Fish in the Bay – June 2021, Anchovy Alert! Spawning Anchovies have returned to Lower South San Francisco Bay.

Anchovies are Spawning again in San Francisco Bay!  Our monthly Anchovy count jumped from 22 in May (with 17 of those loitering far downstream at deep bay station LSB2) to 360 in June with egg-bearing females and/or milting males observed at 12 of 20 stations spread across both the Bay and brackish marshes. 


This is the second year of confirmed Anchovy spawning.  We first discovered egg-bearing female and milting male Anchovies in July 2020:


Egg-bearing Anchovy at LSB1

Northern Anchovies (Engraulis mordax) migrate into LSB from the deeper SF Bay or the ocean from summer through late fall.  They come here to spawn!

Based on this first complete year of observation:  Northern Anchovies appear to continuously spawn from June through first cold-snap in late October/Early November at nearly all locations in Lower South Bay, but with particular intensity in upstream marshes in Alviso Slough and Coyote Creek.   


Water quality where Anchovies spawn.  The table summarizes start-&-finish, and top-&-bottom readings for Dissolved Oxygen (DO), Salinity, and Temperature at all 20 stations during the trawling surveys.  

  • DO was really low at upstream stations on June 5th. DO at station UCoy1 was below 3 mg/l across the board!  Nonetheless, Anchovies were unfazed.  At least some appeared to be in a state of extreme sexual excitement! 
  • I am not aware of any documentation of Anchovy preference for, or even survival in, waters with DO as low as 2 mg/l! 
  • BTW: As near as we could discern, the low DO on June 5th was a natural example of episodic marsh hypoxia.  There were no conspicuous signs of biological disturbance, anthropogenic or otherwise.


Why do Anchovies seek high temperature/low dissolved oxygen brackish marshes to spawn???


Possible schools of Anchovies near Coy1

Many more fish were spotted on sonar.  The fishfinder side-scan sonar indicated schools of small fishes as we trawled through the main channel of Coyote Creek on both days of the June weekend.  We couldn’t confirm that these were Anchovies, but that is the best guess. 


Anchovy trend.  The 10-year record shows that Anchovy numbers regularly increase each summer.  2020 was a particularly good year in LSB, but outside of a few other spurious peaks, there is no clear trend.  Hopefully, 2021 will be as good or better than 2020.

Anchovies were likely spawning right under our noses for years, but we never thought to check until July 2020.  Earlier egg and larval studies indicated that Anchovies spawn in San Francisco Bay generally, but Lower South Bay was always assessed as the least likely location: e.g., see McGowan (1986) Northern Anchovy, Engraulis mordax, Spawning in San Francisco Bay, 1978-79 … 


A blue-backed egg-bearing Anchovy at Coy3

Variation in dorsal colors was striking and characteristic at several stations.  Anchovies ranged from deep blue to greenish-almost-colorless.  Dorsal color has little or no relationship with spawning readiness.


Both young and old Anchovies appear to spawn.  In both 2020 and 2021 we observed slightly more small-colorless (<80 mm, presumably young) egg-bearing females than larger-colorful ones (>90 mm).    


Young golden-green Anchovy with eggs at Coy1.

According to NOAA fisheries, and various other sources,

  1. Northern Anchovies begin spawning at two years of age.
  2. They generally live two to four years with rare individuals living up to seven years.
  3. They grow fast up to seven inches (178 mm).

I suspect that statement #1 might be erroneous.  We seem to see numerous examples of very young-looking Anchovy females bearing eggs.   But, that is only based on eye-ball observation.  We have not yet attempted to determine actual age of young Anchovies. 

With regards to statement #3: We never catch Anchovies longer than about 130mm (5 inches).  Even 120mm is very rare for us.  Those oldest seven-year-old fish are either 1) extremely rare, 2) do not migrate into the Bay, or 3) are too fast and agile to catch in a trawl net.


Intense blue or green dorsal sides indicate recent migration from saltier & less turbid water.

Lessons learned from last year: 

  • Like other Clupeiform fishes, Anchovy dorsal sides change color with salinity: they “blue up” in high salinity, and they “brown down” in fresher water. 
  • Unlike other Clupeiforms that change dorsal color within minutes, Anchovy color persists for days, weeks, or possibly even months in mature Anchovies. Ergo, dorsal color provides a clue about recent Anchovy migration.


Some of the 159 Anchovies at LSB1 on 6 June 2021.

Young Anchovies, 2 to 3 inches (~50 to 80 mm), are more common in LSB.  They develop very little dorsal color and only limited silvering on the sides until they are exposed to higher salinity closer to the ocean.    

These translucent green-backs look very young.


The sizes of individuals exuding either eggs or milt ranged from 59 to 102 mm.


Different year-classes mix and mingle.  We usually find a few to several longer Anchovies (~100mm) with solid green or blue backs amongst groups of youngsters.  The large ones are at least a year older.  Their vivid color must indicate they have visited the ocean at least once. 


Marsh-spawning Anchovies have been studied in Chesapeake Bay and estuaries in Portugal, among a few other places.  From that work, we learned that: 

  • ‘Mature’ female Anchovies batch-release eggs every 7 to 10 days during spawning season. Eggs are scrawny and white as they develop, then become plump and pink as they swell and hydrate prior to release. 
  • Spawning frenzies occur at night within a few hours of midnight.


Green to brownish Anchovies at Alv3 in June.

Please Note:  Many spawning Anchovies may have been missed in our survey. 

  • At each station, only a small portion of Anchovies were checked for eggs or milt in the interest of limiting stress or injury to the fish. Each spawning adult represents at least hundreds for the next generation. 
  • A wise farmer never squanders the seed stock!

We will continue to monitor this Anchovy Spawn throughout the 2021 season.

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