Cannibalistic interactions of juvenile mud crabs Scylla serrata: the effect of shelter and crab size

Transcription

1 Africn Journl of Mrine Science 213, 35(): xxx xxx Printed in South Afric All rights reserved Copyright NISC (Pty) Ltd AFRICAN JOURNAL OF MARINE SCIENCE ISSN X EISSN Cnnilistic interctions of juvenile mud crs Scyll serrt: the effect of shelter nd cr size DO Mirer 1,2 * nd P-O Moksnes 3 1 Keny Mrine nd Fisheries Reserch Institute (KMFRI), PO Box 1651, Moms, Keny 2 Linneus University, School of Nturl Sciences, Box 391 2, Klmr, Sweden 3 University of Gothenurg, Deprtment of Biology nd Environmentl Sciences, Box 61, 53 Göteorg, Sweden * Corresponding uthor, e-mil: dimirer@yhoo.com, dmirer@kmfri.co.ke In the culture of mud cr Scyll serrt, cnnilism is often the gretest cuse of mortlity. A lortory study ws conducted to compre the influence of size clss differences nd shelter on cnnilism nd lim loss in juvenile mud crs (2 7 mm internl crpce width; ICW). Four size clsses of juvenile cr (A: 21 3 mm, B: 31 mm, C: 1 5 mm nd D: 51 7 mm ICW) were tested in ll possile comintions using four different sustrt with vrying degree of shelter (seweed, plstic strings, moo tues nd open snd sustrtum) in h trils. Results suggest tht cnnilistic interctions re hevily influenced oth y size differences of crs nd the vilility of shelter. Cnnilism on the smllest size clss (2 3 mm ICW) incresed out 1 times in the presence of the lrgest cr (51 7 mm) compred with tretment with only sme-size crs (control tretment). Shelter provided little refuge for the smllest crs, wheres cnnilism in lrger size clsses decresed y >5% in ll the shelters compred with the snd sustrtum. The findings suggest tht oth size-grding nd provision of shelter could minimise cnnilism in the culture of mud crs. Keywords: cnnilism Introduction The culture of mud crs Scyll serrt is commercilly developed in South-Est Asi (Keenn 1999, Agyni 21) nd hs een focus over the pst decde in the Western Indin Ocen (WIO) region (Mwlum 22, Rönnäck et l. 22, Mirer 29, 211). Aquculture expnsion in South-Est Asi hs een documented to hve cused decline in wild seed supply tht hs impcted oth culture nd cpture fisheries (Keenn 1999). Reserch on the development of seed production hs chieved significnt progress in South-Est Asi in recent yers (Quinitio et l. 21, Alln nd Fielder 23, Quinitio et l. 27, Quinitio nd Estep 211). However, no effort hs een mde in the WIO region to produce seed in htcheries ecuse frming is minly smll scle. Therefore, the ssumption is tht there re sufficient numers of seed crs in the wild to meet the growing industry. However, this ssumption my not hold for long ecuse of overexploittion of the resource y rtisnl fishers using such trditionl cpture methods s hook sticks, pots, trps nd seine-nets, s in South-Est Asi (Motoh 193, Angell 1992, Kosuge 21). Currently, rtisnl fishers in the WIO cn meet the demnd from tourist hotels, privte homes nd export mrkets, which is growing rpidly (Brnes et l. 22, ACDI-VOCA 25, Richmond et l. 26, Mirer 29, 211). However, eing unregulted, the fishery is vulnerle nd there re indictions of significnt decrese in wild popultions (Frncis nd Bryceson 21). Such decline will hve direct negtive impcts on recruitment nd seed vilility nd the ehviour of the industry in the region, s oserved in other prts of the world (Tkeuchi 2, Hmski et l. 22). Becuse of limited knowledge on the vilility of juvenile mud crs from the wild nd their ehviour in culture conditions, most culture inititives in the WIO grow smll sudults to mrket size over short periods. In South-Est Asi, len crs which hve reched mrket size re cultured to gin weight in process referred to s fttening. In the WIO, cr culture is smll scle nd is minly crried out in individul cges to minimise cnnilism nd ggression (fighting). However, survivl is low nd growth slow in such culture systems (Mirer 211). Recent inititives hve therefore een tken to develop smll-scle, grow-out systems in ponds nd pens, where smll juvenile crs re frmed for longer to ttin mrket size, similr to the system used in South-Est Asi (Mwlum 22, Mirer 29, Mirer nd Mtile 29). Glolly, the fctors ffecting survivl in cr culture re cnnilism, moulting, slinity nd temperture fluctutions, feed, shelter nd stocking density (Trino et l. 1999, Ruscoe et l. 2, Holme et l. 27, Mnn et l. 27, Rodriguez et l. 27, Mirer 29, Quinitio nd Estep 211). Some of these mortlity-ssocited fctors hve een ddressed in South-Est Asi nd survivl is currently etween 5% nd 7% per growth cycle in vrious culture systems (pens, cges nd ponds). Africn Journl of Mrine Science is co-pulished y NISC (Pty) Ltd nd Tylor & Frncis

