Where Are You When I’m Not Looking?

The Lower Missouri River is a large river.  With over 800 miles of river to track, our boats cannot be everywhere, watching every tagged sturgeon at the same time.  A handful of reproductive females with black eggs were tagged with transmitters late in the spring and released near the Platte River, Nebraska.  While USGS and Nebraska Game and Parks Commission crews were busy tracking other females, the spring spawning ritual for these few fish took place while biologists weren’t looking.  However, just because the Comprehensive Sturgeon Research Project (CSRP) biologists were not present at spawning does not mean that we weren’t collecting data.

Each sturgeon tagged with a telemetry transmitter is also implanted with a data storage tag (DST) that records temperature and depth at 15-30 minute intervals everywhere the fish travels during migration and spawning.  Biologists recapture each sturgeon after the spawning season and download the data from the implanted tags.  From the condition of the female sturgeon’s ovaries and the data downloaded from the DST tags, biologists can reconstruct some of what might have happened and gain valuable insight.

Two pallid sturgeon females, PLS11-014 and PLS11-015, were tagged and released in the Missouri River below the mouth of the Platte River on April 28, 2011.  Based upon the condition of their eggs at the time of tagging, spawning should have occurred soon after release.  Both females were recaptured months later in the fall of 2011.  At recapture, biologists determined that they had released their eggs and spawned.  Since neither fish was tracked during her spawning migration, biologists were left to wonder where or when spawning may have occurred.  Closer examination of the temperature data downloaded from their DST tags is revealing.

Comparison of temperatures recorded by the DST inside PLS11-014 with temperatures in the Missouri River and in the nearby Platte River shows that the temperature experienced by the fish very closely matches the temperature in the mainstem Missouri River (Figure 1).  This indicates that PLS11-014 remained in the Missouri River continuously during the reproductive season and likely spawned somewhere in the mainstem river.

Figure 1. Temperature recorded by a data storage tag (DST) inside female pallid sturgeon PLS11-014 indicates that she spawned in the mainstem Missouri River.

The data recorded by the DST inside PLS11-015 is strikingly different (Figure 2). On May 3, five days after she was released below the mouth of the Platte River the temperature data recorded by the DST tag changes.  The temperature inside PLS11-015 no longer matches the Missouri River, but now matches the temperature of the nearby large tributary, the Platte River.  Data indicates that PLS11-015 stayed in the Platte River through the entire month of May.  Based upon laboratory evaluation of her readiness to spawn, it is likely that she spawned somewhere in the Platte River.

Figure 2. Temperature recorded by a data storage tag (DST) inside female pallid sturgeon PLS11-015 indicates that she likely spawned in the Platte River.

By Aaron DeLonay

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