Oxygen Isotope Studies on Sea Scallops ...

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from Browns Bank, Nova Scotia. Can. j. Fish. Aquat, Sci. 45: 1378-1 386. Oxygen isotope records from two sea scallops, Placopectew magelhwicus, collected ...

sotope Studies on Sea Scal ops, Placopeeten magellanicus, from Browns Bank, Nova Scotia F. C. Tan, D. CaiI1 and D. L. podclick2 Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Fisheries and Oceans on 05/25/16 For personal use only.

Physicas and ChernEcaB Sciences Branch, Deprmene of Fisheries and Oceans, Bedford lnseieuke sf Oceanography, P-0. Box 80Q6,Darmohoeh, N.S. B2Y 4A2

Tan, F. C., D. Cai, and D. L. Roddick. 1988. Oxygen isotope studies on sea scallops, Placopecten rnagellanicus, from Browns Bank, Nova Scotia. Can. j. Fish. Aquat, Sci. 45: 1378-1 386. Oxygen isotope records from two sea scallops, Placopectew magelhwicus, collected alive from E3rowns Bank, Nova Scotia, show annual cycles that closely approximate the isotopic composition predicted as a function of observed temperatures and the isotopic composition of the ambient seawater. The external growth lines coincided with the most positive 6880values of the cycles, suggesting their formation during the p e r i d of coldest water temperatures (spring). The SWO results indicate that the growth lines are annual events, consistent with the biological evidence. Les donnees sur la presence de Itisotope dfoxyg$ne dans deux pgtoncles g$ants, Piacopecten magellanicus, capturgs vivants sur le banc de Brown (Nouvelle-Ecosse), nlowtrent des cycles annuels qui se rapprochent de la compsition isotopique predite cornme une fonction des ternpkratures obsew&s et de la composition isohpique de B'eau de mer ambiante. Les courbes de croissance externe cdincidaient avec les valeurs S8Q Ies plus positives des cycles, ce qui donne 3 penser que leur formation a lieu au cours de la pkr"aod ooir les temp4ratures de B'eau sont les plus froides (printemps). Les r&suItatsde 8 ' 8 0 indigguent que les cocrrks de croissance wnt des evenernents annuels, qui sont conforrnes aux indications biologiques. Received Nowmber 30, 1987 Accepted May 7 1, I 986

Rqes le 30 wovembse 8987 Accept6 le 1 1 mai 8988



he sea scdlop, Placgsecfen mgellanicu~,is an important economic resource for Atlantic Cmadim fisheries, providing the second highest income h m landings. Sea scdlops in Atlantic Cmda inhabit water depths from approximately 1 rn below mean low tide to deeper than 100 rn (Cxlliney 1974; M o h et al. 1985). The considerable economic importance and heavy exploitation of sea scdlop stocks underscore the weed for accurate age and growth rate estimates for this species. Such infomatiow is v i a if the sea scdlop fishery is to be managed effectively. The technbques that are presently employed for age deteration of the sea scdlop rely on the interpretation of lines visible on the exterior of the shell or on the hinge ligament (Stevenson md Dickie 1954; M e d l et d. 1965). The extemd lines am assumed to be depsited in early spring each yea. Although the kchique is used extensively by both the Department of Fisheries md Oceans in Canada md the National Marine Fisheries Service in the United States, it c m present pmblems. Shwk marks, due to a sudden change in temperature or injury by fishing gear, may be confused for annuli. Recognition of an m u d ring is, at times, very subjective. The fist m u d ring may not be laid down during the f i t winter, but only in the second winter (Memill et id. 196%).In view of the

