A Rocky Start

If asked to describe the first few weeks of a baby pallid sturgeon’s life in the Missouri River, most biologists will tell you the embryos attach to a hard surface, develop, hatch, and enter the fast-flowing water to drift downstream.  Despite catching a very few free embryos over the last 10 years, no one has actually observed this phase of the sturgeon’s life cycle in the river.  Instead we’ve used scientific deduction and reasoning to come up with a plausible scenario of how a pallid sturgeon begins its life in the river.

Recently, however, laboratory observations of pallid behavior at these earliest stages, together with new data on post-hatch behaviors of other riverine sturgeons, have led to an alternative hypothesis to the hatch-and-drift paradigm.  Our flume studies of 2012 and 2013 have indicated that newly-hatched pallid sturgeon survive for only a few hours at water velocities > 0.10 m/s.  It isn’t until the sturgeon are 12 days old that they can survive at velocities of 0.23 m/s. 

How then can newly hatched pallid sturgeon survive velocities in the range of 1.5-3 m/s, typical of the Missouri River? Recently, scientists working on reproductive ecology of the white sturgeon have shown that free embryos hunker down in the interstices among rocks for a few days and develop fins and swimming muscles, before venturing into the drift.  Might this also be true for the pallid sturgeon?

To test this hypothesis, scientists at CERC are using outdoor artificial streams with two types of bottom substrate onto which the sticky, newly fertilized pallid eggs are ‘seeded’ (Figure 1).  In one stream, the small cobble substrate does not allow the pallids anywhere to hide after hatching so that they immediately drift upon hatching (Figure 2).  In a second stream, the eggs are adhered on and down inside a pile of larger rocks. Upon hatch, the embryos have more opportunity to maintain their position in among the boulders and emerge to drift after a few days of development.  Nets placed downstream of the rocks in both streams will catch the drifting free embryos.  If the pallids do delay their drift, then we would expect there to be a few days difference between the streams in the peak collection of free embryos in the nets.

 

Figure 1. Fertilized pallid sturgeon embryos are gently dispersed over the rock substrate. The PVC pipes are used to support a canopy of shade cloth to more closely mimic the muddy depths of the Missouri River.

Figure 2. Fertilized pallid sturgeon embryos are gently dispersed over the gravelrock substrate. The PVC pipes are used to support a canopy of shade cloth to more closely mimic the muddy depths of the Missouri River.

The vulnerability of these tiny fragile larvae, and their urgent need to find a hospitable environment where they can feed and grow before their yolk is gone, strongly suggests this life stage could be a survival bottleneck for the population.    Although we will likely never be able to simulate in the laboratory the complex and extreme conditions in the Missouri River we must begin to think outside-the-box and creatively find ways to observe ecologically relevant pallid sturgeon early-life behavior.

By Diana Papoulias

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