Aquaculture, 94 ( 1991) 263-278 Elsevier Science Publishers B.V.. Amsterdam

263

Mussel culture in Galicia (N. W. Spain )

A. Perez Carnacho”, R. Gonzalezb and J. Fuentesb aInstiiuio Espaiiol de Oceanograjia, Apdo. 130, La Coruiia, Spain

“Centre Experimental de Vilaxoh, Conselleria de Pesca, Xunta de Galicia, Vilagarcia de Arousa. Spain

ABSTRACT

Perez Camacho, A., Gonzalez, R. and Fuentes, J., 1991. Mussel culture in Galicia (N.W. Spain). Aquaculture, 94: 263-278.

This paper describes mussel culture in Galicia (N. W. Spain) based on our previous studies and data obtained from a survey carried out in 1984 on 337 rafts ( 10% of the total) in the Rias of Arousa, Pontevedra and Vigo. Data are also included on the biology of the cultured mussel, raft characteristics and culture techniques.

The raft culture process is divided into four stages: obtaining the seed, growing the seed, thinning out the ropes and transferring the seed onto new ropes and final harvest and sale. These operations differ among the different rias and the zone of the ria. Therefore seed can be obtained directly from the coastal rocks or from collectors over the rafts, or both. Seed is attached to ropes that will be thinned out twice or three times before commercial size is reached. Collectors are placed in spring and the mussel seed ropes are placed in the sea throughout the whole year, but specially from Novem- ber to March. The thinning out process is usually carried out between June and October.

The mean surface area of the rafts is 261 m* in the Ria de Vigo, 352 m2 in the Ria de Pontevedra and 369 m2 in the Ria de Arousa, with an increase in the last of almost 25% since 1974.

The declared production per ha and year reaches 48.0 t/raft in the Ria de Arousa, 46.9 t/raft in the Ria de Pontevedra and 33.2 t/raft in the Ria de Vigo, while production per m* of raft is 130 kg, 133 kg and 127 kg, respectively.

INTRODUCTION

Marine aquaculture in Spain is carried out along more than 5000 km of coastline. It is characterized, on the one hand, by the wide variety of areas suitable for the culture of very different species and on the other hand by the almost exclusive farming of bivalve molluscs, which make up over 99% of the total production of cultured species (Perez Camacho, 1987).

According to different estimates, between 150 000 and 200 000 t are ob- tained annually from aquaculture in Spain, of which the mussel Mytilus gailoprovincialis Lmk. comprises 97%. The remaining percentage is distrib- uted among the oyster (Ostrea edulis L. ) and other species (Perez Camacho, 1987).

0044-8486/91/$03.50 0 199 1 - Elsevier Science Publishers B.V.

264 A. PEREZ CAMACHO ET AL.

Most of this Spanish production is obtained in the Galician Rias (N.W. of Spain). The annual production accounts for approximately half of the world production of this mollusc, placing Spain foremost among world leaders in aquaculture (Fig. 1) .

Mussels are cultured on ropes hung from rafts. Mussel seed from coastal rocks or collectors hung from the rafts are tied to the ropes using a tine mesh net, which decomposes a few days later. The mussels remain attached to the ropes by means of the byssus threads. At 40-cm intervals, there are wooden pegs placed crosswise between the ropes, which help to support the weight of the mussels, distributing it along the whole rope. After 5 or 6 months, when the mussels have reached a length of 4-5 cm, the ropes are taken up to redis- tribute the mussels onto a greater length of rope. Each rope produces enough seed for about three new ropes (or thinning out ropes) where the mussels remain until they measure 7 to 10 cm, when they are ready to be harvested and sold (Perez Camacho, 1987 ) (Fig. 2).

According to Perez Camacho ( 1987), the extraordinary extent of mussel culture in Galicia can be linked to several factors: the relatively low cost of the rafts, their high production, the price of mussels (making even the exploi-

RIA DE

Rin DE

Rin DE

GALICIA

PORTUGAL

Fig. 1. Map of the Galician Rias (N.W. of Spain).

