Sightings and Depth Distribution of Harbour Porpoise (Phocoena phocoena) off Point Lynas from 2012 – 2018 Boat Surveys

Point Lynas, which is situated on the northeast coast of Anglesey, North Wales, is a headland which provides a vantage point to view a variety of marine wildlife such as harbour porpoises (Phocoena phocoena) (Calderan 2003; Weare 2003; Ake 2005; Shucksmith et al. 2009). The area is thought to provide the oceanographic conditions and environments to allow harbour porpoise to forage, such as headlands and strong currents which concentrate their prey (Shucksmith et al. 2009).  

Harbour porpoises are protected under Annexes 2 and 4 of the EU Habitats and Species Directive. This means that the deliberate disturbance, capture or killing of these animals is illegal (EU Habitats and Species Directive 1992). Special Areas of Conservation (SACs) can also be designated for the species in key locations. In January 2017 the North Anglesey Marine SAC came into effect to specifically protect harbour porpoises and encompasses Point Lynas and surrounding waters. The purpose of the SAC is to maintain the range and abundance of the species (JNCC 2017). The protection of the harbour porpoises means that it is important to monitor the population and gather information on the distribution of these animals. 

Harbour porpoise distribution has been shown to be influenced by both biological and oceanographic factors. For example, a positive correlation between harbour porpoise abundance and physiological features which concentrate prey such as herring has also been found (Watts and Gaskin 1985). Depth has also been shown to have an impact on the distribution of harbour porpoise. A positive correlation between the distribution of harbour porpoises and increasing bottom depth was found in the Bay of Fundy (Smith and Gaskin 1983) (Watts and Gaskin 1985).  Harbour porpoises have been shown to have preferred depth ranges although this seems to be variable for different locations. In Scotland, harbour porpoises have shown preferences for bottom water depths of 50 -150 metres (Booth et al. 2013). Off the Northern coast of the San Juan Islands, 83% of harbour porpoise sightings from a 1-year boat survey occurred in waters deeper than 100 metres (Raum-Suryan and Harvey 1998). At five locations off the coast of Anglesey (including Point Lynas), a 3-year study found that most sightings (65%) occurred at a depth of between 30-45 metres (Shucksmith et al. 2009).  

 Point Lynas is also known for the wide variety of seabirds that can be seen from the rocky headland, such as gannets, guillemots, choughs, and shags.  

Aims and Hypothesis

A transect-based boat survey was conducted every year from 2012 to 2018 (except 2017) in the waters off Point Lynas, with the aim of recording the sightings and determining the depth distribution of harbour porpoises, as well as calculating the probability of a sighting at different depths.

The hypothesis is that most harbour porpoise sightings off Point Lynas across the 6 years will occur in water around 30 to 45m deep.

Methods

The boat survey followed a single platform survey method. Four observers watched for cetaceans, with two on each side of the boat, port and starboard.  The area scanned by each of the pairs of watchers was 90 degrees from their side of the boat to 10 degrees off the opposite side. While scanning one observer on each side scanned the distant water (>200m from the boat) with reticule binoculars, while the other observers used the naked eye to scan for cetaceans in the near water. The fifth observer on board was the data recorder who recorder all data on effort and sightings sheets. Each observer rotated roles every 10 minutes with new teams of 5 observers taking over every hour. Every time the observers rotated jobs a new effort form was completed. The transect number and start time were entered at the start of each survey. At each rotation, the time, boat position and speed were recorded as well as the Beaufort Sea state, the swell and a scale of rain and fog. Once a cetacean was sighted, the data recorder was made aware and the GPS position and time could be recorded. The angle and distance from the boat are then recorded from either the first cue of a single cetacean or the estimated centre of the school of animals. The distance was recorded as distance estimated by eye and using the top marker of a reticle placed on the horizon and counting the number of divisions to the cetacean sighting to get the perceived location.

The shore survey consisted of scanning an area from a vantage point. The species of seabird or cetacean spotted was recorded every 5 minutes throughout the day. The time is noted down and then the number of each species was recorded.

Depth for each survey point was extracted using ArcMap and bathymetry raster data, using the identify tool. The sightings data was then converted into binomial presence or absence data, with presence represented by a 1 and absence represented by a 0. The presence-absence data will then be analysed using a binomial general linear model, to see if the depth is a significant predictor of presence or absence of harbour porpoises. This analysis, however, does not consider the sighting effort i.e. the number of transects surveyed or the length of these transects, both of which will influence the probability of a sighting occurring at different depths.

Results

A total of 18 surveys were completed across the six years with a total of 39 sighting occurrences and 68 harbour porpoises were sighted in this area in the 6 years. Most of the sightings and surveys occurred within the designated SAC for the protection of harbour porpoises (Figure 1).

The binomial general linear model suggest that depth is a significant predictor in the presence or absence of harbour porpoises 

(GLM: F = 0.084, P =0.002)(R²=0.93 (McFadden 1973)) The analysis also showed that there is a 50% probability of spotting a harbour porpoise around Point Lynas in water 42.5 metres deep (Figure 2).

