HELGOL~'qDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresun te r suchungen 35, 329-339 (1982)

Entry of migrating American eel leptocephali into the Gulf Stream system

R. C. Kleckner & J. D. M c C l e a v e

Migratory Fish Research Institute and Department of Zoology, University of Maine; Orono, Maine 04469, USA

ABSTRACT: Abundance , size and dep th distributions of Amer ican eel (Anguilla rostrata) lepto- cephali collected in four transects across the Florida Current be tween the Straits of Florida and Cape Hatteras, North Carolina, were examined in order to assess the relat ive importance of two migrat ion routes into the Gulf Stream system. A nine-fold increase in the abundance and an increase in the m e a n length of leptocephal i collected in the Florida Current north of the Straits of Florida indicate tha t most leptocephal i enter the Gulf Stream system directly from the Sargasso Sea ra ther than by a more southern route th rough the Bahama Islands. Leptocephali were concentrated in the upper 140 m at n ight and upper 350 m dur ing the day. Wide vertical ranges in daytime collections prec luded more refined est imation of vertical distribution. The collection of e ight small leptocephal i (11 to 17 mm total length) in the western Car ibbean Sea is discussed in relat ion to the origin of leptocephal i t aken in the Straits of Florida.

I N T R O D U C T I O N

I n f o r m a t i o n o n t h e m i g r a t i o n r o u t e s f o l l o w e d b y A m e r i c a n ee l , Anguilla rostrata, l e p t o c e p h a l i b e t w e e n t h e i r n a t a l a r e a e a s t of t h e B a h a m a I s l a n d s a n d t h e coas t of N o r t h

A m e r i c a is f r a g m e n t a r y a n d i n c o m p l e t e . S u r f a c e c u r r e n t s a r e n o d o u b t r e s p o n s i b l e for

t h e i n i t i a l m o v e m e n t of t h e s m a l l e s t l a r v a e f r o m t h e s p a w n i n g a r e a ( S c h m i d t , 1922,

1925). H a r d e n J o n e s (1968) s p e c u l a t e d t h a t s u r f a c e c u r r e n t s i n t h i s r e g i o n w o u l d ca r ry

A m e r i c a n e e l l e p t o c e p h a l i n o r t h of t h e B a h a m a I s l a n d c h a i n a n d i n t o t h e G u l f S t r e a m

s y s t e m e a s t of C a p e H a t t e r a s , N o r t h C a r o l i n a . A m e r i c a n ee l l e p t o c e p h a l i a r e c o m m o n i n

s u m m e r c o l l e c t i o n s a l o n g t h e m a i n ax i s of t h e G u l f S t r e a m s y s t e m b o t h n o r t h a n d s o u t h

of C a p e H a t t e r a s ( V l a d y k o v & M a r c h , 1975). L e p t o c e p h a l i a re a l so p r e s e n t i n t h e F l o r i d a

C u r r e n t e a s t of M i a m i , F lo r ida , b e t w e e n M a y a n d A u g u s t (Smi th , 1968). It is no t k n o w n

w h e t h e r t h e s e l e p t o c e p h a l i e n t e r t h e F l o r i d a C u r r e n t f rom t h e e a s t v i a t h e O l d B a h a m a ,

N i c h o l a s a n d S a n t a r e n C h a n n e l s or f r o m t h e C a r i b b e a n S e a v i a t h e Y u c a t a n S t r a i t a n d

G u l f Loop C u r r e n t .

T h e s t u d i e s c i t e d a b o v e s u g g e s t a d u a l i n p u t of A m e r i c a n e e l l e p t o c e p h a l i i n to t h e

G u l f S t r e a m sys t em, o n e v i a t h e F l o r i d a C u r r e n t a n d t h e o t h e r f r o m t h e S a r g a s s o S e a

n o r t h of G r a n d B a h a m a I s l and . In o r d e r to a s s e s s t h e r e l a t i v e i m p o r t a n c e of t h e two

m i g r a t i o n rou t e s , w e h a v e e x a m i n e d (1) t h e a b u n d a n c e , s i ze a n d d e p t h d i s t r i b u t i o n s of

l e p t o c e p h a l i c o l l e c t e d a l o n g four t r a n s e c t s c r o s s i n g t h e m a i n ax i s of t h e F l o r i d a C u r r e n t

b e t w e e n W e s t P a l m B e a c h , F lo r ida , a n d C a p e H a t t e r a s , N o r t h C a r o l i n a , a n d (2) t h e

