Our effects are simple: the more people, the fewer fish

People are bad news for fish - yeah, we've known that for awhile. Just look at the decrease in size of catch off the Florida Keys from 1957 to the 1980s and 2007 (on right) and that conclusion is obvious. But, surprisingly, little research has been able to show how human populations affect a group of fish. Most studies focus on one type of fish or are contained to a small area because their manpower is limited to a small team of scientists diving and recording data themselves. Others simply look hypothetically at what increases or decreases in some variable like pollution would have. And even those that are large in scale work off of fisheries data, which is only collected by certain (usually industrialized) countries, leaving many areas undocumented. Christopher D. Stallings, a biologist from the Zoology department at Oregon State University, decided that we needed to capture a bigger picture of the effects that people are having on fish right now all over - and he found a way, thanks to a motivated organization called REEF.

REEF, the Reef Environmental Education Foundation, was founded in 1990 out of growing concern about the health of the marine environment, and the desire to provide the SCUBA diving community a way to contribute to the understanding and protection of marine populations. What they do - the simplified version - is teach divers how to count and ID fish, then allow them to go out on their own and record species data. With over 127,000 surveys and counting, they've got a lot of data on what species have been seen where and when. And most importantly, the data is available to any researcher who wants to use it.

So Stallings did something that surprisingly few scientists have done and used the REEF survey data to compare the sightings of large, predatory fish throughout the Caribbean. The results, published in PLoS ONE, are clear. When he compared the fish data with the human population densities for the same areas, he found a humbling trend. The more people there were in a place, the fewer large predators there were. The predator populations dropped by 2.2% on average for every 100 people per square kilometer increase in human density. Not only were there fewer large fish, the ones that were there were far less diverse. As human population numbers increased in an area, 15 of the 20 different predatory species he looked at disappeared, and the remaining five were the smallest.

Stallings points to fishing as the most likely cause of this dramatic effect, though certainly other factors could be at play. Since the data doesn't look at why the fish populations changed, there could be other reasons, including increased pollutants or loss of habitat.

It's also hard to say what effects the absence of these predators will have on the reefs themselves. Studies have suggested that losing top predators can have a dramatic, negative impact on a community. The loss can make an ecosystem less resilient to change like increased temperatures or major events like hurricanes. It can also make them vulnerable to invasive species. But studying such effects in detail is difficult, as no one wants to remove predators from an area to see what happens.

This study is important because it highlights just how strong the impact we have on the oceans that surround us. A 2% decrease per 100 people is huge when you think about it. For example, Miami, FL has 2,532.1 people per square kilometer, which is more than double the density of Tampa, FL which has 1,146.7 people per square kilometer. With those numbers, we'd predict that the reefs off of Miami have over 30% less predatory fish that the ones off Tampa - and those are both big cities with a lot of people, major ports and a lot of fishing. Imagine how staggering the difference between those big cities and what the oceans would be like if we weren't there at all.

This study also opens the door for a lot more usage of the incredible data collected by REEF. Finally, we have a way to look at the Caribbean (and other) fish populations that isn't limited in scope. We can, at last, see the big picture, without biasing the results towards industrialized countries, particular habitats, or only a few species. Unfortunately, the bigger picture isn't a pretty one.

Stallings, C. (2009). Fishery-Independent Data Reveal Negative Effect of Human Population Density on Caribbean Predatory Fish Communities PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005333