Scoops of seawater

The Great Barrier Reef World Heritage Area, off the northeast coast of Australia, is “one of the richest and most diverse natural ecosystems on Earth.” Thousands of species of mollusks, more than 1,500 species of fish, and 400 species of coral live within its waters.

There’s something else there, too – herbicides, washed into the ocean via rivers draining agricultural areas.

Herbicides can breakdown in nature (UV rays can degrade the molecules, or microbes can consume them), but it’s unclear how long that process might take under the conditions that occur around the Great Barrier Reef – many of the studies conducted in the past included unrealistic conditions, like unusually low temperatures or levels of herbicides 500 times higher than what researchers typically find there. A group of scientists working in Queensland, Australia, sought to detail the timing of herbicide persistence in seawater from the Great Barrier Reef lagoon kept under more natural conditions; they recently published the results of their study in the journal PLoS ONE.

Working with samples of water scooped up from off the coast of Queensland, the researchers added several different herbicides, then stored the flasks under different combinations of conditions: some in the dark, some under partially lit conditions, some at 77 degrees Fahrenheit (the average temperature of seawater around the Great Barrier Reef over the course of a year), and some at 88 degrees Fahrenheit (the summertime high temperature of seawater near the shore in part of the Great Barrier Reef lagoon). They took samples from the flasks every few weeks for a year to measure the herbicide concentrations, allowing them to estimate how quickly the herbicides broke down.

The estimated half-life of each herbicide – the time it takes for half of the initial amount of the chemical to degrade – ranged from about 150 days to more than 5,000 days. The light and temperature conditions the flasks were stored under didn’t lead to consistent patterns in how the herbicides broke down.

The scientists also added a chemical that stops microbial activity to some of the flasks, and no herbicide degradation occurred under those conditions, suggesting that it was the microbial community present in the seawater collected from the Great Barrier Reef lagoon that broke down the herbicides.

“Chronic exposure of [the Great Barrier Reef] and catchment biota to . . . herbicides (from microbial communities to macrophytes) remains largely unstudied,” the researchers note, “and should be a future focus for research and risk assessment.”

Australia's Great Barrier Reef World Heritage Area stretches over 1,000 miles along the coast of Queensland. 

(Image by Tchami via Flickr: Creative Commons license)

Seal voices

For the first four months of its life, an Antarctic fur seal pup depends on its lactating mother for sustenance. The mother seal spends the majority of her time in the ocean on foraging trips, returning to the land every four to seven days to feed her pup for a few days or less.

During the breeding season, Antarctic fur seals can congregate in dense colonies of over a thousand individuals – and, because mother seals will only nurse their own pups and can be aggressive toward pups that are not their own, a pup’s ability to recognize its mother is crucial.

Scientists know that seals in the family Otariidae, or eared seals (so named because they have external ears), including the Antarctic fur seal, use vocal cues to communicate and recognize one another. Mothers and pups may also use sight and smell to find each other, but auditory clues appear to be their most effective means of reunion. A new paper recently published in the journal PLoS ONE elucidates new details of how an Antarctic fur seal pup recognizes the voice of its mother, and some limits to that auditory recognition.

Researchers working on Courbet Peninsula in the Kerguelen Islands in the southern Indian Ocean studied a colony of 750 pairs of mothers and pups. They recorded the calls of mother seals and played them back to the pups, sometimes with modifications to amplitude and frequency, to determine which acoustic aspects of the calls the pups were using to recognize their mothers. The scientists recorded the number of calls the pups made in response to the recordings, as well as how long it took the pups to respond, and how long it took the pups to look at the loudspeaker standing in for the mother seal.

The researchers also noted that the pups often gathered in groups of about 10 individuals while waiting for their mothers to return to land; they took advantage of these gatherings, and played mother seal calls to them from about 25, 100, and 200 feet away to see how well the pups could discriminate mother seal calls at progressively longer distances.

The pups responded best to mother seal calls that were not modified in amplitude or frequency, suggesting that they use both of those signals in recognizing their mothers’ voice. They also appeared to be better at discriminating mother seal calls at shorter distances – when the scientists played the vocalizations of one of their mothers to the groups of about 10 seal pups, about four pups typically responded from 200 feet away, three from 100 feet away, and just one from 25 feet away. The researchers also note that “[f]or all tested distances, the filial pup of the female chosen for the playback always responded.” In other words, while some pups got it wrong, the pup whose mother they were actually listening to always got it right. 

