Impacts of vessel noise on red drum (Sciaenops ocellatus) spawning choruses in Saint Andrew Bay, Florida, U.S.A.

This study demonstrates that vessel noise in Saint Andrew Bay, Florida, significantly disrupts Red Drum spawning choruses by reducing their sound production levels and prevalence, particularly during peak spawning seasons when vessel traffic is frequent.

The Gist

Imagine a bustling underwater city where the residents are Red Drum fish. Every evening, as the sun sets, these fish gather in the bay to throw a massive "mating party." To find a partner, the males sing a loud, rhythmic drumming song (a chorus) that echoes through the water. It's their version of a love song, and it's crucial for the future of their population.

However, there's a problem: the city is getting noisy.

This study is like a detective story investigating how the constant roar of boats (from commercial ships and recreational speedboats) is ruining the fish's party. Here is the breakdown of what the researchers found, using simple analogies:

1. The Setting: A Noisy Restaurant

Think of Saint Andrew Bay as a popular restaurant where the fish are trying to have a quiet, intimate conversation.

• The Fish: They are the diners trying to whisper their love songs to each other.
• The Boats: They are like a rowdy group of people shouting, slamming doors, and revving engines right next to the diners' table.
• The Problem: The fish calls are low-frequency rumbles (like a deep bass), and boat engines make the exact same low-frequency rumble. It's like trying to hear a whisper while a jet engine is running next to you.

2. The Experiment: Listening in the Dark

The researchers didn't just watch; they set up "underwater microphones" (hydrophones) in the bay for two summers (2021 and 2022). They recorded everything during the "crepuscular" hours—that magical time just after sunset when the fish are most active.

They asked three main questions:

1. Do the fish stop singing when a boat goes by? (The "Disturbance" theory).
2. Do they sing louder to be heard over the noise? (The "Compensation" theory, like shouting at a concert).
3. Do they just ignore the noise and keep singing at the same volume? (The "Nothing Happens" theory).

3. The Findings: The Fish Are Getting Quiet

The results were clear: The fish are getting disturbed.

• The "Hangover" Effect: The study found that if a lot of boats passed by earlier in the evening, the fish sang much quieter later on. It's as if the noise gave them a headache or scared them away, so they lost their voice or left the room.
• The "Shouting Match" Failure: In 2022, when the fish were singing loudly and a boat came by, the total sound level actually dropped. This is a huge clue. If the fish were shouting louder to compete with the boat, the total noise would have gone up. Since it went down, it means the fish either stopped singing, sang much softer, or swam away from the microphone.
• The Overlap: The boats are there almost all the time. In peak spawning season, boats were detected in over 30% of the recordings, and on some days, the bay was 100% covered in boat noise. The fish have very few quiet moments to sing.

4. Why This Matters: A Broken Love Connection

Imagine you are trying to propose to your partner, but every time you speak, a loud siren goes off.

• If you stop talking because you can't be heard, your partner never hears the proposal.
• If you leave the room because it's too loud, you never get the chance to meet.

For the Red Drum, this isn't just annoying; it's a reproductive crisis. If the males can't be heard, they can't find mates. If they can't find mates, the population shrinks. The study suggests that the constant noise pollution is essentially "jamming" their dating app.

5. The Twist: They Don't Just Shout Louder

You might think, "Well, maybe they just get louder!" (This is called the Lombard Effect—like people shouting at a rock concert).

• Some animals do this. But the Red Drum in this study generally didn't. Instead of turning up the volume, they turned down the volume or left the area.
• The researchers noted that in some cases, the fish might be trying to compensate, but the boat noise is just so overwhelming that even if they tried to shout, their signal would still be drowned out. It's like trying to whisper a secret during a thunderstorm; no matter how hard you try, the storm wins.

The Bottom Line

This paper tells us that the Red Drum fish in Florida are living in a world that is becoming too loud for their love songs. The boats are so frequent and so loud that the fish are either getting scared into silence or swimming away from their own spawning grounds.

The Takeaway: Just as we need quiet libraries for studying, these fish need quiet bays for reproducing. If we want to keep the Red Drum population healthy, we need to manage the noise in our coastal waters, or we risk silencing their future.

Technical Summary

1. Problem Statement

Anthropogenic noise pollution, particularly from vessel traffic, is an increasing threat to aquatic life, affecting behavior, physiology, and acoustic communication. While sound travels efficiently in water, overlapping with the hearing ranges of many fish species, in situ research on how noise impacts spawning behaviors remains limited.

• Target Species: Red Drum (Sciaenops ocellatus), a commercially and ecologically significant estuarine fish in the Gulf of Mexico.
• Behavioral Context: Red Drum form dense spawning aggregations during summer and autumn, producing intense low-frequency choruses (100–200 Hz) primarily during crepuscular (dusk) periods.
• The Conflict: Vessel noise in coastal habitats is often low-frequency and overlaps temporally and spectrally with Red Drum calls. This overlap poses a risk of acoustic masking, potentially disrupting courtship, reducing reproductive success, or forcing behavioral changes.
• Research Gap: The study aimed to determine if Red Drum alter their sound production (calling amplitude or cessation) in response to vessel noise, testing three competing hypotheses:
  1. Disturbance: Fish reduce or cease calling during/after noise exposure.
  2. Compensation (Lombard Effect): Fish increase call amplitude or shift frequency to overcome noise.
  3. No-Modification: Fish call at the same intensity regardless of noise.

