Bottlenose dolphin (Tursiops truncatus) behavior, habitat use, and movement patterns in an active Texas ship channel

Date

2023-08

Authors

Mills, Eliza

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Abstract

Anthropogenic activity in coastal areas can damage marine habitats and alter marine mammal behavior and movement patterns. Understanding marine mammal habitat use and behavior near human activities is crucial for advising conservation management to aid in the preservation of marine mammals and the critical environments that they occupy. Port Corpus Christi and Port Aransas, Texas, have experienced extensive infrastructure and oil exportation growth over the past 40 years. This thesis provides insight into common bottlenose dolphin (Tursiops truncatus) behavior, habitat use, and vessel interactions in the active Corpus Christi Ship Channel (CCSC) that has not been studied since the 1980’s. A shore-based digital theodolite was used for one year to research free-swimming bottlenose dolphin: (1) behavioral states (forage, travel, mill, socialize, orient-against current, rest) and habitat use in relation to temporal patterns (season, time of day), tidal patterns (tidal state, current speed), and habitat types (distance to mangroves, seagrass, depth, shoreline) in the CCSC-Aransas Pass confluence area; (2) behavioral state (forage, travel, mill, socialize, orient-against-current, rest) and movement (swimming speed, bearing change) patterns in relation to temporal patterns (season, time of day), group size, number of calves, vessel characteristics (presence, type, size, speed, total vessels, distance to vessels) near Port Aransas in the CCSC. Throughout all seasons and times of day, bottlenose dolphins primarily used the CCSC-Aransas Pass confluence area for foraging, traveling, and milling, while socializing, orienting-against-current, and resting occurred less frequently. Bottlenose dolphins primarily engaged in foraging, traveling, and milling in the deep middle of the confluence area, potentially to avoid fast-moving recreational vessels at the surface by diving or moving between channels. Dolphins also commonly foraged near the active Port Aransas ferry crossing, indicating that dolphins may be acclimated to the ferries’ consistent movement patterns. Socializing, orienting-against-current, and resting infrequently occurred in the confluence area, suggesting that intense vessel traffic may deter specific behavioral states or that dolphins may relocate away from vessel activity in the CCSC-Aransas confluence to engage in certain behaviors. Vessels (e.g., oil tankers, cargo carriers, tugs, barges, ferries, ecotours) were within 300 m of dolphins during 80% of dolphin observations and an average of two vessels were present with each dolphin group. Foraging, milling, and socializing occurred in large group sizes in the confluence area. Dolphins swam slowly and increased bearing changes with increasing group sizes of ? 15 individuals, potentially to increase protection from vessel activity. More than one vessel type and size were found near the majority of dolphin positions. Foraging, socializing, and traveling occurred when ecotourism vessels were present, suggesting that dolphins may be partially habituated, tolerant, or desensitized to dolphin-watching tours in the CCSC. Dolphin swim speed and bearing change varied with vessel type. As the number of vessels present increased, dolphin swim speeds and bearing changes increased possibly to avoid vessel traffic. Despite daily vessel traffic, bottlenose dolphins appear to use the deeply dredged channels, mixing of currents, and human-made seawalls of the CCSC-Aransas Pass area. The CCSC-Aransas Pass area appears to be a key foraging hotspot for bottlenose dolphins despite anthropogenic operations. Dolphins exhibited movement responses to the number of vessels in the CCSC and swam faster and changed bearing/direction as the number of vessels increased. Behavioral and movement patterns identified in this study can direct future research efforts in the CCSC, as continued observation of bottlenose dolphin habitat use and vessel interactions is advised as vessel activity and coastal development increases in the area. Future studies can focus on how dolphin behaviors change with varying distances to vessels and in the presence of ecotourism vessels and how dolphin group size and calf presence change with vessel interactions. Research on seasonal fish abundance and movement through the CCSC, as well as identifying common prey species of dolphins is also recommended. Long-term monitoring of dolphin abundance, habitat use, site fidelity, and responses to increases in anthropogenic activities may lead to improved management practices for dolphin protection in the CCSC.

Description

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Marine Biology

Keywords

behavior, bottlenose dolphin, habitat use, movement patterns, theodolite, vessels

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