SEASONAL CYCLE OF CONDITION INDEX OF OYSTERS IN THE YORK AND RAPPAHANNOCK RIVERS1

By

D exter Haven

ABSTRACT

F ive se r ie s of stu d ies were made to test for d ifferences in condition index of oysters grown in the York and Rappahannock R ivers. O ysters were cultured in elevated trays and on the bottom from 1956 through 1961. All groups came from a s in g le source and were se le c te d for uniformity in s iz e . Rappahannock oysters showed high in d ices in la te spring and fall with a low in la te summer; the lev e l of condition of bottom groups w as below that of trays. York River oysters showed a peak of condition in late spring; in most years there w as no in crease in quality in la te fall; bottom oysters were generally of lower quality than th ose in trays, but d ifferences were not as large as in the Rappahannock. In general, the leve l of quality in the York w as alm ost alw ays lower than in the Rappahannock. D ifferences in le v e l of index between tray and bottom oysters were not a sso c ia ted with presence of Dermocystidium or Pinnotheres ostreum or with age or crowding, but were probably the result of factors a sso c ia ted with the bottom.

INTRODUCTION

Q u a l i ty of o y s t e r m ea ts i s of c o n s id e r a b l e i n t e r e s t to com m erc ia l growers, for it o f ten d e te rm in es the margin of p ro f i t . High q u a l i ty m eats a re u s u a l l y plump w i th a c reamy w h i te c o lo r , and they g e n e ra l ly fi l l the sh e l l c a v i t y . In c o n t r a s t meats of low q u a l i ty a re sh runken w i th a high w a te r c o n ten t and a t r a n s lu c e n t a p p e a r a n c e .

It h a s long b e en known tha t o y s te r m ea ts va ry g rea t ly in q u a l i ty , bu t r e a s o n s for the v a r i a t io n a re not w e l l u nde rs tood . Many com plex fa c to r s a s s o c i a t e d w i th the environment a re involved a s i n d ic a ted in a rev iew by Korringa (1952).

In the p re s e n t s tudy "Cond i t ion Index" w a s s e l e c t e d a s an a c c e p t a b l e m easu re o f q u a l i ty and y i e l d . It is de f ined a s ;

1 C ontr ibu t ion : Virginia In s t i tu te of M ar ine S c i e n c e , Virginia F i sh e r ie s Labora tory , G lo u c e s t e r Poin t , V irg in ia . Con t r ibu t ion N o . 104.

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, , 100 X dry w e igh t (q) o y s te r meatsCond i t ion Index = — TTL r ~ n — T —/— \---------s i z e of the s h e l l c a v i t y (cc)

In e f fe c t , co nd i t ion index (C. I . ) com pares m eats with the i r t h e o re t i c a l maximum s i z e , tha t i s , the volume of the s h e l l c a v i t y . The h igher the numerica l va lue for "C . I . " the g rea te r wil l be the amount and q u a l i ty of m eats from any given b u s h e l . In a d d i t io n , C . I . may s e rv e a s a m easure of the r e c e n t p h y s io lo g ic a l h is to ry of the a n im a l , a s low in d ic e s have b e en shown to be a s s o c i a t e d with d i s e a s e or u n favo rab le e n v i ro n ­menta l co n d i t io n s .

Among fac to rs tha t a p p ea r to lower C . I . a re c e r t a in o rgan ism s a s s o c i a t e d with o y s t e r s : the mud worm, Polydora w e b s t e r i , a cc o rd in g to Lunz (1941); the p a r a s i t i c fu n g u s , D ermocyst id ium m ar inum , a cc o rd in g to Ray e t a K (1953), and u n p u b l i s h e d da ta of Virginia In s t i tu t e of Mar ine S c ien ce ; a t t a c h e d m u s s e l s a cco rd in g to Engle and Chapman (19 53); and the o y s te r c r a b , P inno theres o s t r e u m , acco rd in g to Haven (1959).

Sto rage of r e s e r v e food, p r inc ipa l ly in the form of g ly co g en , and the c o n d i t io n index of o y s t e r s a re both r e l a t e d to the s ex u a l c y c le (Medcof and N e ed ie r , 1941; Engle, 1958). G e n e ra l ly , a long the Atlantic c o a s t , h igh con d i t io n in d ice s occu r in l a t e spr ing and a re a s s o c i a t e d w i th an accu m u la t io n of food r e s e r v e s and with deve lop ing gonads . In d ic e s a re low in summer a f te r s p aw n in g , but improve a g a in during a period of food sho r tage in l a te f a l l . Age a p p a ren t ly ha s some e f fec t on co nd i t ion (unpubli shed d a t a , Virginia In s t i tu te of Mar ine S c i e n c e ) . Examples o f env ironm enta l c o n d i t io n s t h a t may in f luence meat q u a l i ty a re : crowding and a v a i l a b l e food (Korringa, 19 52), w a te r dep th (Nelson , 1950, and Loosanoff and Engle , 1942), low s a l i n i t y (Engle, 1946, and B eaven , 1946), and c h a r a c t e r of the bottom (Ito and Im ai , 1955).

In many p rev ious s tu d ie s it is d i f f icu l t to a t t r ib u te observed d i f f e r en c es in index to known c a u s e s . Frequen t ly the p a s t h is to ry of o y s te r s w as unknown, and c o n s e q u e n t ly in d i c e s w ere s u b j e c t to unknown s o u rc e s of v a r i a t i o n . The p re sen t s tu d y , begun in 19 56, w as d e s ig n e d to e l im ina te or c o m p e n s a te for s e v e ra l known so u rc e s of v a r i a t i o n .

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DESIGN OF EXPERIMENT

C o m para t ive s t u d i e s were made of co n d i t io n index of o y s t e r s c u l tu red in t rays and on the bottom a t four s e l e c t e d a r e a s : the upper York River a t Pur tan Bay, lower York a t T i l l a g e ' s G lo u c e s t e r Po in t o y s t e r p la n t in g , upper R appahannock a t G a r re t t ' s Bowler Wharf o y s t e r p lan t in g , and in the lower Rappahannock a t Urbanna (Fig. 1 and Table 1). Tray and bottom s t a t i o n s were o c cu p ie d in th e lower York and Rappahannock Rivers dur ing the en t i re s tu d y , w ith th e e x c e p t io n of the 1957-58 s e a s o n , w hen bottom o y s t e r s w ere not p l a ce d in the lower Rappahannock . Tray and bo ttom o y s t e r s were cu l tu re d in the upper York and Rappahannock Rivers during th e 1958-59 s e a s o n o n l y .

Tab le 1 . L oca t ion o f s t a t i o n s in York and R appahannock Rivers

S ta t ion M i le s ab o v e mouth

M ean* s a l i n i t y , p p t .

