From the paper: "The traps were baited with ~100 g of mackerel that was enclosed in a mesh bag to allow the development of an odour plume, but prevent the amphipods consuming any of the bait that might otherwise affect POP levels in downstream assays."

So even if the mackerel did contain the pollutants, they weren't transferred to the amphipods.

Thanks for this. I thought it was unlikely they hadn't controlled for pollution in the bait, but simple things sometimes slip through.