Identification and Characterization of Spotted Seatrout Spawning Habitat in Two Mississippi Estuaries: Biloxi Bay and Grand Bay - Utilizing passive acoustics to understand spawning site selection
Spotted seatrout, Cynoscion nebulosus, inhabit estuarine and nearshore Gulf of Mexico waters from the west coast of Florida to the Gulf of Campeche (Pattillo et al., 1997) and is one of the most sought-after saltwater recreational fish species in the southeastern United States. They spawn in coastal and estuarine waters (Mok and Gilmore, 1983; Saucier and Baltz, 1993), primarily in their natal estuary (Bortone, 2002). Off Texas and along the west coast of Florida spotted seatrout spawn in estuaries in association with grassbeds near shallow channels (2-4 m) (Brown-Peterson et al., 1988; Walters et al., 2007). Nothing is known about spotted seatrout spawning habitats in Mississippi coastal waters. However, current trends in Mississippi’s coastal population growth and increased development along the Mississippi coast, post-Hurricane Katrina, could impact fish species that spawn close to shore through increased fishing pressure and spawning-habitat degradation.
Figure 1: The sonic muscle is a specialized muscle that vibrates against the swim bladder to produce sound used to attract females during the spawning season. Photo Credit: FWRI
Fish belonging to the drum family (Sciaenidae) produce seasonal species-specific courtship sounds, with their extrinsic sonic muscles (Figure 1), in geographic areas in which the species spawns (Connaughton and Taylor, 1994). Because spotted seatrout are soniferous, it is possible to detect and map the locations of their courtship sound production in Mississippi estuaries via passive acoustic telemetry.
The courtship sound for each species of fish is as unique as the songs of birds, and similarly can be used in identification. By selecting the links below you can listen to the courtship sounds of a spotted seatrout, silver perch, sand (white) seatrout, and red drum.
Courtship songs of the drum family of fishes. Sound files and illustrations provided by the Florida Fish and Wildlife Research Institute. Illustrations by Diane Rome Peebles.
Scientists at the Gulf Coast Research Laboratory are conducting spotted seatrout spawning habitat research by using passive acoustics, a non-invasive method based on identifying fish sounds. The method involves taking a recording using an underwater microphone called a hydrophone (Figure 2). The advantage of this method of study is that data can be collected without disrupting the environment or sacrificing fish as with traditional capture methods.
Figure 2: Acoustic survey equipment in use. HTI Hydrophone Model 96-min, frequency range 2-30 Hz in the water; a 60-sec sample recorded with a Marantz digital recorder PMD 670 and amplified for in-situ listening via a Marshall amplifier.
Different species of fish spawn at different times of year and at different times of day. Typically, drums spawn in the evening. Spotted seatrout spawn during the summer months and males begin drumming around sunset and continue for several hours, anywhere from three to twelve. Therefore to target this species, GCRL scientists conduct their research between the hours of 1800 and 0200 hr, from May to August. Males have been known to aggregate in large numbers (hundreds) at specific spawning locations during this mating period. Using sound production to attract females is an effective method for these fish because they are able to communicate at night in murky waters when visibility is low. Once males and females are together, both release their gametes (reproductive cells) into the water and the males’ sperm will fertilize the females’ eggs in the water column.
The goal of this study is to identify and characterize spotted seatrout spawning habitat in two Mississippi estuaries: Biloxi Bay (an impacted estuary) and Grand Bay (a pristine estuary) (Figure 3).
Figure 3. Images of (a) Biloxi Bay, impacted by coastal development and commercial boat traffic, and (b) Grand Bay NERR, a pristine estuarine research reserve.
Sampling was conducted once each week from May through August. The study areas were divided into two zones based on geographic and logistic criteria (bold line on map) (Figures 4 and 5). Each zone was further subdivided into half mile grids (squares on map). Since spotted seatrout frequencies have been attributed to lunar influences, the zones were rotated monthly so that each zone was sampled over the various lunar phases. Six grids were randomly selected, within each zone for each night of sampling, and four stations per grid were sampled. Hydrophone recordings and environmental data, such as water temperature, salinity, dissolved oxygen, current speed, and substrata type, were collected at all stations. The estimated distance (directly on-top of, close by, and in the distance) and number of spotted seatrout (0,1-2, 3-5, small aggregation, and large aggregation) producing sound were recorded.
