The human eye versus the GoPro — which is better for identifying open-ocean fish? 

New research evaluates both approaches for gauging the numbers and diversity of fish.

Research Need                        

Scientific sampling of open-ocean fish is challenging. These species live far from shore, with no refuge habitat, and they continually swim to escape predators, to capture their own prey, or to find more favorable environments. Studying their biology and population is costly and logistically complicated. 

Enter the fish aggregating (or aggregation) device. The “FAD” is a manufactured object, either floating or anchored to the sea floor, that fishers use to attract marlin, tuna, mahi-mahi (dolphinfish), and other open-ocean fish. 

Fish choose to swim close to floating objects near the surface of the water in the open ocean, likely for protection. Smaller fish inhabit these objects first, attracting a community of larger, predatory species over time. 

Scientists wondered whether FADs could serve as platforms to assess the status of fish species.

What did they study?

A team of researchers examined fish communities associated with two moored FADs just below the surface in the central Bahamas. The team hoped to learn how quickly fish colonize a new FAD and to understand when (or even if) an FAD reaches a plateau with regard to the fish using it. 

The team used two survey methods (video and visual surveys), so they could compare their relative strengths and weaknesses. Weekly, over the course of two years, a diver would scan 360 degrees for four minutes before deploying a GoPro on the FAD. Two hours later, the diver would retrieve the video and perform another four-minute, in-water scan. 

The team also noted wind speed and direction, cloud cover, and water temperature, to determine the factors affecting the amount and diversity of fish at FADs.

What did they find?

In total, researchers observed 48,965 fish from 27 species. Visual sightings and video surveys performed on the same day identified small prey fishes (mainly juvenile jacks, runners, and scads) close to 86% of the time.

Mathematical modeling showed that the length of time the FADs were in the water (their “age”), the season, and the survey method itself (visual vs. video) affected findings relating to the abundance of fish. 

Although the number of distinct species was higher in video surveys, the overall abundance of fish was higher in visual surveys, except for sharks. Bar jacks, almaco jacks, and scads, to name a few species, were more abundant in warm months, whereas unicorn filefish, wahoo, and mahi-mahi were more abundant during cold months.

Anything else?

While the scientists observed common open-ocean fish — including mahi-mahi, wahoo, and several species of oceanic sharks — tuna and all billfishes were notably absent from all surveys. There were numerous confirmed catches of white marlin, blue marlin, Atlantic sailfish, and yellowfin tuna in the study area. The study thus shows that subsurface FADs may attract a different fish assemblage than surface FADs. 

So what?

Fish appear to colonize FADs quickly and reach peak abundance and diversity (albeit seasonally cyclical) within the first several months. It appears FADs can be useful in standardized surveys for fisheries stock assessments, although these findings highlight the need to tailor the survey methods for specific objectives and targeted fisheries.

Reading

Schneider, E. V. C., Talwar, B. S., Killen, S. S., Russell, S., Van Leeuwen, T. E., & Bailey, D. M. (2023). Colonization, diversity, and seasonality of fishes at pelagic fish aggregating devices. Journal of Fish Biology, 1–12. https://doi.org/10.1111/jfb.15592

The Moore Charitable Foundation/Moore Bahamas Foundation funded this work.

BY SARA MIRABILIO

The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant.