Fish in the Bay – August 2020: Anchovy Spawning Supplemental Report.

Hello folks.  This is the promised Anchovy Spawning Supplement to accompany the regular August Fish in the Bay report. 

The purpose is to rigorously document our spawning Anchovies to the extent possible within the physical limits of time and moral desire to avoid unnecessary fish stress. 

Anchovy spawns occur at many places along the coasts of North and South America, Africa, and Europe, but to find a spawn in progress literally under our feet (beneath our boat) is kind of rare. As far as I have been able to discern, this may be the only specific upstream Anchovy spawning location identified on the West Coast – correct me if I am wrong.   

First, an announcement:  Dr. Levi Lewis receives UC Davis Award. 

Dr. Levi Lewis on the trawling boat.

Dr. Lewis received the UC Davis Award for Excellence in Postdoctoral Research in August.  As you all may know, Levi is the overseer of our trawling effort as well as lead Principal Investigator (PI) and manager of the UC Davis Otolith Geochemistry & Fish Ecology Laboratory (formerly Hobbs Lab). 

Levi Lewis is currently a 4th-year postdoc and Delta Science Fellow in the Wildlife, Fish & Conservation Biology (WFCB) Department. As an undergraduate, he studied under advisor Dr. Peter Moyle and later received his master’s degree at San Diego State University. He completed his Ph.D. at Scripps Institution of Oceanography, UC San Diego, in 2016. 

The latest word is that Levi has now been hired as an “Assistant Research Scientist” by the UC Davis Academic Federation which means Levi is now a member of the university’s research faculty. 

Needless to say, our amazing Anchovy spawning discovery would not be possible without Levi’s guidance, supervision, and leadership.


The Anchovy Spawning Report

Trawl map.

The map above highlights trawling stations where egg-bearing Anchovies were observed on August 1st

Historical Background.

  • Anchovies may have spawned in these waters long ago, but for at least four decades this spawning habitat was restricted to a narrow ribbon of Coyote Creeks with practically no fringing marsh, hence little or no food production.
  • Prior to levee breaching in March 2006, Pond A19 was a barren salt evaporator pond that was constructed in the 1940s.
  • Prior to flood control modifications in 1988, the marshes along Dump Slough were enclosed inside Pond A18; also a barren salt evaporator pond constructed in 1954.

Methodology.  We conducted a second crude survey to document presence of spawning Anchovies on August 1st, 2020. 

  1. Random Anchovies were selected for “egg-checks.”
  2. If eggs were detected by gentle squeeze, photos of eggs and dorsal color were matched and displayed below.
  3. All fish were immediately returned to the marsh after counting, measuring, and egg-checks.
  4. No other documentation was attempted in the interests of time and limiting stress on the fish.


Water quality conditions for Spawning Anchovies on August 1st.

Compared to the previous Anchovy spawning observations on July 12th:

  • Dissolved Oxygen (DO) was generally about 1 mg/l higher,
  • Salinity was 2 to 4 ppt saltier, and
  • Water temperatures were about 1 degree cooler.

These differences may have been mainly due to the stage of tide during each set of trawls:  falling and low tide in July, versus rising and high tide in August.

Literature Review:  Many studies describe spawning Anchovies around the world.  Some links have been provided in earlier blogs.  Here are a few more examples:

Anchovy spawning is generally correlated with warm sea surface temperatures around 19 to 25 degrees C (a few species of ocean-spawning Anchovies spawn in cooler waters) and abundant tiny food.  River freshwater flows enhance the growth of tiny food and are in turn correlated with successful Anchovy spawns in shallow seas.  Some Anchovy populations have been found to spawn far upstream in waters as fresh as 9 ppt (e.g. in Portuguese estuaries).  This is mostly consistent with what we see here in Lower Coyote Creek.   

East Coast Anchovies (Anchoa mitchilli), aka “the Bay Anchovy.”

The Bay Anchovy (Anchoa mitchilli) of the east coast seems to be the best analog for our SF Bay Northern Anchovy (Engraulis mordax).  The Bay Anchovy inhabits estuaries and coastal areas along the US east coast to the Gulf of Mexico from Massachusetts to the Yucatan. Like our own variety, Anchoa mitchilli has been found in extremely fresh waters and regularly spawns in salinity as low as 7 to 9 ppt in the Chesapeake Bay and Hudson River.


1. Spawning Anchovies in Pond A19 on August 1st:

Some of the 251 Anchovies caught at Pond A19-1 (the west side of the pond).

Most fish in Pond A19 were golden-green and very faded, but you can see flashes of green in this photo.  There were also some blue-backed Anchovies in this group as shall be shown.


This first group was caught in the western (downstream) side of the pond.  These fish, like all the others in this study, tended to be much “faded;” with relatively few chromatophores expressing iridescent colors over the dorsal side.  However, unlike the more upstream areas, more fish in this group exhibited bright green or bright blue hues; usually as a bright line of chromatophores along the lateral line.    


A number of fish were also darker:

  • The big anchovy, shown above, stood out. He, or she, was longer and visibly darker than almost all the rest. 
  • Melanophores darken from exposure to sunlight in clear water. It is a fish suntan.  We surmise that this dark appearance probably resulted from this fish swimming in the clear blue ocean until recently.
  • Chromatophores express iridescent colors through an entirely different guanine crystal mechanism that is modulated by salinity. In this example, there are both blue and green patches of chromatophores spread over the entire dorsal side even though the overall appearance is more green than blue. 
  • Did this fish recently arrive from the clear blue Pacific?   


The two fish shown here are solid blue except for green crowns at the head. 

  • But they are not so dark. Perhaps they came from the turbid blue Pacific? 
  • This is complicated!


This random anecdotal survey could not detect any correlation between dorsal color and spawning readiness. 

