Naturespeak A naturalist's view of the world

May 16, 2009

I Have Seen the Future

Filed under: Uncategorized — wykes @ 10:39 am

On my second trip out on the “Big D” (the Lower Detroit River) aboard the USFWS research vessel Sentinel, we failed to catch any sturgeon on the set lines. You can’t blame that, however, on Jim Boase or Dr. Manny, the fisheries biologists in charge of the operation. This is fishing after all and sometimes you get ’em and sometimes you don’t.  This is not to say that we didn’t see any sturgeon that day. In all, you could say that we saw ten fish, but I’ll need to explain this statement. Although our view was completely different than previous trips, it proved to be some of the most exciting sturgeon viewing yet.

The USFWS tagging operation is not the only on-going sturgeon effort taking place on the river. A crew from the U.S. Geological Survey (USGS) was also engaged in survey work in the Fighting Island Channel (see here). Last Fall, an experimental spawning reef was laid across the bottom of the river from the north tip of Fighting Island to the Canadian shoreline at LaSalle. Consisting of a band of mixed gravel and large stones, the reef was intended to provide the type of bottom substrate that sturgeon require for egg-laying. This spring was the first opportunity to see if the great fish would give it their stamp of approval and the USGS team, aboard the tiny but mighty R V Sculpin,  was there to check for sturgeon spawning activity.

Searching for fish eggs can be a needle-in-the-haystack kind of operation, especially given the swift-flowing current and 28 foot depth in that portion of the channel.  Egg mats, rectangular metal frames covered with filter material, are randomly laid across the bottom in order to overcome this sampling difficulty. These units are about as low tech as you can get – a tribute to governmental frugality, as a matter of fact. The material itself is standard furnace filter cloth and the clips used to secure it are of the Office Depot variety. The sticky eggs adhere to these mats and can be viewed when brought to the surface. Members of the research team were actively engaged in this activity as we pulled up alongside (see below & here). 

There were plenty of eggs attached to the filter material. Most of them were Sucker eggs, this being the peak of their spawning season, but the presence of Sturgeon eggs provided ample evidence that the fish were indeed capitalizing on this new spawning venue. One of the crew offered up a small container containing nine brownish-grey eggs as evidence (see beginning and below).  They were about 3 mm. in size and surrounded by a clear “shell.” There was little effort to conceal the obvious joy felt by all at this discovery. These eggs were but a small sampling of the activity going on far below.

Lake Sturgeon breeding is a hit or miss affair. The males don’t participate until they are sexually mature at 15-20 years old, and the females wait until they are a bit older than that. Even when able, individuals don’t spawn every year. Males undergo the effort every 1-3 years and the gals space out their participation at 4 to 6 year intervals. This means that only a small portion of the population actually spawns in any given year.

When spawning does take place, the females – heavy with a full crop of up to 700,000 eggs – arrive over the gravel beds under the escort of several eager males. As the “hen” broadcasts her eggs over the site, the males vie for position in order to fertilize them as they exit her vent.  There’s a lot of bumping and grinding going on as the fish wrestle for position over the course of several days. It’s not an affair for faint-hearted fish! Once fertilized, the eggs become sticky and drop down into the gravel and eventually settle down into the small spaces between the rocks. Some of these eggs, of course, end up landing on the egg mats.

Those eggs that remain on the bottom are oxygenated by the stiff current and hatch within ten days. The emerging fry will burrow down  into the gravel and stay there for an additional ten days. Then, in a co-ordinated motion, they all ascend into the water stream and vanish!  The next time they show up in this part of the river they are two year olds. No one is sure where they go during the first few years of their life.

Before leaving the spawning site, we lowered an underwater camera over the gravel beds to take a look around. For the most part, all I saw was a bumpy view of a field of rocks  looking like a lunar landscape.  Suddenly, out of the murky depths, the camera revealed the shadowy form of a sturgeon over the bed. The fish darted off before any details could be seen, but there was no doubt that the fish were spawning as we drifted over the reef.

Between this sighting and the wonderful handful of eggs, I do believe I have seen the future of a magnificent fish. The future looks good.

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