Flmdevlgen rapportser., 1 , 1984. ISSN 0333-2594 Tbe Propagatlo~ of Cod
G a d u smorhua L.
A RELATIONSHIP BETWEEN THE NUMBER OF GROWTH INCREMENTS ON THE OTGLITHS AND AGE OF LARVAL AND JUVENILE COD.
G a d u s morhuaL.
O.A. Bergstad
Department of Fisheries B ~ o l o g y , University of Bergen P.O. Box 1839, N-5011 NORDNES Norway
ABSTRACT
Bergstad, O.A., 1984. A relationship between the number of growth lncrements on the otoliths and age of larval and juvenile cod,
G a d u s m o r h u aL. In: E. Dahl, D.S. Danielssen, E. Moksness and P. Solemdal (Editors), The Propagation of Cod G a d u s
% c r h u a L. Fl~devlgenrapportser., l , 1984:251-272
Astudy of the relatlonshlp between the number of growth Increments on the otolrths and age was based on larval and juvenile cod,
G a d u s m o r h u cL., from labcratory and pond reared populatlons.
Lapllll were larger than saglttae at hatchlng and remalned so for approximately 25 days. Saglttae became slgnlflczntly larger than lap1111 at the end of the larval perlod. Major growth trends of the otollths corresponded to the growth rn standard length reported for the populatlons. Marked elongatron along the anteroposterlor axls and formatlon of marglnal lobes started lmmedrately prlor to metamorphosrs.
Comparrson of otollth dlameter and the dlameter of the flrst, usually very dlstlnct, growth lncrement lndlcated that zone formatlon started somewhat before hatchlng. The dlarneter of the frrst rncrement was hlgher rn lap1111 than In saglttae.
The number of growth Increments, as recorded uslng llght mrcroscopy, dld not appear to be a constant functlon of larval age between hatchlng and
metamorphosisat age 35-40 d. The larger of the otolrths at any glven age, elther the lap1111 or the saglttae, showed hlgher lncrement numbers than the smaller.
There was some lndrcatlon that the rate of Increment formatlon was related to the lndlvrdual growth rate of the larva In this perlod.
In addltlon
K Othe prxmary lncremenrs recognize? In the lar-
val otallths,
compositebroader zones appeared after metamor-
phosls. These broader zones were on the average formed dally ar
least untrl 140 days after hatchrng. The freauency of deposr-
~ p p e a r affected by ~ n d l v l d u a l ticn of these structures
did
not -growth rate varlatron.
INTRODUCTION
Studies of the early llfe hlsrory of cod, G a d u s m o r h u o
L.,
and other specles have always been limlted by the lack of adeauate methods for determlnlng the exact age of individual larvae or j u v e n ~ l e s . Knowledge of age forms an essentlai basls for studles of the recruitment mechanisms of narural f ~ s h stocks; it 1s requlred for estlmatlng gro%,th and ~ o r t a l l t y rates as well as for obtaining knowledge of the age structure of a cohort in lts first year of llfe. Tradltlonally, morpho- logical and morphometric d a t a , k ~ o w n frow experiments deslgned to study developmental rates under controlled condltlons, have been used to estrmate an approximate age. Thls approach may only seldomiv give verv reliable esriwates, since it 1s highly dependent on knowledge of temperatores experienced bv rhe larvae In the past and slnce precision is acceptable for most uses witnln a usually very narrow age range.Observations lndlcate that otolltns of larval and juvenlle flsh contai? a number of growth increments c s r r e s ~ o n d i n g to the average age, measured as rhe number of days since an early stage In thelr llfe hlstcry (Pannella, 1371, 1974; Brothers et al., 1976; Taubert and Coble, 1 9 7 7 - Barkman, 1978; Schmldx and F a b r ~ z r o , 1 9 8 0 - kadrke and D e a r , 1982; 17ictcr and 9rorhers, 1382; and others). Countlng growth rncrements should then form the basxs for potentially very preclse and rellable ageing In cases where other methods Drove inadequate.
A
routine appllcatlon of lncrement countlng for age determl- natlon rests on several assumptions, among orhers the follow- lng:a) lncrement formation 1s inrtiated at approximately the same age in all larvae.
bl
lncrement deposlrlon rate follcking this age equals 1 Increment per day ( ~ n c r . d-l) 1n cne aqe lnterval srudjed, rsi n d e p e n d e r i t of qinMrn i a r c c f t h e f i s b c s w l r h i q u r d e I ~ r n ~ r s a r a i s u n a f f e c r e d b y r b e e n v i r o n x e n r a l v a r l a b l l i t v l l k e l y t o b e f o u n d i n r h e n a r u r a l n a b l t a r .
Some s t u d l e s h a v e shown r h a t t h e s e assumptions may n o t a l w a y s b e v a l l d , and h e n c e t h a t f o r some s p e c l e s t h e r e may b e a d e f l n i r e n e e d f o r assessing t h e l i r n r t a r l o n s i n h e r e n t I n t h e m e t h o d . T h u s , a g e a t f i r s t i ~ c r e m e n t f o r m a t r o n may v a r y some- w h a t ( P e n n y and A n d e r s o n , 1 9 8 1 ; L a r o c h e e t a l . , 1 9 8 2 ) , d e p o s r -
- 1
t l o n r a t e may deviate f r o m l ~ n c r - d - e v e n d u r l n g p e r r o d s o f p o s l c l v e g r o w t h ( B l a c k e r , 1 9 7 5 ; P a n n e l l a , 1 9 8 0 : G e f f e n , 1 9 8 2 : N e l l s o n a n d G e e n , 1 9 8 2 3 , and s t r u c t u r a l g r o w t h r a r e v a r l a t l o n w l t h l n n o r m a l l e v e l s may a f f e c t d e p o s l t l o n r a t e ( G e f f e n , 1 9 8 2 ) .
