I recently got a letter from the United States Federal Bird Banding Laboratory. (Yes there is such a thing.) I was pretty surprised. In the 1980s I studied seabirds and hawks, and I banded many thousands of birds. But that was a long while back. I no longer have a banding license -- though I miss those days -- and I haven't put a band on a bird's leg in 20 years. So I wasn't expecting bird-banding mail. I opened it and became informed that a Common Tern I'd banded 25 years ago as a chick was breeding this past summer, on an island about 80 miles northeast of where it hatched. Wow. A few years ago, this would have been the species' longevity record, which, I learned, now stands at 27 years.
When I was studying seabirds in grad school, there came a flurry of published studies on the relationship of birds to fisheries. There are 4 general dynamics: 1) birds catch fish, thereby competing with fisheries; 2) fisheries compete with seabirds, catching fish that seabirds need, thus lowering their ability to survive or raise chicks; 3) birds directly benefit from fisheries because they eat fishery discards in such volume that they survive at higher-than-natural densities, expanding to larger-than-natural ranges; 4) fishing gear drowns seabirds at high enough rates to drive population declines in some species, notably albatrosses.
Seemingly conflicting conclusions -- fishing both helps and hurt seabirds -- occur because different kinds of birds do different things. Gulls, which by nature are scavengers and opportunists, can benefit from discards. Gannets and terns eat almost exclusively live fish that they catch by plunging; depleting schools of fishes like herring or sand-lance can crucially diminish their food supply.
Now we have a new paper. Philipe M. Cury and a baker's dozen of colleagues have published, "Global Seabird Response to Forage Fish Depletion -- One-third for the Birds" (Science, December 23, 2011). Under examination: How much food do seabirds need? (Their definition of forage fish includes fishes, squids, and the shrimp-like crustaceans called krill that swarm in cold waters where they feed everything from seabirds to whales.)
The ocean is no untrammeled wilderness. As the authors note, many large fishes and marine mammals have been severely diminished by fishing and other human activities. Meanwhile, there's an intensifying collision between the decline of larger fishes and the increase of human appetites, now seven billion people and still climbing. Fishing for smaller species such as sardines and anchovies -- the food for big fish, many mammals, and birds--is increasing.
The scientists looked at 14 seabird species across 7 ecosystems. They set out "to test the hypothesis that the form of the numerical response between seabird breeding success and forage fish abundance is consistent across species and ecosystems." Bear in mind that the world contains hundreds of seabird species, and for most of them there is no data on the relationship between the birds and their food. So, this paper's generalization may not hold across all species or in all situations. For example, in deep open-ocean waters, some tropical boobies rely heavily on actively feeding tunas to make forage fish accessible by driving them to the surface. What happens to birds when fishing depletes tunas, even if prey such as flying fish are not targeted by fisheries, and remain abundant?
The scientists found that seabird breeding success was fairly high and stable when prey densities were average or above. That's not surprising; it means seabirds are best adapted to average conditions. Below average prey densities, though, there's trouble. The authors refer to just-below-average prey densities as "the threshold at which breeding success began to decline." The "threshold" also happens to be prey densities that are about one-third of the maximum prey abundance observed in the most prey-dense years. And another thing happens at the threshold; seabird breeding success becomes not just lower but more variable, a wider range of good and bad.
One more thing: although seabird breeding success declines strongly when food abundance goes below average, at above-average food densities breeding success reaches a plateau, and does not change, even as food abundance further increases. This appears to be a global pattern.
This implies that chronic food scarcity will compromise long-term seabird breeding success. It also implies that it would be difficult to undo harm done to bird populations after depressing their food abundance. Protecting prey so that they bounced back from depletion (which would be a management feat in itself ) would not allow seabirds to simply bounce back and compensate for population declines caused by food scarcity. Apparently, they can't. Even if there's plenty of food, seabirds whose numbers are depressed by drowning in gill-nets or on longlines, can't simply bounce back after people stop doing what's killing them. (Albatrosses and their relatives, for instance, can only lay one egg per breeding season, no matter how much they're eating.)
Long life spans and slow breeding rates are typical in seabirds. Evolution has forged a palette of sturdy, steady birds built to reflect the sea's ups and downs and to survive and thrive through bouts of meager times. Their longevity is their bridge across troubled waters. But like any bridge, it only goes so far, and there's a limit to how much it can bear.
The authors think their study's main conclusion is, leave "one-third for the birds." That's catchy, but suggesting that we try to manage for one-third of the maximum is unrealistically complicated. I think their main finding is: seabirds need prey to remain around average levels of abundance.
Either way, our assumption is that it's important to have seabirds, to conserve their populations, to help ensure that they can breed successfully -- in short, that it's important to share the world with seabirds. I'd like to think that every bird on the ocean has a shot at living its full life-span, like that 25-year-old Common Tern I met as a chick in its nest in the beach-sand so many summers ago.
And not just seabirds, but, as the authors say, "other predators in marine ecosystems," such as big ocean super-fishes like tunas and billfishes, and marine mammals.
Of course, many assume that rebuilding or maintaining populations of predatory mammals, sharks, and birds is an important goal of fisheries and ocean management. But we have to face a fact that trumps this study's findings, the on-paper goals of management regimes, and my own desires: It's a world of 7 billion and counting, wherein many people couldn't care less about seabirds or other predators -- unless they're tasty.
That is the real issue. And the real solution is education, empowerment, and human dignity. In a discussion of seabirds, human population is not off-topic. It's the context for all these topics.