Fish in the Bay – November 2023, Part 3 – Bat Rays to the Rescue?

We caught 36 Bat Rays in November.  It was unexpected to see so many this late in the year.  With these additional Rays, the 2023 count is 206.  Holy Bat Ray, Batman!  This is a new record Bat Ray year!

Bat Rays arrived late this summer.  The big rainwater flush in January through March lowered salinity Bay salinity.  We presume that discouraged Rays from entering Lower South Bay (LSB).

  • Up until September, only 18 Rays had shown up = a horrible year.
  • The 47 babies caught in September established a decent annual count.
  • Another 105 babies and young adults were caught in October. – A great year.
  • The November catch was a mix of young and big bruiser adults, and a new record year was established.

We counted record numbers of Bat Rays in each of the last four years since 2020.  Each year topped the previous one by a decent margin.


1. Musculista Explosion.

There were fewer Musculistas in November.

Musculista (Asian Date Mussel) count = 341 for November.  2023 is also our record Musculista year – BY FAR!  We have already tallied over 4,000 Musculista as of November.  This is a 10-fold increase over the 2022 total which was the previous record year. 

Riddle me this:  We collected an astounding number of non-native Musculista mussels in August and October 2023.  But, the numbers dropped off rapidly by November and December. 

  • We saw a similar rapid summertime increase, and then fall decrease, in Musculista population in 2022, albeit total numbers were much lower. Where do all those Musculista go?
  • Musculista often exhibit dramatic boom-and-bust cycles. The mussels themselves generally only live for one year.
  • Crooks (1996)
    “Although the maximum life span of M. senhousia is approximately 2 yr, mortality is high and most of the population are only annuals.” 
  • But, other forces could be at work.

Why We Care – Musculista are eco-system engineers. Each bivalve surrounds itself in a bag of byssal threads.  In a dense colony, byssal threads combine to form dense mats over the bottom surface.  Musculista mats convert a soft silty bottom into a more solid structure. This new solidified surface can be beneficial to some organisms and detrimental to others.

  • Crooks (1998)    
    “The primary effect of the mussel and its mats was facilitation of other organisms. Total densities of all macrofaunal individuals as well as species richness were typically higher inside than outside mussel mats. Two species that exhibited large enhancements of densities within mussel mats were the tanaid, Leptochelia dubia, and the gastropod, Barleeia subtenuis. …


2. Bat Rays!

Station Alv3: Bat Ray count = 17:  7 males, 10 females.  All but one were young.

Lower South SF Bay is a Bat Ray nursery.  Pregnant mama Rays come here each summer to give live birth to between 2 to 10 pups.  During the summers, most of the Rays we see are tiny newborns, but we also catch at least a few adults from time to time.    

Sami releasing a baby female Bat Ray at LSB1 on 11 November.

Station LSB1: Bat Ray count = 6:  2 males: 440 & 175 mm.  4 females: 165 to 180 mm.  Our late-summer Bat Ray catches were impressive.


 Big male Bat Ray (440 mm) at LSB1.


Bat Rays at LSB2 – first impression.  Sami and Niko emptying 17 large Bat Rays into the tub.

Station LSB2: Bat Ray count = 12:  These were all big males at station LSB2.  This was interesting.  Do Bat Rays segregate themselves by sex? …  apparently, they do!


Bat Rays at station LSB2 – second impression.

This California State University Long Beach site describes many habits of Round Rays (close relatives of the Bat Ray): 

  • Life History. Round stingrays have an annual reproductive cycle with peaks in mating occurring from May-July.
  • Seasonal Movement Patterns. Females sexually segregate to avoid aggressive interactions during non-mating seasons and to seek warmer coastal waters during gestation (Hoisington and Lowe 2005, Mull et al. 2010, Jirik and Lowe 2012). Males are not found in warmer coastal waters as much as females possibly due to a sex-specific energetic cost from residing in areas of increased temperatures for extended periods (Jirik and Lowe 2012).
  • Fun fact: Stingray spines are like fingernails in that they lack nerves and grow back after a period of time (Lowe et al. 2007).  It takes approximately 3-4 months for stingrays to fully regenerate a replacement spine.


Bat Girl Ray at station Alv1.  This female is the biggest Ray we have seen in LSB.  Her nose-to-tail length was 600 mm.  A big girl Ray would only be so far upstream for one reason: to deliver her pups! 

More info about sexual segregation of Round Rays.

  • Jirik and Lowe (2012)
    “Strong sexual segregation occurred in the restored habitat with mature female U. halleri forming large unisex aggregations in summer, during months of peak seasonal water temperatures, and males only present during spring. … Tagged females typically spent <14 days in the restored habitat, using the habitat less as seasonal water temperatures decreased. Females tended to emigrate from the estuary by mid-August, coinciding with the time of year for parturition. The elevated water temperatures of the restored habitat may confer an energetic cost to male U. halleri, but females (particularly pregnant females) may derive a thermal reproductive benefit by using warm, shallow habitats for short periods of time during months of peak water temperatures.”
  • Brodbeck et al (2023)
    This paper again confirms that female Round Rays spend more time in warmer upstream marsh habitats.


