Habitat preferences and interannual variability in occurrence of the harbour porpoise Phocoena phocoena off northwest Scotland

Abstract

The harbour porpoise Phocoena phocoena is the most common cetacean around the British Isles, but knowledge of its ecology, habitat preferences and inter-annual variability is still inadequate. Here, sightings collected by the Sea Watch Foundation during vessel surveys in West Scotland (August during the years 1993, 1994, 1996 and 1997) were critically analysed and used to construct a predictive habitat model for harbour porpoises in the Greater Minch. Generalised additive models were used to analyse relative abundance in relation to environmental variables; a preference for waters within 15 km from the shore and between 50 and 150 m depth was clearly identified. A relationship between tidal variables and porpoise distribution was also recognised with more sightings predicted for high tidal stream speed areas as well as during times of high tide. Maps constructed from the model were used to identify potential 'hotspots'and compare between years. Four areas with high relative abundance were identified in (1) the region between Ardnamurchan, Coll and the Small Isles, (2) southeast of Barra, (3) northeast of Skye to Gairloch, and (4) west of Pairc Peninsula (Isle of Lewis) to Shiant Islands. Number of sightings fluctuated up to 4-fold between consecutive years; such extreme variability in relative abundance is offered as a bench-mark for comparing trends in the future as well as evidence that the Greater Minch represents only a small part of the effective range of this population. KEY WORDS: Phocoena phocoena · Habitat preference · Relative abundance · Inter-annual variability Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 381: [297][298][299][300][301][302][303][304][305][306][307][308][309][310] 2009 being commercial fisheries. Parties to the Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) have agreed to work towards reducing by-catch levels to below 1.7% per year of the North Sea porpoise population (ASCO-BANS 2000) to ensure that this species is maintained in a favourable conservation status. Robust estimates of population size are necessary to quantify the impact of by-catch; to this end the large-scale surveys known as SCANS I & II (Small Cetacean Abundance in the North Sea) were carried out in 1994 and again in 2005, during the month of July (Hammond et al. 2002, Hammond & Macleod 2006. In addition, different types of disturbance (e.g. noise, chemical pollutions, aquaculture) might result in temporal redistributions of animals among adjacent areas rather than changes at the population level.For conservation measures to be most effective, greater knowledge of habitat use and habitat preference is highly desirable at a variety of spatial scales from the national to the local spatial scale, especially as spatial planning is becoming the framework for management of human activities within the marine realm (Defra 2006). Furthermore, as the harbour porpoise is on Appendix 2 of the EC Habitats Directive (Council ...

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

“…All models in the confidence set included depth (Table 4). Porpoises were more likely to occur in the deeper ranges (> 60 m) of the study area, with a 3-fold difference in occurrence evident See Table 1 Table 1 (Carretta et al 2001, Northridge et al 1995, Weir et al 2000, Marubini et al 2009, Embling et al 2010. Watts & Gaskin (1985) suggested that the animals may in fact be actively avoiding turbulence in shallow (<10 m) areas.…”

“…The harbour porpoise Phocoena phocoena Linnaeus 1758 is a prime example of a species for which predictive power of distribution and habitat preference has been difficult to obtain across large spatial scales (Embling 2008, Marubini et al 2009, Embling et al 2010) because both its environment and its responses to the environment are highly variable. 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 ).…”

“…Such surveys provide a 'snapshot' of the distribution, whereby it is appropriate to average spatial conditions in an area and assume pseudo-equilibrium between the species and its environment (Guisan & Thuiller 2005). 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.…”

Harbor Porpoise, Phocoena phocoena vomerina, in Cook Inlet, Alaska

“…Large fl uctuations in harbor porpoise sightings observed interannually may be attributed to sampling only a portion of their effective range (e.g., Marubini et al, 2009). While a number of harbor porpoise populations found along the coastline of the eastern North Pacifi c seem to have established home ranges (Chivers et al, 2002), the extent of area used by Cook Inlet harbor porpoise is not known.…”

“…Speckman et al (2005) found the highest densities of herring and longfi n smelt in warm waters <40 m deep, but herring peaked in clearer, more saline waters while longfi n smelt peaked in more turbid, fresher waters. Johnston et al, 2005;Marubini et al, 2009;Gilles et al, 2011). Given schooling prey occupy a range of oceanographic domains in the lower inlet, this may explain some of the interannual variability in harbor porpoise encounter rates in offshore versus nearshore areas during systematic surveys (e.g., NMFS beluga whale survey data).…”