By noticing how the fishing line slants in the water, the old man is able to tell what the fish he has hooked is doing.
When he is trying to hook a fish, the old man makes sure that his lines go "straight down into the dark of the water...he (keeps) them straighter than anyone (does) so that at each level in the darkness of the stream there would be a bait waiting exactly where he wish(es) it to be for any fish that (swims) there".
Once he has hooked a fish, the slant of the line continues to give the old man the information he needs to bring it in. Among other things, the slant of the line shows the depth at which the fish is swimming and gives a warning of in which direction the fish will be pulling the boat. By noting changes in the slant of the line, the old man can also tell when the fish is coming up to the surface, when and how widely he is circling, and when he is about to jump.
The great marlin that the old man catches is "two feet longer than the skiff". When he first jumps up out of the water he is
"bright in the sun and his head and back (are) dark purple and in the sun the stripes on his sides (show) wide and a light lavender. His sword (is) as long as a baseball bat and tapered like a rapier",
and his tail has the appearance of a "great scythe-blade".
The marlin's belly is silver, and he has "a great chest fin". In death, he turns "silver from his original purple and silver, and the stripes (show) the same pale violet colour as his tail...wider than a man's hand with his fingers spread and the fish's eye look(s) as detached as the mirrors in a periscope or as a saint in a procession".