P r e v r o u s s t u d l e s o f c o d f o r t h e p u r p o s e o f v a l ~ d a t l n g l n c r e - menr c o u n t l n g on o t o l l t h s a s a n a g e l n g method c o v e r e d t h e v e r y e a r l y a n d t h e l a t e p a r t s o f t h e l a r v a l p e r l o d ( R a d t k e a n d Walwood, 1 9 8 0 ; G ] @ s a t e r , 1 9 8 1 ; G]@sater a n d T l l s e t h , 1 9 8 2 ) . The f l r s t two s t u d l e s w e r e b a s e d on l a r v a e o f known a g e r e a r e d I n t h e l a b o r a r o r y , r n a l a r g e o u r d o o r b a s l n o r a n a t u r a l p o n d , w h e r e a s t h e i h ~ r d i n c l u d e d w r l d l a r v a e s t a g e d a c c o r d l G g t o morphological c r l t e r l a . The r e s u l t s l n d l c a t e d t h a t r n c r e m e n t f o r m a t r o n p r o c e e d e d a t a g e n e r a l r a t e o f 1 l n c r - d - l , t h u s supporting P a n n e l l a ' s ( l 9 7 1 ) suggestion t h a t t h e o b s e r v e d 2 3 6 0 p r l m a r y u n r t s ~ n o t o l l t h s ( s a g l r t a e ) o f young c o d r e p r e - s e n t e d d a i l y g r o w t h l n c r e w n t s .
The p r e s e n t s t u d y was m e a n t a s a 7 a t t e m p t t o t e s t t h e hypo- thesis o f d a l l y I n c r e m e n t f o r r n a t l o n o v e r a w l d e r a g e ~ n t e r v a l . P s s e s s r n e n t of t h e magnitude o i d e v i a t l o r s f r o w t h l s p a t t e r n was made I n o r d e r c o e v a l u a t e how r e l l a b l e I n c r e m e n t c o u n t l n g c a n b e e x p e c t e d t o b e f o r a g e l n g l a r v a e a n d juveniles f r o m n a t u r a l p o p a l a t l o n s . I was a b l e t o r e l a t e t h e number o f I n c r e m e n t s t o r e a l a g e t h r o u g h t h e s u a p l y o f l a r v a e f r o m r e a r r n g e x p e r l m e p t s i n a n a t u r a l pond s u p p l e m e r t e d b y g r o u p s k e p t I n t h e l a b o r a - c c r y .
M P T E 2 I R L S A Y D t4ETHODS
S a m p l e s of l a r v a l a n d
luvenlle
(post-metamorphic) c o d w e r eo b t a l n e d f r o m p o p u l a t i o o s s t o c k e d I n a 6 0 . 0 0 0 n3 n a t u r a l pond a t A u s r e v o l l , k e s t e r n Norway I n l 9 8 0 a n d 1 9 8 1 ( O l e s t a d a n d K v e n s e t h , 1 9 8 1 ; K v e n s e r h , 1 9 8 3 ; K v e n s e r h a n d O l e s t a d , 1 9 3 4 ) . The mean t r m e o f h a t c h l n g was known I n b o t h y e a r s . I n 1 9 8 1 h a t c h l n g o c c u r r e d w l t h l n a p e r l o d o f 5 d a y s ( 7 0 % o f l a r v a e w l t h i n
+
1 d a y o f t h e d a y o f 50% h a t c h l n g ) , a r d t h e s a m p l e s c o n t a r n e d l a r v a e o f mean a g e o f 8 - 1 4 0 d a y s a f t e r h a t c h l n g . I n 1 9 8 0 t h e r a n g e i n h a t c h l n g t l m e was w l d e r ( 8 - 9 d a y s ) a n d t h e a g e - r a n g e a n d s l z e o f s a m p l e s l e s s a d e q u a t e . T h u s , m o s t r e s u l t s a r e b a s e d on t h e 1 9 8 1 s a m p l e s .S r n c e y o l k s a c l a r v a e w e r e n o t l n c l u d e d l n t h e pond s a m p l e s , e g g s f r o m s t r l p p e d f e m a l e s w e r e incubated a t ~ O C ( 2
1°c)
a n d a p h o t o p e r l o d o f 1 2 h o f l l g h t a n d 1 2 h o f d a r k n e s s ( f l u o r - e s c e n t l a m p s ) I n 2 1 c y l l n d r l c a l b e a k e r s . S o r t i n g a t t h e o n s e t o f h a t c h l n g p r o v i d e d g r o u p s o f l a r v a e h a t c h e d w l t h i n a 24 h p e r l o d f o r u s e I n t h e o t o l l t h s t u d l e s . D e v e l o p m e n t w l t h r e g a r d t o y o l k absorption, g u t d l f f e r e n t l a t l o n a n d b o d y g r o w t h ( s t a n - d a r d l e n g t h a n d myotome h e l g h t i was a s d e s c r r b e d b y E l l e r t s e n e t a l . ( 1 9 8 0 ) . No f o o d was o f f e r e d , a n d m a s s starvation o c c u r r e d a f t e r 11-12 d a y s .
7 0 - 95% e t h a n e 1 (pH % 9 d u e t o a d d l t l o n o f NaOH) was u s e d a s t h e p r e s e r v a z l v e f o r f l s h o f a l l l l f e s t a g e s . P r l o r t o d l s - s e c t l o n , t h e l a r v a e w e r e rmmersed l n a d r o p o f d l s t l l l e d w a t e r o n a c l e a n g l a s s s l l d e . E x t r a c t l o n o f l a r v a l o t o l l t h s was d o n e w l t h l n s e c t n e e d l e s m o u n t e d o n wooden r o d s w o r k l ~ g a t 25-40 X m a g n r f l c a t l o n u n d e r a d l s s e c t l n g microscope ( r e f l e c t e d l l g h t , b l a c k b a c k g r o u n d ) . J u v e r l l e o t o l l t h s w e r e most e a s l l y f o u n d a f t e r a f r o n t a l s e c t r o n o f t h e h e a d j u s t d e e p e n o u g h t o e x p o s e t h e b r a l n had b e e n made.
T h e o t o l l t h s w e r e ~ d e v t l f l e d a s s a g l t t a , l a p l l l u s a n d a s t e r - r s c u s a c c o r d ~ n g t o c h e i r p o s l t r o n I n t h e o t l c o r g a n . A l l s l x o t o l r t h s w e r e e x t r a c t e d wheq possible, a l t h o u g h m o s t attention was f o c u s e d on t h e two l a r g e r , t h e l a p l l l u s ( u t r l c u l a r o t o l l t n ) a n d r h e s a g r t t a ( s a c c u l a r o t o l l t h i .
