May 22, 2017 By Ruby Gonzalez
Shellfish farmers using the largest lagoon in southern France can now hope for better oyster recruitment following release of a study by French researcher Dr. Martin Ubertini entitled “Gametogenesis, spawning behavior and larval abundance of the Pacific oyster Crassostrea gigas in the Thau lagoon: Evidence of an environment-dependent strategy.”
“The study was released in February, and we truly hope that it will benefit oyster farmers by highlighting favorable temporal windows for post-larvae harvesting,” Dr. Ubertini told Hatchery International. “Following our description of oyster gametogenesis/spawning, they could deploy harvesting devices at the moment with the highest probability of oyster recruitment.”
Ubertini et al are from Ifremer/ UMR MARBEC, Eureka Modelisation and Ifremer/ UMR LEMAR, all located in France.
Temperature may not be the only factor involved in Crassostrea gigas (Pacific oyster) reproduction according to the study, which was conducted during the first two years of a three-year project, from 2012-2014.
“It appears from our study that the reproduction window within the Thau Lagoon stretches from the beginning of June to the end of September, and is characterized by multiple, asynchronous and partial spawning events.”
“From a quantitative point of view, gametogenesis intensity – GSI, fecundity – was correlated to the abundance of diatoms. Once gametogenesis is complete, spawning can occur according to three types of spawning triggers,” the authors cited. These are locking, synchrony and stressful triggers.
Temperature should be considered as a “locking trigger” with a threshold value above 22°C.
The moon – both full and new – appeared as a “synchrony trigger” that maximizes chances of gamete matching.
The third is an additional trigger, like thunderstorms that act more like a stressful trigger because they may increase the gamete release.
“These new findings should play a part in the conceptual pattern of C. gigas reproduction in non-tidal Mediterranean,” note the researchers.
With a hierarchy of spawning triggers established, they said shellfish farmers in Thau could better forecast spawning events.
The Thau lagoon, the biggest in the South of France, provides 9.5% of the total French C. gigas production. Despite this, the larval recruitment of oysters within the lagoon “is still misunderstood and spat collection remains highly variable.” The situation leaves shellfish farmers to rely mostly on hatchery spat.
The lagoon is about 280 km2 and is almost closed. It is linked to the Mediterranean only through Sete Channel and other small connections that are negligible in terms of water exchange.
The study, the first in this field, aimed to describe reproduction features of C. gigas within the Mediterranean, as well as to explore spawning triggers of this species. It followed the reproduction cycle from gametogenesis to spawning in several locations within the Thau lagoon.
“Temperature was insufficient alone to explain spawning, highlighting other potential triggers such as the moon or lightning strikes. We made the hypothesis of a hierarchy of events triggering spawning, encompassing locking, synchrony and intensifying triggers,” the authors said.
– Ruby Gonzalez
• The reproduction features of Crassostrea gigas were assessed for the first time in a non- tidal Mediterranean lagoon.
- Phytoplankton concentration and assemblages affected gametogenesis, diatoms having a positive effect.
- No spawning event was observed below 22°C, and 23°C appeared as a critical temperature for significant spawning events.
- Full and dark moon in combination with high temperature did enhance spawning events.
- A hierarchy of spawning triggers was established, which can help shellfish farmers to better forecast spawning events.
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