DNA testing of fish eggs taken from the upper Mississippi River last summer confirm they did not derive from invasive Asian carp as once thought, according to the U.S. Geological Survey. The agency had issued a warning in March stating the species was spreading northward.

The eggs actually belong to a native North American species in the same family as carp, according to a press release from the USGS.

Earlier this year, researchers believed the samples came from Asian carp based on a visual analysis of the eggs’ size and shape; the species was also successfully spawning in the northern portion of the Mississippi. USGS scientists alerted the public about the potential impacts on the river system and decided to perform genetic testing to confirm their findings.

“What we have learned from this research is that non-Asian carp cyprinid eggs in the northern portions of the Upper Mississippi can closely resemble Asian carp eggs in size and shape,” said Leon Carl, USGS Midwest Region Director. “These findings underscore the importance of using genetic testing to confirm the results of visual identification.”