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Use of the Lincoln Index Mark-Recapture Method to Estimatethe Population Densities forHarvestable Yabbies, Cheraxdestructor, in Grow-Out PondsPeter Farrell a & Brian Leon Ard aa De part ment of Ap plied Bi ol ogy and Bio technology , RMIT Uni ver sity , GPO Box 2476V, Melbourne, Vic to ria, Aus tra lia , 3001Published online: 14 Aug 2009.

To cite this article: Peter Farrell & Brian Leon Ard (2001) Use of the Lincoln IndexMark-Recapture Method to Estimate the Population Densities for Harvestable Yabbies,Cherax destructor, in Grow-Out Ponds, Journal of Applied Aquaculture, 11:3, 65-74,DOI: 10.1300/J028v11n03_06

To link to this article: http://dx.doi.org/10.1300/J028v11n03_06

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Use of the Lin coln In dexMark-Re cap ture Method

to Es ti mate the Pop u la tion Den sitiesfor Harvestable Yab bies, Cherax de struc tor,

in Grow-Out Ponds

Pe ter FarrellBrian Leon ard

ABSTRACT. The Lin coln In dex mark-re cap ture method was eval u ated for its ac cu racy in es ti mat ing the num ber of harvestable yab bies, Cheraxde struc tor, in earthen ponds. Where a sin gle re cap ture event was used toes ti mate the pop u la tion, a 3 to 6 day in ter val be tween the re lease and re -cap ture of marked an i mals tended to pro vide a more re li able es ti matethan a re lease and re cap ture in ter val of 24 hours. Where two re cap tureevents were used to pro vide an av er age es ti mate, the method tended toun der es ti mate by 20% low den sity pop u la tions, that had been the sub jectof pre da tion by wa ter birds, whereas those es ti mates de ter mined forponds with higher den si ties of cray fish, where avian piscivores were ex -cluded, tended to be more ac cu rate ( ±10%). The ap pli ca tion of pop u la tiones ti ma tion as a tool for ar ti fi cial feed-man age ment is dis cussed. [Ar ti clecop ies avail able for a fee from The Haworth Doc u ment De liv ery Ser vice:1-800-342-9678. E-mail ad dress: <getinfo@haworthpressinc.com> Website:<http://www.HaworthPress.com> 2001 by The Haworth Press, Inc. Allrights re served.]

KEYWORDS. Cray fish, yab bies, Cherax de struc tor, pop u la tion, den -sity, trap ping, feed ing

Pe ter Farrell and Brian Leon ard, De part ment of Ap plied Bi ol ogy and Bio tech nol -ogy, RMIT Uni ver sity, GPO Box 2476V, Mel bourne, Vic to ria, Aus tra lia, 3001.

Pe ter Farrell’s pres ent ad dress is 35 Brock Street, Euroa, Vic to ria, Aus tra lia, 3666.

Jour nal of Ap plied Aquaculture, Vol. 11(3) 2001 2001 by The Haworth Press, Inc. All rights re served. 65

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INTRODUCTION

Farmers of fresh wa ter cray fish in semi-in ten sive sys tems need to knowstock den si ties in or der to op ti mize the use of ar ti fi cial feeds (Farrell and Leon -ard 1999a). How ever, for the yabby, Cherax de struc tor, pop u la tion den si tiescan vary sub stan tially from ini tial stock ing den si ties with in-pond breed ingand re cruit ment and losses due to pre da tion and can ni bal ism. The semi-in ten -sive farmer needs a quick and sim ple method to pe ri od i cally de ter mine cur rentstock den si ties.

