> The dose makes the poison.

Sure, but what is the effective dose for shrimp? As the article says: more research is needed.

I do recall a similar article about Prozac in American rivers, and there it was shown to be enough to affect the local wildlife.

[IANAB] Get the minimal dose for humans or rats and reduce it linearly to the weight of the shrimps. Perhaps it's better to use the concentration of cocaine in the human brain instead of the total dose divided by the total weigh. (Shrimps don't have bones, is that important?, IANAB)

The neurotransmisors are more similar in all animals than expected, so I guess a similar concentration in the brain is a good assumption. 1/1000 of the human concentration is probably safe for shrimps.

An alternative is to study it in the lab. Get a few shrimps and get them high. How hard can it be?

My guess is that this study is more useful for the police to get a general map of drug use than to see if shrimps are in danger. I guess that the shrimps concentrate the drugs, so it's easy to collect some water and filter the shrimps and put them in a blender, than collecting a bigger sample of water and concentrating it.

One you get down to parts per billion/trillion you can find just about everything in ordinary water.

According to some "medical" woo-mongers, that is enough to make it the strongest cocaine ever.

Give it a few years and the detection system will become commonplace and you'll read things like "the suspect's sushi was confirmed to contain opioids using a field testing kit" in police reports.

Well, there already have been cases where overly sensitive testing kits resulted in false positives because people had eaten food with poppy seeds