About 5% of resident bottlenose dolphins in Sarasota Bay bear scars from having been struck by boat propellers. While many people consider dolphins to be too quick and nimble to be struck by boats, this is clearly not the case. For my Master’s thesis, I evaluated the spatial and temporal distribution patterns of recreational boating and bottlenose dolphins in Sarasota Bay to investigate some of the factors that might lead to boat strikes.
Analyses were conducted in a geographic information system (GIS) based on two previously compiled datasets. The first depicts recreational boating patterns based on 1,973 digitized travel routes, totaling more than 30,000 km in length and 1,552 destinations from map-based mail surveys conducted in 2003 and 2005 by the Florida Sea Grant Boating and Waterway Planning Program at the University of Florida. The second dataset consists of monthly, boat-based, bottlenose dolphin photographic identification surveys completed during 2002-2006 by the Sarasota Dolphin Research Program. The dataset includes 1,656 dolphin sighting locations, representing 5,227 dolphins, and 20,006 km of survey boat travel routes. Relative abundance, encounter rate and concentration of dolphins and boating activities were calculated and compared for 4 seasons, 3 depth categories, and 6 habitat types to determine which seasons and areas have a high relative risk of co-occurrence that may lead to injury or disturbance.
Boating activity was highly seasonal, reaching its peak among sampled boaters (primarily Florida residents) during May-July, and dolphin abundance within my subsection of the SDRP study area was highest during August-October. Dolphin concentration was greatest in 1-2.7 m depths and boating concentration was greatest in depths >2.7 m during all seasons. Dolphin concentration was greatest in the habitat “Pass” in all seasons except during May-July, when boating activity peaks, and was also most highly concentrated in the habitat “Pass.” Location Quotient Analyses were performed to evaluate concentration levels in each depth category and habitat type relative to the entire study area. Dolphin concentrations were higher than expected in depths 1-2.7 m and >2.7 m and in “Channels” and “Passes.” Boating concentrations were higher than expected in depths >2.7 m and in “Passes,” “Channels,” and “Open Bay.” Dolphin locations were concentrated in areas where recreational boats follow predictable routes. Also, “Passes” and “Channels” constitute areas where boaters are less likely to engage in other activities than at other habitats (i.e., “Sandflat”), thus, dolphins may quickly resume their activities once a vessel has navigated through rather than enduring its presence for a long time. The results from the present study could be of use in the development of management strategies and educational programs.
This article appeared on page 6 of the 2017 SDRP Annual Report, Nicks n Notches.