L a r v a l o t o l l r h s w e r e d r l e d a t 60°c f o r t h r e e o r more
h o u r s , mounted I n a c l e a r synthetic m o u n t l n g medlum
(EPON,
P r o - T e x x ) , c o v e r e d w l t h a c o v e r s l l p and l e f t t o h a r d e n o v e r n l g h - ci 6 0 O ~ f o r E P O N , room t e m p e r a t u r e f o r P r o - T e x x ) . O t o l l t h s f r o m
o l d e r L a - v a e (SL
>
7 - 8 r n d w e r e r r c u n r e d w l t h t ? c l y c o n c a v e s r d e s D o l r r l n g u p w a r d s .S a g i t r a e f r o m juveniles r e q u l r e a g r l n d l n g t o e x p o s e i r t c r e - m e n t s I n t h e l r c e n t r a l r e g l o n . S a g r t t a l , f r o n t a l a n d r r a n s v e r s e s e c t r o n s , f o l l o w i n g B a n n e l l a ( 1 9 8 0 3 , w e r e made o f s e l e c t e d s a g l t t a e t o f a c r l l t a t e c o Q p a r l s o n . T r a n s v e r s e s e c t l o n s a t tqe f o c a l l e v e l a p p e a r e d t o g r v e t h e m o s t c o m p l e t e p r c t u r e of t h e t o t a l g r o w t h o f t h e o t o l l r h a n d w e r e chose^ f o r r o u t r n e u s e . Corresponding s e c t l o n s w e r e described b y S r e f f e n s e n ( 1 9 8 0 ) .
Whole s a g r t t a e w e r e embedded I n r e c t a c g u l a r b l o c k s o f EPON.
A d e n t r s t ' s d r l l l e a u l p p e d w l t h f l n e - g r l t f r n l s h l n g d l s c s ( 3 % S o f - L e x ) was u s e d f o r g r l n d l n g , w o r k l n g u n d e r a d i s s e c t i n g microscope. E a c h o t o l l t h was g r o u n d f r o m t h e a n t e r l o r t l p t o w a r d s t h e p l a n e o f t h e f o c u s a n d m o u n t e d k l t h t h e g r o u n d s u r - f a c e a g a l n s t t h e s l l d e . 4 f t e r hardening, a t r a n s v e r s e s e c t r o n c o u l d b e o b t a l n e d by g r r n d l n g f r o m t h e posterior t r p t o w a r d s t h e p l a n e o f t h e g l a s s s l l d e . The g r o u n d s e c t l o n s w e r e f l n a l l y mounred i n E P O k a n d c o v e r e d w l t h a c o v e r s l l p .
Makrng m e a s u r e m e n t s on o r o l l t h s u s l n g l i g h t n ' l c r o ~ c o p v 1s d l f f l c u l t d u e t o t h e l r s h a p e s a n d s r r u c t u r a l p r o p e r c l e s . L a r v a l l a p l l l r a n d s a g l t r a e a r e a l m o s t h e m r s p h e r l c a l a n d t h u s a p p e a r circular. 13 s l m p l e m e a s u r e m e n t of t h e l r l o n g e s t d i a m e t e r was f o u n d a c c e p t a b l e f o r d e s c r l b l n g c b e l r g r o w t h p r l o r t o mctamor- p h o s l s
(SL
a 1 2 m m ) . J u v e n i l e s a g l t t d e h e r e m e a s u r e d b e f o r e g r l n d l n g a l o n g r h e l r l o n g e s t a n t e r o p o s t e r r o r a n d d o r s o v e n t r a l a x e s .A r l n g a w p e a r l n a c l o s e r o t h e n u c l e u s was u s u a l l y m a r k e d l y more prominent t h a n t h o s e f o l l o w - n g a n d h a s c h o s e n a s t h e s t a r t r n g p o l n r when counting. The r r n g was c l a s s l f l e d a s ' d l s - r l n c c " o r ' r n d l s t l n c t ' , and n e a s u r c d a s t h e l o n g e s t d l a m e t e r d r a w r t n r o a g h t h e n u c l e u s .
Two k l n d s of increments h e r e d e i r n e d f o r l a r v a l a n d j u v e n l l e o t o i l r h s r e s p e c t r v e i y . L a r v a l o t o l l t h s c o r t c a l n e d increments
c o n s l s r i n g o f a r a r r o w d a r k b a ~ d a n d a w i d e l l g i - t b a n d , s i m l l a r a n d p r o b a b l y c o r r e s p o n d - n g t o d i s c o n r r n u o u s aqd ~ n c r e m e o t a l z o - e s ( 7 a . n a ~ a e t a l . , 19811. J u v e n r i e s a g - c t a e - o r r a r n e J wider c o m p o s l r e a r o ' , t h r r c r e m e n c s , e a r h c o 1 5 1 s t l q q o f
I
-S
3 f t h e n a r r o w e r u n - r ~ s e e n l nthe
i ? r v a L o t o l l t h s .The number of increments on larval otollths was accepted as the maxiaum number of single concentric units ( a s defined above! fro^ the first increment to the edge. In juvenile
sagittae the maximum number of composite bands from the central region to the edge was registered. (The central region is not equivalent to rhe nucleus, rather to the part of the otolith formed prior to metamorphosis).
Counting error, excluslsn of otoliths
Initial tests demonstrated that a standardization of the counting procedures was necessary to obtain acceptable corres- pondence between different persons' counts of increments on the same otoliths (Bergstad,
19833.