2 2 Mirer nd Moksnes However, ggression nd cnnilism, which re rnked s the min cuses of low survivl in semi-intensive nd intensive culture systems, still remin chllenge (Quinitio et l. 21, Willims nd Primver 21, Alln nd Fielder 23, Mnn nd Pterson 23, Pterson et l. 27). Reserch in South-Est Asi hs used htcheryproduced seed crs tht re of similr size t stocking, which reduces cnnilism (Prkes et l. 211). However, there is little informtion on how the interctions etween different size clsses of crs in culture systems influence cnnilism. Informtion on different size-clss interctions is essentil to improve culture success in the WIO, where seed supply is from the wild nd there re mny size clsses eing stocked (Mirer 29, Mirer nd Mtile 29). Cnnilism of juvenile portunid crs is t its gretest when the cnnilising crs re lrge enough to kill other crs in intermoult with hrd exoskeletons (Moksnes et l. 1997, 199, Mrshll et l. 25). To void high rtes of cnnilism in cr quculture, therefore, it is criticl to ensure tht the sizes of crs re kept similr in pens to preclude this type of cnnilism. The im of the present study ws to investigte how size-clss differences ffect cnnilism rtes of juvenile mud crs nd how cnnilism is influenced y the vilility of shelter. The informtion will hopefully guide frmers on possile size-clss comintions to stock nd on suitle shelters to use t the frms. Mteril nd methods A series of lortory experiments ws crried out t the Keny Mrine nd Fisheries Reserch Institute (KMFRI), Moms, Keny, to ssess the interctive effect of size difference nd shelter on cnnilism in juvenile mud crs. As the primry im ws to ssess cnnilism on intermoult crs, we used short experimentl trils ( h) to minimise moulting, nd high densities of crs (75 crs m 2 ) to enhnce cnnilistic interctions etween the crs. Experimentl set-up Locl fishers were hired to collect juvenile crs from the nery mngrove intertidl flt during low spring tides. Cr collection took pproximtely 3 h nd trnsporttion to the lortory nother hour. Sorting ws done in the field, hrd-shelled helthy crs (ctive nd with intct ppendges) eing pcked in plstic uckets with clen, wet snd, covered with mngrove leves nd trnsported to the lortory. At the lortory the juveniles were sorted ccording to size clss (see elow), plced in 5-litre quri with snd t density of pproximtely 3 crs m 2 nd cclimtised for t lest 2 h efore use in the experiment. Crs were fed srdine during cclimtistion. Although communl cclimtistion my introduce superior/ inferior ehviourl trits in crs (Bergmn nd Moore 25, Prkes et l. 211, Bettie et l. 212), the effect should e sme in ll tretments ecuse the cclimtistion ws stndrd for ll size clsses. Crs were mesured (internl crpce width, ICW, nd crpce length, CL) t the strt of the experiment, to enle identifiction of individul crs t the end of trils tht included the effects of moulting. Only helthy crs (ctive nd feeding) with intct ppendges were used in the experiments. The tests were crried out in smll, circulr, sttic tnks (ottom re.3 m 2 ; 1 litres) with 2 mm of sieved (1 μm) ech snd, filled with nturl surfce sewter from Tudor Creek. Tnks were covered with lck polythene sheeting to minimise disturnce. The tnks were provided with mild ertion throughout the experimentl period. Wter ws mintined t room temperture (27 2 C) nd t n verge slinity of 32 ( SD 1.2). The tnks were rrnged in fully rndomised design on verticl metl rcks in two prllel rows on one side of the room. The trils were conducted for h (dy nd night) nd the experiment spred over period of two months. Tretments In ll experimentl tretments, four crs per tnk were ssessed (equivlent to ~75 crs m 2 ), either four crs of the sme size clss (control tretment) or one of lrger size clss (referred to s the predtor cr ) nd three smller crs of the sme size clss (referred to s prey crs ). We used severl prey crs in ech tnk to increse the chnces of detecting differences in the rtes of cnnilism etween tretments. Cnnilistic interctions etween four size clsses of juvenile mud crs (A: 21 3 mm, B: 31 mm, C: 1 5 mm, D: 51 7 mm ICW) were tested in ll possile comintions in four types of shelter tretments: seweed (Eucheum denticultum), plstic strings (undles of string loclly known s kkns ), moo tues, nd snd sustrtum without ny extr shelter (ll tretments hd snd on the ottom of the continer). The numer of size comintions tested differed etween different size clsses; i.e. the smllest size clss (A) ws ssessed in four size comintions, including the control tretment with sme size (AA) nd three tretments with one lrger predtor cr (AB, AC, AD), the second size clss ws ssessed in three size comintions (BB, BC, BD), the third in two (CC, CD) nd the lrgest size clss (D) only with crs of the sme size (DD, i.e. control tretment). In totl, different tretment comintions were ssessed with five replictes for ech tretment. A totl of crs ws used in the experiments. Sustrt were selected sed on their potentil to provide shelter from cnnilism nd on their vilility to cr frmers. Red lge (Eucheum denticultum) for the experiment were otined from community frms long the southern cost of Keny. Four rnches 2 3 mm long with totl weight of 25 g were used for the shelter. The shelter mde of plstic strings ws cut from scks into pieces mm long, with out 2 strings tied t the centre in undle. The ottom ends of the seweeds nd plstic strings were uried in the snd. Dried moo tues were cut into pieces 15 mm long nd with mm internl dimeter nd three pieces were plced horizontlly on the ottom of ech tnk. At the strt of ech tril, the predtor cr (if present) ws plced in experimentl tnks 3 min efore introducing prey crs. The lrger crs were used only once s predtors ut could e used s prey crs in their respective size clsses fter undergoing n dditionl cclimtistion for