An i mal ecol o gists have long been con cerned with es ti mat ing the num ber ofwild an i mals in a pop u la tion and have de vel oped a num ber of tech niques to doso. Mark-re cap ture is one such tech nique, and the ba sic method from whichmod i fi ca tions are de rived is known as the Lin coln In dex (LI) (Southwood1966). The LI is based on the pro por tion of in di vid u als marked in an ini tialsam pling pe riod be ing pres ent as re cap tures in a sec ond sam pling pe riod. Pop -u la tion num bers (P) are es ti mated as fol lows (Fowler and Co hen 1994):

P = M 3 N 4 R

where: M = the num ber of an i mals cap tured, marked, and re leased from a trap -ping event; N = the num ber of an i mals cap tured from the sub se quent trap pingevent; and R = the num ber of marked an i mals re cap tured in the cor re spond ingtrap ping event. It is pos si ble to cal cu late the stan dard er ror for the es ti mate byus ing the fol low ing equa tion (Fowler and Co hen 1994):

SEN M(M R)

R

2

3= × −

There are a num ber of as sump tions that have to be met for the pop u la tiones ti mate to be con sid ered ac cu rate (Southwood 1966): (1) all an i mals musthave an equal chance of be ing cap tured or re cap tured; (2) the marked an i malsmust not be ad versely af fected by the mark (or rather the like li hood of re cap turemust not be af fected ei ther through trap shy ness, ad dic tion or death); (3) themarks must not be lost be fore the re cap ture pe riod; (4) marked an i mals mustmix ran domly with the un marked pop u la tion be fore re-sam pling or, al ter na -tively, sam pling it self must be ran dom; (5) im mi gra tions to and em i gra tionfrom the sam pling area (or from a cap tive pop u la tion) can be ac counted for;and (6) no births or deaths can oc cur in the pe riod be tween sam pling events.

De pending on the type of farm ing op er a tion, some grow ers may not be ableto sat isfy all of these as sump tions at all times of the year, and in any event,some re search ers doubt whether all cray fish in a pop u la tion are ever equallysus cep ti ble to cap ture, given that some an i mals may be trap shy or trap prone

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(Morrissy 1973, 1975; Brown and Brewis 1978; Tay lor 1988). Morrissy(1975) stated that the o ret i cally, if sam pling sites are se lected on a ran dom ba -sis, then all an i mals should have an equal chance of be ing re cap tured, but thenar gued that this as sump tion de pended on all an i mals be ing ho mo ge neouslydis trib uted. Fac tors af fect ing het er o ge neous dis tri bu tion in clude both bi oticand abiotic cir cum stances, such as sub strate and avail abil ity of ref uges; sex;size; molt stage (ecdysis); wa ter flow, depth and qual ity; the avail abil ity of nat -u ral feed; and the de tec tion of pred a tors (Hazlett et al. 1974; Appelberg et al.1993; Lodge and Hill 1994). Black (1963) ob served that only 15% of Pro -cambarus spp. were re cap tured away from the point of orig i nal cap ture andcon cluded that stream dwell ing cray fish oc cu pied a home range from whichthey rarely strayed, and both Black (1963) and Hazlett et al. (1974) have cal cu -lated the home range of stream-based Amer i can cray fish as be ing about 33 m.If the dis per sion of marked cray fish is not high, then re-sam pling must be ran -domly al lo cated, and large num bers of sam ples may be ex pected to gen er atethe cor rect marked: un marked ra tio for pop u la tion es ti ma tion. Romaire andPfister (1983) found that a trap ping reg i men of 100 baited traps/ha workedbetter for har vest ing cray fish than 75, 50 or 25 traps/ha, and Morrissy (1975)ob served that com pet i tive trap ping (sev eral traps at the one site) may al le vi atesome of these prob lems, be cause trap shy an i mals may be more likely to becap tured af ter dom i nant an i mals are re moved. In an ear lier study, Morrissy(1973) ac knowl edged that some an i mals may never be vul ner a ble to cap tureand that this may even be a ge netic or learned re sponse. How ever, the er rors inpop u la tion es ti ma tion may not be se vere as long as steps are taken to min i mizethe vi o la tions of the as sump tions (Southwood 1966).

