Saturday, January 20, 2018

Altered Stream Flow

Sand Shiner. A small fish found in muddy badlands streams. This is from Horse Creek in western McKenzie County, ND. Photo taken 8/11/15.
A few decades ago, there was some debate about the number of words for snow used by inhabitants of the Arctic regions. Some said that Eskimos had 100 words for snow. This would not be a surprise, were it to be true. In English, there are many words for frozen precipitation: snow, sleet, hail, rime. There are three kinds of lava: pahoehoe, aa, and pillow. There are numerous kinds of wetlands: swamp, bog, fen, marsh, shrub-carr, peatland. Our natural world becomes subdivided. Sometimes this can be exploited. At one time there were just a few types of chocolates available, now there are thousands. Almond chocolate, sea-salt chocolate, orange chocolate, mushroom chocolate, beef-flavored chocolate, chocolate McLiver and so forth. The point is, once a person studies something for a few years, that person begins to see distinctions between what had at one time been considered identical objects. This is what perception does to us.

It is the same with fish: They can be salt-water, fresh water, benthic, pelagic. Benthic: at the lowest level of a body of water. Pelagic: at the upper layers of a body of water. Their eggs have distinctions, too. They can be buoyant or can sink. Their buoyancy varies based on "egg specific gravities that tune the egg buoyancy to create specific vertical distributions for each local population" (Sundby 2015). What that means is, the eggs are designed to linger in a water level that is optimum for their development. For some, that's suspended at a particular level in the water column, others are at or near the bottom. Some eggs are adhesive, sticking to objects in the water, often in the benthic zone. Some eggs come in masses and others are single. So we have variables of adhesiveness, buoyancy, and mass. There are other variations, like fish who lay their eggs on beaches or mud, fish that build nests, fish that are mouth brooders and this is quickly getting out of hand and we are running out of words.

This is the story: Around the world, many species of fish are disappearing. Some of these are from the mid-continental rivers and streams in North America. Here is a list of six imperiled fish species from these waters:

Rio Grande Silvery Minnow (Hybognathus amarus) – Endangered
Bluntnose Shiner (Notropis simus) – Endangered
Arkansas River Shiner (Notropis girardi) – Threatened
Rio Grande Shiner (Notropis jemzanus) – near Threatened 
Plains Minnow (Hybognathus placitus) – Presently numerous but in serious decline
Speckled Chub (Macrhybopsis aestivalis) – Substantial long-term decline

There are other species in the same waterways that are not currently imperiled. Why is this?

What the six imperiled species have in common is each one of them produces non-adhesive, semi-buoyant eggs. They are members of what is called a "semibuoyant egg reproductive guild." The buoyancy is sufficient that the egg remains suspended in turbulent or flowing waters of rivers. It is in this buoyant, pelagic state that the egg develops to maturity. Were the eggs of these species to sink to the bottom, they would settle into sediment and die. This is to say that the eggs are dependent upon a river current for success. How much river current?

The eggs take 1-2 days to hatch and the newly hatched larvae must be in suspension for several more days where they "swim up" and develop their gas bladder and absorb their yolk sac. Thus, as they are developing, the eggs and larvae depend upon a current that can keep them in suspension for 3-5 days. Being carried for 3-5 days in a current will transport the eggs a distance downstream that depends upon the flow rate of the river. A conservative drift rate of 3 km/hour would transport the eggs "72-144 km before hatching. Developing protolarvae could be transported an additional 216 km during the swim-up stage" (Platania 1998). Thus, the eggs and larvae would need a total of 288-360 km or 179-224 miles of contiguous moving water.

Therein lies the answer. When dams were placed along the rivers that these species inhabited- Upper Pecos, Middle Pecos, Middle Rio Grande, Arkansas - they often broke the length of free-flowing river into segments less than 179 miles. Long stretches of the river are now behind impoundments and there, the water is nearly still. Any semi-buoyant, non-adhesive eggs that enter the impoundment will stall out, sink down through the pelagic zone to the benthic zone, settle into the sediment, and die. Other alterations in natural stream flow patterns have also thwarted the reproductive success of these species. It is no surprise then, that fish with non-buoyant and adhesive eggs, those that are successful in the benthic zone, thrive.

We are looking for a word that describes the alteration of natural habitats with semi-permanent and non-responsible schemes that randomly endanger species in an offhand way.

1 comment:

Steven Spickerman said...

Nice writing Dave. Miss bumping around the woods with you. Steven S.