Native oyster project

During summer 2012, ROV video surveys have been carried out in Lough Foyle on Barney’s Bank, Redcastle and Bottom of Middle Bed to begin gathering baseline data of habitat availability, quality and biodiversity, and inform habitat enhancement experiments and other future work.
In order to begin investigating questions regarding habitat quality and spat settlement in Lough Foyle, a small-scale trial was established on a 12 ha area of the Redcastle Bed in June 2012.

ROV camera shot showing peacock worms (Sabella pavonina) on the Barney’s Bank bed

Firstly, areas of the bed were “harrowed” – an operation similar to ploughing a field ready for seed sowing, where a bagless oyster dredge is towed across the seabed. The aim is to turn over heavily fouled shell, etc. and exposing cleaner surfaces.
Dredge samples were taken before and after, then the area was closed for the fishing season. Dredge samples will be taken again in Spring 2013 to assess oyster spat settlement in harrowed and unharrowed plots.

Scattered shell on the seabed with a shore crab (Carcinus maenas) swimming out of the way of the ROV

To examine cultch and settlement, creels filled with oyster shell were deployed in the area, 6 with live oysters and 6 without. Creels were used as they occupy a known area of seabed and are easily retrieved.
The pots were lifted from the beds at the end of November 2012 and the contents analysed analysed for settlement of native oyster spat and other species, including fouling organisms such as keel worms (Pomatoceros triqueter)
The intention is to continue these investigations and experiments in 2013.

The ROV occasionally picks up passengers on the way – this shore crab (Carcinus maenas) came up clinging on for dear life!

A survey of native oyster spawning activity during the breeding season (June to September) was carried out by the Loughs Agency on six oyster beds in 2011. The IBIS native oyster project undertook a similar survey from 14/06 to 27/09 in 2012, with the addition of the Southside oyster bed. Plankton samples, live oysters and environmental data such as temperature and salinity were collected on a weekly basis. Bivalve larvae in the plankton were assessed and live oysters were examined for gonad development and brooding of eggs and larvae.

Oyster brooding later stage larvae (the purplish mass). Oysters with larvae in their mantle cavity are referred to as “sick”. There is a lovely observation from the 1600s of the oysters being sick in June but being “better” by August

Data from the survey can be used to inform work such as habitat experiments, landing sizes and broodstock requirements. Observations during the survey have raised ideas for additional areas of investigation and future revisions to survey protocols. The native oyster project will also be investigating molecular methods of identifying larvae in plankton samples.
These surveys may be a useful management tool; monitoring the fishery during the close season is a key audit point and may act as an early warning system by highlighting factors that may be influencing recruitment into the fishery. It is hoped that similar surveys will be carried out in future years to build a longer-term picture of native oyster reproductive cycles in Lough Foyle.

Larvae from the oyster in the image above, viewed via a light microscope