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Tuna are spawning in marine protected areas

Marine protected areas are large swaths of coastal seas or open ocean that are protected by governments from activities such as commercial fishing and mining. Such marine sanctuaries have had rehabilitating effects on at-risk species living within their borders. But it’s been less clear how they benefit highly migratory species such as tuna.

Now researchers at MIT and the Woods Hole Oceanographic Institution have found evidence that tuna are spawning in the Phoenix Islands Protected Area (PIPA), one of the largest marine protected areas in the world, covering an area of the central Pacific as large as Argentina.

The researchers observed multiple species of tuna larvae throughout this protected expanse, suggesting that several migratory species are using these protected waters as a reproductive stopover, over several consecutive years, and even during a particularly strong El Niño season, where PIPA may have provided a critical refuge.

The results, published this week in the journal Scientific Reports, suggest that marine protected areas may be ocean oases for migratory fish, with plentiful nutrients and clean, clear waters that encourage tuna and other migratory species to linger, and spawn often. The study supports the notion that marine protected areas can provide protection to adult fish during spawning, and in this way, help to bolster fish populations — particularly those that, outside protected areas, are in danger of overfishing.

“We have proven that tuna are spawning in this protected area, and that it’s worth protecting,” says Christina Hernández, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences. “There are various types of protection for marine areas around the world, and all those measures allow us to preserve populations better, and in some cases protect highly migratory species.”

Sea change in conservation

The Phoenix Islands Protected Area is part of the territorial waters of the Republic of Kiribati (pronounced Keer-ee-bahs), a sovereign state in Micronesia made up of three island chains in the central Pacific. The islands, if stitched together, would amount to no more than the land area of Cape Cod. However, Kiribati’s ocean territory is vast, extending 200 nautical miles from each of its 32 atolls. The people of Kiribati rely heavily on revenue from tuna licenses that they mete out to commercial fishers. In 2008, however, the republic designated 11 percent of its waters as a mixed-use marine protected area, with limited fishing. Officials ultimately banned all fishing activities in the region starting in 2015, in a conservation effort that — among other things — protected many endangered species, such as giant clams and coconut crab, along with birds, mammals, and sea turtles living within its boundaries.

While fishing vessels have respected the protected territory, keeping their activities outside PIPA’s boundaries, legal fishing efforts surrounding PIPA caused the researchers to wonder whether PIPA might eventually provide an economic gain in the form of “spillover effects.” In other words, if an ecological region is preserved over long periods of time, it might produce more fish that, once full-grown, might cross the territory’s boundaries, benefiting both Kiribati and the regional fishing community.

Hernández’ colleague, Randi Rotjan of Boston University, had been working with the Republic of Kiribati on ways to scientifically monitor PIPA, and wanted to assess whether the protected region might also serve as protected spawning grounds for migratory tuna.

In 2014, the team began yearly expeditions to the central Pacific, to sample within PIPA for tuna larvae, fish younger than 4 weeks old, that would suggest recent spawning activity in the region. The researchers embarked on a 140-foot-long student sailing vessel, owned and operated by Sea Education Association, which also collaborated on this study. Sailing from Hawaii, the ship reached the edges of PIPA after about a 10-day journey. Once within the protected area, the team began sampling the waters for tiny fish, using three different nets, each designed to collect at 100 meters, 50 meters, and skimming the surface.

The team pulled up nets teeming with ocean plankton, including tuna larvae, along with tiny crustaceans, jellyfish, pelagic worms, and anchovies, all of which they preserved and transported back to Massachusetts, where they carried out analyses to extract and identify the number and type of tuna larvae amid the rest of the catch.

From 2015 to 2017, the three years included in the current paper, the researchers analyzed samples from over 175 net tows, and identified more than 600 tuna larvae, covering a distance within PIPA of more than 650 nautical miles, or 1,200 kilometers. Compared with a handful of previous studies on tuna larvae populations, Hernández says the number and density of larvae they found is “pretty on track for what we expect for this part of the Pacific.”

“Larval populations can’t really control how they move, and they get mixed around by ocean currents and dispersed away from each other,” Hernández explains. “As they continue to grow, they start to school and are in denser aggregations. But as larvae, they live at low densities.”

The tuna larvae appeared in about similar abundances over all three years, and even in 2015, when a strong El Niño season dramatically altered ocean conditions.

“That’s something that’s relatively good news, that the protected area seems to be pretty good habitat across environmental conditions,” Hernández says.

The team identified tuna larvae in their samples as species of skipjack, big-eye, and yellowfin.

“These particular fish are not so picky about where they spawn, and they can spawn every two to three days, for a couple of months,” Hernández says. “If they’re thinking the food is pretty good in PIPA, they may stay inside its boundaries for a few weeks, and might have additional spawning events that they wouldn’t have if they were outside the protected area, where they could get caught before they spawn.”

The results are the first evidence that highly migratory species spawn in marine protected areas. But whether such regions encourage species to reproduce more than in other, unprotected waters will require studies over a longer period of time.

“We have to protect these areas long enough to figure out if they are causing an increase in tuna populations,” Hernández says. “The amount of information we have about the Pacific tuna is paltry. And it’s critically important that we study the early life stages of fishes, and that we monitor protected areas, and populations of tuna, as the ocean changes.”

This work was supported in part by the PIPA Trust, Sea Education Association, the Prince Albert of Monaco Foundation II, New England Aquarium, and Boston University.

