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- "I think I made fish too hardcore" --Toady One
These terrible beasts challenge the mighty elephants for the post of King of Beasts. Their insidious nature and vicious strength provide them with the means to drag a fully grown dwarf into the aquatic depths, whereupon they proceed to gnaw at the struggling victim. And this is not all, for the awful carp can stare at the well-meaning fisherdwarf who for fear at these awful descendants of the Old Ones, are sent staggering back, too often straight off a cliff. To make things worse, they will taunt the hunters, who in hopes of achieving eternal glory, will unload whole quivers towards them, oblivious to nearby animals walking on land. Not all hope is lost, for the carp bears an Achilles heel, and will however die after the first winter if one is lucky enough to have a map which freezes. Otherwise, one may try draining all the rivers and lakes to air-drown these water-fiends (but be wary not to water-drown your dwarves in the process).
The carp's status as a cold-blooded dwarf killer is no doubt due to the fact that Toady One simply used a default bite attack for the carp (as with most of the new beings in v0.27.169.32a). The carp, half the size of a dwarf, has a bite attack which inflicts 1-6 damage points and can latch, allowing it to tear off chunks (and potentially entire limbs). Compared to the strength of a dwarven punch (1-2 damage points) this is a bit overkill.
The carp's propensity to attack creatures outside of its element and its ability to scare away dwarves it could never really reach is probably a bug. The same could be said about the hunters ability to target and attack them.
For the first two releases in which they existed (0.27.169.32a and 0.27.169.33a), carp (and other fish) also gained swimming skill (despite being innate swimmers) and corresponding attribute increases, making them significantly stronger, faster, and tougher than normal.
Additionally, dwarves who get attacked by carp tend to end up in the river, either from being pulled in, "dodging" into it, or trying to bull-rush the carp.
Note that while carp found in murky pools seem to fiercely defend their territory (believing themselves to be cornered), those that swim through rivers generally prefer to flee from dwarves rather than fight.
- "So i'm pretty new to dwarf fortress and something.... odd happened. As the migrants were passing a pool of water, about half of them were 'struck down'. Intrigued, I used  to look around, but the only thing in the pool was carp. What killed most of the migrants?" -- Forum User Warhammer651
If you are new to DF, then there is only one thing to do if you come across a river full of carp and you don't want your dwarves to die... Run. Run and never look back. But that's not very dwarvenly, and while others may understand, they'll still mock you. The game can be merciless, some dwarves will die; part of the fun is trying to stop it, if not always succeeding perfectly.
You need to construct a well anyway for tending the wounded - that will solve one small part of the problem.
You can channel out a canal and put protective grates over it, for both fishing and drinking, and designate your fishing/drinking zones there - that will help also. (Channel just the end area, and dig a tunnel to that desired area, and then channeling out the last tile next to the water source to flood it - that will prevent creating a long carp-friendly access river to that area. But better just don't fish and drink only from a well.)
If you have some carp-free water sources, you can use designations (traffic areas ( ) to simply discourage your Dwarves from using the dangerous waters. (Note that if carp are sighted in one part of a long stream, they are everywhere in it.)) then set
A further alternative would be to position your Marksdwarves nearby for some shooting practice. This, combined with the Traffic area solution, works exceedingly well in keeping your dwarves safe and controlling the problem, if not wiping out the Carp population altogether for the long-term safety of a Carp-infested map.
Another option is cheating by modifying the raw files. Carp are in Dwarf Fortress\raw\objects\creature_large_riverlake. In here you can find their damage and attacks and modify them, so that they become weak or benign creatures (or even more hardcore...). Or, if you feel like a cruel god, you can just delete them from existence altogether.
Just in case you don't believe us, here is a little hint:
[CREATURE:FISH_CARP] [NAME:carp:carp:carp] [TILE:224][COLOR:3:0:0] [LARGE_ROAMING] [AQUATIC][UNDERSWIM][IMMOBILE_LAND] [MODVALUE:3] [GENPOWER:3] [PREFSTRING:protruding mouths] [ALL_ACTIVE] [POPULATION_NUMBER:15:30] [CLUSTER_NUMBER:5:10] [BENIGN][MEANDERER][NATURAL] [PETVALUE:50] [BODY:BASIC_2PARTBODY:BASIC_HEAD:SIDE_FINS:DORSAL_FIN:TAIL:2EYES:HEART:GUTS:ORGANS:NECK:SPINE:BRAIN:MOUTH] [ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:1:6:GORE][ATTACKFLAG_CANLATCH] [ATTACK:SECOND:BYTOKEN:TAIL:slap:slaps:1:2:BLUDGEON][ATTACKFLAG_WITH] [SIZE:3] [MAXAGE:20:30] [NO_DRINK] [CHILD:1][CHILDNAME:carp fry:carp fry] [FAT:1] [BIOME:RIVER_TEMPERATE_FRESHWATER] [BIOME:RIVER_TROPICAL_FRESHWATER] [BIOME:LAKE_TEMPERATE_FRESHWATER] [BIOME:LAKE_TROPICAL_FRESHWATER] [STANDARD_FLESH] [HOMEOTHERM:10067] [SWIMS_INNATE][SWIM_SPEED:500] [MUNDANE]