MUSSELCULTURE IN GALICIA (N.W. SPAIN)

IL + I

SEED TIED ON SETTLE~NT ON ROPES h

SETTLEMENT COLLECTORROfES ON ROCKS

it 6 MOETHS

THINNING OUT T T MUSSELS 5CM

REPR~OUCTION

4 11 MONTHS

REPR~OUCTION

T T f%JSSELS

FROM ROCKS

265

Fig. 2. Phases of mussel cultivation in Galicia.

tation of a single raft a lucrative business), the abundance of seed, which can be collected from the rocks by the mussel farmer himself, and low operating costs. Since the mussel feeds mainly on phytoplankton present in the water, the only real operating costs are fuel for the boats and the upkeep of the boat and the raft. The above factors have led to the development of a family busi- ness type structure which is dedicated to farming over 3000 ha in the rias, producing a large quantity of protein-rich food. Previously, this area had been exploited by fishing with a much lower yield, or had simply not been ex- ploited at all.

Various studies have been published dealing with different aspects of the biology and culture of mussels: the biology of the mussel (Aguirre, 1979; Andre& 1958, 1960; Ptrez and RomBn, 1979), biochemical composition (Fraga, 1956, 1959), particle retention and filtration in the rafts (Cabanas et al., 1979; Fraga and Vives, 1960), culture methods (Bardach et al., 1972; Korringa, 1976; MariiIo et al., 1.982 ) and associated epifauna (Roman and PCrez, 1979, 1982).

This paper describes mussel culture in Galicia based on our previous stud- ies and data obtained from a survey carried out in 1984 on 337 rafts ( 10% of the total) in the Rias of Arousa, Pontevedra and Vigo. Data are also included on the biology of the cultured mussel, raft characteristics and culture techniques.

BIOLOGY

Growth PCrez and Roman ( 1979) found that on two rafts located in the inner and

outer parts of the Ria de Arousa, mussels of average length 18 mm and aver- age weight of 0.5 g that were attached to ropes in April reached an average

266 A. PCREZ CAMACHO ET AL.

Fig. 3. Growth in shell length of mussels cultivated on two rafts in the Ria de Arousa (dashed line, raft from the inner part; solid line, raft from the outer part).

length of 48-5 1 mm and average weight of 8-9 g, in August, depending on the area. At this time it becomes necessary to proceed with the thinning-out pro- cess. The same authors state that if the seed is hung on the ropes in the au- tumn, growth is much slower and the mussel does not reach a suitable size for thinning out. until the following spring (Fig. 3 ) . The freshly seeded ropes sup- port 10 kg of mussels. After 4 months of culture they weighed 105- 112 kg.

Once these mussels have been distributed onto other ropes, after only 13 months of culture they reach an average length of 82 mm and an average weight of 42-43 g, depending on the different culture zones. The thinned out mussel ropes with an initial weight of 30 to 43 kg of mussels in August reached a weight of between 96 and 111 kg the following September.

According to these results, seeded ropes with 10 kg of mussels will have increased in weight by ten-fold by the time the mussels are separated onto new ropes, creating about three new ropes, which in turn will triple in weight on reaching marketable size. In addition, the culture period will last an aver- age of 16 months, from 4-6 months on the seed ropes and IO- 12 months on the new thinning out ropes ( Aguirre, 1979; Perez and Roman, 1979 ).

Reproduction and condition In her study on mussels in the Ria de Vigo, Aguirre ( 1979) estimated the

size for sexual maturity at 35 mm and found two spawning periods for this species; the first in spring and the second in autumn. Larval development lasts approximately 50 days, whereas larval settlement takes place from April to June or July. No settlement was observed for the autumn spawning. Based on data pertaining to seed settlement on the culture ropes, Perez and Roman

MUSSELCULTURE IN GALICIA (N.W. SPAIN) 261

( 1979) established the reproductive season for the mussel to be from March- April until July-August; they also observed no settlement in autumn.

Mussel meat content is subject to a cycle which is closely related to sexual maturity (Aguirre, 1979). Although there are marked differences from one year to another and between the different culture areas, generally speaking the lowest values are found in early spring and the maximum in summer and autumn, during which time the fresh meat weight accounts for approximately 45% of the total mussel weight (Perez and Roman, 1979). According to these authors, when gametes are expelled during spawning, the mussel loses ap- proximately 74% of its organic matter, which means that about 6000 t of sex- ual products are released into the sea per year, and this is considering only the case of the Ria de Arousa.

Mortality Perez and Roman ( 1979) found a 33-36% mortality in the seed phase,

which translates to values of between 1.10 and 1.25 for the annual instanta- neous coefficient of natural mortality. The natural mortality estimated by these authors for the thinned out mussels fluctuated between 14% and 15%. The annual instantaneous coefficient values were between 0.15 and 0.16.

