I n t e r n a t i o n a l Council f o r the Explorat ion o f the Sea

C.M. 1991/L:44 Session V

REPRODUCTIVE SUCCESS OF SPRAT (SPRATTUS SPRATTUS) I N GERMAN BIGHT DURING 1987

Jurgen A l h e i t

POLARMAR, Columbus-Center, Burger 20, 2850 Bremerhaven, Germany

and

Andrew Bakun

P a c i f i c F isher ies Environmental Group, Southwest F i she r ies Science Center, NMFS, NOAA, P.0.Box 831, Monterey, C a l i f o r n i a 93942, USA

ABSTRACT

The SARP " w i t h i n year" experiment was c a r r i e d ou t on sp ra t by Alshuth (1988a) i n the German B igh t i n 1987 when product ion o f small larvae was determined over the e n t i r e spawning season from Apr i 1 t o August and, subsequently, t he b i r th-date d i s t r i b u t i o n o f t he j u v e n i l e s which were su rv i v ing u n t i l November was establ ished.

These data were now analysed i n r e l a t i o n t o wind speed, s t a b i l i t y o f the water column and t i d a l impact. The r a t i o s o f the b i r t h - d a t e f requencies, found by count ing d a i l y o t o l i t h marks, t o the w i t h i n - season v a r i a t i o n s o f l a r v a l product ion i n d i c a t e on ly modest s u r v i v a l dur ing l a t e May and e a r l y June even though subs tan t i a l l a r v a l product ion was ind icated. However, a pe r iod o f c o n s i s t e n t l y low wind cond i t i ons occurred from June 9 t o June 27 . Correspondingly , the apparent su rv i Val r a t e dur ing the f i n a l two weeks o f June increased several f o l d over t h a t o f t he preceeding f o u r weeks, However, dur ing the f i r s t h a l f o f Ju l y f o l l o w i n g a pe r iod o f h igher winds from 29 June t o 4 Ju ly the apparent s u r v i v a l r a t e dec l ined somewhat. I n l a t e J u l y and e a r l y August t he apparent s u r v i v a l r a t e again increased t o the h ighest values found dur ing the operat ion, even though some wind events o f lesser i n t e n s i t y than the e a r l y Ju ly event occurred dur ing the per iod.

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INTRODUCTION

Methot (1983) introduced an elegant method when s tudy ing r e c r u i t - ment v a r i a b i l i t y o f t he northern anchovy, Engraulis ringens, i n t h e C a l i f o r n i a Current, a species w i t h a p ro t rac ted spawning sea- son o f about seven months. He recorded the produc t ion o f young anchovy l a rvae over t h e e n t i r e spawning pe r iod from December t o June o f t h e f o l l o w i n g year. Subsequently, he c o l l e c t e d the juve- n i l e s several months a f t e r spawning season and determined t h e i r b i r t h d a t e d i s t r i b u t i o n by count ing d a i l y r i n g s on t h e i r o t o l i t h s . The b i r t h d a t e d i s t r i b u t i o n curve o f t h e j u v e n i l e s was then compared t o t h e l a r v a l product ion curve t o i d e n t i f y per iods o f heavy m o r t a l i t y o r o f except ional s u r v i v a l o f d i f f e r e n t cohorts o f la rvae. These "except iona l " per iods can then be matched w i t h environmental data se ts t o determine the causes o f heavy m o r t a l i t y o r good s u r v i v a l .

So, by ana lyz ing l a r v a l product ion, b i r t h d a t e d i s t r i b u t i o n o f j u v e n i l e s and oceanographic, meteorological observat ions one can record intra-season'al changes i n l a r v a l s u r v i v a l , p i n p o i n t c r i t i c a l per iods o f s u r v i v a l and i d e n t i f y t he causal mechanisms.

Th is h i g h temporal r e s o l u t i o n experiment was r e f i n e d by the South- west F i she r ies Center under the leadersh ip o f Reuben Lasker (Las- ke r 1985) and became known as the "w i th in -year exerc ise" i n SARP s tud ies (Bakun e t a l . 1991). I t has been i d e n t i f i e d as a uniquely promising process-oriented approach f o r analyzing the causal mechanisms and consequences o f short- term l a r v a l s u r v i v a l (Anon. 1983, 1984, 1987). The p a r t i c u l a r advantage o f t h i s approach i s t h a t i t addresses the problem on t h e popu la t ion l e v e l , ra the r than addressing the f a t e o f t y p i c a l o r "average" la rvae which may be i r r e l e v a n t t o n e t reproduc t ive success (Bakun e t a l . 1991).

