ICES mar. Sei. Symp., 199: 99-107. 1995

Feeding activity and contribution of raft culture in the diet of in the Ria de Arousa (Galicia, Northwest Spain)

E. Gonzâlez-Gurriarân, J. Freire, and L. Fernandez

Gonzâlez-Gurriarân, E., Freire, J., and Fernandez, L. 1995. Feeding activity and contribution of mussel raft culture in the diet of crabs in the Ria de Arousa (Galicia, Northwest Spain). - ICES mar. Sei. Symp., 199: 99-107.

Mussel raft culture has brought about an increase in the biomass and production levels of benthic megafauna in the Ria de Arousa. The aim of this study was to determine the diel feeding activity, the effect of feeding activity on trawl catches, and the effect of the and their associated epifauna on the diet of the crabs Necora puber, , depurator, and L. arcuatus (Brachyura, ). There were differences in the consumption of the main diet components between different and areas (raft, non-raft, and beach stations): the epifaunal ( longicornis) was the main component of the diet of N. puber, C. maenas, and L. depurator in mussel raft areas; polychaetes were the main component for L. depurator in non-raft areas; and seaweeds predominated in the diet of L. arcuatus in beach areas. Diel feeding activity varied with the species and the area. Feeding activity increased slightly at dusk and at night, although no clear diel differences were found in stomach fullness. Catches fluctuated considerably between successive trawls; there was no clear re­ lationship between crab feeding activity and catch fluctuation.

E. Gonzâlez-Gurriarân, J. Freire, and L. Fernândez: Departamento de Bioloxia , Universidade da Coruna, Campus da Zapateira sin, E-15071A Coruna, Spain [tel: (+34) 8128 0788, fax: (+34) 8110 4129],

Introduction estimation of food consumption (Pennington, 1985) and the assessment of predator-prey relationships (Ney, The Ria de Arousa (Galicia, Northwest Spain) is an area 1990). This article examines the diel feeding activity of of high biological productivity (Tenore and Gonzalez, the populations in the Ria de Arousa, as well 1975; Tenore et al., 1985) and intensive raft mussel as the relation between feeding activity and fluctuations culture (Mytilus galloprovincialis). The benthic system in trawl catches, with particular emphasis on the changes has changed in relation to this culture - production levels brought about by mussel culture activity. and biomass of the megafauna have increased consider­ ably (Iglesias, 1981; Gonzâlez-Gurriarân, 1982; Olaso, 1982; Romero et al., 1982) due to the food contribution Material and methods provided by the mussels and associated epifauna, as previously reported for several species of demersal fish Sampling was carried out throughout a 24-h cycle (April (Chesney and Iglesias, 1979; Löpez-Jamar et al., 1984) 1989) at four stations located in different areas of the Ri'a and decapod (Gonzâlez-Gurriarân, 1978; de Arousa (Fig. 1); two raft zones (B1 and B5) in the Gonzâlez-Gurriarân et al., 1989; Freire et al., 1990). inner and outer area of the ria respectively; and two Although the feeding dynamics of the megabenthos is zones where mussels are not cultured, M2 in the central the determining factor in the observed changes, the channel and P3 representing a typical beach area. The limited data available are still insufficient to analyse the raft stations, with depths of 10m in B1 and 20m in B5, different aspects of the trophic organization in the ben­ have mud bottoms (especially B l) that have been highly thic system, such as spatial, temporal and ontogenetic transformed by culture activitites. M2 had the greatest changes and how they relate to prey availability. In this depth, about 40 m with a mud bottom, while the beach sense, the analysis of diel feeding activity in marine area was 3-5 m deep with a sandy bottom and an abun­ populations is a key factor because these data allow the dance of green seaweeds. 100 E. Gonzâlez-Gurriarân, J. Freire, and L. Fernandez ICES mar. Sei. Symp., 199 (1995)

ti w

42*11

Figure 1. Rias Baixas of Galicia (NW Spain). Location of raft polygons and sampling stations in the Ria de Arousa. B1 and B5 = raft stations; M2 = channel station with no rafts; P3 = beach station.