uncertainties involved in the accurate interpretation of annuli, additiomd independent methods o f establishing the periodicity of these features would be useful. Furthemore, verification of zmmd formation would have significant mmagement implications, since the exploitation rates are high enough that scdlops tend to be captured in rather early growth phases. The stable oxygen isotope method has recently been used successfully in the interpretation of m u d growth patterns in bivdve shells (Jones et d . 1983; Mmgosim et d . 1987). This method has also been used on two specimens of sea scallops from the coast of Virginia by Kkaxatz et d.(1 984). Growth rates determined from the isotopic records by Krantz et d. (1984) are roughly twice hose estimated from the extemd line method. No explanations were offered for this discrepancy. Furthermore, Kimtz d d. (1984) reported that the external lines, or rings, which we presumed to be deposited muidly in the spring, appear to QCCW in various seasons, according to the inferred seasonality of the isotopic profile. The purpose of this study is to compare the age md timing of formation of e x t e d rings of sea scallops from Browns B d , as determined by examination of the external lines md the oxygen isotope method. This should dso help resolve the discrepancies reported by H(gmtz et 81. (1984).

BPemmenta d h s s : F h t Institute of &emogaphy9 State Oceanic s Republic of China, sheries and Oceans, Bidogicd Sciences Branch, Halifax Fisheries Research h b r a t o q 9 P.0. Box 500, Station M,Halifax, N.S. B3J 2S7.

Living specimens of sea scallops used in this study were collected from Browns Bank, Nova Scotia. This location was cho-


Cart. J. Fish. Aqwt. Sci., Vol. 45, 198%

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FIG. 1. Shells 1 (top) me%2 (bottom) showing sampling lines ( m e e d in black for visibility) md growth lines (mowka&),

Can. J. Fish. &mt. Sci., Val. 45, 1988

Tmm 1. MonhBy m m w-r dieted $'%I fm shell cdcik. Date

Temp. 6°C)

t e r n p m m , salinity at C3, and pm-


6W,, &=, (pkedicted)

Apd 1979

June July

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September W o k Nmemkr December J


kbmq Mmh [email protected]

dune July August

sen because there existed both. co rcid quantities of scdHops md a long-term r m r d of bottom tempratwe a d salinity. Isotopic mdyses were p d o m e d on two specimens collected alive on May 20, 1983, and August 27, 1985 (42"53'20'N, 65"54'5UW at a water depth of 100 rn for spech e n I and 42"50332.9"$V,6637'3 1.2" at a water depth of 85 rn for specimen 2)- The exterior of both shells showed no e of boring or extensive erosion; however, the outer shell of specimen 2 was unreadable, and so the exact age c m o t be accurateHy detemined. It does serve the purpose of coq&ng with the isotopic record from specimen 1. Estima~onof the yearly growth for the left (upper) valve of both shells was made using lines on the exterior of the shell following the method described by Memil1et d. (1965). Growth lines, representing estimated yeas of growth, were marked on each sf the two shells by two exp~encedreaders, B and M, sf the Department of Fisheries and &ems, Halifax, Nova Scotk, and were used for c o m p ~ s o nwith the growth interpretation based on the oxygen isotope method. The two shells used in this study were aged twice by both readers on two separate occasions md without reference to the results &om the oxygen isotope analysis or other readings. Reader D had aged shell 1 when it was f i t collected for mother study in 1983 and again in 1986 as p a t of this study, but before shell smples were obtained for isotopic analysis. Shelt 2 was f i t aged by this reader in 1986, before shell s m ples were obtained, md again in 1987, after sampling. Reader M was d w given the shells to age as a comparison between readers. The fmt occasion was before isdopic sampling in 1986 and the swond t h e was in 1987, after sampling had taken place. On the first masion, reader M was given the shells while he was in the prmess of ageing x d o p samples fmm Gmrges Bank. He was told the two shells were from Browns Bank, an a e a with a slower growth rate than Georges B d (Robert et al. 1985), a d was simply asked to mark on the shells what he wouM consider to be m u d rings. The second time he was asked to carefully study the shells md determine the exact psition of the mnud rings. The two shells were prepard for isotopic analysis by first lightly sanding the exterior of the shell to remove the pems-

