Formation and Seasonal Evolution of Atlantic Menhaden Juvenile Nurseries in Coastal Estuaries

Estuanes Vol, 19, No. 1, p. 105--114 March 1996

Formation and Seasonal Evolution of

Atlantic Menhaden Juvenile Nurseries

Coastal Estuaries

in

K. D. FRIEDIAND Lz D. W. A H ~ . N ' I t O L Z

J. F. Gt:'rrtPav National Marine l:isheries ,Service Beaufort Laboratory Beaufort, North Carolina 28516

ABSTRACT: A hypothesis on the formation and seasonal evolution of Atlantic menhaden (Brevoortla tyrannus) juvenile nurseries in coastal estuaries is described. A series of cruises were undertaken to capture postmetamorphic juvenile menhaden and to characterize several biological and physical parameters along estuarine gradients. The two study systems, the N e u s e and Pamlico rivers in North Carolina, contain important menhaden nursery grounds. Juvenile menhaden abundance was found to be associated with gradients o f phytoplankton biomass as ev idenced by chlorophyll a levels in the upper water column. Fish abundances were only secondarily associated with salinity gradients as .salinity was a factor that moderated primary production in the estuary. The persistence o f spatial and temporal trends in the distribution of phytoplankton in the N e u s e and Pamlico estuaries wigs reviewed. The review suggested that posUnetamorphic juvenile menhaden modify their distribution patterns to match those created by phytoplankton blomass, which in turn makes them most abundant in the phytoplankton maxima of estuaries. Because the location of these maxima varies with the mixing and nutrient dynamics o f different estuaries, so will the location of the nursery.

I n t r o d u c t i o n

During their residence in estuarine nurseries, juvenile Atlantic m e n h a d e n grow rapidly and metamorphose into fiher-feeding fishes. The impor- tance of this per iod has lead to the evolution of physiologic and behavioral adaptations in menhaden to facilitate their locating of and persisting in nursery areas.

Atlantic m e n h a d e n are assumed to spawn in the coastal ocean off Nor th Carolina f rom late fall to early spring (judy and Lewis 1983; Ahrenholz et al. 1987b; Lewis et al. I987). I.arvae f rom these spawns are t ransported to the mouths of estuaries by meteorologic and oceanographic events occur- ring over variable time scales (Nelson et al. 1977; Checkley et al. 1988). Once in the estuary, menhaden larvae are d e p e n d e n t upon t ransport mechanisms to arrive at the nursery grounds (Pietrafesa andJanowitz 1988). Dense aggregations of juvenile m e nha de n reside in estuaries through the summer (Ahrenholz et al. t989). In the fall, t empera ture changes initiate an emigrat ion of the year-class to the coastal ocean (Friedland and Haas 1988). The factors that concentra te pos tmetamorphic menha-

1 Corresponding author. _o Present address: National Marine Fisheries Service, Woods

l-hfle I.aboratory, Woods Hole. Massachusetts 02543.

den into schools, and later define the distribution of schools along various physical and biological gradients within an estuary, de te rmine the seasonal evolution of the nursery.

To unders tand these factors, it is impor tan t to trace the ew)lution o f the species' feeding habits from larvae to juvenile to adult. While on the con- tinental shelf, m e n h a d e n larvae are particulate omnivores, a feeding strategy which is appropr ia te tbr this habitat. The durat ion of this feeding stage is apparently a plastic trait (Maillet and Checkley 1991), thus allowing an ex tended time window for the metamorphosis to filter feeding. This trait is very important because the time necessary for larval menhaden to recrui t to the mouths of estuaries can be quite variable (Warlen 1994). On the nursery grounds, the fish metamorphose into obligate filter-feeding phytoplanktivores (June and Carlson 1971; Friedland et al. 1984). As adults, m e n h a d e n cont inue to filter feed; however, they tend to feed on larger food particles than those utilized by juveniles (Durbin and Durbin 1975).