Natural mortality estimated during the culture period fluctuates between 19% and 20%, although we should also add mussel loss due to detachment from the ropes, generally because of storms. In the more exposed ria areas, these losses may be as high as 40% of the total number of mussels on the ropes, while in the more sheltered zones, values do not exceed 14% (Perez and Roman, 1979).

Filtration and biodeposition In 1979, Cabanas et al. estimated the total volume of water flowing through

a raft to be 7.6~ 10’ m3/day, with an average concentration of particulate organic matter of approximately 306 mg C/m’, 39 mg N/m3 and 1 .O mg chlo- rophyll a/m3. The filtration rate for raft-cultured mussels was estimated to be 5.3 l/h, the feeding rate (mg ash-free dry wt/day removed per g dry flesh of mussel) was 2 1.7%, the biodeposition rate (mg ash-free dry wt/g dry flesh wt per day), the gross ecological efficiency (net production of mussel/food fill- tered ) was 13.5% and the assimilation efficiency [ (feeding rate - biodeposition rate) /feeding rate] was 79%.

The enormous concentration of mussels on the rafts, which can exceed 5 million individuals/raft (Cabanas et al., 1979), causes a considerable de- crease in available food, as the water flows through them. According to these authors, 30% of the carbon, 42% of the nitrogen, and 60% of the chlorophyll contained in the particulate organic matter present in the water is retained (Table 1).

This extraordinary filtration results in a greater production on the ropes

268 A. PEREZ CAMACHO ET AL.

TABLE 1

Retained food, % of filtration and biodeposition in a mussel raft (Cabanas et al.. 1979)

Food in the seawater at the front of the raft

Retained food

Filtration

(%) Biodeposition

232.6 kg C/day 68.8 kg C/day 29.6 14.3 kg C/day 29.1 kg N/day 12.4 kg N/day 41.8 1.7 kg N/day

0.8 kg Chl. a/day 0.5 kg Cl. a/day 59.6 514.2 kgOM/day 152.0 kg OM/day 29.6 3 I .6 kg OM/day

OM, organic matter.

located at the front of the raft than at the rear, and rafts located at the edges of the parks show a higher production than those in the inner zones.

After evaluating all these data and using their laboratory experiments, Perez and Gonzalez ( 1984) consider that the outstanding mussel production in the Galician Rias cannot be explained by taking only primary production data into account. Rather they affirm that detritus and bacteria must be recognised as playing an important role in the diet of the mussel.

On the other hand, mussels produce a great amount of detritus which Ca- banas et al. ( 1979) estimated to be approximately 190 kg dry weight per raft and per day, of which 3 1.6 kg is organic matter, 14.3 kg organic carbon and 1.7 kg organic nitrogen. Logically this creates an enormous accumulation of organic matter on the bottom of the rias.

Epifauna Once the ropes have been submerged in the water, they provide a substrate

for different organisms and are quickly colonized. Roman and Perez ( 1982 ) , in a study in the Ria de Arousa, found a total of 99 invertebrate species on the mussel ropes. Among the most important are crustaceans, mainly Pisidia longicornis and several amphipod species (Phtisica marina, Eurystheus maculatus and Jassa falcata, among others). Similarly, Gonzalez ( 1982 ), in a study on the epifauna of seed ropes, cites a total of 72 species, 30% of which are crustaceans, 27% polychaetes, 12% gastropods, 7% lamellibranchs, and the remaining species are divided among ascidians, sponges, hydrozoans, etc.

According to Roman and Perez ( 1982 ), the presence of the different epi- fauna1 species on mussel ropes is conditioned by many diverse factors. Given the considerable filtering activity of mussels and their high biodeposition rate, only predatory and detritivorous organisms develop successfully in this hab- itat. Filtering species, however, that compete with the mussel for food are quickly eliminated.

Obviously only organisms which are able to swim, and planktonic larvae of benthic species, will have access to the ropes. In the first case colonization is carried out by contagion from the closest ropes. This explains the great amount

MUSSEL CULTURE IN GALICIA (N.W. SPAIN) 269

of organisms found on the ropes 1 month after they were hung in the sea. These organisms are mainly amphipods, Phtisica marina, Eurystheus macu- latus and Corophium sextoni among others.