L. arcuatus L. depurator

o CO 1-°-

i - 0 .5 - CQ ac 0.0 0.0 6 12 18 24 6 12 18 24

N. puber C. maenas

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0.0 0.0 6 12 18 24 6 12 18 24 TIME (GMT) TIME (GMT) Figure 2. Daily evolution of stomach fullness (%BDW ± s.d.) for the different species and stations sampled. i c e s mar. Sei. Symp., 199 (1995) Feeding activity and contribution o f mussel raft culture in the diet o f crabs 1 0 1

Table 1. Catches (number of individuals per tow), number of stomachs analysed (n), percentage of stomachs containing food and stomach fullness (expressed in points and %BDW) for the different species and stations sampled throughout the diel cycle. Mean and standard deviation (in parentheses) are given.

Species/ Time % with station (GMT) N/tow n food Points % BDW

Day 06:49 218.0 (84.9) 50 88.0 64.7 (34.4) 0.79 (0.67) L. arcuatus 13:28 280.0 50 66.0 51.0 (40.8) 0.60 (0.68) P3 Night 19:07 139.0 (33.9) 50 74.0 56.7 (41.8) 0.69 (0.70) 23:46 326.0 50 80.0 56.0 (36.5) 0.55 (0.55) Day 08:26 9.3 (3.2) 29 79.3 50.0 (35.4) 0.44 (0.48) L. arcuatus 14:40 27.5 (0.7) 54 77.8 62.2 (41.6) 0.50 (0.57) Bl Night 19:28 9.0 (4.0) 28 82.1 63.9 (37.6) 0.72 (0.72) 01:00 3.3 (2.2) 10 50.0 42.0 (45.7) 0.35 (0.70) Day 08:26 47.0 (5.3) 50 68.0 38.2 (39.7) 0.29 (0.47) L. depurator 14:40 63.0 (4.2) 50 74.0 41.2 (36.3) 0.28 (0.34) Bl Night 19:28 45.0 (41.0) 50 88.0 43.4 (36.1) 0.34 (0.45) 01:00 80.3 (31.3) 50 68.0 38.8 (37.6) 0.31 (0.42) Day 12:04 171.0 (22.6) 50 54.0 25.0 (33.0) 0.21 (0.40) L. depurator 17:29 104.0 (41.0) 50 72.0 50.2 (42.2) 0.59 (0.78) B5 Night 22:15 162.0 (56.6) 50 52.0 23.6 (29.2) 0.23 (0.41) 03:26 350.0 (31.1) 50 74.0 41.0 (35.2) 0.33 (0.38) Day 10:02 7.3 (1.5) 20 60.0 31.5 (36.6) 0.13 (0.17) L. depurator 15:56 5.7 (2.1) 16 31.3 7.5 (13.9) 0.24 (0.48) M2 Night 20:58 8.3 (2.5) 23 65.2 38.3 (39.7) 0.45 (0.61) 02:39 13.0 (4.2) 27 67.8 42.2 (38.6) 0.49 (0.59) Day 08:26 4.7 (2.9) 13 76.9 31.5 (28.8) 0.16 (0.17) N. puber 14:40 5.0 (2.8) 9 44.4 27.2 (39.5) 0.23 (0.30) Bl Night 19:28 3.7 (1.5) 11 81.8 43.2 (35.5) 0.54 (0.54) 01:00 5.0 (7.8) 15 66.6 29.3 (29.9) 0.37 (0.65) Day 12:04 7.0 (7.1) 13 62.5 32.7 (38.0) 0.24 (0.32) N. puber 17:29 7.3 (7.0) 21 57.1 31.0 (36.0) 0.34 (0.49) B5 Night 22:15 18.0 (26.9) 53 67.9 26.2 (29.8) 0.34 (0.61) 03:26 27.0 (9.9) 39 69.2 36.1 (34.4) 0.35 (0.44) Day 08:26 13.7 (3.2) 41 75.6 32.3 (30.7) 0.15 (0.27) C. maenas 14:40 10.5 (0.7) 21 71.4 18.8 (21.9) 0.11 (0.19) Bl Night 19:28 6.0 (6.9) 16 56.2 21.9 (31.5) 0.14 (0.28) 01:00 10.0 (7.5) 29 75.9 28.1 (26.1) 0.10 (0.15)