3 Africn Journl of Mrine Science 213, 35(): xxx xxx 3 t lest 2 h. During the h trils, crs were monitored dily nd ny mortlity or moulting ws noted. At the end of ech experimentl run, ll shelters were crefully removed to void loss of the experimentl specimens. Wter nd snd were sieved through 5 μm sieve to retrieve crs, moult shells, roken shells nd cr remins. Morphometric mesurements (ICW, CL) were tken for whole moult shells nd live crs were checked for cnnilistic scrs (roken crpce), moulting nd loss of ppendges. Simultneously, the presence of ded nd uneten crs, live crs, moulted crs nd moult, shells s well s evidence of dmged crs, were noted. Sttisticl nlyses All dt were tested for homoscedsticity with Cochrn s C-test (Sokl nd Rohlf 191) nd, if found to e heteroscedstic, were squre-root trnsformed efore the ANOVA ws performed. All dt for the percentge of cnnilised crs were squre-root trnsformed to homogenise the vrince. A posteriori multiple comprisons were crried out with the Student Newmn Keuls (SNK) procedure. To test for the effects of shelter nd cr size differences on cnnilistic interctions, series of ANOVAs ws crried out. As the experimentl design ws not orthogonl, the effect of prey size nd predtor size ws tested in seprte sets of two-wy ANOVA. In ll nlyses, the percentge of cnnilised crs (the percentge of prey eten) nd the percentge lim loss of prey crs (the percentge of surviving crs missing one or more ppendges) were tested s dependent vriles. Lim loss of predtor crs ws negligile nd not included in the nlyses. To ssess the comined effect (shelter nd size), we lso crried out nlysis on the totl percentge of crs cnnilised or missing lim. The percentge of crs found ded nd uneten, nd the percentge of prey crs tht moulted, were lso nlysed s dependent vriles to estlish whether fctors other thn cnnilism ffected survivl, nd if moult rte ws eing influenced y experimentl tretments. The overll effects of predtor presence nd shelter were tested in two-fctor ANOVA model using predtor tretments (control, predtor size clsses B D) nd shelter tretments ( levels) s fixed independent vriles. Ech prey size clss (size clsses A C) ws lso tested in three seprte series of nlyses using the sme two-fctor ANOVA model. The overll effect of prey size (size clsses A D) ws tested in one-fctor ANOVA model using ll dt. To ssess cnnilistic interctions only within cr size clsses, prey size nd shelter were nlysed s independent vriles using only dt from the control tretments, in seprte series of nlyses. Results Experimentl conditions All crs used in the experiment were ccounted for t the end of ech tril. In five of the predtor tretment comintions (.1% of the tretments), the predtor ws ded t the end of the trils, so those dt were excluded from the nlyses ecuse the predtory spect ws lost in the tretment. In 7.5% of the predtor tretment comintions, the predtor cr hd moulted nd these dt were included in the nlyses ecuse there ws predtion in some of the trils. In totl, 3.% of the prey crs were found ded nd uneten t the end of the trils; their individul dt were not used in the nlysis. The reson for the mortlity ws uncler, ut it did not differ significntly (p >.5) etween tretments in ny nlyses (Tles 1 nd 2) nd should not hve ffected the cnnilism results. On verge,.3% of the prey crs hd moulted t the end of the tril. The percentge of prey crs tht moulted did not differ significntly (p >.5) etween predtor tretments. However, the percentge of prey crs tht moulted ws significntly higher in snd sustrtum thn in the other sustrt when nlysing dt from ll experimentl units (Tle 1, Figure 1). The sustrtum pttern in moult crs ws indicted for ech prey size clss, lthough the difference ws not significnt when nlysing the dt seprtely y size clss (Tle 2). A similr result ws found when nlysing the control dt seprtely, where the moult rte ws significntly higher in snd thn in the moo tretment (Tle 2, Figure 3, SNK-test p <.5). Cnnilistic interction Overll, cnnilistic interctions incresed with predtor size, decresed with prey size nd were fewer in the sustrt tht provided shelter. However, the effect of predtor presence nd shelter differed etween prey size clsses. In ll, 51 individul crs were cnnilised, giving n equivlent of 7.5% cnnilism in h for ll predtory tretment comintions. When testing the overll effect of predtor cr nd shelter tretments, the percentge of cnnilised crs ws significntly higher in the snd sustrtum (on verge 15% loss h 1 ) thn in ll sustrt comined (3 % loss h 1 ) in ll predtor tretments (Tle 1, Figure 1). Cnnilism ppered to increse with predtor size, from n verge of 5% in control tretments to 13% in the presence of predtor size clss D, ut this trend ws not significnt (Tle 1). There ws lso trend of decresed cnnilism with incresed prey size, from 11% t prey size A to 5% t prey size C, ut no significnt effect of prey size ws found (one-fctor ANOVA; df 3,191; F 1.6; p.22). The percentge of surviving crs missing one or more lims followed pttern similr to tht of cnnilism. However, predtor tretments hd greter effect thn shelter tretments on lim loss. The percentge lim loss incresed significntly from n verge of 3% in control tretments to 12% in the presence of the lrger predtor size clsses (Tle 1, Figure 1), lthough it ws not significntly ffected y shelter. Totl negtive interctions (i.e. the sum of prey crs cnnilised or missing lim) showed significnt effect of oth shelter nd predtory tretment. Snd sustrtum hd significntly greter degree of negtive interction thn the three shelters used. Also predtor size clss D hd significntly greter percentge of lim loss thn the control tretments (Tle 1; SNK-test t p <.5). Anlyses of ech prey size seprtely demonstrted tht cnnilism on the smllest prey crs ws more

