The Antillean manatee (Trichechus manatus manatus) is an endangered marine mammal that inhabits the Caribbean Sea and riverine systems in Central America. Their acoustic behavior is relevant for individual identification, mating and parental care. Manatees produce tonal sounds with highest energy in the second harmonic (usually 5 kHz), and their audiogram indicates sensitivity from 0.3 kHz to 90 kHz with lowest thresholds in the 16 to 18 kHz range. We recorded manatees in the San San River, a highly polluted riverine system in Panama, using a stereo array. Frequency transmission experiments were conducted in four subhabitats, categorized using riverine vegetation. Incidental interactions of manatees and small motorboats were examined. Acoustic transmission was linearly related to tonal vocalization characters: correlations were stronger in freshwater than in transition and marine environments. Two bands, 0.6 to 2 kHz and 3 to 8 kHz, attenuate similarly in all subhabitats, and these bands encompass F0 (tone) and peak frequency respectively of manatee tonal calls. Based on our data we conclude that frequency transmission depends mainly on river depth and bottom characteristics, also motorboat sounds mask signals from 3.5 kHz to 8 kHz, which overlaps the peak frequency of tonal calls. In spite of differences between acoustic transmission in subhabitats of the San San River, manatees utilize bands that transmit efficiently in all subhabitats.

Although it has been hypothesized that reef fish reduces sound production during extreme environmental events, it is possible that fish sounds are still present but only masked by elevated environmental noise. Although available acoustic technology makes it difficult to address this question, we can still generate hypotheses about the impact of environmental noise on fish sound production. Is there a behavioral response to environmental noise, or are fish sounds present at the same levels but masked and therefore undetectable without adequate filters? We recorded sciaenid (Pisces: Sciaenidae) sound production around Isla Tortuga, Costa Rica, during the “Nortes,” a period of strong winds caused by cold fronts and extreme variation of atmospheric pressure. Sciaenid sounds apparently stopped with almost no emissions during strong winds (56 miles/h) and transitioned to the normal emission rate on the next day when winds receded to 2 miles/h. We hypothesize that fish sounds were present at all times, but masked by wind-produced noise instead of a biological modulation as described by Mann and Grothues (2009) in similar conditions. I present circumstantial evidence in support of this hypothesis and discuss possible methods to test it directly.