Variability in larval settlement of abalone on artificial collectors

12 Pages·1997·5.9 MB·

by  K R Rodda

Moll. Res. 18: 253-264 (1997).
Variability in larval settlement of abalone on artificial collectors
K.R.Rodda,! J.K. Keesing(cid:8217) and B.L.Foureur !
' Lincoln Marine Science Centre, South Australian Research and Development Institute,
PO Box 1511, Port Lincoln, South Australia, Australia, 5606.
2 South Australian Aquatic Science Centre,
South Australian Research and Development Institute,
PO Box 120, Henley Beach, South Australia, Australia, 5022.
Abstract
The spatial and temporal variability in settlement of greenlip abalone, H. /aevigata on
artificial settlement collectors was examined over three years. Pairs of the collectors of an
iSpmaptriaolv esdc aldees.s igAn pweerrieo d deopfl opyeeadk soent ttlheem enste a obcecdu rraetd sibxe tswiteeesn ovOecrt oba ern umabnedr Deocf edmibfeferr enitn
each of the three years at all sites. The similarity in size frequencies between sites at the
settlement peaks demonstrated that spawning was epidemic, occurring over a period of 2(cid:8212)
os3t phdaeawryn si.cn ognT thresa esatsseyodnn cwhriratonhn goetudhs a tf troiofmm i an 0gs. i0t aen md8(cid:8221) kitnmot en1ds.i2is ttyam n?t.o. f OApvdeeualrkt s thdaietsn sfriotauinreg se s ibtoeefst wdweeinetsnih tiinse ist,2e s kndmuum rboiefnr gs e atchohef
lGsayrrCnovocawllhtl rheso centtryotalr teser sos fa iwrneesf rpeeara rw eniudins ndeffegurp loe mna dntedo(cid:8220)no clto ihnoio rnft m sele(cid:8221)oax csaaulfm roiilannldiuognlw gte dta hibena uttn erdrtea-wlnaaoctn ienvwu.eae le ka bvauirnnitdaeabrinvlcailets y aoilfnl oawtbheaed l otnsieemt itnllgea mrevanante./d
Spawning times to be more accurately estimated than with previous methods such as
gonad indices.
: sealed
K *ywords: H. laevigata, larval settlement, larval collectors, spawning, epidemic spawning.
Introduction
mAQMlwT a.tt aii1fnn9iec 9 i5a)gl.r ocuoEplsslt,ei cmtia:o ntrcisl oundh ia: nvogef tlbhoeebe sntr eerl saut(ciBcvoee ostsafhbu lua nnddian n Tcmaeer arsio: nu1fg.r i1ln9a1g8r9 v78a)6et , h aeno Pndhi inlmatblabeo inaptslshido t ny1eat 9 o8 ot(6fee)K sm,etpl eaaosberrlivcaianhlslgi h n saoeettnd htdeeat rl l.mes sspmc1 aea9tnl9i(tea5H l,a orifnNrs coaalmlsladhaer n vayeeilt ts
ol in order to begin to understand metapopulation structures in the area.
fesing et al. (1995) used a modified abalone larval collector base
liotis laevigata Donovan, H. rubra Leach, H.
cC Yo eMlmoebractiesa; llyP erfoinsh eda.n d ThiLse susetuurd,y sHe.t oruote i to Gdreatye rmani n:d e Ht.h e stcealmaproirsa |L eaancdh .s paOtinally vatrhiea bilfiirtsyt itnw ol aravrael
254 K.R. Rodda, J.K. Keesing, B.L. Foureur
greenlip abalone (H. /aevigata) settlement at six sites. We compared the larval abundance at sites of
high and low adult abundance on Taylor Island and McLaren Point. Temporal variation was
measured by comparing larval settlement at several sites over a three year period. By deploying
collectors in the field from August to February some years, we were also able to determine blacklip
(H. rubra) settlement.