In the case of P. longicornis, although colonization may take place by con- tagion from nearby ropes, it mainly occurs from planktonic larvae of this spe- cies. Settlement of larvae from benthic organisms on the mussel ropes is de- pendant upon the ropes being in the water at the moment attachment occurs, and thus, on operations controlled by man. Decisions as to when ropes are to be hung and how long they are to be submerged affect colonization by the different species and the future development of the community.

Another factor which is decisive in determining the specific composition of the epifauna of the rafts is the mussel itself, as it is the dominant organism in the community and represents 95% of the total biomass. It is a very efficient water filter, eliminating a major portion of suspended particles. However, the mussel is not very efficient at assimilation, and this results in a high level of biodeposition. From this we may conclude that the mussel is a strong com- petitor of filtering organisms and will favour the presence of detritivorous organisms. Hence, the great majority of the most abundant species that make up the associated epifauna in mussel culture are detritus-eating organisms such as P. longicornis, Cucumaria normani, Arenicola ecuadata and a number of amphipods, nemerteans and aphrodites, while the filtering organisms, such as Balanus perforatus and several sponge species, ascidians, and young mus- sels, normally comprise less than 10% of the epifaunal biomass.

CULTURE TECHNOLOGY

As pointed out in the introduction, mussel culture began relatively recently in Spain. After early attempts to farm mussels on fixed structures in the 1940s the first floating platforms or rafts were installed in Galicia. After an initial period of moderate development, the number of rafts began to grow rapidly between 1960 and 1970. In 1967 a total of 2600 rafts was registered (Fig. 4). After this time the growth in number slowed considerably, coming practically to a standstill as of 1983, at which time there were 3386 mussel culture rafts (Perez Camacho, 1989).

The rafts are located in different rias along the coast, and are distributed in parks, where each raft is separated from the others by a distance of 80-100 m. The number of parks and number of rafts in each park varies considerably from one ria to the next. The Ria de Arousa accounts for 68.2% of the total number of rafts in Galicia, the Ria de Vigo has 15.69/o, the Ria de Pontevedra, 10.2%, the Ria de Muros-Noia, 3.3% and the Ria de Ares, 2.6% (Perez Camacho, 1989 ) .

270 A. PEREZ CAMACHO ET AL

IWO 1950 1x0 1970 1980

YEAR

Fig. 4. Growth of the number of rafts in Galicia from 1940 to 1984.

25

Fig. 5. Distribution of the size of the rafts (m* ) in 1984 (data from the three rias sampled).

Characteristics ofthe rafts The basic construction of the raft consists of a series of floats that support

a rectangular wooden framework made of eucalyptus, from which the ropes are suspended.

Raft size is highly variable. There are structures measuring less than 100 m* and others that are far greater than 500 m2 (Fig. 5 ) . The average raft size for the three rias studied is 337 2 5.1 m* (mean + standard error). However, raft size differs considerably between the rias under study. Thus in the Ria de Vigo the mean raft size is 2612 16.8 m*, in the Ria de Pontevedra it is 352 2 18.9 m* and in the Ria de Arousa, 369 + 5.1 m*.

According to our data, although the number of rafts has not undergone sub-

MUSSEL CULTURE IN GALICIA (N.W. SPAIN) 271

stantial growth in recent years, raft size has increased greatly, at least in the Ria de Arousa. The average raft size in this ria in 1977 was 297 m2 (Mar-So et al., 1982) whereas in 1984 it was 369 m2, as quoted earlier. This is an increase of almost 25%.

The useable culture area, that is, the total raft area, excepting the space taken up by the floats, is directly related to the number of floats a raft has. Thus rafts with only one float have a useable area of 80%, and those with four or six floats can use 90%.

The number offloats each raft has varies considerably between the different rias. While in the Ria de Arousa 64% of the rafts have four floats and 36% have six, in the Ria de Pontevedra 68% of the rafts have four floats and 13% have six floats, and in the Ria de Vigo 55% of the rafts have only one float, and 40% have four.

There are also differences in the anchoring systems of the rafts. In the Rias de Arousa and Pontevedra they have only one iron chain, which allows the rafts to move with the tides, always facing in the direction of the tide. In the Ria de Vigo, on the other hand, rafts are anchored with two chains (front and rear) and remain fixed during tidal changes.

