Shark in amber

  

Gordon wrote this afternoon to let me know he and a few other folks from Madrone Audubon had come across a small shark at Doran Beach this morning (9 September 2020).  He took a couple of photos and kindly agreed to let me share them with you.

Not only do these photos document an interesting shark stranding, but they also show the unusual amber light in Bodega Bay and elsewhere today caused by the intense smoke from so many large wildfires.

Salmon Sharks (Lamna ditropis) look very similar to White Sharks (Carcharodon carcharias), but they have a shorter snout, the white coloration on the underside extends above the pectoral fin (just behind the gills), and there is a double-keel at the base of the tail fin.  I can't quite see that keel in these photos, but the other characteristics look right for Salmon Shark.

During the late summer and fall, we often receive a few reports of stranded juvenile Salmon Sharks.  Sadly, not too long ago it was discovered that these stranded sharks often have a bacterial brain infection that causes meningoencephalitis (which may result in disorientation).

Here's another photo illustrating the size of the shark, with a pair of footprints for scale.  It appears that the shark was only ~4 feet long or so.

 

 

Many thanks to Gordon for sharing this interesting observation and his photos!

"Sharktober"

I've heard of a few Great White Shark (Carcharodon carcharias) attacks on marine mammals during the last few weeks — off Half Moon Bay, between Bodega Head and Bodega Rock, from the Farallon Islands, and next to the Alcatraz Island pier.

Yesterday I encountered the probable remains of a white shark attack washed up on a local beach.  It was half of a sea lion.  For those interested, I'll show a picture below.  (If you'd rather not see a half-eaten sea lion, then read no further.)

First I'll show the tooth of a Great White Shark that I found in 2005.  I posted this image in 2012.  I'll admit, when I found this tooth 10 years ago, I had no idea exactly how rare it was to find a white shark tooth.  I was new to California, but knew white sharks occurred in this area, so I naively thought, "Nice!  A shark tooth!"  And now I'd say, "WOW!  A SHARK TOOTH!"  I'll show the sea lion picture after the tooth.

 

Okay, here's the sea lion — likely cut in half by a shark:

 

When I first noticed this animal from a distance, I was viewing it head-on and observed the silvery color.  I thought it might have been a harbor seal.  As I approached it, I noticed the long flipper and realized it was a sea lion.  I hadn't known sea lions could look so silvery (I generally think of them as quite brown), but I read that the fur of younger animals can be this color after a molt.

Relatively recent research on white shark movements in this area has revealed that sharks spend time near the Central California coast in late summer/fall (primarily September-November), then migrate 2000-5000 km offshore (primarily from April-July).

If you'd like to learn more about these impressive migrations, you can read more here:

Philopatry and migration of Pacific White Sharks (Jorgensen et al. 2009)

The Leopard vs. the Lion

Who knew that one of the most spectacular local wildlife photos of 2015 would appear so early in the year?

Jeff and Alice were observing and photographing wildlife along the shoreline of Bodega Harbor on 3 January.  They were in the right place at the right time, and Jeff had the skills to capture this amazing image: 

 

That's an adult male California Sea Lion (Zalophus californianus) catching a Leopard Shark (Triakus semifasciatus).

 

The lighting really highlights the Leopard Shark's beautiful patterning.  And you can tell from the splashing and commotion in the water that this was a dramatic moment.  Alice mentioned that there was quite a bit of thrashing and that the sea lion would sometimes toss the shark into the air!

Here's another image, showing the middle and tail end of the shark (below).  The shark's species name, "semifasciata," means "half-banded," and I think you can see why in this picture:

 

The next image is probably the best at helping you visualize the thrashing that was going on.  I wasn't sure how often sea lions ate Leopard Sharks, so I did a quick Internet search.  Several people have commented (and posted pictures or video) about seeing similar behavior in San Francisco Bay and in San Diego.  However, Jeff's pictures are much better.  Way to go, Jeff, for representing Bodega Bay and documenting a fascinating predator-prey interaction!

 

Does seeing these pictures make you wonder what happens next?  How do the sea lions consume the sharks?  Do they tear smaller pieces from them?  And where do sharks rank within the diet of a sea lion?  Are sharks preferred, or do sea lions just capture them when other prey aren't available?  Can sharks use submerged vegetation (e.g., eelgrass beds) to hide from sea lions?  Can they sense a sea lion coming?  Does the wonderful patterning of a Leopard Shark sometimes help to camouflage it from a sea lion?

P.S.  Many thanks to Jeff and Alice for sharing this story and images.

Smooth-hound?

Yesterday, 24 November 2013, County Park staff flagged me down to report a shark that had washed ashore at Doran Beach.  Although no longer alive, it was still in pretty good condition, so here's a picture:

 

The shark was ~5 feet long.  The next image shows a better side profile:

 

Note the relatively long snout and large eyes:

 

And the interesting shape of the tail fin:

 

Park staff had wondered whether this might be a Brown Smooth-hound Shark (Mustelus henlei).  Some of the characteristics seemed like a good match for that species, but there are a few others that I'm not quite sure about.  

