Hi
With a torch light,and a kids scoopnet, after it get dark, as they are nocturnal.
Once you develope a eye for it, and/or fine a spot you go the same place every time.
In day light turn over small rocks and you fine them clinging to it.

I disagree with them being werry hardy, we use the lack of, or decreese in scuds, in known natural habitats,that you normaly would find them, as a early warning system, that some thing? is wrong.

Here some other info links: https://www.fba.org.uk/journals/index.php/FF/article/viewFile/286/188
http://www.pondlife.me.uk/crustaceans/gammarusshrimp.php
http://lhsfoss.org/fossweb/teachers/materials/plantanimal/gammarus.html
http://people.cst.cmich.edu/mcnau1as/zooplankton%20web/Gammarus/Gammarus.htm

PAUL