A beam trawl with a 10-mm mesh and an effective 1981). This method visually estimates the total volume mouth of 4 m was used to take 10-min tows, representing occupied by the food as compared to stomach volume, 800 m2 swept surface. Each station was sampled at 6-h with 100 being the maximum. The number of points is intervals (four times per station during the cycle) and on then divided among the different components depend­ each occasion three tows were done (in station P3 only ing on their relative importance. Wet and dry weights of one tow was taken on two occasions owing to technical the stomach contents - in most cases the stomachs con­ problems). tained a single component - and the wet weight of the The species studied were Liocarcinus depurator crab analysed were obtained. Dry weight of the individ­ (stations B l, B5, M2), L. arcuatus (P3, B l), Necora ual crabs was calculated using previously estimated con­ puber (B l, B5), and Carcinus maenas (Bl). In order version factors (Gonzâlez-Gurriarân, 1982). In crabs to do the stomach fullness and content analysis, a sub­ with more than one food component per stomach, dry sample of 50 individuals for each species, by station and weight per prey was estimated using the total dry weight hour, was taken. In the case of smaller captures, all the of the contents and the relative importance of each item specimens were analysed. The point method was used to as determined by the point method. estimate stomach fullness, as well as the relative import­ Stomach fullness for each sample was estimated using ance of each prey (determined at the lowest possible three indexes: percentage of stomachs containing food, taxonomic level) present in each stomach (Williams, mean value of points, and percentage of body dry weight 102 E. Gonzâlez-Gurriarân, J. Freire, and L. Fernândez ICES mar. Sei. Symp., 199 (1995)

