- Email: [email protected]
Population structure and exploitation of Bolinus brandaris (Mollusca: Gastropoda) off the Catalan coast (northwestern Mediterranean)
Fisheries Research 23 (1995) 319-331
Population structure and exploitation of Bolinus brandaris (Mollusca: Gastropoda) off the Catalan coast (northwestern Mediterranean) Paloma Martin*, Pilar Sdnchez, Montserrat
Ram6n
Institute de Ciencias del Mar (CSIC), Paseo Joan de Borbd s/n, 08039 Barcelona, Spain Accepted 28 September
1994
Abstract The purple dye murex, Bolinus brandaris, a prosobranch mollusc which is very common in the Mediterranean, is a resource of local importance. It is fished all the year round using a special type of dragged gear that is highly efficient for catching this species, which represents 73% of the commercial catch of this gear. The bathymetric size distribution of the mollusc is significantly correlated with depth. The purple dye murex grows relatively less in total length and shell width than in aperture. The species shows a slight positive allometry with regard to the increase in weight. During the year two groups of monthly length frequency can be distinguished: one from April to October and other from November to February. In 1993 the annual catch of the purple dye murex off the Catalan coast was 360 tonnes. The species is fished mainly in the southern half of the littoral (98% of the annual catch). The catches during the year are markedly seasonal, being lowest in summer and highest in late autumn and winter. The hourly yields are significantly correlated with depth, but not with the duration of the haul. Keywords: Bohus brandaris; Mediterranean
Sea; Population structure; Seasonal dynamics
1. Introdnction
The purple dye murex, Bolinus brandaris (Linnaeus, 1758 ), is a prosobranch gastropod mollusc, very common in the Mediterranean, which inhabits sandymuddy bottoms at depths between 5 and 50 m. It is fished at depths from 15 to 25 m, and constitutes a resource of local importance in different zones of its area * Corresponding
author.
0165-7836/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0165-7836 (94)00346-7
320
P. Martin et al. /Fisheries Research 23 (i995) 319-331
of distribution (Fischer et al., 1987 ). At the present time the annual catch on the Catalan coast, from the French frontier to the southern limit of the Ebro Delta (42”20’N 3”20’Eand 40”30’N 0”30’E), is 360 tonnes. Traditionally, various kinds of artisanal fishing gear have been used for catching this species (trammel nets, basket traps, dragged gears). From the mid-eighties, a new type of dragged gear, locally known as ‘rastell’and specially designed for catching the murex, began to be used on the Catalan coast. This gear, a beam trawl modified without skates (see Section 2, Materials and methods), is used by around 60 vessels located mainly in the Ebro Delta zone. This fishing gear is used throughout the year, 5 days a week. A number of rules for the regulation of its use exist, and include the following aspects: gross tonnage and horse power of the vessel, fishing ground, number of fishing hours per day, length of gear, number of tickler chains, minumum shell size (25 mm shell width). Before the catch is sold, a selection by size is made on board, and the smallest specimens are returned to the sea. The catch is marketed fresh. B. branduris constitutes the major part of the commercial trade of this fishing gear. Co-occurring species include the sole (Soleu spp), the prawn (Penaeus keruthurus), the mantis shrimp (SquiZZumantis) and the cuttlefish (Sepia oficinulis). Occasionally, fishermen change the target species, seeking the striped Venus ( Chumeleu gullinu), a very abundant bivalve in the Ebro Delta zone. Available information on B. branduris is scarce. Among the previous studies are those of Barash and Zenziper ( 1980)) describing the spawning, Dalla Via and Tappeiner ( 198 1 ), which deals with the biometry of the species, Bartolome ( 1985), a preliminary study on the biology and fishery, Ramon ( 1986), which discusses the spawning and larval development of various prosobranch gastropods, and several studies on spermatogenesis (Amor and Durfort, 1990a,b, Amor, 1992). A principal drawback in the study of B. branduris is the poor knowledge of both its biology and population dynamics. This study describes the exploitation of B. branduris off the Catalan coast, and also provides information on several characteristics of the population. The main objective is to provide as much information as possible on the species which, at the present time, is being exploited with little previous knowledge.
2. Materials and methods The material was collected from 37 hauls made by commercial vessels of the fishing fleet of Sant Carles, one of the ports on the Catalan coast where the fishery for this resourse is of major importance (Fig. 1) . Sampling was proportional to the fishing effort, and was designed so as to represent the activity of the ‘rastell fleet. Sampling was carried out regularly every month (from November 1992 to October 1993) on the usual fishing grounds (3 1 hauls). Also during the study, a protected zone where the ‘rastell’cannot normally operate was visited and samples taken between 5 and 14 m (six hauls). The minimum depth of the sampling
P. Martin et al, /Fisheries Research 23 (I 995) 319-331
_---_f-
I _^^
wuvl” O”36’ r’
0942.
321
A
O”46’
Fig. 1. Study area with the location of the hauls and the types of bottom indicated. A, Bofinus brandark fishing ground (muddy bottom); B, protected area for the vessels using dragged gear (muddy bottom); C, Chamelea gallina fishing ground (coarse sandy bottom).
Fig. 2. Scheme of the dragged gear modified for catching Bolinus brandaris.
was 3 m and the maximum depth 25 m. In each haul, the fishing ground, the depth, the duration and the total catch were recorded. Vessels that use the special ‘rastell’ fishing gear for B. brandaris were selected.