3. Guitarfish.

Shovelnose Guitarfish.   These were the only two Guitarfish netted in 2023.  Both were tiny babies.  Guitarfish, Bat Rays, and Leopard Sharks all follow a very similar pattern:  Pregnant females swim as far upstream as they can tolerate in mid-to-late summer to give live birth in warm brackish sloughs.

  • Fact Animal. “Shovelnose guitarfish usually reproduce once a year, giving birth to up to 30 live young. … birthing grounds are usually well concealed and in calmer waters.”  (Sadly, the shovelnose guitarfish has recently been assessed for The IUCN Red List as near threatened …)  

Guitarfish and Bat Rays could be working together to control the Musculista scourge.

More studies that document sexual segregation among Bat Rays, Guitarfish, and Leopard Sharks in coastal nurseries:


4. Other Interesting Oddball Fishes in November.

Pacific Tomcod.  4 young Tomcods were caught.  We do not know why young Tomcods started showing up in LSB a few years ago.  Since then, we have regularly seen a few young ones per month in winter.

Common Carp.  This was our 11th Common Carp since August.  We had never seen them in LSB trawls before.  Most of them had damaged tail fins.  We suspect the brackish Bay water habitat may be stressful for them.


5. Snails – native and non-native.

San Francisco Bay has a long history of non-native mollusk invasions, aka ‘mollusk wars.’  A combination of ship ballast water and importation of oysters from the East Coast and Asia over the last 150 years was responsible for bringing many of these non-native species.  The Eastern Mud Snail story is arguably the most extensive invasion.

Eastern Mud Snail (upper left) and California Horn Snail (lower right) at Alv1 on 11 November.

California Horn Snail.  This Horn Snail looked alive!  We occasionally find long-dead shells, but never a live one.  

  • However, I could not be certain without probing or crushing the animal. I took photos and released it.  I should have observed more closely, but there were Staghorns, Longfins, and a big Bat-Girl Ray to deal with.  … Bigger fish to fry, so to speak.
  • With possible exception of small remnant populations, California Horn Snails are effectively extirpated from Lower South Bay and almost all of San Francisco Bay.


Last Refuge of Horn Snails in Lower South Bay?  There is at least one upper mudflat near Palo Alto where Horn Snails still roam free.  (There could be others, but this is the only one I know about.)  Luca and I visited it in August 2022. It was strange to see so many turreted shells crawling over the mud.

Horn Snails used to dominate the Bay’s muddy banks. They were the primary consumers of benthic diatom. They still remain in Bays and inlets farther south in California and Baja, but here, they have been entirely replaced by Eastern Mud Snails over the last 100 years or so.

Fun Fact:  Horn Snails host a lot of trematode parasites.  People who study parasites love Horn Snails.


Eastern Mud Snail – archenemy of the Horn Snail.  The Mud Snail invasion story is well known.  Even so, it is shocking to see the extent of the displacement.  The photos above from December are a typical sight during a neap and ebbing tide: Millions of Mud Snails munching diatom slime at the waterline.   

  • Mud Snails blanket the mud banks whereas Horn Snails are never seen anymore.
  • Global Invasive Species Database – Ilyanassa obsoleta “The eastern mud snail (Ilyanassa obsoleta) introduction to the Pacific Coast of North America has caused a change in the native fauna. In San Francisco Bay the once dominant California horn snail (Cerithidea californica) has been reduced to small populations where habitats overlap. Where salinity is higher populations of California horn snails are able to survive (Race, 1982).”
  • Nemesis, Marine Invasions Lab – Ilyanassa obsoleta “It was first collected in San Francisco Bay in 1907, where it was introduced with Eastern Oysters (Dall 1907, cited by Carlton 1979) and is now widespread and abundant (Cohen and Carlton 1995; Cohen et al. 2005).  … Ilyanassa obsoleta is an omnivore. Young snails in aquaria graze on algae growing on glass and probably feed by this method in the wild. As they grow, they switch to ingesting large quantities of sediment, together with organic matter and benthic diatoms. They also feed on decaying algae, such as Ulva, and are strongly attracted to carrion, such as dead fish and mollusks. … This snail is eaten by fishes, crabs, and birds. It is also a host to many species of parasites.” 
  • An even newer snail invader arrived some decades ago – “the Japanese False Cerith.”  We have not yet seen this one in LSB.

But, at least for now – Musculistas are on the run!
Will Bat Rays and Guitarfish foil the current Musculista invasion? 

Stay tuned – “Same Bat-time, Same Bat-channel.”

  • Batman Theme, 1966-1968: 
  • The “Batusi” dance was actually a thing in the 1960s: 
  • Adam West doing the Batusi: 
  • Fun Fact: In 2016, television critics ranked Batman as the 82nd greatest American television series of all time.

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