To end October and bring to a close our blog on harbour porpoises we hear from Fiona Read who is currently studying bycaught harbour porpoises from the north-west Iberian Peninsula. As with other areas that we've heard about in previous blog entries, harbour porpoises are subject to intense pressure from fisheries in these waters. Fiona is trying to answer a multitude of questions from the information that is provided by the animals that strand in the region. It has become apparent in this last month that some populations of harbour porpoises, although being a common sight in coastal waters, can in fact be seriously threatened and need protection. Over to you Fiona .....
The north-west Iberian Peninsula (NWIP), comprising Galicia (north-west Spain) and northern Portugal, is one of the world’s main fishing regions and a large variety of fishing gears including purse-seines, traps, trawls, gill nets and long-lines are used. The area also has one of the highest rates of marine mammal strandings in Europe and a high number of cetacean by-catches (by-catch is when you catch a non-target species), including harbour porpoises (Phocoena phocoena).
In recent genetic studies, the NWIP harbour porpoise was identified as a unique population and in 2005 SCANS-II (Small Cetacean Abundance in the North Sea and Adjacent waters II) estimated the Iberian population to be 2,600 porpoises. Little work has been conducted on growth and reproductive patterns of harbour porpoises in the NWIP but they are much larger than harbour porpoises found in more northern Atlantic waters such as the Netherlands, the United Kingdom and Greenland. Because porpoises are very shy animals it is not possible to conduct photo-id on them like other marine mammals. Therefore, we have to reply on data from stranded animals. Strandings in Galicia are attended by CEMMA (Coordinadora para o Estudio dos Mamiferos Mariños) and SPVS (Sociedade Portuguesa de Vida Selvagem) attend strandings in Portugal. When a porpoise strands CEMMA and SPVS go to the beach to conduct a necropsy to try and determine why the animal has died. As many as 50 % of stranded harbour porpoises in the NWIP die from interactions with fisheries, mainly fishing gear, where they most likely swim into the net, become entangled and as a result, die from drowning.
Now for the complicated part! The Agreement on the Conservation of Small Cetaceans on the Baltic and North Seas (ASCOBANS) and the International Whaling Commission (IWC) state that an anthropogenic (human induced) removal of more than 2 % of the population is unacceptable. However, in order to know how many porpoises die each year from fisheries interactions we need to know some basic data about the population and this includes the age structure of the population.
So how do we age a porpoise?
Ageing a porpoise is just like ageing a tree! Harbour porpoise teeth are spade shaped and very small (see figure 2). We take a tooth from the stranded porpoise and using a machine called a cryostat we cut very thin sections of the tooth, stain them purple-blue and then look at the sections under a microscope. Just like a tree, the tooth has annual growth layer groups (GLGs) consisting of one dark and one light band. By counting these GLGs we can determine the age of the porpoise. Figure 3 shows a harbour porpoise tooth under a microscope. This animal was 4 years old when it died.
We then use this age-at-death data to make a life-table of the population and determine the annual mortality rate of porpoises. Preliminary data from the NWIP indicates that around 7.5 % of the Iberian harbour porpoise population dies annually from fisheries interactions. This value is very high and most likely unsustainable! This data is used to make conservation plans to help reduce the rate of fisheries mortality and try to conserve the Iberian harbour porpoise population. Stranded porpoises are also used to study life history (e.g. age at sexual maturity), toxicology, parasitological, virology, bacteriology, epidemiology and many other studies.