P. Martin et al. /Fisheries Research 23 (I 995) 319-331
322
Table 1 Summary of the sampling data Haul
Date
Depth (m)
Duration (min)
Fishing zone
Total catch (kg)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
26 Nov. 92 26 Nov. 92 17 Dec. 92 17 Dec. 92 17 Dec. 92 19 Jan. 93 19 Jan. 93 19 Jan. 93 19 Feb. 93 19 Feb. 93 19 Feb. 93 11 Mar. 93 11 Mar. 93 11 Mar. 93 11 Mar. 93 15 Apr. 93 15 Apr. 93 15 Apr. 93 15 Apr. 93 19 May 93 19 May 93 19 May 93 17 Jun. 93 17Ju.93 17 Jun. 93 21 Jul. 93 21 Jul. 93 21 Jul. 93 21 Jul. 93 25 Aug. 93 25 Aug. 93 15 Sep. 93 15 Sep. 93 15 Sep. 93 19 Oct. 93 19 Oct. 93 19 Oct. 93
23 25 23 23 22 25 24 24 6 24 24 6 6 20 24 6 6 7 21 6 21 22 6 6 6 7 6 3 9 10 18 17 17 15 17 19 10
120 90 105 115 70 90 90 80 4 110 105 5 15 110 110 10 5 75 145 32 42 189 10 10 10 10 11 7 31 20 150 90 90 90 90 110 80
A A A A A A A A C A A C C A A C C B A B A A C C C C C C B B A A A B A A B
52.9 90.8 82.3 91.4 38.8 49.2 121.8 17.7 139.9 48.0 39.4 525.3 325.0 31.8 17.1 215.8 114.9 155.9 38.8 48.2 39.8 70.3 129.6 96.8 76.3 201.7 263.1 33.1 68.1 37.5 66.4 61.5 86.7 206.8 32.1 24.7 133.8
B. brandaris catch (kg)
C. gallina catch (kg)
5.0 3.9 6.0 4.7 4.3 8.0 9.0 6.3
n,
n,
84 60 464 215 368 169 115 221
162 321 182
88 83
74 96
83 137
48 65
277 250 149 118 84
251 168 254 245 238
251 116 400 225 257 275 227 300 424
250
149 156
79.2 3.8 4.1 30.8 183.4 2.7 3.0 129.6 68.3 23.8 8.2 3.0 2.2 6.4 35.9 24.0 18.6 42.4 37.8 10.2 31.0 3.4 39.8 23.6 46.3 42.4 12.4 13.5 33.2
586 427 325 275 228 154
A, Bolinus brandaris fishing ground (muddy bottom); B, protected area for the vessels using dragged gear (muddy bottom); C, Chamelea gaflina fishing ground (coarse sandy bottom); n,, number of specimens measured of the total catch; n,, number of specimens measured of the culled catch.
P. Martin et al. /Fisheries Research 23 (1995) 319-331
323
L
Fig. 3. Dimensions considered in the shell of Bolinus brandaris. SL, shell length; SW, shell width; SLWS, shell length without siphon; A, aperture length.
The gear consists of three parts: the mouth, the bag and the cod-end. The total length of the gear varies between 6 and 8 m. The mouth is a rectangular or semioval metal frame, with a 2 m maximum horizontal aperture and a 40 cm vertical aperture. The upper part of the bag consists of netting and the lower part of 1316 tickler chains (Fig. 2). The cod-end (60-70 mm mesh size) measures 1.5-3 m. The lower part of the bag is made of thicker and more resistant mesh, while in the upper part the material is lighter in order to maintain the net open during the trawl. Fishing is done by dragging two gear units side by side from the stem. The boats that use this kind of gear cannot be heavier than 16 gross tonnes. A representative sample was taken from each haul, and its specific composition was determined in the laboratory. A total number of 5440 specimens of B. brandaris from the total catch were measured, and 4654 specimens from the fraction of the catch that was marketed, after selection by size (Table 1). The following shell measurements were taken, with a precision of 0.1 mm, using an automatic vernier caliper: shell length (SL), shell length without siphon (SLWS), shell width (SW ) , aperture length (A) (Fig. 3 ) . Monthly frequency distributions of size ( SL ) were calculated for the total catch, as well as for both the total and culled annual catches (class interval 2 mm SL). Total weight (TW ) was recorded with a precision of 0.1 g. The relative growth of the four dimensions of the shell was studied. Comparison of the different values of the slope (b) was done using Student’s t-test, for a significance level of P< 0.05. Monthly length (SL) distributions of the total catch, smoothed by standard methods (three classes moving average ), were compared. The percentage of similarity ( PS ) was used as a measure of resemblance:
324
P. Martin et al. /Fisheries Research 23 (1995) 319-331
where x and y are the length distribution vectors to be compared (Goodall, 1978 ) . The Kolmogorov-Smirnov test (K-S test) was also applied to test the resemblance between monthly distributions (Press et al., 1988). Daily yield per vessel (kg day- ’) and hourly yield (kg h- ’) , in relation to the depth and duration of the haul, were estimated from the sampling on board the commercial vessels. Data on total monthly landings of B. brunduris along the Catalan coast were available. This information was provided by the fishermen’s associations from 16 fishing ports. These monthly catch statistics form a complete record of B. branduris landings. Data from the fishing port of Sant Carles ( 1985- 1993) was used to show the evolution of landings during the year.
3. Results From the total of 37 hauls made during the sampling, B. brunduris was caught in 26 hauls, six of which were located in the protected zone. In the hauls which were targeted on the purple dye murex, this species made up 2 1% of the total catch and 73% of the commercial catch, which is evidence of the efficiency of the gear. It is worth noting that, although the protected zone is located very close to the C. gullinu fishing ground, and at a similar depth, the hauls performed in the protected zone caught only the purple dye murex. The range of sizes caught varied between 16 and 90 mm SL (Fig. 4). The monthly frequency distributions of size from November 1992 to March 1993 showed a mean size of between 50.2 and 54.0 mm SL. In April two mode peaks were clearly apparent, one corresponding to individuals of 28 mm SL and the other to those of 46 mm SL. In May the mean size of the length distribution (49.5 mm SL) was smaller than in the autumn and winter months. In July the situation was very similar, although with a smaller mean size (42.7 mm SL), due mainly to the absence of large individuals. From August to October smaller individuals, from 16 to 20 mm SL, appeared, and the mean sizes of the population continued to be small (42.0-47.0 mm SL). Results obtained by the comparison of the monthly length frequency distributions of the total catch by means of the PS and the K-S tests were similar (Tables 2 and 3). Highest PS values between monthly distributions corresponded to the highest values of the significance level of the estimated statistic D of the K-S test. According to these results, two different groups of monthly length distributions appeared during the year. The first corresponded to April-October inclusive, and the second to November-February inclusive. The March length frequency distribution had intermediate characteristics between these two groups. On one hand,
P. Martin et al. /Fisheries Research 23 (1995) 319-331
JANUARY
AUGUST
MARCH
OCTOBER
325
APRIL ‘Or
I
Shell length (mm)
Fig. 4. Bolinur brandaris monthly length (SL) frequency distributions vember 1992 to October 1993.
for the total catch, from No-
the March length frequency distribution was similar to that of the previous months and also to that of the following months of April and May, but it was different from the distribution corresponding to July. The size distributions of the total and culled annual catches were clearly different. The marketed fraction (culled catch) showed a marked bias towards the largest sizes (Fig. 5 ). The total annual catch ranged between 16 and 82 mm SL, while the culled catch ranged between 28 and 90 mm SL. Small sized individuals ( < 42 mm SL) were practically absent from the culled fraction. The most frequent sizes were 46 mm SL in the case of the total catch, and 48 mm in the culled catch.