In a pre lim i nary study (Farrell and Leon ard 1999b), car ried out in six0.25-ha ponds that had been ear lier har vested by farm work ers, it was es tab -lished that the LI mark-re cap ture method had the po ten tial to re li ably es ti matecap tive yabby pop u la tions at the very low num bers en coun tered (<50 cray -fish/0.25-ha pond). How ever, field test ing of the method at com mer cial den si -ties was deemed nec es sary be fore the use of the LI could be rec om mendedwith any con fi dence.

The aim of this ex per i ment was to es tab lish a mark-re cap ture pro to colbased upon the LI that might re li ably es ti mate the num ber of yab bies in anearthen pond. One way to im prove the ac cu racy of the es ti mate is to in creasethe num ber of re cap ture events, al low ing an av er age es ti mate to be de ter -mined, and this was also eval u ated in the pres ent study.

MATERIALS AND METHODS

The ex per i ment was con ducted us ing four earthen ponds (0.02-ha each).Ini tially, the four ponds were drained, dried, filled with wa ter, fer til ized,

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limed, and stocked with 1,000 hatch lings from a hatch ery, pro vid ing an ini tialstock ing den sity of 5 cray fish/m2. For the next 70-73 days, these an i mals weregrown semi-in ten sively by pro vid ing over night aer a tion and daily feed ingwith pel lets 6 days a week from day 25. The pel lets were de liv ered on a dietweight per unit area ba sis that was ini tially set at 0.3 g/m2, 1.0 g/m2 at day 39,then 2.0 g/m2 at day 53.

Each of the four ponds had a dif fer ent form of pro tec tion in fra struc ture toas sess stock losses from pred a tory birds, these be ing full-ex clu sion net ting(Pond 4), scare-line mar keted as a bird de ter rent (Pond 3), and the in dus trystan dard of us ing a sub merged reef formed from sec ond-hand tires (Pond 2).Pond 1 was left un pro tected as a con trol. Both above-pond struc tures, that isscare-line and ex clu sion net ting, were erected close to the wa ter sur face(0.5 m). The net ting over Pond 4 was an ex truded plas tic with a 100-mm barlength, and ca ble and/or wire was used to se cure the outer edge to the pondbank. The mono fil a ment scare-line over Pond 3 was se cured at each end of thepond with 150-mm spac ing be tween strands, and its cov er age in cluded thepond bank where it was ap prox i mately 300 mm high. Each of 220 sec ond-hand tires for Pond 2 were cut 10 times with an an gle grinder along their side-wallsto al low drain age dur ing har vest, and were placed in the pond to form two sub -merged reefs, leav ing the cen ter of the pond clear to as sist with har vest.

For the pop u la tion es ti mate, ar ti fi cial diet was with held on the day of sam -pling, and four traps for each pond, equiv a lent to 200 traps/ha, were baitedwith a com mer cial dog bis cuit and emp tied af ter an over night set of 12 hours.The four traps were set from one bank, and were evenly dis trib uted down thelength of the pond. The traps em ployed in this trial were a col laps ible type with ac cess af forded via a sin gle en trance at the top. The bait was held in a tube,which was also used to hold the en trance open. The sides of the trap were im -pen e tra ble to all cray fish size-classes, while the base of the trap was con -structed from 10-mm gal va nized steel mesh, which would re li ably re tain thoseyab bies weigh ing at least 8 g. Smaller an i mals tended to fall through the meshdur ing re trieval of the trap to the pond bank, and these an i mals were judged tobe of unharvestable size and are not a part of the pop u la tion es ti mates in thisstudy. As the traps were re trieved, the cap tured an i mals were marked by cut -ting a straight edge along a uropod with small scis sors and im me di ately re -turned to the pond. Pre lim i nary test ing of the method in aquaria showed it to be quick, sim ple, and re li able for the worker, and com pletely safe for the cray fish(molt ing was un af fected). The num ber of marked an i mals was re corded foreach pond. Twenty-four hours later and again on the first day of har vest ing and fi nal re moval of an i mals in or der to con duct a pop u la tion cen sus (3, 4, 5 and 6days later for Ponds 4, 3, 2, and 1, re spec tively), the traps were re bait ed and re -trieved af ter an over night set. On re triev ing the traps dur ing each ep i sode, acount was made of the to tal cap tures and the num ber of marked (re cap tured)