D ep th Bottomtype

YorkT i l l a g e ' s 6 20 14 Firm s h e l l y mudPurtan Bay 18 12 6 Soft s andy mud

RappahannockUrbanna 14 15 7 Hard s an d y c l a yG a r r e t t ' s 29 10 7 S h e l l s over mud

* D a ta from Andrews and H ew a t t (1957)

In March of e a c h y ea r s e v e r a l th o u s a n d D e rm o c y s t id iu m -f ree o y s t e r s w ere o b ta in e d from W reck Shoa ls in t h e Jam es R ive r . O y s t e r s s e l e c t e d to r a n g e from 55 to 65 mm w ere c l e a n e d of fou l ing o rg a n i s m s , and randomly d iv id ed in to s e v e r a l groups , At e a c h s t a t i o n some of t h e s e o y s t e r s w e re he ld in t rays e l e v a t e d s ix in c h e s from the bottom w h i le a n o th e r group of s im i la r s i z e w a s s c a t t e r e d on na tu ra l o y s t e r bottom b e s i d e the t r a y s . Trays were l i f ted e a c h month and a t t a c h e d fou l ing o rg a n ism s w ere s c r a p e d off the o y s t e r s . After a period for a c c l i m a t i z a t i o n , 25 o y s t e r s w ere rem oved e ac h month for C . I . d e t e r ­m ina t ions . The sam e number of o y s t e r s w e re t a k e n from the bottom g ro u p s , u s in g d iv in g gear during th e warmer months and to n g s in w i n t e r . After abou t 14 m on ths , w h e n sam p l in g and n a tu ra l morta l i ty had removed nea r ly a l l o y s t e r s , new a d d i t i o n a l groups w e re p la ce d a t the s t a t i o n s . In a l l , f ive s e r i e s of o y s t e r s have b e e n s tu d ie d a cc o rd in g to the fo l lowing s c h e d u le :

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F ig . 1 . L o ca t ion of a l l s t a t i o n s o c c u p ie d during s tu d y in York and Rappahannock Rivers .

I . T r an sp la n ted June 1956, sam pled Sep tem ber 1956 through June 1957

II . T r an sp la n ted March 19 57, sam pled June 19 57 through June 1958

II I . T ra n s p la n te d M arch 1958, sam pled April 19 58 through August 19 59

IV. T r an s p la n ted M arch 19 59, sam pled April 19 59 th rough Sep tem ber 1960

V. T r a n s p la n te d M arch 1960, sam pl ing in p r o c e s s

In th e 19 57-58 s tu d i e s (Ser ies II), on t h e lower York River, t r i p l i c a t e t rays 150 f e e t a p a r t w ere m a in t a in e d , i n s t e a d of the u s u a l s in g le t r a y . S e a s o n a l da ta for th i s s t a t i o n a re r e p r e s e n t e d by mean v a l u e s from the th ree t r a y s . D a ta from t h e s e t r ay s w e re a l s o u s e d to s tudy p o s s i b l e v a r i a t io n of C . I . over r e l a t i v e l y shor t d i s t a n c e s .

During e a c h s e a s o n , con t ro l s t u d i e s w ere c a r r ie d out a t v a r io u s c o m m erc ia l o y s t e r g rounds in both r iv e r s . T w e n ty - f iv e m a r k e t - s i z e d o y s t e r s w e re d redged from 16 publ ic g rounds in the York and R ap p a ­h a nnock Rivers dur ing the w in te r and c o n d i t io n i n d i c e s w e re de te rm ined (Table 7).

In r e v i e w , the a t t r i b u t e s of th i s e x p e r im en ta l d e s i g n a re :

1 . C u l tu r in g o y s t e r s in t r a y s m in im izes v a r i a b i l i t y due tobottom ty p e s or d e p o s i t s , to the lo c a l e f f e c t s o f bottom to p o g ra p h y , and o the r b e n th ic f a c t o r s . Tray o y s t e r s might r e a s o n a b l y show the p o te n t i a l C . I . of an y r e g i o n . Exper im en ta l o y s t e r s on the bottom s e r v e to r e l a t e t ray to bottom c o n d i t i o n s .

2 . Exper im en ta l lo t s for any s in g l e s e r i e s w e re made up byrandom s e l e c t i o n from a group of uniform s i z e t a k e n from a point s o u r c e . This t e c h n i q u e g a v e the b e s t a s s u r a n c e th a t i n i t i a l l y the v a r i a t i o n and mean C . I . of e a c h lo t would not be s i g n i f i c a n t l y d i f f e r e n t . Thus mean d i f f e r e n c e s in C . I . of th e p r o c e s s e d o y s t e r s in the fo l lowing per iod of e x p e r im e n t a t io n a re more l ik e ly to be due to lo c a l e n v i r o n ­m en ta l c o n d i t i o n s t h a n to p o s s i b l e i n i t i a l v a r i a t i o n and mean d i f f e r e n c e s .

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3 . Each group of o y s t e r s is fo llowed for only about 14months . This r e s t r i c t e d t ime period h e lp s p reven t la rge d i f f e r en c es in such fac to rs a s s i z e and r a te of in fec t ion w ith p a r a s i t e s , which might d ev e lo p if the o y s t e r s were held for longer per iods .

4 . The t r a n s fe r of exper im en ta l groups from the James to the s t a t i o n s by M arch of e a c h y e a r avo ided movement during pe r iods of s h e l l and meat growth . H ow ever , it must be s t a t e d t h a t in M arch , i n d ic e s of Wreck Shoal o y s t e r s may range from 4 .0 to 5 . 5 , and the re w a s g e n e ra l ly an in i t ia l rap id i n c r e a s e in cond i t io n in d ic e s a f te r t r a n s p l a n t i n g .

LABORATORY PROCEDURE

T ech n iq u es for de te rm ina t ion of C . I . rev iew ed by G a l t so f f e t a l . (1947) w ere som ew ha t m odif ied . In the r egu la r monthly sam pl ing , 25 o y s te r s were p r o c e s s e d a s a u n i t . Each o y s t e r w as d ra ined e x a c t ly one minute a f te r s h u c k in g . After d ra in in g , a l l 25 m ea ts were p la ce d in a s in g le a luminum foil d i s h and dried a t 87 C to a c o n s t a n t w e i g h t . Shell cavity, vo lume w as de te rm ined in the u s u a l manner , by c a l c u l a t i n g the d i f f e r e n c e s in d i s p l a c e m e n t b e tw ee n the en t i re group of o y s t e r s and the i r empty v a lv e s .

