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, pres­ent, 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 technolog­ical developments o f plastics, improved power, improved navigation, and aided by legal implements. Scallop cultivation in hatcheries has greatly contributed to produc­tivity o f spat used as a source for several ranching programmes. Depletion o f natural scallop populations has made it necessary to consider ranching as a means for creat­ing 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 o f wild spat cannot provide a consistent source o f supply. Movements o f 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 o f 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 o f 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: minchin@ indigo.ie.

Introduction

The discipline o f ranching scallops in Europe has evolved during the last 25 years following high exploita­tion levels and population declines. The possibility of restocking using releases o f scallops was inspired by the development of an industry in Japan based on cultivat­ing the scallop Patinopecten yessoensis (Ventilla, 1982; Aoyama, 1989). However, in order to get to this stage, a greatly improved knowledge of scallop biology and han­dling skills was needed. The information that led to the first attempts to cultivate Pecten maximus relied on the basic biological information gained from the 1940s. Nevertheless, more information is still needed in order to improve growth and survival and increase recapture rates. This account examines the development of ranch­ing, with particular reference to what has happened in Ireland. The range for Pecten maximus extends from northern Norway to the west coast o f Africa (Mason, 1983a) and possibly into the Mediterranean Sea, as Pecten jacobeus may be a form of Pecten maximus. The countries principally involved in investigations have been Spain, France, the United Kingdom, Norway, and Ireland, all ICES Member Countries. Although ICES had not evolved a specific programme for the ranching of P. maximus, much of the information enabling this

development was advanced by the endeavours o f those who frequently reported their findings to ICES Statu­tory Meetings. In addition, knowledge gleaned for other scallop species worldwide was avidly assimilated and used in attempts to develop a viable ranching industry.

Exploitation

The scallop has long been regarded as an item of beau­ty on account of its unique shape and has been the sub­ject of artistic impression for many centuries (Cox, 1957). Its shell has been used for vessels and containers for thousands of years and over the last century has become one of the more highly prized o f seafoods, lead­ing to its high level o f exploitation.

In Ireland, some of the earliest records date from about 5470 BC (F. Mitchell, pers. comm.) from shell middens. Shells have also been found elsewhere in un­dated cave sites (Jackson, 1934). There is some indica­tion that scallops may have been commercially fished during the 16th century in Ireland (Stanley, 1967) and were captured on the west coast of Ireland in the mid-to- late 1600s, but not in sufficient abundance to form a trade, according to an Inquiry Report o f 1836. Andrews (1847) reported that small boats took scallops on the

The potential fo r ranching the scallop, Pecten maximus - past, present, and future: problems and opportunities 417

southwest coast, and Brabazon ( 1848) records the use of oyster dredges for catching scallops. These scallops 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 same time, there was a small fishery in Strangford Lough which included boats from Jersey (Welch, 1934).

One method, probably used over a long period of time, was the taking of scallops on calm days using a hoop net on a long pole into which the scallop was tipped (Gibson, 1957). Storms have washed scallops ashore on beaches, and such events still continue to take place from time to time. Oyster dredges were used for their capture, but with the addition o f a toothed bar, scallops could be more efficiently raked from the sedi­ment. Boats either used sail for power or hand-winched a 50-fathom anchor line using a capstan while dragging the dredge (Gibson, 1957). From the 1920s, captured scallops were dispatched in jute sacks to the Billings­gate market in London. This was attributable to the main demand being outside the country and the lack o f home- processing facilities at that time. Production rose during the period o f general food scarcity in England during 1940-1949 (Gibson, 1956). With the development of freezing and processing facilities and increased storage capacity, exploitation in Ireland increased from the 1960s. The use o f outboard motors and then the more powerful inboard motors led to increases in catching ability. In the 1950s, Dick Baird designed a dredge with a diving plane to prevent the dredge from hopping on the seabed (Baird, 1959; Rolfe, 1969) and over two decades, a wide range o f dredge designs ensued. The shallow inshore beds were the first to be exploited. Further increases in power resulted in a greater number of dredges being used to fish progressively deeper waters. The dredges were designed to fish a wide range of substrates by having spring-loaded teeth that would not snag on boulders. Scallops collected in this way were often broken.