4 Mirer nd Moksnes Tle 1: Two-fctor ANOVA model of squre-root trnsformed dt (for ll prey sizes) testing the percentge of cnnilised prey crs h 1, the percentge of live prey crs missing one or more lims, the totl numer of negtive interctions (i.e. the sum of prey crs cnnilised or missing lim), the percentge of moulting prey crs, nd the percentge of prey crs found ded nd uneten (mortlity), s function of predtor nd sustrtum tretment Totl negtive Cnnilism Lim loss Source of vrition df interctions Moult Mortlity SS F SS F SS F SS F SS F Predtor (ns).3 3.3* *..13 (ns) (ns) Shelter *.9 1. (ns) * **.2.7 (ns) Predtor Shelter (ns).1.67 (ns) (ns).7.62 (ns).9.92 (ns) * p <.5; ** p <.1; (ns) p >.5 Tle 2: Two-fctor ANOVA models of squre-root trnsformed dt testing the percentge of cnnilised prey crs h 1, the percentge of live prey crs missing one or more lims, the percentge of moulted prey crs, nd the percentge of prey crs found ded nd uneten (mortlity), s function predtor tretment (control, predtor size B, C nd D) nd hitt tretment (snd lone, seweed, plstic strings nd moo tues), for prey size clss A (21 3 mm), B (31 mm) nd C (1 5 mm ICW), nd for control tretments without predtor crs Source of vrition Cnnilism Lim loss Moult Mortlity df SS F SS F SS F SS F Prey size A Predtor ** (ns) (ns) (ns) Shelter (ns).6.56 (ns) (ns) (ns) Predtor Shelter (ns) (ns).5.36 (ns).6.76 (ns) Prey size B Predtor (ns) (ns).. (ns) (ns) Shelter (ns).1.9 (ns) (ns).2.5 (ns) Predtor Shelter (ns).1.35 (ns).5.62 (ns) (ns) Prey size C Predtor (ns) (ns).1. (ns 1 ) (ns) Shelter **.6.76 (ns) (ns 1 ) (ns) Predtor Shelter (ns) (ns) (ns 1 ) (ns) Control Prey size 3.3. (ns).2.91 (ns)..11 (ns)..22 (ns) Shelter **.1.1 (ns) *.1.3 (ns) Predtor Shelter (ns) (ns)..66 (ns) (ns) ** p <.1, (ns) p >.5; 1 Dt heteroscedstic despite trnsformtion influenced y the size of lrger predtory crs, wheres cnnilism on the lrger prey crs ws more ffected y the shelter ville (Figure 2). Cnnilism on size clss A ws hevily ffected y predtory tretment nd incresed significntly from 2% in control tretments to 27% in the presence of predtor size clss D (Tle 2, Figure 2). In contrst, cnnilism on size clss A ws not significntly influenced y the type of shelter, lthough there ws trend of more cnnilism in snd thn where moo shelter ws ville. The percentge of size clss A missing lim ppered to increse from 1% in control tretments to 1% in the presence of predtor size clss D, ut no sttisticl difference could e detected (Tle 2). Cnnilism of size clss B ws less ffected y the presence of lrger predtory crs (cnnilism nd lim loss vried etween % nd 9% nd % nd 11% respectively). There ws trend of greter cnnilism in the snd tretment (13%) reltive to moos, where there ws no cnnilism (Figure 2). However, no significnt effects were found on either cnnilism or lim loss (Tle 2). Cnnilism of size clss C ws significntly greter in the snd tretment (2%) thn in the shelter tretment ( 2%), wheres no significnt effect of predtor tretment ws found (Tle 2, Figure 2). In contrst, lim loss ppered to increse from 3% in the control tretment to 12% in the predtor tretments, difference tht ws orderline significnt (p.56), wheres no cler effect of shelter ws oserved (Tle 2). Anlyses of dt from control tretments (sme size clss of crs) showed significntly more cnnilism in the snd sustrtum (on verge 11%) thn on other sustrt ( 3%). Although no significnt effect of prey size ws found (Tle 2), trend of less cnnilism in the smllest size clss (on verge 1.%) thn in the lrger size clss (5 %) ws oserved. This difference ws most pprent in snd where cnnilism in the control tretments ws zero for size clss A, ut 2% for the lrger size clsses (Tle 2, Figure 3).

5 Africn Journl of Mrine Science 213, 35(): xxx xxx 5 PERCENTAGE MOULT LOSS CANNIBALISM PERCENTAGE CANNIBALISM Prey size A Prey size B Prey size C 2 5 Control B C D PREDATOR Snd Seweed Plstic HABITAT Bmoo Control B C D PREDATOR Snd Seweed Plstic HABITAT Bmoo Figure 1: Men percentge cnnilism, lim loss nd moult of prey crs h 1 s function of predtor tretment (control, predtor size B 31 mm, predtor size C 1 5 mm, predtor size D 51 7 mm ICW) nd shelter type (no shelter, seweed, plstic strings nd moo tues) pooled cross ll prey tretments (snd ws the sustrtum in ll hitt tretments). Letters ove rs indicte significntly different mens t p <.5 for the SNK-test. Error rs denote SE Figure 2: Men percentge cnnilism of prey crs h 1 s function of predtor tretment (control, predtor size B 31 mm, predtor size C 1 5 mm, predtor size D 51 7 mm ICW) nd shelter type (no shelter, seweed, plstic strings nd moo tues) for three prey size clsses (A 21 2 mm, B 31 mm, C 1 5 mm ICW). Letters ove rs indicte significntly different mens t p <.5 for the SNK-test. Error rs denote SE Discussion In grow-out quculture of portunid crs, cnnilism often constitutes the iggest cuse of mortlity, resulting in loss of up to 9% (Rodriguez et l. 21, Alln nd Fielder 23, Mirer 29, Shelley nd Lovtelli 211). Two pproches to decrese the rtes of cnnilism in culture hve recently een suggested: (1) to size-grde the crs (e.g. nursery culture methods with regulr sorting to seprte smll crs from lrger ones nd hence decrese predtion on smller crs [Mrshll et l. 25, Rodriguez et l. 25, Prkes et l. 211]); (2) to provide structurlly complex hitts tht provide shelter from cnnilism for smller prey crs nd moulting crs (Hutchinson 1999, Mrshll et l. 25, Mnn et l. 27, Ut et l. 27, Prkes et l. 211, Bettie et l. 212). Results from the present study suggest tht cnnilistic interctions etween juvenile mud crs re strongly influenced y oth the difference in size etween the crs nd the vilility of shelter. However, the response of prey crs to experimentl tretments ws very size-specific. Cnnilism on the smllest size clss (2 3 mm ICW) incresed 1-fold in the presence of the