68 JOURNAL OF APPLIED AQUACULTURE

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an i mals. These counts were then used in the two equa tions de scribed to ob taina pop u la tion es ti mate and stan dard er ror for each pond. An av er age pop u la tiones ti mate for the num ber of harvestable an i mals in each pond was de ter minedby av er ag ing the es ti mates ob tained from each sep a rate re cap ture event.

Af ter es ti mat ing the pop u la tions us ing the LI, ac tual counts of the re main -ing cray fish were made as they were re trieved from each pond, first by trap -ping then by drain har vest ing. Each pond was trapped in ten sively for four daysbe fore the ponds were emp tied of wa ter. The baited traps were pulled twice ina 24 hour pe riod and set at a rate of 200 traps/ha for 2 days, and 250 and 400traps/ha for 1 day each. Drain har vest ing was car ried out us ing a 100-mmgas-driven pump to empty the ponds. A 3-mm mesh screen was placed over the pump in let pipe to pre vent losses of harvestable stock. Once the ponds wereempty, the re main ing cray fish were re trieved by hand, and counts of all theharvestable an i mals ( $ 8 g) were made for com par i son with the cal cu lated es -ti mates.

RESULTS AND DISCUSSION

For three out of the four ponds, the es ti mate based upon a re cap ture countmade within 24 hours of the re lease of marked an i mals was an un der es ti mateof the ac tual pop u la tion by be tween 30 and 60% (Ta ble 1). The fi nal pond (4)was an ex cep tion, with the first es ti mate pro vid ing an over es ti ma tion of the ac -tual harvestable pop u la tion by 25%. In the pres ent study, the trap den sityadopted was 200 traps/ha, which con trasted with an ear lier study (Farrell andLeon ard 1999b) that also used a re cap ture in ter val of 24 hours but where thetrap ping den sity was just 32 traps/ha. The use of a higher trap ping den sity onlyap peared to mar gin ally im prove pop u la tion es ti mates, but the data-sets arequite small, and fur ther tri als are re quired to re solve this is sue. In the pres entstudy, pop u la tion es ti mates based on the sec ond re cap ture ef fort ap peared topro vide a more ac cu rate es ti mate of the ac tual harvestable pop u la tion. This ispos si bly a re sult of the re leased an i mals hav ing a lon ger time (3-6 days) to mixinto the gen eral pop u la tion, there fore sat is fy ing the as sump tion of ran dommix ing of marked an i mals into the un marked pop u la tion (Southwood 1966).

Using the cal cu lated es ti mates based on the sec ond re cap ture event, in Pond 1, the pond with the low est ac tual pop u la tion, a 21% over es ti ma tion of the ac -tual harvestable pop u la tion was cal cu lated, con trast ing with Pond 4, the pondwith the sec ond high est ac tual pop u la tion, which was un der es ti mated by ap -prox i mately the same mag ni tude (Ta ble 1). Pond 3 had a slightly higherharvestable pop u la tion than Pond 4, and the ac tual harvestable pop u la tion ofPond 3 was ap prox i mately 2% less than the cal cu lated es ti mate (Ta ble 1). Theac tual pop u la tion of Pond 2, al though low at 285 harvestable in di vid u als, was

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nearly dou ble that of Pond 1 and ap prox i mately one-third of Ponds 3 and 4.The LI cal cu la tion on the sec ond re cap ture event for Pond 2 un der es ti matedthe harvestable pop u la tion by around 9% (Ta ble 1).