Pulses o f landings followed the finding of previously unexploited beds (Mason, 1983a). Following a series of coastal surveys along the Irish coast, several new fish- able beds were found (Lee, 1973a, 1973b). Improve­ments in fishing efficiency and in dredge design togeth­er with vessel modernization eventually resulted in reduced stock numbers and the necessity for conserva­tion. Legislation varied in each country owing to the varying rates of growth and social circumstances. In Ireland, a by-law prevented the more efficient means of procuring shellfish by diving. Some heavily exploited fisheries in France became dependent on a small num­ber o f year classes, and there were concerns for the future of the fishery following a period o f poor recruit­ment (Ansell et al., 1991). Reliance on continued ex­pansion o f the industry based on previously unfished populations was unjustified, and existing stocks were subject to high fishing pressure. As with the measures to cultivate oysters following their depletion almost a cen­

tury before, the idea of managing scallop stocks by en­hancement seemed practical.

Understanding the biology

There were unexplained changes in scallop seasonal abundance together with alleged sightings of swimming scallops that led to the belief that they migrated. This hypothesis was explored by Gibson (1953); he studied the movements of tagged scallops and, following diving investigations with Baird (Baird and Gibson, 1956), suggested that scallops "migrate" from "feeder beds" - areas where young scallops accumulated following set­tlement. Their contentions were, in fact, partly correct. Thirty-five years later, in the same bay, scallop spat were found to gradually disperse to the large expanse of sediment, where adult scallops were found, after detach­ment from marine algae (Minchin, 1992a). The move­ments that followed after byssal release could not be considered as a migration because there was no obvious actively directed movement. Current and wave wash most probably dispersed scallops to the more open and dredged ground. Baird and Gibson also noted that younger scallops were poorly represented in dredge hauls, but Baird was able to recover a greater proportion o f younger scallops when diving on the same grounds (Baird and Gibson, 1956). From these investigations, it was realized that the tooth spacing on the bar used to rake the scallops out o f the bottom was the important measurement for the selection of adult scallops rather than the mesh size of the bag.

Some years later, scallops were found to move up the slope o f large ponds and concentrate near the water sur­face. In the still water environment of a pond, the vec­tors of tidal current and wave surge could not override any specific small-distance movements of scallops. It is interesting to note that concentrations of scallops occur close to the base of scree slopes in fjords. These move­ments may, in fact, be a migration up-slope to better feeding conditions or to form concentrations so as to maximize fertilization (Minchin, 1987).

Growth rings appearing on the flat shell were used to age scallops (Mason, 1957). The reproduction cycle was extrapolated from changes in the size, shape, and colour o f the gonads, verified by histological techniques (Ma­son, 1958), and enabled calculations of natural mortali­ty o f unexploited populations (Gruffydd, 1974a). The identity of larval scallops in plankton samples was made possible following the studies by Rees (1950). He had examined the morphology of bivalve larvae in the North Sea, but more detailed studies o f larvae were to follow (Le Pennec, 1974), and the time at which they settled could be predicted by following their growth in the plankton. This helped in timing the deployment o f col­lectors, thus increasing yields and reducing fouling.

For a very long time, the existence of scallops to thumbnail size was rarely observed. The earliest record-

418 D. Minchin

ed small scallops were those found by the Reverend Barlee, a Church o f Ireland clergyman, in 1845. He col­lected large numbers o f small Pecten maximus from Bertrabouy Bay on the west coast o f Ireland, and these samples were recently rediscovered in the Smithsonian Museum. USA (Dr Thomas Waller, pers. comm). A cen­tury later, Elmhurst (1945) described the finding of some small spat, followed by collections o f spat by Eg­gleston (1962). The diving pioneer Jack Kitching found many spat attached to hydroids and marine algae (Kitching and Ebling, 1967) while working in Lough Hyne, Ireland. Mason (1969) then collected spat in localized areas where scallops had concentrated on the west coast o f Scotland. Here, scallops grew and de­tached and, where there were few currents, would accu­mulate on the sediment nearby. In strong current areas, they were probably widely dispersed at a smaller size, their byssus not being strong enough to retain them; cur­rents easily disperse the light spat. This could explain why they were rarely found in the strong tidal regions off the coasts o f the Isle o f Man and Brittany (Minchin, 1992a).

Gibson (1956, 1959) noted that there were two main settlements on the west and southwest coasts o f Ireland, and this, he claimed, explained the two modes of the first year’s winter growth rings found in these popula­tions. This first growth ring is not easily deciphered from shell markings, and it now appears that the two growth rings may now represent two different year classes (Minchin, 1992a). The modes from the two main periods of settlement in a given year tend to merge, owing to variable growth rates and selective predation, to give a single prominent mode. Most of the errors in the ageing o f populations stem from this early assump­tion.

