Porpoise
The porpoise is the smallest cetacean living in the North Sea. There are around 300,000 porpoises in the entire North Sea. They used to be found in large numbers in the Zuiderzee and the Wadden Sea, when anchovies and other small fatty fish were plentiful. They have grown scarcer since 1950, however reports increased strongly between 1995 and 2006. This was probably due to a food shortage in the northern North Sea, whereby dolphins and porpoises swam into Dutch regions. A porpoise reservation has been established west of Sylt due to many young that are born in that area.
Porpoises have a rather flat head, are dark grey on their back and white on their belly. Males grow to 1.5 meters long and weigh around 45 kilograms. Females are usually larger: up to 1.8 meters and weigh 60 kilograms. The pectoral fins of the porpoise are black and a dark stripe runs from the corners of the mouth to the flippers. The small dorsal fin is more or less triangular.
It is not easy to observe porpoises at sea. Compared to other dolphins, porpoises rarely jump out of the water, so that one does not usually see more than the top part of its back with its dorsal fin when surfacing for a breath of air.
Porpoises live either alone or in groups of three to five animals, sometimes more. If one spots two animals together, it is often a mother and child. During migration, they sometimes form large groups. Just like dolphins, the porpoise orients itself underwater by using echo sounding or 'sonar'.
Mating season
The mating-season is in the period June to the beginning of August. The pregnancy lasts for 11 months, so a birth peak takes place in July. The young are 70 to 80 centimetres long at birth, one half as big as the mother animal. Most female porpoises do not bear young every year. The females are sexually mature at 5 or 6 years old. The calves will first start eating a little bit of fish when they are 4 months old. They still remain nursing by their mother till they are around 8 months old. Only after they stop nursing will their diet consist of fish only. Young porpoises often eat gobies. These are small fish living on the sea bottom.
Diet
The diet for the adult porpoise consists of all kinds of fish. In the Dutch section of the North Sea, the diet is mostly small benthic fish, herring, squid, whiting and cod. On the open North Sea they feed mostly on herring, sprat and mackerel. For porpoises in the German Wadden Sea, flatfish are the most important prey, while gobies are the main diet of porpoises in the Baltic Sea. Eating flatfish can be dangerous: various porpoises that have washed ashore were found to have suffocated after eating large flatfish.
Porpoises eat around 5 kilogram of fish daily. This is 10% of their body weight. There is a theory that porpoises help each other to look for food using their sonar system. This is a strange phenomenon for such animals that live primarily in solitude. If it is true, this would be a form of social foraging. One can imagine that it is easier to find food in a large sea when the group is spread out.
Distribution of the porpoise
The porpoise is found in all shallow waters in the northern hemisphere; that includes water from Norway to western Africa in the eastern part of the Atlantic Ocean. According to counts from 1994, there are between 267,000 and 465,000 porpoises living in the North Sea.In the Netherlands, most porpoises are observed along the North-Holland coast and north to northwest of the Wadden Islands. The best chance of seeing one is between December and April, close under the island coasts. The greatest numbers of porpoises wash ashore during this period.
Back again
It was normal in earlier days to see porpoises along the coast, in the Wadden Sea, the former Zuiderzee and the delta region. Up till the mid 1900s, there were several places where they could be spotted from land, such as the Marsdiep. More than 50,000 porpoises inhabited the Dutch section of the North Sea before the Second World War. In the 1950s and 1960s, the porpoise did not only decline in the Netherlands but along all of the coastal areas of the North Sea. The causes were (PCBs in particular), increasing fisheries resulting in a decrease in available food and entanglement in synthetic fishing nets.
Since 1995, many porpoises are regularly spotted again along the coast. The population in the southern North Sea grew so fast that it cannot all result from new births. Reports at sea support this observation. Extensive studies from sea-going vessels in 1994 (SCANS) and 2005 (SCANS2) showed that the total number of porpoises in the North Sea fluctuated in both years around the 250,000 animals. Two-thirds of the population swam in the northern North Sea in 1994, while just the opposite was found in 2005, where two-thirds were found in the southern North Sea. Because the number of fish-consuming birds in the northern North Sea is declining, the scientists assume that the porpoises left the region to find new sources of food. At any rate, the result is that it is relatively easy in the Netherlands to spot porpoises in the winter from land or a ferry.
Since 2007, reports of beachings along the Holland coast have been declining again. Whether or not they are moving back to the northern North Sea is not yet certain.
Strandings and research
Since 1995, the number of porpoises that wash ashore Dutch beaches increased with a peak in 2006. This increase is related to the growth in the number of porpoises swimming in the southern North Sea.
When a dead porpoise washes ashore, it is reported to Naturalis, the national nature history museum in Leiden. The museum in Leiden has been collecting and processing reports and stranded cetaceans or years. As many animals as possible are stored and investigated. Live porpoises that are too sick to return to the wild are brought to SOS Dolphin, at the Dolfinarium in Hardewijk, where they are given the chance to heal.
Historical studies show that a shift has occurred in the season for strandings. The greatest amount of porpoises washed ashore in the Netherlands in the period 1930 to 1965 in the summer, while nowadays the majority of the strandings take place in the autumn and winter. Studies of the cause of death in 2006 showed that half of the animals died from drowning. Most likely, they probably became entangled in nets from gill, trammel and standing rigging fisheries while in search of food near the bottom.
Studies of the fatty layer from recent beached porpoises showed that it contained a toxic brominated flame retardant (hexabromocyclododecane). This flame retardant is used in insulation foam and furniture textile. That material disrupts the hormonal working of the thyroid gland and the nervous system of the porpoise. Although the material is toxic, it replaces flame retardants that used to be used. According to the scientists, it is not known whether or not this retardant is less detrimental to the environment than former ones.
Victims of standing rigging fisheries
Danish fishery observers sailed together with fishermen in 1994 in order to seen how many porpoises were being caught. The numbers appeared to be frighteningly high: the first estimates were 7000 victims per year.
It concerns specifically the standing rigging fisheries, the most important fisheries in Denmark whereby sole, turbot and cod in particular are caught. However, porpoises are protected and may not be caught. Porpoises are attracted to the fish in the nets. They become entangled as well and drown because they are unable to surface for air.
More and more fishermen are attaching underwater speakers (pingers) to their nets which send out noise to scare the porpoises. Since 2007, vessels larger than 12 meters that fish with gill and trammel nets are required to use pingers.
A porpoise entangled in a net can badly damage the net. That is why many fishermen cut the animal loose as soon as possible. In many cases, fins and/or tails are cut off in the process and the entrails are removed so the animal sinks.
Whale louse is a kind of parasite which causes problems for porpoises
Names:
Dut: Bruinvis (gewone bruinvis, varken)
Lat: Phocoena phocoena
Eng: Harbour porpoise (common porpoise)
Fren: marsouin
Ger: Schweinswal (Braunfisch, Meerschwein, Kleiner Tümmler)
Dan: Almindeligt marsvin
Nor: Nise
Source: de Vleet, Ecomare