At t im es of e x p ec te d maximum and minimum f a t n e s s , 25 to 55 o y s t e r s were p r o c e s s e d in d iv id u a l ly . The s h e l l c a v i ty volume for e ach o y s t e r w a s d e te rm in e d , and the meat dried in d iv id u a l ly in a smal l g l a s s pe tr i d i s h . The mean co n d i t io n in d ic e s of ind iv idua l ly p r o c e s s e d o y s t e r s w ere t e s t e d by the t or t^ t e s t s to ad ju d g e p o s s i b l e s ig n i f i c a n t d i f f e r en c es (Tables 2, 4, 5, and 6).

In c id e n c e of pea c r a b s (P innotheres o s t r e u m ) w as noted during the p r o c e s s in g of s a m p le s . In the l a t e fa ll of e ac h yea r smal l t i s s u e s l i c e s were o b ta in ed for s tudy of D erm ocyst id ium in fe c t io n s . I n c id en ce of t h e s e two o y s te r p a r a s i t e s w as o b se rv ed to de te rm ine the i r p o s s ib l e in f luence on cond i t ion in d e x .

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RESULTS

S e a s o n a l C y c l e s of Tray and Bottom O y s te r s

Lower York River ( T i l l a g e ' s ) : Monthly s a m p le s of t ray and bottom o y s t e r s in d ic a te d a d e f in i te s e a s o n a l c y c l e (F ig . 2 ) . Both g roups had the h i g h e s t C . I . during M ay , June , and ea r ly Ju ly , f o l ­lowed by a d e c l i n e in q u a l i ty during m id - su m m e r . After the spaw ning s e a s o n the re w a s l i t t l e i n c r e a s e in q u a l i ty and both groups of o y s te r s w e n t into the w in te r per iod w i th ab o u t the sam e C . I . they had shown in l a t e s u m m e r .

1 9 5 9 - 6 0

1 9 5 8 - 5 9

Xuaz

19 5 7 - 5 8zo

QZoo

1 9 5 6 - 5 7

OCTA PR JUN AU 6 OCT DEC JUN AUGF E 8 APR

F ig . 2 . S e a s o n a l c o n d i t io n index c y c l e s for four s e r i e s of t ray (open c i r c l e s ) and bottom (solid c i r c l e s ) o y s t e r s grown a t T i l l a g e ' s o y s t e r ground in the lower York R iver .

The s e a s o n a l c u r v e s s u g g e s t t h a t during th e 1957-58 and 1959-60 s e a s o n s , t ray o y s t e r s w ere of b e t te r q u a l i t y than t h o s e from the b o t to m . H o w ev e r , the s e r i e s for 1956-57 and 1958-59 in d ic a ted l i t t l e d i f f e r e n c e b e tw e e n the two g roups .

C o m p o s i t e c u rv e s b a s e d on mean v a lu e s for the four s e r i e s (Fig. 3) i l l u s t r a t e the po in ts p rev io u s ly ou t l ined : maximum in d ic e s in May and June and the a p p a r e n t a b s e n c e of c o m p arab le fa t t en in g during

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12

XUloz

QZoo

A P R JUN AUG OCT DE C FE B A P R J U N

F i g . 3 . C om p o s i te cu rve showing mean s e a s o n a l cond i t io n in dex c y c l e s for the four s e r i e s of tray (open c i r c l e s ) and bottom (solid c i r c l e s ) o y s te r s a t T i l l a g e ' s o y s te r ground in th e lower York from 19 56 through 196 0.

fa ll and w in t e r . They a l s o show an a p p a r e n t super io r i ty of t r a y - c u l tu re d o y s t e r s over t h o s e grown on the bo t tom . H ow ever , s t a t i s t i c a l a n a l y s i s on ind iv idua l o y s t e r s i n d i c a t e s th a t bottom o y s t e r s w ere s ig n i f i c a n t ly lower in q u a l i ty in only two ou t of s e v e n c o m p a r i so n s (Table 2).

The th ree t rays p la ce d 150 fee t a p a r t in the lower York in 19 57 were sam pled regu la r ly a s groups from M arch through November , and ind iv idua l ly on D ecem ber 3 0, 1957 (Table 3) . M ean v a l u e s showed sm al l d i f f e r en c es . In the s t a t i s t i c a l a n a l y s i s of ind iv idua l o y s te r s made in D ecem ber , mean d i f f e r en c es b e tw ee n th e th re e t r ay s w e re not s ig n i f i c a n t (Table 2).

Upper York (Purtan Bay): S tud ies w e re made through one s e a s o n , from April 19 58 through July 19 59, in th i s a r e a . The s e a s o n a l c y c l e (Fig. 4) ap p ea re d s im i la r to tha t found in the lower r i v e r . There w a s a s in g le peak of q u a l i ty in the sp r ing , fo llowed by a d e c r e a s e in summer and l i t t l e c h a n g e during fa ll and w in t e r . The bottom o y s t e r s showed a peak of very shor t du ra t io n , whi le t h o s e in th e t ray rem a ined in good

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Table 3 . C ondi t ion in d ice s for th ree t rays 150 fee t a p a r t , York River

Date(1957)

N o . of o y s te r s

in sample

Tray84

Tray87

Tray88

M ean

M a rch 11 25 4 .9 4 .9 4 .9 4 .9

June 27 25 1 0.3 10 .4 10.4 10 .4

July 3 0 25 7 .7 7 .4 7 .3 7 .5

S e p t . 6 25 8 . 2 8 .1 8 .2 8 .2

S e p t . 3 0 25 7 .8 8 .4 7 .5 7 .6

N o v . 4 25 8 .6 7 .4 8 .2 8 .1

N ov . 27 25 8 . 0 6 .8 7 .2 7 .3

Dec . 3 0 55* 7 .61 7 .3 5 7 .84 7 .6

* Ind iv idua l o y s t e r s t e s t e d for s t a t i s t i c a l a n a l y s i s (see Table 2).

XÜJQ2

2OI -Q2Oo

A P R J UN AUG O C T D E C J U N AUGFEB A P R

F ig . 4 . S e a s o n a l c o n d i t io n index c y c l e for t ray (open c i r c l e s ) and bottom (solid c i r c l e s ) o y s t e r s grown a t Pur tan Bay in the upper York River from April 19 58 through July 1959 .

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co n d i t io n through Ju ly . Again , a s in the lower r iv e r , t ray o y s t e r s s e e m e d to be of b e t t e r q u a l i ty than bottom o y s t e r s , but only the August 1958 v a l u e s were s ig n i f i c a n t ly d i f fe ren t (Table 2) .

Lower Rappahannock (Urbanna): In c o n t r a s t to v a l u e s ob ta ined in the York, co n d i t io n in d ic e s of Rappahannock tray o y s t e r s i n d ic a ted a d e f in i te b imodal s e a s o n a l cu rve (Fig. 5) . There w a s a h igh during M ay , June , and ea r ly Ju ly , and a d e c r e a s e during l a t e sum m er .