Deflning increments according to the prlncinles mentioned and r e s t r ~ c t i n g the choice of mag- nlfication to 400X
for all counts signiflcant3.y reduced varla- tion between counters. l70 Larval otollths were counced by two persons ~ n d e p e n d e n t l y . The otolrths were presented in a random sequence (with respect to age) and larval age \<as not known to the counters. In 6 5 % of the counts the devlatron xas less or equal to one increment, in 83% less or equal to rwo increments while92%
of the counts differed by three or less Increments.A
randomly selected group of ntoliths was counted bp a chird per- son who obt-ained results agreeing well wrth those obtained by the main counters.Ail s a g r t ~ s e from :cven~les (sectlocs) xere coucted by two persons ~ ~ d e p e n d e n x l y . Ce:~lztions varied greatly around a mean of 9 - l0 incremerts ( l 0 - 20% s-,f t'nt meen cumber of c o m p o ~ ~ . t e zones). Large devrarinns couiZ tisKzLly be ex~la!fied by less
;ha?% owtlrnaL q u a l ~ t y of the secirans, zLi-defined trarsltlon between larval increments (central region) an6 the first broad
~ncrerner~l~~; or ~bv:oi:s '"i~s:ri'erprtta~ ~ n a s of str~uotuies i r cnrts 05 tile ci-olit!:~.
Ci:3l?ths bavinq ibnorrnal .chape, rhose 5roken thr9,xqh h a ~ d l - inq a? a r y o o r r i of the prepzrarron ,?nd tkose
+;b?ch
poorseparation made increments visible in parts of the radlus only or made deviation between independent counts exceed certain limits were excluded from further studies. Limits were set ac- cording to the number of increments estimated from two counts.
Thus, when the estimated number was less than or equal to 1 5 , maximum acceptable deviation was l increment, whereas when the estimated number was higher no more than a deviation of
2
in- crements was accepted.In
cases where deviations exceeded these limits, the otoliths were counted again, this time allowing discussions between the counters.Around 1.5% of the otoliths were classified as abnormal. Of otoliths from the pond larvae (1981) not excluded due to abnor- mality or breakage, 15% were excluded since their quality was too poor to allow any estimate of Increment number to b e made.
in addition, 8% were omitted since deviations exceeded limits.
( I t should b e noted, though, that exclusion of an otolith through this procedure did not necessarily mean exclusion of a larvae, since other o ~ o l l t h s of tie same individual often showed acceptable quality).
it was found systematically that otoliths from laboratory kept Larvae had less drsticct Lncrernents that those from larvae from ~ h e pond, agreeing with observatrons made
by
others(Geffen, 1982; Bailey, 1.982; iaroche et al., 1982). This, to- gether with thelr smaller slze often leading to fracture durlng handling* led to higher countrng error and higher numbers of ornrtted otoil~l!s from these groups.
Counts of c o ~ p o s r t e zoQes I F Juvenrle saqittae were made on the s e c t ~ o n havrnq the best quailty,
?:bus
only one otolith per fish (erther the left or rhc rrqht sagrtta) was trezted. The sectlon was drscussed when deviailons between the counts by two persons exceeded15
Increments ( 4 out of 22 sectlons from f ~ s h older than 99 days). All sections were accepted after common treatmen? srnce cbvrous mislnterpretatrons fully explained rhe extensive l n l r i al deviation.DeVelopmePc and growth of otollths
The e m b r y o g e ~ e s l s of the otollths of cod has been descrlhed
by
Dale
( 1 9 8 4 1 . P I ; Larvie rarcled wlrh plar?ocnnvex l a p ~ l i ~ and saglrtae, >*bereas rne asrerzscl were elther very small or nor formed at thls srage. "he laprlll were alwavs larger thar sagltrae ar hatchlng and urril a standard length of approxl- mateiy 6 mm was reached (age s25
days I? thrs case) (Table l).In
the iarer parrs of rhe larval ~ e r l o d , the sagltra became the larger an6 gradually became elongated along lrs anteropost- erior axrs. &t metamorphosis the flrsr margrnal lobes appeared, and ar age 90 days 11 alreadvresembles
the adult saqlcta. Tbe laprllus ?vd the asterlscus slmllarly attarned rhe shapes seen 1n adulcs followrng meramorphosls.Both the larger otoliths grew slowly the flrst 15-20 days compared wlth the rates seen 1v the late larval perlod and after metamorphosrs. Thrs corresponded wlth the changes rn body growth rate of the larvae as reported by Kvenserh (1983) and
@restad and Kvenseth (19811.
Formation of flrsi lncrement
A
promrnent rrcrement close to the nucleus was present on almost all otollths from the pond larvae (91 out of 9 8 larvae had this characteristic on at least 3 out of 4 otclrths). Thls may correspond to a dlstlnct ring observedby
transmission electron microscopy of otoliths from newly hatched larvae( D a l e , 1984). Increments were found closer to the nucleus in only
2
- 3 cases w h ~ c h rndrcated that the heavy rlng usually marked che inrriatlon of Increment format-ion.The first lncrement !.was not always as well deflned on the otollths from the laboratory larvae. Only about 60% of the larvae showed thls characterlsrlc. Tkrs may reflect differences in rearlng condltrons.
In the laboratory larvae the fractlon of larvae having a dlstlnct lncrernenr did not lncrease wlth age from harchrng towards the end of the yolk sac perisd, nor did lts posrtlon relatrve to the focus shou much varlatlon. Also, otcllths wlth or ~ l t n o u r a dlstinct flrst rrng showed the same number of ln- crements a t yolk absorprion. The dlameter of the otollths at hatching, as indlcared r n Table
2
and as measuredby
Radtke andMean dlamerer of otollrhs r e l a ~ e d to mean age. s srandard devlatlon, n
-no. of larvae
a )
Cod larvae from pond and laboratory groups
Age (days) Group
:*leanotolith diameter
(pm)Lapillus s n Sagitta s n
Lab.Pond (81)
b j Juveniles o f
cod from pond experlmenxs, 198i
Age (days) &lean dlarnecer of sasrtca
(mm)Anteroposterlal
s nOorsovenrral s n
a ) T h e n e a n d l a m e t e r o f t n e f l r s r d l s t l r c t i n c r e m e n t o n o t o l l t h s f r o m c o d l a r v a e f r o m p o n d a n d l a b o r a t o r y g r o u p s .
