A Practical Guide to Conducting Underwater ROV Transect Surveys
Underwater transect surveys are a method used in marine biology and ecology to study and monitor underwater environments, particularly coral reefs, seagrass beds, and other marine habitats. The key features of these surveys include:
Transect Line: A transect is a straight line or path along which one counts and records occurrences of wildlife species or habitat features. In underwater transects, a rope or tape measure is often used to mark the line.
Standardized Area: The survey covers a standardized area along the transect line. This standardization helps in comparing different areas or the same area over time.
Species Counting and Identification: Scientists count and identify species found along the transect. This can include fish, coral, seagrass, and other marine organisms.
Monitoring Over Time: Repeating the surveys over time can provide information on how underwater ecosystems or landscapes change, helping to identify trends or the impact of coastal development.
Underwater transect surveys have conventionally been conducted by dive teams, a method that, while effective, often comes with logistical challenges and higher costs. However, with the advent of advanced Underwater Remote Operated Vehicle (ROV) technology, these surveys can now be carried out more efficiently and economically. ROVs present a practical alternative to traditional diving methods, offering several advantages in conducting underwater transect surveys. This guide will dive into the use of ROVs for conducting these surveys, outlining the steps and considerations involved in leveraging this innovative technology for underwater ecological studies.
Step 1: Building Your Transect.
Based on observed experiences in the field, weighted ropes, marked at regular intervals, serve as a practical and cost-effective solution for creating transect lines in underwater surveys. Additionally, a measuring wheel equipped with attached weights can also be used as a transect line. The following example outlines a method for constructing a transect line using a weighted rope, detailing the steps and materials needed to create an effective and reliable tool for underwater transect surveys.
Selecting ideal rope: Choose a durable, high-visibility rope that can withstand underwater conditions. Use sinking (e.g., leaded) rope so that it remains on the bottom while deployed. The rope should also be on the thicker side (at least ¼ inch) so that it is easily visible, even during turbid or low-light conditions.
Cutting to Length: Measure and cut the rope to the length that matches your transect requirements. This length should be determined based on the survey area and the specific goals of your study.
Marking your transect line: Consider marking the rope with colored tape to better keep track of your distance travelled underwater. Colored electrical tape holds up well in underwater environments. For a more refined system, use multi-colored tape: blue tape for 10-meter increments, red tape for 5-meter increments, and black tape at each meter. This strategy provides a clear, organized method for accurately gauging distance along the transect line.
Mark transect with coloured electrical tape to better track distance travelled under water.
Weights and Floats
For effective deployment, each end of the transect line should be anchored with a weight. These weights are then connected to vertical lines (also weighted to avoid excess rope at the surface which can become tangled in the prop), which in turn are attached to surface floats. This arrangement ensures the transect line remains stable and properly oriented underwater, while visible at the surface.
Choosing Weights: Again, based on experience, concrete cinder blocksmake reliable and inexpensive anchors for each end of your transect. Either way, you want to select enough weight so that the transect line remains fixed, while counterbalance the buoyancy of the surface floats. You also need to factor in the potential rise in tide levels when choosing the weight and the length of the vertical line. If your buoy in more positively buoyant than the weight of your anchor, and if you don’t have enough rope, then when the tide rises, your transect lines might float away!
Attaching Weights: Securely fasten the weights at both ends of the ransect lines (an seen in image below. Ensure the connections are strong to prevent detachment under water. Each weight should also be connected to the vertical buoy line (as seen in below imgae).
Photo of concrete black used to anchor surface float and transect line.
Surface Floats: Choose highly visible, buoyant floats. Attach them to the top end of the sinking lines. These floats need to be substantial enough to stay afloat even in choppy conditions. They mark the start and end of your transect and are crucial for the ROV operator to easily find the transect points.
Labeling Floats: It's helpful to label the floats with information about the transect (e.g., transect number and your contact information in case the transect becomes lost).
Photo of surface float attached to transect line.
Step 2: Deploying Your Transect Line
Positioning: Carefully deploy your transect line in the selected survey area. Ensure that the line is laid out straight along the seabed. This straight alignment is important for accurate data collection and consistent measurement along the transect. AVOID DRAGING THE TRANSECT LINE DURING DEPLOYMENT, as this can cause damage to benthic wildlife.
Tides and Currents: Be mindful of the local tidal patterns and water currents when deploying the transect line. Adjust the weight of the anchors and the buoyancy of the floats accordingly to ensure that the line remains stable and in place, regardless of changing underwater conditions. To minimize the risk of entanglement with the propeller, always try to deploy the lines when the vessel's engine is in neutral.
Visibility and Accessibility: Ensure that the surface floats are visible and identifiable from a distance. This aids in navigation and quick location of the transect line for subsequent surveys.
Step 3: Preparing and Launching Your ROV
Equip and Launch Your ROV: Attach necessary tools like cameras and sensors. Ensure all systems are functional. I recommend developing your own check-list to ensure a safe and effective deployment.
Before deployment, observe the current patterns in the survey area to determine the most effective launch point relative to the transect and the current's direction. Launch the ROV down current from the transect. This approach uses the natural water current to your advantage, ensuring that any movement of the ROV or its tether is carried away from the transect line, thus reducing the risk of entanglement.
Begin the survey by identifying the surface float marking the starting point of the transect. Once the float is located, guide the ROV down to the seabed to find the transect's starting point. This step may vary in difficulty, depending on the water's visibility and sea state. If locating the start is challenging, consider an alternate approach: dive in a direction perpendicular to the transect line's expected path until you encounter the line. After locating the line, trace it back to its starting point. From there, you can officially initiate the transect survey.
Step 4: Conducting the Survey
Navigating the ROV Along the Transect Line: Carefully pilot the ROV to follow the marked transect line, ensuring a steady and consistent speed for uniform coverage. As you navigate, focus on observing and documenting marine life and environmental conditions. Use an underwater ruler or scaling tool for precise size measurements of marine organisms or habitat features. Maintain a consistent altitude above the seabed to standardize observations and improve the accuracy of visual data.
Image of marked transect line and Orange sea pen (top left)
Recording Observations and Data: Throughout the transect survey, record the types and numbers of invertebrates, fish species, and other notable marine life observed. Use a predetermined range (e.g., within 1 meter on either side of the transect line) to include observations in your data. This helps in maintaining consistency in the data collection process. Document environmental or habitat features, substrate types, or other notable features. Make notes on the ROV's onboard systems, such as GPS coordinates, depth, and water temperature, to correlate biological observations with environmental conditions.
Completing the Transect: When the ROV reaches the end of the transect, identified by the weight and vertical line connected to a surface float, carefully maneuver the ROV to avoid entanglement. Then, gradually ascend the ROV to the surface, ensuring the tether is properly managed to avoid any entanglement with surface floats or other equipment. Once at the surface, retrieve the ROV, securing it and its tether before moving to the next transect or concluding the survey session.
Step 5: Post-Survey Analysis
Conducting Thorough Video Analysis: After completing the transect survey, review the recorded video footage. This step is helpful for dentifying and documenting species and features that might have been overlooked during the live observation. This analysis also allows for a more accurate count and identification of species, as well as a closer examination of environmental features like substrate type, signs of ecological stress or change, and so on.
Tips and Best Practices:
Tangle Prevention: Be mindful of the ROV tether to avoid entanglement with the transect line or surface equipment.
Advantage Over Diving: ROV surveys allow for complete recording of the transect and detailed post-dive analysis, offering a more detailed and cost effective approach than traditional diving surveys.
Enjoy some more photos from a transect survey conducted earlier this year!