© Biologische Anstal t Helgoland 0174-3597/82/0035/0329/$ 02.00

330 R.C. Kleckner & J, D. McCleave

spec imen and collection data for a group of small Amer ican eel leptocephal i from the Yucatan Strait in the wes te rn Car ibbean Sea, a potent ia l source of leptocephal i for the Florida Current.

MATERIALS AND METHODS

The Florida Current samples were borrowed from the Woods Hole Oceanographic Inst i tut ion collections held by the Depar tment of Fishes, Museum of Comparat ive Zoology, Harvard University, Cambridge, Massachusetts. They were collected be t w e e n 29 July and 16 August 1978, on R.V. Oceanus cruise 49 by R. H. Backus and J. D. Craddock. Four trawl station transects were made perpendicu la r to the historical ma in

8 0 ° 7 5 °

55 °

5 0 °

2 5 °

Cape 9 0 % Romain

" . :%

%,, 80

S t Augustine 96°e% • • • ~8

West Palm B e a c h , 103 ; ~ ,

ee °

h~

Fig. 1. North Atlantic Ocean from Cape Hatteras to the Straits of Florida showing Florida Current transect and sample locations including station numbers

axis of the Florida Current be tween West Palm Beach, Florida, and Cape Hatteras, North Carol ina (Fig. 1). The transects ex tend seaward from slope water, across the Florida Current and into water of Sargasso Sea origin.

All collections were t aken with a 10 m Mult iple O pe n i ng Closing Net and Environ- menta l Sensing System (MOCNESS-10). This system fished a series of nets sequent ia l ly dur ing each obl ique trawl. The mouth area of the MOCNESS-10 was 10 m 2 w h e n the frame of the net was t i l ted to 45 ° dur ing fishing. The ne t t ing was 3.5 m m circular mesh throughout. Sensors on the MOCNESS-10 measured pressure {depth}, speed through the water, frame angle and net open ing and closing. This information was t ransmit ted via

Entry of leptocephal i into the Gulf Stream 331

the towing cable to shipboard displays and recorders. Estimates of the volume of water filtered by each ne t were calculated from this data.

Identif icat ion of Amer ican and European eel leptocephal i (A. rostrata and A. anguilla) followed Smith (1979). Total length was measured from the anterior tip of the teeth to the tip of the caudal fin rays. Total length is approximately 2 to 3 % longer than s tandard length, which is measu red from the tip of the snout to the caudal fin base (F.-W. Tesch, personal communicat ion) . Specimens were fixed and preserved in 10 % formalin.

Crude est imates of the relat ive a b u n d a n c e of Amer ican eel leptocephali at each trawl station were calculated as follows: (1) only nets with all or part of their sampl ing effort in the upper 140 m are inc luded in the estimates (see "Results" for vertical distr ibution range), (2) estimates of the water volume filtered for nets inc lud ing a sampl ing component be low 140 m were proportionally reduced to inc lude only the percentage fi l tered wi th in 140 m of the surface, and (3) the total n u m b e r of leptocephal i collected by all trawls at a station were divided by the total est imated water volume filtered in the upper 140 m by all nets fished at the station.

Tempera ture -dep th data from expendab le ba thythermograph casts and characteri- zations of the associated mesopelagic fish fauna provided by J .D. Craddock were ut i l ized to def ine the water mass sampled at each station. The Sargasso Sea fauna was characterized by the presence of Pollichthys mauli (Gonostomatidae) and Lepidophanes gaussi (Myctophidae), the Gulf Stream system fauna by tropical species, such as Lepidophanes guentheri, Diaphus dumerillii, and D. luetkeni (Myctophidae), and the slope water fauna by temperate species, such as Ceratoscopelus maderensis and Ben- thosema glaciale (Backus et al., 1970, 1977).