As l a r v a l p roduc t ion can vary on small temporal scales, f requent surveys are requ i red t o e s t a b l i s h a v a l i d l a r v a l p roduc t ion curve. Because o f t he h igh demand o f sh ip t ime and o ther opera t iona l resources requ i red , no f u l l y elaborated SARP "w i th in -year " exer- c i s e , inco rpo ra t i ng a f u l l s u i t e o f environmental and b i o l o g i c a l data, has been completed t o date (Bakun e t a l . 1991).

A f i r s t at tempt t o apply the "w i th in -year " experiment t o North Sea sp ra t , Sprattus sprattus, was made i n the German B igh t i n 1987 (A lshuth 1988a, 1988b). Larval p roduc t ion o f s p r a t i n the German B i g h t was monitored and the b i r t h d a t e d i s t r i b u t i o n o f t h e Juven i les was es tab l i shed subsequently. However, no ana lys i s o f causal mechan ims o f t he d i f f e r e n t i a1 " w i t h i n-season" 1 a rva l m o r t a l i t y was done. The o b j e c t i v e of t h i s study i s t o r e l a t e the r e s u l t s o f A lshuth 's s tud ies (1988a) t o wind speed, t i d a l impact and s t a b i l i t y o f t h e water column.

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LARVAL PRODUCTION AND BIRTHDATE DISTRIBUTION OF JUVENILES

Larval product ion was determined by e i g h t f o r t n i g h t l y c ru i ses over the e n t i r e spawning season from the second h a l f o f A p r i l t o the f i r s t h a l f o f August on a g r i d of 14 sampling s t a t i o n s spread evenly over the German Bight (F ig . 1 ) . The larvae were c o l l e c t e d by obl ique bongo tows and only larvae UP t o 5 mm l eng th were con- s idered f o r t he product ion est imate (Alshuth 1988a). Juveni les were c o l l e c t e d i n October and November w i t h a RMT and t h e i r age was determined subsequently by count ing d a i l y r i n g s on the oto- l i t h s (Alshuth 1988a). A comparison of t he b i r t h d a t e d i s t r i b u t i o n of t he j u v e n i l e s and the l a r v a l product ion curve c l e a r l y i nd i ca tes d i f f e r e n t i a l "within-season'' m o r t a l i t y o f the d i f f e r e n t l a r v a l cohor ts ( F i g . 2 ) . The almost i d e n t i c a l b i r t h d a t e d i s t r i b u t i o n o f j u v e n i l e s caught i n mid-October and those caught f o u r weeks a f t e r t h a t shows t h a t there was no d i f f e r e n t i a l m o r t a l i t y dur ing t h i s j u v e n i l e phase.

DISCUSSION

This i n i t i a l SARP f i e l d study o f sp ra t i n the German B igh t was more a "proof o f concept" f o r t he development o f a short- t ime scale " s u r v i v a l index" than a complete SARP "w i th in -yea r " exer- c i s e . That i s , i t d i d no t include a comprehensive s u i t e o f equa l l y h igh frequency measurements o f t he phys ica l and b i o l o g i c a l envi- ronment, h i s t o l o g i c a l measures o f s t a r v a t i o n (Thei lacker 1986), immunoassays o f predator stomach contents (Thei lacker e t a l . 1986), e t c . Thus, t he data a v a i l a b l e f o r i n t e r p r e t a t i o n o f the i nd i ca ted sho r t t ime-scale v a r i a t i o n s i s minimal. S t a b i l i t y o f the water column, t i d a l impact and l o c a l wind cond i t i ons were analyzed i n r e l a t i o n t o the s u r v i v a l index o f the j u v e n i l e s .

Analysis o f the t i d a l regime and the s t a b i l i t y o f the water column d i d n o t i n d i c a t e any r e l a t i o n s h i p w i t h the s u r v i v a l index. Howe- ver , v a r i a b i l i t y i n l o c a l wind cond i t i ons showed i n t e r e s t i n g r e l a - t i o n s h i p s t o the d i f f e r e n t i a l s u r v i v a l index o f sp ra t j u v e n i l e s i n the German B igh t .

