Study uses photographed crabs as a quick bioindicator of estuary health

By on October 27, 2014

A fiddler crab photographed on an Australian beach. Similar species were assessed as bioindicators in a study in Estuaries and Coasts. (Credit: Denise Chan, via Flickr/CC BY-SA 2.0)

Maybe “intertidal mudflat” fails to evoke the sand, sun and fun of the less esoteric term “beach,” but the crabs that inhabit these low-lying estuarine habitats full-time have a lot more to say about the health of their homes than the words we humans use to define them.

A study published in Estuaries and Coasts demonstrates the potential of crabs as bioindicators for their habitats’ well-being. With just a camera, some crustaceans and a little skill with statistical modeling, it’s not hard to tell a healthy estuary from a flagging one — if you know where to look.

“The crabs are very abundant, and they’re a food item for many fish and birds in the mangroves,” said Peter Vermeiren, a freelance ecologist who co-wrote the study as a doctoral student at James Cook University in Australia. “They rework the sediment — they’re very useful organisms in estuaries.”

Vermeiren knew of other studies that had examined the feasibility of crabs as bioindicators, but those had only been done on small scales, he said.

“There was a disconnect between these studies and what managers needed because managers work more on the large scale,” Vermeiren said.

The study integrates Vermeiren’s own ecological research and the findings of researchers before him into a modeling system that can help managers make rapid assessments of estuary health based on crab distribution patterns. Vermeiren photographed crab distributions at 10 estuaries along the coast of North Queensland, Australia, from a dingy in the main channel. Focusing on the species that characterized each particular habitat, he compared his observations to predictions from his statistical models. He calls his methods a “first-level assessment of estuarine health.”

Lorenna, a volunteer, photographs on-shore crab distributions. (Credit: Peter Vermeiren)

Rather than provide a detailed analysis of a given estuary’s health, Vermeiren says this type of bio-assessment is best geared toward managers who need to quickly determine which of the estuaries under their stewardship are in the greatest need of care. Unusual species distributions can signal potential issues in a habitat. The method is cost-effective, and since it relies on photography, there’s little risk of frightening the crabs into hiding.

“With a photographic method, you can stay at a distance and you don’t interfere with them,” Vermeiren said. “You can just go ‘click-click-click’ and photograph an estuary in one day.”

One alternative to Vermeiren’s method requires catching crabs by hand and tallying their numbers. While this technique provides a better opportunity to count eggs and determine sex, it’s not very convenient — and quite invasive. Vermeiren performed a hand-catch at one estuary to collect data on three species for inclusion in his models. He pulled no punches with his two-word summary of the experience:

“Very muddy,” Vermeiren said, laughing. “It’s a slow method and it’s a very destructive method. You disturb the sediment and it has a lot larger impact.”

Hand-catching crabs was muddy, inefficient work compared to photographic surveys. (Credit: Peter Vermeiren)

Vermeiren hopes he can build his sample base to include other habitat systems. With an expanded dataset, the ecologist believes he could identify patterns that characterize specific types of pollution threats instead of merely indicating a zone of concern. Furthermore, Vermeiren says his model and methods could be used with other species.

“You could use the method for other sedentary organisms, barnacles for example,” he said. Perhaps, he suggested, his work could influence citizen science monitoring efforts in coastal habitats.

“I want to extend the photography to citizen volunteers,” Vermeiren said. “It would be interesting for fishermen to use this method — and also to see if they can analyze the data themselves.”

Top image: A fiddler crab photographed on an Australian beach. Similar species were assessed as bioindicators in a study in Estuaries and Coasts. (Credit: Denise Chan, via Flickr/CC BY-SA 2.0)

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