Freshwater fish migrate, but we do not know where and why.
According to Brenda Pracheil, a University of Wisconsin-Madison post-doctorate research fellow in the Limnology Department, scientists lack comprehensive knowledge on the habits of migratory fish species.
There is one freshwater fish species whose migration patterns have been studied—the paddlefish. The padddlefish is a member of the pre-historic sturgeon family and one of the most unique fish in the world. It is known for its spoon shaped snout and this is perhaps why the studies began.
“Probably because [paddlefish] are so unique, a group of 22 states decided to band up and study them,” Pracheil said.
She is talking about MICRA, the Mississippi Interstate Cooperative Resource Association, a large coalition of states and government agencies. In 1995, MICRA started a massive mark recapture study, in which animals are captured and tagged with hopes of recapturing them to gain insight on their movements. Paddlefish were marked throughout their range—the Mississippi River and connected large tributaries such as the Ohio, Missouri and Tennessee Rivers.
Scientists have been catching paddlefish, marking new fish and recording tagged fish for a central MIRCA database ever since. The results: paddlefish can really move.
Some paddlefish have been captured in Kentucky and recaptured in South Dakota. That is quite a distance for a fish to move and it has implications on how we should manage paddlefish and other migratory fish.
Paddlefish ranges often fall under the jurisdiction of multiple state governments, each of which has different management laws. Often the rivers they live in are boundaries between states. The most extreme example is the section of the Ohio River between Kentucky and Ohio; paddlefish cannot be harvested in Ohio, but in Kentucky commercial fishing of paddlefish is allowed.
This is problematic because paddlefish are not an abundant species. Nobody really knows how many paddlefish are out there, but their caviar is valuable and so it drives harvest.
They are internationally recognized by CITES, the Convention of International Trade of Endangered Species, as a species threatened with extinction. However, they are not on the United States Endangered Species List, so management is in the hands of states.
“We need management units that correspond with the biology of the fish,” Pracheil said.
In other words, a given population of paddlefish that moves between states should be managed in one unit, rather than being controlled under multiple jurisdictions.
The same can be said for migratory fish around the world. Yet worldwide migration habits are not known for any freshwater fish other than the paddlefish.
“This is the highest resolution study available on migratory freshwater fish,” Pracheil said. “A ton of effort went into this, and we still don’t know a lot.”
It would be very difficult to map the migratory habits of more freshwater fish species. The studies involved require a great deal of time, resources and tireless work. But they are crucial to making informed management decisions on the large rivers of world. The balance of our greatest freshwater fisheries is at stake.