Wind e f f e c t s are involved i n many o f t he c u r r e n t l y popular hypo- theses concerning regu la t i on o f recru i tment v a r i a b i 1 i t y . N u t r i e n t enrichment o f t he i l l u m i n a t e d upper zone o f the ocean by wind- d r i ven upwel l ing o r mixing may be important i n mainta in ing the t r o p h i c base f o r product ion o f l a r v a l food. A wind-based index o f coasta l upwel l ing (Bakun 1973) has been e m p i r i c a l l y r e l a t e d t o popu la t i on dynamics o f a number o f commercial f i s h stocks o f the C a l i f o r n i a Current and other eastern ocean upwel l ing systems (Bakun and Par r i sh 1981, Shepard e t a l . 1984, Bakun 1985, Bakun i n press) . The hypothesis o f Lasker (1975 , 1978, 19881, which ad- dresses det r imenta l e f f e c t s o f d i spe rs ion o f f i n e scale food p a r t i c l e s t r a t a by wind-induced t u r b u l e n t mixing, has been very i n f l u e n t i a l i n the development o f SARP. Lasker 's hypothesis i s supported by the analys is o f Peterman and Bradford (19871, who e m p i r i c a l l y r e l a t e d i n te ryea r v a r i a b i 1 i t y i n l a r v a l anchovy m o r t a l i t y r a t e o f f C a l i f o r n i a t o frequency o f calm per iods of

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s u f f i c i e n t du ra t i on f o r f i n e scale s t r a t a o f food organisms t o form. Ro thch i l d and Osborn (1988) have suggested t h a t c e r t a i n l e v e l s o f turbulence could a c t u a l l y enhance feeding e f f i c i e n c y o f small marine organisms. Pa r r i sh e t a l . ( 1 9 8 1 ) surveyed repro- duc t i ve h a b i t s of f ishes o f the C a l i f o r n i a Current and found a p a t t e r n o f avoidance of det r imenta l e f f e c t s o f wind-induced o f f sho re Ekman t r a n s p o r t , which would c a r r y eggs and larvae away from the favourable coastal h a b i t a t . I d e n t i f i c a t i o n o f s i m i l a r p a t t e r n s i n o ther types o f f i s h stocks and reg ional s e t t i n g s , i n d i c a t i n g wide-spread importance o f "phys ica l r e t e n t i o n areas", have lead t o S i n c l a i r ' s (1988) f o rmu la t i on o f the "member/vagrant" hypothesis. P a r r i s h e t a l . (1983) show a pervasive tendency, i n reproduct ive h a b i t s o f eastern ocean coasta l pe lag ic f i shes , f o r simultaneous min imizat ion o f exposure o f larvae t o both o f f sho re t r a n s p o r t and turbulence. Cury and Roy ( 1 9 8 9 ) have used new non- 1 i near methods (Mendelssohn and Cury 1987 1, t o empi r i c a l 1 y demon- s t r a t e a cons is ten t "opt imal environmental window", where repro- duc t i ve success i s favoured a t an in termediate range o f wind speed, n e i t h e r t o o low f o r adequate upwelling-based enrichment nor so h igh t h a t excessive of fshore t r a n s p o r t and t u r b u l e n t mixing occurs. Roy ( 1 9 9 0 ) showed the same p a t t e r n o f apparent s e l e c t i o n f o r an in termediate i dea l c h a r a c t e r i s t i c wind speed i n a survey o f the reproduct ive h a b i t s of West A f r i can coasta l pe lag ic f i shes .

I n d i c e s o f wind-re la ted processes

Several s e r i e s o f wind-related i nd i ces ( F i g . 3 A , E, C) r e l a t e d t o the var ious hypotheses enumerated above, have been prepared. The bas ic data source i s t he f i l e o f 6-hourly nor thern hemisphere synopt ic atmospheric pressure/wind analyses produced a t the U.S.Navy's F l e e t Numerical Oceanography Center i n Monterey, C a l i - f o r n i a . From these a s e r i e s o f est imates o f t he l o c a l surface wind i n the German B i g h t was produced according t o the method o f Bakun ( 1 9 7 3 ) . The "wind mixing index" ( F i g . 3 A ) i s an est imate o f the r a t e o f t r a n s f e r o f wind-derived t u r b u l e n t mixing energy through the sea surface; i t i s formed as the bi-weekly average o f t he t h i r d power (cube) o f t he wind speed (Bakun and Par r i sh 1980) .