P. Martin et al. /Fisheries Research 23 (1995) 319-331
326
Table 2 Monthly size distribution (SL) to monthly size distribution similarity-coefficient of similarity) for the period November 1992 to October 1993 Dec. Jan. Feb. Mar. Apr. May Jul. Aug. Sep. Oct.
0.810 0.899 0.856 0.773 0.639 0.691 0.478 0.638 0.517 0.556
0.858 0.913 0.817 0.664 0.775 0.475 0.716 0.505 0.597
0.886 0.788 0.644 0.722 0.475 0.637 0.507 0.564
0.860 0.703 0.788 0.521 0.715 0.553 0.628
0.815 0.849 0.639 0.808 0.672 0.759
0.793 0.7 11 0.829 0.789 0.819
0.666 0.849 0.661 0.784
0.689 0.884 0.854
0.707 0.820
0.874
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May
Jul.
Aug.
Sep.
Table 3 Kolmogorov-Smimov Dec. Jan. Feb. Mar. Apr. May Jul. Aug. Sep. Oct.
matrix (percentage
0.9624 1.OOoo 0.9840 0.3774 0.0115 0.1264 0.0000 0.0127 0.0002 0.0009
two-sample test results: significance level of the estimated statistic D
0.9940 1.oooo 0.5749 0.0218 0.2748 0.0000 0.0798 0.000 1 0.0032
0.9971 0.3615 0.0126 0.1348 0.0000 0.0102 0.0001 0.0010
0.8527 0.0588 0.4320 0.0002 0.0772 0.0007 0.0080
0.5963 0.9997 0.0110 0.6444 0.0271 0.1986
0.3596 0.4742 0.8232 0.6879 0.9926
0.0235 0.9796 0.0206 0.3224
0.0464 0.9994 0.8565
0.0667 0.5679
0.9139
Dec.
Jan.
Feb.
Mar.
Apr.
May
Jul.
Aug.
Sept.
Nevertheless, 38.7% of the specimens of total catch ranged between 46 and 54 mm SL, whereas 60.3% of the specimens of the culled catch were included in this interval. The species showed a size distribution that correlated significantly with depth (linear correlation, r= 0.378, d.f. = 15 10, P-c 0.00 1). The small and medium-sized specimens were found distributed in the range of depths covered by the sampling, while the larger ones ( > 65 mm SL) were found mainly from a depth of 20 m (Fig. 6), and were practically absent in the hauls made in the protected zone (<15mdepth). B. branduris showed isometric growth when considering the SL and SLWS relationship. However, the relationships of the other three dimensions considered, SL-SW, SL-A and SW-A, showed positive allometry (Table 4). The purple dye murex grows relatively less in shell length and shell width than in aperture. The species showed a slight positive allometry with regard to the increase in weight. The annual catch of the purple dye murex off the Catalan coast was 360 tonnes in 1993. Catches do not present a homogeneous distribution along the littoral,
P. Martin et al. /Fisheries Research 23 (1995) 319-331
321
15 50
30
10
90
70
Shelllength(mm) Fig. 5. Bolinm brandaris annual length (SL) frequency distributions, (A) and the culled catch (B).
in percentage, for the total catch
5O10
20
30
60 40 50 Shell length (mm)
Fig. 6. Bolinus brandaris distribution
70
60
by depth (m) and size (mm, SL).
being highest in the fishing ports located in the southern half. The catch of this species is very small in the ports located in the northern half (less than 2% of the annual catch). The fleets from the ports of Sant Carles and Vilanova, in the southern zone, which are the ports that have the greatest number of boats using the ‘rastell’, obtain about 80% of the annual total catch.
328
P. Martin et al. /Fisheries Research 23 (I 995) 319-331
Table 4 Estimated values of the parameters
SL vs. SLWS SL vs. SW SL vs. A SW vs. A SL vs. TW
(a and b) of the allometry relationships
for Bolinus brandaris
a
b
r
n
t
0.5554 0.3012 0.1710 0.6644 3.7E-5
1.019 1.108: 1.195* 1.061* 3.093*
0.952 0.934 0.926 0.938 0.959
1089 501 501 809 2827
1.92 5.67 8.95 4.43 5.24
*Indicates positive allometry (P-c 0.05 ) . SL, she11 length; SW, shell width; SLWS, she11 length without siphon; A, aperture length; TW, total weight; r, correlation coefficient; n, number of specimens; t, Student’s t value.
-
B. brandaris
.-... C. gallina
Fig. 7. Monthly catches (tonnes) of Bolinus brandaris and Chamelea gallina from vessels of the port Sant Caries during 1985-1993, showing the seasonality of the catches of both species in the course of the year.
The catches of B. brandaris showed a pronounced seasonality. The minimum during the year in Sant Carles, between 5 and 7 monthly tonnes, occurred in the summer (June-July); the maximum, which varied between 15 and 30 monthly tonnes during the 1985-l 993 period, occurred in the late autumn and in the winter. Catches showed an increasing trend until 1989- 1990. The vessels which use dragged gear for purple dye murex devote their fishing time during the year mainly to this species, but alternately this gear is used for fishing the striped Venus C. gallina. The periods of the year with the largest catches of striped Venus corresponded to those months when less purple dye murex was caught (Fig. 7 ) . From 25 hauls in which the target species was B. brandaris, a daily yield, per vessel, of 34.7 kg and a mean hourly yield of 8.5 t 8.7 kg h- ’were estimated. The hourly yields ranged between a minimum value of 1.5 kg h- ’and a maximum of 30.8 kg h- ‘. The mean duration of these hauls was 94 2 40 min, with a minimum duration of 20 min and a maximum duration of 189 min. The highest yields were
P. Martin et al. /Fisheries Research 23 (1995) 319-331
329
Depth (m)
Fig. 8. Hourly yields of Bolinus brandaris versus depth (A) and duration (B) of the haul.
obtained at the upper limit of the protected zone in Sam Carles. It should be noted that, in the hauls made at shallower depths in the protected zone, the yields were higher ( 17.7 + 8.5 kg h- ’) than in the usual commercial fishing grounds (6.2 t- 7.0 kg h- ’). On the other hand, similar hourly yields were obtained in hauls of fairly distinct durations (Fig. 8). The hourly yields were significantly correlated with depth (linear correlation, r= - 0.604, d.f. = 23, P< 0.05 ), but not correlated with the duration of the hauls (r= - 0.177).
