Fishmouthing looks like buckling but it is usually random on the roof. The front edge of the shingle is raised, and tapers back into the shingle. Although it does not usually affect the durability of the shingle, it should be addressed. Possible Causes:

• Moisture build-up in the attic can cause wetting & drying cycles in the shingles. Improving attic ventilation can prevent this.
• Installing wet shingles on a dry day, or dry shingles on a wet day will almost guarantee the appearance of fishmouthing.
• Nails that are 'popping' out can also cause fishmouthing. Simply correct the position of the nail.

This phenomena is mainly an aesthetic issue that can be repaired in most cases. The most common repair method would be to use hot melt adhesive to glue down the distorted shingle rendering it flat. To proceed, the sealant bond of the affected shingle should be broken first. These types of repairs are best carried out in mild (not too hot) weather conditions.

- http://www.dreamroofs.com/en/02/p_02_061.aspx?id=19 -