I n addressing Lasker 's hypothesis, i t i s probably n o t the average mix ing i n t e n s i t y t h a t i s c r u c i a l , bu t t he existence o f adequate temporal "windows" (Bakun and P a r r i s h 1980) dur ing which the product ion o f t u r b u l e n t mixing energy remains low f o r a long enough pe r iod f o r concentrated food p a r t i c l e s t r a t a t o accumulate and f o r larvae t o accomplish successful "f i r s t - f e e d i n g " . These calm per iods have been c a l l e d v a r i o u s l y "Lasker events", "Lasker gates", e t c . Here we spec i f y a "Lasker ( x , y ) window" as being a f u l l day i n which the wind speed d i d n o t exceed "x " m s 8 - 1 , which d i r e c t l y f o l l o w s an unbroken sequence o f a t l e a s t " y " preceeding days w i t h wind speeds n o t exceeding the same " x " m-1 l i m i t . Peterman and Bradford (1987) chose 10 m s-1 f o r t he wind speed l i m i t f o r t h e i r study of C a l i f o r n i a anchovy. Mendelssohn and Mendo ( 1 9 8 7 ) chose 5 m s-1 f o r t h e i r study o f Peruvian anchovy. We show both ( 5 , 4 ) and ( 1 0 , 4 ) Lasker windows f o r t he per iod and l o c a t i o n of t he 1987 German B igh t "Sprat-SARP" opera t i on i n F ig . 3 .

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The s t r e s s exer ted by the wind on the sea surface was computed from t h e synopt ic surface wind est imates using t h e standard quadra t ic drag law and assuming a constant drag c o e f f i c i e n t o f 0.0013. The r e s u l t i n g s t ress est imates where then averaged by component f o r each bi-weekly increment o f t he study (e.g. , F ig . 3C).

The s u r v i v a l index (F ig . 30) was defined as t h e r a t i o o f t he per- centage o f t he sampled l a r v a l b i r t h d a t e s (F ig . 2A) i n each b i - weekly increment t o t h e corresponding l a r v a l p roduc t ion est imates ( F i g . 2A and F ig . 3E). The su rv i va l index (F ig . 3D) was de f ined as t h e r a t i o o f t he percentage o f the sampled l a r v a l b i r t h d a t e s (F ig . 2 ) i n each biweekly increment t o t h e corresponding l a r v a l p roduc t ion est imates (F ig . 2 and F ig . 3 E ) .

We can examine these se r ies f o r apparent con fo rm i t i es t o t h e hypo- t h e t i c a l l inkages. However, l ack ing co r robo ra t i ng data on sub-sur- face processes and e f f e c t s , t he ana lys i s i s a t t h i s p o i n t merely c o r r e l a t i v e and sub jec t t o a l l well-known l i m i t a t i o n s o f a cor- r e l a t i v e approach (e.g. Bakun 1985). Note a l s o t h a t t h e pe r iod o f subs tan t i a l l a r v a l product ion conta ins on ly f i v e bi-weekly inc re- ments (F ig . 3E) w i t h perhaps two o the r data p o i n t s ( f i r s t h a l f o f May and f i r s t h a l f o f August) having very uncer ta in r e l a t i v e ma- gn i tude o f s u r v i v a l because o f very low ind i ca ted l a r v a l produc- t i o n bu t a t l e a s t a m i n d i c a t i o n t h a t s u r v i v a l may have been ra the r good. Accordingly, t h e degrees o f freedom a v a i l a b l e f o r any s o r t o f emp i r i ca l ana lys i s are extremely l i m i t e d . Thus the b r i e f analy- s i s t h a t f o l l o w s should be proper ly regarded as merely an exerc ise i n hypothesis generation. No i m p l i c a t i o n t h a t t h i s dis-cussion i n any way c o n s t i t u t e s a v a l i d hypothesis t e s t i s intended.

