Bottom cultures are for the species living on the bottom of the sea, such as ark shells, Manila clams, etc. The basic operations of the bottom culture are based on the seed spreading and keeping the culture ground until harvesting. The parameters affecting the site selection are water depth, water temperature, algal foods, bottom composition, and natural appearance of the species to be cultured. Normally, water depths for bottom culture are within 20 meter for most of the species or within 40 meters specially for ark shell.

1. Bottom culture for Ark shell

The seeds of ark shells are spreading on the bottom below the intertidal down to 40 meter in depth. The wild seeds of 2∼3cm in shell length are scattered through on the selected farming ground in March to May. The preferred seedling is of density 500,000 spats per ha and at the time between the two tides when no tidal currents are evident. The ark shells can be harvested after I to two years after seedling.

1-1. The seed collection and on-growing

In wild condition the ark shells spawn in July to September. The preferred times for seed collections are 25∼30 days after fertilization with the larval sizes of 240∼280μ. Therefore, continuous monitoring of the wild spawning behaviors are necessary. For the larval attachment, natural or artificial products as a collector are submerged in the middle of the water column. For example, a location of 20∼30m in depth, the collectors are set in depth of 10∼20m for best attachment of the larvae.

The attached seeds are kept in the seed collectors for a month when the spats reach 0.2∼2.6mm in shell length. The spats are wrapped with polyethylene nets and moved to nursery system which is normally set in 3∼5m in depth until the sizes of 2∼3cm in shell length. After the nursery, the seeds are scattered onto the farming grounds

2. The bottom culture for Manila clam

The spawning of the clam occurs from late May to early November with main spawning seasons of 3 months starting June; July to August in the western waters and June to August in the southern waters. Farming grounds are normally selected muddy-gravel bottoms from the intertidal zone to 2 meters deep. Seeds sized 1∼2cm in shell length can be collected from wild zones. However, considerable farms are inducing larval settlement of the species on their farming grounds by simply standing branches of trees on the grounds which can reduce the velocity of the tidal current to enhance larval settlement. The collected seeds are scattered at the time between two tides when no tidal current is manifest. The seeds are scattered evenly on the basis of a million per ha. The on-growing goes for a year to achieve market sizes.

                          
A typical view of farming ground for Manila clam on the western coasts of Korean peninsula.
 

                   
The bottom culture system for Manila clam and a view of harvesting the clam during the low tide