No Les essais de repeuplement en coquille Saint-Jacques

ISSN 0704-3716 Canadian Translation of Fisheries and Aquatic Sciences 4. No. 5076 Experiments in scallop restocking J.C. Dao, and H. Didou Origin...
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ISSN 0704-3716

Canadian Translation of Fisheries and Aquatic Sciences 4.

No. 5076

Experiments in scallop restocking

J.C. Dao, and H. Didou

Original title:

Les essais de repeuplement en coquille Saint-Jacques

In : Présenté à un colloque, Brest, France, 2-7 novembre, 1982

Original language:

French

Available from: Canada Institute for Scientific and Technical Information National Research Council Ottawa, Ontario, Canada KlA 0S2

1984

6 typescript pages

C T /Y

5o7(e.

TRANSLATION FROM FRENCH - 1 -

EXPERIMENTS IN SCALLOP RESTOCKING by J C Dao and H Didou Paper read at a symposium in Brest, France, 2-7 November 1982

20.09.82

Scallops are subject to intensive fishing using 6 to 20 metre boats equipped with dredges. The life cycle of these bivalve mollusks is coming to be well understood, though research work in France is of fairly recent date. It is now known how to manage scallop stocks in the short run using forecasting models, and release of juveniles can be used as a restocking method. An experiment in scallop restocking is being carried out in the harbour at Brest. Table 1 shows the principal aspects of the process, which allows the scallop population to renew itself at the same time as large numbers are fished.

1. Production of juveniles Generally the first activity in a restocking procedure is the production of juveniles. Restocking is often simply equated with the release of juveniles. Two methods have now been developed:

1.1 Spat collectors The first method is to capture spats in collectors suspended in the water. The collector is a small-mesh net that traps larvae, and prevents predators from entering and the young scallops from leaving once they have grown a certain amount. Basic to this method is the biological knowledge needed to determine when and where to position the collectors. In Japan this method has eliminated the need to use hatcheries, but there does have to be a well-stocked bed with a large number of spawners. Such a situation existed with regard to the 'pétoncle noir'* in the harbour at Brest, where many thousands of juveniles have been taken in spat collectors. With regard to the scallop**, such a situation exists at Mulroy Bay in Ireland, where the method is now being developed. The first batch of spats was imported in spring 1982, and .the experiment now underway is having very encouraging results.

* I could not find this species name in any French source. However 'pétoncle' includes Chlamys islandicus, Pecten magellanicus, Pecten opercularis and other species. ** French "coquille Saint-Jacques": either Pecten maximus (in the Atlantic) or Pecten jacobaeus (in the Mediterranean). 'Coquille Saint-Jacques' is the subject of this paper. Both 'pétoncle' and 'coquille Saint-Jacques' are normally rendered as 'scallop' in English. --Tr

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1.2 Hatcheries This second method is still in a preliminary stage of development but in the near future it should be possible to produce several million spats. After several years of experimentation that saw massive mortality rates once the?veligers*** reached 1 mm, positive results were finally obtained in 1982 at the two hatcheries working on this method (SATMAR, COB and CLPM Brest)****. Since the problems of developing the artificial nursery stage have not yet been resolved, the veligers are fed with unfiltered sea water.

*** literally "post-larvae" **** I have no idea what these letters stand for. appears to be a COB publication.

--Tr

However this article

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- 2 1.3 Maturation Since the young scallop thus produced are not yet sufficiently large to be released, the rearing process continues. Beginning at 10-20 mm (for captured juveniles) or 1-2 mm (for those from the hatcheries), the scallops are matured in protective enclosures until they reach 30 mm. For those originating in hatcheries, the enclosures must in the initial period have a very fine mesh. The enclosures are suspended from one of two types of fixed structure: a sub-surface line* attached to the shore at a depth of 8 metres (this is used in Brest harbour and in Saint-Brieuc bay), or a tray** at greater depths. In either case, fouling must avoided: water must be able to circulate freely through the fine mesh, which can rapidly become plugged. In addition, there is the problem that certain animals can ,be responsible for high mortalities: predators entering the enclosures as larvae (crab, starfish) or epibionts which can inhibit development (Polydora, mussels). Site and time selection are important determinants of success in this respect.

2. Controlled release of juveniles and dealing with predators No full-scale restocking operation has been possible, because there has not been an adequate supply of juveniles for this purpose. Spot experiments have been carried out, or are underway, but not on a scale sufficient to affect the development of a scallop bed. The biggest experiment occurred between 1977 and 1980: 26,000 juveniles captured in Saint-Brieuc bay in 1976, and transported in early 1977 to Brest harbour, were released in March 1977 in an experimental area selected by CLPM Brest and COB. In June 1978, 50% of them were still alive and they reached commercial size over the sumker. In late spring 1980, 8000 scallop weighing an average of 200 g each were caught in the area. This result is being verified by a further experiment at Brest with spats from Scotland (introduced in 1980), from Ireland (1981) and from hatcheries, and also by an experiment in Saint-Brieuc bay. What is appearing in these experiments is the importance of the relation between the nature of the ocean bottom, the size of the released juveniles, and the types of predator present. It is hard to do anything about predators whose biology and population dynamics are but poorly understood, so we are depending on the size of the scallops, the fact that they can conceal themselves by burrowing a short distance into certain types of bottom, and the fact that possible release sites vary greatly as regards which predators and competitors are present.

