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valves may vary by a factor of two during the course of the year (Cayford & Goss-Custard 1990, Zwarts 1991, Zwarts & Wanink 1993). there is an opportunity to test whether seasonal changes in the profitability curves accurately predict a shift in size selection. Cayford & Goss-Custard (1990). for example, analysed the seasonal variation in the size selection for Mussels on the Exe estuary. SW. England. The birds actively selected those si/e classes predicted io maximize intake rale lot mosl months of the year. However, in spring, the birds fed on smaller Mussels than were predicted from their profitability and availability. Cayford Ac (ioss-Custard (1990) suggested that size selection may be influenced at certain times of the year by still unmeasured differences between size classes in prey quality and energy content. However, other apparently contradictory results can be reconciled more easily with the optimal foraging model. Handling time does not increase as the flesh contenl s>\~ a bivalve increases because the time taken actually to eat the llesh is only a small fraction of the total (Wanink & Zwarts 1985). Most of the handling time is spent in lifting the bivalve io the surface, stabbing or forcing the bill between the valves, or hammering a hole in the shell. Hence the profitability of a given prey size may be as much as twice as high in early summer as in winter. The rejection threshold, therefore, should be lower in summer than during the test of the year (Fig. 8). In fact, in Macoma. the reverse was found (Fig. 4B), as Oystercatchers were more selective in summer than at other times of the year. However, all data on si/e selection in early summer refer to breeding Oystercatchers which have the rather high intake rate of 3 mg s ' (Ens el al. 1992, Bunskoeke el al. 1996). Such a high rate should have the effect of raising the rejection threshold for Macoma. thus countering the opposite effect on the rejection threshold of an increased prey condition (Fig. 8). Thus, in this case, two tendencies may be working in opposite direct ions. The intake rates of Oystercatchers. measured over periods of several hours usually vary between 1 and 3 mg dry llesh s' 1 (Fig. 13). When they feed al a rate of 1 mg s ', they would be predicted to accept all Scrobicularia or Mytilus larger than 15 mm long, whereas when the intake rate is three times higher, the rejection threshold should be raised to 20 and 25 mm. respectively (Fig. 10). The simple way to test these predic PREY SIZE SELECTION AND INTAKE RATE 168 tions would be to compare observed rejection thresholds with the lower thresholds predicted from the intake rale. There is. however, a methodological problem. In the field studies summarized in Fig. 13. intakerate is closely correlated with the size of the prey taken (see below): the expected dependence of the lower si/e threshold on intake rate may thus be attributed simply to size selection itself. The intake rate is determined by a combination of three variables: the searching time, the handling time and ihe dry llesh weighl of ihe prey taken. The search time primarily depends on prey density (Hulscher 1976, 1982. Sutherland 1982b. Wanink & Zwarts 1985). Both handling time and prey weight increase with prey size (Fig. 9), whereas there is a seasonal variation in the llesh content of individual prey. The major factor determining iniake rate in the reviewed studies was the si/.e of the prey (Fig. 14); for instance. 59' I oj the variance in intake rate on Scrobicularia could be explained by prey size. A covariance analysis performed on the 197 studies which measured intake rate (Zwarts el al. 1996b) revealed lhat prey weight explained 16% of the variance and lhai both Ihe prey species and season had significant effects (R 2 = 0.24 and 0.19. respectively). When prey of different species, but of similar size, were compared. Cerastoderma and Macoma were shown to yield a higher intake rate than the other bivalves. The reason is that, for a given size class. Cerastoderma and Macoma are more profitable. This is due to their more globular shape (Zwarts & Blomert 1992). so that they contain more llesh than corresponding size classes in the other three bivalves (Fig. 10). The increase in iniake rate with prey size (Fig. 14) is thus due to the high profitability of the larger size classes (Fig. 10). The yield of the smallest prey taken (Fig. 10) is below the average iniake rate of 2 mg S '. So even if the birds could eat these small prey continuously, without spending any time in searching for them, the average intake rate would fall to the low level of 1 or 2 mg s'. In contrast, the larger size classes can be handled at a rate of about 10 mg s" 1 (Fig. 10). enabling the birds to attain an intake rale of 2 mg s'. even though as much as 80% of the feeding time is spent in searching. In fact, the searching lime must have been less than this, as Oystercatchers taking large bivalves have an intake rate of 3 or 4 mg s ' (Fig. 14).
The finding that the intake rate predominantly depends on si/e selection makes it difficult to interpret the field data on si/e selection. The prediction of the optimal foraging mmlol is thai the threshold si/e of acceptable prey should depend on the intake rate. However, the threshold level itsell affects the intake rate. One way to break this circle is to examine studies where the density of the most profitable prey, and si i the iniake rale, varied considerably. When this was PREY SIZE SELECTION AND INTAKE RATE - Scrobicularia y=l S3X-4.70 p=0.009 r=0.77 done, the lower size threshold changed in the direction predicted by the model: for example. Oystercatchers were more selective when the density of large Macoma was high (Fig. 4A). Similarly, Sutherland (1982c) found that Oystercatchers took fewer small Cockles as the abundance of ihc large ones increased. Such find ings allow the direction of causality to be established. On the assumption that these findings can be applied across all studies, we conclude that, as predicted by the - Cerastoderma y=0.95» a.33 . p
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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
Page 114 and 115: face and so expose themselves to a
Page 116 and 117: surface in die containers was varie
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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 158 and 159: and maiiv are too thick-shelled to
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 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
Page 210 and 211: 2 4 6 8 10 time on feeding area (h)
Page 212 and 213: 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
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Pekkarinen M. 1984. Regeneration of
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lion of Mussels Mytilus edulis hy O
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lulal Dal esiuanes: a comparison of
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