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Fish: Small Mouth Yellow

Find, identify & discuss the marine species of SANParks
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onewithnature
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Fish: Small Mouth Yellow

Unread postby onewithnature » Thu Dec 25, 2008 5:38 am

I'm by no means an expert on fish, so hungry for information on the following. Not in a SANP, but driving recently over the Orange River bridge on the N1, stopped for a scan, and noticed a bunch of fish lined up parallel to each other, heads facing the onrushing current. The Orange River (is it now the Gariep, although the sign still said "Orange"?) is murky, so it was difficult to identify many markings on these aquatic denizens, but I would say half a metre in length, silvery-grey, and muted scale patterns.

For me, what was odd was that they were divided into two groups about 20 metres apart, each group comprising about ten individuals, and all hovering side by side, but JUST where the railing shadow of the bridge formed a horizontal line across the river. They all remained in that shadow, side by side, moving neither forwards nor backwards, the length of their bodies slightly shorter than the breadth of the shadow.
Any ideas as to why this would occur? Any information would be great! Thanks.
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Re: Fish in the Shadow

Unread postby adrianp » Mon Dec 29, 2008 9:51 am

I would have to guess, small mouth yellow fish.

Your options for fish of that size in the river system are:

Carp.... prefer slower water, and are fat and lazy.
Large mouth Yellowfish .... rare, teritorial predatory.
Mud fish ... bottom dwelling, unless when spawning, but a bit smaller than yellows or carp.
Small mouth yellowfish... prefer the faster moving waters, and tend to travel in shoals/pods of about ten to twenty fish. They often congregate at the base of structures/obstacles, when spawning, and are visible from the surface.

The preference for shadows may have something to do with visibility, either to avoid predation, but also, to increase the visibility of the food items that float downstream. Often bight sunlight will obscure the view of food items, especially in silted water.

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Re: Fish in the Shadow

Unread postby Johan van Rensburg » Tue Jan 06, 2009 4:06 pm

adrianp has a good theory going here and I'd say he is spot-on with the ID-guess.

Just as an aside... there must have been some lee in the current at the point where these fish were keeping (stationary). All fish species would look for an easy spot to hold station while waiting for food to be washed toward them, so preserving energy. Lees, washes and eddies are not easy to read for us, but the presence of batches of fish holding in a spot is indicative of such a place in a stream. If there are similar lees in full sunlight, then the lee in the shade would be more attractive and therefor hold more fish because of the comfort level (protection from direct sunlight) and camouflage provided (both from within the water and from above against prey items and attack from birds of prey, otters, etc).

Another possibility is that these fish may have been resting up between bouts of spawning (this is THAT time of the year). If that was the case, then the fish could be mudfish or mud mullet as well... or even largemouth yellowfish. But I still think Smallmouth yellowfish are the most plausible call in spite of the description onewithnature uses "silvery-grey". Generally smallmouth yellows tend to be deep yellow-green in clear water, but tend to become much lighter, more silver, in murky water. All the species mentioned above would fit that colour description in murky water.
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