For example, the maximum size listed for Brown Smooth-hounds is ~3.2 feet, so this shark appears to be larger.  And I don't know how consistent tail shape is in this group of sharks, but when I compared pictures of Brown Smooth-hound tails online, this individual's tail was different enough for me to hesitate.

So now I'm going to have to ask for help.  Are there any shark experts out there who can assist with this identification?  I'll circulate the photos and will report back if anyone offers additional ideas.

At least two species of smooth-hound sharks (Gray and Brown) are known to occur along the West Coast from northern California down to the Gulf of California.  Brown Smooth-hound Sharks are apparently common in shallow water bays such as Humboldt, Tomales and San Francisco bays.  Gray Smooth-hound Sharks (Mustelus californicus) occur in bays and along rocky shores.  For the record, Gray Smooth-hounds do get larger than Browns (up to at least 5.3 feet).

There is more information about both of these sharks on this California Department of Fish and Wildlife web page.

ADDENDUM (6 December 2013): Sorry it's taken me so long, but I need to update the identification of this shark!  Several experts have confirmed this as a Soupfin Shark (Galeorhinus galeus).  I had started to wonder about this when I checked Bay Fishes of Northern California (Bane and Bane 1971) because they mentioned that small Soupfins could look similar to smooth-hounds...and several of the characters looked right.  So a Soupfin Shark it is!

Ridges and grooves

Here's a fun set of mystery close-ups for you.

 

All of these images were taken under a microscope.  (There are some sand grains in the pictures, too.  For example, the amber-colored grains above.)

 

All of the pictures are from the same animal, just from different parts of the body.

 

Here's one more close-up, and then the next picture following this one will give the answer away, so consider yourself warned!

 

Okay, here we go.  Are you ready?  Do you have a guess?

 

All of the close-ups were the scales of a shark, probably a juvenile Salmon Shark (Lamna ditropis).  You can see some of the scales directly below the teeth in the image above.  I just found out about this shark at the end of the day, so I'm not sure I heard all of the details right...but it sounds like Matt and the Coastal Oceanography class discovered it on Salmon Creek Beach.

When I looked at the shark's skin with my hand lens I was so impressed with the scales that I couldn't resist taking a few pictures under the microscope to share with you (the shark was very small).  Note that some of the scales have very obvious longitudinal ridges.  It's thought that these ridges and grooves (running parallel to the water flow) reduce drag and thereby improve swimming performance.

This shark was in poor condition — it had probably been on the beach for a while and was partly scavenged and severely dried (next image).  But I've heard about a few other recent Salmon Shark strandings, so it's an opportunity to provide a little information about these events.

 

 [For scale, this shark was only ~2 feet long.]

For at least 20 years, juvenile Salmon Sharks have stranded on California and Oregon beaches in the late summer and early fall.  Until recently, the cause was unknown.  But last year, several researchers published a paper in which they discovered that stranded sharks had bacterial brain infections.  (It still isn't clear how or where the sharks acquire the bacteria.)

Schaffer, P.A., B. Lifland, S. Van Sommeran, D.R. Casper, and C.R. Davis. 2013. Meningoencephalitis associated with Carnobacterium maltaromaticum-like bacteria in stranded juvenile Salmon Sharks (Lamna ditropis). Veterinary Pathology 50: 412–417.

If you'd like to learn more general information about Salmon Sharks, check out the web pages provided by the Pelagic Shark Research Foundation and the Florida Museum of Natural History. 

Toothy

You're not going to believe this.  I found what I thought was a dead shark on the beach.  I turned it over to get a closer look at the teeth, and it tried to bite me!

No, just kidding — Happy April Fools' Day!

But I did encounter the head of a shark washed up on the beach, most likely a Leopard Shark (Triakis semifasciata).  Here are photos from above and below (taken on 30 March 2012).  Note the black bands and spots above, and the large nares (nasal openings) in front of the mouth.

 

 

Leopard Sharks are locally common in Bodega Harbor and Tomales Bay.  They feed on benthic invertebrates, e.g., crabs, shrimp, clams, and innkeeper worms.  As adults, they measure about 4-5 feet long.  They give birth to live young (~8 inches long) during the spring/summer season (primarily in April and May), often in eelgrass beds.

I've read that Leopard Sharks are sometimes eaten by Great White Sharks (Carcharodon carcharias).  Great Whites are known to spend time in the Bodega Bay region.  Here's a tooth from a Great White that I found on a local beach in 2005.  It's worn like sea glass, so the serrations along the edges aren't as visible in this scan (although you can still feel them on the tooth).

The genus, Carcharodon, means sharp or jagged tooth.  White sharks are impressive apex predators.  They can reach 20 feet in length and may weigh over 5,000 lbs.  Not kidding this time!