PROBLEMS WITH PLANKTON

 

When people consider the things that can threaten our fisheries, they typically think of the obvious problems, like overfishing, predation, habitat loss, or a decline in forage.

Sometimes, they also consider the sort of things that don’t necessarily lead to population declines, but can make fishing harder or move fish elsewhere, like warming waters.

But more and more, as scientists look at the problems besetting various fisheries, they realize that the issues often lie at the very base of things, that is, with the tiny plants and animals, the plankton that lie at the very bottom of the food web, and hold the whole thing together.

And it seems that plankton has been becoming more and more problematic in the last decade or so.

A recent article in the New York Times noted that the distribution of phytoplankton, the single-celled plants that convert sunlight into the makings of life, has changed in recent years.  Satellite surveys of plankton abundance have found that there is less phytoplankton in the open ocean than there used to be, but more close to shore, particularly in more northerly regions, where a warming ocean seems to be creating conditions that favor plankton growth.

The increase in inshore phytoplankton has certainly been apparent in my local waters south of Long Island, New York, particularly to those of us who fish for tuna, shark, and other pelagic fishes.  Fishing for such big fish of the open sea is often called “blue water fishing,” because once one gets away from the shoreline—a distance that, depending on place and time, might be anywhere from a mile to 30 miles or so off the beach—the greenish, nutrient-laden inshore waters give way to blue water so clear that we could sometimes see the outlines of a hooked fish when it was still 50 or 60 feet beneath the boat.

I recall drifting near the Yankee wreck, in 20 fathoms of water, on an early September day, and watching a mako come in to a bait that was drifting at least 40 feet under the surface, and seeing it all as clearly as I might have watched someone swim beneath the surface of a backyard swimming pool.  Although green water sometimes extended farther from shore, it was unusual not to have blue water by the time a boat crossed the 30-fathom line.

Today, that’s no longer the case.  Often, these days, when I fish for sharks in 20 or 25 fathoms south of Long Island, I’ll have an angler fight a fish up to the boat, and we’re still not able to see it when my gloved hands get their first grip on the 15-foot wire leader that’s attached to the hook.  Last July, I was 40 miles from the inlet and still fishing in murk, finding bluefish instead of the bluefin tuna that I was seeking.

The green, plankton-filled water has certainly had a negative impact on fishing for billfish and the various tunas, fish which, as sight feeders, generally need clear blue water to locate and run down their prey.  Although there are still quite a few tuna around, we’re running ever farther, trying to find that blue water that holds them.

But in those cases, phytoplankton are merely an inconvenience that makes fish harder to find.  In other cases, they become a real risk to the health of both people and fish stocks.  Off the southwest Florida coast, so-called “red tides—blooms of harmful phytoplankton—are a regular enough occurrence that the state maintains a web page advising people of “Red Tide Current Status,” which advises viewers of where fish kills take place, and also notes that

“Respiratory irritation suspected to be related to red tide was reported over the past week in Southwest Florida (Pinellas, Manatee and Sarasota counties,”

which is probably not the news that tourists want to see as they plan their winter vacations.  Still, the Visit Florida website dutifully informs potential visitors that

“A red tide is a higher-than-normal concentration of a microscopic alga (plant-like organism).  In Florida, the organism that causes most red tides is Karenia brevis (K. brevis).  This organism produces a toxin that can affect the central nervous system of fish.  At high concentrations (called a bloom), the organisms may discolor the water a red or brown hue.  The water can even remain its normal color during a bloom…

“Some people may experience respiratory irritation (coughing, sneezing, and tearing) when the red tide organism is present along a coast and winds blow aerosolized toxins ashore…

“If you experience respiratory irritation, wear a mask, such as a painter’s mask, that covers the nose and mouth to filter out marine aerosol particles that carry the red tide toxins…Always seek medical care if your symptoms worsen.  For your home or motel room, keep your windows closed, the A/C on and check/change the unit’s filter.”

