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A dozen or so small grey fish next to a ruler.
Information icon Adult saltmarsh topminnows. Photo by Ronald Paille, USFWS.

Looking for the saltmarsh topminnow in coastal Louisiana

The Fish and Wildlife Service has been petitioned by WildEarth Guardians to list the saltmarsh topminnow under the Endangered Species Act. Not much is known about the topminnow’s distribution and biology so the Service is researching this species.

According to scientific literature, the topminnow occurs in marshes along the northern Gulf of Mexico coast. It is a small non-migratory estuarine fish which reaches up to three inches long. It forages on the marsh surface during high tides, and retreats to small tidal creeks and rivulets during low tide.

The topminnow has a unique spawning biology. During the warmer months, it deposits eggs on the surface of higher elevation marshes during the highest lunar tides of the month. The eggs are exposed to the air until the next monthly high tide inundates them, at which time they hatch.

In routine fisheries sampling and research conducted by the Louisiana Department of Wildlife and Fisheries, the National Marine Fisheries Service, and Louisiana State University, the topminnow has been captured at a number of locations throughout Louisiana. After compiling and mapping those capture locations, it was apparent that there were some coastal Louisiana locations where there are no records.

Water flowing through a salt marsh
A typical small tidal creek used by saltmarsh topminnows. Photo by Ronald Paille, USFWS.

Given the topminnow’s preference for small tidal creeks and rivulets, it is not readily captured by standard fish sampling gear such as trawls which cannot be used in these small tidal creeks. To determine if the apparent distribution gaps are due to gear type limitations or actual gaps in topminnow distribution, staff members from the Ecological Services Field Office in Lafayette, Louisiana, are collecting fish samples from marshes that appear to be suitable topminnow habitat, but where they have yet to be documented.

A 15-foot-long minnow seine with a one-quarter-inch mesh is being used to collect fish samples in these small tidal creeks. When the seine is pulled in the typically soft bottomed marsh creeks, the fish avoid the seine by swimming ahead of it. Consequently, rather than pulling the seine, the seine is placed diagonally across the mouth of a small tidal creek. As the seine is being set up, another team member walks upstream and wades down the creek toward the net to drive fish down the creek and into the seine. When the driver reaches the open end of the seine, the open end is closed and the sample captured. This method is most effective on very low tides when the water has dropped well below the marsh surface, otherwise topminnows will be in the marsh where they are much more difficult to capture.

Periods of low water following passage of cold fronts are targeted for this sampling as the typically strong north winds result in predictable and persistent low water level conditions.

Sampling has successfully collected topminnows in some areas where there were no records of them. In other areas, no topminnows have been caught despite the presence of apparently suitable habitat. Sampling of these areas will continue.

Samples of fish reveal that the topminnow is not limited to saltwater marshes as one might assume from its name. It is abundant in some brackish and low-salinity marshes. Researchers with Louisiana State University also have found them to be abundant in the freshwater marshes of the Atchafalaya River Delta.

Projects to construct small tidal creeks in recently created marshes would create the preferred habitat used by topminnows and other marsh resident fish and shellfish. These small resident fish and shellfish serve as prey species for red drum, spotted seatrout, and other commercially and recreationally important fish. Thus tidal creek construction projects would benefit not only the topminnow, but would also provide forage for red drum, spotted seatrout and other sought after species.

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