Figure 4: Map depicting oyster and artificial reef habitat in Biloxi Bay/p>
Figure 5: Map depicting oyster and submerged aquatic vegetation habitat in Grand Bay NEER
In order to assess what makes the spawning habitat unique, and to assess differences in site selection for the two regions, we compared the environmental data for the locations where spotted seatrout were heard to those where they were not. Though spotted seatrout sounds were heard in both sampling regions, they were more prevalent in Grand Bay than Biloxi Bay. During the two years of the study (08-09), 704 stations were sampled: 356 in Biloxi Bay and 348 in Grand Bay. Spotted seatrout sounds were heard in 44% of the sampled stations in Grand Bay (Figure 6) and 19% of the stations in Biloxi Bay (Figure 7).
Figure 6: Map depicting locations of spotted seatrout sound production. Color indicates estimated distance—Red is “on-top-of”, Yellow is “nearby”, Green is “in the distance”. Size of circle indicates estimated number of spotted seatrout producing sound—small circles are “1-2” ranging up to the largest circles representing large aggregations. Black dots indicate no sound was detected.
Figure 7: Map depicting locations of spotted seatrout sound production. Color indicates estimated distance—Red is “on-top-of”, Yellow is “nearby”, Green is “in the distance”. Size of circle indicates estimated number of spotted seatrout producing sound—small circles are “1-2” ranging up to the largest circles representing large aggregations. Black dots indicate no sound was detected.
The habitat type selected by spotted seatrout in Grand Bay (Figure 7), was sandy substrate near seagrass beds; whereas in Biloxi Bay (Figure 6), spotted seatrout utilized artificial reefs and oyster beds for their nightly calling. Other studies have stated that spotted seatrout have been shown to prefer seagrass habitat as their spawning location, and it is likely that the lack of seagrass habitat in Biloxi Bay may be the cause of the reduced courtship sounds production and the variation in site selection. In both locations, the temperature was warmer (> 27°C), the salinity was higher (>15 psu) and the water was deeper (>2 meters) at sites where spotted seatrout sounds occurred compared to where no sounds were detected.
Figure 8: Pie chart depicting the expected habitat usage if spotted seatrout were utilizing the available habitat proportionally to what was sampled (left) compared to the actual habitat usage observed throughout the 08-09 sampling season (right) in Grand Bay.
Figure 9: Pie chart depicting the expected habitat usage if spotted seatrout were utilizing the available habitat proportionally to what was sampled (left) compared to the actual habitat usage observed throughout the 08-09 sampling season (right) in Biloxi Bay.
In addition to the mobile survey, long-term acoustic recording systems (LARS) (Figure 11) were deployed in each bay during 2009 to determine the variability of spotted seatrout spawning activity. Analysis of the LARS data (Figure 12) indicated that the duration of aggregation courtship calls averaged around 4.5 hours, but ranged from 1.3 – 7.4 hours in Biloxi Bay and 0.5 – 7.8 hours in Grand Bay. The duration of aggregation calls also appeared to increase around the full moon phase. In Biloxi Bay, spotted seatrout sounds were detected until the last day of deployment at the end of September; however in Grand Bay, if spotted seatrout were present they were masked by the sounds of other species, and thus were only detected until 8/31/09.
|Figure 11: The long-term acoustic recording systems (LARS), by Loggerhead Instruments, includes a hydrophone, DSG recorder, and SDHC memory card inside of a waterproof, PVC housing rated to 100-m. In the 2009, sampling season, one of these systems was placed in each study area, in a region of where spotted seatrout were heard “on-top of”.|
Figure 12: Graph depicting the duration of spotted seatrout calling duration in Biloxi Bay (pink line) and Grand Bay (Blue line). Circles above indicate the full (hollow) and new (filled) moon phase.
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