“The world that I grew up in is gone,” says Kristin Kaschner, a marine ecologist at the University of Freiburg in Germany. Between 1890 and 2001, the population of all whale species declined from more than 2.5 million to under 880,000. While the population of some whale species has rebounded since the global whaling moratorium in 1986, many are still endangered. And while the majority of fish stocks are fished in a way that allows them to maintain or grow their populations, just over 34 percent of them are overexploited, which means we’re removing so many fish from a certain area that their populations cannot recover. Some of the fish stocks being overexploited include Japanese anchovy, Alaska pollock, and South American pilchard. “I think we are moving towards a world where the default is not a natural ecosystem in which everything is as you had it before there was human exploitation and intervention,” says Kaschner.
Although the picture isn’t rosy at the moment, looking at the size spectrum of marine organisms could be a helpful indicator of ocean health, says Julia Blanchard, an ecologist at the University of Tasmania in Australia. Blanchard has studied coral reefs and found that when the Sheldon spectrum seems out of whack, it’s a sign that the reef ecosystem is no longer healthy. “If we’re looking at improving that, what we might do is ask what would be a level of fishing that would maintain the size spectrum,” she says.
One problem is that fisheries often target what scientists call BOFFFFs: big, old, fat, fecund, female fish. Their large bodies are prized by fishers, but BOFFFFs are a vital source of new baby fish. Take these away and the size spectrum quickly veers out of kilter. One way to manage this is to encourage the fishing industry to target medium-size fish, allowing mature ones to replenish depleted populations.
Of course, overfishing isn’t the only challenge that marine populations are facing. A worst-case scenario of 5 degrees Celsius of warming would be too hot for 50 percent of fish species, and even 1.5 degrees of warming would still be too much for 10 percent of fish, according to one study. Overfishing means these populations are starting from a much weaker point than they would otherwise be. Take too many fish out of the ocean and you reduce genetic diversity, weaken food webs, and allow ocean habitats to degrade, all of which makes an individual ecosystem more vulnerable to changes. “What’s important is that as you fish out a system and then it’s warmed, it’s much less resilient to that warming,” says Blanchard.
The good news is that fish species can bounce back. “They are extremely resilient,” says Ken Andersen, a marine ecologist at the Technical University of Denmark. In September, the International Union for the Conservation of Nature moved four tuna species further down its list of threatened species after their populations started to recover, thanks to stricter fishing quotas and crackdowns on illegal fishing. “It’s easier to stop overfishing than it is to stop climate change,” says Galbraith. “If we fish less, if we allow ecosystems to recover, we can maintain that.”
More Great WIRED Stories