Depredation and fisheries interactions involving bottlenose dolphins
By Jessica Powell, MS Candidate, University of South Florida
C835 leaps as Jessica Powell photographs
during a sighting in Sarastoa Bay.
Dolphins in the Sarasota Bay area have been observed and reported stealing bait or catch off fishing lines, stalking fishing boats to eat catch thrown back by fishermen, and even begging from boats. Such angler-interaction behaviors are defined as depredation: the act of stealing or damaging a prey item already captured by some other process. The consequences of depredation behaviors became evident in the spring of 2006 when 2% of the resident dolphin community died from ingestion of lures, hooks, and other recreational fishing gear. Simulation modeling by Dr. Bob Lacy of the Chicago Zoological Society showed that such losses to this population were unsustainable. In an effort to reduce this mortality, this project aimed to investigate a number of factors that could contribute to the increase in dolphin-fishing interactions. It was also our goal to help mitigate the problem by developing and distributing an informational card, “Dolphin-Friendly Fishing and Viewing Tips.” (see inside back cover).
Kristen Burtch and Kim Atwater collecting
data during a behavioral focal follow.
From May 2007 through April 2008, we monitored local fishing piers, both visually and acoustically, for dolphin presence, depredation events, and fishing effort. Findings showed that depredating dolphins are not attracted to specific bait, catch, or overall fishing effort. The depredation behavior from piers appears to be opportunistic or involve specific animals, such as adult male FB106, suggesting that this is a learned foraging strategy. Acoustic monitoring for depredating dolphins proved to be relatively ineffective; however, it did reveal that FB106 was echolocating during some depredation events. We were surprised to see that this animal used a high-energy sensory process like echolocation for depredation, which appeared to be a low-cost foraging strategy.
F109 stalks a fishing boat in New Pass.
In summer 2007 and 2008, we conducted focal behavioral follows on pre-selected individuals in order to better understand the behavior of depredating animals. Overall, depredating dolphins allotted different amounts of time for specific habitat and activity use than did control animals (non-fishing interacting dolphins) of the same sex and similar age. Depredating dolphins spent less time traveling and naturally foraging and more time milling. In addition, fishing interaction dolphins spent more time in channels and passes and near fishing boats and lines than did non-fishing interaction animals. Currently, we are working on an analysis that will allow us to evaluate hearing capabilities and socialization patterns of depredating animals.
F232 stalks a fishing boat in Longboat Pass.
Unfortunately, analysis of long-term data has shown that dolphin-angler interactions have continued to rise since 2005. The greatest numbers of dolphin-angler interactions are recorded in March, a time of year in Sarasota when there are high numbers of tourists and seasonal residents present.
In an effort to raise public awareness and support on this issue, “Best Fishing Practices for Avoiding Interactions with Wild Dolphins” (developed by NOAA, working in conjunction with Mote Marine Laboratory, the Chicago Zoological Society, and Hubbs SeaWorld Research Institute) were incorporated into a 3x5 laminated informational card, “Dolphin-Friendly Fishing and Viewing Tips.”
A boater illegally feeds BEGR in Venice.
Since January 2008, over 197,000 cards (English and Spanish) were distributed primarily throughout the state of Florida to fishing piers, bait and tackle shops, marinas, aquariums, boat rental facilities, and organizations with the ability to reach the target audience. Overall, the goal is to reduce human impacts on wild bottlenose dolphins so that populations will continue to thrive in Florida waters. Furthermore, since dolphin depredation and fisheries interactions are a problem worldwide, we hope our methods can serve as a template for evaluating and solving the same issues in other study areas.
This project was made possible by funding provided by the USF College of Marine Science Graduate Assistantship and the Disney Wildlife Conservation Fund.
A fishing boat looks on as BEGR takes a fish.