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and maiiv are too thick-shelled to be opened (Kg, 2). like slabbing Oystercatchers. hammering birds reject Mussels below 20 to 25 mm long. bin. in contrast to slabbers. Mussels larger than 60 mm are laken only infrequently. When a correction is made for the prey fraction thai is unavailable due to shell thickness and barnacle cover (Fig. 2), the proportion or each large prey class taken roughly coincides with their available density (Meire & Ervynck 1986). or explains al least a part of the deviation between observed size selection and frequency distributioii of si/e classes on offer (CayfordcS Goss-Custard 1990). Size selection and optimal foraging Predicted 'active si/e selection' The random touch model tests the assumption that birds lake prey al random. In fact, as the results in the previous section show, the observed prev selection often deviates from the predictions of the model. Oystercatchers apparently prefer some si/e class to others. Why? Prev size selection in Oystercatcher is analysed here within the framework of optimal foraging theory (Enilen 1966. Mac Arthur & Pianka 1966. Krebs & Kacelnik 1991). The basic assumption is thai predators are able to rank prey according to their profitability, defined as ihe intake rate while prey are being handled. They are predicted to reject prey for which the profitability is below the current average intake rate over both handling and searching combined. The decision rule governing the rejection threshold is therefore based on the relative profitability of handling compared with continuing to search, i.e. whether the bird can achieve a higher net intake rate by continuing to search for more profitable prey than it could achieve by handling a less profitable, although more frequently encountered, prey. This leads lo the prediction thai. when the profitability of the prey remains the same, the rejection threshold should increase as ihe intake rate increases. The rejection threshold should also increase when the iniake rale remains the same but the profiiabiliiv of all prey types decreases: lor instance, because the prey are lean. The rejection threshold should therefore be flexible within clearly defined limits, as illustrated in Fig. 8. PREY SIZE SELECTION AND INTAKE RATE 162 10 20 30 40 length of prey (mm) intake rate lotworwiil cangal Fig. 8. The optimal pre) size selection model. The two slopes delimit the seasonal variation in profitability (mg a ' handling) as a Innclionol prev si/e I'rev lor which ihe prolil.ihililv is below Ihe illlake rale line s' feeding, i.e. during both searching and handling) should be rejected. Which pre) shook) he rejected thus depends on the level oi the intake raie .is well a-on ihe length profitability slope. The .lark shaded Held shows the expected range within which the lower acceptance threshold should he lound when Ihe intake rale varies between I and 4 mg s ' feeding. Small prey are less profitable Even after a correction has been made for the small 'effective touch area" oi small prey,Oystercatchers appear lo lake fewer oi them than would be expected on Ihe basis of the frequency with which they are encountered (Figs. 2 to 7). The birds always completely reject prey less than about 10 mm long in Macoma and Cerastoderma and about 15 to 20 mm long in Scrobicularia. Mya and Myiilus. There might be a very simple explanation for this. Oystercatchers are specialized to open hard-shelled prey before they eat the llesh. in contrast lo Knot Calidris canutus or Bar-tailed Godwits limosa lapponica which swallow ihe prey whole and crush them in the stomach. There must be a size below which Oystercatchers are hardly able to separate the flesh from the shell. Whether or not this is close to ihe observed rejection threshold has still to be tested. On the other hand, several papers have used the optimality approach to explain the size rejection threshold of Oystercatchers. The prediction is that, below a certain size threshold, prev are simply noi worth taking since their energv value is too low given the time required to handle them: i.e. to open them and eat 50
ihe llesh i/waris and Dreni 1981. Ens 1982. Hulscher 1982. Sutherland 1982c. Zwarts and Wanink 1984, Meire & Ervynck 1986, Sutherland & Ens 1987, Cayford & Goss Custard 1990). Handling times have therefore been measured in both captivity and in the field. When determining the relationship between handling time and prey size in the field, prey sizes have to PREY SIZE SELECTION AND INTAKE RATE 60 80 100 10 length of the shell (mm) be estimated by eye against something of known length, normally bill lengih or the size of a colour ring. Fortunately, observers are quite consistent in their estimates nnii simple corrections are sufficient lo give an accurate estimate of the si/e taken (Ens 1982. Blomert el al. 1983, Goss-Custard et al. 1987. Boates and Goss-Custard 1989). 40 50 60 70 Fig. 9. Dry llesh weighl ling ash-tree dry weight; shaded) and handling lime (s; symbols) as a function of shell length (mm) in different studies The seasonal variation in prey weights arc from Cayford & Goss-Custard (1990) and Zwarts 11991). Data arc from the follow Macoma. Blomert a at. (1983) and Hulscher (1982 and unpubl). Scrobicularia: Blomert el al. (1983). Boates & Goss-Custard (1989). Habekotte (1987), Wanink ft Zwarts (1985. 1996). Cerastoderma: Ens el al. (1996b & c). Hulscher (1982 and unpubl.). Meire (1996b), Sutherland (1982c), Swennen ci al. 1989). Triplet 11990). Mya: Wanink & Zwarts (1996). Zwarts & Wanink (19X4). Msiilus i slabbers i: Blomert el al. I 198 i i. Cayford & Goss-Custard (1990). Ens ei ul. (1996c). Hulscher (unpubl.), Koene (1978). Linders (1985). Speakman 119X4ai. Sutherland • The alloineinc relations are given, y = Inthandling lime. s). * = Im shell lengih. mm). 163
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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:
Page 108 and 109: prey risk for an animal al a depth
Page 110 and 111: Table 6. Size al which there is a m
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Page 114 and 115: face and so expose themselves to a
Page 116 and 117: surface in die containers was varie
Page 118 and 119: 50 OOF 310.00 I 5.00 ^ 1.00 3=" 0.5
Page 120 and 121: Tahle 1. Macoma and Scrobicularia.