Larva l r e t e n t i o n hypothesis

The most ev ident v i sua l c o r r e l a t i o n appears t o be w i t h eastwards wind s t r e s s (F ig . 3C). The orthogonal northward component showed no perceptabel degree o f c o r r e l a t i o n . Eastwards s t r e s s would pro- duce southward surface Ekman t ranspor t (Ekman 1905) i n a deep ocean s i t u a t i o n . However, i n a shal low area such as the coastward p o r t i o n s o f t h e German B igh t , f r i c t i o n a l e f f e c t s would probably cause t h e t r a n s p o r t t o be angled somewhat more d i r e c t l y downwind than t h e "900 t o the r i g h t " s p e c i f i e d by t h e "pure" Ekman t rans- p o r t d e r i v a t i o n . Thus, t he near surface t r a n s p o r t f rom an eastward wind s t r e s s would, i n a l l l i k e l i h o o d , be d i r e c t e d from the Nor th Sea a t la rge , d i r e c t l y i n t o the German B igh t . Th i s would tend t o h o l d d r i f t i n g la rvae w i t h i n the shal low p o r t i o n s o f t he German B igh t , r e t a r d i n g t h e i r d ispers ion i n t o t h e open North Sea, and he lp ing t o keep them as "members" r a t h e r than becoming "vagrants", according t o t h e terminology o f S i n c l a i r (1988). I n t h e shal lower p a r t s o f t h e German B igh t , la rvae may f i n d improved feed ing condi- t i o n s w i t h i n the f r o n t a l systems produced by t h e f r e s h water i n - f l ows and perhaps f i n d some refuge from predat ion , e t c . I n addi- t i o n , s ince the b i r t h d a t e d i s t r i b u t i o n s which deter-mine the sur- v i v a l index are c o l l e c t e d l o c a l l y , t h e v a r i a t i o n o f t h e p ropor t i on o f "members" t o "vagrants" may be o f comparable s i g n i f i c a n c e t o v a r i a t i o n s i n ac tua l s u r v i v a l i n determining t h e v a r i a b i l i t y o f t h e s u r v i v a l index.

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Stab le ocean hypothesis (Lasker 's hypothesis)

The most no tab le d e v i a t i o n from a d i r e c t Visual p r o p o r t i o n a l i t y o f s u r v i v a l index ( F i g . 3D) t o eastward wind s t ress (F ig . 3C) i s a pe r iod of r e l a t i v e l y enhanced apparent s u r v i v a l dur ing the second h a l f o f June, Th is pe r iod corresponds t o a per iod o f c o n s i s t e n t l y low winds y i e l d i n g eleven Lasker (5 ,4 ) windows w i t h i n the 15-day pe r iod (dark shading F ig . 38). We note t h a t the spawning pe r iod o f s p r a t ( F i g . 3E) i s conf ined t o the seasonal pe r iod o f low t u r - bu len t mix ing energy product ion by the wind (F ig . 3A). The one two-week pe r iod where an i n d i c a t i o n o f subs tan t i a l s u r v i v a l (F ig . 3D) corresponds t o a pe r iod o f r e l a t i v e l y h i g h mixing index (F ig . 3A) i s du r ing the f i r s t h a l f o f May where presence o f f o u r "Lasker (4 ,5 ) windows" i n d i c a t e s an extended pe r iod o f calm cond i t i ons even al though the mean turbulence generat ion, f o r t he pe r iod as a whole, was h igh .

SUMMARY

Thus, l ook ing on ly a t p o t e n t i a l wind e f f e c t s , the r e s u l t s o f t he 1987 German B i g h t Sprat-SARP exerc ise appear t o be cons is ten t w i t h a combination o f favourable e f f e c t s o f ( i ) l a r v a l r e t e n t i o n ( P a r r i s h e t a l . 1981; S i n c l a i r 1988) and o f ( i i ) water column s t a b i l i t y du r ing e a r l y l a r v a l l i f e (Lasker 1975, 1978). There i s l i t t l e evidence here f o r any subs tan t i a l favourable e f f e c t o f increased tu rbu lence generation, a c t i n g e i t h e r t o s t imu la te food p a r t i c l e p roduc t ion by mixing n u t r i e n t s f rom the pycnocl ine i n t o the upper mixed laye r o r t o increase contac t ra tes (Rothch i ld and Osborn 1988) between feeding la rvae and appropr ia te food par- ti c l e s .

REFERENCES

Anon. 1983. Workshop on the I R E P Component o f t he I O C Programme on Ocean Science i n Re la t i on t o L i v i n g Resources (OSLR). I O C Workshop Rep. No. 33. Intergovernmental Oceanographic Commission, Unesco, Pa r i s . 51pp.

Anon. 1984. IOC-FA0 Guiding Group o f Experts on the Programme o f Ocean Sciencwe i n Re la t i on t o L i v i n g Resources, F i r s t Session. Reports o f Meetings o f Experts and Equ iva len t Bodies SC- 84/WS/18. Intergovernmental Oceanographic Commission, Unesco, Pa r i s . 34pp.

