Fish in the Bay – December 2021: Longfin Alert! Longfin Smelt have commenced Spawning in Lower South San Francisco Bay!

The first spawning-ready Longfin Smelt of the 2021/22 spawning season have been detected.  This typically happens in December, but only in very few places around San Francisco Bay anymore.   

You may know the story: Longfins were so common through the 1970s that people would dip-net for them in San Francisco Bay.  Their numbers plummeted in recent decades.  Now they are quite rare and under consideration for “Endangered” status.


Downstream Trawls – Saturday, 4 December.

We collected two large adults at the upstream station on Alviso Slough (Alv1).  Neither were checked for spawning readiness, but Longfins bearing eggs or milt have been found at this station in previous years. 

Long-term sustainability of our Longfin spawn greatly depends on older returning adults. Large older fish produce vastly more eggs and milt. 

  • Smelt around 80 mm long or longer are at least two-year-old fish.
  • Longfins greater than 100 mm long are considered to be late in year 3.


Three younger Longfin Smelt (top) above a Northern Anchovy and some shrimp at Pond A21-1

We found only one Longfin at each of stations Alv2, Alv3, Coy2, and Coy3.  They measured respectively 85mm (a female), 55mm, 60mm, and 70mm.  Aside from the larger female, the young of year stragglers will play only a minor role in this current winter’s spawn.


The first signs of staging for spawning were spotted in Pond A21.   Twelve Longfins were netted on the north side of the pond and thirteen more on the east side. 

The Pond A21 Longfins came in all sizes, ranging from 54 to 108 mm.  Nine of them were over 80 mm.  Sex could not be determined even in the largest fish.  We presume that these two groups were mostly female because 1) Spawing-ready males are easy to identify from the long anal fin and adjacent “belly ridge,” and 2) Male Longfins tend to stage farther upstream where they select and defend spawning sites. 


 Upstream Trawls – Sunday, 5 December.

Sunday trawls started in Artesian Slough.  This slough is heavily influenced by the freshwater plume of tertiary-treated wastewater from the San Jose-Santa Clara Regional Wastewater Facility.  And, the slough has been consistently documented as both a Longfin and Northern Anchovy spawning ground for each of the last several years.

Double Good News: 

  1. Twenty Longfins were caught in Artesian Slough.  Five were 80 mm and longer.  This included one male at 112 mm.  He was visibly exuding milt!
  2. Catches of non-native Inland Silversides in Artesian Slough dropped from 277 in November to just 12 in December. Until this month, we were very concerned that the population explosion of these pests last summer would consume all eggs and larvae.  Falling temperatures appear to have hammered both Inland Silversides and Shimofuri Gobies too.


The large male shown above has probably selected his spawning site amongst tule reeds and roots along the banks of Artesian Slough.  Large males like him use their long anal fins to clean ideal spawning sites.  Females generally feed and fatten up downstream as they swell with eggs.

  • No egg checks were conducted on these fish in accordance with sampling protocols. But, we noted sex of the fish and presence of eggs or milt whenever visible.


Some of the large adult Longfins from the north side of Pond A19 (station A19-1).

Additional Longfins were caught at every station farther upstream.

  • 12 Longfins in Dump Slough. This included seven adults over 80 mm.  One of the females at downstream Dmp-2 showed visible eggs!
  • 5 Longfins at UCoy stations. Three were over 80 mm.
  • 77 Longfins in Pond A19. 35 and 42 were caught respectively on each side of the pond. 


More Longfins plus a few Anchovies from the east side of Pond A19 (station A19-4).

The most Smelt were caught in Pond A19.  Some of these were shockingly large:  All but 12 of the 77 were over 80 mm. Thirteen were 100mm or greater! 


Both egg-bearing females and milt-emitting males were observed on both sides of Pond A19!

We continue to study Pond A19 and other nearby spawning hotspots to learn what environmental and biological factors sustain this fish.   Both Ponds A19 and A21 were breached and commenced restoration in March 2006.  These restored ponds are now our most important staging and spawning sites for threatened Longfin Smelt in winter … and Anchovies in summer!


Longfins are spawning here for yet another year! … Rejoice!

Comments are closed.