4. Discussion The presence of B. brandaris together with the absence of C. gallina (Sanchez and PDPEM, 1993) in a protected zone which is located beside the striped Venus fishing ground and at the same depth, emphasizes the importance of the granulometry of the substratum in the distribution of both species. Due to the different nature of the bottom, mud in the closed area and coarse sand in the striped Venus fishing ground (Guillen, 1992), there is no overlap in the distribution of these two species. Small individuals of B. brandaris appeared mainly from July to October and during the month of April. Bartolome ( 1985) noted that, in a zone close to the study area, spawning of this species takes place in the spring, and there is a period of 2 months of development in ootheca. In the eastern Mediterranean, Barash and Zenziper ( 1980) likewise found spawn masses during a period that lasts several months (from May to July). In this study spawn masses were found in Au-
330
P. Martin et al. /Fisheries Research 23 (1995) 319-331
gust. Also, according to the results of the comparison of the monthly length frequency distributions, two different groups can be established. The former corresponds to the period from November to March, and the latter from April to October. All these data suggest a lengthy spawning period off the Catalan coast. The results of depth and size distribution, with the largest specimens caught in the deepest hauls and practically absent in the hauls performed in the protected zone ( < 15 m depth; Fig. 6)) where the fishing fleet does not operate, suggest that the species carries out an ontogenic migration towards deeper waters. The seasonality observed in the B. branduris catches, which is a characteristic they share with other fish resources exploited in the Mediterranean, remained constant during the whole period 1985-1993, and can be attributed to changes in the species abundance throughout the year. The times of the year corresponding to the minimum and maximum catches were the same in all years, despite the changes in the fishing pattern undertaken in the last years. In the last 4 years, the number of vessels using ‘rastell’has remained fairly constant (around 50 vessels). However, from 1989 the striped Venus catches, obtained by the same vessels fishing for the purple dye murex, increased markedly with respect to the previous years. Nevertheless, seasonality of B. brandaris catches in these last 4 years was not affected, highest monthly catches being obtained in the same period as during 1985-l 989, when the only target species was B. brandaris. The lack of any significant correlation between the duration of the haul and the hourly yield suggests that the distribution of the purple dye murex is not homogeneous. In fact, at least during the reproductive period, the muricids form groups in order to spawn together (D’Asaro, 1970). On the other hand, the fact that the hauls made in shallower waters corresponded to the hauls made in the protected zone-i.e. a zone where the dragged gear for purple dye murex cannot operateexplains the significant correlation found between depth and hourly yield. Hence, the species is more abundant in the protected zone. Several factors suggest continued resource availability of the purple dye murex. The existence of a calcareous exoskeleton provides the molluscs significant protection. Furthermore, selection by size is done on board and the discard, made up of the small-sized individuals ( < 45 mm SL), is immediately returned to the sea. A common practice among fishermen consists of maintaining alive on board a part of the purple dye murex catch in a tank with sea water, so as to market it the following day, in order to get a higher price, given that prices change depending on the day of the week. Moreover, this species has no survival problems when they are introduced into aquaria after having been fished (Ramon, personal observation). Considering these facts together, it seems that the fishing strategy applied to B. brandaris would not affect the small-sized individuals, thus favouring the renewal of the resource. Acknowledgements
This study was financed by the Generalitat de Catalunya, Research Contract Ref. PCC 30020/92, and carried out by the Mediterranean Population Dynamics Group of the ICM.
P. Martin et al. /Fisheries Research 23 (199s) 319-331
331
References Amor, M.J., 1992. Ultrastructural study of the mucosa of the male gonoduct of Murex brandaris (Hexaplex brandaris) (Gastropoda, Prosobranchia). Invertebr. Reprod. Dev., 2 1: 149-l 60. Amor, M.J. and Durfort, M., 1990a. Changes in nuclear structure during eupyrene spermatogenesis in Murex brandaris. Mol. Reprod. Dev., 25: 348-356. Amor, M.J. and Durfort, M., 199Ob. Atypical spermatogenesis in Murex brandaris. Mol. Reprod. Dev., 25: 357-363. Barash, Al. and Zenziper, Z., 1980. Egg masses of mollusca from Mediterranean waters of Israel and notes on reproduction of the freshwater species Theodoxus jordani and Melanoides tuberculata. Veliger, 22: 299-317. Bartolome, Ch., 1985. Contribution a l’etude du gasteropode Murex brandaris (Linnaeus, 1758) dans le Golfe du Lion. Diplome d’Etudes Approfondies, Universite des Sciences et Techniques du Languedoc, 92 pp. Dalla Via, G.-F. and Tappeiner, U., 198 1. Morphological and functional correlates with distribution of Murex trunculus L. and Murex brandaris L. (Mollusca, Gastropoda) in the northern Adriatic. Boll. Zool., 48: 191-195. D’Asaro, C.N., 1970. Egg capsules of prosobranch mollusks from South Florida and the Bahamas and notes on spawning in the laboratory. Bull. Mar. Sci., 20: 4 14-440. Fischer, W., Bauchot, M.-L. and Schneider, M. (Editors), 1987. Fiches FAO d’ldentilication des Esp&es pour les Besoins de la P&he. Mtdittrranee et Mer Noire. Vol. I. Vegetaux et Invertebres. FAO, Rome, 760 pp. Goodall, D.W., 1978. Sample similarity and species correlation. In: H. Whittaker (Editor), Ordination of Plant Communities. Dr. W. Junk, The Hague, pp. 99- 149. Guilltn, J., 1992. Dinamica y balance sedimentario en 10s ambientes fluvial y litoral de1 delta de1 Ebro. Ph.D. Thesis, University of Barcelona, 580 pp. Press, W.H., Flannery, B.P., Teukolsky, S.A. and Vetterling, W.T., 1988. Numerical Recipes. Cambridge University Press, 8 18 pp. Ramon, M., 1986. Estudio y description de las puestas, desarrollo larvario y protoconcha de algunos gasteropodos prosobranquios de la costa catalana. Tesis de Licenciatura, University of Barcelona, 157 pp. Sanchez, P. and PDPEM, 1993. Analisi de l’impacte de la pesca amb rastell sobre les poblacions d’organismes marins. Generalitat de Catalunya, Research Project Contract Ref. PCC 30020/92, 132 pp.