Page 122 and 123: siphon would not have to reach the
Page 124 and 125: known siphon weight and burying dep
Page 127 and 128: ACCESSIBLE PREY ARE OFTEN IN POOR C
Page 129 and 130: 1 2 3 4 burying depth (cm) Kin. I.
Page 131 and 132: I—1 • • : : : : ! - ! -|-r 0
Page 133 and 134: Chapter 7 DOES AN OPTIMALLY FORAGIN
Page 135 and 136: OPTIMAL FORAGING AND THE FUNCTIONAL
Page 137 and 138: 100 200 300 prey density (n-m-2) Fi
Page 139 and 140: Table I. Results of five one way an
Page 141 and 142: Table 2. Results of eight iwo-wav a
Page 145 and 146: prey density II III Fig. 12. Freque
Page 149: PREY SIZE SELECTION AND INTAKE RATE
Page 152 and 153: Predicted 'passive size selection'
Page 154 and 155: lig. 2. Larger Mussels are less ava
Page 156 and 157: Fig. 5. Scrobicularia plana. Size c
Page 160 and 161: The measurements of handling time i
Page 162 and 163: 1 2 3 4 5 6 7 probing depth (cm PRE
Page 164 and 165: valves may vary by a factor of two
Page 166 and 167: optimal foraging model, the minimum
Page 168 and 169: their gut is full? Do they reduce t
Page 171 and 172: PREY PROFITABILITY AND INTAKE RATE
Page 173 and 174: catchers to take only certain prey
Page 175 and 176: licult error of estimate arose if p
Page 177 and 178: Counts of feeding and non-feeding b
Page 181 and 182: 20 30 40 50 shell length (mm) PREY
Page 185 and 186: Nereis Arenicola tipuia earthwo'm M
Page 187 and 188: take rate. We might therefore expec
Page 189 and 190: prey species, using parallel slopes
Page 191 and 192: 5.0 4.0 1-3.0 a & • % * • * •
Page 193 and 194: time during the feeding period. As
Page 195 and 196: winter. Similarly, handling time in
Page 197 and 198: 5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
Page 199 and 200: situation arrived in the western pa
Page 203 and 204: • ' • 14 PREY PROFITABILITY AND
Page 205 and 206: Notes lo appendix: PREY PROFITABILI
Page 207: 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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Two sites (N and M in Fig. 2) were
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20 30 VARIATION IN FOOD SUPPLY OF K
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20 l i r'l ' i •o j^y^T n ^ ' " ^
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July ' Aug ' Sept Fig. 9. Proportio
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available. There must, however, be
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Chapter 14 SEASONAL TREND IN BURROW
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BURROWING AND FEEDING IN NEREIS SEA
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Table 2. Results of a 2-way analysi
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June 1981 June 1982 is * N.MUD •
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8 10 I 12 JZ 5. -g 1*» •e o i 16
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6 - n=!080 5 • g 4 CP > i 3 CO CD
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1985) and Curlew (/wails ,v, Lsseli
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Chapter 15 VERSATILITY OF MALE CURL
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Smid! 1951. Muus 1967, Wolff 1973,
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0.6 0.8 1.0 1.2 1.4 1.6 jaw lengih
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Na oi both Nrieck Fig. 7. Numenius
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6 8 10 12 14 16 18 worm length (cm)
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too (Fig. 11 A). Indeed, the averag
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VERSATILITY OF CURLEWS FEEDING ON N
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total time spent al the surface. Th
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2 4 . • 2.2 • 30 .-.2.0 to Q. 1
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PREY DEPLETION BY OYSTERCATCHER AND
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the rule: the observed lower limit
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to locate the clams after they had
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Curlew, by bury ing some hundreds o
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PREY DEPLETION BY OYSTERCATCHER AND
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SAMENVATTING 345
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Voorgeschiedenis In 1960 verscheen
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maken. Als gebied A een grotere voe
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omdat ze nog vrijwel niets wegen. l
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kunnen opzuigen. of diep leven en z
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van de uitgerekte sifo over het wad
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het toen niet gemakkelijk hebben ge
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doende nonnetjes van 10 tot 15 mm l
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aangevoerde voedsel en de wulp past
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zijn dan twee jaar en wulpen geen s
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was dan ook dank/ij de inzet van al
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REFERENCES 367
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Allen RL. 1983. Feeding behaviour o
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BreyT. 1989. Der Einfluss physikali
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latie tol chironoiiiiilen en de wai
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huch der Vogel Milteleuropas, Band
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llolmann II. & H. Huerschelmann 196
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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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