J 9 5 9 - 6 0

1 9 5 8 - 5 9

XUJQZ

ZOh- 1 9 5 7 - 5 8ozoo

1956-5 7 !

AUG O C TJ U NA P RF E BA PR J U N O C T DECAUG

F ig . 5. S e a s o n a l c o n d i t io n in dex c y c l e for four s e r i e s of tray (open c i r c l e s ) and bottom (solid c i r c l e s ) o y s t e r s grown a t Urbanna in the lower R appahannock .

A s e c o n d maximum in l a t e fa l l or w in te r app rox im a ted th e spring peak in a m p l i t u d e . In June 19 58 and in the w in te r of 19 59 bottom o y s t e r s did not show a s e a s o n a l i n c r e a s e in c o n d i t io n in d ex c o m p ar ab le to th a t shown by the t ray o y s t e r s . C o m p o s i te c u r v e s b a s e d on a l l a v a i l a b l e d a ta (Fig. 6) i l l u s t r a t e th e s e a s o n a l b imodal cu rv e for both s e t s of o y s t e r s .

Bottom o y s t e r s a p p e a re d to h a v e a c o n s i s t e n t l y lower index th a n t ray o y s t e r s . C om par ison of mean v a l u e s for co n d i t io n in d ic e s of in d iv id u a l ly p r o c e s s e d tray and bottom o y s t e r s a t e x p e c t e d pe r iods of maximum and minimum f a t n e s s show ed t h a t , w i th one e x c e p t io n , t ray o y s t e r s in th e lower r ive r s t a t i o n s w ere s i g n i f i c a n t ly of b e t t e r q u a l i ty than bottom o y s t e r s (Table 4) .

- 5 2 -

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- 5 3 -

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F ig . 6 . C o m p o s i t e cu rve showing mean s e a s o n a l c o n d i t io n index c y c l e s for the four s e r i e s o f t ray (open c i r c l e s ) and th re e s e r i e s of bot tom (solid c i r c l e s ) o y s t e r s a t Urbanna in the lower R appahannock from 19 56 to 196 0.

i

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A P R OEC0 CT FEB AUGJ U N AUG JUNA PR

F ig . 7 . S e a s o n a l c o n d i t io n index c y c l e for t ray (open c i r c l e s ) and bot tom (so lid c i r c l e s ) o y s t e r s grown a t G a r r e t t ' s o y s t e r ground in th e upper R ap pahannock from M arch 19 58 th rough July 19 59 .

- 5 4 -

Upper Rappahannock (Garre t t ' s ) : O y s t e r s grown in t r ay s showed a bimodal s e a s o n a l index curve (Fig. 7) s im i la r to th a t for the lower r iv e r . The maximum for l a t e fa ll and w in te r , h ow ever , w a s more prominent than th a t for the s p r in g . In th i s r e s p e c t the upper r iver d i f fered from the lo w er .

Again, a s in the lower r ive r , c u rv e s for t ray and bot tom o y s t e r s fo llowed s im ila r t r e n d s , w i th bottom o y s t e r s a p p a r e n t ly in d ic a t in g a c o n s i s t e n t l y lower i n d e x . S t a t i s t i c a l s t u d i e s of ind iv idua l o y s te r s showed th a t d i f f e r e n c e s in th e mean c o n d i t io n in d i c e s b e tw e e n t ray and bo ttom groups w ere s ig n i f i c a n t dur ing spr ing and w in te r , but no d i f f e r en ce w as a p p a r e n t in l a t e summer (Table 4).

Com par ison of Tray O y s t e r s in the Two Rivers

The c u rv e s in F ig . 8, w h ich show four y e a r a v e r a g e s for tray o y s t e r s in the two r iv e r s , s u g g e s t th a t t h o s e c u l tu red in the lower York had a c o n s i s t e n t l y lower co n d i t io n in dex during the w in te r months than t h o s e grown in the lower R ap p ah an n o ck . During sp r ing and summer, p o s s i b l e d i f f e r en c es b e tw e e n th e two groups w ere not o bv ious . S t a t i s t i c a l c o m ­p a r i s o n s a t pe r iods of maximum and minimum f a t n e s s confi rm t h e s e o b s e r ­v a t io n s (Table 5). During eac h of the four w i n t e r s , York River t r ay o y s t e r s w ere s ig n i f i c a n t ly lower in q u a l i ty than th o s e in the Rappahannock . No d i f f e r e n c e s w e re e v id en t during ear ly spr ing and only one of the two t e s t s in l a t e summer showed York River o y s t e r s to h ave a lower i n d e x .

JUNAPRAPR DECJUN OCT FEBAUO

F ig . 8 . C o m p o s i te cu rve showing mean c o n d i t io n in dex cu rv e for the four s e r i e s of t ray o y s t e r s a t T i l l a g e ' s in the lower York (c losed c i r c l e s ) and the four s e r i e s of t ray o y s t e r s a t Urbanna (open c i r c l e s ) in the lower Rappahannock from 19 56 th rough 1960.

- 5 5 -

Table 5 . S t a t i s t i c a l summary of th e groups of tray o y s t e r s p r o c e s s e d in d iv id u a l ly for con d i t io n index in the York and Rappahannock Rivers

Date S ta t ionlo c a t io n

Samples i z e

Time in r i v e r - months

M eanc o n d i t io n

index

Value t or t* df

I . Firs t S e r ie s

1 / 3 / 5 7 Urbanna 50 7 10 .4 3 .1 5 * * 491 / 8 / 5 7 T i l l a g e ' s 50 9 . 0

5 / 2 8 / 5 7 Urbanna 50 12 13 .8 1 .02 986 / 3 / 5 7 T i l l a g e ' s 50 13 .3

I I . Second Se r ie s

8 / 2 7 / 5 7 Urbanna 50 6 8 .4 0 .2 8 1039 / 1 3 / 5 7 T i l l a g e ' s 55 8 .3

2 / 3 0 / 5 7 U rbanna 55 10 1 2 .2 12 .4 * * 1082 / 3 0 / 5 7 T i l l a g e ' s 55 c* 7 . 8

I I I . Third Se r ie s

5 / 2 7 / 5 8 U rbanna 49 3 11 .5 1 .25 976 / 3 / 5 8 T i l l a g e ' s 50 12.3

5 / 2 7 / 5 8 G a r r e t t ' s 48 3 1 0 .0 1.83 966 / 3 / 5 8 Pur tan Bay 50 11 .2

8 / 2 8 / 5 8 Urbanna 25 6 8 .5 3 .18** 488 / 2 7 / 5 8 T i l l a g e ' s 25 7 . 0

