The care that is generally taken to preserve the quality of products during harvest, sorting and transport has already been mentioned. Since the emphasis is in marketing the product in as fresh a state as possible, only very simple preservation and processing techniques are employed for aquaculture products, except for those that are meant for export.
Salmon is probably a unique aquaculture product where extreme care is taken in handling, slaughtering and preservation. The pre-cautions taken to reduce stress at harvest to preserve the meat quality have been described earlier. An additional means of maintaining
good meat quality is by bleeding the fish when alive. Besides slowing the degenerative process, this improves the appearance and taste of the meat. The usual procedure is to stab each fish with a knife behind the gills, in order to cut the major blood vessels. The carcass is then placed in tanks containing cold running water or in small floating nets, to bleed for a few minutes. Bleeding is done either on the farm itself or on arrival at the packing plant. The bled carcasses are generally gutted and cleaned and packed between layers of flake ice in polystyrene boxes. Such high-quality fresh fish fetches premium prices. Only when the market is depressed or consumers specially require it will good Atlantic salmon be frozen. In contrast, a major part of the trout harvested is frozen in plastic wrappings.
Where there is a demand for smoked salmonids, the fish are gutted, split and smoked for transport to markets, packed individually in styrofoam packets. Soft or hard smoking is adopted, according to market preference. The value of the product is significantly increased by this process. Smoked milkfish has a good market in the Philippines and smoked oyster meat is eaten in some countries like Japan.
Despite the preference for live or fresh products, freezing becomes necessary in certain cases because of market conditions or when the presentation of the product will improve by the process. For example, the colour of American catfish is not very attractive to many consumers and so they are headed, gutted, skinned and then frozen for transport. Skinning is particularly difficult and labour-intensive when done by hand, but mechanical skinning devices have been developed. The same appearance problem and the general perception of it as a poor-quality fish have led to the preference for raising a red-coloured hybrid variety of tilapia, which can be sold more readily in frozen fillets. They are often marketed under more attractive trade names, such as fresh-water snappers, fresh-water perch, St. Peter’s fish, etc.
When long-distance transport is involved, especially for export, shrimps are preserved by freezing, following the usual shrimp processing procedures. They may be headed, shelled and deveined before freezing. Oysters, when not sold in the shell, are generally shucked by hand using an iron spatula. Mechanical shucking
devices are also used in many areas. The shucked oysters are kept under refrigeration for one or two days, before packing and transport to markets.
Historically, the harvesting and processing of seaweeds have been small-scale manual operation but, with the rapid expansion of seaweed farming, mechanical devices are increasingly used. The commonly farmed seaweed Porphyra sp. (nori) is processed into dry sheets. The collected weeds are drained of all the water and then chopped with knives manually or with mechanical cutters that can be adjusted to cut thin or thicker slices, depending on the hard-ness of the thalli. A suitable quantity of water is added to the chopped weeds (usually about 13–14 l fresh water for 1kg weeds) and mixed thoroughly. The mixture is poured on to a simple moulding device, which consists of a bamboo matting on which a frame is set up to keep the mixture in the form of a uniform sheet. The mixture spreads within the frame and the water drains through the holes in the mat, leaving the nori in a thin sheet. To reduce the labour involved, seaweed sheeting machines are now widely used. The mats with the sheets are dried in the sun or in indoor driers. When well dried, the sheets are removed from the mats and made into convenient bundles.
Canning or other types of preservation are seldom practised on any significant scale for aquaculture products, other than for mussels and oysters. Small-scale attempts have been made to find new markets for less popular species like the silver carp and tilapia by canning, with the main idea of reducing problems with fine bones. In countries like Spain, where large-scale mollusc farming is practised, it is difficult to market all the production in a live, fresh or even frozen state, so a sizeable canning industry for molluscs, particularly mussel meat, has developed. Speciality products such as stuffed carp or gefilte fish in cans and oyster sauce in bottles are sometimes made from cultured species.