D
- Mean d r a m e r e r ( p m , a l l a g e s ) , s - s t a r d a r d d e v l a t l o n ,Q - n o . o f o t o l l t h s
G r o u p S a g l t t a L a p i l l u s
-
-D s n D s n
Pond
( 1 9 8 G ) 1 6 . 1 1 . 9 2 9 2 0 . 9 2 . 2 2 9Pond
( 1 9 8 1 ) 1 6 . 1 2 . 1167
2 1 . 1 2 . 3 l 5 1Lab.
( 1 ) 1 8 . 3 2 . 0 1 5 2 1 . 9 2 . 1 2 5Lab.
( 2 ) 1 6 . 9 2 . 3 2 6 2 3 . 5 1 . 8 2 3b)
T h e mean d l a m e t e r o f t h e o t o l r t h s a t h a t c h l n g . Cod l a r v a e f r o m l a b o r a t o r y g r o u p s . S y m b o l s a s l n a )G r o u p S a g l t t a L a p r l l u s
-
-
D
S n D s nL a b . ( l j 2 0 . 7 1 . 7 1 0 2 8 . 4 1 . 4 9
Lab.
( 2 ) 2 2 . 2 2 . 2 9 3 0 . 6 1 . 7 9W a l w o o d ( 1 9 8 0 ) . a p p e a r s a l w a y s s o m e w h a t l o n g e r t h a n t h e d l a - meter o f t h e f l r s r l ~ c r e m e n t . T \ r s s u g g e s r s t h a r t h e f r r s t l n - c r e m e n t ( b e l t d l s t l n c r o r n o t ) 1s f o r m e d r m r n e d l a t e i y p r l o r t o o r a t h a t c h l n g a s a l s o f o u n d b y R a d t k e a n d Lvarkood ( 1 9 8 0 ) . I t s h o u l d b e n o t e d , t h o u g h , t n a t s o m e l a p i l l i s h o w e d a d i s t i n c t I n c r e m e n t f o l l o w e d b y 2 - 3 I n c r e m e n t s a t h a t c h l n g , w h l c h p r o - b a b l y m e a n s t h a t t h e f o r m a t i o n o f t n e f l r s t r i n g 1s independent
of
t h e p r o c e s s o f h a t c h l n g .T h e r e g u l a r o c c u r r e n c e o f a d l s t i ~ c t s t r u c t u r e c o r r e s p o n d r n g r o r h e e n d o f y n l K a b s o r p r i o n , a s s u q g e s r e d
bv
R a d t K e a n d W a ~ w o o d ( 1 9 8 0 1 , w a s n o t s u p p o r t e doy
my o b c e r v a r _ o n s o f l a b o r a - t o r y a n a p o n d l a r v a e .Increment nunber vs
age -Larvae
No systemarlc
differencebetween left and rrghr otollrhs of the same klnd, wlth regard ro slze or the number of Increments, was found (Bergstad, 1983). Thus, the mean of counts for the tdo was used as a basls for
calculatingzhe age-group averages shown In Flg.
3and Table
3.From the laboratory larvae, only one otolrth of each klnd was examlned (Fig. l).
No dlrect correspondence between the mean number of lncre- ments and mean age was fouqd. Thls was true In both the labora- tory kept and pond reared groups. The only
exceptionwas the average number of Increments on saglttae from 31 days old larvae.
A devlatlon from dally Increment
depositionIn the labora- tory larvae may not be surprrsing, at least not passed day six when most yolk reserves had been absorbed. Thus, most conclu- sions
concerninglncrement deposltlon were based on rhe larvae from the pond.
For larvae yoanger rhan ,bout 18 d , rhe number of lncrements has generally lower than what would correspond ro a dally depo-
TABLE
3Mean number of growrh lncremenrs on sagltta
( S )and laplllus
( L
of cod larvae from pond (1981). Results of t
-tests for
palred
comparisonsw i r h l ~ each age are shown
Age (days) No. of incr.
S L
td.f. P L vs. S
* . V
I ) . * = . c
I
C L A P I LLUSN = 8 O
AGE
(DAYS
AFTER HATCHING )Fig.
1. T h e number of growth increments on the utricular (upper, L a p ~ l l u s ) and saccular ( L o w e r , saqitta) otollths vsage.
Cob larvae from laboratory groups 1 and2
(fllled and open squares respectrvely).N
- ro. of larvaes r t i o n r a t e s i n c e t i l e t i m e o f
50%
i1iitci.3ni;. T h i s c o i l l d e i t h e r b e r e a l ; i . e . t h e o t o l i t h s i n d e e d c o n t a i n e d l e s s increments t h a n e x p e c t e d , o r t h e c o u n t s w e r e b r a s e d t o w a r d s t o o l o w n u m b e r s . T h e l a s t a l t e r n a t i v e c o u l d r e s u l t f r o m s y s t e m a t r c r n - a b i l i t y t o o b s e r v e v e r y f a i n t r c c r e m e n t s . T h i s may b e a s e r r o u sp r o b l e m when t h e o t o l r t h s g r o w a s s l o w l y a s i n d i c a t d .ln T a b l e l ; r h e d a i l y i n c r e a s e I n r a d i u s b e i n g n o m o r e t h a n . 3 5
-.40
p m . S i n c e t h e r e s o l v i n g p o w e r o f t h e m i c r o s c o p e w a s r n t h e r a n g e.35-.45
pm, c h a n c e s a r e t h a t " d a i l y " i n c r e m e n t s , i f p r e s e n t , w o u l d n o t a l w a y s b e s e p a r a b l e . I n d e e d , i n c r e a s i n g t h e r e s o l v i n g p o w e r d o e s g i v e o v e r a l l s o m e w h a t h i g h e r c o u n t s , a l t h o u g h n o t h i g h e n o u g h t o make " i n c r e m e n t c o u n t " e q u a l t o a g e . O n e p r o b l e m i s t h a t i n t h e p r o c e s s o f i n c r e a s i n g r e s o l v i n g p o w e r , c o n t r a s t g e t s l o w e r a n d o p t i c a l a r t i f a c t s t e n d t o r n c r e a s e t h e p o s s i - b i l i t y o f m r s i n t e r p r e t a t i o n . N e v e r t h e l e s s , i t 1s p r o b a b l y n o t s a f e t o c o n c l u d e c h a t t h e o v e r a l l low c o u n t s w e r e e x c l u s i v e l y c a u s e d b y l o w f r e q u e n c y o f deposition. U n f o r t u n a t e l y , I w a s n o t a b l e t o u s e scanning e l e c t r o n m i c r o s c o p y t o c l a r r f y t h r s p r o - b l e m a n y f u r t h e r .P a s s e d d a y 2 0 , h o w e v e r , t h e o t o l i t h s g r e w f a s t e n o u g h p e r d a y t o e x p e c t " d a i l y " r n c r e m e n t s , i f p r e s e n r , i o b e w i d e e n o u g h t o b e r e s o l v e d . An a v e r a g e o f 3 0 i n c r e m e n t s o n s a g i t t a e f r o m t h e l a r v a e o f mean a g e
