The line appears in Robinson Jeffers's poem called "Salmon Fishing." At this point, near the end of the poem, the fishermen's catch of salmon have been pulled up out of the water and lay on rocks, twitching as they die, no longer able to

wander at will
the wild Pacific pasture nor wanton and spawning
race up into fresh water.

You note the alliteration. Another figure of speech or literary device used here is metaphor. A metaphor is a comparison not using the words like or as. In the line you cite, the Pacific ocean is compared to a pasture, a strip of grazing land.

Further, the word "wild" describing this "pasture" is an example of double entendre. The ocean is wild as in a wilderness area or the "wilds." Such an area can be very peace and tranquil, simply untamed by humans. But wild can also refer to violent behavior. In this case, the "wild" Pacific is both a wilderness and violent—the poem starts with a description of the south wind "shout[ing] to the rivers," which indicates that the weather is tempestuous or wild.

The line also includes assonance. This is the use of repeated vowel sounds to create a pleasing sense of rhythm. We can hear this in the repetition of the short "a" sound within "Pacific, "pasture," "wanton," and "spawning."

Finally this passage also uses imagery. Imagery is description using any of the fives senses of sight, sound, taste, touch, or smell. Here, Jeffers uses visual imagery. We can picture in our mind's eye the salmon in the wild sea.