ICES Marine Science Symposia, 215: 416—423. 2002 The potential for ranching the scallop, Pecten maximus - past, present and future: problems and opportunities Dan Minchin Minchin, D. 2002. The potential for ranching the scallop, Pecten maximus - past, pres­ ent, and future: problems and opportunities. - ICES Marine Science Symposia, 215: 416-423. Ranching scallops requires a full knowledge of their biology, and this has only evolved during the last half-century. This knowledge needed to be merged with the technolog­ ical developments of plastics, improved power, improved navigation, and aided by legal implements. Scallop cultivation in hatcheries has greatly contributed to produc­ tivity of spat used as a source for several ranching programmes. Depletion of natural scallop populations has made it necessary to consider ranching as a means for creat­ ing a sustained resource. Few areas currently have sufficient natural settlements; when these occur, they vary in intensity from year to year. As a result, collections of wild spat cannot provide a consistent source of supply. Movements of spat may need to be controlled to maintain the diversity present in some isolated populations and to reduce disease, disease agents, and parasite transfers. Future opportunities exist for ranching scallops provided there is an improved knowledge of their interactions with other biota. Developments in biotechnology and reduced predation rates are likely to lead to significant increases in production. Flowever, the spread in the range of toxic algal events and exotic species could modify such expectations. Keywords: biology, culture, ranching, scallops. Dan Minchin: Marine Organism Investigations, 3, Marina Village, Ballina, Killaloe, Co. Clare, Ireland; tel: +35J 86 60 80 888: e-mail: [email protected]. Introduction development was advanced by the endeavours of those who frequently reported their findings to ICES Statu­ The discipline of ranching scallops in Europe has tory Meetings. In addition, knowledge gleaned for other evolved during the last 25 years following high exploita­ scallop species worldwide was avidly assimilated and tion levels and population declines. The possibility of used in attempts to develop a viable ranching industry. restocking using releases of scallops was inspired by the development of an industry in Japan based on cultivat­ ing the scallop Patinopecten yessoensis (Ventilla, 1982; Exploitation Aoyama, 1989). However, in order to get to this stage, a greatly improved knowledge of scallop biology and han­ The scallop has long been regarded as an item of beau­ dling skills was needed. The information that led to the ty on account of its unique shape and has been the sub­ first attempts to cultivate Pecten maximus relied on the ject of artistic impression for many centuries (Cox, basic biological information gained from the 1940s. 1957). Its shell has been used for vessels and containers Nevertheless, more information is still needed in order for thousands of years and over the last century has to improve growth and survival and increase recapture become one of the more highly prized of seafoods, lead­ rates. This account examines the development of ranch­ ing to its high level of exploitation. ing, with particular reference to what has happened in In Ireland, some of the earliest records date from Ireland. The range for Pecten maximus extends from about 5470 BC (F. Mitchell, pers. comm.) from shell northern Norway to the west coast of Africa (Mason, middens. Shells have also been found elsewhere in un­ 1983a) and possibly into the Mediterranean Sea, as dated cave sites (Jackson, 1934). There is some indica­ Pecten jacobeus may be a form of Pecten maximus. The tion that scallops may have been commercially fished countries principally involved in investigations have during the 16th century in Ireland (Stanley, 1967) and been Spain, France, the United Kingdom, Norway, and were captured on the west coast of Ireland in the mid-to- Ireland, all ICES Member Countries. Although ICES late 1600s, but not in sufficient abundance to form a had not evolved a specific programme for the ranching trade, according to an Inquiry Report of 1836. Andrews of P. maximus, much of the information enabling this (1847) reported that small boats took scallops on the The potential for ranching the scallop, Pecten maximus - past, present, and future: problems and opportunities 417 southwest coast, and Brabazon ( 1848) records the use of tury before, the idea of managing scallop stocks by en­ oyster dredges for catching scallops. These scallops hancement seemed practical. were caught and used for soup, but more especially they were important for baiting longlines; the curved gonad is well shaped for placement on a hook. At about this Understanding the biology same time, there was a small fishery in Strangford Lough which included boats from Jersey (Welch, 1934). There were unexplained changes in scallop seasonal One method, probably used over a long period of abundance