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Harbour Porpoise

Harbour Porpoise by Chris Bielby

The Harbour Porpoise Phocoena phocoena is the most common cetacean inhabiting the seas off the North East coast of England. It is compact and robust with a rotund body and blunt head and reaches a maximum length of 1.9 metres and weight of 70 kg. The females tend to be slightly larger than the males and the newborns 67 cm to 90 cm with a weight of around five kg.

It has dark grey-brown upperparts with the pale of the throat and underparts sometimes reaching up on to the flanks. The young tend to be slimmer, darker and more uniform. The Harbour Porpoise has a rounded head sloping to a small indistinct beak above a mouth containing 19-28 teeth on each side of each jaw and a mouthline which slants gently upwards giving a smiling expression. There is an inconspicuous fine dark line joining mouth to flipper (Hoyt, 1984). It has a short, broad-based triangular dorsal fin, usually low and blunt but very rarely slightly falcate, set just behind the mid-point of its back. There is a slight dorsal ridge leading from the dorsal fin to the tail stock. The tail flukes are all dark with a concave trailing edge, blunt tips and a small median notch. The flippers are small, dark and slightly rounded.

Harbour Porpoises are usually shy and retiring and rarely bow-ride, but they can sometimes be attracted by slow-moving and quiet boats. They are capable of swimming at about 14 mph when pressed, but usually travel much more slowly. When rising to breathe they give the impression of a slow, forward rolling motion as if the dorsal fin is fixed on the circumference of a large rotating wheel. They usually surface at 10-20 second intervals, before diving for two to eight minutes while feeding at depths down to 200 metres. They are typically in small loose groups of two to eight individuals, mother and calf pairs or singles, although numbers can accumulate at feeding frenzies. They feed by foraging near the seabed, catching schooling fish, cephalopods and crustaceans (Carwardine, 1995).

They are usually relatively slow swimmers with a characteristic “rolling” action as they surface to breathe with an invisible but audible “puff” type blow. They can occasionally swim more vigorously, even leaping out of the water and tail-slapping but this is rare and often associated with social interaction. In calm conditions they can rest or bask on the surface for some time. Harbour Porpoises become sexually mature between three and five years. Most calves are born between May and August after a gestation of 10-11 months. Calves are weaned between four and eight months, but the mother may become pregnant again whilst still lactating. Their maximum lifespan is up to 24 years though 12 years is more typical (Evans et al, in Harris and Yalden, 2008).

Harbour Porpoises inhabit the areas over the continental shelf at depths of less than 200 metres and seem to prefer the more turbulent and tidal waters around headlands, islands and even into estuaries and up rivers. Sightings suggest that there are resident populations around the more favoured locations, although numbers increase at times of food abundance which suggests a willingness to migrate or perhaps some transient individuals in the population.

Porpoises occur around the coasts of the whole of the North Atlantic, the Baltic and Black Seas, as well as the North Pacific. It was estimated that the population in northwest European waters was around 340,000 in 1994 (Shirihai and Jarrett, 2006). Analysis of various data sources such as the Sea Watch Foundation and the Sea Mammal Research Unit indicated that Harbour Porpoise numbers are relatively low adjacent to the North East coast roughly between Scarborough and Berwick-on-Tweed compared to other areas of the North Sea coast further north and south (Reid et al, 2003).

Many of the North East records of Harbour Porpoise come from strandings. In the cases where cause of death has been ascertained, bye catch accounted for roughly half of the deaths, with starvation, pneumonia and parasitic infections being other notable causes.

The scarcity of records before 2000 makes it impossible to judge any trends accurately, although it seems clear that there has been a resident population off our coast throughout the period, albeit largely unrecorded. There was a particularly large count on 12 September 1993 which was from a chartered birdwatching boat some five miles east of the Farne Islands in perfect calm conditions when many small family groups totalling at least 100 animals were apparently feeding on shoaling fish, accompanied by many seabirds and two Minke Whales Balaenoptera acutorostrata (author’s own observation).

Since 2005 there has been a relatively consistent effort in recording Harbour Porpoises. Total numbers in excess of 200 animals were recorded in 2006, 2007, 2008, 2009 and 2011. These data indicate a substantial population living off the North East coast and only continued and co-ordinated survey efforts will enable an accurate picture of trends to be established. This should form merely a baseline for further studies.

The recorded distribution reflects to a large degree the location of observers who post their records, rather than the actual distribution of the Porpoise population. Note also that the Durham region has a shorter coastline with fewer headlands offering good viewing platforms than the others.

Records of Porpoise off the North East coast show a general trend of low numbers during May followed by an increase to maximum numbers in September. Sightings then level off until a dip in January followed by a rise again through to March which is followed by the decrease in April to the minimum in May. This reduction in sightings in May coincides with the time when Porpoises are recorded as usually giving birth and there may be as yet undiscovered “maternity ward” areas close to the region but away from regular observers to which the pregnant females migrate as they approach their full term. Certainly there is an increase in sightings of mothers with young calves in early summer when they return to our region.

It is not possible to judge how much influence the variation in observer effort affects these results, but the decrease in the population in the early summer coincides with plenty of seawatching activity from the birdwatching fraternity, many of whom contribute records. This would suggest that the May decrease is a genuine decrease in the inshore population of Porpoises off our coasts. Much more research is needed to find out whether this is due to migration or other behaviour linked perhaps to breeding habits. Certainly the increase during July to October could well be linked to the migration of the Mackerel Scomber scombrus and Herring Clupea harengus shoals through the North Sea. In recent years these shoals have been very large attracting other species of Cetaceans including whales. The absence of a large commercial fishing effort against these shoals at present bodes well for the larger predators which seem to be making a comeback in recent years.

The population of Harbour Porpoise off our coast seems to be reasonably healthy and numbers are apparently fairly static. The apparent increase in potential prey species, like the summer run of Herring shoals down the North Sea, could be seen as a positive trend for the future of Porpoises and other cetaceans. Similarly, the increase in Salmon Salmo salar entering the Tees has attracted individuals as far upstream as the Barrage. The reduction in pollution in our rivers can only be good but there are still residuals in the ecosystem which pose threats to health.

Threats to Porpoises are still very real and include the use of monofilament gill nets which are set close inshore where they frequently feed. Similarly, recent surveying and engineering activities in Tees Bay in connection with Offshore Wind farms may have an unforeseen influence on local populations of both prey and Porpoises. The loud thumping noise produced by the pile-drivers laying the windmill bases coincided with a decrease in sightings off Hartlepool Headland, although small numbers of animals returned after the disturbance stopped (author’s own observations). The increasing use of speedboats, high speed Jet-skis and even Kite-surfers close offshore must pose a threat of both physical damage and at least disturbance to this species. Recent research on Bottlenose Dolphins Tursiops truncatus has shown that they can be very aggressive towards Porpoises to the point of killing them on occasions. Killer Whales Orcinus orca also regard them as prey species, but as neither of these large predators is very common off our coasts they do not currently pose a significant threat to the general population.

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Written by Chris Bielby (last updated Nov 12)