Population structure and exploitation of Bolinus brandaris (Mollusca: Gastropoda) off the Catalan coast (northwestern Mediterranean) Paloma Martin*, Pilar Sdnchez, Montserrat
Ram6n
Institute de Ciencias del Mar (CSIC), Paseo Joan de Borbd s/n, 08039 Barcelona, Spain Accepted 28 September
1994
Abstract The purple dye murex, Bolinus brandaris, a prosobranch mollusc which is very common in the Mediterranean, is a resource of local importance. It is fished all the year round using a special type of dragged gear that is highly efficient for catching this species, which represents 73% of the commercial catch of this gear. The bathymetric size distribution of the mollusc is significantly correlated with depth. The purple dye murex grows relatively less in total length and shell width than in aperture. The species shows a slight positive allometry with regard to the increase in weight. During the year two groups of monthly length frequency can be distinguished: one from April to October and other from November to February. In 1993 the annual catch of the purple dye murex off the Catalan coast was 360 tonnes. The species is fished mainly in the southern half of the littoral (98% of the annual catch). The catches during the year are markedly seasonal, being lowest in summer and highest in late autumn and winter. The hourly yields are significantly correlated with depth, but not with the duration of the haul. Keywords: Bohus brandaris; Mediterranean
Sea; Population structure; Seasonal dynamics
1. Introdnction
The purple dye murex, Bolinus brandaris (Linnaeus, 1758 ), is a prosobranch gastropod mollusc, very common in the Mediterranean, which inhabits sandymuddy bottoms at depths between 5 and 50 m. It is fished at depths from 15 to 25 m, and constitutes a resource of local importance in different zones of its area * Corresponding
author.
0165-7836/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0165-7836 (94)00346-7
320
P. Martin et al. /Fisheries Research 23 (i995) 319-331
of distribution (Fischer et al., 1987 ). At the present time the annual catch on the Catalan coast, from the French frontier to the southern limit of the Ebro Delta (42”20’N 3”20’Eand 40”30’N 0”30’E), is 360 tonnes. Traditionally, various kinds of artisanal fishing gear have been used for catching this species (trammel nets, basket traps, dragged gears). From the mid-eighties, a new type of dragged gear, locally known as ‘rastell’and specially designed for catching the murex, began to be used on the Catalan coast. This gear, a beam trawl modified without skates (see Section 2, Materials and methods), is used by around 60 vessels located mainly in the Ebro Delta zone. This fishing gear is used throughout the year, 5 days a week. A number of rules for the regulation of its use exist, and include the following aspects: gross tonnage and horse power of the vessel, fishing ground, number of fishing hours per day, length of gear, number of tickler chains, minumum shell size (25 mm shell width). Before the catch is sold, a selection by size is made on board, and the smallest specimens are returned to the sea. The catch is marketed fresh. B. branduris constitutes the major part of the commercial trade of this fishing gear. Co-occurring species include the sole (Soleu spp), the prawn (Penaeus keruthurus), the mantis shrimp (SquiZZumantis) and the cuttlefish (Sepia oficinulis). Occasionally, fishermen change the target species, seeking the striped Venus ( Chumeleu gullinu), a very abundant bivalve in the Ebro Delta zone. Available information on B. branduris is scarce. Among the previous studies are those of Barash and Zenziper ( 1980)) describing the spawning, Dalla Via and Tappeiner ( 198 1 ), which deals with the biometry of the species, Bartolome ( 1985), a preliminary study on the biology and fishery, Ramon ( 1986), which discusses the spawning and larval development of various prosobranch gastropods, and several studies on spermatogenesis (Amor and Durfort, 1990a,b, Amor, 1992). A principal drawback in the study of B. branduris is the poor knowledge of both its biology and population dynamics. This study describes the exploitation of B. branduris off the Catalan coast, and also provides information on several characteristics of the population. The main objective is to provide as much information as possible on the species which, at the present time, is being exploited with little previous knowledge.
2. Materials and methods The material was collected from 37 hauls made by commercial vessels of the fishing fleet of Sant Carles, one of the ports on the Catalan coast where the fishery for this resourse is of major importance (Fig. 1) . Sampling was proportional to the fishing effort, and was designed so as to represent the activity of the ‘rastell fleet. Sampling was carried out regularly every month (from November 1992 to October 1993) on the usual fishing grounds (3 1 hauls). Also during the study, a protected zone where the ‘rastell’cannot normally operate was visited and samples taken between 5 and 14 m (six hauls). The minimum depth of the sampling
P. Martin et al, /Fisheries Research 23 (I 995) 319-331
_---_f-
I _^^
wuvl” O”36’ r’
0942.
321
A
O”46’
Fig. 1. Study area with the location of the hauls and the types of bottom indicated. A, Bofinus brandark fishing ground (muddy bottom); B, protected area for the vessels using dragged gear (muddy bottom); C, Chamelea gallina fishing ground (coarse sandy bottom).
Fig. 2. Scheme of the dragged gear modified for catching Bolinus brandaris.
was 3 m and the maximum depth 25 m. In each haul, the fishing ground, the depth, the duration and the total catch were recorded. Vessels that use the special ‘rastell’ fishing gear for B. brandaris were selected.