Culture o f scallops

Through knowing the reproductive cycle of scallops, it was possible to spawn them in the laboratory (Comely, 1972; Sasaki, 1979; Gruffydd and Beaumont, 1972; Buestel et al., 1982). Settlements were obtained, often with difficulty because of varying egg quality (Cochard, 1987), in part determined by their lipid content (Paulet, 1989). With further knowledge o f broodstock manage­ment, it was possible to control reproductive activity (Lubet et al., 1987), and more predictable settlements became possible by using correct sperm concentrations (Gruffydd and Beaumont, 1970). The production of scal­lop spat over a long season enabled production o f large numbers on which to develop an industry.

For some laboratories, there has been a high depend­ence on antibiotics, which may reflect the geographical variations of water quality. Attempts at producing chea­per spat by spawning scallops in situ in sea loughs have been made; the fate o f the produced embryos is unknown (Minchin, 1992b), but could be explored fur­

ther (Minchin, 1993). The occasional success obtained through rearing larvae to settlement in mesocosms immersed in the sea (G. Burnell, pers. comm.) or in silos on land (Anderson et a l , 2000) may result in cheaper spat production once techniques are perfected. Advances in feeding recently settled scallops in nursery culture have also reduced costs (Rodhouse et al., 1981).

Reports o f scallop spat collections in the wild for dif­ferent countries were based on activities mainly in the 1970s in France (Buestel, 1976; Buestel et al., 1979), Ireland (Minchin, 1976), Scotland (Ventilla, 1977; Fra­zer and Mason, 1987), England (Pickett, 1978), Isle of Man (Brand et al., 1980), and Spain (Roman et al., 1985), although recruitment was found to be quite vari­able (Boucher, 1985). Spat were obtained using differ­ent collector designs ranging from onion bags holding small sections of confiscated drift salmon netting to "humbug" collectors made up of stiff tetrahedra o f plas­tic containing net fillers. It was clear that many o f these studies would not have advanced, but for the develop­ment o f plastic products used in pearl nets, lantern nets, trays, cages, and synthetic rope. Some of the plastics used were originally designed for other purposes, such as the dishwasher tray later to become the well-known NW Plastic trays, used extensively in shellfish culture during the 1980s and 1990s.

Environmental factors were also o f importance in the management o f stock (Gruffydd, 1974b; Wilson, 1987). The layout of longlines tested and used in cultivation in Japan was employed. Although the efficiency of handling scallops gradually improved, the predation on scallops and fouling o f collectors and trays persisted. Some solu­tions to these problems, for example, the mortalities resulting from the binding of scallops with the byssal threads from mussels, could be considerably reduced by introducing dogwhelks (Nucella lapillus) to the trays (Minchin, 1989).

A substance recommended in the early 1980s for con­trolling fouling on shellfish cages was tributyltin. This is highly toxic and impaired shellfish broodstock. It was also used on fish cages, and this almost certainly led to the poor and failed settlements o f scallops in Mulroy Bay, Ireland (Minchin et a l , 1987). In addition, its usage by ships as an antifoulant on hulls may have led to declines o f scallop stocks in enclosed harbours such as Cork Harbour and the Rade de Brest. Legislation banned its use on craft < 25 m from 1982, and this ac­tion resulted in improvements in water quality.

Ranching

Early studies in the 1970s in Ireland and France consist­ed o f broadcasting hundreds to thousands o f scallops. It was a concern that the sowing o f such small numbers would create a situation whereby there would be a dis­proportionate mortality from immigrating predators. Large-scale programmes were favoured. Survival esti­

The potential fo r ranching the scallop, Pecten maximus - past, present, and future: problems and opportunities 419

mates could be made from the commercial returns, but obtaining an indication of the survival before this time was difficult because only small areas could be sur­veyed by diving, often with poor underwater visibility and with poor sea states. The main studies were under­taken in the Rade de Brest and in the Baie de St Brieuc, France (Buestel et al., 1987; Dao, 1990) and more recently in Norwegian fjords (Strand, pers. comm). Following sowing, scallops would take about two fur­ther years to reach market size. The regions chosen for sowing should take account o f the natural growth of scallops in these areas (Minchin and Ni Donnachada, 1995) and if transplanted, gonadal development may follow a different pattern (Latrouite and Claude, 1979), probably reflecting differences in the genetics o f differ­ent populations. In several experiments, unauthorized removal o f scallops may have contributed unduly to pes­simistic estimates of survival.

Mortalities were associated with all ranching experi­ments, and different bays appeared to have different predatory problems (Paul et al., 1987) which changed with time. In the Baie de Saint Brieuc prior to the extended frosty period in 1963, the octopus was an im­portant predator of scallops (Dao, pers. comm.), but since that time, the hermit crab has become more impor­tant. In shallow inlets, scallops may also be killed by extended cold periods (Minchin, 1985a). Scallops ex­posed to temperatures below 4°C become torpid and will eventually expire. Over the last century, the biota within many European inlets may have changed consid­erably. The great reduction of predation by the exploita­tion of skates and rays may have resulted in increases in their natural prey, such as crabs, which are known to be avid predators of sown scallops. Dispersal from the fish- able area also has implications for the economic viabil­ity of ranching (Howell and Frazer, 1984).