6 6 Mirer nd Moksnes PERCENTAGE CANNIBALISM MOULT A B C D SIZE CLASS lrgest predtory crs (51 7 mm) reltive to the control tretments. Shelter hd less influence on controlling cnnilism in tht size clss. In contrst, cnnilism on lrger prey crs (31 5 mm CW) ws less influenced y the presence of lrger predtors ut decresed y >5% in the presence of shelters rther thn simple snd sustrtum. The results suggest tht size-grding nd shelter could oth e importnt in minimising cnnilism in grow-out quculture ut tht size-grding is prticulrly importnt for smller crs, wheres shelter is more importnt for the lrger size clsses. Effects of cr size In rchyurn crs, the reltive difference in size etween predtor nd prey crs is importnt in influencing the rtes of cnnilism. When crs re of similr size, cnnilism on intermoult crs is miniml, lthough it is greter in newly moulted, soft, defenceless crs. However, ove criticl size difference, cnnilism cn ffect hrd intermoult prey crs, usully resulting in rtes of cnnilism tht re much greter (Kurihr et l. 19, Moksnes et l. 1997, Fernández 1999, Moksnes 2). To ddress this sitution, size-grding is prctised in nursery culture of mud crs to increse the survivl rtes of juveniles (Cerezo 21, Rodriguez et l. 25, Mnn et l. 27). Therefore, culture Snd Seweed Plstic HABITAT Bmoo Figure 3: Men percentge cnnilism nd men percentge moult within ech prey size clss h 1 s function of cr size tretment (A 21 3 mm, B 31 mm, C 1 5 mm, D 51 7 mm ICW) nd shelter type (no shelter, seweed, plstic strings nd moo tues). Letters ove rs indicte significntly different mens t p <.5 for the SNK-test. Error rs denote SE of mud crs needs to im to seprte crs of different sizes prior to stocking s well s during culture, to preclude cnnilism on hrd intermoult crs. In the present study, the effect of reltive size differences on cnnilism ws demonstrted in the smllest prey size clss (2 3 mm). In control tretments, where the reltive size difference etween prey nd predtor ws <5%, cnnilism ws low (<2% h 1 ), ut it incresed with incresing size difference to 27% mortlity h 1 in the presence of the lrgest predtors, in which there ws men size difference of ~1%. High rtes of cnnilism on smller crs y lrger ones ( 5 mm ICW) suggested tht hrd shelled (intermoult) crs of the smllest size clss (A 2 3 mm ICW) were preyed upon t men size difference of % etween prey nd predtor. The findings suggest tht the criticl size difference etween prey nd predtor is etween 5% nd % for tht prey size, nd tht juvenile mud crs 2 3 mm ICW hve to e seprted from crs > mm ICW to decrese cnnilism. This is consistent with the findings of Rodriguez et l. (27) on the preference of frmers in South-Est Asi for cr >3 mm ICW to ensure fster growth nd reduced mortlity. In lrger prey sizes (31 5 mm ICW), the rtes of cnnilism did not increse in the presence of lrger predtors, suggesting tht cnnilism ws minly of newly moulted crs. The low rte of cnnilism on hrd, intermoult crs could in prt e explined y the smller men size difference etween prey nd predtor crs in these tretments (2 7%) compred with tht in the smllest size clss, ut lso tht greter size difference is required to llow cnnilism on hrd crs y the lrger prey crs. Although the presence of lrger predtory crs did not increse the rtes of cnnilism for the 31 5 mm ICW prey size clss, the percentge lim loss did increse y n verge of four times in tretments with the lrgest predtor size reltive to the control tretments. The findings therefore support the elief tht gonistic interctions my e mnifest in lim loss where size differences re smll. This implies tht size-grding crs etween 31 nd 7 mm CW my hve only smll effects on cnnilism nd survivl, ut it could decrese the numer of incidents of lim loss tht impct cr qulity. Effects of hitt In nturl popultions of rchyurn crs, cnnilism etween juvenile size clsses is thought to e mjor cuse of mortlity for smll juvenile stges tht often ggregte in structurlly complex shelters such s segrss nd mussel eds, where predtion rtes y lrger predtors re minimised (Fernández et l. 1993, Moksnes et l. 199, Moksnes nd Heck 26). Providing shelter for juvenile mud crs t quculture frms my therefore hve strongly positive effects on their survivl. This suggestion hs received support in the present study ecuse cnnilism, oth within nd etween size clsses, ws less for the three shelter tretments (i.e. seweeds, plstic strings nd moo tues) thn for the snd sustrtum without shelter. Although cnnilism did not differ sttisticlly etween shelter type, trend of less cnnilism ws oserved where moo tues were used s shelter. This

