Riding the tide: use of a moving tidal-stream habitat by harbour porpoises

Abstract

Tidal-stream habitats present periodically fast-flowing, turbulent conditions. Evidence suggests that these conditions benefit top predators such as harbour porpoises Phocoena phocoena, presumably allowing them to optimise exploitation of prey resources. However, clear demonstration of this relationship is complicated by the fact that strong tidal flows often occur near-simultaneously across a wide area. The Great Race of the Gulf of Corryvreckan (western Scotland, UK) is a jetting tidal system where high-energy conditions persist across a broad range of tidal phases in a localised, moving patch of water. Porpoises can therefore actively enter or avoid this habitat, facilitating study of their usage of adjacent high-and low-energy environments. The distribution of harbour porpoises was studied using passive acoustic porpoise detectors (C-PODs) deployed on static moorings (~35 d) and on Lagrangian drifters moving freely with the current (up to ~48 h). This dual approach provided complementary perspectives on porpoise presence. C-PODs moored in the path of the Great Race registered a significant increase in detections during the passing of the energetic tidal jet. Encounter durations recorded by drifting C-PODs were longer than those recorded by moored C-PODs, suggesting that porpoises tended to move downstream with the flow rather than remaining stationary relative to the seabed or moving upstream. The energetic, turbulent conditions of the Great Race are clearly attractive to porpoises, and they track its movement with time; however, their structured movements in response to the evolving tidal situation cannot simply be represented as a direct relationship between current speed and porpoise presence.

Acoustic monitoring reveals the times and tides of harbor porpoise (Phocoena phocoena) distribution off central Oregon, U.S.A.

“…In agreement with our results, porpoises in Land's End, Cornwall, U.K. and southwest Wales, U.K., were also found to prefer strong ebbing tidal flows for foraging (Pierpoint 2008, Jones et al 2014. Benjamins et al (2016) hypothesized that peak tidal flows may disrupt the ability for fish to stabilize their position in the water, therefore creating opportunities for predators to take advantage of disoriented prey. In addition, Jones et al (2014) suggested porpoises had adopted a foraging strategy of intercepting or "ambushing" prey during ebb tide that was concentrated on coastal and benthic topography.…”

“…Benjamins et al . () hypothesized that peak tidal flows may disrupt the ability for fish to stabilize their position in the water, therefore creating opportunities for predators to take advantage of disoriented prey. In addition, Jones et al .…”

“…In our study, PPMs were detected more often at the reef site during peak ebb and flood flow. Studies show that variables, such as tidal height, tidal speed, or tidal phase have a direct influence on the distribution (Marubini et al 2009, Jones et al 2014, Benjamins et al 2016) and behavior (Johnston et al 2005, Pierpoint 2008) of porpoises. However, no consistent pattern has emerged and it seems the preferred tidal height, speed or phase for porpoises is site specific.…”

“…, Pierpoint , Benjamins et al . ). However, the influence from diel and tidal cycles are region‐specific, and this is likely related to the availability of dominant prey in the area.…”

“…Although it remains unclear whether reported movement patterns are related to seasonal inshore-offshore movements or because observation conditions are poor and it is harder to survey and detect them. Harbor porpoise populations in other regions display habitat-use patterns relative to 166 season (Simon et al 2010, Gilles et al 2011, diel cycle (Carlström 2005, Todd et al 2009, Schaffeld et al 2016, and tides (Johnston et al 2005, Pierpoint 2008, Benjamins et al 2016. However, the influence from diel and tidal cycles are region-specific, and this is likely related to the availability of dominant prey in the area.…”