “brown-down” to golden and eventually to colorless/brownish when salinity drops below roughly 10 or 11 ppt.


The exact salinity at which Anchovies transition from green to golden/clear is difficult to determine in the field.  And, color changes cannot be tested post-mortem.  Unlike other Clupeiform fishes, Anchovy dorsal color disappears completely within about 5 minutes after death.   


Anchovies clearly follow the same general salinity-induced color change thresholds as Shad, Herring, and Sardines, but not always consistently. 


Anchovies develop dorsal colors after exposure to consistently higher salinity.  With exposure to salinity, reflective and iridescent guanine crystals form in dorsal chromatophores. 

  • Dorsal color develops first at the crown of the head in post-larval Anchovies soon after metamorphosis.
  • A thin band of color then spreads along the lateral line toward the tail and eventually up to cover the entire dorsum at or near marine levels of salinity.


In Anchovies, the process of color development is also reversible.  At low or variable salinity, guanine crystals dissipate.  Dorsal color fades to clear, brown, or gray with only traces of iridescent color remaining, if at all. 


At above 19 or 20 ppt, dorsal colors in Clupeiform fishes transition from green to blue.   

However, evaluating Anchovy dorsal color is difficult when it is faded.  The fish can appear clear/colorless or even the wrong hue when remaining iridescence is minimal.    


In addition, individual Anchovies often break the color rules.  In the example shown here, salinity at LSB1 was well above the green-to-blue threshold at 25.5.  Some anchovies appeared blue as expected, but others were unexpectedly green.   What is going on here?


In this final example at LSB2, salinity was again well above the green-to-blue threshold, but many of the 190 Anchovies appeared to express the full range of colors from blue to golden. They were painted like Easter Eggs!  Once again, Anchovies broke the color rules! 

Several photos were taken, but ultimately further inquiry was aborted in the interest of releasing Anchovies unharmed.  This mystery remains unsolved.


3. Possible Tricolored Blackbird Sighting in Dump Slough.

Freshwater from last winter’s storms facilitated the spread of California Bulrush this year.  The extensive Bulrush stands are highly attractive to at least four or five species of “blackbirds,” including European Starlings, Brown-headed Cowbirds, Red-winged Blackbirds, and threatened Tricolored Blackbirds.  On August 12th, they were roosting and hovering over tall Bulrush like clouds of locusts.    


Native Red-winged and Tricolored Blackbirds depend on fresh-to-brackish marshes for nesting and feeding sites.  The numbers of both species have declined in recent decades due to constant loss of cattail and bulrush habitat.

  • It is difficult to distinguish Red-wings from Tricolors from a distance, but we caught sight of at least three potential Tricolors amongst the flocks. This is the first time in almost 3 years that we have spotted any at all.
  • Tricolored Blackbirds:   

California Bulrush and Cattail in Artesian Slough on 12 August 2023.

The quality of marsh bird and fish habit is heavily influenced by California Bulrush.

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