Sneaky shrimp wreak havoc on Great Lakes

They’ve been on experts’ Most Wanted lists since 1998. Since the 1970s, investigators have been tracking their movements, waiting for them to burst on the scene and wreak havoc at any moment.

And finally, this November, it happened.

But these outlaws are no ordinary criminals. In fact, they’re no more than half an inch long, and from a distance, could be mistaken for fish.

For more than thirty years, scientists have been keeping tabs on Hemimysis anomala, one of seventeen species of shrimp living in European waters frequented by U.S.-bound cargo ships. It was only a matter of time, experts feared, before these minute crustaceans strayed across the Atlantic and invaded North American fresh water environments.

And now, Hemimysis has been the first to do just that. The members of the National Oceanic and Atmospheric Administration (NOAA) who first spotted this warm-water species of mysid shrimp in Lake Ontario in November were stumped at first. How had these fresh-water dwellers traversed the thousands of miles of salt water separating them from their home in Eastern Europe’s Ponto-Caspian region?

Could ballast tanks be to blame? A ship’s ballast is a large tank that can be filled with water to adjust the ship’s stability and center of gravity. Water can be added or released from the tank as needed; if water is taken in one area and released in another, it would be easy to inadvertently transport aquatic organisms.

The shipping industry’s NOBOB – “no ballast on board” – rule was designed to avoid just this problem. According to David Reid of the NOAA, more than 90% of the ships entering the Great Lakes since the mid-1980s have been NOBOBs.

However, these very NOBOBs are probably still to blame for the shrimp invasion. Closer investigation has proved that it’s impossible to expel the last few gallons from the bottom of a ship’s ballast – and even one gallon is one too many. Ships are now required to completely rinse out their tanks with salt water, thus totally displacing the fresh water.

But it is too little too late. The Hemimysis are already quite at home in the warm surface waters of Lake Ontario. McGill University researchers predict that the species will compete with native fish for the microscopic organisms at the bottom of the aquatic food chain, potentially causing serious problems in the native food web. In turn, the shrimp themselves could be a tasty new source of food for larger fish species. Unfortunately, their high fat content makes them prone to bioaccumulation of PCBS and other pollutants in the lake waters. These toxins, which accumulate in the fatty tissues of the animals that consume them, are then passed on to the fish that eat the shrimp, and the humans that eat those shrimp.

Hemimysis’ tiny bodies may be almost clear, but they are far from invisible. The introduction of this diminutive species will continue to impact the Great Lakes’ ecosystem for decades to come. The full extent of the damage remains to be seen, but scientists predict Hemimysis to be long-term problem. It looks like Louisiana isn’t the only place whose shrimping has seen better days.

by Sara Kate Kneidel


Keywords:: shrimp, Hemimysis, H. anomala, introduced species