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.

Aaron Carlisle, a postdoctoral scholar in biology and lead author of the study, said, “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.” 

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 of Sciences 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 Pelagic 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," said 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."

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. 

Senior Fellow, by courtesy, Barbara Block (Biology), 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."