Where the fish? Maybe we already caught them.

There's a human tendency to blame someone else's behavior for problems that may have complex causes, but a new study suggests that in declining fish populations, the obvious conclusion is the right one.

(Image: Ulua cruising. Credit: Dr. Anthony R. Picciolo, NOAA NODC.)

The key player in the decline of Hawai'i reef fish...is fishing, says the study—the largest-ever assessment of reef fish populations in the main Hawaiian Islands.

And the proofs are pretty clear.

The study, published in the journal Environmental Conservation, is entitled, “Assessing the importance
of fishing impacts on Hawaiian coral reef fish assemblages along regional-scale human population gradients.”

Its authors are Ivor Williams of the Hawai'i Cooperative Fishery Research Unit at the University of Hawaii and the state Division of Aquatic Resources; Alan Friedlander of the Oceanic Institute and NOAA National Ocean Service; William Walsh and Kosta Stamoulis of the Hawaii Division of Aquatic Resources; and Robert Schroeder and Benjamin Richards of the University of Hawai'i's Joint Institute for Marine and Atmospheric Research.

One of the key proofs is this: Across the state, where there is heavy fishing, the population of the kinds of fish anglers are seeking is down far more than the ones they don't target.

“This study shows that the reef fishes most coveted by fishers, such as uhu, ulua and redfish, are severely depleted, and it points to fishing as the main driver of those declines,” Williams said.

Over the years, evidence to this effect has been piling up. This study is a major advance. It looked at coastal areas across the state, sampling fish populations at 89 locations. A number of pieces of the fishery puzzle came to light.

This is important because you can't solve a problem if you don't clearly understand its causes, just as you can't fix a backfiring car until you know whether it's a fuel issue, a spark issue or some other problem. A mechanic's first challenge is to narrow down the causes.

In reef fish populations, there are lots of potential problems: sediment that chokes corals, oil spills, chemical runoff, physical damage from dredging and dragging anchors, and, of course, fishing, are among them.

“Humans can impact coral reef fishes directly by fishing, or indirectly through anthropogenic degradation of habitat. Uncertainty about the relative importance of those can make it difficult to develop and build consensus for appropriate remedial management,” the study authors said in their paper summary.

One assumption of the study was that large-scale environmental problems should affect most of the different fish stocks. When the results showed that only the fished fish like uhu and ulua were down, and other species like hawkfish, small triggerfish, surgeon fish and others were still doing okay, that tended to implicate fishing.

“If the chief cause of fish declines was habitat loss or environmental degradation related to development and pollution, then we would have seen fish declines across the board. Instead, fish declines along human population trends were only really apparent for species preferred by fishers,” Williams said.

Other pieces of information: Targeted fish populations tended to be reduced in places where there are lots of people fishing—meaning urban areas compared to very rural areas. But also, the target species tend to be healthier—even in urban areas—where it's difficult for anglers to get to the water.

“It did not seem that proximity to human populations by itself was associated with fish population declines, but rather that the crucial factor was proximity to human populations who were able to readily access, and therefore fish, nearshore waters,” the authors wrote.

How bad is it?

“We found that herbivores are enormously depleted. The biomass on Oahu reefs is only about three percent of that in remote parts of the state. Parrotfishes are massively impacted by fishing,” Williams said. Herbivores are plant-eaters like uhu or parrotfish, as distinguished from carnivores or meat-eaters like ulua or jacks.

The paper doesn't give coastal pollution a pass. It says that reefs impacted by uncontrolled urban activity result in degraded reefs that support fewer fish. But overfishing makes problems worse.

“Where significant habitat or environmental degradation occurs around heavily populated locations, its likely effects will be to exacerbate already severe impacts of intensive fishing, rather than being the main driver of any local declines in target fish stocks,” the paper says.

©2008 Jan W. TenBruggencate