Decoding the Information Content of Fish Sounds and How Fishes Extract Information from Sounds: Insights from the Plainfin Midshipman and Beyond

Abstract

Many species have evolved the ability to produce sound for communication. One of the most common types of sounds produced by animals is advertisement calls made by males to attract females for mating. These calls often contain information about morphometric parameters that indicate the quality or reproductive potential of the male. Acoustic communication is commonly observed in ray-finned fishes. While most sonic fish species produce short-duration advertisement calls (~1s or less), the plainfin midshipman fish (Porichthys notatus) produces calls averaging ~10 minutes and up to 2 hours, making them some of the longest vocalizations in the animal kingdom. Despite its long-standing role as a model organism for neuroethology research on acoustic communication and social behaviors, it was unclear if the long-duration hums produced by type I (singing) males contain information about male quality. In Chapter 1, I demonstrate that the acoustic features of the hums produced by type I males are correlated with morphometric parameters indicative of quality, such as body size and condition. This suggests that these hums contain information that females could potentially use in mate-choice decisions. Female midshipman are effective at localizing these hums, following local particle motion cues to find the source. However, there is a 180-degree ambiguity in determining sound direction from particle motion. It has been proposed that gas-filled swim bladders, which detect acoustic pressure, help resolve this ambiguity. Yet, how the swim bladder affects the motion of the fish's inner ears remains unclear. In Chapter 2, I used the finite element method to predict how the swim bladder affects the motion of the otoliths in the inner ear of the midshipman for sounds incident from various directions. I showed that the swim bladder likely resolves the 180-degree ambiguity in directional hearing at behaviorally relevant frequencies for the plainfin midshipman. These predictions can be tested using advanced experimental methods. Many fish do not actively produce sounds but can hear, suggesting that fish hearing may have originally evolved to extract information useful for survival and reproduction from ambient environmental sounds. However, most bioacoustic studies on fishes have focused on communication sounds. In Chapter 3, I review cases where natural ambient sounds serve as sources of information for fishes. I highlight various sources of ambient sound in aquatic environments and hypothesize how they could act as beneficial cues. I also found evidence of natural sounds functioning as noise, disrupting the detection of important signals. This review aims to encourage more studies on ambient sounds and their impact on fish, which is crucial for understanding the effects of underwater noise pollution. Fishes are attracted to sounds such as conspecific advertisement calls. In Chapter 4, I developed Sound-bait, an acoustic trapping method to selectively capture fish species using species-specific attractive sounds. This low-cost method has implications for reducing bycatch in fishing and determining the biological function of fish sounds. In summary, my dissertation provides insights into fundamental questions about fish hearing and acoustic communication, and offers practical applications of this knowledge to aid wildlife conservation.