31
d w o u l d s e e m t o i n d i c a t e a mean d a i l y i n c r e m e n t s d e p o s i t i o n s i n c e h a t c h i n g . A t t h e s a m e a g e , t h e c o u n t s o n l a p i i l ~ s e e m e d t o o l o w . No-,:, b o t h t h e s e f r g u r e s may b e underestimates o f t h e t r u e n u m b e r o f i n c r e m e n t s s r n c e , 3 f t h e a r g u m e n t s m e n t i o n e d a b o v e a r e c o r r e c t , c o o c t s i n t h e r c n e r - m o s t r e g r o n s o f t h e o t o l i t h s a r e t o o l o w . T h u s , r n c r e m e n t d e p o - s i t i o n r a t e a p p e a r e d t o I n c r e a s e a f t e r d a y 2 0 , a l t h o u g i - i p r o - b a b l y n o t a s much a s s u g g e s t e d b y t h e p l o t I n F l g . 2 .On t h e s a g i t t a e , r t a p p e a r s t h a r m o r e t h a n o n e r n c r e m e n t w a s f o r m e d e a c h d a y . Xo o b v l o u s g r o u p i n g r n r o " d a l l y " a n d m o r e d i f f u s e " s u b d a i l y " u n r t s i s e n s c T a u b e r t a n d C o h i e ,
1975:
C a m p a n a a n d N e r l s o n , 1 9 8 2 ; a n d o t h e r s ) s e e m e d p o s s r b l e . A c o m p a r i s o n o f r n d e p e n d e n t c c u n t s o n s a q i t t a e a n d i a p l l l i f r o m th,e s a m e r ~ d r v l d ~ ~ a l r e v e a l e d s r y n l f r c a n r d r f f e r e n c e s ( T a b l e 3 ) . I l a p r l l u s usua!.ly b a d s l i g h r i y m o r e i n c r e m e n t s t h a n s a g r f t a ~n t b c e a r l y l a r v a l stages, w h e r e a s r h e c o n v e r s e w s s
F L ~ . 2. The mean nlmber of
q r o d t nrncremenrs on sagitca and laplllus vs. aye. Cod Larvae from pond experxmenz 11981).
Dots - aqe-gro~~p
mearis,bars - range, -1ertrcal
I r ~ ? e s - z2
s(standard de- atio ion), N - no. of larvae.
true at the end of the larval period. This corresponds wlth the size differences of the two otoliths as seen in Table la.
Counting bias cannot b e excluded as explanation for the slight differences in the early parts, while the differences at 31 d seem real.
The conclusions to be drawn from these studies depend strongly on the magnitude of counting bias during the period of slow groh-th; a factor which also makes age estimates less reliable for routine use. Counting bias can, however, hardly account for all the deviation seen. The average rate of incre- ment formation appears lower than l incr d-l the first
20
d a y s , after which it increases. This contrasts somewhat with indications of an average daily increment formation found by others (Radtke and Waiwood, 1980; GjGszter, 1981: GjGsater and Tilseth, 1982). The discrepancy may not b e very severe, though, since a comparison of the actual data reveals only slight differences (differences In conclusions can stem from the very narrow age intervals covered In the earlier studies).Low rates of Increment deposition during the early larval perrod were reported for herring,
Ciupea harengus,
by Lough et al. (l982 i.
Also, rates hrgher than l incr d-l have been observed (Geffen 1982; Neilson and Geen, 1982).In the study of cod larvae, general rates of Increment for- matlon, otolith growth (Table la) and body growth (Kvenseth, 1983) all increased from a low level rhe first
20
days toa
higher level the rest of the larval perrod. Thls suggests that there is an overzll posltive ccrrelation between oto1it.h growth rate (and hence, larval growth rate) and Increment deposition rate.That otolith growth is correlated with body growth has been shown for several species (Taubert and Coble, 1977; Barkman, 1978; Methot, 1981; Radtke and D e a n , 1982; Neilson and G e e n , 1982: Laroche et al., l9821 and is thus not unexpected.
A
posr- tive c o r r e l a t ~ o n between bcdy growth rate and increment depo- sition rate was demonstrated by Geffen (19821 for herrrng(Clupea harengusl
and turbot (ScophtkaZmusmaximus)
larvae, w k ~ l e Taubert and Coble 11977), Sarkman (1978) and Radtke and Dean (1982) found no such tendencrts for green sunfish[ L z p c m e s e d m n e l l h s i , Arlaqtlc sllversldes ( Y e n z d t a m e n c d z a ) and the m u m m ~ c h o g ( F ~ n d u Z a s h e z e r o c l - 6 ~ ~ s ) respectruely, as long as growth was nor severely suopressed.
The results for cod larvae presented here glve no frrm basrs for concludlrg that a correlatron between otolxth growth rate and rrng deposltron rate 1s rhe main euplanatlon for the lack of llnearrty rn the lncremenr vs age relarronshrp. In chrs cast, ~ o r k l n g wrtn a srngle popularlon onlv, tbe range of counts between larvae of tke same mean age have to b e wxder than what can reasonably be expected from random varratron (countxng error, age varratron a ~ d varlable age at flrst rrng deposrtron) to be able to detect any such correlatron.