For analyses of length and relat ive abundance , samples of leptocephal i were grouped wi th in transects by their associated types of mesopelagic fish fauna. Those samples whose fauna inc luded slope water fauna or Sargasso Sea fauna as wel l as Gulf Stream fauna were grouped with samples whose fauna was a pure Gulf Stream one. A two-way analysis of variance of m e a n length by transect and faunal assemblage type was conducted us ing the genera l l inear model for u n b a l a n c e d data. Because the interact ion be tween transect and faunal type was signif icant (P < 0.001), the following one-way analyses of var iance were done: length by transect for Gulf Stream faunal type only, l ength by transect for Sargasso Sea faunal type only, and length by water type for each of the three nor thern transects. Duncan ' s test was appl ied if there were more than two levels of the i n d e p e n d e n t variable. Significance criterion was P < 0.05. Similar analyses were not performed on the relative abundances , because the estimates of a b u n d a n c e are crude.

Spec imen and collection data for the Car ibbean Sea samples were provided by D. G. Smith of the Universi ty of Texas Mar ine Biomedical Institute, Galveston, Texas. These samples were collected with a two-meter r ing net of 570 ~m Nitex mesh at 20 ° 30' N, 87 ° 00' W be tween the Yucatan Pen insu la and Isla Cozumel.

RESULTS

Amer ican eel leptocephal i were concentra ted in the upper 140 m at n ight in the transect collections. Ninety-six percent of all specimens taken in nets sampl ing discrete depth ranges at n igh t were collected in this layer (Table i). Unfortunately, the narrowest

332 R . C . K l e c k n e r & J. D. M c C l e a v e

Table 1. Vertical distribution of American eel leptocephali in MOCNESS-10 collections in the Florida Current

Depth range Number of Volume of water Catch rate (m) American eel filtered (No./104 m 3)

leptocephali (104 m 3)

Day

Night

0 - 350 65 (91.5 %) 36.8 1.77 340- 750 5 (7.0 %) 89.6 0.06 670-1200 1 (1.4 %) 52.4 0.02

0 - 140 340 (95.8 %) 67.5 5.04 90- 400 12 (3.3 %) 18,8 0.64

140-1000 2 (0.6 %) 67.9 0.03

d e p t h r a n g e f i shed in t he sur face w a t e r s d u r i n g the d a y w a s 0 to 350 m. Ne t s f i shed o b l i q u e l y b e t w e e n 0 a n d 350 m c o l l e c t e d 92 % of t he s p e c i m e n s t a k e n in d a y t i m e

co l l ec t ions (Table 1). C lea r ly , A m e r i c a n ee l l e p t o c e p h a l i in t he se co l l ec t ions d id not

m i g r a t e d i u r n a l l y to d e p t h s b e l o w 350 m. T h e q u e s t i o n of d i e l v e r t i c a l m i g r a t i o n w i t h i n t he u p p e r 350 m is no t r e s o l v e d by t h e s e data .

T h e m e a n to ta l l e n g t h of A m e r i c a n e e l l e p t o c e p h a l i in co l l ec t ions a s s o c i a t e d w i t h

Gul f S t r e a m m e s o p e l a g i c f ish f a u n a i n c r e a s e d f rom sou th to nor th (Fig. 2). C a p e H a t t e r a s l e p t o c e p h a l i ( samples 115-118) w e r e s ign i f i can t ly l o n g e r t h a n C a p e R o m a i n l ep to -

c e p h a l i ( samples 85-87) w h i c h w e r e in tu rn s ign i f i can t ly l o n g e r t h a n St. A u g u s t i n e a n d Wes t P a l m B e a c h l e p t o c e p h a l i ( samples 91-94 and 99-106 , r e spec t ive ly ) . S i m i l a r l y t he

m e a n l e n g t h of l e p t o c e p h a l i a s soc i a t ed w i t h Sa rgasso Sea f a u n a w a s s i g n i f i c a n t l y

g r e a t e r at C a p e H a t t e r a s ( samples 119-123) t h a n a t C a p e R o m a i n a n d St. A u g u s t i n e ( samples 80-83 a n d 97-98 , r e spec t ive ly ) .

