Abstract
Currently, there is sparse information available on the near-field effects of tidal in-stream energy conversion (TISEC) devices on marine mammals. There is also little data on the temporal presence and activity of marine mammals in the upper Bay of Fundy (Tollit et al., 2011). Overall, harbour porpoise (Phocoena phocoena) are listed by COSEWIC as a species of special concern and represent the most commonly occurring species of cetacean in Minas Passage/Basin, seen year-round in small pods, while white-sided dolphins (Lagenorhynchus obliquidens) are believed to visit periodically in the summer. While the risk of direct collision or turbine strike remains a potential concern for marine mammals (Wilson, Batty, Daunt, & Carter, 2007), behavioral or activity level modifications or loss of foraging habitat due to anthropogenic noise disturbance (notably noise during TISEC turbine operation, but also during any foundation construction) and indirectly due to changes in prey populations (such as reef effects due to TISEC turbine presence) are considered two significant data-gaps that need biological assessment before any defensible build-out occurs (Ryan, 2010). The collection of baseline data prior to further TISEC deployments is considered vital in any subsequent post-deployment assessment of changes in cetacean activity levels or spatial use.
Prior to this Research Project, FORCE funded SMRU Ltd (University of St Andrews) and Acadia University to undertake a three month pilot baseline study (10 August 2010 – 23 November 2010) during which three autonomous Passive Acoustic Monitoring (PAM) devices, specifically C-POD hydrophones (autonomous cetacean echolocation click detectors manufactured by Chelonia Ltd), were deployed and recovered in the FORCE demonstration area using custom-made bottom moorings fitted with acoustic releases (Tollit et al., 2011).
This Final Report describes work undertaken in 2011 and 2012, during which seven C-PODs were deployed to expand the spatial and temporal coverage of the pilot baseline study. C-PODs were deployed in a gradient design (Ellis & Schneider, 1997), believed most appropriate to study impacts like noise disturbance and indirect prey effects. During both years of study, 2 devices were located within the demonstration area and 5 outside the area (dependent on recovery of C-PODs), ensuring coverage of shallower waters north and deeper waters south, where prey availability may concentrate cetacean foraging. Battery operated CPODs required multiple (3) deployments per year to cover the time period May to November.
The main objectives of the Research Project were to determine the baseline activity patterns and behavior of key cetaceans (porpoises and dolphins) in the Minas Passage during spring, summer and fall, and to assess how these vary temporally (with respect to time of day, weeks, months and across years), spatially (within and outside the FORCE demonstration area) and with current patterns (tidal cycles and current velocity). A secondary goal was also to assess (if possible) how these activity patterns vary subsequent to the deployment of TISEC devices or cable-laying operations. This secondary goal could not be achieved as there were no TISEC deployments during the study period. Instead, we conducted a pilot study to assess the performance of another hydrophone technology that was co-deployed with C-PODs in the FORCE test area.
In summary, cetacean baseline data was collected for all three seasons (May – January), but a variety of technical difficulties prevented complete coverage for some units outside of the demonstration area. In spite of this, a total of 1,342 days of data were collected (1,932,410 minutes). Data collected in 2011 and 2012 indicate a daily (98% of days had detections) but typically low level presence of harbour porpoise in Minas Passage (median DPM/day = 22 or 1.5% of each day). No other odontocete cetacean species (i.e., dolphins) was detected during either year of the study.
Porpoise presence was highest during the month of May and lowest during the months of July and August coinciding with the seasonal movement of the summer harbour porpoise population into the Bay of Fundy. A final peak in porpoise presence occurred in late October. While it appears that porpoise activity decreases during winter, winter monitoring would need to be conducted to verify this. The tidal variables of velocity and height had a large impact on porpoise presence in Minas Passage. Porpoise presence peaked at flood tidal velocities from 0.5 to 2.5 m/s and tidal heights of 1.5 to 3.5 m above mean tidal height. At higher flood velocities, porpoise detections decreased. On ebb tides, porpoise presence decreased with increasing tidal velocity. Tidal height had a similar pattern with porpoise detections decreasing as tidal height became more negative. The location of C-POD units did have an effect on porpoise detections. The most plausible explanation for this is the depth at which the C-POD was deployed. The deeper the C-POD unit, the higher the number of porpoise detections. The C-PODs within the FORCE area had relatively high porpoise detections (depth ranges from 52-56 m) while those units in deeper water to the south outside of the FORCE area (84 m) had higher predicted detections. The shallowest unit to the north of the FORCE site (27 m) had the lowest porpoise detections.
Diel (day-night) effects were also evident in the data. Porpoise detections were highest in the early morning hours, just after midnight, while they were at their lowest during early afternoon, just after midday. Sediment noise caused by large current velocities within Minas Passage interfered with our ability to detect porpoise by causing memory saturation of the C-POD units. This impact varied by location with the majority of locations impacted most during spring flood tides (i.e., the fastest tidal velocities). However, two locations outside the FORCE lease area were heavily impacted on both ebb and flood tides and were thus avoided in year two of this study. In spite of the impact of sediment noise on porpoise detections, we were able to control for sediment noise by including it as a covariate in our statistical modelling and therefore still make predictions at high tidal velocities.
A pilot project to compare the porpoise detection performance of C-PODs with a digital hydrophone and recorder, the icListenHF, was conducted over a one month period in August 2012. This study showed that flow-induced noise, which varies with tidal height and current speed, limits Harbour porpoise detection by both hydrophone types, especially during spring tide cycles. Click train detections per minute in August were greatest on the ebb tide at night during a neap tide. Further concurrent deployments using a combination of C-PODs and digital hydrophones are recommended for comparative and complementary purposes and for greater spatial coverage, especially during peak activity periods (spring and fall). Range testing of hydrophones under different flow scenarios would assist the interpretation of the datasets collected.
Minas Passage is a very challenging location in which to monitor biological and physical processes of importance to testing TISEC devices. In spite of these challenges this two year study on marine mammal presence has succeeded in measuring baseline levels of marine mammal usage of Minas Passage which will be crucial for monitoring potential impacts from TISEC devices after they are installed at FORCE.