Materials and Methods
Collectors
At the original site used by Keesing et al. 1995 (McLaren Point, South Australia), we compared
the larval collecting ability of three artificial collector designs, including the original laserlight plate
design (type 1(cid:8212) see Keesing et al. 1995). Collector type 2 was constructed of 13 layers of a black
polycarbonate material which had a complex surface area (3 m?) and a volume of 0.3m} (Fig.1). The
surface of neighbouring sheets formed channels which provided a variety of surface orientations and
an enhanced trapping capability. It also provided a variety of light regimes to incorporate any larval
preference. Type 3 was constructed of 10 layers of fluorescent diffuser material held separate with a
central rod which directed the collector into the current.
Study sites
Once comparisons were made between collectors, the most efficient collector design was chosen
and used to establish spatial variation in larval settlement at five sites in Thorny Passage, near Port
Lincoln. In addition, the relationship between adult abundance and the number of larval abalone
Figure 1. Collector design 2 used in this study.
Variability in abalone larval settlement
22
acne was compared by choosing sites of varying adult abundance. Four of the sites (two high and
Sa a adult abalone abundance) were located 2 km apart on Taylor Island (136°1'00", 34°54'00").
Clark itional site (Passage site) with no known adult population of H. laevigata within 500m (P.
ia son pers. comm.) was set 2 km west of Taylor Island. The remaining site (McLaren Point (cid:8212)
1'00", 34°47'90") with a high adult abundance, was 8 km north of Taylor Island (Fig.2).
= each site, six collectors were placed one metre above the seabed. The collectors were fixed in
i to a solid yoke attached to a cement-filled tyre. During the experimental period we changed the
et eh of the yoke from PVC piping to a flat mild steel. This was especially useful in areas of high
an movement where the PVC piping was prone to break.
io . distance between pairs of collectors varied from 2-6 m. The collectors were placed at a water
an of 5-6 m at four sites and at 11-12 m at the remaining two sites. Five of the sites were on the
ea -reef interface near an adult greenlip abalone population and the remaining site (Passage site)
S On a sandy bottom.
The collectors were conditioned naturally in the sea with diatoms and bacteria which created a
Australia
STUDY AREA VU
x Passage site
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, Taylor North high
Taylor Island * taylor North low
x Taylor South high
(cid:8220)Taylor South low
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Bure 2. Study site locations in Thorny Passage, near Port Lincoln, S.A.
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mee Se for settlement and growth of juvenile abalone (Nash et al. 1995, R.Grove- Jones
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d index determined by the following equation (after
Shepherd and Laws 1974):
weight of gonad
(cid:8216)Gonad |ind (cid:8364)Xt:=Sccceneneennann x 100
weight of gut + gonad
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Variability in abalone larval settlement aa?
Results
Collector type
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51.5 P=0.79, n=70), thus justifying the utility of the collectors as a quantitative tool.
Larval settlement
arteilmR.tmsae ih Peaa o kgemoengreaSloeyS e s se i eennt Stn t pi ) eo onuetod ntihma fsho ektrb a1b uhe iset9myespln,erw9o rta H tsas4eest.stws t/nstwh luh s e 1eieimtio ntl9mlrw tcthad9feeeyo o ee5n op cmvst tfeealipyat h a titgehletaat noeaa elbneeHrcratetpad le.dsrcaeell s u i o airniosonkrsn sri(rfve eu ttFee e ttebhaisrdscshter ge t. feo la. tul ovea rtdtotusmAeeylh w3neulrs(,ee) endbpet Am.at esd hceutc sce ngooBhohtpqteunuen ifiru st cgnrveistpi aheaotin)nneut t hm1t ug eaes 9.g ots- ke 9ahouet s yt3eDnthnsueww adet aatoma t lsrdh wsmiaelsec at eam e coe sivreo lsar oennl tlr8eldHfitleelp i. es c pllksc c tyoeiettswmonsfrcosuarhl ca yir dssaothonss yd eoew.dreci vsf n tosh(mi ( ttF nrgatFAfaliwooahirotngefegetunrk t.om. ar so reet te undht(36w isthtl))Mos(hea .y n cte O oa atLsTcstontithTahpt lt dthraeerehoae essocyle e bsiluitrlcn egf hegf ooi nTersaohlrriiatt su-loP rf tyhreuD(o irtSl cFtiecyehtoosin aeTercoiurgtn eaazre.t t)teresy mhh yl I(cid:8217) le 3ebyss(o oc)a wle Fhr.froaeyirdi sd mnerga)g.itdap.et Ifh h ro afse ,s e5lsT dssi)raiThsite.eenstetaeh ntdphesi tTyot o ,lo,hl rlwow npietseoiyhe mis dtnreteea heci d(nrkseslnFt eIs, pptui, s aeeydg dltnaea.iataais4aknnthranae)ragsedesld
.