54-

52- E E - v

. =50 - -

~ 4 8 - -

5 -~46- I - - -

44-

6 - 5 - 5- 51J- 29- 12 t5- 14- 8A 4A 6A IA 51J A I4A 15A

I I I I I I 1 I

I05 --I--

5O

25

-I- GS GS SS

i

West 9'~- 9'~ Palm 94 98

Beech St, Augustine

112 95 104 -{-

÷ -I- t8

GS SS SL GS SS

8'5- B'o- l lz- ris- ib- 87 83 114 1t8 125

Cape Romain Cape Hatteras

Fig. 2. Mean total lengths, standard errors and sample sizes of American eel leptocephali in Florida Current transect samples. Samples within transects are grouped on the basis of their associated mesopelagic ichthyofauna (SL = slope water fauna, GS = Gulf Stream system fauna, SS = Sar-

gasso Sea fauna). The upper caption lists the day and month (J = July, A ---- August)

Entry of l ep tocepha l i into the Gulf Stream 333

This south to north increase in l eng th is apparent ly due to the input of la rger

spec imens from the wes te rn Sargasso Sea. Leptocephal i t aken at the eastern ends of the

St. Augus t ine and Cape Hatteras transects, in collect ions with a Sargasso Sea faunal composi t ion (samples 97-98 and 119-123), were s ignif icant ly longer than those col lected

to the wes t on their respect ive transects. Leptocephal i t aken at Cape Romain were not s ignif icant ly different in mean leng th from west (samples 85-87) to east (samples 80-83).

Leptocephal i associated wi th slope water fauna were t aken only at Cape Hatteras;

their m e a n length was only sl ightly grea te r than that of lep tocephal i t aken much farther

south at St. Augus t ine and West Palm Beach. The observed length distr ibutions among and a long transects apparent ly reflect real

geograph ic differences, rather than tempora l ones, at least in the Florida Current itself. The m e a n leng th of lep tocephal i col lec ted at the western end of the Cape Romain

transect on Ju ly 31 and Augus t 1 (samples 85-87, 48.1 ± 0.48 mm, m e a n ± s tandard

error) was not s ignif icant ly different from that of lep tocephal i col lected there 9-10 days later (sample 110, 49.1 _ 0 . 6 8 r a m ) (t test, P>0.10) . Similar ly the m e a n length of

lep tocephal i col lec ted at Cape Hat teras on August 13 (sample 115, 46.6 ± 1.24 ram) was

not s ignif icant ly different at the same location 3 days later (sample 124, 46.9 __ 0.87 ram)

(P > 0.10).

T h e m e a n ca t ch o f A m e r i c a n ee l l e p t o c e p h a l i p e r u n i t e f f o r t i n t hose c o l l e c t i o n s

character ized by a Gulf Stream system fish fauna increased dramat ical ly nor thward from

I05 I

E I 0 "

o

0 o - o;i

Sample number I01,

102 104-106 I00 99 I I I I

GS GS GS GS

0.8 1.3 0 I I o.5

I00

200 . g

~ 500 o,_

a 4 0 0 .

5 0 0 -

600

26°C

~ 2 2

_

I "" [~':;i "[:'" I I I I I 0 I0 20 30

Distance (neut. mi.)

Fig. 3. West Palm Beach transect. Bar graph shows catches of American eel leptocephali per 104 m 3 of water filtered. Sample numbers and characterizations of their associated mesopelagic ichthyo- fauna are listed at the top of the figure (? = no mesopelagic fauna caught, GS = Gulf Stream system fauna). The depth to selected isotherms at each sample location is shown in the bottom half of the

figure. The horizontal scale is the distance from west to east along the length of the transect

334 R .C . Kleckner & J. D. M c C l e a v e

I o E

0 -+,,

+7>

9695 91

it ,,,,,,, I I, I

I O " ? G S

o / o o ill

ioo;

2 0 0

7 -':300" o . (0

° 4 0 0 .