P. Martin et al. /Fisheries Research 23 (I 995) 319-331
322
Table 1 Summary of the sampling data Haul
Date
Depth (m)
Duration (min)
Fishing zone
Total catch (kg)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
26 Nov. 92 26 Nov. 92 17 Dec. 92 17 Dec. 92 17 Dec. 92 19 Jan. 93 19 Jan. 93 19 Jan. 93 19 Feb. 93 19 Feb. 93 19 Feb. 93 11 Mar. 93 11 Mar. 93 11 Mar. 93 11 Mar. 93 15 Apr. 93 15 Apr. 93 15 Apr. 93 15 Apr. 93 19 May 93 19 May 93 19 May 93 17 Jun. 93 17Ju.93 17 Jun. 93 21 Jul. 93 21 Jul. 93 21 Jul. 93 21 Jul. 93 25 Aug. 93 25 Aug. 93 15 Sep. 93 15 Sep. 93 15 Sep. 93 19 Oct. 93 19 Oct. 93 19 Oct. 93
23 25 23 23 22 25 24 24 6 24 24 6 6 20 24 6 6 7 21 6 21 22 6 6 6 7 6 3 9 10 18 17 17 15 17 19 10
120 90 105 115 70 90 90 80 4 110 105 5 15 110 110 10 5 75 145 32 42 189 10 10 10 10 11 7 31 20 150 90 90 90 90 110 80
A A A A A A A A C A A C C A A C C B A B A A C C C C C C B B A A A B A A B
52.9 90.8 82.3 91.4 38.8 49.2 121.8 17.7 139.9 48.0 39.4 525.3 325.0 31.8 17.1 215.8 114.9 155.9 38.8 48.2 39.8 70.3 129.6 96.8 76.3 201.7 263.1 33.1 68.1 37.5 66.4 61.5 86.7 206.8 32.1 24.7 133.8
B. brandaris catch (kg)
C. gallina catch (kg)
5.0 3.9 6.0 4.7 4.3 8.0 9.0 6.3
n,
n,
84 60 464 215 368 169 115 221
162 321 182
88 83
74 96
83 137
48 65
277 250 149 118 84
251 168 254 245 238
251 116 400 225 257 275 227 300 424
250
149 156
79.2 3.8 4.1 30.8 183.4 2.7 3.0 129.6 68.3 23.8 8.2 3.0 2.2 6.4 35.9 24.0 18.6 42.4 37.8 10.2 31.0 3.4 39.8 23.6 46.3 42.4 12.4 13.5 33.2
586 427 325 275 228 154
A, Bolinus brandaris fishing ground (muddy bottom); B, protected area for the vessels using dragged gear (muddy bottom); C, Chamelea gaflina fishing ground (coarse sandy bottom); n,, number of specimens measured of the total catch; n,, number of specimens measured of the culled catch.
P. Martin et al. /Fisheries Research 23 (1995) 319-331
323
L
Fig. 3. Dimensions considered in the shell of Bolinus brandaris. SL, shell length; SW, shell width; SLWS, shell length without siphon; A, aperture length.
The gear consists of three parts: the mouth, the bag and the cod-end. The total length of the gear varies between 6 and 8 m. The mouth is a rectangular or semioval metal frame, with a 2 m maximum horizontal aperture and a 40 cm vertical aperture. The upper part of the bag consists of netting and the lower part of 1316 tickler chains (Fig. 2). The cod-end (60-70 mm mesh size) measures 1.5-3 m. The lower part of the bag is made of thicker and more resistant mesh, while in the upper part the material is lighter in order to maintain the net open during the trawl. Fishing is done by dragging two gear units side by side from the stem. The boats that use this kind of gear cannot be heavier than 16 gross tonnes. A representative sample was taken from each haul, and its specific composition was determined in the laboratory. A total number of 5440 specimens of B. brandaris from the total catch were measured, and 4654 specimens from the fraction of the catch that was marketed, after selection by size (Table 1). The following shell measurements were taken, with a precision of 0.1 mm, using an automatic vernier caliper: shell length (SL), shell length without siphon (SLWS), shell width (SW ) , aperture length (A) (Fig. 3 ) . Monthly frequency distributions of size ( SL ) were calculated for the total catch, as well as for both the total and culled annual catches (class interval 2 mm SL). Total weight (TW ) was recorded with a precision of 0.1 g. The relative growth of the four dimensions of the shell was studied. Comparison of the different values of the slope (b) was done using Student’s t-test, for a significance level of P< 0.05. Monthly length (SL) distributions of the total catch, smoothed by standard methods (three classes moving average ), were compared. The percentage of similarity ( PS ) was used as a measure of resemblance:
324
P. Martin et al. /Fisheries Research 23 (1995) 319-331
where x and y are the length distribution vectors to be compared (Goodall, 1978 ) . The Kolmogorov-Smirnov test (K-S test) was also applied to test the resemblance between monthly distributions (Press et al., 1988). Daily yield per vessel (kg day- ’) and hourly yield (kg h- ’) , in relation to the depth and duration of the haul, were estimated from the sampling on board the commercial vessels. Data on total monthly landings of B. brunduris along the Catalan coast were available. This information was provided by the fishermen’s associations from 16 fishing ports. These monthly catch statistics form a complete record of B. branduris landings. Data from the fishing port of Sant Carles ( 1985- 1993) was used to show the evolution of landings during the year.
3. Results From the total of 37 hauls made during the sampling, B. brunduris was caught in 26 hauls, six of which were located in the protected zone. In the hauls which were targeted on the purple dye murex, this species made up 2 1% of the total catch and 73% of the commercial catch, which is evidence of the efficiency of the gear. It is worth noting that, although the protected zone is located very close to the C. gullinu fishing ground, and at a similar depth, the hauls performed in the protected zone caught only the purple dye murex. The range of sizes caught varied between 16 and 90 mm SL (Fig. 4). The monthly frequency distributions of size from November 1992 to March 1993 showed a mean size of between 50.2 and 54.0 mm SL. In April two mode peaks were clearly apparent, one corresponding to individuals of 28 mm SL and the other to those of 46 mm SL. In May the mean size of the length distribution (49.5 mm SL) was smaller than in the autumn and winter months. In July the situation was very similar, although with a smaller mean size (42.7 mm SL), due mainly to the absence of large individuals. From August to October smaller individuals, from 16 to 20 mm SL, appeared, and the mean sizes of the population continued to be small (42.0-47.0 mm SL). Results obtained by the comparison of the monthly length frequency distributions of the total catch by means of the PS and the K-S tests were similar (Tables 2 and 3). Highest PS values between monthly distributions corresponded to the highest values of the significance level of the estimated statistic D of the K-S test. According to these results, two different groups of monthly length distributions appeared during the year. The first corresponded to April-October inclusive, and the second to November-February inclusive. The March length frequency distribution had intermediate characteristics between these two groups. On one hand,
P. Martin et al. /Fisheries Research 23 (1995) 319-331
JANUARY
AUGUST
MARCH
OCTOBER
325
APRIL ‘Or
I
Shell length (mm)
Fig. 4. Bolinur brandaris monthly length (SL) frequency distributions vember 1992 to October 1993.
for the total catch, from No-
the March length frequency distribution was similar to that of the previous months and also to that of the following months of April and May, but it was different from the distribution corresponding to July. The size distributions of the total and culled annual catches were clearly different. The marketed fraction (culled catch) showed a marked bias towards the largest sizes (Fig. 5 ). The total annual catch ranged between 16 and 82 mm SL, while the culled catch ranged between 28 and 90 mm SL. Small sized individuals ( < 42 mm SL) were practically absent from the culled fraction. The most frequent sizes were 46 mm SL in the case of the total catch, and 48 mm in the culled catch.
