Exemption under section 13(b)(4), like exemption under section 13(a)(5), depends upon the employment in the actual activities named in the section, and an employee performing a function which is not necessary to the actual conduct of a named activity, as explained in § 784.106, is not within the exemption. It is also essential to exemption that the operations named in section 13(b)(4) be performed on the forms of aquatic life specified in the section and not on other commodities a substantial part of which consists of materials or products other than the named aquatic products. Application of these principles has been considered generally in the earlier discussion, and further applications will be noted in the following sections and in the subsequent discussion of particular operations mentioned in the section 13(b)(4) exemption.