During the 6-year boat surveys, most sightings (36.8%) occurred in water between 31 -35 metres deep. While 76.5% of sightings occurred at depths of 26 to 40 metres deep (Figure 3). No sightings occurred in water between 5 to 10 metres. The next lowest number of sightings across the 6 years occurred in a water depth of 16 -20 metres.

Of the 6 years surveyed 2018 had the lowest number of harbour porpoise sightings per unit effort (by the number of transects sampled) while 2012 had the highest number of sightings per transect. There is a general declining pattern with less harbour porpoise sightings occurring per transect from 2012 to 2018 (Figure 4)

Common Name	Scientific Names	Sightings
Herring Gull	Larus argentinus	31
Razorbill	Alca torda	5
Shag	Phalacrocorax aristotelus	28
Guillemot	Uria aglge	7
Gannet	Morus sp.	14
Black Guillemot	Cepphus grille	3
Lesser black back gull	Larus fuscus	2
Cormorant	Phalacrocorax carbo	1
Harbour Porpoise	Phocoena phocoena	34

Table 1 – Table showing the Common and Scientific names of the animals (predominantly seabirds) which were sighted on the 2018 shore survey from Point Lynas, as well as the number of sightings of each species.

Of the seabirds, herring gulls and the shags were the most frequently sighted seabirds in the 2018 shore survey, with the cormorant being the least sighted seabird.  

Discussion

Harbour Porpoises are a protected species in North Anglesey, making it important to monitor the population abundance and range of this species.  It was found and expected that depth would be a predictor of presence or absence of harbour porpoise as positive correlations between depth and harbour porpoise abundance (Smith and Gaskin 1983; Watts and Gaskin 1985). The main hypothesis was that most harbour porpoise sightings off Point Lynas across the 6 years will occur in water around 30 to 45m deep, based on previous surveys done around Point Lynas (Shucksmith et al. 2009). It was found that across the six years most sightings did occur within this range, this is supported by some other surveys which found similar depth distribution (Carretta et al. 2001). However, this is contrary to other surveys which have found that harbour porpoises are sighted more often or spend more time in deeper water 50 to >100 metres depending on the individual geographic location (Read and Westgate 1997; Raum-Suryan and Harvey 1998; Booth et al. 2013). There seemed to be a declining trend in harbour porpoise sighting per unit effort in the six years surveyed however this may be due to other factors, such as weather and visibility as the 2018 survey days were particularly rainy and windy reducing the visibility and therefore the sightings.

References

Ake H (2005) Abundance and habitat use by Harbour porpoise (Phocoena phocoena) at two sites on Anglesey. Thesis MSc -- University  Wales, Bangor  Marine Mammal Science

Booth C, Embling C, Gordon J, Calderan S, Hammond P (2013) Habitat preferences and distribution of the harbour porpoise Phocoena phocoena west of Scotland. Marine Ecology Progress Series 478:273–285. doi: 10.3354/meps10239

Calderan SV (2003) Fine-scale temporal distribution by harbour porpoise (Phocoena phocoena) in North Wales: acoustic and visual survey techniques.

Carretta JV, Taylor BL, Chivers SJ (2001) Abundance and depth distribution of harbor porpoise (Phocoena phocoena) in northern California determined from a 1995 ship survey. In: Fishery Bulletin. Accessed 4 May 2018

EU Habitats and Species Directive (1992) Council Directive 92/43/EEC.

JNCC (2017) North Anglesey Marine. http://jncc.defra.gov.uk/page-7244. Accessed 4 May 2018

McFadden D (1973) Conditional logit analysis of qualitative choice behavior.

Raum-Suryan KL, Harvey JT (1998) Distribution and abundance of and habitat use by harbor porpoise, Phocoena phocoena, off the northern San Juan Islands, Washington. Fishery Bulletin 96:808–822.

Read AJ, Westgate AJ (1997) Monitoring the movements of harbour porpoises  with satellite telemetry. Marine Biology 130:315–322. doi: 10.1007/s002270050251

Shucksmith R, Jones NH, Stoyle GW, Davies A, Dicks EF (2009) Abundance and distribution of the harbour porpoise (Phocoena phocoena) on the north coast of Anglesey, Wales, UK. Journal of the Marine Biological Association of the United Kingdom 89:1051–1058.

Smith GJ, Gaskin DE (1983) An environmental index for habitat utilization by female harbour porpoises with calves near Deer Island, Bay of Fundy. Ophelia 22:1–13.

Watts P, Gaskin DE (1985) Habitat index analysis of the harbor porpoise (Phocoena phocoena) in the southern coastal Bay of Fundy, Canada. Journal of mammalogy 66:733–744.

Weare J (2003) Abundance and habitat use of harbour porpoise (Phocoena phocoena) off Point Lynas, Anglesey. PhD Thesis, MSc. Thesis, University of Wales, Bangor. School of Biological Sciences. Marine Mammal Science