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Learning from fish that sing in the kelp forest

In 2022, Washington State gave highest priority to the protection and restoration of kelp forests in the Salish Sea. At the same time, state agencies acknowledged that apart from mapping the geographic extent of the kelp canopy, they lacked affordable methods for monitoring the ecological functioning of kelp forests. This project explored the use of “soundscapes,” i.e., monitoring of fishes and other soniferous marine organisms acoustically, as a means of reliably quantifying wildlife diversity and abundance beneath the kelp canopy. We hypothesized that fishes that either build and defend nests in rocky reefs beneath kelp canopies or migrate in schools would be likely to communicate acoustically and, based on studies conducted elsewhere, that they would likely produce low-frequency buzzing or high-decibel popping. Necessary first steps were to identify soniferous fishes associated with Salish Sea kelp forests; document their characteristic sounds; and demonstrate that existing machine-learning software could recognize and quantify these sounds in automated underwater recordings under real-world conditions.

To accomplish these objectives, targeted recordings were made of fishes in two public aquaria and in well-studied nearshore habitats of Lopez Island, WA. Aquarium recordings were found to be dominated by mechanical and anthropogenic noise in the target frequency range to an extent that confounded pattern-recognition software. In addition, our priority species (rockfishes, greenlings that nest under kelp canopies) were being kept in multispecies tanks and aquaria staff were unable to segregate them even for short periods of recording time. Field recordings were limited by weather and seasonal migrations of target fish species, and challenges were encountered with instrument availability and compatibility.

Despite these complications, useful recordings were successfully made and there is strong evidence suggesting the capture of sound production by copper rockfish in the aquarium and Pacific herring in the field. Copper rockfish, one of the most widespread and faithful residents of kelp forests, appear to emit a low-frequency “song” consisting of a what seems to be a species-specific distinctive pattern of humming vocalizations. Pacific herring, widely recognized as a keystone forage fish in Salish Sea ecosystems, likely can be detected acoustically, and the size of their schools estimated, by a distinctive popping or crackling sound that is probably produced, like the “farting” sound of Atlantic herring, by discharges from their swim bladder. These findings are being discussed with colleagues engaged in marine bioacoustics research and the assessment of marine noise in the Salish Sea.

With improved instrumentation, more extensive field recording efforts are underway to collect more examples of rockfish and Pacific herring sonifery, improve sound quality, and record additional kelp-associated species. With a more robust reference library to train sound-recognition software, we will be able to efficiently analyze almost 500 hours of existing hydrophone recordings in an extensive kelp forest along southern Lopez Island. This pilot project demonstrated the potential to develop a bioacoustics approach to monitor marine wildlife activity beneath the kelp canopy continuously, remotely and inexpensively, addressing a significant need for Puget Sound conservation and recovery.