First the science!
The surface temperature of a dry ice pellet when unpacked and placed in a room at ambient temperature will be -78.6 deg C. This surface temperature will be maintained as the pellet sublimes and cools its surroundings. As soon as the pellet has sublimed to nothing, the cold area it created will rapidly return to the room conditions.
OK, so knowing this, how can I use dry ice it to keep something cold while I transport it ?
If you take a batch of frozen fish from a freezer at -25 deg C, place it in an insulated container and cover it with dry ice pellets they will try to cool the fish they are in contact with to -78.6 deg C. Whether the fish will ever get this cold, or even remain frozen over a period of time, will depend on many factors:
The weight of fish in the container.
The weight of dry ice added.
The insulating properties of the box.
The configuration of dry ice and fish in the container.
The external conditions around the box.
By controlling all these factors it is possible to configure a container of frozen fish packed with dry ice to reach a destination still frozen within a fixed period of time, but although some formulas can be found to predict how much ice is needed for a specific task it is always best to make a test to be sure before sending large valuable consignments in this way.
Point to watch are:
Do not have the dry ice in contact with the produce.
Keep the dry ice on top of the produce with a small airspace between e.g bubble wrap.
Allow for some circulation around the produce.
Add more ice in warm weather, and always err on the plus side.
If in doubt call 0116 224 0072 for guidance.