Table 2. ANCOVA results comparing stomach fullness (food contents) between stations and times in each zone. Body dry weight) in the different species between stations and times dry weight was used as the covariate (Jenkins and in each station. In all cases, except for L. depurator, data were log10 transformed to obtain homogeneous variances (Bartlett Green, 1977). The differences between the consumption test, p > 0.05). of the different prey (as %BDW) between stations for each species, between species at the same station, and df F P between times for each species/station, were analysed using the Mann-Whitney and Kruskal-Wallis tests. An Liocarcinus arcuatus Covariate 1 2.143 0.144 analysis of variance (ANOVA) was used to study the Station 1 0.650 0.421 day/night changes in catches for each species/station. Total 320 Linear correlation was used in the study of the relation­ Bl Covariate 1 1.788 0.184 ship between catches and stomach fullness. Time 3 1.091 0.356 Total 120 P3 Covariate 1 0.767 0.382 Time 3 1.462 0.226 Results Total 199 Feeding activity Necora puber Covariate 1 2.961 0.087 In general, no significant variation in the feeding activity Station 1 0.514 0.474 throughout the diel cycle was found. Fullness tended to Total 173 increase at dusk (Fig. 2, Table 1), except in C. maenas at Bl Covariate 1 0.770 0.385 B l. Maximum fullness values for L. arcuatus in P3 were Time 3 1.055 0.378 Total 47 found at dusk as well as at dawn. This tendency of the B5 Covariate 1 1.249 0.182 %BDW index was also observed using the data obtained Time 3 0.551 0.500 by the point method. No significant differences were Total 125 observed (ANCOVA, p > 0.05; Table 2) in the degree of fullness in any of the species between stations, and the Liocarcinus depurator Covariate 1 1.465 0.227 only significant diel differences that appeared were in L. Station 2 0.812 0.445 depurator at station M2 (greatest fullness in the night Total 485 samples, p < 0.01). Bl Covariate 1 8.116 0.005 The lowest fullness levels were in C. maenas (mean Time 3 0.275 0.843 Total 199 %BDW 0.13), increasing in N. puber (0.32), and L. B5 Covariate 1 0.263 0.609 depurator (between 0.33 and 0.42 for the different Time 3 1.890 0.133 stations), and reaching the highest levels of %BDW in Total 199 L. arcuatus (0.50 in B l and 0.65 in P3). M2 Covariate 1 0.144 0.705 Time 3 3.188 0.028 Total 85 Diet composition Carcinus maenas There were important variations in diet composition Bl Covariate 1 0.365 0.547 depending on the area and species (Fig. 3). In the mussel Time 3 0.363 0.780 Total 106 culture stations (Bl and B5) the dominant prey was the decapod Pisidia longicornis, the main component of the epifauna of the raft ropes. This species reached values of about 80% of the %BDW in N. puber; in L. depurator it [%BDW = (dry weight content/body dry weight) x made up 40-65% of the diet, and represented 53% of 100]. The diet composition was expressed as %BDW per the diet of C. maenas. Only L. arcuatus in B l consumed type of prey: less P. longicornis, although it was still important (15%). P. longicornis took on greater importance in the diet in B5 (where this species was more abundant, Fer­ %BDWj = (Xn %BDWjj) / ( £ n %BDWi) nandez et al., 1990) than in the inner zone B 1. In station i = l i = l Bl other components that were important were the eggs where %BDWj; is the percentage of body dry weight of of the gastropod Nassa spp. (especially to N. puber and prey j in stomach i, %BDWj is the percentage of body L. arcuatus), the mussel, M. galloprovincialis (es­ dry weight of the content for individual i, and n is the pecially for L. depurator and N. puber), and ulvacean number of stomachs analysed. seaweeds (for L. arcuatus). An analysis of covariance (ANCOVA) was used to In the non-raft areas, the diet of the decapods varied compare the degree of fullness (dry weight of stomach substantially. At the beach station P3, the diet of L. i c e s mar. sd . Symp., 199 (1995) Feeding activity and contribution of mussel raft culture in the diet of crabs 1 0 3

Table 3. Results of the Kruskal-Wallis (K-W) and Mann-Whitney (M-W) tests comparing the consumption of the diet components between stations for each species and between species in the same station, ns is not significant, p > 0.05. When p < 0.05, results of the comparisons two by two a posteriori are shown (Mann-Whitney test). The samples are grouped from left to right for the increasing values of consumption for each prey. LDE = L. depurator, LAR = L. arcuatus, NPU = N. puber, CMA = C. maenas.

LAR LDE NPU Bl B5 Diet components (M-W) (K-W) (M-W) (K-W) (M-W)

Fishes ns ns ns LAR CMA NPU LDE NPU LDE Natantia Bl P3 ns ns ns ns Brachyura ns ns ns ns ns Pisidia P3B1 M2 B l B5 ns LAR CMA LDE NPU LDE NPU Other crustaceans ns Bl B5 M2 ns ns NPU LDE Mytilus P3B1 M2 B5B1 B5B1 LAR CMA NPU LDE ns Nucula ns B5 M2 Bl ns LAR NPU CMA LDE ns Other bivalves ns B5B1 M2 ns CMA LDE NPU LAR ns Nassa eggs P3B1 ns B5B1 CMA LDE NPU LAR ns Gastropods P3B1 Bl B5 M2 ns ns ns Ophiuroids ns Bl B5 M2 ns ns NPU LDE Psammechinus ns ns ns LDE LAR CMA NPU LDE NPU Aslia P3BI ns ns ns ns Nereis Bl P3 B1B5 M2 ns ns ns Other polychaetes ns Bl B5 M2 ns ns ns Ulvaceans Bl P3 ns B5B1 LDE NPU CMA LAR ns Other seaweeds ns ns ns ns LDE NPU Zostera ns ns ns CMA LDE NPU LAR ns Others Bl P3 ns ns LAR LDE NPU CMA ns Undet. ns B5 M2 Bl ns LAR NPU LDE CMA ns