500-

600"

S a m p l e n u m b e r 92 95 94 97 98

I I I I I

GS 6S+ GS+ SS SS SS SS

4.0

0 . 5

I i

0 20 40 60 80 D i s t a n c e ( n a u t . m i . )

Fig. 4. St. Augustine transect. Bar graph shows catches of American eel leptocephali per 104 m 3 of water filtered. Sample numbers and characterizations of their associated mesopelagic ichthyofauna are listed at the top of the figure (7 = no mesopelagic fauna caught, GS = Gulf Stream fauna, SS = Sargasso Sea fauna). Catch rate was not calculated for leptocephali collected in sample 91 due to gear failure. The depth to selected isotherms at each sample location is shown in the bottom half of the figure. The horizontal scale is the distance from west to east along the length of the

transect

the Straits of Florida. Compared wi th their abundance in the West Pa lm Beach collec-

tions (Fig. 3: samples 99-106) Ieptocephal i were approximate ly three t imes more abun-

dant at St. August ine (Fig. 4: samples 92-94}, six t imes more abundant at Cape Romain

(Fig. 5: samples 85-88) and nine t imes more abundant at Cape Hatteras (Fig. 6: samples 115-118) .

At Cape Romain Amer ican eel. lep tocephal i were approximate ly three t imes more

abundant in collect ions associated with a Sargasso Sea mesope lag ic fish fauna (Fig. 5:

samples 80-83) than they were to the west in collect ions with a Gulf Stream system fauna (samples 85-88). Sample 84, which also had a h igh lep tocepha lus catch rate, did not

inc lude mesope lag ic species. The eastern Cape Romain samples, inc lud ing sample 84, were all associated wi th a p ronounced th icken ing of the "18°C wate r" typical of the

nor thern Sargasso Sea (Pickard, 1979). Such h igh lep tocepha lus catch rates w e r e re-

pea t ed only in Cape Hat teras t ransect sample 122 (Fig. 6). Other samples charac ter ized

by a Sargasso Sea fauna at Cape Hatteras and at St. Augus t ine (Fig. 4) took lep tocepha l i at catch rates approximate ly equa l to their respect ive samples wi th a Gulf S t ream system fauna.

Amer ican eel l ep tocepha l i were not taken in slope water collect ions at St. Augus t ine

(Fig. 4: samples 95 and 96) or Cape Romain (Fig. 5: samples 89 and 90). At Cape Hat teras

E n t r y of l e p t o c e p h a l i i n to t h e G u l f S t r e a m 335

2 0 - 18-

ro E 16-

~" 14- 0 - - 1 2 -

6- 4- 2-

I 0 0 -

2 0 0 - g

~: 5 0 0 -

®

' - ' 400 -

5 0 0 -

6 0 0

90 8 9 8 8 I I I

? ? GS

Sample number 87 86 85 84 83 82 81 8 0

I I I I I I I I

"~ GS GS ? SS SS SS SS

0 0 0 , i i i i i i i i I i

0 20 4 0 6 0 8 0 I 0 0 D i s t ance (naut . mi.)

16.7 16.5

'i 0__411 'i, i

Fig. 5. Cape Romain transect. Bar g raph shows catches of Amer ican eel leptocephal i per 104 m 3 of water filtered. Sample numbers and characterizat ions of their associated mesopelagic ichthyofauna are l isted at the top of the figure (? = no mesopelagic fauna caught, GS = Gulf Stream system fauna, SS = Sargasso Sea fauna). The depth to selected isotherms is shown in the bottom half of the

figure. The horizontal scale is the dis tance from west to east a long the length of the transect

t h e y w e r e t a k e n i n l o w n u m b e r s i n s l o p e w a t e r c o l l e c t i o n s a t t h e w e s t e r n e n d of t h e

t r a n s e c t (Fig. 6: s a m p l e s 112 a n d 113). S a m p l e 114, a l so c h a r a c t e r i z e d b y a s l o p e w a t e r

f a u n a l c o m p o s i t i o n , h a d a s u b s t a n t i a l l y h i g h e r c a t c h (Fig. 6). T h e c o m p r e s s e d d o w n w a r d

s l o p i n g i s o t h e r m s e a s t of s t a t i o n 114 d e l i n e a t e t h e w e s t e r n e d g e of t h e F l o r i d a C u r r e n t .