stracm md my foreign matter. Discrete smples of shell. material were then ground from the outer shell layer using a 8.5nun D ~ m ebit. l The calcium cahnate powders were collected appmxhately every 1 dong the axis of maximum growth from a b u t 10 nun from the umbo to the ventral magin (Fig. 1). Samples were taken only from the outer shell layer which is deposited sequentidly dong the shell margin during growth of the scallop. This was later verified using thin sections of the shells taken though the sampling lines. Attempts to smple close to the rawabo were not successful, as the arg0ni.c pdlid myosbacum layer was too near the surface to get a discrete sample from the outer shell layer that was large enough for analysis. The stable isotopic cornpsition of each shell powder s m p k was determined using a mdification of techniques described in Tm md Hudson (1974). Appmximately I rng of sample was placed in a p p x thimble and roasted in vacuo at 4WC for 1 h. Once cool, roasted samples were reacted in vacuo with 100% phosphoic acid at 25°C for 12 h. The oxygen isotopic composition of the evolved CO:, gas was determined using a HIosect. isotopic ratio mass spctrometer, equipped with a triple collector and an on-line data processing calculator. The isotopic vdues are reported in the conventiond delta (8) notation as the enrichment m depletion of (parts per thousmd, 760) relative to the Pedee k l e m i t e (PDB) cabonate standard (Epstein et d. 1953)- The precision of the isotopic determinations was evaluated by analyzing an internal inorganic cabonate s~nchrdwith every batch of 20 samples. The sverdl single standard deviation was 2 8.11%0(n = 39). The oxygen isotopic composition of the seawater smples, used to determine the composition of the bottom water, was determined by the method described by Tan and Strain (1980) with a precision of & 0.1960 (I SD).

Average monthly tempratwe a d salinity data for April 1979 to August 1980 were obtained from station C3 (42"51'24'%, 65"49'30rW), 8 rn from the bottom near the study site on Browns Bank (Table 1). The kghest salinity (34.1760)occ in fall, while the lowest vdue (32.7%0)occurred in spring. monthly temperawes show similar changes, with the highest temperature (1 1.6"C) occMng in (46°C) in March. Detailed tempe other yem me not available fa- th nearby station, C2 (43'03 ' 18'N, 6945 ' 18W), me compmed in Fig. 2a md 2b, up to May 1983 when shell 1 was collected. The C2 record follows the pattern shown at station C3 in 119'9% 80. Temperature md salinity vdues fm the near-bottom water in the surrounding m a (42°48.20' to 42'5 8.20fN, 65'49.50' to 65"59.5OpVduring I9 so have similar ranges md vari&ions as in 1979-80 ( t of Fisheries md Oceans, Marine Envirmmental Data Service, Otbwa, Ont ., unpubl. data). The oxygen isotopic composition of the bottom water a%the study site was obtained by direct measurement of the near-bottom water in May and August 1985 and by meawement of tiB8Oand sdhity of b s a m water in the nearby area (41"44.36'" to 43'03 .72'N, 65%. $2' to 67"20.10rW)during October 1986. The expected isotopic composition sf the shell calcite can be cdcaalated by using (1) the galeokmpera- quation of Epstein et d.(1953) md later mdified by Craig (19651, (2) the mean

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Jan Apr July Oct Jan Apr July Ocf Jan Apr July Od Jan Apr July Oct Jan Apr 1979 1980 1981 1982 1983

31 Jan Apr July &t Jan Apr July Oct Jan Apr July Oct Jan Apr July C k t Jan Apr



mon&ly bottom water salinity and kmpramre absemd during A p d 1979 to August 1980, and (3) the. oxygen isotopic cornpsition of the bottom water at the study site- The predicted te cm be estimated by oxygen isotopic cornpsition o ustion of Craig (1965) solving the modifid pdeote using the qasadraa~cfornula, such that 52(16.9-T) 1 14.2-d 17 8186c&i,, = 8 L B O t W ~ 1 0.26 where T = ambient tern (degrees Celsius). ition md t e m p r a ~of the The oxygen isotopic water were substituted into the quation to obtain the average oxygen isotopic comp~itionof shell cdcik d e p s i t d during each month from April 1979 to Aupst 1980 (C3) md Novemk r 1978 to May 1983 (C2).


