PERTUIS. In a few decades, the life of Crassostrea gigas changed a lot. The study conducted by Ifremer on the Marennes-Oléron basin details the changes in a complex environment where fresh and marine water, currents and winds, air temperature and flows interact. The Charentais sluices are the best field of study. “With the Arcachon basin, this area is the only one in France where the oyster meets the ideal conditions to reproduce itself and it is the first in Europe for the production“recalls Patrick Soletchnik, a researcher at Ifremer's laboratory,” Resources Environment of the Charentais “, which is the first time in the world that such work has been conducted on aquaculture production.
Since the 1960s, Ifremer has been measuring the physical parameters of the oyster basin every year. From the first week of June to the last of August, the scientific organization also carries out oyster larvae sampling to help oyster farmers optimize the installation of their collectors, these plastic discs on which spat a thousandth of a millimeter are fixed to start making their shell. It is in summer that oysters release into the marine environment billions of gametes, the expulsion of this “milt” costing the animal two-thirds of its weight. These time series now allow Ifremer to compare data from two periods, 1977-1991 and 2000-2015.
Estuary temperatures rise and rivers weaken
SALINITY. The first lesson is no surprise. The temperature of the air like water has increased between the two chosen decades. For the atmosphere, the rise is 1.4 ° C since the beginning of the industrial era. For water, the measurement is more precise. “The surface marine waters of the Marennes-Oléron basin increased by more than 1 ° C between 1985 and 1995 and have remained since the 2000s at more than 2 ° C compared to the end of the XXe century“, says Patrick Soletchnik.
The second result is more surprising: the salinity of the water of the openings increases between the two periods from 0.6 to 0.9 grams of salt per liter (g / l) to reach 32.3 g / l (the ocean is on average at 40g / l). This estuarine environment “marinises” due to a decrease in freshwater inputs. Rivers internal to the Pertuis (Charente and Seudre) bring 58% of the water and those outside (Garonne, Loire) 42%. “However, a drop in rainfall associated with increasing human samples means that the inflow to the sea has decreased by 10 billion m3 between the two periods studied.“, continues Patrick Soletchnik.
New oyster colonization to Denmark
PHYTOPLANKTON. The last parameter studied is the abundance of phytoplankton, the food of the oyster. From an indicator, “chlorophyll a”, the researchers were able to demonstrate a 46% increase in this biomass. The explanation is due to the combined rise in salinity and water temperatures. More food means better health for the animals, which produces more gametes. Spat production is therefore 50% higher than XXIe century that in XXe. But this reproductive success could also be the consequence of the decline in oyster production. A quarter of the farms have indeed disappeared since 2000 mainly for lack of buyers. The stock of shellfish thus rose from 80,000 to 100,000 tonnes in the 1990s to 37,000 tonnes in 2012. “At 100,000 tons, oysters are able to filter the total water volume of the sluices in just a few days, assures Patrick Soletchnik. We hypothesize that accidental spat collection by oysters themselves decreased, increasing the chances of larval survival There is a shadow on the board, however, and the risk of encountering pathogens, including the herpes virus that causes high mortality in the last decade, is higher.
As an additional favorable element, the reproduction period becomes longer. In XXe century, it was concentrated on the months of July and August. It starts today in early June and ends in September, from 7 to 14 weeks. ” The production of gametes does not know any more peak in July but spreads throughout the month of June whose weather in Charente Maritime is not more spring but summerThe oyster seems to be an animal that benefits from climate change, so biologists thought that Crassostrea gigas could never breed north of the Loire estuary. However, there have been colonizations in … Denmark!