T h e r e l a t i v e a b u n d a n c e of A m e r i c a n e e l l e p t o c e p h a l i a t t h e s e g e o g r a p h i c l o c a t i o n s

i n t h e F l o r i d a C u r r e n t , d i d no t c h a n g e d r a m a t i c a l l y o v e r s h o r t t imes . F o u r s a m p l e s t a k e n

d u r i n g a 39 h p e r i o d a t W e s t P a l m B e a c h (Fig. 3: s a m p l e s 101 a n d 104-106) t o o k f rom 0.5

to 2.6 l e p t o c e p h a l i p e r 104 m 3 of w a t e r f i l t e red . At C a p e R o m a i n s t a t i o n 86, c o l l e c t i o n s

t a k e n t h r e e d a y s a f t e r t h e i n i t i a l s a m p l e (Fig. 5) t o o k 1.7 l e p t o c e p h a l i p e r 104 m 3 a n d a t

C a p e H a t t e r a s s t a t i o n 115, c o l l e c t i o n s t a k e n n i n e d a y s a f t e r t h e i n i t i a l s a m p l e (Fig. 6)

t o o k 3.5 l e p t o c e p h a l i p e r 104 m 3.

A t o t a l of 58 E u r o p e a n e e l l e p t o c e p h a l i w e r e t a k e n in t h e F l o r i d a C u r r e n t t r a n s e c t

co l l ec t i ons . T h e y w e r e d i s t r i b u t e d as fo l lows : (1) W e s t P a l m B e a c h t r a n s e c t = 1 s p e c i -

m e n , (2) St. A u g u s t i n e t r a n s e c t = 0 s p e c i m e n s , (3) C a p e R o m a i n t r a n s e c t , G u l f S t r e a m

f a u n a s t a t i o n s 85 to 88 = 8 s p e c i m e n s a n d S a r g a s s o S e a f a u n a s t a t i o n s 80 to 83 = 16

336 R . C . K l e c k n e r & J. D. M c C l e a v e

E

0 \

112 I

SL 20- 18-

16- 14-

12- I0-

Sample number 117,

115 114 115 118 116 119 120 121 122 125 I I I I I I I I I I

SL SL SL+GS+ GS SS SS SS SS SS GS SS 18.2

I a 8" 7.3 7,4 1 _ e . 7 ~ 6 . 7

6 4.4 4 2,8

100-

200-

z 3 o o -

m 4 0 0 -

500- I0 16

6 o o \ , 0 ' 4 '0 6 0 8'0 '

Distance (naut. mi.)

Fig. 6. Cape Hatteras transect. Bar g raph shows catches of Amer ican eel leptocephal i per 10 4 m 3 of water filtered. Sample numbers and characterizations of their associated mesopelagic ichthyofauna are l isted at the top of the figure (SL = Slope water fauna, GS = Gulf Stream system fauna, SS = Sargasso Sea fauna). The depth to selected isotherms is shown in the bottom half of the figure.

The horizontal scale is the distance from west to east a long the length of the t ransect

s p e c i m e n s , a n d (4) C a p e H a t t e r a s t r a n s e c t , G u l f S t r e a m f a u n a s t a t i o n s 115 to 118 = 16

s p e c i m e n s a n d S a r g a s s o S e a f a u n a s t a t i o n s 119 to 123 = 17 s p e c i m e n s . 0 - g r o u p l ep -

t o c e p h a l i a v e r a g e d 38.4 m m i n l e n g t h (SE -- 0.319, n - - 4 8 ) a n d 1 - g r o u p l e p t o c e p h a l i

a v e r a g e d 50.1 m m i n l e n g t h (SE = 0.795, n = 10}. C a t c h r a t e s of E u r o p e a n e e l l e p t o c e -

p h a l i i n t h e s e c o l l e c t i o n s w e r e too l o w to p e r m i t a d e t a i l e d a n a l y s i s of d i s t r i b u t i o n .

