Harbour porpoise (Phocoena phocoena) foraging strategy at a high energy, near-shore site in south-west Wales, UK

Abstract

Harbour porpoises (Phocoena phocoena) visit Ramsey Sound at specific states of tide to feed. The preferred foraging location is high-energy habitat in South Ramsey Sound where a tide race, overfalls and upwelling zones form during the ebb phase. Seabed topography and tidal currents combine to create a foraging resource exploited by harbour porpoises at regular and predictable intervals. Porpoises are observed surfacing repeatedly directly into the tidal stream above and adjacent to, a steep-sided trench on the seabed. Tidal currents and the steep walls of the trench are believed to concentrate prey which is funnelled towards the waiting porpoises. Schools with calves prefer areas on the periphery of the fastest flowing water, where current speeds are lower. The presence of harbour porpoises is restricted almost entirely to the ebb tidal phase, but porpoises regularly occupy this site for the entire ebb phase. Harbour porpoise foraging in a tide race habitat is widely reported from the geographical range of the harbour porpoise and raises issues regarding foraging strategy and the net energetic value of hunting prey in high-energy environments.

Harbour porpoise habitat preferences: robust spatio-temporal inferences from opportunistic data

“…For example, although harbour porpoises are known to eat a wide variety of prey across their entire range, their diet tends to be dominated by only 2 to 4 main prey species in any given area ). There are consistent reports of the associations of harbour porpoises with dynamic, high productivity zones such as upwellings or high energy coastal locations (see for example Watts & Gaskin 1985, Borges & Evans 1997, Weir & O'Brien 2000, Johnston et al 2005, Tynan et al 2005, Pierpoint 2008). Such ephemeral aggregations of prey may be particularly important for the harbour porpoise owing to its small body reserves, cold living environment and energetically demanding reproductive schedule (Read & Westgate 1997, Reed et al 2000, Read 2001, Lockyer 2003, Johnston et al 2005.…”

“…Johnston et al (2005) showed that focal regions of tagged animals tracked by satellite overlapped with localized fronts where herring Clupea harengus and euphausiid zooplankton aggregate, and that a large eddy and a frontal system extended to these areas (see also Watts & Gaskin 1985, Read & Westgate 1997. In Pem brokeshire, foraging hotspots in the vicinity of islands (Pierpoint 2001(Pierpoint , 2008) may indicate a similar process, as was found within Mousa Sound, southeast Shetland Islands (Borges & Evans 1997). …”

“…Tidal variables were present in all of the best 33 models, but no single tidal variable appeared to fully explain the effects of tide on harbour porpoise occurrence (Table 4). Similarly, tidal variables are supported in the literature, but the preferred tidal phase or speed appears to vary across studies (Johnston et al 2005, Pierpoint 2008, Marubini et al 2009, Embling et al 2010. Instead of particular tidal conditions or depth per se, porpoises are likely to select a range of current regimes and topography that enhance relative vorticity (Johnston et al 2005) and concentrate prey in available high quality patches (Borges & Evans 1997).…”

“…Indeed, different study scales may reveal species− environment relationships at different levels in a resource that is structured hierarchically in space, where small, high density patches are nested within large, low density patches (Wiens 1989, Embling 2008, Marubini et al 2009). Fixed-position land obser vations have documented that harbour porpoises swim against the prevailing current in association with foraging sea birds at certain tidal states (Evans & Borges 1995, Pierpoint 2008. In salmonid fishes the trade-off between drifting food availability and swimming cost results in a 'sliding spatial niche' across variable flow conditions (Heggenes 1996).…”

Integrating science and management for marine mammal conservation

“…A number of papers in the present volume investigated the effects upon marine mammals of particular human activities, including ship strikes on large whales (particularly fin whale and grey whale) in the Pacific north-west of the United States (Douglas et al, 2008); disturbance from high-speed pleasure craft on bottlenose dolphins in the Mississippi Sound (USA) (Miller et al, 2008); incidental capture of Amazonian manatee calves in shrimp trawls and gillnets in NE Brazil (De Meirelles, 2008) (Pierpoint, 2008). Information on cetacean habitat requirements can help us to understand niche partitioning between species, as well as shifts in distribution related to climate variation and change.…”