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Anon. 1987. IOC-FA0 Guiding Group o f Experts on the Programme of Ocean Sciencwe i n Re la t i on t o L i v i n g Resources, Second Session, Reports o f Meetings o f Experts and Equivalent Bodies SC- 87/WS/41. Intergovernmental Oceanographic Commission, Unesco, Pa r i s . 37pp.

Larven und Juven i len der Sprot te, SPrattUS s p r a t t u s L . . Ph. 0. Thesis, U n i v e r s i t a t Bremen. 191pp.

Alshuth, S . 1988a. Bestimmung von A l t e r und Wachstumsraten be i

Alshuth, S. 1988b. Seasonal v a r i a t i o n s i n length-frequency and b i r t h d a t e d i s t r i b u t i o n o f j u v e n i l e s p r a t ( S p r a t t u t s p r a t t u s L . ) . I C E S C.M. 1988./H:44, 12pp.

Bakun, A . ( i n p ress) . The C a l i f o r n i a Current, Benguela Current, and Southwestern A t l a n t i c Shel f Ecosystems: A comparative approach t o i d e n t i f y i n g f a c t o r s r e g u l a t i n g biomass y i e l d s . I n Large Marine Ecosystems. K.Sherman, L.M.Alexander, and B.Gold (eds . ) American Associat ion f o r the Advancement o f Science.

Bakun, A . 1973. Coastal upwel l ing i nd i ces , west coast o f North America, 1946-71. NOAA Tech. Rep. NMFS SSRF-671, 103 pp.

Bakun, A . 1990. Global c l ima te change and i n t e n s i f i c a t i o n o f coas ta l ocean upwel l ing . Science 247: 198-201.

Bakun, A and R.H . Par r i sh . 1980. Environmental i npu ts t o f i s h e r y popu la t ion models f o r eastern boundary c u r r e n t regions. pp. 67- 104. I n : G . D . Sharp (ed) Workshop on the E f f e c t s o f Environ- mental V a r i a t i o n on the Surv iva l o f Larval Pe lag ic Fishes, Lima, Peru, 20 Apr.-5 May, 1980. I O C Workshop Rep. 28: 323 pp.

Bakun, A . , J . A l h e i t and G. Kul lenberg. 1991. The Sardine-Anchovy Recruitment P r o j e c t (SARP): r a t i o n a l e , design and development. I C E S C.M. 1991/L:43, 16pp.

Cury, P. and C . Roy. 1989. Optimal window and pe lag i c f i s h recru i tment success i n upwel l ing areas. Can J . F ish . Aquat. Sc i . 46: 670-680.

Ekman, V.W. 1905. On the in f luence o f t he e a r t h ' s r o t a t i o n on ocean cu r ren ts . Ark. Mat. Astron. Fys. 2(11): 1-55. ( r e p r i n t e d i n J. Cons. i n t . Explor. Me 20(2), 1954).

t he r e l a t i o n betwen inshore ch lo rophy l l maximum l a y e r s and successful f i r s t feeding. Fish. B u l l . U . S . 73: 453-462.

Lasker, R . 1978. The r e l a t i o n between oceanographic cond i t i ons and l a r v a l anchovy food i n the C a l i f o r n i a Current: I d e n t i f i c a t i o n of t he f a c t o r s lead ing t o recru i tment f a i l u r e . Rapp. P.-v. Reun. Cons. i n t . Explor. Mer. 173: 212-230.

Lasker, R . 1985. What l i m i t s c lupeo id product ion? Can. J. F ish . Aquat. Sci . 42: 31-38.

Lasker, R . 1975. F i e l d c r i t e r i a f o r s u r v i v a l o f anchovy la rvae:

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Lasker, R . 1988. Studies on the nor thern anchovy: b io logy , recru i tment , and f i s h e r y oceanography. pp. 24-41. I n Studies on F i she r ies Oceanography. Proceedings o f t h e 25th Anniversary Symposium "F i she r ies and Fishery Oceanography i n the Coming Century". Tokyo, 10-13 November, 1986. Japanese Society o f Fishery Oceanography.

Mendelssohn, R . and P. Cury. 1987. F luc tua t i ons o f a f o r t n i g h t l y abundance index o f t he I v o r i a n coas ta l pe lag i c species and associated environmental cond i t ions . Can. J . Aquat. Sc i . 44: 408-421.