7 Africn Journl of Mrine Science 213, 35(): xxx xxx 7 ws possily ecuse moo provides greter protection on the seed thn plstic strings or seweed lge, which provide more shelter within the wter column. The tues my lso provide the lrger prey sizes of crs with more effective refuge from cnnilism from predtor crs, s indicted y the lck of cnnilism where moo shelters were used for prey size clsses B nd C (Figure 2). The positive effects of pproprite shelter re consistent with the findings of Ut et l. (27), who used clm shells nd ricks s shelter to increse the survivl of juvenile Scyll prmmosin reltive to crs on simple snd sustrtum. However, in the present study, the effect of shelter differed etween prey sizes. Cnnilism ws on verge eight times greter on the snd sustrtum thn where ny of the three shelter types were used in the control experiments. This difference ws strongly influenced y the lrger size clsses of crs, which suffered high rtes of cnnilism where there ws no shelter (on verge 2%), wheres there ws zero cnnilism for the smllest size clss. The size-specific shelter effect ws oserved lso in the predtor tretments, cnnilism eing greter with no shelter thn where shelter ws provided, lthough this ws significnt only in the lrgest prey clss (D). Cnnilism on the smllest prey size (A) ws similr in ll shelter tretments. This result is not consistent with other studies on portunid crs, however, where the survivl of young juvenile stges ws mny times higher in structurlly complex hitts reltive to snd or mud hitts (Pile et l. 1996, Moksnes et l. 1997, 199). The present study used smll experimentl continers nd high densities of crs (75 crs m 2 ) to enhnce gonistic interctions nd hence to determine mesurle cnnilism rtes within short period. Although this pproch provides comprle reltive rtes of cnnilism, s well s estimtes y size of solute refuge from cnnilism on intermoult crs, it my decrese prt of the refuge vlue of shelters nd provide greter interctions for igger crs ecuse of the limited spce. This my e ecuse prey nd moult crs were lwys within detectle distnce of lrger predtors in the smll experimentl continers, so the concelment function of the shelter tht is provided in igger environment with lower cr densities might hve een lost. The results from the present study my therefore reflect minly the solute hitt refuge of the shelter mteril used, where the prey cr is reltively inccessile to the predtors. This my explin why the moo-tue shelter ppered on verge to provide etter refuge thn the others, s discussed ove. The lck of shelter refuge for the smllest size clsses should e interpreted with cution, however, ecuse the shelters my provide greter refuge from cnnilism in nture nd t lower cr densities in quculture systems. However, despite this potentil rtefct tht my hve decresed the differences in the efficcy of the shelters provide, cnnilism did decrese nd survivl incresed y n verge of 6% in the three shelter types reltive to the tests with no shelter. This effect of shelter is consistent with previous studies on juvenile portunid crs crried out in quculture systems in which n increse in survivl of ~5% is common when shelters re provided (Mnn nd Pterson 23, Mrshll et l. 25, Mnn et l. 27, Ut et l. 27). The densities of the crs used in the experiments were higher thn the stocking densities normlly used in grow-out pond culture of mud crs using wild crs etween nd mm ICW (Mnn et l. 27, Rodriguez et l. 27), ut similr to the densities used in nursery cultures where mud cr meglope re grown to ~25 mm ICW (Rodriguez et l. 21). Although our resulting solute rtes should not e compred with the mortlity rtes t frms with lower stocking densities, they do provide reltive rtes of cnnilism to e compred etween shelter types, nd lso provide estimtes of solute refuge y size from cnnilism. The moulting rte of juvenile crs ws lso ffected y the presence of shelter, eing greter in the no-shelter tests thn where shelter ws provided in oth control nd predtory tretments. This result ws unexpected nd not consistent with other studies of portunid crs tht show greter rtes of moult in hitts tht provide shelter from predtors reltive to suoptiml sustrt such s open snd, where the moult is delyed (Moksnes et l. 1997, 23). However, the moult pttern oserved ws likely not result of cnnilism removing moulting individuls ecuse tht should hve produced the opposite pttern (see Figure 1). Implictions for the culture of mud crs Cnnilism constitutes mjor source of mortlity in mud cr culture ut is poorly understood glolly (Alln nd Fielder 23, Mnn nd Pterson 23, Mnn et l. 27, Ut et l. 27, Mirer 29, Shelley nd Lovtelli 211). Results from the present study suggest tht oth size-grding nd the provision of suitle shelter cn decrese the rtes of cnnilism in grow-out culture of mud crs. To void high rtes of cnnilism on hrd intermoult crs, this study suggests tht the reltive size difference etween crs rered together should e smll, i.e. ll nimls should e within 2% of ech other y length. This is prticulrly importnt if smll (<3 mm ICW) juvenile seed crs re used. In South-Est Asi, juvenile seed crs > mm ICW re preferred y frmers ecuse they need less time to rech commercil size reltive to smller seed crs. However, ecuse the nturl mortlity of juvenile portunid crs is mny times higher in smll newly settled crs thn in lrger ones (Pile et l. 1996, Moksnes et l. 199) nd is often density-dependent (Moksnes et l. 1997, Moksnes 2), the impct on the fished popultion will e minimised nd quculture more sustinle if the smllest juvenile crs ville re used. To void exceeding the criticl size difference t which cnnilism strts, it is importnt to stock ech culture with crs of similr size. However, ecuse of the gret differences in the growth rtes of individul crs (Mnn et l. 27) nd the hevy dependence on wild supply, continuous size-grding of crs would likely still e required during their culture. According to the results presented here, size-grding would lso decrese lim loss, which hs negtive effects on growth, s well s the qulity of the crs nd their mrket vlue when one or oth chelipeds

