Sturgeon spawning

At seven and a half feet long, the fish took up most of the circular 12-foot tank she was slowly circumnavigating, the rows of thorny scales lining her back and her pointed snout giving her the appearance of a dinosaur. She was an Atlantic sturgeon, caught in the Chesapeake Bay at the mouth of the Choptank River in the spring of 2007, and the lab where I worked at the time, the University of Maryland’s Horn Point Laboratory, was buzzing with the news of her arrival.

Atlantic sturgeon, a ‘prehistoric’ species more than 120 million years old that can grow up to 14 feet long, were fished down to a fraction of their former population in the 1800s and 1900s, primarily because of the profits to be made by selling their eggs as caviar. Since 1998 there has been a moratorium on harvesting the fish on the U.S. Atlantic coast, but fish that are caught accidentally in the Chesapeake Bay can be turned in for a reward (pdf); these days, they are generally tagged and released back into the water where they were caught.

The fish that I saw at Horn Point was there because she was a mature female, full of eggs – the lab is involved in sturgeon restoration efforts, and planned to fertilize her eggs and, eventually, release her progeny back into the Chesapeake Bay.

In order for restoration efforts to succeed, it’s necessary for scientists to learn as much as they can about how Atlantic sturgeon spawn and reproduce in the wild – and new research recently published in the journal PLoS ONE suggests that the timing of sturgeon spawning might be more variable than previously thought.

Atlantic sturgeon are anadromous, like salmon – they are born in freshwater, migrate to estuaries or the ocean to grow, then return to the streams where they were born to spawn. (Unlike some species of salmon, sturgeon typically make several spawning trips during their lifetime.) Previous research documented Atlantic sturgeon returning to freshwater in the spring and summer.

A team of scientists monitored Atlantic sturgeon in the James River, which empties into the Chesapeake Bay, during the spring and fall between 2008 and 2014. They documented four adult sturgeon during the spring, and 369 adults during the fall sampling trips.

The scientists implanted tags into some of the fish, which allowed them to follow their movements throughout the river. They identified two predominant patterns: the one fish that they were able to tag in the spring swam upstream – presumably to the spawning grounds – in May, then quickly left the river. The fish tagged in the fall typically swam into the lower river in June for an ‘extended staging period,’ then swam upstream in September and October, apparently to spawn, suggesting that the Atlantic sturgeon in the James River have two spawning runs, one in the spring and one in the fall.

The scientists note that further study of the timing and location of the two spawning groups “is required to develop informed sampling and tagging protocols to better estimate population size,” a number that sturgeon researchers and managers are very interested in getting right. The discovery of a previously overlooked fall spawning run is also important for fish conservation; “i.e., dredging moratoria in the spring alone cannot be effective when most of the population is in the spawning reaches in the late summer and fall.”

The Atlantic sturgeon I saw at Horn Point in 2007 was part of a much larger conservation and restoration effort, one that continuing research on the timing of sturgeon spawning can’t help but improve.

As a species, Atlantic sturgeon were swimming under the waves when dinosaurs walked the Earth, more than 120 million years ago. 

(Image by Mauro Orlando via Flickr/Creative Commons license)

Peeps and chirps

One of my favorite parts of summer is sleeping with the windows wide open – and, if there’s a pond or a marshy spot nearby, drifting off to the peeps and chirps of frogs calling to one another.

The global chorus of frogs, however, is getting quieter – amphibian populations are declining around the world due to many factors, including habitat loss, climate change, and disease. The deadliest disease amphibians face is chytridiomycosis, an infection caused by a type of fungus called Batrachochytrium dendrobatidis, or Bd for short.

Bd infections can devastate amphibian populations and even drive them to extinction, though some locations and species seem to be resistant to the fungus. Scientists believe that environmental differences in temperature, altitude, and moisture influence the ability of some amphibians to survive infection, or avoid it altogether. New research reported in the journal Freshwater Biology suggests that the presence of predators might play a role as well.

Scientists exposed a group of wood frog tadpoles to ‘predator cues’ – in other words, they added the excretions of predacious beetle larvae that had been fed a diet of tadpoles to the tanks containing the experimental tadpoles, signaling to the tadpoles the presence of a predator. Another group of tadpoles were kept under similar conditions, but not exposed to predator cues. The researchers also added Bd, the fungus responsible for chytridiomycosis, to some of the tanks.

The two groups of tadpoles exposed to Bd – those that experienced predator cues and those that didn’t – had equal rates of Bd infection; about thirty percent of the tadpoles were infected with Bd. The infected tadpoles in the predator-present group, however, had less than half as many fungal spores in their bodies as the tadpoles that weren’t exposed to predator cues.

The scientists suspect that the stress-inducing predator cues primed the tadpoles’ immune systems, allowing them to lower their Bd infection loads. “This is an important result,” the researchers note, “because virulence is often associated with Bd pathogen load.” In other words, lower infection loads could translate to higher survival rates.

Amphibian decline is a complex, worldwide problem, but studies on chytridiomycosis and the fungus that causes it move us closer to a solution – and a world in which nocturnal frog choirs continue to sing.

