Canst thou fill his skin with barbed irons? - Referring to its thickness and impenetrability. A common method of taking fish is by the spear; but it is here said that the leviathan could not be caught in this manner. The common method of taking the crocodile now is by shooting him; see the notes at Job 41:1. Nothing is more remarkable in the crocodile than the thick and impenetrable skin with which it is covered; and the description here will agree better with this animal than with any other.
Or his head with fish spears - The word here rendered "fish-spears" (צלצל tselâtsal) means properly a "tinkling, clanging," as of metal or arms, and then any tinkling instrument. Here it evidently refers to some metal spear, or harpoon, and the name was given to the instrument on account of its clanging noise. The Septuagint renders this strangely, referring it to the "Phenicians," or merchants mentioned in the previous verse - "With their whole fleet they could not carry the first skin of his tail, nor his head in their fishing-barks."

Canst thou fill his skin with barbed irons? - This refers to some kind of harpoon work, similar to that employed in taking whales, and which they might use for some other kinds of animals; for the skin of the crocodile could not be pierced. Herrera says that he saw a crocodile defend itself against thirty men; and that they fired six balls at it without being able to wound it. It can only be wounded under his belly.

Canst thou fill his skin with barbed irons? or his head with fish spears? This seems not so well to agree with the whale; whose skin, and the several parts of his body, are to be pierced with harpoons and lances, such as fishermen use in taking whales; and their flesh to be cut in pieces with their knives: but better with the crocodile, whose skin is so hard, and so closely set with scales, that it is impenetrable; See Gill on Ezekiel 29:4. Or if the words are rendered, as by some, "wilt thou fill ships with his skin? and the fishermen's boat with his head" (n)? it makes also against the whale; for this is done continually, ships of different nations are loaded every year with its skin, flesh, and the bones of its head.
(n) Vid. Schultens in loc.

His hide is not penetrable, as that of fishes.

Fill - A whale's you may: but the skin of a crocodile is so hard that an iron or spear will not pierce it.