Groundfish chasing the light for science

Counting fish isn’t easy – especially when counting live fish that are swimming around in the depths of the ocean. But since 2015, CCIRA has been doing just that by towing a video camera equipped with lasers behind a boat. Using data from this work, CCIRA scientists have published a paper in the journal Biological Conservation about correcting bias in fish surveys so that we can better understand how fisheries and environmental change impact groundfish stocks over time. But there is a hitch.

The laser beams project as dots on the seafloor or on fish. The known distance between the dots helps observers estimate the size of the fish and habitat features like boulders and corals. But, like a fishing lure, some fish species may confuse the laser dots for food and actively chase them which could bias the counts towards those species.

To overcome this potential source of error, CCIRA’s Science Coordinator, Alejandro Frid, CCIRA’s Fisheries Coordinator, Madeleine McGreer, and co-author Twyla Frid analyzed the video footage from those surveys. They determined that fish with relatively short lifespans—such as lingcod and kelp greenling, which are risk-takers that prioritize finding food and reproducing in the short-term—were most likely to follow the laser dots. Longer-lived species like yelloweye, which can live a century or more, take fewer risks for a short-term meal and were less likely to follow the dots. Instead, these fish prioritize staying alive to reproduce every year of their long lives.

Armed with this new knowledge, Alejandro and Madeleine developed a method to correct for this bias in fish counts. Applying this correction factor will strengthen future research efforts and lead to better conservation outcomes.

CCIRA’s growing body of scientific publications can be found on our Reports page.