Hard science takes a backseat to poetic meditation on the known and unknowable on the “Nature” (7 p.m., PBS) presentation of “The Mystery of Eels.” The documentary is narrated by artist and author James Prosek (“Trout: An Illustrated History”), whose fascination with eels grew after visiting Ray Turner, the proprietor of Delaware Delicacies near Hancock, N.Y.

Turner keeps an eel weir, an ancient device for trapping the snakelike fish. Weirs consist of two stone walls that form a V in the river. A wooden box located at the point of the V catches eels as they migrate back down the river on the way to the Sargasso Sea in the North Atlantic.

While Turner and the other weir keepers employ a “technology” at least hundreds of years old, the market for eels is as fluid and lucrative as today’s stock market. Eels, considered a delicacy in Asia, are big business. Recently, Chinese companies have developed facilities where young fish known as “glass eels” are raised to adulthood. Prosek visits fishermen and women in Maine, where catching glass eels for the Chinese has made some millionaires.

In Japan, Prosek meets with scientists spending millions on ways to breed eels artificially. So far, it has been in vain. Few know exactly where eels do their spawning, or why they swim inland to fresh water rivers only to return to the sea as adults. Their mysterious nature has made eels figures of legend and worship among New Zealand’s Maori tribe members.

Prosek is not a professional narrator, and at times I’d wished he had hired one. All the same, he has made a beautiful little movie about an unusual subject that some find disturbing, if not repellent. I highly recommended it.

Tonight’s other highlights:

• Rollins’ sister shows evidence of abuse on “Law & Order: Special Victims Unit” (8 p.m., NBC).

• Evolution proceeds on hour two of “Australia’s First 4 Billion Years” (8 p.m., PBS).

• Jenn’s twin needs help on the season finale of “Boston’s Finest” (8 p.m., TNT).

• Safecracking on “Elementary” (9 p.m., CBS).

• The silent treatment on “Chicago Fire” (9 p.m., NBC).

• Ryman calls on “Nashville” (9 p.m., ABC).

• John barely copes on the season finale of “Southland” (9 p.m., TNT).

• Events back in the U.S.S.R. rock Elizabeth’s world on “The Americans” (9 p.m., FX).