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From the BBC (3/1/10): "Thousands of dead octopuses have washed up on a beach in northern Portugal, in what is being called an environmental disaster. They cover a 5-mile stretch of Vila Nova de Gaia beach - no reason has yet been found for their appearance. The authorities have warned the public not to eat them. " Angel Guerra (Instituto de Investigaciones, CSIC, Vigo) suggested an explanation in two e-mails circulated on the fastmoll list. I have summarised his comments here: This might be a massive mortality produced by a sharp change of salinity. If it has rained intensely several months, the contribution of fresh water that would have entered the estuary of the Tagus would be enormous. A decrease of the salinity from 35 to 29 could produce the death of these animals. A massive mortality of octopuses of the same species occurred with animals cultivated in suspended cages in the north part of the Ria deVigo (Galicia), which is also an estuary, a few years ago, when the contribution of fresh water closet for the rivers due to the intense rains increased. The appearance of the dead animals was similar in both cases (see Chapela et al 2006): the animals that died in the sea cages due to low salinity also displayed the arms rolled outwards and a very pallid colour. Oceanographers at IIM have noted that this autumn (2009), and especially the beginning of winter, has been characterized by heavy rains affecting the Douro and Minho Rivers (North west Spain and North Portugal). At certain times,for example dams upstream are open, the layer of fresh water transported by the rivers be several metres deep. Northerly or northeasterly einds can push this layer of fresh water against the south margin of the estuaries (for example where Vilanova de Gaia beach is located or where the suspended cages in the Ría de Vigo are located). This sudden influx of fresh water can produce high mortalities in sessile organisms such as bivalve mollusks (e.g. clams), as occurred in some parts of the Ría de Arosa in Galicia, and might also cause massive mortalities in animals with relative low capacity for displacement, as it is the case for octopuses, which are quite stenohaline organisms. |