Naturespeak A naturalist's view of the world

February 16, 2011

Something Fishy in the Detroit

Filed under: Uncategorized — wykes @ 11:35 am

Fish don’t often make the news, but when they do it usually involves lots of them – dying in droves and stinking up the place. Unfortunately, more often than not, this type of calamity is due to some form of accidental chemical or thermal discharge. In other words, it is usually our fault. Recent reports of massive Gizzard Shad die-offs in the Detroit River and the shallow waters of western Lake Erie, however, have a totally different spin to them. The silvery fish are showing up and dying in biblical quantities. Earlier in the winter a massive influx of the fish clogged the intake gates at the Monroe Power plant and their mangled bodies mucked up the pumps.  There were so many fish that their collective weight actually bent the iron bars supporting the grid.  Were this one of the ancient plagues, the Pharaoh would definitely have considered releasing Moses’ people!  Large windrows of the fish are showing up where the ice has retreated from the shore. But none of this is our fault – nor can we blame it on global warming, polar bears, or an angry God. We can pin most of the blame on the fish themselves.

Gizzard Shads, you see, have the spineless constitution of gelatin on a hot August day, although it is the cold January days that do them in. They are native North American fish, but are nevertheless highly susceptible to cold and oxygen deprivation. Harsh winters (like the current one) create a solid ice cover which, in turn, leads to a lowering of the dissolved oxygen in the water. This in turn leads to lots of dead gizzard shad.

This year, there are a few additional factors that make it an exceptionally bad shad (hey, that rhymes!) season. A few years ago, it was a good shad year (a super shad season) and lots of little shads were born. These little fellows are now coming of age and clogging up the river space. There is so much shad sushi out there that nearly every living creature, including traditional vegetarians such as Mute Swans, Canvasback Ducks, and Canada geese are eating them. Some of these birds are even fighting each other over the fish! Add to this a very cold winter and the apparent spread of a fish virus called VHS, and you have the makings of a great shad smackdown.

Usually it is the smaller individuals that take the brunt of these natural evils. Most of the dead fish are less than 6 inches long. Gizzard Shad, however, can get quite large – attaining a length of 20 inches or more. Because they are such odd looking fish, and currently news-worthy, I thought this would be a great opportunity to show you one of the big ones that recently came into my possession.

What one sees upon first gazing at a big dead Gizzard Shad is platter shaped silver fish with a big eye and small head (see above). What one smells I will leave to your imagination. The mouth is quite small for a fish of this size (this one was about 18 inches long) because they are filter feeders. They have no teeth per se.  Instead, they simply take in microscopic plankton and filter it through a set of comb-like gill rakers. The “gizzard” part of their name comes from their muscled stomach which functions much like a bird gizzard to crush and grind the collected mass of micro-life.  They are members of the herring family – a group which includes herrings, shads, sardines, menhaden, and anchovies – but are they are unique in possessing this gizzard-like stomach.

Like their cousins, however, they have a ridge of saw-like scales along the belly (see above) and they lack a lateral line -that row of vibration sensitive openings usually spanning the length of most fish.  This is somewhat hard to see on the specimen, but easier to see (or, in this case, not to see) on my drawing of it (at beginning). Unlike their relations, they are not considered tasty by humankind (although I am not personally a fan of anchovies – nor they of me).

Externally, the Gizzard Shad has one more claim to fish design fame. The last ray on their dorsal, or top, fin is very long (see above). It extends several inches beyond the trailing edge. Why? Well, there doesn’t appear to be any “reason” other than looks. Even a glass-eyed shad needs some fashion flare in order to look good when their pictures start appearing in the morning news.

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