Exploring kelp forest soundscapes using ad hoc frequency-based metrics

Abstract

The kelp forests off the southern California coast are alive with sound, yet their soundscapes remain poorly studied. The kelp forest soundscapes of two sites off La Jolla, CA were surveyed: one within a marine protected area, the other outside of the protected area. Four frequency-based metrics were created to explore how the sounds of the kelp forest vary throughout the day and through the seasons. Two of the metrics tracked frequencies associated with putative fish choruses (60–130 Hz and 300–500 Hz), one metric tracked the long-duration humming of plainfin midshipmen (tonal calls with a fundamental frequency at 85–95 Hz, and harmonics at 175–185 and 265–275 Hz), and the final metric tracked snapping shrimp snaps (2.5–7.5 kHz). The two metrics that tracked the fish choruses exhibited both diel and seasonal periodicity, with strong spectral peaks in the late spring and early summer coinciding with the presence of the choruses. Similar to these two metrics, the metric that tracked the humming of the midshipmen exhibited nightly spectral peaks in the late spring and early summer during their spawning season. The metric that tracked snapping shrimp exhibited strong spectral peaks during dawn and dusk hours yet had little seasonal variability.