Abstract
The drive towards sustainable energy has seen rapid development of wave and tidal stream (MRE) energy. However, little is known of any environmental and ecological effects [1]. The FLOWBEC-4D project developed an upward facing sonar platform to investigate how currents, waves and turbulence at MRE sites may influence the behavior of marine wildlife, how important collision risks might be, and how MRE devices (MREDs) might alter the behavior of wildlife [2]. Foraging efficiency (the capture of prey by a predator) is considered to be the major ecological driver of population dynamics, as it controls both adult and juvenile survival and condition [3]. Information was gathered on predator and prey use of MRE sites to identify and quantify which type of habitats (depth of water column, speed of tides, etc.) predators predictably use for foraging.
Although b oat surveys can provide high-resolution coverage along specific tracks [4], it is not logistically feasible to monitor a high-energy site continuously at high-resolution for a 14-day tidal cycle. Wind, waves and tide reduce positional accuracy such that boat surveys cannot monitor fine-scale interactions of individual targets at the precise location of MREDs and the costs of long duration surveys are high. Surface platforms [5, 6] can reduce cost but are similarly limited because of their instability in high-energy sites. In both cases, there is also the risk that the boat/platform presence and noise (in air and under water) could affect the species being studied.
Mounting instruments on the MRED provides a stable mounting and simplifies power and data requirements for longer duration surveys. The interactions of fish with tidal turbines have been imaged using cameras but visibility (turbidity and illumination) limits both the range and survey time [7]. Active lighting will directly affect animal behavior. Acoustic instruments mounted on the MRED are adversely affected by turbulence within a few meters from the MRED itself, which can mask the presence and interactions of wildlife [8]. Conversely, an independent platform allows the instruments to be positioned a short distance from the MRED, recording the interactions of wildlife and also conducting baseline studies elsewhere under similar conditions, e.g. in an area free from MREDs or prior to MRED installation.
The FLOWBEC seabed platform (Figure 1) addressed these issues by integrating a number of instruments to record information at a range of physical and trophic levels. Data were recorded at several measurements per second, for a duration of 2 weeks to capture an entire spring-neap tidal cycle at wave and tidal energy sites at the European Marine Energy Centre (EMEC). An upward-facing multifrequency Simrad EK60 echosounder (7° beamwidth, 38, 120 and 200 kHz) is synchronized with an upward-facing Imagenex Delta T multibeam (MBES) (120° x 20° beamwidth, 260 k Hz) aligned with the tidal flow. An ADV measures current and turbulence, and a fluorometer measures chlorophyll (a proxy for plankton) and turbidity. The latest revision has integrated a Nortek Signature broadband 5-beam ADCP, an upward facing color video camera, and passive acoustic monitoring (PAM). The platform is self-contained with no cables or anchors, facilitating rapid deployment and recovery in high-energy sites and allowing baseline data to be gathered. Measurements from the subsea platform are complemented by a 3D hydrodynamic model and concurrent shore-based marine X-band radar and ground-truth wildlife observations.
The benefit of combining information from multiple instruments to increase coverage, sensitivity and the information available has been recognized elsewhere, as it also allows one instrument to trigger the recording of another [9]. In the case of FLOWBEC, co-registration of targets seen across acoustic instruments greatly increases the information available. The EK60 alone provides quantitative measures and patterns of target distribution [10], yet co-registration of the same target on the MBES, allows concurrent behavior and predator-prey or target-MRED interactions to be monitored. Targets co-registered on both instruments can be used as a training dataset to aid classification of targets detected on a single instrument.
Single/split beam, MBES and acoustic cameras have been evaluated previously for use in tidal sites [5, 11, 12]. However, turbulence can both mask ecological targets, and compound classification. This paper describes the development of novel processing techniques to mask surface-connected turbulence, extract biological targets for parameterization and tracking, and an example of the information gains from co-registering data between instruments.