8 / 2 8 / 5 8 G a r r e t t ' s 23 6 9 . 7 1 .26 468 / 2 7 / 5 8 Purtan Bay 25 10.3

2 / 3 / 5 8 U rbanna 50 9 11.1 6 .22* 73. 1 / 2 8 / 5 8 T i l lag e ' s 25 7 .6

. 2 / 3 / 5 8 G a r r e t t ' s 50 9 14 .6 8 .4 0 * * 49

. 1 / 2 8 / 5 8 Purtan Bay 25 9 . 6

IV. Fourth Se r ie s

.1 / 2 / 5 9 U rbanna 50 8 11 .8 7 .3 3 * * 97

.1 /9 /5 9 T i l l a g e ' s 49 7 .9

* Tray c u s e d , s e e Tab le 2* P < 0 .05

** P < 0 .01

- 5 6 -

If c u rv es for the upper r ivers in the 19 58-59 s e r i e s a re compared (F igs . 4 and 7) it is s e e n th a t tray o y s t e r s in the upper Rappahannock were u n q u e s t i o n ab ly s u p e r io r .

Com par ison of Bottom O y s t e r s in the Two Rivers

S t a t i s t i c a l c o m p ar i so n s (Table 6) show tha t in two out of th ree w in t e r s , bottom o y s t e r s in the lower Rappahannock had h igher in d ic e s than t h o s e grown in the lower York . C om par isons during the spr ing of 19 58 show th o s e in the lower York to be super io r , w h i le no d i f f e r en ce w a s ev iden t in the spr ing of 19 57 . In the summer of 19 58 a s in g le com par i son showed bottom o y s te r s in the Rappahannock to h ave higher in d ic e s .

Exper im enta l and Commerc ia l O y s t e r s

E x tens ive s tu d i e s on o y s t e r s from publ ic o y s t e r ro c k s w ere made a s a b a s i s for c om par i son w ith exper im en ta l r e s u l t s . Samples of 25 o y s t e r s e a c h were ta k en from beds tha t cove red the range of com m erc ia l o y s te r product ion in both r ive rs . Cond i t ion in d ic e s w ere de te rm ined in w in te r when o y s t e r s were be ing h a r v e s t e d for m a rk e t . R esu l t s a re summarized in Table 7 .

Between March 19 56 and January 1960, York River o y s t e r s from com m erc ia l p la n t in g s showed l i t t l e trend in C . I . up or down r ive r , and no la rge v a r ia t io n from year to y e a r . During th i s en t i re period many in d ic e s w ere be low 6 . 0 and it is our e x p e r ie n c e tha t o y s t e r s with in d ic e s rang ing a round 5 .5 a re not d e s i r a b l e from an economic s t a n d ­p o in t . G enera l ly m ea ts from t h e s e o y s t e r s a re f l a c c id , s e m i - t r a n s p a r e n t , and w i th a high w a te r c o n t e n t . Y ie lds a s repor ted by oy s te rm en a re u s u a l l y abou t four p in ts of meats per "cu l led" b u s h e l . During th e w in te r of 1960-61 , how ever , the re w a s a d i s t i n c t c h a n g e in qu a l i ty and y ie ld of York River o y s te r s i n d ic a ted by an i n c r e a s e in in d ic e s in the D e c e m ­ber 1960 s a m p l e s . O y s t e r s r e a ch e d a s i x - y e a r h igh in q u a l i t y , but a s in p rev ious y e a r s in d i c e s w ere be low th o s e in the R ap p ah an n o ck .

C om parab le s t u d i e s in the Rappahannock showed in d i c e s c o n ­s i s t e n t l y higher than in the York, o y s t e r s be ing of a v e r a g e or above a v e r a g e q u a l i t y . Again , a s in the York River, q u a l i ty and y i e ld s r e a ch e d a s i x - y e a r high during the w in te r of 1960-61 , a s in d ic a ted by the D ecem ber 1960 s a m p l e s .

- 5 7 -

Table 6 . S t a t i s t i c a l summary of the groups of bottom o y s t e r s p ro c e s s e d ind iv idua l ly for cond i t ion index in th e York and RappahannockRivers

D a te Sta t ionlo c a t io n

Samples i z e

Time in r iv e r - months

M eanc o n d i t io n

index

Value t or 11 df

I . Firs t Se r ie s

1 / 3 / 5 7 Urbanna 50 8 8 . 51 / 8 / 5 7 T i l lage ' s 50 9 . 0 1 .63 98

5 / 2 8 / 5 7 Urbanna 50 13 10 .86 / 3 / 5 7 T i l l a g e ' s 50 11 .8 1 .6 0 98

I I I . Third Ser ies

5 / 2 7 / 5 8 Urbanna 50 3 7 .16 / 3 / 5 8 T i l l a g e ' s 50 11 .6 8 .6 * * 98

5 / 2 7 / 5 8 Ga rrett ' s 50 3 7 .96 / 3 / 5 8 Purtan Bay 50 11 .4 6 .75** 49

8 / 2 8 / 5 8 Urbanna 25 6 8 .68 / 2 7 / 5 8 T i l lage ' s 24 7 . 1 4 .0 4 * * 47

8 / 2 8 / 5 8 G a r re t t ' s 25 6 8 . 88 / 2 7 / 5 8 Purtan Bay 25 8 . 8 48

1 2 / 3 / 5 8 Urbanna 49 9 8 .81 1 / 2 8 / 5 8 T i l l a g e ' s 25 8 . 0 2 .04* 72

1 2 / 3 / 5 8 Garre t t ' s 50 9 13 .51 1 / 2 8 / 5 8 Purtan Bay 24 8 .5 7 .79** 72

IV. Fourth Se r ie s

1 1 /2 /5 9 Urbanna 49 4 7 .51 1 /9 /5 9 T i l lag e ' s 49 6 .3 2 .7 1 * * 96

* P < 0 .05** P < 0 .01

- 5 8 -

Table 7. Cond i t ion index of c o m p o s i te s am p le s from publ ic o y s t e r beds

York River

M i le s above M a r . Jan . A p r . Dec . J a n . Dec .mouth 1956 1957 1958 1958 1960 1960