That might not sound like an enjoyable vacation, but things are even worse for the fish, which live in the region year-round without the benefits of windows or air conditioning that might combat at least a fraction of the red tide toxins.  And things aren’t likely to get better at any time soon.  A 2022 article in Inside Climate News noted that

“A task force organized by Florida Gov. Ron DeSantis to address the state’s algal bloom crisis concluded in a recent report that ‘without hard work and careful planning’ adverse human health impacts and widespread wildlife mortality would most likely ‘worsen’ because of climate change and the state’s growing population.”

While phytoplankton can have undesirable impacts when it becomes too abundant, zooplankton can have negative impacts on wildlife when it becomes too scarce, or when its abundance shifts to a time when it is no longer available to the creatures that need it.

I first learned of that problem a decade ago, when a paper titled “Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery” appeared in the journal Science.  The thrust of the paper was that as waters warmed in the Gulf of Maine, the number of cod reaching maturity declined.  The researchers raised the possibility that the increasing water temperatures have led to a decline in the abundance of zooplankton that larval cod depend on for food.

One important element of the missing Gulf of Maine zooplankton seems to be a copepod—a type of tiny crustacean—known as Calanus finmarchicus.  The New York Times reported that

“Calanus hibernate through winter, hiding from predators in the dim light of deeper waters…

“Dr. [David] Fields [a zooplankton ecologist] calls the layer of sleeping Calanus the ocean’s fat layer, a valuable resource for other life.  ‘That’s the whole reason the Gulf of Maine runs the way it does, because of that beautiful fat layer,’ he said.”

The Times also noted that

“specimens [of Calanus finmarchicus] had big oil sacs, full of the calorie-rich lipids that fish and right whales seek out.  In experimental studies, Dr. Fields and his colleagues have found that as the temperature rises, Calanus get smaller and have less fat relative to their body size.”

The decline of Calanus copepods in the Gulf of Maine may also be having a negative impact on the Gulf of Maine stock of American lobster.  While adult lobster remain abundant in the region, since 2012, biologists have noted a marked decline in the numbers of younger individuals.  In particular, they have been noting a dearth of lobster at the stage where they cease being free-floating larvae and settle on the ocean floor.

Some biologists suspect that a shortage of Calanus finmarchicus is again at the root of the problem. 

Historically, Gulf of Maine lobster began hatching in early June, with the peak of the hatch occurring a month or so later.  A new generation of Calanus finmarchicus appeared in April and were abundant into early fall.  The timing was perfect for the young-of-the-year lobster, which could feed on the copepods until the lobsters were ready to settle to the bottom of the sea.  

But warming waters have changed the timing of both events.  Now, the lobster are hatching earlier, while the copepods are spawning at the same time, but the copepod population’s annual decline begins much sooner, so that, but the time the young lobsters need them, there are about 70 percent fewer copepods available than there were three decades ago.  Lacking their once-reliable food source, fewer larval lobsters are surviving to settle on the bottom, a result that can only lead to fewer adult lobster in the not-too-distant future.

And such problems aren’t limited to the Gulf of Maine. 

In Maryland, striped bass have experienced their sixth consecutive year of poor reproduction.  While biologists have, for a while, associated poor striped bass spawns with unfavorable environmental conditions, the availability of food, in the form of zooplankton, is now believed to play an important role. 

The Maryland Department of Natural Resources has noted that

“One theory for low spawning success is known as the ‘mismatch hypothesis.’ This suggests that the food source—blooms of cold-water zooplankton—are not matching up with the first time larval striped bass need to eat, as winter temperatures in the Chesapeake increase.  If the zooplankton blooms don’t align with the first-feeding larvae, feeding success is too low for good survival.”

To make things a little more complicated, it’s not clear that any sort of zooplankton will do.  A biologist who has decades of experience working with Maryland striped bass recently stated that one of his former colleagues believed that, of all the zooplankton present in the Chesapeake Bay, the juvenile striped bass focused their feeding on only two or three species.  It’s not clear that the fish would shift their focus if the preferred species were not available.

So yes, to perpetuate a cliché, small things can sometimes make a big difference.  And the presence or absence of plankton—among the smallest forms of life in the sea—can have a very big impact on the health of marine resources all along the coast.