Mendelssohn, R . and J . Mendo. 1987. Exp lo ra to ry ana lys i s o f anchoveta recru i tment o f f Peru and r e l a t e d environmental se r ies . p . 294-306. I n D. Pauly and I. Tsukayama (eds.) The Peruvian Anchoveta and i t s Upwell ing Ecosystem: Three Decades o f Change. ICLARM Studies and reviews 15. I n s t i t u t o de l Mar de l Peru, (IMARPE), Cal lao, Peru; Deutsche Gese l lschaf t f u r Technische Zusammenarbeit (GTZ), GmbH, Eschborn, Federal Republic o f Germany; and I n t e r n a t i o n a l Center f o r Aquatic Resources Management (ICLARM), Manila, Ph 351 pp.

Methot, R . D . 1983. Seasonal v a r i a t i o n i n s u r v i v a l o f Engrau 7 i s mordax estimated from t h e age d i s t r i bu t j u v e n i l e s . F ish . B u l l . , U.S. 81: 741-750.

Pa r r i sh , R.H. , A . Bakun, D.M. Husby and C.S . Nelson. Comparative c l imato logy o f se lec ted environmental

L i v i n g 1 i ppines,

1 a rva l on o f

1983. processes i n

r e l a t i o n t o eas t re rn boundary c u r r e n t pe lag i c f i s h reproduct ion. I n Proceedings o f t h e Expert Consu l ta t ion t o Examine Changes i n Abundance and Species Composition o f N e r i t i c F i sh Resources. pp. 731-778. Ed. by G . D . Sharp and J . Csirke. FA0 F ish . Rep. 291: 1224 pp.

mechanisms and reproduc t ive success o f f i s h e s i n t h e C a l i f o r n i a Current. B i o l . Oceanogr. 1: 175-203.

Pa r r i sh , R . H . , C . S . Nelson and A . Bakun. 1981. Transport

Peterman, R.M. and M . J . Bradford. 1987. Wind speed and m o r t a l i t y r a t e o f a marine f i s h , t he nor thern anchovy (Engrau7is mordax). Science 235: 354-356.

Rothch i ld , B . J . and T.R. Osborn. 1988. Small-scale turbulence and p lank ton contac t ra tes . J . Plankton Res. 10: 465-474.

Roy, C . 1990. RBponses des stocks de poissons pelagiques d l a dynamique des upwel l ings en Af r ique de l ' o u e s t : analyse e t modbl isat ion. These de Doctorat. U n i v e r s i t h de Bretagne Occidental , Bres t , France. 147 pp.

Shepard, J.G., J.G. Pope and R . D . Cousins. 1984. Va r ia t i ons i n f i s h stocks and hypotheses concerning t h e i r l i n k s w i t h c l imate . Rapp.P.-v. Reun. Cons. i n t . Explor. Mer 185: 255-267.

S i n c l a i r , M. 1988. Marine Populat ions: an essay on popu la t ion r e g u l a t i o n and spec ia t ion . U n i v e r s i t y o f Washington Sea Grant Program ( D i s t r i b u t e d by Un ive rs i t y o f Washington Press. Sea t t l e and London.). 252 pp.

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Theilacker, G.H. 1986. Starvation-induced mortality o f young sea- caught jack mackerel, Trachurus symet r icus , determined with histological and morphological methods. Fish. Bull., U . S . 84: 1 - 1 7 .

Theilacker, G.H., A . S . Kimball and J . S . Trimmer. 1986. Use o f ELISPOT immunoassay to detect euphausid predation on larval anchovy. Mar. Ecol. Progr. Ser. 30: 127-131.

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F i g . 1 : Study a r e a i n German B i g h t w i t h l o c a t i o n o f sampling s t a t i o n s .

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B

C

F i g . 2 : A - Seasonal d i s t r i b u t i o n o f abundance o f l a r v a l s p r a t i n German B i g h t i n 1987.

B - Seasonal d i s t r i b u t i o n o f b i r t h d a t e s o f j u v e n i l e s p r a t c o l l e c t e d 13/14 October 1987 i n German B i g h t

C - Seasonal d i s t r i b u t i o n o f b i r t h d a t e s o f j u v e n i l e s p r a t c o l l e c t e d 1 1 November i n German B i g h t

( m o d i f i e d a f t e r Alshuth 1988 a , b )

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Fig. 3: A , B , C - Series of wind-related indices.

D - Survival Index of juvenile sprat.

E - Larval production o f sprat (as Fig. 2 A ) .