In other words, people from across all sectors of society are coming together to address the most pressing challenges facing our global ocean. Pioneering chemist and astronaut Cady Coleman put the challenge this way: “We are, all of us, the crew of Spaceship Earth. This is our charter, and we must do the work.”
The power of partnership
A delegation from Monterey Bay Aquarium is in New York City this week to help do that work. We’re partnering with organizations, governments and businesses to reduce plastic pollution, address ocean acidification and improve the sustainability of global fisheries and aquaculture.
That spirit of partnership is the heart and the promise of the U.N. Ocean Conference. “I’m sure that you’re aware that the ocean is in deep trouble,” said Peter Thomson, president of the U.N. General Assembly. “The good news is that we’re working on solutions.”
After chowing down a big meal, you might feel your belly warm as your stomach muscles and digestive organs set to work breaking your food into smaller and smaller pieces rich in nutrients. A bluefin tuna’s stomach experiences a similar spike in temperature when it gulps down a mouthful of juicy sardines.
Now, scientists at Stanford University, Monterey Bay Aquarium and the National Oceanic and Atmospheric Administration (NOAA) have devised a way to measure that internal temperature increase in the fish – and connect it to how much the tuna ate and where it consumed its meal.
Pacific bluefin tuna are superbly streamlined, bullet-shaped fish, with powerful swimming muscles capable of powering transoceanic travels. Unlike most other bony fishes, they are warm bodied, able to elevate their internal tissue temperatures above that of the surrounding water.
Bluefin tuna remain warm by capturing the metabolic heat produced as their swimming muscles contract with every tailbeat. This happens via specialized net-like blood vessels, called counter-current heat exchangers, in their muscles and digestive organs that prevent heat loss through the gills. Maintaining warmer-than-water body temperatures allows the fish to swim more efficiently and spend less energy digesting food, and enables them to thrive in a wide range of ecological niches.
The researchers focused on this thermal characteristic to measure energy intake, and from that to surmise the animals’ daily foraging habits. Researchers implanted small data-logging tags in more than 500 tunas off the coast of southern California and Mexico, and recorded the fishes’ body temperature, ambient water temperature, and their locations and diving patterns as they searched for prey. With the help of fishers, the researchers recovered more than one-third of the tags, containing data records as long as three years as the fish made seasonal migrations from the waters off Mexico to Oregon.
Previously, observation work led by Rebecca Whitlock, a postdoctoral scholar at Stanford, made with fish held at the Tuna Research and Conservation Center, had created a model to translate changes in tuna heat signatures into caloric intake. At the center – a partnership between Stanford and the Monterey Bay Aquarium – researchers could count single sardines or squid consumed by individual tunas and match the warming signal in the stomach to the energy value of the prey item digested.
Documenting their dining
The thermal data showed exactly when the tunas ate a meal, and the researchers estimated how much energy a free-swimming bluefin receives per unit of time, as well as how temperature changes impact that energy intake.
“We’ve been able to follow what Pacific bluefin tuna do in the open sea and record their feeding and meal size, every day for up to three years,” said Whitlock, the lead author of the new paper. “Combining laboratory observations with electronic tagging can provide amazingly rich data and insights into the life of a wild marine predator.”
Tag data showed that wild tunas consumed prey on 90 percent of the days observed during the study. The empirical data analyses and energetic model output allowed scientists to chart precisely how much the fish ate – typically sardines and squid – and the total energy they consumed as they journeyed through the ocean.
A roadmap in the ocean
From this, the scientists mapped the position data from the tags to satellite observations of sea temperature, chlorophyll levels and ocean currents – all factors that can combine to create nutrient-rich feeding grounds. The location of these feeding grounds coincided well with successful tuna feedings, though interestingly the fish didn’t always camp out at places with the best conditions to take advantage of the buffet.
“Foraging success was correlated to environmental features,” said co-author Elliott Hazen, a research ecologist with NOAA’s Southwest Fisheries Science Center. “Tuna may use the oceanography as a roadmap to move from hotspot to hotspot, and temperature appears to be the most important environmental cue.”
Interestingly, the study demonstrated a potential tradeoff between feeding in the richest areas and avoiding the physiological constraints associated with feeding in waters that are either very cold (which slows the heart) or very warm (which are energetically taxing). This answered a long-standing question about the species’ traditional range limits, from north of Oregon to south of the Baja Peninsula, despite the fact that close relatives of bluefin (yellowfin and albacore tuna) thrive outside of those latitudes.
Good places to digest
Tag data showed that tuna tend to stay in waters where they can remain at an optimum temperature to promote rapid digestion. Too high or too low an ocean temperature, and the increased demands of digestion can strain the cardiovascular system.
“Digestion is metabolically costly, and the bluefin are doing it most efficiently,” Block said. “Our results suggest that physiological constraints on the tunas’ whole organismal performance constrain their thermal distribution, and thus the latitudinal distribution of the fish.”
Block calls this portion of the Pacific Ocean the “Blue Serengeti,” an open ocean ecospace where currents concentrate nutrients and plankton, attracting forage fish such as sardines or anchovies, which in turn lure larger predatory fish such as bluefin tuna.
Understanding the locations of Blue Serengeti “watering holes” for these large migratory fish remains a mystery, but is a key part in planning better conservation efforts that protect essential habitat. Linking the regions both physiologically and to environmental correlates has been an objective of the research team.
The new work helps close that gap by identifying feeding hotspots (areas of highly successful feeding) for Pacific bluefin tuna: along the Baja Peninsula in June and July, off Northern California from October to November, and near Central California in January and February.
“Our results add to our understanding of predator-prey dynamics in the California Current,” Block said. “By understanding where bluefin forage most, we can help protect these places and improve efforts to rebuild Pacific bluefin tuna stocks.”
– Bjorn Carey is a life sciences public information officer for Stanford News Service
Reference: Rebecca E. Whitlock, Elliott L. Hazen, Andreas Walli, Charles Farwell, Steven J. Bograd, David G. Foley, Michael Castleton and Barbara A. Block. “Direct quantification of energy intake in an apex marine predator suggests physiology is a key driver of migration.” Science Advances 25 Sep 2015: Vol. 1, no. 8, e1400270 DOI: 10.1126/sciadv.1400270
Bluefin tunas are among the most remarkable fishes in the ocean – apex predators that migrate across ocean basins for long distances at top speeds. Biologically, economically and culturally significant at a global scale, they have long been the target of lucrative fisheries. Today, they face a range of threats worldwide.
From January 18-20, 2016, this three-day gathering will bring together the world’s foremost bluefin science and management experts to discuss issues that will shape a sustainable future for the planet’s bluefin tuna populations, and to consider a future global vision for bluefin tunas.
The program will cover the latest scientific knowledge for all three species, current and new fisheries management tools, the economics of the bluefin tuna industry and trade, the emerging role of tuna aquaculture, and the impacts of climate change. It’s all with the goal of shaping a vision for healthy, sustainable bluefin populations.