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October 25, 2012

Saltwater Marsh Troubles: Who's to Blame?

Debunking the claim that recreational anglers are the cause for saltwater marsh die-offs.

Not long ago, I received an e-mail from the president of the American Sportfishing Association, Mike Nussman, who asked for some help with a study that had just appeared in Ecology, the publication of the Ecological Society of America. It was titled: “A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing.” If you like to saltwater fish, that title will get your attention.

Having never seen the study by Andrew Altieri, Mark Bertness, Tyler Coverdale, Nicholas Herrmann and Christine Angelini, I dove into it. This paper hypothesized that “a trophic cascade is leading to increased grazer (herbivorous crab) populations and the rapid die-off of salt marshes in the northeastern United States.” It further hypothesized that “recreational fishing is triggering this trophic cascade because we observed that marsh die-off is highly localized and consistently occurs only where recreational anglers have access to marshes.” If true, that could have some far-reaching implications.

For a non-scientific layman, I’ve deduced that the study seems to have a lot of controls to test the researchers’ various hypotheses. The researchers’ conclusion is that recreational fishermen cause localized depletion of apex predators, which they determined to be striped bass, blue crabs and smooth dogfish. I may be out of it, but I don’t know of a lot of recreational dogfishing in the marshes. The authors “found that the localized depletion of top predators at sites accessible to recreational anglers has triggered the proliferation of herbivorous crabs, which in turn results in runaway consumption of marsh vegetation. This suggests that overfishing may be a general mechanism underlying the consumer-driven die-off of salt marshes spreading throughout the western Atlantic.”

The study included visual observations of recreational fishing in areas where there was extensive die-off. In those and where they had observed recreational-fishing activity, they used aerial photographs dating back many years to calculate the increase of infrastructure such as docks and boat slips over the years. This increase was used as a proxy to demonstrate the increase in fishing pressure. None of the marsh sites that did not exhibit die-off had any infrastructure to allow access.

The more I read the report, the more concerned I got. But some questions began to creep into my head, questions that could have an impact on the results of this study.

The first question regarded the use of the increase in infrastructure to determine the amount of fishing pressure in any one area. The increase in docks and boat slips does indicate an increase in boating activity, and a lot of boats are used for fishing, but the actual location of fishing activity is not determined by the location of the infrastructure. As a proxy for overall boating activity, though, it is probably a good indicator.

The next question concerned the number of fishing trips, which came from NOAA Fisheries. I agree that this is a good indicator of increased fishing activity, but again, it does not pinpoint the location of that fishing activity.

One of the big assumptions was that since there were no observations of recreational fishing in the marsh areas not exhibiting die-off, it was thought there was no fishing there. If their overall conclusion is correct, in those marsh areas there would be “normal” rather than depleted levels of apex predators. Then why weren’t recreational anglers there? That assumes that in one marsh, anglers can figure out where the fish are, and in others, they can’t. I don’t buy it. Along the same line of reasoning, what happened during the 1970s and ’80s when the striped bass populations were in the dumpster? Why weren’t all the marsh areas seeing die-off?

Currently, most striped bass are released either as regulatory releases or voluntarily. Those striped bass found in the marsh areas are likely to be smaller fish and thus more likely to be released.

I discussed this study with Dr. David Ross, Scientist Emeritus at Woods Hole Oceanographic Institution. He made some of the same observations I did but made a few more that warrant discussion. He noted that the study indicated “die-off marshes had half the biomass of top-level predators (e.g., striped bass).” Then he added: “So we conclude that our recreational fishermen fish only in areas where the biomass (stripers, etc.) are in small numbers, and absolutely avoid the areas where the fish are. If true, then how can recreational fishermen be the cause of the die-of ?”

He also noted that the pictured vegetation die-off was “an amazingly straight line. Animals usually don’t do such neat work.”

It was comforting to get some respected scientific response to this study. I do not know if Ecology does peer-review work prior to publication, but I suspect that this study will not just fade away. When recreational fishing has caused problems in the past, responsible leaders have made great efforts to correct these problems. We would do the same here, if this is validated.