The age ofthe rafts at the time of our study ( 1984) fluctuated between 0 and 30 years, with an average of 8.2 t 0.4 1. Forty-one per cent of the rafts in ex- istence in 1984 were constructed between 1980 and 1984, and 66% were con- structed between 1975 and 1984. In the Ria de Arousa these percentages were 48% and 75%, respectively.

Culture ropes

The number ofropes on a raft, according to Fig. 6, fluctuates between 200 and 700, with a mean value of 4 18 ? 10.2. This number is substantially lower than estimates made by Andreu ( 1958), Figueras ( 1976) and Korringa ( 1976), as indicated by MarSo et al. ( 1982), and represents a mean of 1.24 ropes per square meter of raft, markedly less than the mean observed in 1977 by the same authors.

The mean number of ropes per raft also varies considerably from one ria to another. Maximum values were reached in the Ria de Arousa with an average of 455 ? 5.6; average values in the Rias de Pontevedra and Vigo were 334 2 8.2 and 294 + 10.2 respectively.

Rope length. According to Marifio et al. ( 1982 ), ropes used in mussel culture consist of two parts; an upper or lashing rope which is fastened to the grid, and is hung as deep as 1 m below the water surface, and the rope where the

A. PeREZ CAMACHO ET AL.

ROPES PER RAFT

Fig. 6. Distribution of the number of ropes per raft in 1984 (data from the three rias sampled).

mussels are tied. The second rope is totally submerged and has wooden or plastic pegs 30 cm long which are inserted at 40-cm intervals. The purpose of dividing the ropes into two parts is to make them last longer, as the upper part, which is exposed to air and sunlight, has a shorter life (2.4 years) than the part of the rope which is underwater (5.8 years).

The mean length of the ropes where the mussels are tied is 9.8 -+ 0.10 m (Fig. 7). The most extreme measurements observed were 4 and 16 m. In keeping with the observations made by Marifio et al. ( 1982), this marked difference in length is related to the depth of the area where the raft is moored. However, even in the deeper zones, ropes do not usually measure more than 12 m. In the cases where longer ropes are used, the mussels are found only on the lower half and they are inserted between normal length ropes, so as to increase the number of ropes on the raft and double the volume of water used, and consequently the amount of food available to the mussel.

Rope length does not change substantially from one ria to another, and the mean rope length is 10.0 2 0.26 m on the rafts in the Ria de Vigo, 1 1 _ I -I- 0.3 1 m on the rafts in the Ria de Pontevedra and 9.7 2 0.15 m on the rafts in the Ria de Arousa. In the latter a 12% increase was observed over the mean esti- mated length recorded in 1977 (Mariiio et al., 1982).

In the early years the ropes were made of esparto grass, but nowadays they are made of synthetic material. Either standard ropes or custom-made aqua- culture ropes with irregular surfaces, designed to facilitate the attachment of the mussel by byssus theads, are used.

MUSSELCULTURE IN GALICIA (N.W. SPAIN) 273

4

25

2c

15

10

5

Fi

e

I _

c _dl 4 5 6 7 I

ig. 7. Distribution of the length of the ropes (m) in 1984 (data from the three rias sampled).

L 2 13 I4 15 16 m

CULTIVATION

The raft culture process is divided into four stages: (i) obtaining the seed, (ii) growing the seed, (iii) thinning out the ropes and transferring the seed onto new ropes and, (iv) final harvest and sale.

Obtaining the seed Seed for culture is obtained from the coastal rocks or from collector ropes

which are hung during the months of March, April and May. These seed-col- lecting ropes may be hung coiled up into a ball or completely straight from the front of the rafts. They remain underwater at a depth of l-2 m during the reproductive season of the mussel, which allows larvae to settle, and they are uncoiled later. As indicated earlier, although some authors claim that the mussel has two reproductive periods, one in the spring and the other in au- tumn, massive mussel attachment in fact occurs in the summer, both on raft ropes and on coastal rocks, whereas in autumn practically no settlement takes place (Marifio et al., 1982).

Most rafts (66%) use only seed obtained from the rocks, which in a great number of cases (75%) is collected by the mussel farmer himself or bought at prices which fluctuate between 25 and 40 pesetas/kg; 8% of the rafts use seed obtained exclusively from the collectors and the remaining 25% use seed ob- tained from both procedures in varying proportions. The use of seed taken from the collectors is closely related to the culture zone, and it is far more common on the rafts in the outer areas of the rias, specially in the Ria de Arousa.

