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Curlew when ihey both feed on clams, because of the small overlap between the prev sizes which are available as well as profitable for both bird species (Fig. 3). Since Curlews select larger elams than Oystercatchers, there is a segregation in lime if the two species are to feed on clams Irom the same year of spatlall. However, this potential partitioning of resources does not tell us anything quantitatively about competition for food between the species. Oystercalchers might deplete the clam stock completely before the shells have reached the size at which diey can be harvested profitably by Curlews. Depletion of the food stock Since we started our sampling programme of the maerobenihie fauna in 1977. there has been one successful spatfall of clam (1979): the previous one was in 1976. Spatfall occurs during the summer. First-winter clams reach a size of c. 8 mm and are thus still too small to be utilized by Oystercatchers. During the second growing season most animals pass the lower acceptance threshold for Oystercatchers. but not until the next year do they become profitable for Curlews (Fig. 5A). During the growth of the shell, clams bury deeper. The size-depth relationship (Fig. 3), can be used to derive the average depth of each cohort (Fig. 5B), but also allows calculation for all sampling date of the numbers of clams above the lower acceptance size threshold and which are accessible lo both bird species (big. 5C). Good years for clam-eating Oystercalchers (the winters of 1977-78 and 1980-81) precede the rich years forCurlews i ihe winters of 1978-79 and 1981-82). The predation pressure by Oystercatchers and Curlews appears to be high enough to explain the greater part of the loss of clams after the second growing season. In October and November 1980 all Oystercatchers present on the mudflats preyed upon clams. From the work of Hulscher we know that the average intake was 3.63 clams/min (n - 870 min). Because the waler level was measured continuously, we know that the mudllats were exposed for 15 900 min in daylight during these two months and for 21 500 min at night. During daytime low-water periods. Oystercalchers fed for 88% of the time that the Hats were exposed, at a den PREY DEPLETION BY OYSTERCATCHER AND CURLEW 342 sity of 1.26 birds/ha. Combining these data, we calculate that in two months the Oystercatchers took 6.4 clams/m 2 by day. Bird counts were made also al night. with infra-red binoculars, and from this it is known lhat Oystercatchers remained to feed at night, hut their feeding rate could not be measured. Assuming ihe same feeding rale as hv dav. the total predation in Octoberand November amounted to 15.1 clams/m-. If the birds which were present during the rest of the winter continued to feed on clams and did not switch to the only alternative prey available. Macoma balihica. the resultant predation pressure in the period 1 October 1980 to I March 1981 could be estimated at 20 clams/m-* by day only, or 49 clams/nr in total if the predation rate by night was the same as by day. This value is a minimum, since we have omitted one bird count which took place just after a cold spell, during which the mudflats were frozen and many Oystercalchers fed on dying bivalves. During this period the density rose to 30 birds/ha. 20 times as high as ihe average density that winter. This situation lasted for between 3 and 5 days. In that short period as many as 20 clams/m 3 might have been eaten if the birds had achieved the same feeding rale as in autumn. Between 1 October 1980 and 1 March 1981 there was a decrease from 110 lo 20 clams/m 2 (Fig. 5C). At maximum. Oystercatchers look 69 of the 90 clams/m-' which disappeared. A smaller proportion were eaten by Common Gulls Lams carats, present after the cold spell, and by some of the Curlews which started to swallow the small elams. No detailed observations are available for the winlet of 1977-78 when ihe density of preferred size classes was much higher but also decreased dramatically during the winter (Fig. 5C). Counts of birds on the feeding area are available, however, and we also know that clams were the main (and perhaps the only) prey taken because we found many clams recently opened by Oystercatchers. The average Oystercatcher density in the winter of 1977-78 was 5.1 times as high as in the winter of" 1980-81. Assuming the same intake rate by the birds, the predation pressure in the period October-March would have been 351 clams/m suming equally heavy predation by day and by night. The decrease which was found -from 440 to 120 clams/m 2 - was in fact below the estimated impact of the Oystercatchers.