E i g h t A m e r i c a n e e l l e p t o c e p h a l i , 11 to 17 m m t o t a l l e n g t h , w e r e c o l l e c t e d d u r i n g

m i d - A p r i l o v e r t h e c o n t i n e n t a l s h e l f of t h e Y u c a t a n P e n i n s u l a a b o u t 110 k m s o u t h of t h e

Y u c a t a n C h a n n e l . T w o l e p t o c e p h a l i 45 a n d 48 m m t o t a l l e n g t h w e r e c o l l e c t e d a t t h i s

l o c a t i o n i n N o v e m b e r .

DISCUSSION

T h e n i n e f o l d i n c r e a s e i n t h e a b u n d a n c e of A m e r i c a n e e l l e p t o c e p h a l i i n t h e F l o r i d a

C u r r e n t b e t w e e n t h e n o r t h e r n S t ra i t s of F l o r i d a a n d C a p e H a t t e r a s c l e a r l y i n d i c a t e s t h a t

m o s t A m e r i c a n e e l s e n t e r t h e G u l f S t r e a m s y s t e m d i r e c t l y f r o m t h e S a r g a s s o S e a r a t h e r

Entry of leptocephal i into the Gulf Stream 337

than by a more southern route. This in terpre ta t ion is supported by the increase in m e a n length of Amer ican eel leptocephal i in Hor ida Current collections taken north of the Straits of Florida and by the increase in specimen length from west to east a long the St. Augus t ine and Cape Hatteras transects.

The largest inpu t of Amer ican eel leptocephal i into the Gulf Stream system south of Cape Hatteras occurs north of 30 ° N. This coincides with a predicted increase in the volume transport of the Florida Current be tween Jacksonvil le, Florida, and Cape Hatteras (Knauss, 1969; Richardson et al., 1969; Richardson & Knauss, 1971; Worthing- ton, 1976; Stommel et al., 1978). The abundances and depth distr ibutions of leptocephal i support the idea that some of the increase in volume transport must be due to inpu t of surface waters from the western Sargasso Sea.

These leptocephal i p robably drift passively to the eastern edge of the Horida Current from a spawning area east of the Bahama Islands. The presence of a strong, broad and persis tent Anti l les Current f lowing northwest outside the Bahama Islands (Wrist, 1924) has b e e n cha l lenged on the basis of geostrophic flow calculations (Ingham, 1975). Rather, a var iable s luggish flow to the north is predicted (Ingham, 1975; G u n n & Ingham, 1977). The latter is more consistent with the transport of developing lepto- cephali to the edge of the Florida Current dur ing the six month interval be tween the predicted spawning peak in February (Kleckner & McCleave, 1980) and the dates of collection of the samples used in our study.

The dis tr ibut ion of a b u n d a n c e concentrat ions and size groups of Amer ican eel leptocephal i in these samples is in t r igu ing w h e n considered relative to average summer flow pat terns of surface currents in the western Sargasso Sea (Naval Oceanographic Office, 1978). The eastern half of the Cape Romain transect was in an area of b izonal flow with observat ions of surface currents directed to the northeast and southwest near ly equal in frequency. South and southwest of this area the prevai l ing flow is to the southwest, whi le to the north and northeast a prevai l ing northeaster ly flow paral lels the ma in axis of the Florida Current to the west. East and southeast of the Cape Romain transect the surface currents are variable. The e levated concentra t ion of leptocephal i at the eastern end of the Cape Romain transect thus lies in an area of opposing current pat terns from which dispersal could take place to the southwest or northeast. The similari ty in m e a n length be tween these leptocephal i and those t aken at the eastern end of the St. Augus t ine transect, at the wes tern end of the Cape Romain transect and at the center of the Cape Hatteras t ransect suggests that these groups may be d rawn from the same source.

