In which many slugs commit suicide
I can't think of any other plausible ways in which the slugs would be ending up in my pond, dead, other than having deliberately slithered in there themselves and drowned through failing to get out again. Slugs don't have much in the way of predators - and if they did, those predators wouldn't normally be dunking them in the pond and then abandoning them. It seems very unlikely that the neighbours have been chucking (pre-killed?) slugs into my pond. But the current rate of dead slug removal from the pond is running at about 3 per day and holding steady for the past week or so.