together with alleged sightings of swimming time, was the taking of scallops on calm days using a scallops that led to the belief that they migrated. This hoop net on a long pole into which the scallop was hypothesis was explored by Gibson (1953); he studied tipped (Gibson, 1957). Storms have washed scallops the movements of tagged scallops and, following diving ashore on beaches, and such events still continue to take investigations with Baird (Baird and Gibson, 1956), place from time to time. Oyster dredges were used for suggested that scallops "migrate" from "feeder beds" - their capture, but with the addition of a toothed bar, areas where young scallops accumulated following set­ scallops could be more efficiently raked from the sedi­ tlement. Their contentions were, in fact, partly correct. ment. Boats either used sail for power or hand-winched Thirty-five years later, in the same bay, scallop spat a 50-fathom anchor line using a capstan while dragging were found to gradually disperse to the large expanse of the dredge (Gibson, 1957). From the 1920s, captured sediment, where adult scallops were found, after detach­ scallops were dispatched in jute sacks to the Billings­ ment from marine algae (Minchin, 1992a). The move­ gate market in London. This was attributable to the main ments that followed after byssal release could not be demand being outside the country and the lack of home- considered as a migration because there was no obvious processing facilities at that time. Production rose during actively directed movement. Current and wave wash the period of general food scarcity in England during most probably dispersed scallops to the more open and 1940-1949 (Gibson, 1956). With the development of dredged ground. Baird and Gibson also noted that freezing and processing facilities and increased storage younger scallops were poorly represented in dredge capacity, exploitation in Ireland increased from the hauls, but Baird was able to recover a greater proportion 1960s. The use of outboard motors and then the more of younger scallops when diving on the same grounds powerful inboard motors led to increases in catching (Baird and Gibson, 1956). From these investigations, it ability. In the 1950s, Dick Baird designed a dredge with was realized that the tooth spacing on the bar used to a diving plane to prevent the dredge from hopping on rake the scallops out of the bottom was the important the seabed (Baird, 1959; Rolfe, 1969) and over two measurement for the selection of adult scallops rather decades, a wide range of dredge designs ensued. The than the mesh size of the bag. shallow inshore beds were the first to be exploited. Some years later, scallops were found to move up the Further increases in power resulted in a greater number slope of large ponds and concentrate near the water sur­ of dredges being used to fish progressively deeper face. In the still water environment of a pond, the vec­ waters. The dredges were designed to fish a wide range tors of tidal current and wave surge could not override of substrates by having spring-loaded teeth that would any specific small-distance movements of scallops. It is not snag on boulders. Scallops collected in this way interesting to note that concentrations of scallops occur were often broken. close to the base of scree slopes in fjords. These move­ Pulses of landings followed the finding of previously ments may, in fact, be a migration up-slope to better unexploited beds (Mason, 1983a). Following a series of feeding conditions or to form concentrations so as to coastal surveys along the Irish coast, several new fish- maximize fertilization (Minchin, 1987). able beds were found (Lee, 1973a, 1973b). Improve­ Growth rings appearing on the flat shell were used to ments in fishing efficiency and in dredge design togeth­ age scallops (Mason, 1957). The reproduction cycle was er with vessel modernization eventually resulted in extrapolated from changes in the size, shape, and colour reduced stock numbers and the necessity for conserva­ of the gonads, verified by histological techniques (Ma­ tion. Legislation varied in each country owing to the son, 1958), and enabled calculations of natural mortali­ varying rates of growth and social circumstances. In ty of unexploited populations (Gruffydd, 1974a). The Ireland, a by-law prevented the more efficient means of identity of larval scallops in plankton samples was made procuring shellfish by diving. Some heavily exploited possible following the studies by Rees (1950). He had fisheries in France became dependent on a small num­ examined the morphology of bivalve larvae in the North ber of year classes, and there were concerns for the Sea, but more detailed studies of larvae were to follow future of the fishery following a period of poor recruit­ (Le Pennec, 1974), and the time at which they settled ment (Ansell et al., 1991). Reliance on continued ex­ could be predicted by following their growth in the pansion of the industry based on previously unfished plankton.