Two approaches to the management o f crabs have been undertaken. One involves the optimum foraging theory of Hughes (1980), whereby the crabs select scal­lops and test their strength before investing any energy in opening them. The other method examines the maxi­mum size of scallop preyed upon by crabs o f different sizes (Minchin, 1991). This last approach enables a cal­culation of a sowing size based on the sizes o f crabs trapped in the area to be sown. Still, it has been a puzzle as to why crab predation has been so devastating; shell strength of cultivated scallops, however, does seem to be much less than that o f wild ones. Sometimes the shell of the cultivated scallops has only half the strength of the wild ones (G. Haugum, unpublished).

In most sowing studies, it was noticed that there was an initial high mortality of scallops. Gradually, it be­came apparent that the condition o f scallops at the time of sowing was important; their vitality appeared to be reduced with prolonged exposure to air. Moving scal­lops to the sowing area from the region in which they have been in intermediate cultivation involves some ex­posure to air. Should this be prolonged, i.e. >12 hours.

mortalities may subsequently be high. Scallops held out of water for periods exceeding 3 h at > 12°C show a noticeable reduction in vitality (Minchin et al., 2000).

Pandora’s sea chest

Exotic species continue to expand their ranges because of the opportunities o f free trade, introductions o f "new" culture species, importation o f shellfish, ballast water releases, and hull fouling (Carlton, 1987; Carlton and Geller, 1993). These are the most prominent vectors, but many others exist. These seemingly innocent activities, when undertaken without controls, can lead to the ex­pansion of undesirable species belonging to a wide range o f taxa. It is not always possible to predict their harmful effects once introduced to a new geographic region; many are benign, and their establishment may go unnoticed. However, some are likely to modify pro­duction and sales o f scallops. The recent appearances of amnesic shellfish poisoning (ASP) on the west coasts of Scotland and Ireland are of grave concern, and further toxins continue to be described in cultivated shellfish. The naked dinoflagellate Gyrodinium aureolum has been associated with purges o f bivalve larvae from the plankton and possible mortalities o f spat (Minchin, 1985b). The ICES Code o f Practice (ICES, 1994) covers those areas where species may be introduced in trade or as exotic species for culture. The advice provided in this Code arose over the course o f 25 years, and this Code is also of interest to managers in countries outside the ICES community. Controls for the management of exotics by shipping are, in part, dealt with by Inter­national Maritime Organization guidelines. Unfortu­nately, these guidelines are not sufficient, and control or sterilization methods for the management o f ballast water need to be urgently examined. Many introductions have also been attributed to ships’ hull fouling. Measures to control the usage of TBT antifouling paints are likely to be implemented, with a possible banning of its use within the decade. The efficacy of alternative antifoulant methods, however, must be at least as effec­tive as that of TBT. The improved management in the control o f toxic discharges into port areas by industry will almost certainly make these areas more suitable for the invasion o f exotic species releases in ballast water or for the progeny arising from brood releases or spawning o f hull-attached biota.

The scientific community

The early studies by the pioneers Mason, Gibson, and Baird undoubtedly provided the basic information used in the management o f scallop populations today. Apart from the technological developments that aided these advances, the most important arose from the remarkable good will and open discussion o f fellow scientists. The

420 D. Minchin

Table 1. Advantages and disadvantages o f scallop ranching.

Disadvantages

Large mortalities due to cold winter events (1754, 1947, 1963) TBT contamination in some inshore bays ( 1980s)Poaching from divers and fishermen (from 1980s) Development of other culture activities (from 1970s)Apparent spread o f algal toxins (from 1970s?)