T i l l a g e ' s* 6 6 .2 7 .4 5 .7 6 .6 5 .6 6 . 7

Green Rock 8 ----- ----- ----- 7 .1 5 .2 -----

Pages Rock 11 6 .1 7 .4 6 . 0 6 . 7 6 .1 8 .6

Aberdeen Rock 14 5 .8 7 .6 5 .8 6 . 0 5 .3 8 . 0

Purtan Bay 18 ----- ----- 5 . 7 ----- ----- 10 .7

Pig Rock 22 4 . 4 ----- 5 .9 ----- ----- 7 .8

Bell Rock Shallow 24 5 .8 5 .7 6 .3 7 . 0 4 .6 9 . 2

Bell Rock D eep 24 5 .7 5 .2 ----- 6 . 7 4 . 4 -----

Average 5 .7 6 . 7 5 .8 6 .7 5 .2 8 .5

Rappahannock River

M i le s a bove N o v . mouth 1956

Dec . 1957

Dec . 1958

Dec . 1959

Dec . 1960

Broad Creek 1 ----- 9 . 0 1 0 .0 ----- -----

Parrots Rock 6 7 .6 7 .6 8 .1 8 .1 10.3

Drummond Ground 12 8 .5 7 .5 8 .5 8 .6 6 . 8

Hogg House 14 7 . 0 9 .4 8 .3 7 .8 12 .3

Smokey Point 19 6 .3 8 .5 9 . 1 7 .4 1 2 .0

Punch Bowl 21 6 . 7 ----- ----- ----- -----

M ora t t ico 24 7 .7 9 .3 8 .6 6 . 2 9 . 8

Bowlers Rock 29 ----- 9 . 5 12.6 8 .6 9 . 7

Average 7 .3 8 . 7 9 .3 7 .7 10.1

* Pr ivate o y s t e r ground

When o y s t e r s from the publ ic o y s t e r ro c k s (Table 7) were c o m ­pared w i th the bottom o y s t e r s p lan ted in the exp e r im en ta l s tudy (Table 6 and F i g s . '2 and 5), the l a t t e r showed h igher in d ic e s in both r i v e r s . D i f f e r e n c e s were sm al le r in the R appahannock than in the York . These d i f f e r e n c e s a re p o s s ib ly due to the g re a t e r a v e r a g e a g e of the o y s t e r s from the na tura l ro c k s .

Ef fec ts of O y s t e r P e s t s on C ond i t ions

In the c o u r s e of the l abora to ry work , mature s t a g e V pea c r a b s (P innotheres o s t r e u m ) were removed from the o y s t e r s a m p l e s and c o u n t e d . This p a r a s i t e h a s b e e n shown to lower c o n d i t io n in dex (H aven, 1959). Rates of in f e s t a t i o n in expe r im en ta l o y s t e r s (Table 8) show th a t p r e s e n c e or a b s e n c e of the p e s t did not a c c o u n t for d i f f e r e n c e s b e tw e e n t ray and Dottom o y s t e r s . For i n s t a n c e , in the R a p p a h a n n o ck , w h e re maximum d i f f e r e n c e s b e tw e e n the two g roups were o b ta in e d , t ray o y s t e r s showed more c r a b s but h ighe r co n d i t io n index than did the bottom o y s t e r s .

In fec t ion w i th D erm ocys t id ium marinum has a l s o been a s s o c i a t e d with lower c o n d i t io n in dex (Ray et ¿U., 19 53) . T e s t s for th i s fungus were made during fa l l c o l l e c t i o n s . R esu l t s (Table 9) in d ic a te d th a t in c id e n c e in bottom o y s t e r s w as only s l i g h t ly h igher than in the t rays . D e ta i l e d s tudy in the s e r i e s of o y s t e r s p r o c e s s e d in d iv id u a l ly in d ic a te t h a t , even if the few in fe c ted o y s t e r s were r em oved , the la rge d i f f e r en c es in C . I . b e tw e e n tray and bottom o y s t e r s would r e m a in . H ow ever , it is e m p h a s iz e d th a t d i f f e r e n c e s in c o n d i t io n index b e tw e e n s t a t i o n s , b e tw ee n r iv e r s , and b e tw e e n y e a r s a re ve ry p robably s u b j e c t to la rg e v a r ia t io n from D e r m o c y s t i d i u m . This s t a t e m e n t is b a s e d on s t u d i e s by Andrews and H ew a t t (19 57), who d e m o n s t r a t ed th a t r a t e s of in fe s t a t i o n with the fungus may vary w i th the s e a s o n and with geo g rap h ic l o c a t io n a n d , a s p re v io u s ly po in ted ou t , i n c id e n c e of the fungus h a s a d e f in i t e in f luence on co n d i t io n in d e x .

Fif th S e r ie s of Tray Bottom S tud ies

D a ta for the fif th s e r i e s were not c o m p le ted in t ime for i n c lu s io n in th is r e p o r t , b u t p re l iminary r e s u l t s c a n be b r ie f ly su m m a r iz e d . In the R appahannock a t U rb a n n a , tray and bottom in d ic e s conf i rmed p r e ­v io u s f in d in g s ; h o w e v e r , both g roups showed in d i c e s h ighe r than in the p rev ious f i v e - y e a r period .

In the York, t ray and bottom o y s t e r s s t i l l m a in ta ined the same r e l a t io n to eac h o ther a s p rev io u s ly o u t l in e d , w ith th e t ray groups a p p a r e n t ly hav ing a s l ig h t ly higher in d e x . In d ice s of the York River

- 6 0 -

Table 8. P e rcen tage of o y s te r s con ta in in g s t ag e V P inno theres os t reum

Sta t ion Tray Bottom

York - T i l lage 'Fi rst Se r ies 1956-57

' s 8 .4 16.7Rappahannock - Urbanna 6 . 5 5 .7

York - T i l lage '

Second Se r ie s 1957-58

' s 17.1 2 2 .2Rappahannock - Urbanna 7 . 5 X

York - T i l lage '

Third S e r ie s 1958-59

' s 8 .1 10.9York - Purtan Bay 9 .0 3 . 6Rappahannock - Urbanna 5 .2 2 .8Rappahannock - G a r r e t t ' s 5 .3 2 .8

York - T i l lage '

Fourth Se r ie s 1959-60

s 8 . 0 5 .6Rappahannock - Urbanna 7 .2 6 .4

Table 9 . W eig h ted in c id e n c e * of Derm ocys t id ium marinum in o y s t e r s

Sta t ion Tray Bottom

Second S e r ie s 1957-58(tes ted O c t . 1957)

York - T i l l a g e ' s 0 .48 -----Rappahannock - Urbanna 1 . 17 -----

Third S e r ie s 1958-59( tes ted S e p t . 19 58)

York - T i l l a g e ' s 0 .0 0 0 .4 0York - Purtan Bay 0 .0 0 0.16Rappahannock - Urbanna 0 .0 0 0 .2 8Rappahannock - G a r re t t ' s ; 0 .0 0 0 .0 0

Fourth S e r ie s 1957-60( tes ted O c t . 1959)

York - T i l l a g e ' s 0 .76 1 .00Rappahannock - Urbanna 1 .12 0 .96

* See Andrews and H ew at t (1957) .