An adult wood frog showing off inflated vocal sacs.

(Image by Dave Huth via Flickr/Creative Commons license)

Salmon eggs

Over 100 years ago, in 1910, workers began construction on the first of two hydroelectric dams that would eventually be built on the Elwha River, on Washington State’s Olympic Peninsula. Before the dams were built (the lowest just five miles from the river’s outlet on the Strait of Juan de Fuca), the Elwha was home to robust populations of several species of Pacific salmon. After the dams were built, most of the river was cut off from the ocean – salmon could no longer migrate back to freshwater to spawn, reproduce, and nourish the streams where they were born.

Last August, three years after the largest dam-removal project ever conducted in the U.S. began, the last section of the uppermost dam was demolished, and just weeks later, salmon were back in the upper Elwha.

Scientists anticipate that salmon will continue to follow their migrations upstream and recolonize the upper Elwha River; as they do so, some will encounter a mysterious population of fish living in Lake Sutherland, a small lake connected to the Elwha River by a creek that comes in above the location where the lower dam used to be.

Those mysterious fish are Oncorhynchus nerka, also known as sockeye salmon or kokanee. Sockeye salmon and kokanee are distinct populations of the same species that tend to either migrate to the ocean and return to freshwater streams to spawn, a trait scientists call ‘anadromy’ (sockeye salmon), or spend their entire lives in freshwater (kokanee). The population of Oncorhynchus nerka in Lake Sutherland was landlocked by the Elwha Dam for a hundred years, but researchers were not sure whether their ancestors were sockeye salmon trapped above the dam when it was built, or kokanee that might never have migrated to the ocean and back at all.

Now, a team of scientists believes they have the answer, and they came to their conclusion based on a humble clue – the size of the eggs the Lake Sutherland Oncorhynchus nerka produce.

As the researchers recently reported in the journal Ecological Research, they compared the Lake Sutherland fish eggs to eggs produced by several populations of sockeye salmon and kokanee from Alaska, Washington, British Columbia, and New Zealand. Kokanee eggs tend to be smaller than those of sockeye salmon (just as kokanee adults tend to be smaller than sockeye salmon adults). The Lake Sutherland fish themselves were typically about a foot long, the same size as the adults of the kokanee populations and half as big as the sockeye salmon adults.

Their eggs, however, were much larger than the kokanee eggs – and well within the range of the sockeye salmon eggs. The growth of the adult fish in Lake Sutherland appears to have been limited by their inability to access the ocean, and, based on the size of their eggs, it’s likely that the Oncorhynchus nerka in Lake Sutherland are descendants of sockeye salmon.

The scientists note that this has “immediate relevance to the restoration” of salmon in the Elwha River, because “it would mean that other traits linked to anadromy might also remain in the population, facilitating the resumption of anadromy in this population.”

Now that the dams on the Elwha River have come down, several species of salmon will once again be able to migrate upstream to spawn – and some Oncorhynchus nerka may finally be able to make it downstream to the ocean, before returning to freshwater to start the cycle anew.

The Elwha Dam in October 2011, about a month after the removal project began. 

(Image by Sam Beebe via Flickr/Creative Commons license)

Turtle footage

In several of the video clips, a loggerhead sea turtle, shell studded with barnacles and matted with an undulating crop of green algae, is flanked by an entourage of fish, apparently eating the bounty of food growing on the turtle’s shell or perhaps using the large turtles for cover (adult loggerhead sea turtles are typically about three feet long).

In another clip, a turtle, evidently in response to the shadowy presence of a shark, flips over so its shell is facing the threat and swims away.

These and other natural loggerhead sea turtle behaviors were captured on video by a remotely operated vehicle, or ROV, deployed off the coast of New Jersey, Delaware, and Maryland, by a group of researchers from the Coonamessett Farm Foundation and the Woods Hole Laboratory of the Northeast Fisheries Science Center, both based in Massachusetts. They recently published their findings in the Journal of Experimental Marine Biology and Ecology – and, as the scientists note in their paper, the “study represents the first example of an ROV for tracking sea turtles.”

The scientists spotted the turtles from a boat, focusing on areas where loggerhead sea turtles have been active in the past; when they found one, they launched their ROV, tethered to the boat, and followed the turtle wherever it went. Over their 10 research trips (during which they recorded footage of 70 turtles), the researchers found that they could maneuver the ROV to within about three to five yards of the turtle without disturbing it.

From that vantage point, the scientists were able to observe a number of apparently natural behaviors which would have been difficult to capture by other means, such as human divers, cameras attached to turtle shells, and tracking tags implanted into turtles, all of which have been used to study sea turtles in the past. “[T]he ROV add[s] a new technique that complements existing technologies while overcoming several of the limitations,” the researchers write.

ROVs appear to be a new and useful tool for scientists attempting to understand how loggerhead sea turtles behave in their natural environment, and how they interact with each other and other animals.

Loggerhead sea turtles are typically about three feet long and weigh about 250 pounds.

(Image by Richard Ling via Flickr/Creative Commons license)