aTo©n oPena sbtsuiatmg iean lgws iatoyef.s poecacku rsreetdt ldeumrenitn g otfh eH .l at/ea esvpirgiantga oart etahrel yT asyulmomre rI sl(aenadr lsyi tDese cveamrbieedr o1v9e9r 3,t hela tteh rOecet oybeearr
1 ae43 a annmddo d4mail7d 5 umDmeea cnSe Lm sbioezner toh1fe9 9p45o-)s.wt e-leakr vaceol :l weactso r 2r5e0trmie veSdL 2o nw eethkes 2l-awteere. k Thciosl lgeactvoer ar emteraine vegd roownt h1 2r aNteo vo.f
T able 1. Tcohlel ecrteolrast.i onLsahrivpa l bedetnwseietny wdaenss ittaiekse n (pferro mm ettheer pseqauka rsee)t tolfe mreenptr oodcuccutrirvien ga douvletr Hs, umlameevri.g ata and post-larvae on the
TaylorSouth Taylor South Taylor North Taylor North Passage McLaren
1E199E99E45 ppoossAAAttddd--uuulllllaatttrrssvsv a aee 60601.56..5..4110509 0403..(cid:8212)-.32 378 00701..0.6-44.2 .92507 030301.7..517.64..576203 310050-:... .0067 050305..6.4264.4..450577 ,
post-larvae 57.0 =
~ing;
Ndicates that no data was taken
260 K.R. Rodda, J.K. Keesing, B.L. Foureur
Taylor South high Taylor South low
400 300
350 250
300
5 250 ry ee
2 200 (cid:8212) 150
2 150
= 100
100 50 {|
50
On 0 +(cid:8212)+ A |a | + +
o = N = oO - N x
= S = aq S <= SS N
oO wv (=) N co t+ (oe) N
N N N N N N
Taylor North high Taylor North low
250 - 450
400
200 4
350
me <. 300
i 150 +
2 250
2 1004 3 200
150
50 4 100
50
0 +(cid:8212)+ 0 +(cid:8212)+(cid:8212)+(cid:8212)-++(cid:8212) t(cid:8212)+(cid:8212)4
oO - N - Oo = N A
= = = a = <= = N
oO wv oO N oO wt (=) N
N N N
350 5 Maclaren Point
300 +
250 -
& 2004
E 150
Cc
100 +
50 +
O Han | Pea ee
a
13/10 26/10 12/11 N
Figure 5. Total number of abalone found on the collectors at all sites in the period October 1993(cid:8212)January 1994.
261
Variability in abalone larval settlement
Taylor South high Taylor North high
250 80
. 60
5 200
2 Dn
2 150
5
5 100
20
50
OF mae SSS 0 bh
Tay N = N - N = N
oa) oS \ 3 oS
Passage
Taylor North low
100 - 250
200
3 g 150
5 100
50
0 / (cid:8212)(cid:8212)- ie |
a - N oO = = N eS = N 3 x= = Ww
i) re oS 9° SE 3 G ©
Maclaren Point
350
300
5 250
Q 200
§
= 150 .
100
50
0 om (cid:8212)_|
5=s 88 Ss @(oY R Vv oe S&e AY
N +t
Figure6 . Total number of abalone found on the collectors at all sites in the period November 1995-February 1996.