- 6 1 -

o y s t e r s rem ained be low t h o s e in the R appahannock , a s in the p rev ious period; h ow ever , they a l s o r e a c h e d a f i v e - y e a r h igh in q u a l i ty w i th in d i c e s a v e rag in g abou t 1 .5 u n i t s h ig h e r . The only dep a r tu re from the g e n e r a l pa t te rn w a s th a t bo th t ray and bo ttom o y s t e r s in the York showed a high c o n d i t io n index during the fa l l and w in te r of 1961 which w as a lm o s t equa l to the spr ing peak .

DISCUSSION

An a n a l y s i s of c a u s e s of v a r i a t io n in c o n d i t io n in dex w ith s e a s o n or among s t a t i o n s w a s not the primary p u rpose of th i s s t u d y . The in te n t w a s to de te rm ine w h e th e r the re a re s e a s o n a l c y c l e s , and if s o , w he ther s e a s o n a l c y c l e s va ry with geograph ic l o c a t i o n , w i th method of c u l t i v a ­t io n , or from y ea r to y e a r .

The s e a s o n a l co n d i t io n c y c l e s in th e two r iv e rs w e re not s imi la r during m os t of the s t u d y . In the R appahannock the bimodal cu rv e d e m o n ­s t r a t e d by t ray and bottom o y s t e r s w a s th e s am e a s th a t t y p i c a l ly found in C a n a d ia n w a te r s by M edcof and N eed ie r (1941) and in upper C h e s a p e a k e Bay by Engle (19 58) . The r e l a t i o n s h ip of th i s t y p i c a l cu rv e to the s ex u a l c y c l e and to food s to ra g e h a s b e e n d i s c u s s e d by the p re v io u s ly c i t e d au th o rs .

The d o m inance of the spr ing peak of index shown by York River o y s t e r s dur ing the 1 956-1960 per iod did not a g r e e w i th r e s u l t s of a p rev ious s tudy by G a l t so f f e t a_U (1947) on th e p o s s i b l e in f lu en c e of s u l f a t e pulp mill w a s t e s on o y s t e r s in the York R iver . S e a so n a l d a ta from th a t rep o r t , on per c e n t dry w e ig h t of o y s t e r m ea t s for n ine l o c a t io n s in the York River from 193 5 to 193 7, i n d i c a t e d a b o u t eq u a l q u a l i ty dur ing sp r ing and f a l l . In c o n t r a s t , during the p r e s e n t s tu d y , eq u a l i n d i c e s in sp r ing and fa ll w e re in d i c a t e d only during the w in te r of 196 0 -61 , or in on ly one out of the f ive s e r i e s of t e s t s .

The num er ica l v a lu e for index v a r i e d s e a s o n a l l y , bu t pe rhaps the most impor tant c o n s i d e r a t i o n w a s t h a t , r e g a r d l e s s of s e a s o n a l v a r i a t i o n , York River tray and bottom o y s t e r s a p p e a r e d to h a v e a c o n - s i s t e n l y lower index th a n th o s e in the R appahannock dur ing the w in t e r .It is not p o s s i b l e now to g ive r e a s o n s for t h i s v a r i a t i o n , but c e r t a in ly d i s s i m i l a r env i ronm en ta l c o n d i t i o n s a re i n d i c a te d . O ne of the most o bv ious d i f f e r e n c e s is in d i c a t e d by Tab le 1, w h ic h show s a h igher mean s a l i n i t y a t T i l l a g e ’s in the lower York, w hen com pared to Urbanna in the lower R a p p a h a n n o c k . H o w ev e r , i t is not p o s s i b l e to s t a t e th a t s a l i n i t y per s e is r e s p o n s i b l e for o b s e r v e d d i f f e r e n c e s in in d ic e s b e tw e e n the two a r e a s , and u n d o u b ted ly a c o m p lex of env i ronm en ta l f a c to r s a s s o c i a t e d w i th s a l i n i ty a re in v o lv e d .

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The in c r e a s e in the co nd i t ion in d ice s of a l l ex p e r im en ta l and commerc ia l groups in both the Rappahannock and the York during the w in te r of 1960-61 to a s i x - y e a r high is of g rea t i n t e r e s t . It i n d i c a t e s th a t the g enera l ly lower in d ic e s noted during the four p rev ious y ea r s a re not permanent ly a s s o c i a t e d with the two r ive rs . Envi ronmental c h a n g e s may o ccu r w hich in f luence both r ive rs s im u l ta n e o u s ly , but it is noted tha t in d ic e s in the lower York were s t i l l be low th o s e of com parab le groups in the lower R appahannock .

Various f a c to r s have been c o n s id e re d a s r e s p o n s i b l e for the lower in dex of bottom o y s t e r s in co m p ar i so n to t h o s e grown in t rays . H igher in c id e n c e of the o y s t e r c rab (P innotheres o s t r e u m ) and D erm ocyst id ium have been e l im ina ted a s p o s s i b l e c a u s e s . Frequent i n s p e c t io n of the bottom by means of d iv ing gear d e m o n s t ra ted tha t bot tom o y s te r s were never a s heav i ly fouled or a s c rowded a s t h o s e in t r a y s , so t h e s e v a r i a b le s probably do not a c c o u n t for o b se rv ed d i f ­f e r e n c e s . The method by which o y s t e r s were s e l e c t e d for the tray and bottom groups e l im in a te s a g e , s i z e , or v a r ia t io n in sou rce a s c a u s e s of d i f f e r en c es b e tw een in d ic e s . It is c o n c lu d e d tha t v a r i a b l e s not m easured in the p re s en t s tu d y , but w h ich a re a s s o c i a t e d with the bottom env i ronm ent , a re r e s p o n s i b l e .

T h e o re t i c a l ly , it a p p ea r s lo g ic a l tha t bottom o y s t e r s may be s u b je c t to h igher tu rb id i ty loads than tray o y s t e r s , thus r educ ing feed ing ra te and c o n s e q u e n t ly cond i t ion in d e x . Supporting e v id e n c e is g iven by Loosanoff and Engle (1947), Loosanoff and Tommers (1948), and Lund (19 57), who s t a t e tha t an i n c r e a s e in tu rb id i ty c a u s e d by m icroorgan ism s or by inan im a te matter u s u a l l y c a u s e s a d e c r e a s e in pumping ra te a n d , th e re fo re , f e e d in g . Harmful s u b s t a n c e s a c c u m u la te d and r e l e a s e d by bottom d e p o s i t s may be de t r im en ta l to growth , a s o u t ­l ined by Ito and Imai (1955) . Or tray and bottom o y s t e r s may be a f fec ted by d i f f e r en c es in cu r ren t v e lo c i ty or a v a i l a b l e food (Lund, 19 57) .

