Image of a large red fish underwater.

Listening in on fishing vessels

Source: Boyle, K. S., Hightower, C. L., Nelson, T. R., & Powers, S. P. (2022). Use of passive acoustic monitoring to estimate fishing effort on artificial reefs in Alabama during the recreational red snapper fishing season. Fisheries Research, 249 (December 2021), 106262.

Sounding off on overfishing

As fishing technology has improved and fishing intensity increased in the last several decades, stocks of large healthy adult fishes have been depleted, leading to widespread fisheries collapses – a problem referred to as overfishing. The fisheries of the Gulf Coast of the United States are no exception. Like many others, the red snapper fishery collapsed entirely in the late 1980s and has only recently begun to recover under strict management efforts.

State-level management efforts for the recreational red snapper fishery in Alabama include strict catch limits and a limited fishing season, including “open” and “closed” days. Catch limits set upper bounds on the number of fish caught by each person, while “closed” days mean that no red snapper fishing is allowed, giving the population time to recover. Additionally, artificial reefs (man-made structures that serve as reefs) made of bridges and pipes have been constructed offshore to give snapper a place find shelter and reproduce. While some of these reefs have been advertised on Alabama’s state website for recreational fishers, others remain “unpublished,” with the goal of reducing fishing in these areas. But, you may ask, how can we tell if these efforts are actually working? Researchers from the University of New Orleans and George Mason University had an idea- harnessing the sound made by boats to determine when and where fishing is occurring.


Casting for boat noise

Current methods of monitoring fishing effort include photos and videos of boat launches and fishing sites, as well as in-person surveys. While these methods are certainly useful, they are limited to daylight hours and a small spatial area. Passive acoustic monitoring (PAM), in which an area is monitored through one or more sound recorders, can operate for long stretches of time and at all hours of the day. Additionally, acoustic methods are well-suited for monitoring of marine vessels, whose motors produce loud, distinctive signals. This is especially true for vessels fishing on offshore reefs, which maintain their position by switching in and out of gear, creating a series of intermittent boat noises.

The researchers took advantage of this signal by applying a custom-designed signal detector to over 2600 hours of recordings taken in the summers of 2017 and 2018. Automated approaches are necessary for analyzing large datasets such as these; sorting through months of continuous recordings by hand is incredibly time-consuming! This detector identified places in the recordings where the amplitude (volume) of the sound dropped to less than half of the original amplitude in the span of 0.2 seconds corresponding to sounds a fishing vessel would make. Researchers then randomly verified a set of those instances identified by the detector.

The recordings themselves were taken by hydrophones (underwater microphones) that were set up about 30 meters below the surface on both published and unpublished reefs. In 2017, the recordings were continuous, while in 2018, the hydrophones were programmed to record for one minute out of every five.

Image of a hydrophone on the seafloor surrounded by schools of fish. The hydrophone is a cylindrical instrument that sits vertically on a wide, X-shaped base.
A hydrophone deployed on the seafloor (image from NOAA National Ocean

Catching a signal

By examining the proportion of recordings containing at least one vessel signal, the researchers detected patterns of boat presence across time and space. Boats were recorded during the daytime with the highest levels of activity around mid-morning and mid-afternoon, consistent with recreational vessels fishing in daylight with breaks for lunch or movement around noon. The number of detections per day decreased as the fishing season progressed, possibly because of high enthusiasm at the beginning of the season or declining number of fish throughout the summer.

Boat detections were higher on “open” days than “closed” days, as the researchers expected. However, as they broke down the data further, some interesting patterns emerged. For 2017, there was strong overlap in vessel detections between “open” and “closed” days. These patterns could be from illegal recreational fishing but are more likely explained by the presence of private charters and anglers targeting other species, both of which were permitted to operate at these times

Angling towards the future

Great news: detecting vessels on reefs using PAM worked! Although the signal detector used in this study occasionally identified boat noises where there weren’t any (mostly caused by small invertebrates bumping against the hydrophone), screening and filtering eliminated most of these effects. The authors have even suggested that similar data could be used to identify the types of motors detected, which may clarify who is utilizing the reefs at various times. While there are metrics that cannot be measured through acoustic means, such as number of fish caught, PAM is a valuable addition to the toolbox of fisheries management methods.


Cover image: Red snapper, NOAA fisheries

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