A marine research probe is anchored to the seabed by a steel tether. Wind-induced currents cause transverse stationary waves to form on the tether. Initially, the tension in the tether is $T$, and the fundamental frequency (first harmonic) of these oscillations is $f_0$.

To stabilize the probe against rising waves, the winch system increases the tension in the tether to $25T$ without altering the length or mass of the tether.

Which harmonic of the tether has a frequency of $15f_0$ after this adjustment?