The ocean off Southern California is a known birthing ground for great white sharks. It also holds toxic chemical concentrations of polychlorinated biphenyls (PCBs) and the insecticide DDT that were dumped into coastal waters decades ago, especially off the Palos Verdes Peninsula.
The legacy of those chemicals remains evident today as marine biologists from California State University, Long Beach (CSULB); the Monterey Bay Aquarium and the Southern California Marine Institute discovered that the tissues of young Southern California white sharks have some of the highest levels of contaminants among all shark species.
Moreover, the babies likely absorb the chemicals from their mothers, who acquire them through the food chain. Yet the chemicals seem to cause little physiological damage to the young sharks, the researchers found.
Their findings appear in the April 30 edition of the online science journal PLOS ONE, (http://dx.plos.org/10.1371/journal.pone.0062886).
The researchers examined the remains of infant and juvenile white sharks incidentally caught in commercial fishing nets, said lead author Christopher Mull, now a Ph.D. candidate at Simon Fraser University in Burnaby, British Columbia. He first became involved in the study while completing his master’s degree at CSULB’s Shark Lab, directed by Professor Christopher G. Lowe, a co-author of the report and an internationally recognized shark and marine fisheries expert.
Other co-authors are CSULB master’s student Kady Lyons, CSULB master’s graduate Mary Blasius, Chuck Winkler of the Southern California Marine Institute, and John B. O’Sullivan of the Monterey Bay Aquarium’s Project White Shark research program.
White sharks are considered apex predators, at the top of the region’s marine food chain. “Young white sharks are likely exposed to DDT and PCBs when they feed on fish in Southern California. Adults, however, feed heavily on harbor seals, elephant seals and California sea lions that forage in Southern California and are known to have high levels of DDT and PCBs,” Mull said.
The young whites had the highest levels of DDT and second highest levels of PCBs ever reported for a shark, Mull continued. “This was particularly alarming given their young age, some likely weeks to months old, and the fact that other shark species with comparable levels were often decades old. Even though we know Southern California is a DDT hotspot, we suspected that these levels couldn’t have been obtained solely by young sharks feeding, and that mothers were a likely source.”
To demonstrate their hypothesis, “We ran a simple computer model simulation where we fed a virtual newborn shark pup—assuming that it had zero contaminants at its time of birth—a meal of the most contaminated fish it could eat every day,” Lowe said. The results showed the simulated levels were nowhere near as high as what was found in necropsy results from incidentally caught sharks. “What this clearly demonstrates to us is that there is no way that these young sharks are acquiring these contaminants based on their own feeding, which means the only other way they can get them is from their mom.”
Mothers passing contaminants to children is a phenomenon common to many species including humans, but such transfers haven’t previously been studied in white sharks, which are pregnant for about 12 months and bear large, well-developed pups nourished from eggs.
Lowe said an interesting aspect of white shark pregnancy is that the developing embryos consume their egg yolks long before birth. The mothers continue to release unfertilized eggs rich in fats and oils (lipids) that the embryos eat while still in the uterus. Because the shark mothers acquired chemicals from eating contaminated fish and marine mammals, they pass these acquired contaminants to their offspring via the eggs, Lowe said. “This is the first study that has really quantified this maternal offloading ability in a shark that’s analogous to a mammal.”
The researchers also discovered promising findings among their data.
“While the evidence of maternal offloading and the elevated levels of DDT and PCBs in young white sharks is disconcerting, surprisingly we found no evidence of physiological impairment from the contaminants,” Mull said. “Despite the high degree of exposure for white sharks as pups and adults, the population actually appears to be rebounding. Historically white sharks were fished, and populations of their prey—seals and sea lions—declined. Now that both sharks and marine mammals are protected, there is evidence of a recovery from historic lows.”
Lowe explained, “We compared the robustness, or the body girth of these sharks, which is just a measure of how fat they are at birth, to sharks that had low levels and high levels of contaminants, and there is no difference. It doesn’t look like a shark that has high contaminant loads is weaker or more sickly than those with low levels. In addition, when we did the necropsies, we found no signs of internal cancers or lesions that are typically indicative of contaminant exposures, especially at the levels that we’ve seen.
“It’s possible that despite the fact that these sharks have these very high loads, that they have ways of dealing with these contaminant levels that other animals like mammals and birds clearly have not,” Lowe said. “Sharks have a unique immune system compared to other vertebrates and they may have some tricks for dealing with these high contaminant loads that higher vertebrates don’t. Nevertheless, it’s an issue we need to keep an eye on for the sake of future generations.”
The contaminant studies are one dimension of a decade-long collaboration among CSULB, the Monterey Bay Aquarium and the Southern California Marine Institute to better understand the life history and migrations of juvenile great white sharks in waters off Southern California and Baja California.
The research partners have also used electronic tracking tags to document shark migrations between waters off California and Mexico, and to better identify coastal habitats used by young white sharks.