PREY DEPLETION BY OYSTERCATCHER AND CURLEW Ki(*. ft. A. Depth of six individual clams on an intertidal llat between April and November 19SI. measured wilh ihe aid of a ihin nylon thread attached on the shell Size was determined in April and November. Maximal probing deplh for Oysiercatchers is given. Note the relationship between si/e increment in the three second-year clams and ihc vulnerable pericxl. B. Changes in depth of three clams plained at a shallow depth. lo show lhat hie clams have nol lost the ability to burrow. The predation pressure by Curlew is more difficult to measure, since there is a large variation in the food choice of different individuals. The majority of male Curlews, for instance, never lake clams, nor do some of the females. The best data at hand concern the predation by the marked female 20Y within her territory. The density of the accessible clams of the preferred size classes amounted to 8 clums/nr in July 1978. In the next nine months, she took at maximum about half of Ihe food slock in her territory of 6000 nr. 143 Discussion: prey risk and depth The mortality rate of the clam decreases with age (Brousseau 1978). It is well documented that Plaice exert heavy predation on the spat of the clam (Schmidt 1951. de Vlas 1979a). In our study area bird predation on the spat is quite low. Knot Calidris caniiius lake them, but this wading bird was usually rather rare in our area. The data presented above show that the lower ac-
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WADERS AND THEIR ESTUARINE FOOD SUP
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plia^ohi. WADERS AND THEIR ESTUARI
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Waders and their estuarine food sup
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WADERS AND THEIR ESTUARINE FOOD SUP
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figuren: Dick Visser omslagfoto: Ja
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15 Versatility of male curlews (Num
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That science is making progress, ma
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INTRODUCTION The remnants of brushw
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When hcnlhic bivalves extend llien
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the burrow depths and on the fracti
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INTRODUCTION Ms,i invest 409 of (he
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able lo sw itch 10 oiher prey which
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Long-term, broadly-based research c
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Chapter 1 SEASONAL VARIATION IN BOD
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SEASONAL VARIATION IN BODY WEIGHT O
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Tahle 1 Weight loss ('•- ± SE) m
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i 60 50 40 30 20 10 0 • INFESTED
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240- SEASONAL VARIATION IN BODY WEI
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20 10 0 -10 -20h 2 3 4 5 6 7 8 9 se
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a E 400 - S. plana 35 mm 320 240 16
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Chapter 2 HOW THE FOOD SUPPLY HARVE
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FOOD SUPPLY HARVESTABLE BY WADERS H
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Methods The study sites were situat
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less than that of bivalves, partly
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I Fig. 3). Peak condition was reach
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total biomass since most of those t
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on the basis of the preceding and t
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However, for obvious reasons, we ma
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prey. A decrease in the prey densit
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Tahle 2. The intake rate ol < lyste
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* ' % -. . ; a X - . - * • ^ •
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(Zwarts & Wanink 1989). In quiet we
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general law relating handling time
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nearly twice as much time (0.79 s).
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4 5 6 7 8 9 size class of Corophium
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and annually. Second, the lower siz
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Prey switching Waders feeding on ti
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autumn and spring, and the contrary
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where they switch to surface-living
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Chapter 3 BURYING DEPTH OF THE BENT
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DEPTH AND SIPHON CROPPING IN SCROBI
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I DEPTH AND SIPHON CROPPING IN SCRO
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DEPTH AND SIPHON CROPPING IN SCROBI
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Table 3. Three-Way analysis Of vari
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Chapter 4 SIPHON SIZE AND BURYING D
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15 20 size (mm) Fig. 3. Cerasioderm
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10 size (mm) SIPHON SIZE AND DEPTH
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o, 7 01 5 | * CL 5- SIPHON SIZE AND
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4 Q) Si •a a. 16- 20- A predator:
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prey risk for an animal al a depth
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Table 6. Size al which there is a m
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* ' 1 /-/ "».-^*«C
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face and so expose themselves to a
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surface in die containers was varie
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50 OOF 310.00 I 5.00 ^ 1.00 3=" 0.5
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Tahle 1. Macoma and Scrobicularia.
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siphon would not have to reach the
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known siphon weight and burying dep
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ACCESSIBLE PREY ARE OFTEN IN POOR C
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1 2 3 4 burying depth (cm) Kin. I.
