Scallop aquaculture is the commercial activity of cultivating (farming) scallops until they reach a marketable size and can be sold as a consumer product. Wild juvenile scallops, or spat, were collected for growing in Japan as early as 1934.Kinoshita T (1935) A test for natural spat collection of the Japanese scallop. Report of the Hokkaido Fish Research Station, 273:1-8. The first attempts to fully cultivate scallops in farm environments were not recorded until the 1950s and 1960s.Shumway SE & Parsons GJ (2006). Scallops: Biology, Ecology and Aquaculture. Elsevier B.V., Amsterdam. Traditionally, fishing for wild scallops has been the preferred practice, since farming can be expensive. However worldwide declines in wild scallop populations have resulted in the growth of aquaculture. Globally the scallop aquaculture industry is now well established, with a reported annual production totalling over 1,200,000 metric tonnes from about 12 species. China and Japan account for about 90% of the reported production.
Cultured species There are varying degrees of aquaculture intensity used for different species of scallop. Therefore, cultured species can be divided into operations that are commercially well-established, those in the early commercial stages, those in development or experimental stages and those where potential for commercial farming has been expressed. Some species fall under multiple categories in different world regions.
Established commercial operations
Aequipecten opercularis (United Kingdom, northern France and Spain, Norway) Argopecten irradians (China) Sub species A. irradians irradians (eastern USA) Sub species A. irradians concentricus (eastern USA) Argopecten purpuratus (Chile) Chlamys farreri (China) Chlamys islandica (eastern USA) Chlamys nobilis (Japan, China) Mizuhopecten/Patinopecten yessoensis (eastern RussiaJapan, China, Western Canada [hybridized with Patinopecten caurinus]) Pecten fumatus (Australia) Pecten maximus (United Kingdom, northern France and Spain, Norway) Placopecten magellanicus (eastern USA)
Early commercial operations Argopecten ventricosus (Mexico) Chlamys islandica (Norway) Crassadoma gigantea (western North America)
Developmental or experimental
Aequipecten tehuelchus (Argentina) Aequipecten opercularis (Norway) Euvola ziczac (Venezuela) Nodipecten nodosus (Brazil, Venezuela)Cervigon, Fernando (Editor), 1983: La acuicultura en Venezuela. Caracas. 123p Patinopecten caurinus (western North America) Pecten maximus (China)
Species with potential Amusium balloti (Australia) Amusium pleuronectes (Philippines) Chlamys varia (northern Europe) Chlamys islandica (northern Europe) Euvola vogdesi (Mexico) Euvola ziczac (Brazil) Flexopecten flexuosus (northern Europe) Nodipecten subnodosus (Mexico)
Other species of note Attempts at cultivation of Chlamys hastate and Chlamys rubida in western North America have been halted due to the small size and slow growth of both species. Initial attempts made at cultivation of Pecten novazelandiae in New Zealand were hampered by large levels of fouling by mussels and by competition from a largely successful natural fishery.
Methods of culture There are a variety of aquaculture methods that are currently utilized for scallops. The effectiveness of particular methods depends largely on the species of scallop being farmed and the local environment.
Spat collection Collection of wild spat has historically been the most common way obtaining young scallops to seed aquaculture operations. There are a variety of ways in which spat can be collected. Most methods involve a series of mesh spat bags suspended in the water column on a line which is anchored to the seafloor. Spat bags are filled with a suitable cultch (usually filamentous fibers) onto which scallop larvae will settle. Here larvae will undergo metamorphosis into post-larvae (spat). Spat can then be collected and transferred to a farm site for on-growing.
Spat collectors will be set in areas of high scallop productivity where spat numbers are naturally high. However, to establish where the most appropriate areas to collect spat are, trial collectors will often be laid at a variety of different sites. Well-funded farms can potentially set thousands of individual spat bags in collection trials.
Hatcheries Scallop hatcheries provide a number of advantages over traditional spat collection for supplying seed to aquaculture operations, most notably in selective breeding and genetic manipulation, as well as providing a regular supply of spat at a low price. While initial attempts to culture scallops in hatcheries were fraught with extremely low spawning and high larval mortality rates,Dabinett PE (1989). Hatchery production and grow-out of the giant scallop Placopecten magellanicus. Bulletin of the Aquaculture Association of Canada, 89(3):68-70. a number of successful techniques have now been developed.Neima PG (1997). Report on commercial scallop hatchery design. Canadian Technical Report of Fisheries and Aquatic Sciences, No. 2176. 55 pp.