274 A. P!?REZ CAMACHO ET AL.

Seed procured from the rocks comes preferably from open-sea areas or the mouths of the rias, and generally in areas near the rafts.

Growing the seed The mussel ropes are seeded by winding the mussels obtained from the rocks

or from the collectors around the ropes with a line-mesh rayon net, which decomposes after it has been in the water for a few days, giving the mussels enough time to attach themselves with their byssus threads. Although ma- chines exist that can attach mussels to ropes, this work with the seed is gen- erally done by hand.

The number of mussel seed ropes per raft varies between 50 and 250, with a mean value for the three rias of 137 ? 2.6, which means that the total num- ber of ropes/number of mussel seed ropes is 3.1. The mean number of mussel seed ropes on the rafts is 142 + 3.1 in the Ria de Arousa, 146 2 6.6 in the Ria de Vigo, and 96 + 3.1 in the Ria de Pontevedra, and thus the total number of ropes/number of seed ropes is 3.2, 5.5 and 2.0 for the rafts in the Rias de Arousa, Pontevedra and Vigo respectively.

The mussel seed ropes are placed in the sea throughout the whole year, but especially from November to March (Fig. 8 ) . In this case, we have also ob- served some differences between the different rias. Forty-two percent of the mussel seed ropes are hung out on the rafts in November and December in the Ria de Arousa, while only 9% and 17% are hung out in this season in the Rias of Pontevedra and Vigo respectively. This difference may be attributed

I

30

20 HARVEST AND MARKET

10

S 0 M J S D M J S D M

Fig. 8. Mussel cultivation operations in Galicia throughout the culture period.

MUSSELCULTURE IN GALICIA (N.W. SPAIN) 275

to the fact that in the Ria de Arousa, seed from collectors is more widely used and, given its more rapid growth, can be attached to the ropes earlier.

When the ropes are placed in the water, their weight fluctuates between 10 kg and 25 kg, the average weight being 14.1+ 0.46 kg. Mussel size is highly variable, depending on origin, although mean length is usually around 2 cm.

Thinning out the ropes After the seed ropes have been in the sea for about 6 months, they have

undergone a considerable increase in weight, and it is necessary to proceed with the thinning out process. At this time ropes weigh between 100 and 200 kg, depending on the raft, with a mean weight of 135 + 2.1 kg. From each of these ropes an average of three new ropes with transferred mussels is obtained.

The new ropes weigh between 30 and 60 kg, the mean weight being 46.1? 1.17 kg, somewhat higher than reported by Marifio et al. ( 1982). Ac- cording to these authors, mussels generally measure between 45 and 55 mm.

Thinning out is usually performed between June and October. During this time 8 1% of the rafts (Fig. 8 ) undergo this process.

Harvesting The mussel is harvested throughout the year (Fig. 8 ). This harvesting ac-

tivity is greatest from October to March, at which time 69% of the annual mussel production in Galicia is marketed. There is another, although less in- tensive, harvesting period from April to September, when the remaining 3 1% is harvested.

The mussel harvesting season is closely related to the mussels’ meat content and consequently, to the reproductive season, which usually occurs in spring. Another decisive factor in determining when the mussel is to be sold is the type of market it is bound for. Mussels to be canned, 31% of the total, are generally harvested during the summer, and mussels to be sold fresh, 69% of the annual production, are harvested in autumn and winter.

At harvest time, the ropes hold between 100 and 200 kg of mussels, meas- uring 70 to 100 mm in length, depending on the culture area, the position of the raft within each park and rope length. Production per raft and per year varies between 20 and 100 t, with an average annual yield of 46.9kO.97 t/ raft (Fig. 9 ) .

Average annual yield is similar in the Rias of Arousa and Pontevedra (48.0 + 1.07 t/raft and 46.9 -+ 2.50 t/raft respectively), and much lower in the Ria de Vigo, 33.2 + 2.45 t/raft. This lower yield is mainly due to the smaller size of the rafts in the Ria de Vigo. In fact, if annual yield is related to raft area for each Ria, the values obtained are very similar: 130 kg/m2, 133 kg/ m2 and 127 kg/m2, for the Rias of Arousa, Pontevedra and Vigo respectively. Similarly, yield per raft has not changed when compared with the survey taken in 1977, where yield was estimated at 46 t/raft per year.

