Smaller Fish – The Decade’s Most Underreported Story?

What was the underreported environmental story of the decade? In my book it was about fish, humans, and evolution.

In 2000, Nobel Prize winner Paul Crutzen coined the term “Anthropocene” for the current epoch in recognition of humans as a dominant force for environmental change on the planet. (More on Crutzen here.) In the past decade’s intervening years we saw a lot of new science on the environment, much of which documented the growing influence we humans are having on the globe.

One indication that we’re now in the Anthropocene is the rapid decline in biodiversity; humanity is causing major and irreversible changes in our planet’s genetic makeup by destroying habitat and literally wiping species off the face of the Earth. That was news, important news, but it was hardly underreported.

But what about a more subtle change, like causing the fish of the ocean to literally evolve before our very eyes? That’s my pick for the decade’s most underreported environmental story. Here’s the scoop as I, admittedly no geneticist or population biologist, understand it. 

Evolution

According to biologyonline.org, evolution is “the change in genetic composition of a population over successive generations, which may be caused by natural selection, inbreeding, hybridization, or mutation.”

There can be no doubt that evolution occurs. In fact over the past decades Darwin’s evolutionary theory put forward in works like his own Origin of Species and beyond has been painstakingly confirmed by modern genomics.

We usually think of evolution as a slow, orderly process occurring over millennia, on what we call geologic time scales. But genetic changes in a population can occur much more rapidly. The influenza virus is a great example. So is the development of antibiotic resistant bacteria. And believe it or not, the evolution of fish now appears to be another.

Fisheries-Induced Evolution

Natural selection typically drives evolution by weeding out the weak and old and leaving the fittest to survive and pass on their genetic makeup to the next generation.

But it doesn’t have to work that way. Imagine a situation where a species is suddenly confronted with a new a predator, a predator that wipes out maybe 50 percent of its population each year (a predation rate many times larger than the population had evolved under) and does so in a very selective way — by taking the largest of the species. Under that type of selection pressure, population biologists tell us that the genetic makeup of the species will quickly evolve into one that favors more rapid maturation and smaller size at maturation.

Theory says it works like this:

  • Size-selective fisheries remove a large percentage of fish that are above a certain size and typically mature enough to have produced offspring.
  • With their loss, reproduction of the population favors individuals that invest energy in maturing faster and reproducing earlier in life rather than in growing larger.
  • The result: over the course of just a few generations we have a population of smaller individuals that mature and reproduce more rapidly.

Interesting theory? Yeah. And one that microbiologists tell us applies to short-lived, rapidly mutating organisms like bacteria. But what about more stable populations? Like fish?

It turns out we have been busy unwittingly carrying out our own little experiment to see if the theory really works. It’s called fisheries-induced evolution. By fishing for the largest fish and overfishing, we appear to have single-handedly engineered a rapid genetic change in the ocean’s fish — a change characterized by declining age and size of fish at maturity.

Fish Getting Smaller: A 20th Century Theory Manifest in the 21st Century

The concept of fisheries-induced evolution is not a new idea. Dr. Mikko Heino of the University of Bergen, Norway, who co-edited a special issue of Evolutionary Applications on the subject last August, puts it like this:

“The first suggestion that fishing may drive evolution is more than a century old, and there has been sporadic interest ever since. Interest started to increase in late 1980s, but the discussion remained rather academic because empirical evidence was very inconclusive and models were too rough. The real wake-up happened after year 2000. Several things fell into place around that time: methods to analyze data from wild fish stocks were improved and started to suggest that evolutionary changes are commonplace, fast evolution was demonstrated in tank experiments, and more realistic models were developed. This happened against the background of fisheries management entering the era of ecosystem approach, and growing recognition among academics that contemporary evolution is commonplace.”

The signs of fisheries-induced evolution are mostly circumstantial (it’s hard to isolate the impact of fishing from other environmental factors) but they are also striking.

For example, scientists have found that changes in maturation traits (age and size) correlate significantly with fishing intensity; the most heavily exploited stocks experienced the largest declines. Such evidence supports the idea that fishing is driving evolutionary change as these changes should be most apparent in the most heavily exploited stocks.

Duke Marine Lab’s own genomics expert Tom Schultz says that direct genomic evidence of fisheries-induced evolution is hard to come by in part because such research is difficult and expensive and only recently have scientists even had the tools to look in that direction. So most of the genetic evidence showing that selection practices epitomized by fishing can lead to rapid genetic changes comes from laboratory experiments that isolate heritable traits while controlling for other variables that would be in play in natural environments. This work shows that traits can evolve with fishing pressure. To date there has been one study that linked molecular changes in wild fish to fishing pressure.

What’s Next in the Evolutionary Line?

For me evidence of human-induced evolution of fish occurring on decadal timescales is big news for geneticists, population biologists, and environmental scientists. It’s all kind of relevant to fishers and those of us who like to eat fish. And given the amount of attention it has gotten (not much), it rates right up there as one of the most underreported environmental stories of the past decade.

So what’s the next scientific challenge? Here’s an idea; how about doing another human-induced evolutionary experiment to be sure we’ve got it right. What about changing fishing habits (e.g., adding marine reserves, limiting fish catches, fishing sustainably) to see if we can reverse the process and re-establish a fish genome that is more like what we had before. Is that possible? Well, initial research suggests that at best recovery rates will be quite slow. But science is fun and the world can do some surprising things. Let’s do the experiment and see what happens.

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