Big predatory fish inhabit the highly productive waters of the Southern California Bight in the summer. While commercial fisheries target those like tuna and the Pacific swordfish (Xiphias gladius), blue sharks (Prionace glauca) and shortfin makos (Isurus oxyrinchus) are often unintentionally taken due these animals sharing the surface waters at night. In fact, between 2000 and 2014 an annual average of 43 metric tons of shortfin makos were landed commercially on the western coast of the USA, a little over half being caught in drift gillnets targeting swordfish; blue sharks had an annual average of four metric tons landed and it is estimated about fifteen metric tons were discarded at sea. While blue sharks have low market value and are typically discarded, makos are marketed on the USA western coast despite their endangered status. The International Commission for the Conservation of Atlantic Tunas (ICCAT) scientists have recently recommended to ban all retention of shortfin mako sharks without exceptions to give this species a shot at recovery.
Scientific evidence shows that sharks spend considerable time in the upper water levels of the ocean, driven by both physical and ecological needs such as preferred temperature, oxygen concentration, light levels, timing (to forage successfully), and avoiding predation. But surface waters don’t always have what these large epipelagic fishes need, and oftentimes they need to dive deeper. Shark movements can be classified as horizontal movements and vertical ones, with the latter being more commonly referred to as diving. These dives are characterized by rapid changes in temperature, light level, hydrostatic pressure, and in many cases, low oxygen conditions as they get deeper. Like many pelagic sharks, shortfin makos and blue sharks into the inky darkness.
These two sharks have physiological differences that can affect how deep they can go, which means that although their movement patterns are pretty similar to begin with, the deeper they go may show differences. For example, mako sharks are famous for their enhanced metabolic activity that allows them to be the fastest swimming sharks while blue sharks are known to search for food using their enhanced olfactory bulbs to find prey at depth. But pressure isn’t the only thing that changes as a fish swims closer to the ocean floor. Just as oxygen levels shift with elevation, oxygen at sea varies with depth. The Southern California Bight, which is the curved coastline between Point Conception and San Diego, and encompasses the Channel Islands, is known for having a strong oxygen minimum zone centered at about 600 m (1968 ft).
Scientist Thompson Banez of Scripps Institution of Oceanography at the University of California San Diego, wanted to further understand the oxygen limits for these sharks. Three mako and three blue sharks were double tagged with pop-off satellite archival tags (PSAT) and satellite position only tags (SPOT) in the Southern California Bight to shed some light on this deep mystery. This research, never been carried out due to PSAT tags lacking oxygen sensors, saw the six sharks swimming the Cooperative Oceanic Fisheries Investigation survey grid, a long-term program that collects biological and oceanographic data, including oxygen data at every meter up to 500 meters (1640 feet) deep. Using this information, Banez linked the movements to specific oxygen sensors.
So what was found? It turns out that the tagged blue and mako sharks spent little time in low-oxygen water levels. Not surprising as they need oxygen to survive! But some shark species, like the epaulette sharks, are regularly exposed to low-oxygen environments. In fact, these sharks have adapted to low oxygen conditions when the tide goes out in their shallow habitat by lowering their energy demands. “While all sharks (and their relatives) require a certain amount of oxygen to survive—and higher levels are better—some species can tolerate low levels of oxygen for prolonged periods of time,” Rachel Skubel, Ph.D., student at the Abess Center for Ecosystem Science and Policy and researcher at the Shark Research & Conservation Program at University of Miami, told the American Lung Association.
Researchers at Georgia Institute of Technology recently have found that oxygen levels started dropping in the 1980s as ocean temperatures began to climb. Falling oxygen levels impacts the habitat of marine organisms worldwide – in fact, habitat loss associated with oxygen minimum zones (OMZs) are a very real threat! Termed hypoxia-based habitat compression, you may have heard of these zones due to the more frequent “hypoxic events” that have killed many marine species as these OMZs seem to be expanding and changing depth. “The higher-oxygen surface waters are important for active species of sharks and their prey, so a reduction in ‘vertical habitat’ could restrict their access not only to food, but also to cooler deeper waters to regulate their body temperature,” commented Skubel.
It was interesting to note that while both species didn’t stay in these low-oxygen environments for too long, blue sharks made more dives and stayed there for longer than the shortfin mako sharks. However, all six sharks spent the majority of their time near the surface and Banez suggests “fishing at deeper depths, in particular during daylight hours, may be a way to lessen bycatch” of these pelagic sharks.