CLAM CULTIVATION. Clams include cockles, razor-clams, manila clams, and quahogs among others. The most important of these species in a commercial sense is the Manila clam ( Tapes philippinarum ) which accounts for 70.57% of all global clam landings (FAO, 2007 ).
The majority of world clam landings come from wild capture fisheries, with many stocks being regulated in order to prevent over-exploitation. Regulatory measures are often placed on the fisheries restricting the number of licences issued, gear used, season, minimum landing size (MLS) and quotas. Examples of regulated fisheries are the cockle (Cerastoderma edule) fisheries in the Netherlands and the UK. In some regions of the world there are fisheries for spat which are then relayed in shellfish parks for cultivation, where there is more control over environmental conditions.
Quantities of commercially harvested clams a re difficult to derive and aren’t precisely known as they are often not distinguished from wild caught clams when landed or reported. This problem is compounded by the grey area in defining what is classed as cultivated product and what is cl assified as r egulated fisheries (Spencer, 2002).
The source of cultivated clams can come from either wild seed or hatchery produced
seed. In general, clam seed is easy to produce in a hatchery, however there are only a
few species for which hatchery produced seed is available in commercial quantities. Hatchery production of seed tends to be for species which locally suffer from unpredictable spatfall in the wild. In the UK there is hatchery production of Tapes philippinarum and Tapes decussates (Groo ved Carpet Shell) on a regular basis with
Mercenaria mercenaria (Hard Shell clam) occasionally being produced (Spencer, 2002). The Manila clam was introduced into the UK and as such is unable to reproduce in the natural environment. The cultivator is therefore reliant on hatchery
produced seed, which can be obtained at a range of sizes between 5mm and 30mm shell length.
The collection of wild seed can range from simple hand collection using rakes and
sieves to the mechanised use of hydraulic dredges which allow access to deeper water where dense patches of seed may occur. Once obtained, the seed are sown in culture plots at more productive densities; however, in order to obtain improved production levels the stock must be protected from predators. One of the main users of wild captured seed for clam cultivation is China, with the focus being on three main species; the Manila clam, the blood cockle, and the razor clam.
Shallow ponds are created low on the intertidal zone, each several hectares in area, to collect the settling spat. The ponds are inoculated with algae such as Chaetoceros in order to provide the larvae and spat with a food source. The tidal flushing of the ponds is controlled in order to prevent the loss of larvae, spat and food. The yield of such ponds is typically between 750-1500 clams (5mm SL ) per m2 . The small clams are re-laid into prepared plots at densities of 180 per m2 , and can produce yields of 2-4kg m-2 within a year (Spencer,2002).
As with the Manila clam, shallow ponds are used for the collection of spat and initial
growout stage. If spatfall is particularly dense in an area then thinning may be required where the spat are re-laid in intertidal beds. Growout is usually around 2 years, with cockles reaching 20mm SL (Spencer, 2002) .
Plots are conditioned by loosening and smoothing of sediment in intertidal areas.
Larval content of the water column is monitored to predict the time of spatfall, and 3-5 months following the predicted spatfa ll the seed are collected. At this stage they are approximately 10mm in length, and are re-laid into growout beds at a density of 900-1800 per m2 . After 6 months the clams have reached 50mm SL and are harvested (Spencer, 2002)
Siting of Grow out Plots
Clam cultivation plots are generally in more sheltered areas away from extreme wind
and wave action. Although cultivation is possible in more extreme environments, the
structure s need to be more robust, with service and labour costs increased. There is
als o a danger that net-covered clams can be smothered before the problem can be rectified. Access to the culture plots is also an important aspect of site choice. If the sediment is too soft then the use of boats is required to access the plots, whereas har der substrates can support wheeled and tracked vehicles, as in France. Many suitable substrates can be found in estuarine areas, however some species such as the Manila clam and the Palourde prefer salinities above 24 psu, and as such are not suited to estuarine areas where the salinity is likely to drop below this level on a daily basis. Although clams can grow effectively high on the intertidal zone, improved growth is achieved further down the shore where they are covered by water for longer periods an d hence feeding for longer periods.
There are different methods of grow out for clam cultivation, each being categorised
under a different production system described in the introduction. The simplest form
of culture is the re-laying of spat into culture plots. With little or no modification to
the plots such methods can be classified as type 2 production systems; i.e. restocking. The use of growbags to culture clams, generally on trestles, can be considered as type 1 production, with wild harvested spat being cultured through a grow out phase. The final method is similar to re-laying spat in the sediment; however the use of netting to reduce predation creates a favourable environment and can be classified as a type 3 production system.