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I—1 • • : : : : ! - ! -|-r 0
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Chapter 7 DOES AN OPTIMALLY FORAGIN
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OPTIMAL FORAGING AND THE FUNCTIONAL
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100 200 300 prey density (n-m-2) Fi
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Table I. Results of five one way an
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Table 2. Results of eight iwo-wav a
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prey density II III Fig. 12. Freque
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PREY SIZE SELECTION AND INTAKE RATE
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Predicted 'passive size selection'
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lig. 2. Larger Mussels are less ava
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Fig. 5. Scrobicularia plana. Size c
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and maiiv are too thick-shelled to
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The measurements of handling time i
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1 2 3 4 5 6 7 probing depth (cm PRE
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valves may vary by a factor of two
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optimal foraging model, the minimum
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their gut is full? Do they reduce t
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PREY PROFITABILITY AND INTAKE RATE
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catchers to take only certain prey
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licult error of estimate arose if p
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Counts of feeding and non-feeding b
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20 30 40 50 shell length (mm) PREY
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Nereis Arenicola tipuia earthwo'm M
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take rate. We might therefore expec
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prey species, using parallel slopes
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5.0 4.0 1-3.0 a & • % * • * •
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time during the feeding period. As
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winter. Similarly, handling time in
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5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
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situation arrived in the western pa
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• ' • 14 PREY PROFITABILITY AND
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Notes lo appendix: PREY PROFITABILI
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Chapter 10 WHY OYSTERCATCHERS HAEMA
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2 4 6 8 10 time on feeding area (h)
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80- 60- o 80 60- 40- 20- INTAKE RAT
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est of bod\ behind: an estimated 22
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INTAKE RATE AND PROCESSING RATE IN
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Wanink 1993). The energy content of
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(7) Age All studies dealt with adul
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irds over long periods. As an examp
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Discussion There is no difference i
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comparison between the weight of ih
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. 1', L ! •
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Introduction PREDICTING SEASONAL AN
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PREDICTING SEASONAL AND ANNUAL FLUC
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1000 r 1 II" 1 - r^Jan'80 PREDICTIN
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IOO BO 60 40 20 0 PREDICTING SEASON
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WHY KNOT TAKE MEDIUM-SIZED MACOMA W
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in Zwarts & Esselink 1989). The len
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shell length (mm) FIR. 3. Peringia.
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fused, specimens larger than this w
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50 100- s s I — f = S. plana, n=2
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area is twice as large as the touch
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10 15 lower size threshold (mm) Fig
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lime (Hughes 1979). This would furt
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WHY KNOT TAKE MEDIUM-SIZED MACOMA T
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Chapter 13 ANNUAL AND SEASONAL VARI
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VARIATION IN FOOD SUPPLY OF KNOT AN
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Page 293 and 294: July ' Aug ' Sept Fig. 9. Proportio
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Page 297 and 298: Chapter 14 SEASONAL TREND IN BURROW
Page 299 and 300: BURROWING AND FEEDING IN NEREIS SEA
Page 301 and 302: Table 2. Results of a 2-way analysi
Page 303 and 304: June 1981 June 1982 is * N.MUD •
Page 305 and 306: 8 10 I 12 JZ 5. -g 1*» •e o i 16
Page 307 and 308: 6 - n=!080 5 • g 4 CP > i 3 CO CD
Page 309 and 310: 1985) and Curlew (/wails ,v, Lsseli
Page 311: Chapter 15 VERSATILITY OF MALE CURL
Page 314 and 315: Smid! 1951. Muus 1967, Wolff 1973,
Page 316 and 317: 0.6 0.8 1.0 1.2 1.4 1.6 jaw lengih
Page 318 and 319: Na oi both Nrieck Fig. 7. Numenius
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Page 322 and 323: too (Fig. 11 A). Indeed, the averag
Page 324 and 325: VERSATILITY OF CURLEWS FEEDING ON N
Page 326 and 327: total time spent al the surface. Th
Page 328 and 329: 2 4 . • 2.2 • 30 .-.2.0 to Q. 1
Page 331 and 332: PREY DEPLETION BY OYSTERCATCHER AND
Page 333 and 334: the rule: the observed lower limit
Page 335 and 336: to locate the clams after they had
Page 337: Curlew, by bury ing some hundreds o
Page 341 and 342: SAMENVATTING 345
Page 343 and 344: Voorgeschiedenis In 1960 verscheen
Page 345 and 346: maken. Als gebied A een grotere voe
Page 347 and 348: omdat ze nog vrijwel niets wegen. l
Page 349 and 350: kunnen opzuigen. of diep leven en z
Page 351 and 352: van de uitgerekte sifo over het wad
Page 353 and 354: het toen niet gemakkelijk hebben ge
Page 355 and 356: doende nonnetjes van 10 tot 15 mm l
Page 357 and 358: aangevoerde voedsel en de wulp past
Page 359 and 360: zijn dan twee jaar en wulpen geen s
Page 361 and 362: was dan ook dank/ij de inzet van al
Page 363 and 364: REFERENCES 367
Page 365 and 366: Allen RL. 1983. Feeding behaviour o
Page 367 and 368: BreyT. 1989. Der Einfluss physikali
Page 369 and 370: latie tol chironoiiiiilen en de wai
Page 371 and 372: huch der Vogel Milteleuropas, Band
Page 373 and 374: llolmann II. & H. Huerschelmann 196
Page 375 and 376: l-cndrem D.W. 1984. Flocking, feedi
Page 377 and 378: Pekkarinen M. 1984. Regeneration of
Page 379 and 380: lion of Mussels Mytilus edulis hy O
Page 381 and 382: lulal Dal esiuanes: a comparison of
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