The range of counts rncreased from
5
rpcrements at age l5 days (laplllus) to l3 rncrements at age 31 days (saglttal. T o test the consrstency of counts, the two marn counters repeated thelr counts of the 15 days lap1111 and the 31 days sagrttae after about 3 nonths. The counts were hlghly consistent, whlch clearly showed char rhe marn source of Jarlance wlthln an age group has real d r f f e r ~ n c e s rn rlny ~ u m b e r s between lndlvrdual larvae (2.e. not countrng error).Yax~rnum devlarron from mean trme of hatchrng was
+ 2
d a y s , and tne age ar frrst Increment formatron (although not deter- rnlned exactly) appears to vary only sllghtly among larvae.Thus, these factors alone can hardly explarn the extensrve and lncreaslng varlatron observed wlthrn age groups. Thls may rndlcate that real d r f 2 r e n c e s rn rlng numbers among larvae of the same aqe exrst as a consequence of rndrvrdual varlatron ln forrnatlon rates.
As usual, a srze hierarchy soon developed wrthln the popu- latlcns lndrcatrng some rndlvldual varlatron rn growth rate (5lestad and Kvenseth, 1981; Kvenseth, i983). Table
4
shows wlthrn age group correlatlon coeffxcrents between otolrth slze and the number of growth lncrernents ( r 2 rs assumed to relate the grohtn of the otolrrh after hatchlng ro Increment ?umber).Although sample srzes are small and wrrhln age group varratlon rn otollth slze rs Ilmrted, the analysrs suqgests an overall p o s ~ t ~ v e correlatlon between the
variables.
Thus, thls may s ~ r e n g t h e n ,he ~rnpresslon thar rhere isan
effect of ~ n d i v i d u a lCorrelation b e t w e e n o r o l i r h d i a m e t e r a r d number o f I n c r e m e n t s w r t h l n a g e - g r o u p s . Cod l a r v a e f r o m p o n a ( 1 9 8 1 ) .
r l - o t o l l t h d l a m e t e r v s n o . o f I n c r e m e n t s
r - ( o t o l l t h d r a m e t e r - d l a m e r e r o f f l r s t i n c r e m e n t ) v s n o . o g I n c r e m e n t s
*
c a s e s I n w h l c h H. : p = 0 1s r e j e c t e d , p<
0 . 0 5 n - n o . o f l a r v a eAge ( d a y s ) L a p i l l u s S a g i t t a
g r o w t h r a t e l e a d l n g t o h l g h e r formation r a t e s I n f a s t - g r o w i n g l a r v a e ( g e t t r n g l a r g e o t o l l t h s ) c o m p a r e d t o s l o w - g r o w l n g l a r v a e . I t d o e s n o t , h o w e v e r , p r o v e t h a t a c o r r e l a t i o n b e t w e e n l a r v a l g r o w t h a n d r l n g d e p o s l t l o n r a t e 1s a l w a y s p r e s e n t z n c o d l a r v a e , b u t c l e a r l y e r n o h a s l z e s t h e n e e d f o r f u r t h e r t e s t l n g o f t h l s hypothesis.
I n c r e m e n t number v s a g e - J u v e n r l e s
The means o f two p e r s o n s i n d e p e n d e n t c o u n t s o f b r o a d , com- p o s l t e I n c r e m e n t s o n s a g l t t a e f r o m ~ n d l v ~ d u a l p o s t - m e t a m o r ~ h l c c o d a r e shown zn F l g . 3 . The r n c r e m e n t s c o u n t e d a r e s r m l l a r t o t h o s e r e p o r r e d by S t e f f e n s e n ( 1 9 8 0 ) f r o m 0 - g r o u p E a s t B d l t l c c o d , b y Campana and N e l l s o n ( 1 9 8 2 1 f r o m juvenile s t a r r y f l o u n d e r ( P i a t z c h t h y s s z e l l c z u s l
and
b y B a l l e b ( 1 9 8 7 ) f r o m~ a c l f l c h a k e i M e r Z u c c z u s p r o d u c z u s ) . The mean nurnDer of t h e s e s t r u c t u r e s a p p e a r e d t o c o r r e s p o n d t o t h e number o f d a y s p a s t
AGE
(DAYS AFTER HATCHING)F i g . 3. The number o f w l d e c o r n p o s l t e g r o w t h r n c r e m e n t s o n s a g r t r a
vs.
a g e . J u v e n l l e c o d f r o m p o n d e x p e r r m e n t ( 1 9 8 1 ).
D o t s
-
means o f c o u n t s f r o m s l n g l e f i s h , N - n o . o f 2 u v e n r l e s .t h e a v e r a g e a g e o f m e t a r n o r p h o s r s . A s t r a r g h t l l n e f l t t e d t o t h e d a t a had a s l o p e n o t s l g n l f l c a n t l y different f r o m u n l t y , a n d I n t e r s e c t e d t h e a g e a x l s a t 3 7 . 5 d a y s a f t e r h a t c h l n g w n r c h 1s v e r y c l o s e t o t n e a g e o f m e c a m o r p h o s l s ( 0 r e s r a d a n d K v e n s e t h , 1 9 8 1 ; own observation). T h u s , on a v e r a g e , o n e b r o a d r n c r e m e n t was d e p o s r t e d e a c h d a y , a n d t h e s t r u c t u r e s r e c o g n i z e d may b e characterized a s " d a l l y " l n c r e m e n t s s o m e r l m e s c o n t a l n l n g n a r r o w
" s u b d a r l v " l n c r e m e n t s .
The T l a r i a t l o n h r t o r ~ a g e g r o u p s was e o n s l d e r a b l e a n d k a s p r o b a b l y m a i n l y c a u s e d
by
c o u n t r n q e r r o r . D e f i n l n q t h e s t a r t l n g p o l n t was d l f f r c u l t a q d t h e r p c r e m e n L s w e r e r o t a l b a y s d e l lCorrelatlov berween dorsovenrral dlamerer of sagltra and rhe number of dlde growth rncrements wrthrn age-groups. Juvenlle cod from pond
(1981) . n - no. of larvae
Age (days) r n
4
0- 0.395 4
4 7
-0.087
69
3- 0.132 l0
l24 0.422 5
140 - 0.333 7
separated.
S o m e improvementof the prepararron of sectlons may reduce countlng error varrablllty and Importance.