Hydration sensor could improve dialysis

For patients with kidney failure who need dialysis, removing fluid at the correct rate and stopping at the right time is critical. This typically requires guessing how much water to remove and carefully monitoring the patient for sudden drops in blood pressure.

Currently there is no reliable, easy way to measure hydration levels in these patients, who number around half a million in the United States. However, researchers from MIT and Massachusetts General Hospital have now developed a portable sensor that can accurately measure patients’ hydration levels using a technique known as nuclear magnetic resonance (NMR) relaxometry.

Such a device could be useful for not only dialysis patients but also people with congestive heart failure, as well as athletes and elderly people who may be in danger of becoming dehydrated, says Michael Cima, the David H. Koch Professor of Engineering in MIT’s Department of Materials Science and Engineering.

“There’s a tremendous need across many different patient populations to know whether they have too much water or too little water,” says Cima, who is the senior author of the study and a member of MIT’s Koch Institute for Integrative Cancer Research. “This is a way we could measure directly, in every patient, how close they are to a normal hydration state.”

The portable device is based on the same technology as magnetic resonance imaging (MRI) scanners but can obtain measurements at a fraction of the cost of MRI, and in much less time, because there is no imaging involved.

Lina Colucci, a former graduate student in health sciences and technology, is the lead author of the paper, which appears in the July 24 issue of Science Translational Medicine. Other authors of the paper include MIT graduate student Matthew Li; MGH nephrologists Kristin Corapi, Andrew Allegretti, and Herbert Lin; MGH research fellow Xavier Vela Parada; MGH Chief of Medicine Dennis Ausiello; and Harvard Medical School assistant professor in radiology Matthew Rosen.

Hydration status

Cima began working on this project about 10 years ago, after realizing that there was a critical need for an accurate, noninvasive way to measure hydration. Currently, the available methods are either invasive, subjective, or unreliable. Doctors most frequently assess overload (hypervolemia) by a few physical signs such as examining the size of the jugular vein, pressing on the skin, or examining the ankles where water might pool.

The MIT team decided to try a different approach, based on NMR. Cima had previously launched a company called T2 Biosystems that uses small NMR devices to diagnose bacterial infections by analyzing patient blood samples. One day, he had the idea to use the devices to try to measure water content in tissue, and a few years ago, the researchers got a grant from the MIT-MGH Strategic Partnership to do a small clinical trial for monitoring hydration. They studied both healthy controls and patients with end-stage renal disease who regularly underwent dialysis.

One of the main goals of dialysis is to remove fluid in order bring patients to their “dry weight,” which is the weight at which their fluid levels are optimized. Determining a patient’s dry weight is extremely challenging, however. Doctors currently estimate dry weight based on physical signs as well as through trial-and-error over multiple dialysis sessions.

The MIT/MGH team showed that quantitative NMR, which works by measuring a property of hydrogen atoms called T₂ relaxation time, can provide much more accurate measurements. The T₂signal measures both the environment and quantity of hydrogen atoms (or water molecules) present.

“The beauty of magnetic resonance compared to other modalities for assessing hydration is that the magnetic resonance signal comes exclusively from hydrogen atoms. And most of the hydrogen atoms in the human body are found in water molecules,” Colucci says.

The researchers used their device to measure fluid volume in patients before and after they underwent dialysis. The results showed that this technique could distinguish healthy patients from those needing dialysis with just the first measurement. In addition, the measurement correctly showed dialysis patients moving closer to a normal hydration state over the course of their treatment.

Furthermore, the NMR measurements were able to detect the presence of excess fluid in the body before traditional clinical signs — such as visible fluid accumulation below the skin — were present. The sensor could be used by physicians to determine when a patient has reached their true dry weight, and this determination could be personalized at each dialysis treatment.

Better monitoring

The researchers are now planning additional clinical trials with dialysis patients. They expect that dialysis, which currently costs the United States more than $40 billion per year, would be one of the biggest applications for this technology. This kind of monitoring could also be useful for patients with congestive heart failure, which affects about 5 million people in the United States.

“The water retention issues of congestive heart failure patients are very significant,” Cima says. “Our sensor may offer the possibility of a direct measure of how close they are to a normal fluid state. This is important because identifying fluid accumulation early has been shown to reduce hospitalization, but right now there are no ways to quantify low-level fluid accumulation in the body. Our technology could potentially be used at home as a way for the care team to get that early warning.”

Sahir Kalim, a nephrologist and assistant professor of medicine at Massachusetts General Hospital, described the MIT approach as “highly novel.”

“The development of a bedside device that can accurately inform providers about how much fluid a patient should ideally have removed during their dialysis treatment would likely be one of the most significant developments in dialysis care in many years,” says Kalim, who was not involved in the study. “Colucci and colleagues have made a promising innovation that may one day yield this impact.”

In their study of the healthy control subjects, the researchers also incidentally discovered that they could detect dehydration. This could make the device useful for monitoring elderly people, who often become dehydrated because their sense of thirst lessens with age, or athletes taking part in marathons or other endurance events. The researchers are planning future clinical trials to test the potential of their technology to detect dehydration.

The research was funded by the MGH-MIT Strategic Partnership Grand Challenge, the Air Force Medical Services/Institute of Soldier Nanotechnologies, the National Science Foundation Graduate Research Fellowships Program, the National Institute of Biomedical Imaging and Bioengineering, the Koch Institute Support (core) Grant from the National Cancer Institute, and Harvard University.