| The Marine Mammal Protection Act (MMPA), prohibits activities that disrupt marine mammals' behavioral patterns and communication. Florida's Indian River Lagoon (IRL) has a very shallow depth profile (mean 1.5 m) and is utilized by 40,000 registered boaters within Brevard County. However, it is unknown whether these conditions would elicit boat avoidance responses in the resident bottlenose dolphin (Tursiops truncates) population or if these animals are adequately protected under the current MMPA general safe viewing guidelines (30 minutes boat exposure, 50 m approach distance). Underwater sounds were recorded from Sebastian Inlet, Grant and Eau Gallie to determine boat signal to noise ratio (SNR), frequency range (kHz), and the percentage of boat noise per recording. Dolphin behaviors (surface rates, group spacing, direction of movement, activities) and vocalizations (whistle rates, frequency range, SNR) were collected before, during, and after boats passed within 900 m. A four hydrophone array was used to record whistles. Boat noise was audible for 30 minutes/hour and mean boat traffic was 35 boats/hour. Sebastian Inlet had the highest boat traffic (58 boats/hour), SNR (max 25 dB), and percentage of boat noise (44 min/hr). Watercrafis were present in 53% of dolphin behavioral observations and passed at a rate of 2.6 boats/min from a mean distance of 185.5 in. When vessels were at distances between 151-900 m, dolphins avoided boats 40% of the time, increasing to 69% when boats were within 150 in. Dolphins significantly decreased their surface rates by 29% during boat presence. While foraging, surface rates decreased by 40% during boat encounters. The spatial relationship between pod members did not change significantly during boat encounters. Although whistle rate was twice as high after boat approaches and SNR decreased during approaches, they were not significant (large SE). IRL dolphins are significantly reacting to boats at greater distances (mean 185 m) than suggested by the MMPA and boat noise exposure levels in Sebastian inlet surpass the current maximum viewing time. As the results here clearly point to dolphin 'harassment', the general 50 m approach distance should be extended to mitigate the current levels of anthropogenic disturbance specific to this shallow lagoon. |