Table 4. ANOVA results comparing catches (number of indi­ consumption of the different prey between stations for viduals per tow) for each species/station between day and night. the same species or between species for the same station were compared (Kruskal-Wallis or Mann-Whitney test) Species/station df2 Fl.dO P (Table 3), we found significant variations in the con­ L. arcuatus /B l 10 1.642 0.241 sumption of a large number of components, particularly L. arcuatus /P3 10 36.488 0.001 the most abundant ones; the exception was N. puber, N. puber/ Bl 5 0.501 0.553 which had a very similar diet at stations B l and B5. N. puber/B5 10 0.152 0.708 L. depurator /B l 7 17.658 0.014 L. depurator /B5 10 6.251 0.041 L. depurator / M2 10 0.039 0.850 Fluctuations in the catch C. maenas!Bl 9 1.838 0.224 N. puber and L. depurator showed higher densities in the outer rfa raft station (B5). C. maenas only appeared in the inner raft station (Bl) and L. arcuatus was the only arcuatus was dominated by plant components (es­ species present in the beach area P3, where high den­ pecially the ulvaceans (52%)), and polychaetes (20%, sities were found. composed mainly of Nereis spp.). In M2, L. depurator Catches were analysed for each species (number of had a more diversified diet, made up principally of individuals per tow) throughout the diel cycle (Fig. 4, polychaetes (41%), molluscs, crustaceans, and gobies. Table 1) in relation to the feeding rhythm and time. Only The prey typical of the raft areas did not appear at all or L. depurator at M2 showed a significant correlation (p < were very scarce. 0.05) between catches and stomach fullness level (r = Within each species and station, the variation in the 0.64 for %BDW, r = 0.61 for mean value of points). composition of the diet throughout the day was not Catches of L. depurator and N. puber at the raft important. There were only significant differences station B5 and the middle station M2 were higher during (Kruskal-Wallis test) in some of the secondary diet com­ the night tows (only significant for L. depurator; N. ponents. Of the dominant prey, the only ones that puber showed high variability in the trawls taken at showed statistically significant fluctuations (p < 0.05) specific hours). In B l, N. puber, L. depurator, and C. were P. longicornis for L. depurator in B5 and the maenas has insignificant fluctuations. L. arcuatus ulvaceans for L. arcuatus in P3. However, when the showed significant differences at this station, due to an 104 E. Gonzâlez-Gurriarân, J. Freire, and. L. Fernandez ICES mar. Sei. Symp., 199 (1995) L. arcuatus L. depurator \ ULVACEANS L - J SEAWEEDS

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08 14 19 01 T 0« 13 19 23 T 08141901 T 1 2172203 T 10182102 1 Bl B5 M2

N. puber C. maenas I

□ SEAWEEDS CZD SEAWEEDS

Q POLYCHAETES ^ 3 POLYCHAETES

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081419 01 T 12 1722 03 T 08 14 19 01 T Bl B5 TIME/STATION TIME/STATION Figure 3. Diet composition by species, station, and time (GMT) expressed as %BDW. Also shown is the diet for each species and station based on total samples taken of the diel cycle (T).