  • Spawning readiness was determined by gently squeezing the abdomens of randomly selected fish (the “egg-check”). If eggs were expressed, a photo was taken.
  • Eggs are visually assessed here as being either “Scrawny & White” or “Plump & Pink.” No further evaluation of spawning readiness was attempted.
  • Like the overall sampled population, Anchovies confirmed as bearing eggs generally had highly faded dorsal colors ranging from bluish-green to golden-green.


Most of the egg-bearing fish appeared to be young (small) and with very little melanophore or chromatophore pigmentation.  This suggests that they could be year 0 or year 1 marsh residents.


Egg condition ranged from “Scrawny & White” to “Plump & Pink.”

  • Anchovy literature from around the world reports that Anchovies are “multiple-spawners.” Females often lay eggs every 5 to 7 days over a period of weeks or even months.
  • “Scrawny & White” eggs are probably not ready for release.
  • “Plump & Pink” eggs will likely be released in the next day or two.


Tiny food is critical!

  • All Anchovy literature agrees that both spawning intensity and larval survival are highly correlated with food availability.
  • Adult anchovies eat tiny crustaceans like mysids, copepods, copepod nauplii, and cladocerans. Dinoflagellates are critical for larval “first feeding.”  


Blue and green Anchovies deeper in the tub.

This last batch of fish from the first A19 trawl was noticeably bluer and greener.  The self-selected group was slightly larger and more energetic.  They evaded the first passes of my small dip net in our big tub, and so they were the last to be counted. 

Notice how much darker pigmented they are as well.  Did these big adults just arrive from the Deep Bay or sea?


Eggs were confirmed in only two of the last batch of anchovies, but only 4 or 5 were checked. 

This female has the long body of an adult plus dark melanophore pigmentation even though the blue chromatophores were greatly faded.  She was one of the clearest examples showing that adult blue Anchovies spawn amongst the young greens and golds in these waters.


Interestingly, Anchovies caught on the east side of Pond A19 were a bit more golden brown. This is the upstream side of the pond, so it stands to reason that water should be fresher. 


Pond A19: a comparison of the two sides.  This is Anchovy spawning territory!  

Dense stands of California bulrush also indicate fresher water here on the east side of Pond A19. 

However, instantaneous readings from our instruments consistently show very little salinity difference between the two sides of the pond.  This is an ongoing mystery.


A highly faded blue Anchovy with scrawny white eggs.


Another faded blue Anchovy.  This one had plump and pink eggs.

It was difficult to hold a wriggling fish in one hand then flip it over to photograph eggs, while holding a camera in the other hand,   … and doing it quickly.  Many fish flipped into the marsh during the process.   Much data was lost!


2. Spawning Anchovies in Upper Coyote Creek (UCoy1):

Anchovies at UCoy1 were golden-green to brown with only a few greener exceptions.


Four egg-bearing Anchovies from UCoy1.


Two more egg-bearing Anchovies.


3. Spawning Anchovies in Dump Slough (aka, Coyote Creek Bypass Channel):

Micah prepares to release the net in Dump Slough.


Two green-backs – a comparison.  Top fish was caught in Pond A19. Bottom fish from Dump Slough.

The above comparison shows the effect of dark melanophores (the fish suntan) on the overall green-back appearance.  The goldish-green dorsal hues are similar, but top fish has very little melanophore darkness.  Bottom fish has a fairly strong underlayment of darkness from melanophores that make the goldish-green chromatophores “pop” more distinctly.  Melanophore darkness seems to persist in a fish for longer time.


  • Top fish also appears more youthful to my untrained eye. Body length appears shorter compared to head size. 
  • Bottom fish, but for the goldish (freshwater) tone, appears to be a Deep Bay or ocean migrant.
  • Two possibilities: 1) Young Anchovies may migrate to the Deep Bay after their second or third year, or 2) Year-round marsh resident Anchovies raised in warmer waters will not attain the longer body lengths of their sea-going cousins, per Carl Hubbs (1925).


The spawning Anchovies of Dump Slough seemed to be consistently greenish and much faded. 


Their color and degree of fading (loss of guanine crystals in chromatophores) is consistent with the generally lower salinity in this slough.


As in the other sloughs, egg condition in Dump Slough ranged from “scrawny & white” to “plump & pink.” 


Snowy Egrets perched on the A18 hydraulic structure in Artesian Slough at the end of the trawls.


4. Final Notes.

Anchovies were clearly ready to drop eggs in waters in, and adjacent to, Coyote Creek in July and August.  But, will spawning continue through this warm season?

  • Very young juvenile Anchovies were caught just downstream of this area in January 2019. See: Fish in the Bay – January 2019, UC Davis Trawls – Anchovy Spawn, a belated report. This strongly indicates that Anchovies spawned very nearby as late as October or November in 2018.
  • Very young juveniles have also been caught at various times throughout warm seasons of previous years, again suggesting that spawning may be fairly continuous a good portion of the year.

Recruitment success of our Anchovy spawns is the next question.

  1. Predation.  The Alviso marshes team with small predators ready to eat fish eggs and larvae.  The Three-spined Stickleback is a notorious egg-eating example. Anchovies themselves are known to cannibalize their own eggs and larvae. 
  2. Recruitment. Anchovy recruitment success is highly dependent on availability of tiny food for larval fish.  This is particularly important for “first-feeding” immediately after yolk sack absorption, roughly 2 days after hatch.  Availability of dinoflagellates (not diatoms) has been identified as a key factor in hatchling Anchovy survival:

Thus, it is possible that Anchovy spawning may be continuous over much of the year, but recruitment success only occurs when predation is suppressed and/or tiny food (dinoflagellate) production surges.

… Time and future trawls will tell.

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