Un der the con di tions of the pres ent study, there does not seem to be a re la -tion ship be tween the ac tual pop u la tion den sity and the ac cu racy of the es ti -mates based on a sin gle re cap ture event when us ing an ex tended pe riod of timebe tween mark and re cap ture, nor do the dif fer ent in ter vals of time be tween thefirst and sub se quent re cap ture ep i sode, 3, 4, 5, and 6 days, for Ponds 4, 3, 2 and1, re spec tively, show any dis cern ible pat tern.

Av er aging the two pop u la tion es ti mates for a pond pro vided an ac cept ablees ti mate (within 10%) of the harvestable pop u la tion for ponds where pop u la -tions were rel a tively high (Ponds 3 and 4, Ta ble 1). On the other hand, av er ag -ing pro duced 20% un der es ti mates of the harvestable pop u la tions where den sitywas low (Ponds 1 and 2). There fore, it is pos si ble that these low pop u la tionden si ties led to a sys tem atic un der es ti mate of the ac tual pop u la tion size. Al ter -na tively, pre da tion pres sure may have led to a pro por tion of the pop u la tion be -

70 JOURNAL OF APPLIED AQUACULTURE

TABLE 1. Pop u la tion es ti mates of cap tive harvestable-size ( $8 g) yabby pop u -la tions from four ponds pro vided with bird-in hib it ing in fra struc tures, based onthe LI (Lin coln In dex) method, com pared to ac tual counts of the har vest -able-size pop u la tions fol low ing trap ping and drain har vest ing. A sec ond pop u -la tion es ti mate was made on the first day of each pop u la tion cen sus.

CountsPonds Catch #1 Catch #2 Re cap tured Es ti mate Ac tual(infrastr.) (M) Catch #3 (R) LI±SE Pop:n Diff. (%)

(N)

1a 11 4 0 60±57 146 258.9(none) 16 1 176±167 +20.5

mean 118±112 219.2

2 23 17 2 196±132 285 231.2(tires) 34 3 260±140 28.8

mean 228±136 220.0

3 177 198 65 539±53 776 230.5(scare-line) 216 44 868±113 +1.6

mean 704±83 29.3

4 54 86 5 929±403 745 +24.7(net ted) 66 6 594±372 220.3

mean 762±388 +2.3aBecause no re cap tures were made, +1 was added to M, N, and R to cal cu late the LI.

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ing some what cir cum spect in their for ag ing ac tiv ity. Pond 1 (no pro tec tivein fra struc ture) and Pond 2 (sub merged tire reefs) were sub ject to high pre da -tion by pred a tory birds. Re duced for ag ing be hav ior as a re sult of high pre da -tion could lead to the sort of trap-shy phe nom ena de scribed by Morrissy(1975), with the re sult that a pro por tion of the pop u la tion is not sub ject to sam -pling and, hence, not in cluded in the LI es ti mate. Brown and Brewis (1978),when they com pared net ting with trap ping as a method of mark-re cap ture,found that trap ping con sis tently pro duced a three-fold un der es ti ma tion of theac tual pop u la tion of the na tive Brit ish cray fish, Austropoamobius pallipes, residing in an aq ue duct, pre sum ably due to trap shy ness. How ever, the trap- based method of pop u la tion es ti ma tion adopted in this study pro duced rea son -ably re li able es ti mates ( ±10%) when the two pop u la tion es ti mates were av er -aged and pred a tory birds were ex cluded from the ponds (Ta ble 1).