increase in the catch of the 15-h sample, which coincided Lawton, 1987; Wassenberg and Hill, 1987). We also with high tide. Although there appeared to be greater observed this tendency in our data where stomach full­ catchability at high tide at the beach station P3, the small ness was generally greater at dusk, but of the species number of trawls that were carried out prevented us studied L. depurator at station M2 was the only one that from being able to evaluate it properly. showed significant variation in the degree of fullness throughout the day at station M2, where there were no rafts and the dominant prey had no hard parts. Discussion Gastric evacuation of seaweeds and the calcified parts of the prey is relatively slow. Choy (1986) found values The brachyuran species in the Ria de Arousa do not of 36-72 h for N. puber, and Hill (1976) estimated generally show great fluctuations in the degree of evacuation to be several days for Scylla serrata. Clear­ stomach fullness throughout the day. Likewise, other ance rate varies considerably depending on the prey studies on the feeding activity of decapod crustaceans, (Sardà and Valladares, 1990), so stomach content analy­ based on stomach contents analysis, did not provide a sis may not give an appropriate reflection of the feeding clear indication that feeding has a daily periodicity rhythm, especially if the percentage of stomachs con­ (Mori, 1982; Stevens et al., 1982; Wassenberg and Hill, taining food is used as the fullness index. Modelling 1989; McTigue and Feller, 1989). However observations stomach repletion dynamics in terms of activity rhythms on the behaviour of these organisms, both in the labora­ and evacuation rates (Eggers, 1977) could be the basis tory and in the natural environment, do suggest there for the assessment of the gut fullness as an index of are activity cycles, with maximum values found gener­ present and past feeding activity, and for interpretation ally at dawn and dusk (Lipcius and Herrnkind, 1982; of the field data. ICES mar. sd. Symp., 199 (1995) Feeding activity and contribution of mussel raft culture in the diet of crabs 105 L. arcuatus L. depurator

30 120'

20 80

10 40

400 300 B5 P3 300 Q 200 200 CO _j_ 100 100

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18 24 0 12 18 24 TIME (GMT) TIME (GMT) Figure 4. Daily fluctuations in catches (number of individuals per trawl ± s.d.) for the different species and stations sampled.

