Didymo and Missouri Trout Streams

[Brian Wise]

Below is a pretty broken up email that I received from AJ Pratt our fisheries biologist on the North Fork of the White.

>>> John Ackerson 08/10/07 11:11 AM >>>

Biologists,

FYI from the US Forest Service on didymo. We better watch out for this

in our rivers.

John

Didymo, a.k.a. "rock snot" is formally known as Didymo geminata - a

diatom

which is a type of one-celled, microscopic algae. The nearest

occurrences

to Missouri are in four nothern Arkansan lakes. Here is a map of its

current distribution:

http://www.epa.gov/region8/water/didymosph.../na_dis.map.pdf

How it gets started

It is primarily transported via fishing gear such as boats, canoes,

kayaks,

waders (especially felt-soled), trailers, bait buckets, life jackets,

etc.

Diving gear is also problematic.

Since it is one-celled, this microscopic algae can be spread very

easily -

even in a single drop of water. Furthermore, Didymo can remain viable

for

several weeks if kept moist.

Didymo is generally a northern circumpolar species, where it is found

in

colder, low nutrient, high clarity streams (similar to the Eleven Point

and

Current Rivers here in Missouri). However, it is now being found in

warmer

waters with less clarity.

Impacts

Didymo is not a human health risk, though people in contact with water

containing D. geminata might notice itchy eyes or irritated skin from

the

silica in the algal cells.

Since one picture is worth a thousand words, here is a picture of a

Didymo

infestation in the Mararoa river in New Zealand (go to bottom of the

website page): http://www.geocities.com/ken_sims_98/nzffa/didymo.html

The main adverse effects come from the organism*s ability to smother

the

riverbed, thus changing the bottom appearance, structure, and food web

of a

the stream. Didymo forms dense mats, which can cover up to 100% of

the

available bed to a depth of 8 inches or more. These mats can kill

aquatic

insect life and essentially starve out fish populations in the area.

In

addition, ecologically common macroinvertebrates (such as crayfish)

found

on the bottoms of well-oxygenated streams will change to more worm-like

and

snail populated communities.

Such reductions of available and preferred food have seen trout

populations

severely reduced or disappear altogether in some overseas streams. In

some

parts of the western USA, fisheries have declined by 90% in 2003 and

2004.

There have also been reports of trout and salmon parr absent from

traditional rearing areas due to gill irritation or clogging, or to

the

changes in food availability.

Didymo is also suspected to be the causal agent in weight loss of trout

(

http://www.hoaff.org/newsletters/NL2007-04.htm) in Arkansas. The

relative

weight of brown trout below Bull Shoals dam in November 2006 was the

lowest

that has ever been recorded. Relative weight for both brown and

rainbow

trout have been falling in Bull Shoals tailwater since 2004, which

coincides with the appearance of Didymo. This decrease in relative

weight

has been documented in other trout waters around the world where didymo

has

occurred. It is believed that blooms of didymo in upper Bull Shoals

tailwater have had the predicted detrimental effect on trout forage

and

trout foraging. Didymo carpets the bottom of the river and makes it

impossible for native macro invertebrates to colonize or survive.

Further,

trout cannot forage efficiently in didymo and get no nutritional value

from

ingesting it.

There have also been reports overseas of the dense mats blocking water

intakes, irrigation intakes, and the intake screens on hydroelectric

dams.

Prevention/Control

Effective prevention and control techniques should include the

cleaning

(check-clean-dry) of any item that is transported from one water body

to

another. For example, i

n 2004 didymo was discovered in New Zealand,

the

first time it was found in the southern hemisphere. To restrict its

spread,

the whole of the South Island of New Zealand was declared a controlled

area

in December 2005. All items, such as boats, fishing gear, clothing,

and

vehicles, that have been in a stream, river or lake, must be cleaned

before

they enter another waterway.

Recent experiments have identified a copper-based compound that may be

an

effective and safe weapon against the invasive alga didymo, although

further tests are needed to determine its effectiveness in rivers (

http://www.niwascience.co.nz/ncabb/abb/2006-19/copper).

Here is a good synopsis of Didymo:

http://www.des.state.nh.us/wmb/exoticspeci...didymo_faqs.htm

Here is the US EPA website information for Didymo:

http://www.epa.gov/region8/water/didymosphenia/

David Moore

Forest Ecologist (Plants)

Mark Twain National Forest

401 Fairgrounds Road, Rolla, MO 65401

ph (573) 341-7457; fax (573) 364-6844