262 K.R. Rodda, J.K. Keesing, B.L. Foureur
19m day!. Comparable growth rates were found by comparing the size frequency of abalone 0(cid:8221)
several other 2 and 4-week collectors.
Annual surveys over three years produced mean adult densities of 0.66 m* (SE 0.11, n=6) and
0.36 m? (SE 0.16, n=5) at the high and low density sites, respectively. At McLaren Point, adult
density was 0.51 m? (SE 0.06, n=3), and at the Passage site, adult density was zero (Table 1). The
density of settled larvae at peak settlement ranged between 10.7 and 70.0 mand was not related t0
the density of reproductive greenlip abalone (ANOVA : F=5.31, P<0.001).
Discussion
The synchronous timing and intensity of the peaks of settlement at the Taylor Island sites within 2
km of each other contrasted with those found at a site 8 km distant. Although our samples represen
the cumulative number of surviving settlers over two weeks, the similar size composition of settle
abalone collected during the peak of settlement activity (Fig. 4) is indicative of an intense period 9
settlement perhaps lasting just 2-3 days and suggestive of a epidemic spawning some days before:
The low or background density of settlement before and after the peak may represent widely dispersed
settlers from a more distant locality or settlers from a local asynchronously spawning pare?
population.
Were the larvae captured by the collectors conspecific? The reef habitat along Taylor Island
supports low to medium density populations of H. scalaris and H. rubra, and H. laevigata can be
found at high (Taylor South high, Taylor North high) and low (Taylor South low and Taylor North
low) abundances. H. cyclobates is common at two sites (Taylor South high, Passage site) while H.
roei is uncommon at all but one site (McLaren Point). To derive which abalone species were likley '°
be spawning, we examined gonad indices of the more abundant species present at Taylor South high.
With the exception of the 1996/1997 season, the reduction in gonad indices, corresponding wit
spawning, suggests that spawning episodes of the 4 species occur at different times of the year. H.
scalaris(cid:8217) peak reproductive activity occurs in March(cid:8212) April, and H. cyclobates spawned durin&
winter (1994) and during early to late summer in 1993/1994 and 1995/1996 (unpublished data). A.
roei is in reproductive condition all year round, but has a major spawning season in April (Keesiné
et al. 1995) and the larvae settle in shallow reef habitat (1-2 m deep) (Shepherd and Laws 1974). It
is therefore unlikely that larval settlement of these three species would be recorded on the collectot
during the experimental period, or be confused with H. /aevigata or H. rubra settlement peaks.
The differences in settlement recorded between sites 8 km distant may be due to differences !(cid:8221)
timing of spawning at the two sites. Water temperatures and food availability differ substantially
between the two sites and may induce different spawning periodicities. Water temperature am!
weather variables at the sites are being assessed as environmental cues for spawning in abalon®
(Rodda and Keesing, in prep.).
The timing of peak settlement in H. Jaevigata at the Taylor Island sites varied slightly over the
three year study although it always occurred in late spring or early summer (early December 1993;
late October 1994 and mid December 1995). Similarly, at the same site in 1987, Shepherd et al.
(1992) found that spawning took place in early December. The reproductive condition of the adults }
influenced by the available nutrition during the months preceding spawning. The gonad index in /*
laevigata begins to increase to around 50% by July each year (see Fig. 3), some 4-5 months prior !°
_ spawning, peaks at around 75% prior to spawning and tends to fall abruptly after peak settlement.
Some declines in gonad indices were not reflected in settlement peaks. Explanation of su¢
incongruities include: the small gonad sample sizes, which may not accurately reflect the natur4
range of reproductive activity occurring at the sites, the resorption of gonad material by individuals
or a light settlement on the collectors. Changes in periodic gonad indices, as described in this pap
derived from necessarily small sample sizes, are a valuable guide to spawning activity, but are 1°
definitive. At our study sites, for example, they indicated likely differential settlement peaks of /*:
rubra and H. laevigata and also excluded the likelihood of larvae of H. cyclobates being confound?