Can. J- Fish. Aqut. Sci., VoL 4Ss8988




Since the expected isotopic cornpsitian of shell calcite is det and the oxygen isotopic comis essential to h o w the relative po impormce of these two envkomewd p m e k ~To . this end we have used a constant 6'Q value of - 0.95%~ (mean 6180 values of wakr during 197S80) a d calculated the e x p t e d isotopic composition of shell calcite. The expected 6180curve is plotted in Fig. 3b.

Resdb and Dbeumion mading by reader M (shell 1, 1986)differs fmm the o readings (Table 2) a d shows the high degree of subjectivity involved in the ageing process. Expecting to fmd rings closer together on shell 2 than on those from Georges 8388

area of shell 1 is more ddis-

Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Fisheries and Oceans on 05/25/16 For personal use only.

md sdinity as de

waters has only relatively minor effect. for specimen I (Fig. 4) were fitted so that the April values comespond to the D-1986 ring positions, with the rest of the pints h e y fit, with no attempt $0easmpensde for seasonal gmwth di height of 120 mm9 wa

ge sf shell removed for sample prior to ageing.

shell. so shows several cycles, 9 to 1 . 3 %(Fig. ~ 51, ues for the whole shell to the r a g e observed ngs dso coincide with the

the difference between the two readers in assigning one versus Can. J. Fish. Aqmf. Sci., Vd. 45, 1988

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Jan Apr July Oct Jan Apa July Od Jan Apr July Oct Jan Aps July Oet Jan Apr 1979 1980 1981 1982 1983

Apr May June July Aug Sept Od Nov Dec Jam Feb Mar Apr May Jtrne Jtrly Aug

1979 FIG. 3. Predicted isotopic vdues of shell calcite without a constant 8180seawater vdue.

mean values of temp re md salinity. Broom and Mason (1978) showed that for ChHmys opercularis, dthough a shell lamella is probably laid down in I d, a h e l l a is not necessarily attribute a cessation of shell growth d availability. Thus if growth is only days in the month, there would values from that predicted with monthly man temperatures. h o t h e r possible factor that m y contribute to the differences in isotopic amplitudes is h t there may be a slight difference in ambient temperatures %randthe isotopic composition of the water between the C3 and C2 sites m d the actual point of capture of the scallops. Further studies would be required to resolve this. There. is a lot of within-cycle variability in the 6180signal as well. This may be explained by the within-month variation in temperature at these sites. Although the monthly m a n ternpratures give a smooth cycle, there is considerable withinCan. J. Fish. Aqnsar. Sci., VoB. 45, 1988

1980 (a) stations C2 and C3 md (b) station C3 with md

in the Browns Bank region. At (November 1978 to May 1983) the standard deviation of temperature within each month (usudly sampled every half how) averaged Q.8IeC with a range of 6.2772 for July 198l to 2.66OC for N w if calcite is only being deposited in the a month a large variation in the 81s0 si Ambient temperatures during shell f o r n a ~ o nmay be estimated from the calcite paleate rature quation of Craig (1965) using the 6lQ vdues of the individual carbonate s m ples and an average water ti8% value of -O.95%0. The estimated m g e of temperalures d h g shell deposition is 5.6l%.tj°Cfor shell 1. This is very close to the minimum (46°C) md maximum (11.6"C) temperatures observed during April 1979 to August 1980 (Table 2). The good agreement between the isotopic and obsmed temperatures suggests that the shell carbonate was deposited in isotopic equilibrium with the ambient water. month variation in temper

h e C2 site for the study


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Distance From Umbo (warn) RG.4. Obseedl and predicted isotopic vdues for shell 1 along with the ring positions assigned by the readers as in Table 1. 'The shaded areas are subjectively assigned to o b s m d isotopic peaks (cold water te

