Tagging of Pelagic Predators

Whale shark

Researchers use new tools to understand the lives of salmon sharks

Salmon sharks congregate near the mouths of rivers in Alaska to feed on salmon returning to their natal rivers to spawn. Credit: Image courtesy of Scot Anderson.You are what you eat, or so the saying goes. Researchers at Stanford University's Hopkins Marine Station are using this fact to better understand the life history of the far-ranging salmon shark, an important apex predator and toothy cousin of the white shark, which roams the entire North Pacific Ocean, from the chilly subpolar waters of Alaska to the warm sub-tropics of Hawaii and the Baja Peninsula.

In all animals, the specific chemical composition of the food they eat gets incorporated into their tissues. In some structures like teeth, hair or feathers, new tissue is laid down in layers as the animal grows, somewhat like rings in a tree. In each of these layers, the chemical composition of their food leaves an indelible chemical "fingerprint" which remains with that animal through its entire life. 

In the case of salmon sharks, whose skeletons are made up entirely of cartilage, annual growth bands are laid down in their vertebrae. By sampling tissues from within the layers of these bands found in the vertebrae of adult sharks, and comparing their chemical "fingerprints" to that of known prey items found in different parts of the of the North Pacific Ocean, the researchers were able to piece together the regions where these sharks were feeding -- all the way back to the very beginning of their lives.

Dr. Aaron Carlisle, a postdoctoral scholar at Stanford University and lead author of the study, says, “Historically, it has been very difficult or impossible to study highly migratory pelagic species, sharks in particular, throughout their life history. They are simply too mobile and their habitats too remote for scientists to study effectively. As a result, much of what we know about the life history of these species focuses on particular age classes, and we simply do not have the ability to track how the diet and habitat of an individual animal changes over the course of its life - information that is critical for management. That was one of the most exciting aspects of this study; by using this retrospective analysis to reconstruct the lives of these pelagic sharks we were able to start to pull back the curtain on the entire life history of the salmon shark.” 

A vertebral section from a ten year old salmon shark, with the annual growth bands labeled.  The band labeled U represents in-utero growth, while bands labeled 0-10 identify tissue deposited during each year of life.   Credit: Kenneth Goldman. One of the most interesting aspects of this study was that the researchers were able to use the chemical composition of the early growth bands to identify the primary nursery area of juvenile salmon sharks and that the transition from their nursery grounds to the habitats used by highly migratory adults is very clearly recorded in their growth rings, showing that as they mature they shift from juvenile oceanic habitats to increased use of coastal habitats as adults. 

This study, which was published in the Proceedings of the Royal Academy B, adds to a growing body of knowledge about the biology of salmon sharks, much of which was carried out using electronic tags. Through the Tagging of Pacific Predators (TOPP) program, which was part of the international Census of Marine Life, over 100 salmon sharks were tagged with electronic tags in the Gulf of Alaska. This allowed researchers to follow their journeys for up to four years, and the tracks revealed that they make annual migrations as far south as the Hawaiian Islands and the southern tip of Mexico’s Baja Peninsula.

"Although tagging has told us a lot about the life history of these animals," says Carlisle, "it only tells us about what they do after they've been tagged, providing a snapshot in terms of their overall lives.  And because we have only been able to tag older animals, we know very little about their younger years. By combining what we learned from tagging with these types of biogeochemical analyses, we can now look back in their life history to see what individual sharks were doing during each year of their life. And by looking at a large number of animals we were able to start to identify patterns that were consistent across the population."

A salmon shark swims through the cold waters of Prince William Sound, Alaska. Credit: Image courtesy of Scot Anderson.Salmon sharks, like many shark species, have been removed from the oceans in large numbers. One report indicated that as many as 100,000 young salmon sharks per year were caught at one time in open ocean gillnets. Currently, the relative amount of bycatch is poorly understood, but may be considerable. 

Stanford University Professor Barbara Block, senior author on the paper, said, "In the past decade we’ve changed the game, with tagging and isotopes discerning how these apex predators utilize our oceanic offshore habitats. Like many of the large predatory sharks, salmon sharks are relatively slow-growing and produce only a few young at a time. Because of this, it is important that we learn as much as we can about them now, and apply this knowledge to ensuring that they are protected in the wild."

About stable isotopes...

The different isotopes of a given element have different numbers of neutrons. So most carbon atoms, for example, have 6 protons and 6 neutrons, for a total atomic mass of 12 – which is called “carbon-12,” denoted 12C. But a small number of carbon atoms have 6 protons and 7 neutrons, for a total atomic mass of 13 – making it 13C.  This form of carbon is stable – hence the name “stable isotope.” (14C is not stable, and is therefore a “radioactive isotope.”)

The different isotopes are chemically identical but have different weights.  This difference in weight is what leads to tissues becoming enriched or depleted in the heavy isotope through various physical and biochemical processes, and these patterns are used by researchers to understand physiological and biological processes.

The “stable isotope ratio” of a substance, then, is calculated as the ratio of one stable isotope to another, as compared to that of a known standard. So for carbon, the stable isotope ratio is calculated as the ratio of 12C to 13C, as compared to the same ratio in a mineral called “Vienna Pee Dee belemnite.” Similarly, the stable isotope ratio of nitrogen is calculated as the ratio of 14N to 15N relative to that found in air.

Salmon sharks feed on smaller fishes, squid and crustaceans. Because the isotopic values of animals and plants vary from one part of the ocean to another, the stable carbon and nitrogen isotope ratios of these prey species tend to closely reflect the regions within which they reside and feed. So as a salmon shark moves from one region of the ocean to another, the isotopic composition of their vertebral growth bands become almost like the pages of a passport - "stamped" with a progression of layers of differing stable isotope signatures, reflecting the unique chemistry of their prey during that period of their life. This gives scientists the means to re-construct broad scale patterns of use of different regions in which the shark has resided through the course of its life history. 

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