Dolphin Deaths Linked to Entrapment Stress

Even if spared from drowning in tuna nets, Pacific Ocean dolphins may still die prematurely or reproduce less because of the stress of the temporary entrapment, a La Jolla scientist suspects.

Wildlife biologist Al Myrick began exploring the question of long-term dolphin stress based on his studies of a “stress signature” in dolphin teeth--combined with a wide survey of scientific literature.

If his continuing studies end up proving that point, they could complicate even further the controversial question of how the Pacific tuna fleet affects the population of northern spotted dolphins.

“I don’t want to make my position one of an advocate of any particular ‘pro’ or ‘con,’ ” said Myrick, part of the federal government’s Southwest Fisheries Science Center in La Jolla. “I just want scientists to start looking at it more critically, instead of just saying, ‘Yeah, we’re saving the dolphins, and as soon as they get out of the net they’re OK’ ”

An estimated 100,000 dolphins die in the Pacific every year after becoming entangled in tuna fishing nets. Major U.S. tuna canners announced in April that they would no longer buy tuna from fishermen who used nets from which dolphins cannot escape, a victory for environmentalists concerned about the dolphin deaths.

But Myrick suggests that even the stress of pursuit and temporary capture may be resulting in delayed deaths or other problems in dolphins.

Myrick bases his concern on his discovery that dolphin teeth show distinct banding patterns--much like the growth rings in trees--that can be related to stressful life events.

Stressful events that he has been able to reliably link with the calcium loss in captive dolphins include illness, capture and pregnancy. Overall, seemingly random stress bands are more common in dolphins associated with tuna fishing than in other dolphin populations, he has found.

During stress, the animal’s body releases elevated levels of the hormone cortisol, lowering blood calcium and causing the body to release parathyroid hormone, which in turn induces the body to “borrow” calcium from the teeth, he has concluded.

An intensely stressful event can cause such a large calcium depletion from muscle, which also contains calcium to regulate contraction, that zoo animals have been known to die of it during transporting, he noted. Indeed, an Andean condor intended for experimental release in Ventura County two years ago was “frightened to death” in this way.

The string of studies that led Myrick to these conclusions fit together like a puzzle.

First, in 1986, he realized that the teeth from 2,500 dead dolphins on which he had been doing aging studies bore mysterious bands not related to regular growth. The longitudinal slices of the conical teeth had chevron-shaped bands in them.

Myrick found that, in about 200 bottlenose dolphins not associated with the tuna industry, the bands were areas where calcium had been lost and then filled back in with calcium later.

But, in the spinner, spotted and common dolphins that fishermen commonly use to find schools of tuna, these bands were filled in with collagen fibers instead of being remineralized with calcium phosphate. The calcium depletion patterns were irregular, and thus not seasonal in nature.

He thought the reason might be fishing-induced stress, but wasn’t sure. So Myrick began looking for similar patterns in the teeth of other mammals that undergo regular cycles of natural stress.

He found them in bears, associated with hibernation and cub birth; elk, related to low winter food supply; and a bobcat, related to extreme weather during one year of its life.

These results made Myrick confident that increasing cortisol levels in the blood had set off the calcium-depleting cycle, but he still had not discovered the chain of events that followed.

So Myrick looked at blood samples from 40 spotted dolphins. Taken by observers on a purse-seine vessel in the mid-1970s, the blood showed below-normal levels of calcium in the blood.

In a later refinement of the study by Eric Archer, a graduate student at Scripps Institution of Oceanography, the blood also showed a decline in phosphate levels as the fishing operation progressed.

This suggested that the parathyroid hormone had been at work, causing calcium phosphate to be robbed from teeth and bones. Chronic stress could result in cortisol continuing to deplete the blood of free calcium--preventing the redeposition of calcium in the teeth later.

The latest research supporting his hypothesis is a 1,000-dolphin tooth study comparing the teeth of dolphins in heavily fished areas of the northeastern tropical Pacific, to those of rarely fished southeastern Pacific dolphins. Preliminary results show significant differences in calcium repair patterns between the two groups, he said.

To some extent, Myrick’s hypothesis requires leaps of faith, but it also offers hints of how scientific dogma can be turned on its ear by a new way of looking at a question.

For one thing, a corollary would put him in direct opposition to some of the most cherished tenets of population biology: That animals will react to a reduction in their population by reaching sexual maturity sooner, reproducing more, and living longer.

Myrick’s hypothesis would have the dolphin population in tuna fishing areas responding in the opposite way, because of the stress-induced cortisol levels.

Steroids such as cortisol not only delay maturity, he noted, but also cause a decrease in luteinizing hormone, a hormone necessary for ovulation.

Myrick suggests such processes might also be operating in highly stressed humans, contributing to infertility and osteoporosis.

Has any of this been proven? No, Myrick concedes. But the questions that these studies of dolphin teeth and blood have raised are just too tantalizing to ignore, he believes.

“We have to shake up the old thoughts with new thoughts, to get people thinking about this,” he said.

Myrick will present a seminar on his ideas at 10:30 a.m. Tuesday at the fisheries center’s large conference room. It will be open to scientists and the public.