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Occurrence of Depredation by Common Bottlenose Dolphins

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Occurrence of Depredation by Common Bottlenose Dolphins Occurrence of depredation by common bottlenose dolphins (Tursiops truncatus) on reef fish captured and released by rod and reel fisheries in the northeastern Gulf of Mexico by Corie E. Grewal Advisor: Dr. Andrew J. Read Prepared for: Jessica Powell, National Marine Fisheries Service April 30, 2021 Masters project submitted in partial fulfillment of the requirements for the Master of Environmental Management degree in the Nicholas School of the Environment of Duke University Executive Summary In the Gulf of Mexico, common bottlenose dolphins (Tursiops truncatus) have been observed depredating fish from rod and reel anglers. Depredation is defined as the removal of captured fish or bait by a predator, in this case bottlenose dolphins. Depredation can cause serious injury and mortality for dolphins who may become entangled or hooked in fishing gear and also increases the costs of fishing for anglers by forcing them to replace gear and bait, as well as the loss of catch to dolphins. Protected species managers in the Gulf are receiving an increasing number of reports about depredation from anglers and are hoping to gain more insight into how such interactions can be reduced. To better understand the nature and extent of these interactions, I examined potential factors that could influence the probability of depredation and the spatial distribution and frequency of such interactions. I analyzed data collected from the Florida Fish and Wildlife Conservation Commission (FWC) fisheries-dependent monitoring at-sea observer program to determine which factors played a role in rod and reel dolphin depredation of rod and reel fisheries between 2009 and 2020. Every observation in the dataset represented a fish caught by a rod and reel angler, and depredation was recorded in the dataset after fish were discarded and observed being consumed by a dolphin. I chose potential predictive factors based on their known or hypothesized influence on dolphin behavior and used a generalized linear model (GLM) to examine relationships between instances of depredation and these predictive factors. The variables that I examined included year, depth, geographic location binned by latitude and longitude into fishing zones, vessel type, fishing mode, the number of anglers fishing, taxonomic family of captured fish, fish fork length and whether or not the fish was vented. To reduce dependence between individual observations that may have taken place on the same trip or at the same location, I also created and included a lag variable. The most parsimonious model was the best fit to the data, and included year, the number of anglers fishing, two geographic areas surrounding Panama City and Destin, Florida, and the fish families Lutjanidae (snapper family), Carangidae (jack family), Serranidae (grouper and sea bass family), and a binned group of other species that were individually captured less frequently. All of these variables were significant predictive factors in the model. The results suggest that food provisioning in the Florida panhandle is a driver of depredation and that the incidence of depredation has increased over time. A spatial analysis of i the location of depredation events indicates that the northern and eastern coastal stocks, which occur over the near-shore shelf off the Florida gulf coast, have become conditioned to exhibit a range of low cost foraging techniques, such as scavenging, begging, and depredation. Snappers were observed in 78% of depredation events and this family was the most significant predictive factor in the model, revealing a regional prey preference for snapper by dolphins of the northern coastal stock. To deter dolphins from depredation, a number of mitigation techniques such as acoustic deterrents, gear modifications, and avoidance are possible, but each technique has advantages and disadvantages that must be considered. Acoustic devices have been shown to be effective in deterring some other marine mammal species, but are likely to draw bottlenose dolphins towards fishing activities, acting as a “dinner bell”. Gear modifications and avoidance may create additional economic burdens for anglers, but are more promising and deserve further study. The analysis presented here can help inform management and mitigation of bottlenose dolphin depredation in the Gulf of Mexico and provides a baseline for future scientific study. This study was the first to analyze the factors influencing the incidence of bottlenose dolphin depredation using data from fishery observers. The use of previously existing data reduced the time and funding necessary for this study, but also created limitations. The dataset was created for fishery managers and