Searching for a needle in a haystack

Female pallid sturgeon with eggs are very rare on the Lower Missouri River.  When one spawns she will release as few as 8,000 to 20,000 eggs over a 12 to 36 hour period.  Those eggs are released in swift water 5 to 8 meters (15-24 feet) deep and perhaps over as much as 1/4 mile of river bottom.  So, trying to collect pallid sturgeon eggs or the larvae that hatch from those eggs has been compared to that old saying about searching for a needle in a haystack – it seems impossible!  In hopes of increasing our chances of success, we’ve combined our knowledge gained from previous laboratory and field experiments with intensive telemetry tracking of reproductive female pallid sturgeon.

In 2009, 2010, and 2011, we followed telemetered female pallid sturgeon to their suspected spawning sites and sampled for hatching and drifting larvae immediately downstream.  Pallid sturgeon eggs develop in the crevices of rocks or attached to the river substrate.  The time required for pallid sturgeon to develop and hatch depends on the temperature of the water.  We have learned from laboratory studies that pallid sturgeon eggs will hatch in 4-8 days and larvae will emerge at 7-9 mm in length at the temperatures typical in the Lower Missouri River during the spawning season.  From laboratory and field studies we have learned that sturgeon larvae begin drifting downstream from the spawning location in the lower half of the water column soon after hatching.  Therefore, by knowing the water temperatures after spawning we can calculate when we should expect larvae to begin drifting downstream and where we should place our nets to have the best chance of collecting them.

The contents of the net are flushed into the cod end, or the trailing end of the net, where the fish are finally caught. The cod is then emptied into black bowls for sorting.

Biologists on the Lower Missouri River begin sampling for larvae 4 to 5 days after a suspected spawning event and sample for a minimum of two days.  Scientists work from an anchored boat equipped with a winch on each side, and a pair of very fine mesh nets attached to 100 pound lead weights are lowered into the water column.  After a set period of time (5 to 15 minutes, depending on flow conditions) the nets are simultaneously brought to the surface, emptied and sorted, and Acipenseriformes (sturgeon and paddlefish) larvae are hand-picked and preserved in alcohol.  All Acipenseriformes specimens are returned to the laboratory where they are identified as sturgeon or paddlefish.  Sturgeon specimens are sent to a genetics laboratory to determine if they are pallid sturgeon or shovelnose sturgeon, and if the telemetered female sturgeon was the parent.

The contents of one 10 minute sample collected on May 24, 2011, at river mile 216.7 in the Lower Missouri River. Note the fish eggs embedded with the coarse organic debris. The fish eggs belong to another riverine species, not sturgeon.

Breaking apart and diluting the samples stuffed with organic debris is essential to finding small larval fish. Luckily, the yolk sac of newly hatched Acipenseriformes stands out against the black sorting bowls.

About Emily Pherigo

Emily is no longer with the Comprehensive Sturgeon Research Project. When she was here, she was a biologist contracted to the Comprehensive Sturgeon Research Project. Most of her time was spent at a computer performing QA/QC on data or updating figures and graphs most used by Aaron DeLonay. However, she occasionally made it to the river, where she enjoyed seeing pallid sturgeon and was reminded why she entered the natural resources field.
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