Bfn Design and installation: verschil tussen versies

Versie van 4 okt 2017 om 12:41

The research was conducted in two parts, a laboratory experiment and a field experiment. The combined effect of different food supply and inundation time were tested in a laboratory experiment (Figure 1.) where oysters were exposed to inundation times of 62%, 82% and 100% inundation and water of different ratios of organic (phytoplankton) and inorganic (silt) material (10000 cells/ml algae, (Rhodomonas salinas) with 0mg, 100mg or 200mg silt).

Figure 1. Actual experiment as set up in the SeaLab. (Van den Brink en Capelle 2017).

A field experiment was set up to test the effects of inundation time (50% or 70% and hydrodynamics (low and high) on the growth and survival of the oysters. The breakwaters at Schelphoek and at Goese Sas were chosen as locations on which to place a new series of oyster sacks. These areas are both accessible for researchers, but are not frequented by the public. Unfortunately the field experiment was much afflicted by damage and washing away of equipment as well as theft of the oysters so that the oysters at Goese Sas were not found at the end of the project to no data was available for the whole 10 months. The oysters at Schelphoek did remain long enough to gather data over 10 months and these are presented below. At each location three oyster sacks were attached to a 1 x 2 m steel mesh sheet with cable ties, and placed on the 'kreukelberm'. At each corner the steel sheets were attached with cable ties onto a steel anchor point set in a concrete block (each weighing around 27 kg). These blocks fit in between the rocks on the kreukelberm to help secure the mattress from water induced movement. (Figure 2.) On each set of oyster sacks a sign was attached to dissuade potential thieves (Figure 3.).

Figure 2. Three oyster sacks were attached to a steel mesh sheet with cable ties. (Van den Brink en Capelle 2017).

Figure 3. On each set of oyster sacks a sign was attached to dissuade potential thieves. (Van den Brink en Capelle 2017).

Unfortunately the signage was not enough to dissuade potential thieves. To increase security cages or gabions, made of the same steel mesh as the mattresses were constructed to replace the mesh sheets with lids that could lock with high grade marine pad locks. Oysters were then placed in the gabions which were set on the 'kreukelberm' and weighted with the same concrete blocks to prevent them washing away (Figure 4.).

Figure 4. Oyster sacks were placed in steel gabions at two heights and at two levels of hydrodynamics at De Schelphoek. Oyster sacks as placed at both heights (low for 70% inundation time and high for 50% inundation time) on the low hydrodynamic side of the breakwater (left), and on the high hydrodynamic side of the breakwater (right). (Van den Brink en Capelle 2017).

The oysters were measured three times during the summer of 2016. The oyster sacks were removed from the gabions and opened carefully. Each individual sack was emptied into a photo tray (Figure 5.) and the empty sack was weighed with an electronic balance. The number of live and dead oysters was recorded for each sack.

Figure 5. Oyster sacks were emptied into a photo tray, where they were counted, weighed and the number of dead individuals recorded. (Van den Brink en Capelle 2017).

Locations for the second attempt at the field experiment. Rectangles indicate approximately where the oysters were placed, red: high hydrodynamic area, blue: low hydrodynamic area. (Van den Brink en Capelle 2017).

The live oysters were then weighed separately before being returned to the oyster sack along with the empty shells of the dead oysters to maintain the scouring effect. The sack was sealed with a cable tie and put back into place in the cages. The average weight per living oyster could hereby be calculated and extrapolated to estimate growth rate (Van den Brink en Capelle 2017).