276 A. PeREZ CAMACHO ET AL.

%

30

20

10

0 0 70 80 90 loo

METRIC T

Fig. 9. Distribution of the annual production (t per raft) in 1984.

Culture process The exploitation of a raft, according to observations made by Marifio et al.

( 1982 ), is a process determined mainly by the fluctuation in the sales volume during the different seasons of the year. Thus, for optimum exploitation, where a raft is full of ropes most of the year, ropes with half-grown mussels can only be hung out when the sale of marketable-size mussels has left a place free on the raft, and thinning out can only be done when the half-grown mussels have reached a suitable size. In addition, the total duration of the culture, aside from the quality of the mussel itself and the area and location of the raft, is dependant upon sales possibilities.

Therefore the duration of the culture process is variable, within broad lim- its, although it averages approximately 17 months, of which 5 correspond to the seed stage and 12 to the thinning out phase.

Obviously, as the market process fits into a yearly cycle, the raft is most profitably utilized if the seed ropes and thinning out ropes are made to coin- cide on the raft. Also, if we take into account that it is possible to obtain three new ropes from one seed rope, only 75% of the raft will be available to hold the ropes in the final production stage. Accordingly, the mean yield per rope for the rafts studied as a whole would be 140 kg, and 14.5 kg/meter of rope; in the different rias, the greatest yield would be in the Ria de Pontevedra with 187 kg/rope and 16.9 kg/m, the next in the Ria de Vigo with 151 kg/rope and 15.1 kg/m and lastly, the Ria de Arousa with 140 kg/rope and 14.5 kg/ m.

MUSSEL CULTURE IN GALICIA (N.W. SPAIN) 277

If, instead of considering the estimates made by the mussel farmers them- selves on the duration of the mussel production process, we choose to repre- sent the sequence of the different culture stages in the rafts as a whole, the result is quite similar. And if we examine Fig. 8, we can verify that in 87% of the cases, seed ropes are hung out between the months of November and March, and 87% of the thinning out activities take place between the months of June and October. This means that the seed rope stage lasts approximately 7 months, slightly longer than the results of previous studies suggested (Perez and Roman, 1979; Maritio et al., 1982).

It is more complicated to determine the time that thinned out ropes remain in the water, owing to the considerable flexibility of the marketing season (Fig. 8). However, it is possible that the average duration, estimated using this procedure, would be between 10 and 15 months.

FINAL CONSIDERATIONS

On comparing the results of this survey with those obtained by Ma&o et al. ( 1982)) there are several factors that come to attention: in the 7 years that have passed between the two studies ( 1977-l 984), in the Ria de Arousa, where 70% of the total production in Galicia is concentrated, raft size has grown considerably (24%), and the rope length has also increased ( 12%); that is, water volume and food available to mussels on the rafts has increased, at least in theory. However, yield per raft and number of ropes have remained the same, which may imply that this ria is approaching its production limit in relation to the amount of surface exploited.

On the other hand, we observe that in the ria with the greatest number of rafts (Ria de Arousa), the lowest yield per square meter is obtained, while in the ria having the smallest number of rafts (Ria de Pontevedra) the highest yield is found.

Consequently, any attempt to increase production (if such a plan were commercially viable), should be directed towards areas that, at the present time, are not subject to intensive exploitation. In the areas where culture den- sity is high, an increase in the number of rafts would be inefficient and could even have effects contrary to those intended.

REFERENCES

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Figueras, A., 1976. Cultivo de1 mejillon Mytilus edulis, y posibilidades para su expansion. In: T.V.R. Pillay and W.A. Dill (Editors), Advances in Aquaculture. Fishing News Books Ltd, Farnham, UK, pp. 362-37 1.

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Perez, A. and Roman, G., 1979. Estudio de1 mejillon y de su epifauna en 10s cultivos flotantes de la Ria de Arosa. Il. Bol. Inst. Esp. Oceanogr., 5 ( 1): 2 I-42.

Roman, G. and Perez, A., 1979. Estudio de1 mejillon y de su epifauna en 10s cultivos flotantes de la Ria de Arosa. I. Estudios preliminares. Bol. Inst. Esp. Oceanogr., 5: 9-l 9.

Roman, G. and Perez, A., 1982. Estudio de1 mejillon y su epifauna en 10s cultivos flotantes de la Ria de Arosa. IV. Evolution de la comunidad. Bol. Inst. Esp. Oceanogr.. 7(2): 279-296.

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