The determining factor in the diet of benthic mega­ epifauna were found in in different areas of the fauna (crustaceans and demersal fish) in the Ria de ria (Löpez-Jamar et al., 1984). The diet of the species in Arousa was the influence of mussel culture activities, as the areas where mussels are cultured overlapped, which shown in earlier papers (Gonzâlez-Gurriarân, 1978; may be an indication of decreasing competition due to Chesney and Iglesias, 1979; Löpez-Jamar et al., 1984; the abundance of food in these zones. In addition, the Gonzâlez-Gurriarân et al., 1989; Freire et al., 1990). In fact that L. arcuatus had a differentiated diet, compared the raft areas, P. longicornis was the dominant species, to the other brachyuran species at the raft stations, was in the epifauna of the culture ropes (Român and Pérez, the reason for its high consumption of seaweeds and 1982) as well as in the diet of the crustaceans. Because of smaller intake of P. longicornis. In spite of the changes the greater mobility of demersal fish, prey from the raft in habitat, L. arcuatus maintained a high consumption of 106 E. Gonzâlez-Gurriarân, J. Freire, and L. Fernândez ICES mar. Sei. Symp., 199 (1995) seaweed, as in other zones (Stevcic, 1987). N. puber, Crustacea, Vol. 7, pp. 107-162. Ed. by F. J. Vernberg and however, displayed strong variations, with a decrease in W. B. Vernberg. Academic Press, New York. 338 pp. Eggers, D. M. 1977. Factors in interpreting data obtained by the consumption of plants in rocky subtidal areas (Choy, diel sampling of fish stomachs. J. Fish. Res. Bd Can., 34: 1986; Norman and Jones, 1990). Mussel culture was also 290-294. a key factor in the diet changes experienced by L. depur­ Einer, R. W. 1981. Diet of green crab Carcinus maenas (L.) ator and C. maenas, compared to other areas (Abellö from Port Hebert, Southwestern Nova Scotia. J. Shell. Res., 1: 89-94. and Cartes, 1987; Hall etal., 1990; Einer, 1981). Fernandez, L., Gonzâlez-Gurriarân, E ., Freire, J., and Muino, The results obtained do not indicate a direct relation­ R. 1990. Abundancia y distribueiön de Pisidia longicornis ship between catch at different hours and feeding (Linnaeus, 1767) (: Anomura) en relaciön con la activity in raft areas; the degree of stomach fullness did dinâmica del cultivo de mejillön en la Ria de Arousa (Gali­ not reflect a clear feeding cycle. In addition, at the raft cia, NW Espana). Bol. Real Soc. Esp. Hist. Nat. (Biol.), 86: 181-193. stations the large amount of objects and organisms de­ Freire, J., Fernândez, L., and Gonzâlez-Gurriarân, E. 1990. posited on the bottom gives shelter to the epibenthic Influence of mussel-raft culture in the diet of Liocarcinus animals, changing their behaviour patterns and giving arcuatus (Brachyura: Portunidae) in the Ria de Arousa rise to a smaller selection of gear. The only positive (Galicia, NW Spain). J. Shell. Res., 9: 45-57. correlation found between the two parameters was in L. Gonzâlez-Gurriarân, E. 1978. Introduction al estudio de la alimentation de la nécora, Macropipus puber (L.) (Deca­ depurator in the middle area of the ria, where there were poda, Brachyura). Bol. Inst. Esp. Oceanogr., 4: 81-93. no rafts and where the changes in stomach fullness were Gonzâlez-Gurriarân, E. 1982. Estudio de la comunidad de also greater. This was the only species that showed crustâceos decâpodos (Brachyura) en la Ria de Arousa significant diel catch variations in B5 and M2, probably (Galicia, NW Espana) y su relaciön con el cultivo de mejillön en batea. Bol. Inst. Esp. Oceanogr., 7: 223-254. related to the greater night feeding activity. Gonzâlez-Gurriarân, E., Freire, J., Fernândez, L., and Poza, The cycles in crustaceans generally comply with E. 1989. Incidencia del cultivo de mejillön en la dieta de several factors (DeCoursey, 1983), among which is feed­ Liocarcinus depurator (L.) (Brachyura: Portunidae) en la ing activity. Therefore it is not easy to find a clear Rîa de Arousa (Galicia, NW Espana). Cah. Biol. Mar., 30: 307-319. correlation between this activity and catches (Stevens et Hall, S. J., Raffaelli, D., Robertson, M. R., and Basford, D. J. al., 1984). Also in this study, the wide variability be­ 1990. The role of the predatory crab. Liocarcinus depurator, tween tows taken in the same area and at the same time - in a marine food web. J. Anim. Ecol., 59: 421-438. an indication of the aggregated spatial distribution of Hill, B. J. 1976. Natural food, foregut clearance-rate and these organisms - makes it difficult to determine trends activity of the crab Scylla serrata. Mar. Biol., 34: 109— 116. in the catches. It would be necessary to carry out a Iglesias, J. 1981. Spatial and temporal changes in the demersal greater number of tows in order to be able to make an fish community of the Ria de Arousa (NW Spain). Mar. accurate evaluation of the existing fluctuations. Biol., 65: 199-208. Jenkins, B. W., and Green, J. M. 1977. A critique of field methodology to determining fish feeding periodicity. Env. Biol. Fish., 1:209-214. Acknowledgements Lawton, P. 1987. Diel activity and foraging behavior of juvenile American lobsters, Homarus americanus. Can. J. Fish, This research was sponsored by the Conselleria de Pesca aquat. Sei., 44: 1195-1205. of the Xunta de Galicia through FEUGA. It also Lipcius, R. N., and Herrnkind, W. F. 1982. Molt cycle alter­ received the collaboration of the Conselleria de Educa- ations in behavior, feeding and diel rhythms of a decapod crustacean, the spiny lobster Panulirus argus. Mar. Biol., ciön e Ordenaciön Universitaria of the Xunta de Gali­ 241-252. cia. We thank C. P. Teed for preparing the English Löpez-Jamar, E., Iglesias, J., and Otero, J. J. 1984. Contribu­ version of the manuscript. tion of infauna and mussel-raft epifauna to demersal fish diets. Mar. Ecol. 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