Shark eyes tell tales

Sightings of leopard sharks (Triakis semifasciata) are common along California’s coast this time of the year. Growing up to six feet long with a distinctive spotted pattern that earned them their name, the sharks typically gather in late summer and fall in the warm, gentle waters of the state’s estuaries. One of their largest populations resides in San Francisco Bay. But these harmless predators — which can live up to 24 years — are facing a silent crisis, one whose particular relevance marine biologist Jonathon Kuntz has learned to detect in an unexpected place: their eyes.

Kuntz, whose research was funded by California Sea Grant, has pioneered a new method for studying shark ecology. The technique relies on the fact that shark eye lenses grow in layers throughout their lives, much like tree rings. By carefully peeling apart these layers — "like an onion," as Kuntz describes it — scientists can now reconstruct a shark's complete life history, all the way from birth to death.

"We can't put a satellite GPS tag on a baby shark because the tags are simply too large," says Kuntz, who conducted this work as a California Sea Grant Research Graduate Fellow at the University of California, Davis. "But with this method, we're getting a chemical signal of where the animals have been and what they've eaten throughout their entire lives."

A Chemical Record Through Time

The technique relies on stable isotope analysis: the distinct chemical fingerprints that different environments leave in an animal’s tissue. For example, the types of carbon that end up in an animal's body tissues differ depending on whether it spends time in fresh or ocean water. Similarly, the way nitrogen builds up in an animal's body typically reveals where it came from in the food chain — for instance, whether a shark has mainly fed on bottom-dwelling creatures like clams and worms, or has been hunting fish.

In Kuntz's study, he analyzed around 40 leopard sharks collected from four bays and estuaries along the Northern California coast, specifically San Francisco Bay, Tomales Bay, Bodega Bay and Drakes Estero. What he discovered surprised him. While Northern Californian leopard sharks are thought to migrate seasonally between different habitats, the isotopes in their eye lenses told Kuntz that the San Francisco Bay population was largely resident — they spend their entire lives in the estuary.

The chemical analysis revealed something else unexpected: sharks in Tomales Bay, Bodega Bay and Drakes Estero may also begin their lives in San Francisco Bay. The eye lenses suggest that these waters serve as critical nursery habitat from which leopard sharks disperse to form the broader Northern California population.

These findings took on particular urgency when Kuntz paired them with separate research he conducted with the California Department of Fish and Wildlife. "We're finding some pretty large declines of leopard sharks in San Francisco Bay over the last four or five decades," Kuntz says. Fisheries data revealed that populations in the area started plummeting after 1980 — around the time when three major wastewater treatment facilities in the South Bay began operating.

Pollution Leaves Its Mark

Today, the San Francisco Bay estuary receives effluent from 38 sewage treatment plants, each with the capacity to pump more than 10 million gallons of wastewater daily, the equivalent of 500 normal-sized swimming pools per facility. 

This, too, is something that Kuntz could see in the sharks’ eyes. Wastewater is rich in nitrogen, creating what Kuntz calls an "anthropogenic” signal in the fishes’ tissues.

Given the declining numbers and the observation that San Francisco Bay seems to replenish leopard shark populations elsewhere, Kuntz argues that it might be time to rethink management policies. Currently, California regulations allow recreational fishers to take three adult leopard sharks per day. Over the entire state, sport anglers harvest an estimated 59,000 leopard sharks annually.

Kuntz fears that this quota might be unsustainable, considering the species' slow reproductive rate. Unlike most fish — including some other shark species — which lay thousands of eggs, leopard shark mothers hatch live pups, between seven to 36 per litter, each about seven inches long.

The animals don't reach reproductive maturity until around ten years of age, and it takes females close to a year to carry a litter to term. "We might need to start looking at San Francisco Bay as a unit of conservation and maybe change the policy a bit," says Kuntz. “Taking adults that took ten years to become reproductively mature out of the system in these numbers could be really detrimental to the population."

A Methodological Revolution

The eye lens technique Kuntz tested out represents a major advancement in shark research. Previously, scientists wanting to study a shark's full life history had to analyze vertebrae — a process that requires specialized diamond saws, mounting materials, chemical treatments and considerable expertise.

The special tools and effort required made studying vertebrae "not attractive for conservation managers or others that can't put in those resources," says Kuntz.

Eye lens analysis, by contrast, requires only basic lab equipment. Scientists simply remove the lens, peel apart the natural layers, dry them in an oven and they're ready for analysis. The process takes hours rather than days. The method has been validated in other fish species over the last decade, but Kuntz's work represents the first successful ecological application to sharks.

The simplicity of the approach could boost conservation research for other species too — nearly half of all shark and ray varieties “are considered data deficient," notes Kuntz. Now an Alaska Sea Grant State Fellow, he continues to collaborate on shark research back in California. 

Meanwhile, the fish that inspired his breakthrough continue their seasonal patterns. As leopard sharks gather once again in California's estuaries, their eyes hold more than the ability to navigate murky bay waters. They contain decades of environmental history and, perhaps, a roadmap for their conservation.