Despite the threat of overfishing in many fish populations, fisheries managers often lack accurate plans and quota systems to allow a sustainable harvest.

Traditional methods of estimating fish populations can be costly, intrusive and laborious, so a team of researchers tested the use of acoustics to survey the Gulf corvina fish.

The researchers found that deploying passive acoustic detection devices near spawning aggregations was a cost-effective and easy-to-use method of estimating abundance through data collected on the fishes’ vocalizations.

Just as humans and many terrestrial animals do, marine creatures use sound for communication in breeding, feeding, social structure maintenance and survival.

However, the sounds produced by marine species have also put them at risk of capture by commercial fisheries.

Scientists have recognized over 100 families of marine fish that produce sound. Many fish species produce species-specific calls while mating, and the populations aggregate during spawning events. Aggregations enhance the sound’s volume. Fishers use listening devices to detect the sounds produced by the masses of fish to pinpoint the fishes’ location, making it easier to catch large numbers.

Doctoral student at the Scripps Institution of Oceanography Timothy Rowell and his colleagues within the Marine Biology Research division published a study in June 2017 in which they adapted the way that the sounds were used. Rather than exploiting the population by catching the aggregated fish, they located the fish spawning aggregations (FSAs) to measure the abundance for conversation purposes.

“As a group, we feel that researchers and managers need to greatly improve our understanding of the complex life histories of fishes and develop better, more efficient ways to monitor their presence and abundances,” Rowell said in an interview with Mongabay-Wildtech.

The team studied the Gulf corvina (Cynoscion othonopterus) fishery in the Colorado River Delta in the northern region of the Gulf of California, which harvests the species during its spawning aggregations.

According to Rowell, the specialized sounds produced by corvinas aided their exploitation by fishers, who locate the spawning events prior to deploying their nets.

“So we said, let’s do that too; let’s listen for the fish and try to map their reproductive habitat and estimate their abundances,” Rowell said. “We sought to see if louder regions actually have more fish and if there was a way to quantitatively estimate their abundances from just sound levels.”

Pros and cons of measuring abundance using acoustics

To manage stocks, fisheries managers establish quota systems according to independent data that estimate stock abundance and catch levels for the spawning season. Some of the fisheries’ data collection methods, such as mark-recapture, visual census and trawls, can be ineffective as well as costly, invasive and laborious.

It is difficult to accurately measure fish abundance and set quotas for entire populations for several reasons. FSAs minimize several complications in measuring the abundance of fish because they happen at predictable times and locations. The researchers measured the abundance of corvina in the FSAs and developed a model to quantify relationships between the spawning sound levels and fish densities.

Using acoustic devices can benefit both the fisheries management and the fish because they are noninvasive yet can survey fish in both shallow and deep waters with a range of visibilities.

Active acoustics, which the researchers measured using an echosounder, are essentially sonar beams that, when emitted from boats, bounce back after encountering an object. Larger fisheries often already use and trust active acoustic methods for collecting data on fish stocks. Smaller-scale fisheries have not yet picked up the method because of the cost and complexity of the collecting and processing data.

The sound produced by FSAs also allows fishers to use passive acoustics to identify the timing and location of spawning. Passive acoustic devices listen for sounds underwater at specific frequencies, and the researchers used one type, a hydrophone or underwater microphone, to test whether fisheries could use passive acoustics to measure sound production and infer the fish abundance.

“The use of passive acoustics to survey fish populations would be an easily transferrable method to pass onto local resource managers for long-term use, allowing them to estimate fish numbers in near-real time,” Rowell said.

The research team hypothesized that comparing measures of fish density and sound levels would result in a linear increase (i.e., larger numbers of fish produce greater sound energy) and that a strong relationship exists between the two.

Do more fish vocalizations reflect more fish?

Generally, assessing the corvina specifically using fisheries-independent methods presents its own problems when using fisheries-independent methods because these fish tend to aggregate in shallow waters with low visibility and extensive tidal ranges. These conditions make visual surveys difficult to complete and the more intrusive methods, such as trawling and mark-recapture, more appealing to fisheries management.

According to the researchers, however, Gulf corvinas were the ideal fish species to study because they are croaker fish, which produce unique sounds during FSAs. Their species-specific sounds simplify the process of developing and testing relationships between the calls detected and the fish density measurements.

To better estimate corvina abundance, the researchers looked to establish acoustic methods that are more reliable and widely available. Rates of harvesting the species are high, even though stock sizes are relatively unknown and the management plan and quota system underdeveloped.

The researchers focused their studies on the two days of peak spawning during FSA events, which take place two to three days before new and full moons. During these times, the team surveyed corvina populations using both active and passive acoustics twice a day, on the outgoing and incoming tides.

Using active acoustics, the scientists conducted eight surveys to measure the density, length, abundance, biomass and spatial distribution of the fish during FSAs. The team used an echosounder that emits an acoustic beam into the water and bounces back echoed signals when the beam encounters objects.

They recorded the ambient sound at several specific locations along each 300-meter transect and identified corvina sounds while removing environmental “noise” by visually examining the resulting echogram images.

They used passive acoustics to measure the spatial distribution of the sound levels attributed to the fish sound production. A separate boat collected the data measurements using a portable hydrophone, an underwater microphone that records all the sounds in the sample area. The researchers then compared the results of passive sound collection with those of the active acoustics to model the relationship between measurements of density and sound production.

Toward a more sustainable corvina harvest

The croaker fisheries bring in tens of billions of U.S. dollars every year, in part because they are relatively easy for fishers to detect. However, overfishing threatens these revenue-generating fish.

“Unfortunately, it has come apparent that a large proportion of fish populations are being harvested at unsustainable levels across the world,” Rowell said, “Thus, it is the responsibility of fisheries researchers and managers to gather more information about the health of fish populations and adjust current practices.”

The team conducted active and passive acoustic surveys to measure the abundance of the Gulf corvina, and the results showed that both can offer information to the fishery managers.

“…This improved knowledge and addition of multiple assessment methods will facilitate better informed management decisions that protect fish populations and the livelihoods of fishers by establishing sustainable harvest levels,” Rowell said.

In the study, the active acoustic methods could accurately and non-invasively measure fish density, abundance and biomass. When measuring sound levels of the FSAs using passive acoustics, the research team found that their model accurately predicted fish density, demonstrating that the passive acoustics can provide the fisheries with a new method to independently assess FSAs.

Active acoustic monitoring is a more costly and complex method for estimating fish abundance. This research suggests that cost-effective passive acoustics could be employed by small-scale fisheries to assess entire spawning grounds over a longer period of time, across several FSAs.

“The continuation, improvement, and expansion of these methods across multiple species will validate passive acoustics as a frontier tool for the management and conservation of fish populations,” the study said.

The research team is using their results to help the Gulf corvina fishery by sharing their methods with the communities that work with the corvina and holding training sessions on how to use the technology and apply it to other species.

Banner image is of a fisherman tossing a Gulf corvina fish across his boat. Photo credit: the Gulf of California Marine Program.

Citations

Radford, A.N., Kerridge, E., and Simpson, S.D. (2014). Acoustic communication in a noisy world: can fish compete with anthropogenic noise? Behavioral Ecology, 25 (5), 1022–1030.

Rowell, T.J., Demer, D.A., Aburto-Oropeza, O, Cota-Nieto, J.J., Hyde, J.R., and Erisman, B.E. (2016). Estimating fish abundance at spawning aggregations from courtship sound levels. Scientific Reports, 7 (3340).