8 Mirer nd Moksnes re lost (Junes nd Smith 1995, Pterson et l. 27, Mirer nd Mtile 29). Continuous loss of chelipeds even from juvenile crs leds to disproportionl cheliped to ody size rtio tht will impct the mrket vlue in terms of wet weight. Rtes of cnnilism could e reduced further y providing shelters tht reduce the encounter rtes of crs in ddition to providing shelter to smll or moulting crs. The results here suggest tht shelter is prticulrly importnt in reducing cnnilism in the more ggressive lrger size clsses of mud crs. In this study nd in others (e.g. Ut et l. 27), severl types of shelter reduced cnnilism notly, suggesting tht frmers could use severl loclly ville sustrt s shelter for the crs, including seweeds (Grcilriopsis) nd moo (Trino et l. 1999). In the present study, the commercil red lg Eucheum denticultum, which is esily otined from locl seweed frms, reduced cnnilism y ~5% reltive to tht which occurred where there ws no shelter. However, there is need first to estlish whether tht species cn flourish in the rckish wter ponds used for mud cr quculture, in similr mnner to other seweed species such s Grcilriopsis spp. nd Culerp spp. (Trino et l. 1999, Putr et l. 213). If so, the lg itself might produce extr income for the frmers of mud crs. Acknowledgements This reserch ws funded y the Western Indin Ocen Mrine Science Assocition (WIOMSA), Mrine Science for Mngement (MASMA), through project entitled Smll-scle mriculture of mud crs (Scyll serrt) in Est Afric. We thnk the Keny Mrine nd Fisheries Reserch Institute (KMFRI) where the lortory work ws done nd Linneus University, Sweden, for the PhD scholrship nd provision of lirry services in the writing-up process of the mnuscript. We lso thnk the internship students t KMFRI, Venny Mwinge nd Emily Wful, nd lortory technicins Justus Wkili nd Mwendw Mruk, for their support, s well s the rtisnl cr fishers who provided the juvenile crs for the experiment (Adhllh Mtile nd Dickson Mwmure). References ACDI/VOCA. 25. Su-sector nd vlue chin nlysis for mud crs for Tng costl elt. ACDI/VOCA project report. Agyni RF. 21. Production economics nd mrketing of mud crs in the Philippines. Asin Fisheries Science 1: Alln G, Fielder D. 23. Mud cr quculture in Austrli nd Southest Asi. Proceedings of the ACIAR cr quculture scoping study nd workshop 2 29 April 23, Joondourri Conference Centre, Briie Islnd, Queenslnd, Austrli. ACIAR Working Pper No. 5. Cnerr: Austrlin Centre for Interntionl Agriculturl Reserch. Angell CA Summry of the proceedings of the seminr on mud cr culture nd trde. In: Angell CA (ed.), Report of the seminr on the mud cr culture nd trde held t Swt Thni, Thilnd, Novemer 5,1991. Mdrs: By of Bengl Progrmme. pp 1 2. Brnes DKA, Dulvy NK, Priestly HS, Drwll TRW, Choisel V, Whittington M. 22. Fishery chrcteristics nd undnce estimtes of the mngrove cr Scyll serrt in southern Tnzni nd northern Mozmique. South Africn Journl of Mrine Science 2: Bettie CL, Pitt KA, Connolly RM Both size nd gender of mud crs influence the outcomes of interference interctions. Journl of Experimentl Mrine Biology nd Ecology 3 35: 1 6. Bergmn DA, Moore PA. 25. The role of chemicl signls in the socil ehvior of cryfish. Chemicl Senses 3 (Suppl. 1): i35 i36. Cerezo RB. 21. Effect of different types of shelter on the production of mud cr Scyll serrt juveniles in concrete nursery tnks. MSc thesis, University of the Philippines Visys, Philippines. Fernández M Cnnilism in Dungeness cr Cncer mgister: effects of predtor prey size rtion, density, nd hitt type. Mrine Ecology Progress Series 12: Fernández M, Irirne O, Armstrong D Hitt selection y young-of-the-yer Dungeness cr, Cncer mgister, nd predtion risk in intertidl hitts. Mrine Ecology Progress Series 92: Frncis J, Bryceson I. 21. Tnznin costl nd mrine resources: some exmples illustrting questions of sustinle use. In: Ahmed J, Bergstrøm C, Bryceson I, Child B, Frncis J, Khn P, Ousmne BG, Price TL, Senrtn S, Treen N, vn Dm C (eds), Lessons lerned cse studies in sustinle use. Glnd: IUCN. pp Hmski K, Supryudi AM, Tkeuchi T. 22. Effect of dietry n-3hufa on lrvl morphogenesis nd metmorphosis to meglops in the seed production of mud cr, Scyll serrt (Brchyur: Portunide). Suisn Zoshoku 5: Holme HM, Southgte CP, Zeng C. 27. Survivl, development nd growth response of mud cr, Scyll serrt, meglope fed semi-purified diets contining vrious fish oil: corn oil rtios. Aquculture 269: Hutchinson K Shelter relted ehviour of the mud cr (Scyll serrt). Honours thesis, University of Queenslnd, Austrli. Junes F, Smith DL The ecologicl consequences of lim dmge nd loss in decpod crustcens: review nd prospectus. Journl of Experimentl Mrine Biology nd Ecology 193: Keenn CP Aquculture of the mud cr, genus Scyll, pst, present nd future. In: Keenn CP, Blckshw A (eds), Mud cr quculture nd iology, proceedings of n interntionl scientific forum, 21 2 April 1997, Drwin, Austrli. ACIAR Proceedings No. 7. Cnerr: Austrlin Centre for Interntionl Agriculturl Reserch. pp Kosuge T. 21. Brief ssessment of stock of mud crs Scyll spp. in Mtng mngrove forest, Mlysi nd proposl for resources mngement. Journl of Jpn Agriculture Reserch Qurterly 35: Kurihr Y, Sekimoto K, Miyt M. 19. Wndering ehvior of the mud cr Helice tridens relted to evsion of cnnilism. Mrine Ecology Progress Series 9: 1 5. Mnn DL, Askw T, Kelly B, Lindsy T, Pterson B. 27. Stocking density nd rtificil hitt influence stock structure nd yield from intensive nursery systems for mud crs Scyll serrt (Forsskl 1775). Aquculture Reserch 3: Mnn DL, Askw T, Pizzutto M Development of htchery system for lrve of the mud cr Scyll serrt t the Briie Islnd Aquculture Reserch Centre. In: Keenn CP, Blckshw A (eds), Mud Cr Aquculture nd Biology. ACIAR [Austrlin Centre for Interntionl Agriculturl Reserch] Proceedings 7. Cnerr, Austrli. pp Mnn D, Pterson B. 23. Sttus of cr seed production nd grow out in Queenslnd. In: Alln G, Fielder D (eds), Mud cr quculture in Austrli nd Southest Asi. Proceedings of the ACIAR cr quculture scoping study nd workshop 2 29 April 23, Joondourri Conference Centre, Briie Islnd, Queenslnd, Austrli. ACIAR Working Pper No. 5. Cnerr: Austrlin Centre for Interntionl Agriculturl Reserch. pp Mrshll S, Wrurton K, Pterson B, Mnn D. 25. Cnnilism