P. Martin et al. /Fisheries Research 23 (1995) 319-331
326
Table 2 Monthly size distribution (SL) to monthly size distribution similarity-coefficient of similarity) for the period November 1992 to October 1993 Dec. Jan. Feb. Mar. Apr. May Jul. Aug. Sep. Oct.
0.810 0.899 0.856 0.773 0.639 0.691 0.478 0.638 0.517 0.556
0.858 0.913 0.817 0.664 0.775 0.475 0.716 0.505 0.597
0.886 0.788 0.644 0.722 0.475 0.637 0.507 0.564
0.860 0.703 0.788 0.521 0.715 0.553 0.628
0.815 0.849 0.639 0.808 0.672 0.759
0.793 0.7 11 0.829 0.789 0.819
0.666 0.849 0.661 0.784
0.689 0.884 0.854
0.707 0.820
0.874
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May
Jul.
Aug.
Sep.
Table 3 Kolmogorov-Smimov Dec. Jan. Feb. Mar. Apr. May Jul. Aug. Sep. Oct.
matrix (percentage
0.9624 1.OOoo 0.9840 0.3774 0.0115 0.1264 0.0000 0.0127 0.0002 0.0009
two-sample test results: significance level of the estimated statistic D
0.9940 1.oooo 0.5749 0.0218 0.2748 0.0000 0.0798 0.000 1 0.0032
0.9971 0.3615 0.0126 0.1348 0.0000 0.0102 0.0001 0.0010
0.8527 0.0588 0.4320 0.0002 0.0772 0.0007 0.0080
0.5963 0.9997 0.0110 0.6444 0.0271 0.1986
0.3596 0.4742 0.8232 0.6879 0.9926
0.0235 0.9796 0.0206 0.3224
0.0464 0.9994 0.8565
0.0667 0.5679
0.9139
Dec.
Jan.
Feb.
Mar.
Apr.
May
Jul.
Aug.
Sept.
Nevertheless, 38.7% of the specimens of total catch ranged between 46 and 54 mm SL, whereas 60.3% of the specimens of the culled catch were included in this interval. The species showed a size distribution that correlated significantly with depth (linear correlation, r= 0.378, d.f. = 15 10, P-c 0.00 1). The small and medium-sized specimens were found distributed in the range of depths covered by the sampling, while the larger ones ( > 65 mm SL) were found mainly from a depth of 20 m (Fig. 6), and were practically absent in the hauls made in the protected zone (<15mdepth). B. branduris showed isometric growth when considering the SL and SLWS relationship. However, the relationships of the other three dimensions considered, SL-SW, SL-A and SW-A, showed positive allometry (Table 4). The purple dye murex grows relatively less in shell length and shell width than in aperture. The species showed a slight positive allometry with regard to the increase in weight. The annual catch of the purple dye murex off the Catalan coast was 360 tonnes in 1993. Catches do not present a homogeneous distribution along the littoral,
P. Martin et al. /Fisheries Research 23 (1995) 319-331
321
15 50
30
10
90
70
Shelllength(mm) Fig. 5. Bolinm brandaris annual length (SL) frequency distributions, (A) and the culled catch (B).
in percentage, for the total catch
5O10
20
30
60 40 50 Shell length (mm)
Fig. 6. Bolinus brandaris distribution
70
60
by depth (m) and size (mm, SL).
being highest in the fishing ports located in the southern half. The catch of this species is very small in the ports located in the northern half (less than 2% of the annual catch). The fleets from the ports of Sant Carles and Vilanova, in the southern zone, which are the ports that have the greatest number of boats using the ‘rastell’, obtain about 80% of the annual total catch.
328
P. Martin et al. /Fisheries Research 23 (I 995) 319-331
Table 4 Estimated values of the parameters
SL vs. SLWS SL vs. SW SL vs. A SW vs. A SL vs. TW
(a and b) of the allometry relationships
for Bolinus brandaris
a
b
r
n
t
0.5554 0.3012 0.1710 0.6644 3.7E-5
1.019 1.108: 1.195* 1.061* 3.093*
0.952 0.934 0.926 0.938 0.959
1089 501 501 809 2827
1.92 5.67 8.95 4.43 5.24
*Indicates positive allometry (P-c 0.05 ) . SL, she11 length; SW, shell width; SLWS, she11 length without siphon; A, aperture length; TW, total weight; r, correlation coefficient; n, number of specimens; t, Student’s t value.
-
B. brandaris
.-... C. gallina
Fig. 7. Monthly catches (tonnes) of Bolinus brandaris and Chamelea gallina from vessels of the port Sant Caries during 1985-1993, showing the seasonality of the catches of both species in the course of the year.
The catches of B. brandaris showed a pronounced seasonality. The minimum during the year in Sant Carles, between 5 and 7 monthly tonnes, occurred in the summer (June-July); the maximum, which varied between 15 and 30 monthly tonnes during the 1985-l 993 period, occurred in the late autumn and in the winter. Catches showed an increasing trend until 1989- 1990. The vessels which use dragged gear for purple dye murex devote their fishing time during the year mainly to this species, but alternately this gear is used for fishing the striped Venus C. gallina. The periods of the year with the largest catches of striped Venus corresponded to those months when less purple dye murex was caught (Fig. 7 ) . From 25 hauls in which the target species was B. brandaris, a daily yield, per vessel, of 34.7 kg and a mean hourly yield of 8.5 t 8.7 kg h- ’were estimated. The hourly yields ranged between a minimum value of 1.5 kg h- ’and a maximum of 30.8 kg h- ‘. The mean duration of these hauls was 94 2 40 min, with a minimum duration of 20 min and a maximum duration of 189 min. The highest yields were
P. Martin et al. /Fisheries Research 23 (1995) 319-331
329
Depth (m)
Fig. 8. Hourly yields of Bolinus brandaris versus depth (A) and duration (B) of the haul.
obtained at the upper limit of the protected zone in Sam Carles. It should be noted that, in the hauls made at shallower depths in the protected zone, the yields were higher ( 17.7 + 8.5 kg h- ’) than in the usual commercial fishing grounds (6.2 t- 7.0 kg h- ’). On the other hand, similar hourly yields were obtained in hauls of fairly distinct durations (Fig. 8). The hourly yields were significantly correlated with depth (linear correlation, r= - 0.604, d.f. = 23, P< 0.05 ), but not correlated with the duration of the hauls (r= - 0.177).