needed to be intensively edited for use with this depredation case study. Furthermore, the relatively small number of depredation events compared with the total number of observations, and the manner in which depredation events were recorded complicated this assessment. Future research should employ experimental designs that are designed to address depredation events. ii Table of Contents Introduction………………………………………………………………………………………..1 Materials and Methods……………………………………………………………………………5 Data Source……………………………………………………………………………… 5 Data Preparation………………………………………………………………………….8 Modeling………………………………………………………………………………....10 Model Selection and Analysis …………………………………………………………....10 Spatial Analysis…………………………………………………………………………..10 Results…………………………………………………………………………………………....11 Discussion………………………………………………………………………………………..15 Acknowledgements……………………………………………………………………………....24 References………………………………………………………………………………………..25 iii Introduction Over the past few decades, depredation by common bottlenose dolphins (Tursiops truncatus) has emerged as a complex conflict for marine resource management. In rod and reel angling (private and for-hire) fisheries, depredation events can consist of a dolphin taking a target fish off a hook as the angler is attempting to reel it in, but can also include taking bait off the line or scavenging to collect fish that are thrown back after being caught (Tixier et al. 2020, Zollett and Read 2006). Depredation has negative consequences for bottlenose dolphins because dolphins that become conditioned to depredate have a higher chance of injury or mortality (Christiansen et al. 2016). It can also lead to bycatch because depredating dolphins are more likely to become entangled or ingest fishing gear (Read 2008, Wallace 1985). Bycatch is non- target catch that becomes trapped in gear during fishing (Read 2008). Bycaught animals may be difficult to free from gear, causing serious injury or death to the individual (Read 2008, Wells and Scott 1994). Dolphins can also be injured or killed due to vessel strikes when depredating (Wells and Scott 1997). Depredation causes additional economic burdens for private and for-hire anglers who lose out on profitable catch or need to purchase extra bait or replace gear to account for losses to marine mammals (Read 2008, Tixier et al. 2020). Frustration stemming from interactions with dolphins can lead to harassment and retaliation efforts (Department of Justice 2006, Department of Justice 2007, NMFS 2016, Read 2005, Zollet and Read 2006). The Gulf of Mexico has been referred to as a hotbed for retaliation against depredating bottlenose dolphins and several documented observations of retaliation have been observed (Department of Justice 2006, Department of Justice 2007, NMFS 2016, Vail 2016, Wursig 2017). In the Gulf of Mexico, there have been numerous anecdotal reports of bottlenose dolphin depredation events in rod and reel (also known as hook and line) fisheries. In Florida, there have been several documented cases of dolphin depredation in various locations such as Destin, 1 Panama City, and Sarasota Bay, and some private and for-hire anglers report increased depredation over time (Cunningham-Smith et al. 2006, Powell et al. 2018, Powell and Wells 2011, Samuels and Bejder 2004, Shippee et al. 2017, Zollet and Read 2006). There are several potential reasons for depredation, which likely interact with one another in the Gulf. One reason is overfishing, which has depleted fish stocks sufficiently to create direct competition between fisheries and dolphin populations (DeMaster et al. 2001, Read 2008, Reichmont et al. 2018). In the Gulf of Mexico, stable reef fish populations lead to healthy ecosystem functions such as balanced predator-prey interactions (Coleman and Koenig 2010). However, the effects of climate change and other anthropogenic disturbances further exacerbate this balance (Coleman and Koenig 2010). In the northeastern Gulf, competition is also driven by declines in fish populations after periodic harmful algal blooms, increased fish predation by the invasive lionfish, and the 2010 BP Deepwater Horizon Oil Spill (Green et al. 2012, Lewis et al. 2020, Powell and Wells 2011). Another potential reason for depredation by dolphins in Florida is intentional feeding. Higher incidence of food provisioning escalates depredation because conditioned dolphins learn that humans offer the potential opportunity for a meal (Powell et al. 2018, Samuels and Bejder 2004). This conditioned behavior may be passed on to social associates and younger individuals through cultural transmission (Christiansen et al. 2016, Herzing 2005, Wells 2003). While bottlenose dolphins are not threatened or endangered under the Endangered Species Act, they are protected under the Marine Mammal Protection Act (MMPA) of 1972 and require conservation consideration in the
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