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"Some people talk to animals. Not many listen though. That's the problem."

-A.A. Milne

Identifying Atlantic cod spawning grounds using novel passive acoustic research approaches

Atlantic Cod

Cod (Photo credit: Dieter Craasmann)

The passive acoustics group at the Northeast Fisheries Science Center is studying the acoustic behavior of fishes in Massachusetts Bay. Primary efforts have focused on two species of gadoid fish: the Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), with particular focus on Atlantic cod during the spawning seasons.

Why study cod acoustics?

Cod spectrogram and waveform

(a) Spectrogram and (b) waveform of an
Atlantic cod

Atlantic cod is a demersal predatory fish that is important ecologically, economically, and culturally in the Northeast. It has been targeted by both commercial and recreational fisheries for centuries, resulting in a major crash in the North Atlantic in the late 1980s/early 1990s. Since Atlantic cod stocks are still recovering, there has been much research into the genetic structure of cod stocks and their movements based on tagging studies. Research into the reproductive behavior of cod is also of increasing importance, since understanding this portion of the cod life cycle could provide valuable information that could improve management of cod stocks.

While the reproductive behavior of cod has been studied for several decades in captive studies, there have been few studies that looked at these behaviors in the wild. During the spawning season, male cod produce low frequency sounds called grunts (broad band signals ranging between 50 and 200 Hz). These are thought to function either as a courtship display to females or an aggressive display to competitors. By understanding these spawning behaviors, we can listen in on where and when these events are occurring. As a result, passive acoustic monitoring offers a novel perspective from which to investigate the occurrence, spatial extent, and duration of spawning cod aggregations. Our aim is to provide information (and integrate it with active acoustic work on cod in this region) that will help put in place, time, and space closures relevant to this species in order to aid their recovery.

What we have done so far?

In a collaborative effort between the passive acoustics group at NEFSC, the Stellwagen Bank National Marine Sanctuary (SBNMS), the Massachusetts Division of Marine Fisheries (DMF) and Cornell University's Bioacoustic Research Program (BRP), a pilot study was conducted by placing a recording unit in the Spring Cod Conservation Zone (SCCZ), a seasonal conservation zone where a spawning aggregation of cod was known to occur during the spring. Cod were recorded on 98% of recording days from the deployment, although the spawning season continued for at least another month after the MARU was retrieved (based on a DMF tagging study). Despite producing sound throughout the deployment, a daily pattern of calling was apparent in the data: cod tended to grunt more often during the day than during the night or twilight hours. The results of this analysis have been published as: Hernandez, K. M., Risch, D., Cholewiak, D. M., Dean, M. J., Hatch, L. T., Hoffman, W. S., Rice, A. N., Zemeckis, D. and Van Parijs, S. M. (2013). Acoustic monitoring of Atlantic Cod (Gadus morhua) in Massachusetts Bay: implications for management and consevation. ICES Journal of Marine Science .
Graph: daily occurence of cod grunts

Figure a) Daily occurence of cod grunts

Map: Daily Presence of cod grunts from 3 glider deployments

Daily Presence of cod grunts (yellow dots) on all 3 gliders

Continuing our collaboration with the Stellwagen Bank National Marine Sanctuary (SBNMS), the Massachusetts Division of Marine Fisheries (DMF), and Cornell University's Bioacoustic Research Program (BRP), we also partnered with The Nature Conservancy and local fisherman to look at winter spawning aggregations in Massachusetts Bay, in an area known as the Winter Cod Conservation Zone (WCCZ). The aims of this project were to describe both the spatial and seasonal extent of winter cod spawning activity in Massachusetts Bay, based on a combined synthesis of the acoustic telemetry (Massachusetts DMF) and passive acoustic monitoring data, from both fixed station and mobile autonomous glider deployments.

For three successive winters, a total of 17 MARU's (5 in 2013, 6 in 2014, and 6 in 2015) were deployed alongside the Massachusetts DMF "active acoustics" receivers in Massachusetts Bay. These recorded continuously at a sampling rate of 2000 Hz. Slocum gliders were also deployed to track cod presence in Massachusetts Bay. Because cod vocalizations can only be heard 50-100 meters from the source, the gliders can help identify possible cod spawning activity in areas outside of the range of our MARUs.

Due to the large time and effort required to analyze large acoustic data sets, an automated cod detector was developed, significantly reducing the time required to find and validate the presence of cod grunts, published in the paper: Urazghildiiev, I. & Van Parijs, S. (2016). Automatic grunt detector and recognizer for Atlantic cod (Gadus morhua). Journal of the Acoustical Society of America, 139(5): 2532-2540.

Cod detector snapshot

Cod grunts detected using the cod grunt detector as viewed in Raven Pro 1.5. True positive detections have been labeled in the grid with a "y".

Results from the study showed the temporal distribution of cod spawning activity to have some inter-annual variability, but based on the results from all three years, spawning activity primarily occurred during early November through January with a peak in mid-December. The spatial distribution of spawning activity was generally consistent among years and concentrated in areas deeper than 50 meters. There were multiple hotspots of spawning activity, including just west of the northwest corner of Stellwagen Bank as the primary focal point of spawning activity, with other lesser focal points inside the WCCZ and just east of the Neptune LNG terminal. Learn more here: Stanley, J. A., Van Parijs, S. M., Hatch, L. T. 2017. Underwater sound from vessel traffic reduces the effective communication range in Atlantic cod and haddock. Scientific Reports, 7: 15633.

What are we currently doing?

Further cod work

Maps: Cod grunts in Massachusetts Bay for each year 2007-2016

Map: Extent of Atlantic cod vocalizations during each winter spawning season in Massachusetts Bay from 2007-2016. Yellow circles are proportional in size to total number of cod grunts throughout the study (e.g. nearly 100,000 cod grunts were recorded at one site in 2015)

To complement this research, we are also analyzing data collected in and near the Massachusetts Bay Winter Cod Conservation Zone (WCCZ) from 2007-2012 for the presence of spawning cod. Since there is still uncertainty as to where current spawning grounds still persist, prospecting archival acoustic data for cod grunts over multiple seasons will show how consistent they are, and may provide insight into new locations of spawning aggregations that can then be protected.

While the main goal of the archival data sets is to identify the temporal and spatial occurrences of cod spawning aggregations, the data is also useful for other things. By examining the vocalizations in detail, call type can be analyzed to see if there are certain dialects, and whether these match call types from other locations. We also plan to investigate whether grunt rates can be used as a proxy to estimate the density of grunting cod on a spawning ground. Density estimates are a useful piece of information, and could complement estimates derived from traditional trawling surveys.

Partners: MASS Department of Marine Fisheries, UMass Dartmouth SMAST, The Nature Conservancy, NOAA Stellwagen Bank National Marine Sanctuary, Woods Hole Oceanographic Institution, Cornell University, and Rutgers University.

Primary Funders: NMFS Co-operative Research Grant, NOAA Saltonstall-Kennedy
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