An important a s p e c t in the co m p ar i so n of tray and bo ttom o y s te r s is th e i r p o s s ib l e u t i l i ty in future s tu d i e s involv ing c o n d i t io n in d e x . It is obv ious th a t both tray and bottom groups may show s im i la r s e a s o n a l t rends a t a s in g le s t a t i o n , bu t the i r l e v e l of co n d i t io n in dex may be co m p le te ly d i f f e r e n t . There fore , in fu ture s tu d i e s the u s e of both groups is c l e a r ly i n d i c a t e d .

Bottom o y s te r s wi l l be of u s e in e s t im a t in g the l e v e l of cond i t ion index in co m m erc ia l ly grown o y s t e r s . H ow ever , b e c a u s e of the v a r ia b le na ture of the bottom, r e p l i c a t e groups should be p laced randomly in each t e s t a rea .

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Tray o y s t e r s should n e c e s s a r i l y be inc luded in fu ture s tu d i e s a s they probably show the p o te n t ia l of th e w a te r in an a rea to produce o y s t e r s of h igher q u a l i t y , e l im ina t ing many "bottom e f f e c t s . " I n a d d i t i o n , d i f f e r e n c e s in l e v e l s of in d ic e s b e tw ee n t ray and bottom groups g ive an e s t i m a te of the i n t e n s i ty of the in f lu en ce of bot tom c o n d i t io n s .

O y s t e r s grown in t rays a re f r equen t ly u s e d by b io lo g i s t s to e s t i m a te growth and m or ta l i ty , and p re s en t da ta in d ic a te tha t t ray o y s t e r s may h ave more meat per un i t of s h e l l c a v i ty than t h o s e grown on the b o t to m . In a d d i t i o n , da ta to be repor ted in a no the r paper in d i c a t e tha t t ray o y s t e r s may h a v e the more rap id growth r a t e . C o n s e q u e n t ly , the p re s en t s tudy i n d i c a t e s tha t r e s u l t s o b ta in e d from t ray s tu d i e s should not be r e l a t e d to b o t to m - cu l tu red groups w i thou t an a d e q u a t e k n ow ledge of the p o s s i b l e d i f f e r e n c e s b e tw ee n the two groups .

S e a so n a l in format ion o b ta in ed from bo th t ray and bottom s tu d ie s wi l l a id com m erc ia l g ro w e r s . Andrews and H e w a t t (1957) s t a t e th a t James River s ee d o y s t e r s tend to r e a c h maximum b io m a s s on the b a s i s of s h e l l w e igh t in Ju n e , abou t 24 to 36 months a f t e r s e t t i n g . In the York River, co n d i t io n index g e n e r a l ly r e a c h e s a s e a s o n a l h igh dur ing June and c o n s e q u e n t l y y i e l d s and meat q u a l i ty a re h i g h e s t dur ing th i s m on th . The re fo re , sp r ing h a r v e s t of York River o y s t e r s is c e r t a in l y i n d ic a ted .

In th e R ap p ah an n o ck , da ta s u g g e s t tha t y i e ld s of m ea t s per b u s h e l and q u a l i t y w i l l be ab o u t eq u a l dur ing spr ing and f a l l . T h e re ­fore , o y s t e r s shou ld be h a r v e s te d in June if t h i s s e a s o n c o r re s p o n d s w i th maximum b io m a s s . If h a r v e s t is d e la y e d un t i l f a l l , then it should not be a t t e m p ted befo re O c to b e r .

Although sp r ing h a r v e s t of o y s t e r s is m os t p r a c t i c a l for b i o ­lo g i c a l r e a s o n s , c o n su m e r demand may be low in the spr ing when o y s t e r s a re in peak c o n d i t i o n . A p o s s i b l e so lu t io n to th i s dilemma would be to f r e e z e or p r o c e s s o y s t e r s h a r v e s t e d in June for co n su m p t io n in w in te r w h en dem and is h ig h .

ACKNOWLEDGMENT

The au thor w i s h e s to e x p r e s s h is g ra t i tu d e for th e a s s i s t a n c e of M r . John N o rc ro s s of th i s l abora to ry in the s t a t i s t i c a l s e c t i o n of th is pape r , and to M is s Evelyn W e l l s for her v a l u a b l e c r i t i c i s m . Thanks a re a l s o g iven to D r . J . D . Andrews for a d v ic e and he lp during th i s p r o j e c t .

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LITERATURE CITED

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Beaven, G . F . 1946. Effect of S u sq u eh an n a River s t ream flow onC h e s a p e a k e Bay s a l i n i t i e s and h i s to ry of p a s t o y s te r m o r ta l i t ie s on upper bay ba rs . Proc . N a t l . S h e l l f i s h . A ssoc . 1 9 4 6 : 3 8 - 4 1 .

Engle , J . B. 1946. Commerc ia l a s p e c t s of the upper C h e s a p e a k e Bay o y s te r ba rs in the l igh t of r e c e n t o y s t e r m o r t a l i t i e s . Proc .N a t l . S h e l l f i s h . A s s o c . 1 9 4 6 : 4 2 - 4 6 .

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H a v en , D . S . 1959. Effects of pea c r a b s P inno theres os t reum ono y s t e r s C r a s s o s t r e a v i rg in ica . Proc . N a t l . S h e l l f i sh . A ssoc .49 (1958): 7 7 - 8 6 .

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Loosanoff , V . L . and J . B . Engle . 1947 . Effect of d i f f e ren t c o n c e n ­t r a t io n s of mic roorgan ism s on the feed ing of o y s t e r s (O . v i rg in ica . ) U . S . F ish & W i l d . Serv . F ish . B u l l . 51 : (42): 3 1-57 .

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Loosanof f , V. L . and F . D . Tommers . 1948. Effect of s u s p e n d e d s i l tand o th e r s u b s t a n c e s on ra te of feed ing of o y s t e r s . S c ie n ce 1 0 7 : 6 9 - 7 0 .

Lund, E . J . 19 57 . A q u a n t i t a t i v e s tudy of c l e a r a n c e of a turb id medium and feed ing by the o y s t e r . P u b l . I n s t . M a r . S e i . U n iv . T exas 4 : 2 9 6 - 3 1 2 .

Lunz , G . R . 1941. P o ly d o r a , a p e s t in South C aro l ina o y s t e r s . J . E l isha M i tc h e l l S e i . Soc . 5 7 : 2 7 3 - 2 8 3 .

M e d c o f , J . C . and A . W . H . N eed ie r . 1941. The in f lu en ce of t e m ­pe ra tu re and s a l i n i t y on the c o n d i t io n of o y s t e r s (Ostrea v i r g i n i c a ) . J . F ish . Res . Board C an a d a 5: 253-257 .

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