9 Africn Journl of Mrine Science 213, 35(): xxx xxx 9 in juvenile lue-swimmer crs Portunus pelgicus (Linneus, 1766): effects of ody size, moult stge nd refuge vilility. Applied Animl Behviour Science 9: Mirer OD. 29. Mud cr (Scyll serrt) culture: understnding the technology in silvofisheries perspective. Western Indin Ocen Journl of Mrine Science : Mirer OD Trends in exploittion, development nd mngement of rtisnl mud cr (Scyll serrt-forsskl-1775) fishery nd smll-scle culture in Keny: n overview. Ocen nd Costl Mngement 5: 55. Mirer OD, Mtile A. 29. A preliminry study on the response of mngrove mud cr (Scyll serrt) to different feed types under drive-in cge culture system. Journl of Ecology nd Nturl Environment 1: 7 1. Moksnes P-O. 2. Self-regulting mechnisms in cnnilistic popultions of juvenile shore crs Crcinus mens. Ecology 5: Moksnes P-O, Heck LK. 26. The reltive importnce of hitt selection nd predtion for the distriution of lue cr meglope nd young juveniles. Mrine Ecology Progress Series 3: Moksnes P-O, Hedvll O, Reinvld T. 23. Swimming nd settlement ehvior in shore cr meglope: why do postlrve emigrte from nursery hitts? Mrine Ecology Progress Series 25: Moksnes P-O, Lipcius R, Pihl L, vn Montfrns J Cnnil prey dynmics in young juveniles nd postlrve of the lue cr. Journl of Experimentl Mrine Biology nd Ecology 215: Moksnes P-O, Pihl L, vn Montfrns J Predtion on postlrve nd juveniles of the shore cr Crcinus mens: importnce of shelter, size nd cnnilism. Mrine Ecology Progress Series 166: Motoh H Trditionl devices for cpturing crs used in the Philippines tody. Reserches on Crustcens 12: Mwlum J. 22. Pen culture of the mud cr Scyll serrt in Mtwp mngrove system, Keny. Western Indin Ocen Journl of Mrine Science 1: Prkes L, Quinitio ET, Le Vy L Phenotypic differences etween htchery-rered nd wild mud crs, Scyll serrt, nd the effects of conditioning. Aquculture Interntionl 19: Pterson B, Mnn D, Kelly B, Brchiesi M. 27. Lim-loss in pond-rered lue swimmer crs Portunus pelgicus (L.): effect of growth in n indoor shedding system. Aquculture Reserch 3: Pile AJ, Lipcius NR, vn Montfrns J, Orth JR Density dependent settler: recruit: juvenile reltionships in lue crs: mechnisms nd effects of tropicl storm. Ecologicl Monogrphs 66: Putr NSSU, Lpong I, Rimmer MA, Rhrjo S Culerp culture in South Sulwesi n lterntive for rckishwter pond culture. Aquculture Asi Pcific 9: 5. Quinitio ET, de Pedro J, Prdo-Estep DF. 27. Ovrin mturtion stges of the mud cr Scyll serrt. Aquculture Reserch 3: Quinitio ET, Estep, FDP Survivl nd growth of mud cr, Scyll serrt, juveniles sujected to removl or trimming of chelipeds. Aquculture 31: Quinitio ET, Prdo-Estep DF, Millmen OM, Rodriguez ME. 21. Seed production of mud cr Scyll serrt. Asin Fisheries Science 1: Richmond MD, Mohmed A, de Villiers KA, Esseen M, LeVy L. 26. Smllholder fisheries enterprises trils, Rufiji District, Tnzni. Finl Report. Rufiji Environment Mngement Project (REMP), IUCN Estern Afric Regionl Office, Niroi, Keny. Rodriguez EM, Prdo-Estep DF, Quinitio ET. 27. Extension of nursery culture of Scyll serrt (Forsskl) juveniles in net cges nd ponds. Aquculture Reserch 3: Rodriguez EM, Prdo-Estep DF, Quinitio ET, Millmen MO, Mingw M, de Pedro J. 25. Rering of mud cr Scyll serrt meglop in rckishwter ponds. Terminl Report 25, Tigun, Iloilo: SEAFDECD/AQD. Rodriguez EM, Quinitio ET, Prdo-Estep DF, Millmen MO. 21. Culture of Scyll serrt meglops in rckishwter ponds. Asin Fisheries Science 1: Rönnäck P, Bryceson I, Kutsky N. 22. Costl quculture development in estern Afric nd the western Indin Ocen: prospects nd prolems for food security nd locl economies. AMBIO 31: Ruscoe IM, Shelley CC, Willims RG. 2. The comined effects of temperture nd slinity on growth nd survivl of juvenile mud crs (Scyll serrt Forsskl). Aquculture 23: Shelley C, Lovtelli A Mud cr quculture. A prcticl mnul. FAO Fisheries nd Aquculture Technicl Pper 567. Rome: Food nd Agriculture Orgniztion. Sokl RR, Rohlf FJ Biometry. New York: WH Freemn. Tkeuchi T. 2. A review of studies on the effect of dietry n-3 highly unsturted ftty cids on lrvl swimming cr Portunus trituercultus nd mud cr Scyll trnqueric. Proceedings of the JSPS-DGHE Interntionl Symposium on Fisheries Science in Tropicl Are, August , Bogor, Indonesi. pp Trino AT, Millmen MO, Keenn C Commercil evlution of monosex pond culture of the mud cr Scyll species in the Philippines t three stocking densities. Aquculture 17: Ut NV, Le Vy L, Nghi TT, Hnh TTH. 27. Development of nursery culture techniques for the mud cr Scyll prmmosin (Estmpdor). Aquculture Reserch 3: Willims MJ, Primver HJ. 21. Choosing tropicl portunid species for culture, domestiction nd stock enhncement in the Indo-Pcific. Asin Fisheries Science 1: Mnuscript received Ferury 213, revised August 213, ccepted August 213