Night sampling for pallid sturgeon free embryos (The Fish Don’t Sleep and Neither Do We)

Science Center Objects

July 2, 2016

By Dr. Robb Jacobson and Aaron DeLonay

The sample design for the Upper Missouri River Pallid Sturgeon Drift Experiment depends on nearly around-the-clock sampling for free-embryos, the life stage from hatch to the initiation of feeding.  It can be a grueling process under the best of conditions, but for some of the crews there is the added complication of sampling at night.  The challenges of wind, rain, and swarming insects are intensified in the chill of a dark Montana night.

The night sampling effort starts at about sunset – 8 pm in this part of Montana in the summer – and continues 12 hours until crews are relieved the next morning (figure 1).  The crews transit to the pre-selected sampling sites before it gets too dark to avoid grounding on sandbars (see previous blog entry Boating the Upper Missouri River is not for the faint of heart).

USGS fish biologists launch at sunset on the Upper Missouri River for a night of sampling for pallid sturgeon free embryos.

Figure 1. USGS fish biologists launch at sunset on the Upper Missouri River for a night of sampling for pallid sturgeon free embryos.

(Credit: Aaron DeLonay, U.S. Geological Survey. Public domain.)

Sampling requires winching a pair of heavily weighted, very fine-mesh nets (in figure 2) to the bottom of the Missouri River for a 10-minute collection, then winching it back up, emptying the contents of the net into a shallow sorting pan, and then sending the nets down again as quickly as possible.  The nets sample just above the bottom of the river and collect a large amount of detritus and organic matter. The crew quickly picks through the contents of the nets for free-embryo sturgeon and experimental beads.  The free embryos are counted and retained for genetic analysis while the number and color of beads are recorded.  The up and down cycle of the nets repeats all night.

USGS fish biologist Dr. Pat Braaten and student contractor Garrett Cook inspect contents of a net during night sampling

Figure 2. USGS fish biologist Dr. Pat Braaten and student contractor Garrett Cook inspect contents of a net during night sampling on the Upper Missouri River.

(Credit: Aaron DeLonay, U.S. Geological Survey. Public domain.)

It’s an intensive and strenuous process, and a bit eerie working from the light of headlamps while anchored in the middle of the river (figure 3).  In this type of sampling there are many net hauls that have zero free embryos or beads, so the rewards are not always immediate. The zero catches are important, however, because they indicate that the experimental release has not yet reached a site, or that all of the free embryos have passed.  When the first free embryos and beads show up in the samples (figure 4), the shouts of success carry in the night from one headlamp to the next, and across the river to the companion boat anchored in the darkness—then the night is not so dark, nor as cold, as it was before the crews found what they were looking for.

USGS biologist Dave Combs searches through net contents for free embryos during night sampling on the Upper Missouri River.

Figure 3. USGS biologist Dave Combs searches through net contents for free embryos during night sampling on the Upper Missouri River.

(Credit: Aaron DeLonay, U.S. Geological Survey. Public domain.)

Typical contents of a net deployment showing a fish free embryo, possibly pallid sturgeon at the tip of forceps.

Figure 4. Typical contents of a net deployment showing a fish free embryo, possibly pallid sturgeon at the tip of forceps. 

(Credit: Aaron DeLonay, U.S. Geological Survey. Public domain.)