* called "floating line" on Table 3. ** literally "submerged table" --Tr

-3-3. Stock management 3.1 Once historical data on fishing in the various parts of a bed are available, it becomes possible to forecast, a year ahead of harvesting, any changes in the numbers of animals; appropriate measures can be then be suggested, and an idea can be had of what the effect of these measures will be. It also becomes possible, once the type of conservation or economic development measures are decided on, to make even more precise forecasts of what the numbers will be a few months down the road; on this basis, scallop dredging schedules can be drawn up. To make such stock management possible, two sorts of information are required: - the volume and location of catches (to be determined from record sheets distributed to masters of fishing vessels) and the composition of catches (to be determined by samplings at the point of unloading); this will provide a map of scallop resources and of dredging activities; - the volume and location of the very young age classes (the scallops that will be fished one or two years later). 3.2 The latest knowledge takes too long to get into the hands of fishermen. They need to know that the sea is like a desert with a few oases of resources here and there, and that the average quantity of scallops on the bottom needs to be at least 1 kg per 100 square metres. Scallop resources need to be treated with caution. An awareness of the need for stock management is vital to fishermen's futures. To provide them with the information they need, there is going to have to be a large amount of basic research work, the results of which may not be very spectacular in the initial years. 4. Maintenance of stock of spawners The knowledge on which short-term management is based is not sufficient to determine the level beyond which the scallop resource is being overexploited. This is a very important question in view of the fact that the scallop population does not automatically reconstitute itself (in Brest harbour 1500 tonnes a year were taken before 1963 and only 50-150 tonnes thereafter). Research required in this regard will take a long time, and progress will depend on studies being conducted throughout France and Europe. The potential fecundity of each animal is very high (several tens of millions of eggs per adult). Natural renewal would therefore require only a very small number of spawners if it were not for massive mortality rates arising from environmental conditions. Thus one could assume that the production of juveniles depends mainly on annual fluctuations in environmental conditions, or alternatively one could assume that the natural production of juveniles depends on the maintenance of a minimal stock of spawners.

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If a relation could be determined between the size of such a stock on the one hand and recruitment on the other, then long-term management models would become possible, but this approach assumes environmental conditions play only a small or moderate role. If we look at the results obtained with a related species in Japan, we find that the stock of spawners was the factor determining the restoration of scallop resources. There is of course the textbook example of Mutsu bay, but there are also examples in the Sea of Okhotsk, in open zones where beds were totally reconstituted (cg the Sarufutsu co-operative). 5. Conclusions Table 2 shows two stable situations, one corresponding to low scallop productivity (#1) and one to high productivity (#2). The tendency is for intensive fishing to turn situation #2 into situation #1. A rigorous program of stock management then becomes necessary. Now that the principal biological factors at work are beginning to be understood, it is important to move toward a full management program to develop the scallop resource. This is the goal of the five-year plan for restocking Brest harbour, the specifics of which (see table 3) have been discussed with the fishermen.

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Maintenance of stock of spawners

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Importation of spats; Hatchery

Production of juveniles

Controlled releases Resource management

1

Dealing with predators

le Production Table

1 - Principal factors in scallop restocking

SITUATION

no 1

Production of juveniles (spat collection or hatchery)

Sparse stocks

Low productivity (0-20 kg/hour/dredge)

Small numbers of spats collected

LI [ SITUATION

no

2 1

rl Large stocks

High productivity (100-1000 kg/hour/dredge)

Huge numbers of spats collected

Table

2 - Objectives of scallop restocking

Table 3: Program for scallop restocking in Brest harbour Maintenance of stock of spawners How big a stock

- 500 tonnes (2.5 - 3 million spawners in 5 years),

Objectives

- Start collecting spats again. Further fishing to be regulated.

Stock managment Conservation

- No fishing of newly released juveniles.

Monitoring restocking progress

- Experimental collection program.

Productivity

- Estimates of quantities created by restocking. Further fishing to be done by rotation,

Controlled release of juveniles Time and place of release; density

- Keep in protective enclosures for winter and spring releases. - Select locations on the basis of fishermen's experience (biological and economic criteria), - 2 to 5 juveniles per m2 (heavy but not excessive stocking),

Numbers released

- About 1 million 30mm juveniles a year (with 50% survival rate over 18 months).

Dealing with predators

- Avoid areas with starfish. Release scallops during period of low crab activity,

Production of juveniles Source

- Local scallops, by artificial reproduction (COB hatchery and Tinduff nursery), - Importation (collection) ?if successful.

Farming practices

- Floating lines and submerged trays. Removal of juveniles to remove foulings.