Fish in the Bay – December 2020: Longfin Alert! Longfin Smelt are Spawning in Lower South San Francisco Bay!

Trawl map highlighting the December Longfin Spawning Zone.

1. Background.

In 2017, UC Davis researchers first confirmed Longfin Smelt spawning in the Alviso Marsh Complex.  It was the culmination of many years of desperate search to locate exact spawning locations and for a fish that has been in sharp decline since at least the 1990s. 

Longfins used to be one of the most numerous fish in San Francisco Bay at least through the 1960s.  They were commercially harvested in the 1800s.  Every year, cold weather triggers Longfin Smelt to swim upstream into brackish marshes for spawning.  But, good spawning places are hard to find …

The UC Davis team tracks the remaining pockets of spawning Longfins in marshes near Napa-Sonoma marsh, Petaluma River, and in the North Delta  … and here in Alviso marshes as well. 

In the last few years, Alviso Marsh Complex stands out as the most robust of the last spawning places for Longfin Smelt in San Francisco Bay.


2. Longfin weekend – Saturday Trawls.

Only 12 Longfin Smelt were caught in November, so expectations were low.  By noon on Saturday, we had trawled 6 of the 10 stations and collected only three smallish Longfins – 55, 68, & 73mm.  We needed to find big mature adults.   At Pond A21, our luck started to change.

One of the 54 Longfins caught on NW side of Pond A21, 12 Dec 2020. 

The trawl on the north-west side of Pond A21, netted 54 Longfins. This was clearly a Longfin meeting place.  They were a mix of big two-year-olds with many yearlings.  Sizes ranged from 53mm (one-year-old) to 107mm (2-year-old adult).  Longfins over 90mm are considered definitely mature and optimal for spawning.  Seven fish were over that threshold.


Longfin Smelt male, Pond A21, 12 Dec 2020.

Some of the big adults had the darkened blue-emerald dorsal coloring, lengthened anal fin, and extended belly-ridge that marks a spawning-ready male.  Males use their long anal fin to sweep sediment away from nesting spots in hopes of attracting females to deposit their eggs there.  The tattered edge of the fin tells us that this male has been busy prepping a site.


Station Coy2, slightly upstream from Pond A21, was our next trawl.  Only two Longfins were netted at this station, both were adults (90 & 102mm!).   The fat-bellied female was literally dripping eggs!

We found EGGS!!!   The Longfin spawn of 2020/21 has begun!


3. Sunday Trawls.

Six of eight adult Longfins at Art2.

16 Longfins were found in Artesian Slough early the following day: eight at Art2 and eight more at Art3. 

The two largest adults (80 and 81 mm standard length) caught at Art2 showed darkened dorsal color consistent with spawning readiness despite their sub-optimal size.  However, these fish were not closely examined in the field for any other indications of sex, age, or status.   


American Shad (top), three Longfins, and a Yellowfin Goby (right) at Art3, 13 Dec 2020.

The situation at station Art3 was similar: eight fish, one adult at 90mm.  Two tiny eggs were observed with one fish – This result is pending lab analysis! 


Big male Longfin showing a belly-ridge at Art3.


Four Longfins from Dmp1.  Adult male at top.

A total of 6 Longfins were caught at the two Dump Slough stations.  The largest was the spawning-ready 98mm male, shown above, with a possible harem of smaller partners (60, 73, & 73 mm) at the upstream Dmp1 station. 


A mix of Anchovies, Shad, and Longfin Smelt at Pond A19 – east side, 13 Dec 2020. 

Pelagic Fish Paradise in Pond A19.  We encountered a mix of fish at both stations in Pond A19:  the combined Pond count was 60 Anchovies, 20 American Shad, 8 Threadfin Shad, and 75 Longfin Smelt! 

Whatever causes Pelagic Organism Decline (POD) in Northern California was not happening in this pond, not this day at any rate!


Egg-bearing female from Pond A19 – east side, 

Pond A19 – East Side was where the action was!  The Longfin cohort at station A19-4 included eight adults over 95mm.  Three females (95, 97, &103 mm) were exuding eggs!  One male (95mm) was seeping milt!  I will not sugar-coat this.  … I believe we may have interrupted some of these big fish in the act!   

Longfin experts will have to evaluate the reproductive readiness of the many smaller dark-backed juniors in the group.  Could the little ones also produce significant eggs or milt? 

HOWEVER, I don’t think we yet know how Longfins time their spawn.  Anchovies are well documented as night-time spawners, for example.  Day-time spawning would be a risky activity in a marsh full of piscivorous birds.  –  more investigation needed!


Egg Closeup.  Ripe Longfin eggs are plump and white.

Longfin eggs turn plump and white when ready to be laid.  The sticky eggs adhere to sand, rocks, and even marsh plants.

Note:  Per Longfin research protocol, egg-checks were NOT performed.  Field examinations are constrained by rules governing these threatened fish: Longfins are measured for standard, fork, and total length only.  Additional analyses will be performed at the lab: e.g. examination of otoliths, stomach contents, liver, and gonads.   


Another big male Longfin from Pond A19 East Side, 13 Dec 2020

How can we help these fragile fish survive and multiply? 



Longfins are Spawning!!!   – Micah celebrates with some trash mylar balloons we found in the marsh.

Folks, Longfins have hope, but time is short. They spawn in this marsh. They are spawning now! … We must Focus!


Comments are closed.