The un der es ti ma tion of the harvestable pop u la tion in ponds sub ject to pred -a tory birds was also re flected in the rel a tive suc cess of baited traps used to har -vest each pond at the end of the study, al though it might be ar gued that this issim ply a re flec tion of the rel a tively low den si ties in these ponds. Af ter drainhar vest ing, it was de ter mined that the four days of prior in ten sive trap ping hadre trieved an av er age of 50% of the harvestable pop u la tion from the four ponds. This is com pa ra ble to the find ing of Smallridge et al. (1990) who re trieved50-70% of the harvestable pop u la tion from four com mer cial ponds us ingbaited traps. How ever, the pres ent study ap pears to in di cate that the pro tectedand un pro tected ponds per formed dif fer ently from each other with re gard tobaited trap har vest ing. Where the stock had been sub ject to the at ten tion ofpred a tory birds, an av er age of 38.1±2.7% of the pop u la tion was trapped fromthe con trol pond (Pond 1) and the pond con tain ing sub merged tires (Pond 2).This pro por tion was con sid er ably less than the 62.1±6.7% of the pop u la tionre trieved from those ponds pro tected from pred a tory birds ei ther by scare-line(Pond 3) or ex clu sion net ting (Pond 4). These lat ter pop u la tions were not un der pre da tion stress and might have been more ac tive as a re sult, and/or their en -ergy re quire ments might not have been met by the sched uled feed-rate, due tothe high pop u la tion den si ties, lead ing to a higher pre pared ness to en ter trapsfor the bait. How ever, none of the an i mals in the pres ent study ap peared to beun der fed (see be low).

The pop u la tion es ti mate can be ex tended to cal cu late stand ing bio mass byweigh ing a sam ple of an i mals cap tured and mul ti ply ing the pop u la tion es ti -mate by the mean weight of sam pled an i mals (Farrell and Leon ard 1999b).From this point it is rel a tively sim ple to pro vide ar ti fi cial feed at an ap pro pri ate rate, cal cu lated as a per cent of bio mass (Farrell and Leon ard 1999a), ratherthan feed ing to a pre set sched ule as was done for the pres ent study. At the timeof the pop u la tion es ti mates in this study, the av er age weight of the harvestablecray fish was 12 g, which means that the av er age bio mass es ti mates for Ponds

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1, 2, 3, and 4 were 1.4, 2.7, 8.5, and 9.1 kg, re spec tively. Based on these fig -ures, the sched ule-based feed ing rate of 2.0 g/m2 (0.4 kg/pond) that was in useim me di ately prior to har vest pro vided feed at 28.6, 14.8, 4.7, and 4.4% of thees ti mated harvestable bio mass for Ponds 1, 2, 3, and 4, re spec tively. A dailyfeed-rate of more than 4% of the stand ing bio mass should have ad e quately met the en ergy re quire ments of all cray fish in Ponds 3 and 4, while Ponds 1 and 2were clearly over fed (Farrell and Leon ard 1999a). To con tinue feed ing out atthe same rate would have pro duced very poor FCRs (Feed Con ver sion Ra tios)for Ponds 1 and 2, which might have had ad verse af fects on wa ter qual ity andthe con di tion of the pond bot tom in those ponds. While it is pos si ble that over -feed ing of Ponds 1 and 2 could have led to low trappability when us ing baitedtraps, lead ing to un der es ti mates of pop u la tion size us ing the LI, there is no ev i -dence of the re verse, that is, that low feed ing led to a higher trappability inPonds 3 and 4. This sug gests that pro cesses other than food avail abil ity are re -spon si ble for any dif fer ences in trappability in this study.