Antarctic fur seal pups at Salisbury Plain, on South Georgia Island, in the southern Atlantic Ocean.

(Image by Liam Quinn via Flickr/Creative Commons license)

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)

Native or not?

“I think it’s important for people to rethink how we see lakes and the things that live in ’em,” says Dr. Curt Stager, professor of biology at Paul Smith’s College, nestled in the heart of New York’s Adirondack Mountains. Stager says we’re used to thinking of lakes as playgrounds and resources, places where we go to catch fish or cool down on a hot summer day, but we should also recognize that lakes, and the organisms that live in them, might have something to teach us. “They’re marvels of evolution,” Stager says. “They’ve got amazing stories.”

One lake in particular has a story to tell, a story that has been collecting in the sediment at the bottom of the lake for more than 2,000 years. It’s a story that the humans living on the shores of the lake haven’t been able to hear, until now.

Yellow perch have long been considered non-native to the Adirondacks – the range map for the species shows a big blank spot in the northeastern corner of New York State, where the Adirondacks are located. Because of its non-native status, the New York State Department of Environmental Conservation has eradicated yellow perch (and other non-native fish) from several ponds to make way for brook trout, a highly prized native species which has declined in recent decades.

But what if yellow perch aren’t non-native to the lakes and ponds of the Adirondacks?

“I was skeptical that perch were not native to the Adirondacks,” Stager says. “It just doesn’t make sense that they’re in mountainous areas all over eastern North America . . . and then there’s just one little spot here where they’re not supposed to be.”

So Stager and a team of researchers from Paul Smith’s College decided to investigate the claim that yellow perch were introduced to the region, rather than a native species. They recently published the results of their study in the journal PLoS ONE.

As aquatic plants and animals living in a lake go about their lives, shedding cells and scales and waste products, that material falls through the water and builds up on the bottom as sediment – the top layer of sediment contains the most recently shed cells and scales, and deeper layers hold progressively older records of what used to live in the lake.

On a bright and cold winter day, Stager and his team drilled through the snow and ice covering Lower St. Regis Lake, on the edge of Paul Smith’s College’s campus, and used a long, skinny tube to extract a core of sediment, just under four and a half feet tall, from the bottom of the lake. The deepest part of the sediment core they lifted out of the lake was between 2,131 and 2,315 years old – well before the relatively recent era of non-native fish introductions in the Adirondacks.

The researchers took samples from the middle of the core, where the edges of the tube couldn’t have smudged and mixed the sediment, and analyzed them for yellow perch DNA.

Study co-author Dr. Lee Ann Sporn extracting samples from a sediment core in the lab. 

(Image by Dr. Curt Stager)

They found evidence of yellow perch throughout the entire sediment core – meaning that yellow perch have likely been living in Lower St. Regis Lake for over 2,000 years. (They also analyzed samples from cores taken from lakes without any yellow perch, to make sure that they hadn’t contaminated their tools with perch DNA, and those samples all came back negative.)

Based on the evidence contained within its sediment, Lower St. Regis Lake is telling us that yellow perch are native to the Adirondacks after all.

Still, something has been changing in the lakes of the Adirondacks – as brook trout declined, yellow perch became more numerous than they were in the past. In the 1800s and early 1900s, there were so few perch in the Adirondacks that naturalists usually didn’t find them at all during net surveys – which is why they were thought to be non-native in the first place – but as Stager says, “a net survey is not a reliable way to tell if something’s not there. If you catch it, you know you have it, but if you don’t catch it, it doesn’t mean it’s not there.”

So why have yellow perch done so well in recent decades? Stager says there are lots of possible explanations – climate change and warming lake temperatures, overfishing of brook trout, human activity fueling ecosystem production, and probably others, too, all of which created conditions that were detrimental to brook trout but advantageous to perch.

Stager and his team only looked for yellow perch DNA in the core from Lower St. Regis Lake, but they have big plans for the future. Brook trout are cherished partly because there are unique strains of the fish that exist only in the Adirondacks; now that they know that yellow perch have been there for thousands of years, the researchers wonder if they could have evolved unique strains, too. Stager believes he can use the DNA stored in sediment cores to reconstruct the history of entire lake communities.

And he hopes that people will start paying closer attention to the stories that lakes can tell us. “I hope this kind of thing really helps us see there’s a much richer heritage we’ve got here than just something to snag on a hook.”

The researchers drilled through snow and ice to extract a sediment core from Lower St. Regis Lake in the Adirondack Mountains of New York State.

(Image by Dr. Curt Stager)