Plankton Tow


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Normally a solitary species, whale sharks aggregate in the Gulf of Mexico each summer to filter feed on an abundance of fish eggs. Researchers are currently studying these eggs in an effort to promote whale shark conservation…

Several times each year, the Georgia Aquarium whale shark research team does ‘plankton tows’ in Quintana Roo, and I had the opportunity to shadow them during a recent expedition. As you can see in the above image, these waters are teeming with fish eggs, making it an excellent field location for roe studies.

This process includes “using a plankton net to collect plankton samples,” explains Harry Webb, Senior Laboratory and Research technician at the Georgia Aquarium. The collected eggs “will be processed for nutritional content, as well as DNA analysis to identify the species of plankton present.”

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This special square frame plankton net grazed the surface of the water in an effort to mimic the whale sharks’ natural feeding behavior. The net was dragged behind the boat for three minutes at a flow meter of .7 knots – information that helps “back analyze how much water was filtered to collect that specific amount of eggs,” says Al Dove, Senior Biologist at the Georgia Aquarium.

The collected fish egg samples were stored in ethanol or formelyne or frozen entirely, and then divided between DNA sequencing labs, the Georgia Aquarium and local whale shark researcher Rafael de la Parra, Project Domino Research Coordinator and Biologist.

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With these particular eggs, conservationists hope to determine the exact fish species, DNA and nutrient content of the eggs that these whale sharks feed on. “We expect it to be the same sequence as last year,” explains biologist Dove. “We also take a ‘voucher’ – or fixed specimen – to help analyze the results. If it turns out there are more than two species of eggs, we will need to divide and sequence the DNA separately.”

But what do fish eggs have to do with whale shark conservation? Well, surprisingly little is known about the whale shark species, despite the fact that they are the ocean’s largest fish. But studying fish roe can determine specific measures to help protect whale sharks’ known feeding sites. Additionally, potential environmental and human-related threats abound (like the recent oil spill), raising concern about these whale sharks that frequent the Gulf of Mexico. “It’s about contamination of their food web… even if they [whale sharks] survive this oil disaster themselves,” explained Dr. Robert E. Hueter, Director, Center for Shark Research at the Mote Marine Laboratory at the recent Whale Shark Festival.

In essence, it’s about understanding the food chain and how one species directly affects another, and researchers hope that learning more about fish eggs will lead to greater knowledge about whale sharks.

Learn more about whale sharks check out this whale shark feeding!


Great Migrations continues on the National Geographic Channel this Sunday November 14 starting at 8P et/pt.

Photo Credits: Dr. Al Dove/Georgia Aquarium