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Native Oyster Spawning Assessments

The native oyster fishery in Lough Foyle is completely natural and wild, and  un-enhanced by human interference. The oysters depend on natural conditions for growth and survival, and can be impacted by many environmental factors including weather and pollution.

Loughs Agency carries out surveys to identify patterns and changes so that we can develop management strategies to offset natural fluctuations.

Sampling starts during April and May. This allows us to gather data on, for example, water temperatures and plankton when the oysters are getting ready to breed.

From early June to the end of September when the oysters are breeding, we conduct surveys of their reproductive activity. A regular supply of juveniles is needed every year to replace the oysters lost to natural and fishing mortality, and to sustain the population. Building a long-term data set helps us monitor this.

In summer, we gather weekly samples from five of the main oyster beds. Thirty adult oysters are collected from each site and their length and weight are measured. The oysters are then opened so we can assess and record the development stage of the gonad or brood. This helps us to find out the proportion of brooding females in the population, and shapes decisions about retaining spawning stock.

We also collect a seawater sample from each site using a plankton net. This allows us to estimate and record the number of native oyster and other bivalve larvae present, as well as types of plankton.

We test water for temperature, salt, dissolved oxygen and cloudiness. A CTD  (Conductivity, Temperature and Depth) instrument is lowered into the lough on a line from a boat. This also provides us with a depth profile. All this information, and more, is used to assess the effects of environmental conditions on native oyster reproductive success.

At the beginning of the breeding season, we set up frames in the lough mounted with an array of spat, or juvenile oyster, collectors. When the season is ending, we examine them for newly-settled juveniles. Daily temperatures are also recorded throughout the season.

You can read the results of our stock assessments in our publications section.

You can read the results of our stock assessments in our publications section.

Her Majesty Queen Elizabeth II

21 April 1926 to 8 September 2022