Factors that promote cultivation in Northern Europe

Efficient recapture methods and decline o f stocks (from 1950s)Development o f durable plastic products (culture systems, rope, packaging) (from 1960s) Processing facilities close to production regions (from 1960s)Development o f safe diving practices (from 1960s)Improved understanding o f scallop biology (from 1950s)Large expanses in navigation zones in port areas available for cultivationThe success o f Japanese, New Zealand, Chilean, and Chinese culture-based productions

W hat has yet to be achieved

Successful returns from scallop ranching Development o f ranching that promotes enhancement Improved understanding o f predator-prey interactions Improved understanding of dispersal mechanisms Identification o f optimal sowing regions Understanding o f gigantism Legislation to enable private ownership Rapid assessment o f scallop quality

teams o f Baird and Gibson (1950s), Mason and Drinkwater ( 1960-1970s), Buestel and Dao (1970s), and Gruffydd and Beaumont (1970s) are examples of how the expertise evolved and the centres of excellence in scallop studies developed, notably in Brest, Aberdeen. Port Erin, Menai Bridge and Conwy, La Coruna, Dublin, and more recently in Bergen. Exchanges of ideas became more frequent with the Scallop (Pectinid) Workshops that first met in Baltimore, Ireland, in 1976; the 11th gathering took place in Bergen in 1999. Meet­ings normally took place every two years and represent­ed all matters relating to scallops from science to mar­keting. Many o f the presentations from these workshops have been published, and reports o f some o f these meet­ings were presented to ICES (Minchin, 1976; Ma-son, 1983b; Beaumont and Mason, 1987). The ICES Working Group on Assessment o f Scallop Stocks held in 1979 (ICES, 1979) contributed to the overall knowl­edge, as have the many aquaculture symposia and ICES Statutory Meetings via the Shellfish Committee. In the 1990s, the European Union sponsored a programme to learn more about scallop ranching, and it clearly point­ed out the difficulties with scallop spat quality and pre­dation that required further attention (Fleury et al., 1997). Some of these matters were examined in a fur­ther EU programme run by Norway under the Large Scale Facility, a programme that supported leading researchers from elsewhere in Europe to work with Nor­

wegian colleagues using the research facilities in Norway. Experience gained on scallops from all areas of the globe was deemed to be of interest, and this infec­tious enthusiasm still persists. The success o f the large- scale production and ranching o f the ecomorphological- ly similar Pecten novaezelandiae in Golden Bay, New Zealand, demonstrated that ranching scallops with a recessing habit could be achieved (Bull, 1987).

Future developments

To date, ranching o f Pecten maximus has met with more failures than successes. Nevertheless, these experiments have provided new information with which to improve our knowledge, a summary account o f which appears in Table 1. One o f the issues appears to be spat quality, which may explain the great variation in the ensuing mortalities that follow a sowing. The impact o f long­term stresses on scallops requires investigation and may be a further quality variable that will need to be taken into account, as will stocking densities and water flow through different cage designs. There are some strong behavioural responses in scallops, and some are remark­able. For example, cultured scallops once sown, recess at similar rates to wild scallops. This may indeed be for­tunate because should they not do so, survival rates may be poor. Scallop-predator interactions require further

The potential for ranching the scallop, Pecten maximus - past, present, and future: problems and opportunities 421

detailed study, and these may include some unexpected species such as sea anemones (Minchin, 1983), small spider crabs, and gobies (P.-G. Fleury, pers. comm.). The biology o f some predators remains poorly known; for example, little is known about the sea star (Mar- thasterias glacialis), an important scallop predator. Continued investigations on various crab species are needed to complete our understanding o f their impact on sown scallops, as there are likely to be diurnal and seasonal variations in predation intensity.

With the development o f more efficient dredges, a decline in stocks has taken place, and the spatial pat­terns o f scallops have obviously been changed. The poor and reduced recruitment in many populations may arise from an increase in nearest neighbour distances, and so a reduced fertilization ability with fewer embryos being produced. Should sowing of scallops include, as an objective, an enhancement o f settlement, there may be less dependence on sowing activities in future years. With the increase in dredge efficiency would come a better recovery rate for sown scallops, and high recovery rates would make ranching projects more profitable because o f higher returns. The sowing o f scallops could also lead to recreational fisheries for snorkellers, where­by sown scallops released by a local community in selected areas may promote tourism and increase bed nights. In the shallower waters, it may be more efficient to consider commercial harvesting using divers as there would be less damage to the sea floor. Perhaps changes in licensing could promote the possibilities of ranching in the future, enabling divers where this is presently pro­hibited and enabling usage o f the seabed by promoting private and cooperative leasing of the seabed.

With improved understanding o f the requirements of scallops, a satisfactory and economic return will be real­ized. Reduced mortality and improved production could be achieved by farming scallops that exhibit gigantism; such individuals do naturally occur and may attain a weight of about 1 kg (Minchin, 1994).

With improved knowledge of local water movements and stock identities, the management of scallop popula­tions will be further improved. Forecasting of settle­ments may become possible, and the sowing o f scallops in specific regions within certain bays may promote the enhancement o f natural settlements. Although results to date have been disappointing, with improved knowledge o f the environment and with further technological devel­opments, a future for scallop ranching would seem assured.

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