This study made use of hatch ery-raised cray fish pro duced some two months in ad vance of the nor mal in-pond re cruit ment. It was ex pected that dif fer encesin pop u la tion sizes be tween ponds would be due to the rel a tive ef fec tive ness of the in fra struc ture pro vided to in hibit the ac tiv i ties of pred a tory birds. Prior tothe study, ev ery ef fort was made to re move the pre vi ous oc cu pants of theponds, but this met with mixed suc cess, since some in-pond re cruit ment oc -curred, as was ev i denced by the pres ence of un der sized ju ve niles and the hand -ful of ma ture adults found in each pond by the end of the study. In Pond 1, there were four adults and 95 un der sized re cruits re trieved af ter the drain har vest, inPond 2, nine teen adults and 285 un der sized re cruits, in Pond 3, sev en teenadults and 520 un der sized re cruits, and in Pond 4, six adults and 165 un der -sized re cruits. The num ber of un der sized re cruits may be a func tion of the ra tio of ma ture males and fe males, but this can not be con firmed, since no ef fort wasmade to de ter mine the sex of the adults. The un der sized ju ve niles were <2 g,and so do not ap pear as part of the harvestable pop u la tions re corded in Ta ble 1, al though the ma ture adults are in cluded. A smaller gauge mesh on the trapswould have made it pos si ble to re tain these re cently re cruited ju ve niles, butthen the method of mark ing might have be come an is sue. A pro por tion of theharvestable pop u la tions of the same size as the hatch ery stock might haveemerged from bur rows when the ponds were flooded or might have mi gratedinto the ponds from else where. If so, it is ex pected that this pro por tion wassmall, and in any case, has no rel e vance to the LI es ti mates.

Nei ther tag ging, or brand ing was con sid ered an ap pro pri ate method ofmark ing cray fish, since a quick method is re quired that might be adopted byfarm ers, and marks need to be un ob tru sive if the an i mal is to be mar ketedshortly af ter mark ing. Cut ting a straight edge on a uropod was found to be verysat is fac tory. One prob lem that may be en coun tered is that small, rap idly grow -

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ing ju ve niles, like the newly re cruited ju ve niles in the pres ent study, may loseex ter nal marks quickly af ter re peated molt ing. How ever, at 12 g, if re cap ture is done within 6 days as for the pres ent study, this did not ap pear to be a sig nif i -cant is sue. A sec ond prob lem may oc cur if a sec ond mark-re cap ture es ti mate is made at a later pe riod of time. Large an i mals may hold a mark for a long time,and marked an i mals from the first es ti mate could be mis taken for re cap turesdur ing the sec ond es ti mate, when in fact they were not in the marked set dur ing the sec ond de ter mi na tion. The so lu tion to this would be to en sure that se quen tialde ter mi na tions use dif fer ent sorts of marks (e.g., left uropod, right uropod, etc.).

The LI used in the pres ent study showed prom ise as an ap pro pri ate methodof re li ably es ti mat ing the num ber of harvestable yab bies in a com mer cialpond, and re quires only the use of equip ment and fa cil i ties that one would ex -pect to find at any yabby farm. It is ex pected that bio mass could be es ti matedsoon af ter the com mence ment of first feed ing with an ar ti fi cial diet, at a timewhen the molt ing fre quency has slowed. It is likely that those farm ing op er a -tions where bio mass var ies in a rather un pre dict able way will get the most ben -e fit from the mark-re cap ture tech nique. In the pres ent study, it is pos si ble thatpred a tory pres sure in hib ited the trappability of yab bies. Farmers con tem plat -ing us ing LI to es ti mate cray fish pop u la tions are ad vised to con duct their ownfield test, as a num ber of other fac tors can in flu ence the ba sic as sump tions re -quired (Southwood 1966) and hence the re li abil ity of the method. Fac tors af -fect ing cray fish trappability in clude trap type, trap ping du ra tion, and bait type(Bean and Huner 1978; Westman et al. 1978; Smallridge and Gray 1989;Taugbøl et al. 1997), time of year, tem per a ture, and the num ber of an i mals(Abrahamsson 1981).

ACKNOWLEDGMENTS

The authors would like to thank Chris Sleigh, the Man aging Di rec tor of theSleigh Group, and Dale McAliece and Eddie Green wood, the cur rent and pastMan agers of Shepparton Na tive Fish Hatch eries, Ki alla Vic to ria, Aus tra lia fortheir sup port of this work.

REFERENCES

Abrahamsson, S. 1981. Trappability, lo co mo tion and diel pat terns of ac tiv ity of thecray fish Astacus astacus and Pacifastacus leniusculus Dana. Fresh wa ter Cray fish5:239-253.