The absence of s ignif icant var ia t ion in m e a n leng th across the Cape Romain transect may be due to a persistent and p ronounced seaward deflection in the wes tern marg in of the Florida Current caused by the Charles ton Bump (Brooks & Bane, 1978; Legeckis, 1979). The Charleston Bump, located 100 km ups t ream from the Cape Romain transect, is also apparent ly responsible for cylic waves and eddies downst ream (Legeckis, 1979). This may have caused lateral mix ing across the Florida Current; however, such mixing is not indica ted by faunal characterizations {Fig. 5).

The s ignif icant ly longer Amer ican eel leptocephali collected along the eastern half of the Cape Hatteras transect must be from a source with a different history. Cold-core eddies f requent ly pass south through the area east of the Florida Current at Cape Hatteras (Richardson, 1980). However, the tempera ture-depth plot for this transect does

338 R .C . Kleckner & J. D. McCleave

not have the e leva ted isotherms associated with a cold-core eddy {Fig. 6). In the absence

of sampl ing further to the south and east we have no explana t ion of the or igin of this group,

Factors responsible for the range of 9 m m in m e a n leng th found among collect ions of

Amer ican eel l ep tocepha l i in the Florida Current and wes te rn Sargasso Sea are unknown. Possibly the smal ler lep tocephal i that en te red the Florida Current th rough the

Straits of Florida or through the Bahamas were the p rogeny of later spawning. Al terna- t ively these smaller l a rvae may have ha tched and drifted in regions whose env i ronmen-

tal characterist ics caused s lower growth.

The e ight 11 to 17 mm Amer ican ee l lep tocephal i col lec ted southwest of the Yucatan Channe l are the smal les t spec imens ever repor ted from the Car ibbean Sea. Spec imens

longer than 30 mm have previous ly b e e n repor ted (Schmidt, 1925; Vladykov & March,

1975). The presence of such small l ep tocephal i in the Car ibbean Sea raises the possibil- ity that Amer ican eels may occasional ly reproduce outside of the southwestern Sargasso

Sea. Representat ions of Car ibbean Sea surface currents der ived from long term averages

of ship-drift observat ions (Wrist, 1964) and average geostrophic circulat ion calculat ions

(Gordon, 1967) may be used to sugges t that lep tocephal i col lected a long the east coast of

the Yucatan Peninsula could have b e e n transported by the Car ibbean Current from a spawning area in the central or eas tern Car ibbean Sea or in the North Atlant ic to the east

of the Lesser Antil les. Data from the trajectories of sa te l l i te - t racked drif t ing buoys (Molinari et al., 1981) and recover ies of drift ing tags (Brucks, 1971; Murphy et al., 1975;

Duncan et al., 1977) support this interpretat ion. However , the recovery a long the eas tern

Yucatan Peninsula of drift ing tags re leased in the Mona Passage (Metcalf et al., 1977) and near the Windward and Jama ica Passages (Murphy et al., 1975; Grant & Wyatt,

1980) opens the possibi l i ty that these leptocephal i may have or iginated at spawing areas north of Hispaniola.

Irrespect ive of thei r natal area, Amer ican ee l Ieptocephal i in the Car ibbean Sea could be carried to the Straits of Florida by the Car ibbean and Gulf Loop Currents

(Murphy et al., 1975; Maul, 1977). Leptocephal i col lected at West Palm Beach may have

en te red the Florida Current by this route or by drift ing west from the Sargasso Sea

through the Bahama Island chain to the Straits of Florida. Drift bottle studies by Day

(unpubl ished manuscript , 1954) es tabl i shed the feasibi l i ty of the latter. The re la t ive impor tance of these routes may not be ascer ta ined from our data.

Acknowledgements. We thank Dr. J. E. Craddock for temperature-depth data and characterizations of the associated mesopelagic fish fauna, Ms. B. D. Gustafson for laboratory assistance and Dr. J. H. Power for comments and suggestions on the manuscript. This study was supported by the National Science Foundation, grant number OCE-7719440, and by the National Geographic Society.

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