One of the most important aspects of hatchery rearing is obtaining and maintaining broodstock. Broodstock must be conditioned so to stimulate gonad development leading up to spawning and much research has been devoted to identifying the best diets and water quality requirements for broodstock. Once broodstock have been conditioned, spawning can be induced. This is most commonly achieved by varying water temperature, increasing water circulation, or by an injection of serotonin (a neurotransmitter). Following spawning, scallop eggs will develop into the "D" larval (shelled) stage in 2 to 4 days post-fertilization. As larvae, they continue to grow and can be fed a variety of microalgal diets with mixed algal diets being reported as giving higher growth rates than single species diets.Ryan CM (2000). Effect of algal cell density, dietary composition, growth stage and macronutrient concentration on growth and survival of giant scallop Placopecten magellanicus (Gmelin, 1791) larvae and spat in a commercial hatchery. MSc Thesis. Memorial University, Newfoundland. Settlement of larvae in hatcheries typically occurs between 35 and 45 days after fertilization of the scallop eggs when larvae are approximately 250 μm in size. Following settlement, the larvae undergo metamorphosis where they rearrange their body form to begin their life as a seafloor dwelling juvenile scallop. Mortality rates are often highest during metamorphosis as larvae go through a series of behavioral and anatomical changes such as loss of the velum (the larval feeding structure) and development of new filter-feeding mechanisms and gills. Post-settlement spat may be further grown in nursery tanks before being transferred to an on-growing system such as pearl nets.
Grow out stage There are two recognized systems for the grow out stage of scallops. These are hanging culture and bottom culture. Each has its own benefits and drawbacks in terms of cost, ease of handling and quality of the finished product. Enclosed culture systems still have not yet been fully developed in a commercial context and development continues in this area. Such a system would have large advantages over other forms of culture as tight control of feed and water parameters could be maintained.
Hanging culture Hanging culture relies on either a raft or longline system (with buoys and lines) that floats on the sea surface from which the cultured scallops are suspended, usually on ropes to which they are attached in some manner. Rafts are considerably more expensive than the equivalent distance of longline and are largely restricted to sheltered areas. However, raft systems require much less handling time. Longlines have proved effective for most farms to date and have the added advantage of being able to be completely submerged (with the exception of marker buoys) so to reduce visual pollution. From a raft or longline a variety of culture equipment can be supported. The main advantage of any form of hanging culture is in the exploitation of mid-water algal populations that cannot be fully utilized in other forms of culture.Hardy D (1991). Scallop Farming. (ed. D Hardy) Fishing News Books, Blackwell Science, Oxford.
Pearl nets Once scallop spat have been collected, the most common way of growing them further is in pearl nets (small pyramid shaped nets usually about 350mm across with 2–7mm mesh). Here, they are usually grown to approximately 15mm in high stocking densities. Pearl nets are typically hung ten to a line and have the advantage of being light and collapsible for easy handling. Scallops are usually not grown to larger sizes in pearl nets due to the light construction of the equipment. Once juveniles have reached a desired size they can be transferred to another form of culture.
Lantern nets Lantern nets were first developed in Japan and are the most common method of growing out scallops following removal of juveniles from pearl nets. They allow the scallops to grow to adulthood for harvest due to their larger size and more sturdy construction. Lantern nets are employed in a similar fashion in the mid-water column and can be utilized in relatively high densities. Flow rate of water and algae is adequate and scallops will usually congregate around the edges of the circular net to maximise their food intake.
Ear hanging Ear hanging methods were developed to be a cheaper alternative to lantern nets. Subsequently, research has shown that growth of ear-hung scallops can also be higher than those in lantern nets. Ear hanging involves drilling a hole in the scallop ear (the protruding margin of shell near where the two shells join) and attaching it to a fixed submerged line for growth. Such a process can be relatively labor-intensive as each scallop must be individually handled and drilled (however, many operations now have machines for this process). Furthermore, high mortality rates can result from drilling if scallops are too small, are drilled incorrectly, or spend too much time out of water and become physiologically stressed. This has resulted in research being conducted into the optimal drilling size. This size has been shown to be species specific with small species not having good survival rates. As such, ear hanging is an effective method of growing out larger scallop species. If ear hanging is an appropriate method, scallops can be densely stocked in pairs on lines with as little as 100 mm between pairs. Scallops are maintained in this fashion for two to three years until harvested. A variety of attachment products are constantly being tested with the best growth so far being obtained with a fastener called a securatie.
Rope culture Rope culture is very similar in principle to ear hanging with the only difference being the method of attachment. In rope culture, instead of scallops being attached by an ear hanging, they are cemented by their flat valve to a hanging rope. This method results in a similar growth and mortality rates as ear hanging but has the advantage of being able to set scallops from a small size. New cementing technologies are being continually developed with the aim of producing quicker setting adhesives to minimize the time scallops spend out of water so to minimize stress.
Pocket nets Pocket netting involves hanging scallops in individual net pockets. Pockets are most often set in groups hanging together. Pocket nets are not used extensively in larger farms due to their cost. However, handling time is low and so can be considered in smaller operations.
Hog rigging Hog rigging involves netting pockets of three or four scallops tied around a central line. This method is quick and cost effective and has been used to a great extent in the European Queen Scallop (Aequipecten opercularis) industry. However, its success in larger scallop species has been limited.
Plastic trays