The practice of specimen carp anglers to sack big carp is really a personal choice as some anglers do it and some don't. Some take a photo of the fish in any weather conditions, even in rain and at night, but some like to have a clear trophy photo of their catches sacking it while waiting for rainy weather to clear or light of day.
The practice of removing a carp from the warmer water to very cold air during winter nights can really be dangerous to the fish because its eyes can suddenly freeze up and the gills can get a huge shock from the cold. In this case it may be better to sack the carp in the safety of the landing net without removing the fish from the water. The sacked fish can then be removed from the net and the cord of the sack tied to a secure pole or bankstick in deep enough water, while the angler waits for warmer weather the following day to take the fish out of the water to weigh and secure a trophy photo.
The following information serves as guidelines to sack carp safely:
1. Only sack carp if it is really necessary.
2. Decide right at the beginning of your session, where the best place will be in your swim, to sack the carp, if the need arises.
3. Use a proper carp sack; preferably with large enough holes/mesh so that water can pass through it easily. A sack that is made of too fine mesh material can actually hinder the carp's gills from opening and closing freely as it prevents water flowing through the material easily, making it hard for the carp to "breath".
4. Wet the carp sack before placing the fish in it. Make sure the sack is big enough to hold the carp and that the sack has a long properly secured cord, long enough so that the carp can reach deeper water when needed.
5. Make sure you have a VERY secure pole / bankstick etc. in the water to secure the carp sack to. If the sack comes loose during the night, its a dead carp for sure. Its good practice to add a marker float to your carp sack in case it comes off the securing line. Then at least you can see where it's swimming around and you can recover it again.
6. Ensure when lifting the carp up when in the carp sack, that its fins are tucked into its flanks/body.
7. If the water contains otters, other furry inhabitants, has a crab problem, etc. dont sack the fish as it will surely be in danger. I've seen crabs make holes in a sack.
8. Always sack a carp in as deep as possible water. Do NOT sack a carp in shallow water, especially in the summer when the sun is out and the shallower water is normally low in oxygen.
9. Do not sack a carp overnight in weedy areas. During the day most types of weed / weed beds release oxygen (photosynthesis), but the opposite happens at night. A carp can actually "drown" if there is too little oxygen.
10. Always keep checking on the sacked carp to ensure its ok, upright, etc. If the carp is tilted onto its side, there is a problem! You need to take the carp out and nurse it back to full strength!
11. When lifting the carp sack out of the water for photos etc, make sure the carp is held securely, fins tucked in. The carp will most probably be very lively and you dont want it to flip out of your hands and fall onto the hard ground.
12. After the session it is necessary to wash the carp sack in an antibacterial solution to curb the spreading of diseases from one fish / lake to another. It should also be done with your weigh sling and unhooking mat.
Please keep in mind that it is still safer for the fish to be released as soon as possible after capture rather than to be sacked. Enjoy your big carp fishing!