2. Methodology

The study was conducted in Saint Andrew Bay, Panama City Beach, Florida, during the spawning seasons of 2021 and 2022 (July–October).

• Data Collection:
  • Passive Acoustic Monitoring (PAM): A single hydrophone (SNAP recorder) was deployed at ~9m depth in a busy navigation channel.
  • Recording Strategy:
    • 2021 & early 2022: 45-second recordings on a 15% duty cycle.
    • October 2022: Continuous recording (45-second files, no pauses) for 5.25 days to capture acute responses to individual vessel events.
  • Abiotic Data: Hourly water temperature, daylength, tidal events, and lunar phase/illumination were recorded to control for environmental variables.
• Analysis Period: A two-hour "crepuscular period" (CP) was defined as 30 minutes prior to and 90 minutes post-sunset, the peak window for Red Drum spawning.
• Acoustic Processing:
  • Files were bandpass filtered (0–600 Hz) to isolate the Red Drum call spectrum.
  • Sound Exposure Level (SEL): Calculated for fish calls ($SEL_{fish}$) and vessel noise ($SEL_{vessel}$) using power summation equations, subtracting estimated background noise.
  • Screening: Visual and aural screening using Raven Lite 2.05 to categorize files by the presence of vessel noise and fish calls.
• Statistical Modeling:
  • Generalized Least Squares (GLS) Models: Used to assess the relationship between fish calling SEL and predictors (vessel noise SEL, date, daylength, moon phase, tide, temperature). Models accounted for temporal autocorrelation (AR1, AR2).
  • Hypothesis Testing:
    • Prior Exposure: Tested if vessel noise in the first 105 mins of the CP predicted fish calling in the final 15 mins.
    • Simultaneous Exposure: Compared SEL during vessel noise vs. fish-only periods using Wilcoxon sign-rank tests, specifically analyzing evenings where fish calling SEL was high enough to be distinguishable from vessel noise.
    • Acute Response: Analyzed continuous recordings to compare SEL immediately before, during, and after individual vessel events.

3. Key Results

• Temporal Overlap: Vessel noise was highly prevalent, detected in >31% of recordings in both years, reaching 100% of recordings on specific dates in peak spawning season (September–October).
• Disturbance Hypothesis Supported:
  • Prior Exposure: In both 2021 and 2022, higher vessel noise SEL during the first 105 minutes of the CP significantly predicted lower fish calling SEL in the final 15 minutes. This suggests a cumulative disturbance effect where fish do not acclimate but rather reduce activity or vacate the area over time.
  • Simultaneous Exposure (2022): On evenings where fish calling SEL was high (>124.6 dB), the SEL during vessel noise periods was significantly lower (median difference of 2.5 dB) than fish-only periods on the same evening. This indicates fish reduced calling amplitude or fled during noise events.
  • 2021 Anomaly: In 2021, when fish calling was high, SEL during noise did not differ significantly from fish-only periods. The authors suggest this may be because vessel noise levels were slightly higher in 2021, masking any potential reduction in fish calls.
• No Evidence of Compensation: There was no statistical evidence that fish increased call amplitude (Lombard Effect) to compensate for noise. In cases where vessel noise was low, fish did not appear to increase amplitude; in cases where noise was high, fish reduced amplitude or ceased calling.
• Abiotic Drivers: Fish calling was positively correlated with moon illumination and tidal differential in 2021. In 2022, temperature and moon phase also influenced calling, but vessel noise remained a significant negative predictor of calling intensity in the "prior exposure" analysis.

4. Key Contributions

• In Situ Evidence: Provides rare field evidence that Red Drum alter spawning behavior in response to chronic vessel noise, supporting the disturbance hypothesis over compensation.
• Cumulative Impact: Demonstrates that the impact of noise is not just immediate; prior exposure within a single evening significantly suppresses subsequent calling activity.
• Methodological Rigor: Utilizes SEL calculations and GLS modeling to disentangle the effects of noise from natural abiotic drivers (tides, moon, temperature) in a complex acoustic environment.
• Behavioral Plasticity: Suggests that while Red Drum may not permanently vacate noisy habitats (as they were present throughout the season), they exhibit acute behavioral suppression (reduced calling or movement away from the recorder) during noise events.

5. Significance and Implications

• Reproductive Fitness: Since Red Drum spawning is tightly linked to acoustic signaling, the observed reduction in calling amplitude or cessation during vessel noise likely leads to wasted signaling effort and reduced mating success.
• Management Relevance: With fishing pressure and Red Drum abundance increasing in the Florida Panhandle, the high prevalence of vessel noise (up to 100% overlap in peak months) poses a significant threat to stock recovery and sustainability.
• Ecological Context: The study highlights that coastal habitats, critical for spawning, are disproportionately impacted by anthropogenic noise. The findings suggest that current noise levels may be acting as a selection pressure, potentially altering population dynamics.
• Future Directions: The authors recommend future studies using hydrophone arrays to distinguish between fish moving away from noise sources versus reducing call rates, and experimental approaches using controlled vessel passes to isolate specific noise thresholds.

Conclusion: The study concludes that vessel noise in Saint Andrew Bay is a significant stressor that disrupts Red Drum spawning choruses, primarily through disturbance (reduction of calling) rather than compensation, potentially jeopardizing reproductive success in this commercially important species.