Nevertheless, wlthrn age group varlatson seemed hlgh to be accoupted for by countlng error alone (at least
10the older groups). There could be room for some rndlvidual varratron.
Thls could not be related to varlaticn rn otollth slze ln thrs case (Table
51,even though a wlde range of otollth srzes occurred (reflectrng rhe range of lengths of the flsb).
Studres of otollths from two popularlens of cod larvae left ro feed or natural food sources exposed to amblent llght con- dltlons
17a large natural oond did nor support tke hypothesis of a coqstanr dally deposlrlon of growrh Increments throughout the larval perlod. Thls could, at least partly, be due to In- ability to resolve very narrow lncremencs formed durlng a perrod of i l o ~ ~ growtn. The need for testlng to whrch extent varratron
10nody groc-h rate affects
incrementdeaosrtlon rate
&as demonstrated. Compared wlth growrh and developmenral
patterns seen ln other large systems (Ellertse? et al., 1 9 8 1
qamblc and Houde,
i 9 H S irhe
oondpopdlatlovs agpeared
rqthernormal. Cn rhls oasls ~t must be concluded that lncrernent cobnrs mav nox always glve as unblased age esrlrnates as suq- gested by prevlous studles.
There seems to be some basls for determlnlng the number of days after metamorphosrs by countlng wlde
compositelncrements on
juvenilesaglttae. The preclslon of an age estlmate based on such counts may not be very hlgh slnce, ln addltlon to countlng error, the age at metamorphosls may vary
considerablybetween and wlthin populatlons (Laurence, 1978; Ellertsen et al., 1981;
Kvenseth, 1983).
ACKNOWLEDGEMENTS
I gratefully acknowledge advlce offered by Dr.
J.Gj@sater,
Mr.S. Tllseth, Mr. T. Dale, Mr. H. Gj@sater and Dr. A. Geffen.
I thank Mr. P.G. Kvenseth and Mr. V. 01estad for the supply of materlal from the poqd
experiments,and MISS I.M. Beck, Mr. T.
J0rgensen and my wlfe, Randi, for helpful asslstarce. Thls work was accepted as partlal
fulfilmentof the requrrements for the Cand. sclent. degree rn Flshery Blology at the Unrverslty of Bergen, Norway.
REFERENCES
Balley,
K . M . ,1982. The early llfe hlstory of the Paclflc hake, MerZucezus productus. Flsh. Bull., U.S., 80: 589-598.
Barkrnan, R.C., 1978. The use of otollth growth rrngs to age young Arlantlc sllversldes, P'enzdta menzdza. Trans.
Am.Flsh. Soc., 107: 790-792.
Bergstad,
O.A.,1983. En unders0kelse av relasjonen mellom antall vekstsoner
1otolrttene og alder hos larver og yngel av torsk, Gadus morhua
L.Thesls, Unrverslty of B e r g e ~ , Norway, l21 pp. + app.
Blacker, R.V., 1975. Stereoscan
observationof a plalce otollth.
J.Cons.Int. Explor. Ner., 36: 184-187.
Brothers, E.B., Mathews,
C.P.and Lasker, R., 1976. Dally growth lncrements on otollths from larval a ~ d adult flshes.
Frsh. 9ull., U.S., 74: 1-8.
CamDana, S.E. and Nellson,
J.D.,1982. Dally growth lncrements
11
otollths of starry flounder, (PZattehthys stellatus)
and the r ~ f l u e n c e of some envlronmental varraoles rn thelr
production.Can.
J.Flsh. Aquat. Scl., 39: 937-942.
C o r r e l a t ~ o n berween dorsoventral dlameter of saglcta and the -umber of wrde growth Increments wlthrn age-groups. Juvenlle cod from pond (1981). n - no. of larvae
Age (days) r
nseparared. Some lrnprovement of the preparatron of sectrons may reduce countlng error varlablllty and importance.
Kevertheless, wlthln age group varratlon seemed hlgh to be accounted for
bycountlnq error alo?e (at least in the older groups). There could oe room for some lndlvldual varlatlon.
This could not be related to varlatron in otollth srze in thrs case (Table
5 ) ,even thougfi a wrde range of otollth slzes occurred (reflectlng the range of lengths of the frsh).
Studles of otollths from two p o ~ u l a t i o n s of cod larvae left to feed on natural food sources exposed to ambrent llght con- d l t ~ o ~ s ln a large natoral p o ~ d dld not support the nvpothesrs of a constanr d a l l ~ d e p o s ~ t l o n of gro,@rh Increments throughout the larval period. Thls could, at least partly, be due to In- abrllty to resolve very narrok increwenrs Eorned during
aperlod of slow growth. The need for testlng to which extenr varlatron ln oody growth rate affects increment d e p o s l t i o ~ rate was demonstraced. Compared drth arolwi*~ acd developrnenral Dattern- seen
lrother large sysrems (Ellertsen er al., 1981.
Caxblc and Houcte 1984)
the pondpopularlons appeared rath-r
normal. On tnls basis rt musr be concluaeu tnat lrcrcment cobnts may not always give as
unbiasedage esclmates as sug- gested by prevlous scudres.
There seems to be some basls for determining the number of days after metamorphosls by countlng wlde
compositeIncrements on juvenile saglttae. The preclsron of an age estimate based on such councs may not be very hlgh since, in addltion to countlng error, rne age at metamorphosls %ay vary
considerablybetween and wltnrn populatlons (Laurence, 1978; Ellertsen et al., 1981;
Kvenseth, 1983).
ACKNOWLEDGEMENTS
I gratefully acknowledge advlce offered by Dr. J. G30sater, Xr. S. Tllseth, Mr. T. Dale, Mr. H. Gj0sater and Dr. A. Geffen.
I thank Mr. P.G. Kvenseth and Mr. V. mlestad for the supply of material from the pond
experiments,and Mlss I.M. Beck, Hr. T.
JGrgensen and my wife, Randi, for helpful assistance. Thls work
*as accepted as partlal f u l f i l m e ~ t of the requlrernents for the Cand. sclent. degree In Flshery Brology at the
Universityof Bergen
,Norway.
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