4. Discussion The presence of B. brandaris together with the absence of C. gallina (Sanchez and PDPEM, 1993) in a protected zone which is located beside the striped Venus fishing ground and at the same depth, emphasizes the importance of the granulometry of the substratum in the distribution of both species. Due to the different nature of the bottom, mud in the closed area and coarse sand in the striped Venus fishing ground (Guillen, 1992), there is no overlap in the distribution of these two species. Small individuals of B. brandaris appeared mainly from July to October and during the month of April. Bartolome ( 1985) noted that, in a zone close to the study area, spawning of this species takes place in the spring, and there is a period of 2 months of development in ootheca. In the eastern Mediterranean, Barash and Zenziper ( 1980) likewise found spawn masses during a period that lasts several months (from May to July). In this study spawn masses were found in Au-
330
P. Martin et al. /Fisheries Research 23 (1995) 319-331
gust. Also, according to the results of the comparison of the monthly length frequency distributions, two different groups can be established. The former corresponds to the period from November to March, and the latter from April to October. All these data suggest a lengthy spawning period off the Catalan coast. The results of depth and size distribution, with the largest specimens caught in the deepest hauls and practically absent in the hauls performed in the protected zone ( < 15 m depth; Fig. 6)) where the fishing fleet does not operate, suggest that the species carries out an ontogenic migration towards deeper waters. The seasonality observed in the B. branduris catches, which is a characteristic they share with other fish resources exploited in the Mediterranean, remained constant during the whole period 1985-1993, and can be attributed to changes in the species abundance throughout the year. The times of the year corresponding to the minimum and maximum catches were the same in all years, despite the changes in the fishing pattern undertaken in the last years. In the last 4 years, the number of vessels using ‘rastell’has remained fairly constant (around 50 vessels). However, from 1989 the striped Venus catches, obtained by the same vessels fishing for the purple dye murex, increased markedly with respect to the previous years. Nevertheless, seasonality of B. brandaris catches in these last 4 years was not affected, highest monthly catches being obtained in the same period as during 1985-l 989, when the only target species was B. brandaris. The lack of any significant correlation between the duration of the haul and the hourly yield suggests that the distribution of the purple dye murex is not homogeneous. In fact, at least during the reproductive period, the muricids form groups in order to spawn together (D’Asaro, 1970). On the other hand, the fact that the hauls made in shallower waters corresponded to the hauls made in the protected zone-i.e. a zone where the dragged gear for purple dye murex cannot operateexplains the significant correlation found between depth and hourly yield. Hence, the species is more abundant in the protected zone. Several factors suggest continued resource availability of the purple dye murex. The existence of a calcareous exoskeleton provides the molluscs significant protection. Furthermore, selection by size is done on board and the discard, made up of the small-sized individuals ( < 45 mm SL), is immediately returned to the sea. A common practice among fishermen consists of maintaining alive on board a part of the purple dye murex catch in a tank with sea water, so as to market it the following day, in order to get a higher price, given that prices change depending on the day of the week. Moreover, this species has no survival problems when they are introduced into aquaria after having been fished (Ramon, personal observation). Considering these facts together, it seems that the fishing strategy applied to B. brandaris would not affect the small-sized individuals, thus favouring the renewal of the resource. Acknowledgements
This study was financed by the Generalitat de Catalunya, Research Contract Ref. PCC 30020/92, and carried out by the Mediterranean Population Dynamics Group of the ICM.
P. Martin et al. /Fisheries Research 23 (199s) 319-331
331
References Amor, M.J., 1992. Ultrastructural study of the mucosa of the male gonoduct of Murex brandaris (Hexaplex brandaris) (Gastropoda, Prosobranchia). Invertebr. Reprod. Dev., 2 1: 149-l 60. Amor, M.J. and Durfort, M., 1990a. Changes in nuclear structure during eupyrene spermatogenesis in Murex brandaris. Mol. Reprod. Dev., 25: 348-356. Amor, M.J. and Durfort, M., 199Ob. Atypical spermatogenesis in Murex brandaris. Mol. Reprod. Dev., 25: 357-363. Barash, Al. and Zenziper, Z., 1980. Egg masses of mollusca from Mediterranean waters of Israel and notes on reproduction of the freshwater species Theodoxus jordani and Melanoides tuberculata. Veliger, 22: 299-317. Bartolome, Ch., 1985. Contribution a l’etude du gasteropode Murex brandaris (Linnaeus, 1758) dans le Golfe du Lion. Diplome d’Etudes Approfondies, Universite des Sciences et Techniques du Languedoc, 92 pp. Dalla Via, G.-F. and Tappeiner, U., 198 1. Morphological and functional correlates with distribution of Murex trunculus L. and Murex brandaris L. (Mollusca, Gastropoda) in the northern Adriatic. Boll. Zool., 48: 191-195. D’Asaro, C.N., 1970. Egg capsules of prosobranch mollusks from South Florida and the Bahamas and notes on spawning in the laboratory. Bull. Mar. Sci., 20: 4 14-440. Fischer, W., Bauchot, M.-L. and Schneider, M. (Editors), 1987. Fiches FAO d’ldentilication des Esp&es pour les Besoins de la P&he. Mtdittrranee et Mer Noire. Vol. I. Vegetaux et Invertebres. FAO, Rome, 760 pp. Goodall, D.W., 1978. Sample similarity and species correlation. In: H. Whittaker (Editor), Ordination of Plant Communities. Dr. W. Junk, The Hague, pp. 99- 149. Guilltn, J., 1992. Dinamica y balance sedimentario en 10s ambientes fluvial y litoral de1 delta de1 Ebro. Ph.D. Thesis, University of Barcelona, 580 pp. Press, W.H., Flannery, B.P., Teukolsky, S.A. and Vetterling, W.T., 1988. Numerical Recipes. Cambridge University Press, 8 18 pp. Ramon, M., 1986. Estudio y description de las puestas, desarrollo larvario y protoconcha de algunos gasteropodos prosobranquios de la costa catalana. Tesis de Licenciatura, University of Barcelona, 157 pp. Sanchez, P. and PDPEM, 1993. Analisi de l’impacte de la pesca amb rastell sobre les poblacions d’organismes marins. Generalitat de Catalunya, Research Project Contract Ref. PCC 30020/92, 132 pp.