Appelberg, M., B. Söderberg, and T. Odelstrom. 1993. Pred a tor de tec tion and per cep -tion of pre da tion risk in the cray fish Astacus astacus L. Nordic Jour nal of Fresh wa -ter Re search 68:55-62.

Farrell and Leonard 73

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Bean, R.A., and J.V. Huner. 1978. An eval u a tion of se lected craw fish traps and trap -ping meth ods. Fresh wa ter Cray fish 4:141-152.

Black, J.B. 1963. Ob ser va tions on the home range of stream-dwell ing craw fishes.Ecol ogy 44:592-595.

Brown, D.J., and J.M. Brewis. 1978. A crit i cal look at trap ping as a method of sam -pling a pop u la tion of Austropotamobius pallipes (Lereboullet) in a mark and re cap -ture study. Fresh wa ter Cray fish 4:159-163.

Farrell, P., and B. Leon ard. 1999a. Es ti mating daily feed-rate for the yabby ( Cheraxde struc tor )–Part 2. Austasia Aquaculture 13(3):55-56.

Farrell, P., and B. Leon ard. 1999b. Cal cu lating an i mal bio mass in fresh wa ter cray fishponds, us ing the yabby ( Cherax de struc tor ). Austasia Aquaculture 13(2):47-48.

Fowler, J., and L. Co hen. 1994. Prac ti cal Sta tis tics for Field Bi ol ogy. John Wiley &Sons, Bris bane, Aus tra lia.

Hazlett, B., D. Rittschof, and D. Rubenstein. 1974. Be hav ioural bi ol ogy of the cray fishOrconectes virilis I. Home range. The Amer i can Mid land Nat u ral ist 92:301-309.

Lodge, D.M., and A.M. Hill. 1994. Fac tors gov ern ing spe cies com po si tion, pop u la tionsize, and pro duc tiv ity of cool-wa ter cray fishes. Nordic Jour nal of Fresh wa ter Re -search 69:111-136.

Morrissy, N.M. 1973. Nor mal (guassian) re sponse of ju ve nile mar ron, Cherax ten -uimanus (Smith) (Decapoda: Parastacidae) to cap ture by baited sam pling units.Aus tra lian Jour nal of Ma rine and Fresh wa ter Re search 24:183-195.

Morrissy, N.M. 1975. The in flu ence of sam pling in ten sity on the “catchability” of mar -ron, Cherax tenuimanus (Smith) (Decapoda: Parastacidae). Aus tra lian Jour nal ofMa rine and Fresh wa ter Re search 26:47-73.

Romaire, R.P., and V.A. Pfister. 1983. Ef fects of trap den sity and diel har vest ing fre -quency on the catch of craw fish. North Amer i can Jour nal of Fish eries Man age ment3:419-424.

Smallridge, M., and L. Gray. 1989. Trap com par i sons in com mer cial yabbie [sic]ponds. SAFish, July-Sep tem ber:18-21.

Smallridge, M., L. Gray, M. Penberthy, and M. Geddes. 1990. The Gerard YabbieFarm. SAFish, April-June:13-14.

Southwood, T.R.E. 1966. Eco log i cal Methods. Methuen and Co., Lon don, Eng land.Tay lor, R.C. 1988. Pop u la tion dy nam ics of the cray fish Procambarus spiculifer ob -

served in dif fer ent-sized streams in re sponse to two droughts. Jour nal of Crus ta cean Bi ol ogy 8:401-409.

Taugbøl, T., J. Skurdal, A. Burba, C. Munoz, and M. Sáez-Royuella. 1997. A test ofcray fish pred a tory and nonpredatory fish spe cies as bait in cray fish traps. Fish eriesMan age ment and Ecol ogy 4:127-134.

Westman, K., M. Pursiainen, and R. Vilkman. 1978. A new fold ing trap model pre -vents cray fish from es cap ing. Fresh wa ter Cray fish 4:236-242.

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