The results for shell 1 indicate a g d apement between the 9 y=of p w t h h k p ~ k from d the e x t m d lines and the 89 record. F d e m o ~the , external lines, with the exception sf ring 3, which are presumed to have formed m u d l y h the spring, coincide with the most positive 6'Q vdues of the isotopic cye1es. This suggests that the external lines are mud events and that they were formed during the coldest part (spring) of the year. The m a of shes 1 in which the redem have assigned a ~g number 3 is probkmdc. Biological literature indicates that it ~ p ~ too~]Barge n anbheremerit to be consided a single yew's pd.One explaahon the presence sf two small shock at If shell growth ceased during these ]perids Ie may not have k n recorded in the shell. 'This would also explain the p a t variation in ass Both readers felt that there should be a k g and yet are not consis%ent in where they place it. The reason could be that the slow p w t h band is missing. In she112, however, there is a similar section in which both mdem have no evidence of shock marks in this otopic cycle h m 35 to 60 would not agree with what i a b u t the growth rates for this area (Robert et 198%).'This 1384


is either an extremely fast-growing individud and both readers are wong in assigning a ring to this m a , or for some envkonmenM or physiologicd reason, here is a 8IQ cycle not coded in the shell. More work wwM be q u k d to r e s o h this.

N ~ o u g hthe outer edge of specimen 2 is unreadable, its isompie record does provide 8 good comparison with the record Spechen 2 dso show of fro to a s B height of 12 the shell (0.4 to 1.8960) mQ the very similar to those of specimen 1, The 6'" m g e is Ass similar to the predicted 8'" range of shell calcite. The lwahons of the rings again coincide with the most positive 6180 values of the cycles. Our results are not consistent with those of &an& d d. (1984) who reported tha the external lines am not mmd events a d that they were formed at different seasons of the year. Mthou& the psition of the rings in the that found by Pasgay and Marill (1979) same m a , their isotopic In the two shells sampled to coincide with seasonal Cm. 9. Fish. &wt. Sci., Val. 49, 8988

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Distance From U m h (mm) FIG.5. Observed isotopic values for shell 2 dong with the firm8 psitiom assigned by the readers as in Table 1. The shaded areas comespnd fQ isotopic peaks (cold w&er te

s as well, but ody after the second year of grow?& i. e. ~ g 1,3, s md 5 in their specimen PM26 and ring 1 in PM18. The reason for the difference in the results of the two studlies m y , therefore, be due to the temperature regimes of the study r, more southern site m y p U c e a su growth check during ma% allowing for good p w t h in h g years; h e more norther1 as has uudly k n rep-dl in the literature. y g n isotopic m o d s from two scallop shells collected dive from Browns B a d show m u d cycles that closely cornpsion predicted as a function and isotopic composition of the water. appear to coincide with the most p s itive 8lQ d u e s of the cycles, suggesting their formation dwing the peaid of colde ~suHt.sindicate that the tent with the biologicd used as rn independen g of the formation of extend M ~ S for Pkaeopeeten mgelkna'em. Cm. J. Fish. A q ~ d d ~ dSci., . Vol. 4.5$ 1988

The diffe~ncein our results and hose of &mtz et d (1984) suggests that for different m a s , an independent method, such as isotope mdysis, should be used to verify that the ~ g ms m u d events. This implies that having a reader that is witfa shells h m a specific m a may greatly impmve accumcy when ageing shells by counting '"ud9' pings.

The authors would like to && M.R. Lively of the Atlantic Oceanographic Labratoq, B d o d Institute of Ocmomphy, for providing the wrm~lytemperature providing shell 1, md M. & m s , Halifax, for doing one of the sets of shell ageing.


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