The sample should be placed in a plastic bag with no water. Tape the cooler shut. Samples need to arrive at the lab no longer than 18 to 24 hours after collection. Fish can be frozen for transport, but only as a last resort. Little can be determined from a fish that has been frozen.
Frozen samples are acceptable for pesticide analysis.
Be sure to contact the diagnostic laboratory prior to shipping. Relate all information needed along with the estimated time of sample arrival.