It has become increasingly evident that feeding and distribution behaviors are closely linked for At- lantic menhaden . Early trawling studies demon- strated that juvenile m e n h a d e n were not distrib- uted consistently with salinity or t empera ture gradients within an estuary (Massmann et al. 1954;

�9 1996 Estuarine Research Federation 105

1 0 6 K.D. Frledland et al.

Wilkens and Lewis 1971; T u r n e r and Johnson 1973). Biological de terminants of distribution were first speculated on by June and Chamberl in (1959) and Weinstein (1979). However, it was not until the distribution of adult m e n h a d e n was compared with satellite te lemetry observations of phytoplankton biomass (Kemmerer et al. 1974) and juvenile distributions were compared with phytoplankton biomass gradients (Friedland et al. 1989) that a par- adigm emerged that m e n h a d e n distribution was def ined by phytoplankton gradients. Therefore , it could be hypothesized that the juvenile m e n h a d e n nursery is pa t te rned by these biological gradients. This hypothesis was partially tested by Friedland et al. (1989), but because this study only examined distributions in creek systems and did not survey the associated estuaries, fiarther evidence is required. If it could be demonst ra ted that juvenile menhaden respond to pat terns of phytoplankton biomass th roughou t the estuary, a more complete test of the above hypothesis would be achieved.

In this paper, we describe the results of a field survey designed to fu r ther test the hypothesis that the distribution patterns of juvenile m e n h a d e n dur ing their estuarine phase are de t e rmined by spatial and temporal changes in phytoplankton biolnass (Friedland et al. 1989). We also present our ideas on the general mechanisms affecting juvenile m e n h a d e n nursery format ion and the seasonal evolution of the nursery in estuaries.

Methods and Materials

JI_~ENiLF MENHADFN DISTRIBUTION AND BIOI,OGICAL AND PHYSICAL PARAMETERS

Relative abundance of pos tmetamorphic juvenile m e n h a d e n , salinity, t e m p e r a t u r e , secchi depth, and chlorophyll a concent ra t ion were measured dur ing a series of three cruises in the Neuse and Pamlico estuaries during the spring of 1984 (Fig. 1). The Neuse and Pamlico estuaries are tid- al ly-dominated, ver t ica l ly-homogenous estuaries that empty into Pamlico Sound, Nor th Carolina (Hobbie 1974; Hobble and Smith 1975). The cruises took place on the Neuse River on May 2 and 15 and the Pamlico River on May 16; 14, 26, and 20 stations were sampled, respectively.

Relative abundance of pos tmetamorphic juvenile menhaden was de te rmined using a surface trawl. The net (width: 1.2 m; height: 0.6 m; length: 3.5 m; construction: 3.2-mm multif i lament knot ted mesh) was towed f rom the R /V Onslow Bay during the May 15 and 16 cruises, thus it was pulled with a single tow line. However, the cruise on May 2 was done f rom two small boats, thus, the net was pulled by two tow lines. We ment ion this fact because we believe the latter towing a r rangement p roduced a

C2~ P a m l i c o ,~ R i v e r 0

United States "3,

36N

i--Neuse~~/, f - . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5 N Rive r

~' ' , A t l a n t i c O c e a n ,,

' ', 5 0 km ,,

7 7 W 7 6 W

Fig. 1. Map of coastal North Carolina showing location of Neuse and Pamlico rivers.

crowding effect on the lish and may have increased the catch rates. Standard ~ m i n tows were con- ducted and catches (and trawl volumes) were ad- justed to a s tandard volume using flow meters hung in the trawl mouth.

Water samples for salinity, tempera ture , and chlorophyll a measurements were taken by pump at 1.0 m depth. Salinity was de t e rmined with a Beckman model RS5-3 induct ion sal inometer (to the nearest 0.1%o). Tempera tu re was measured with a glass thermometer . Chlorophyll a concen- tration was de t e rmined using the methods described in Friedland et al. (1989). Secchi dep th was recorded to the nearest centimeter.

Length frequencies of fish captured dur ing each cruise were developed. Each length f requency represents a composi te for a cruise and is not weight- ed for individual tows. In addition, a similar length f requency tor juvenile m e n h a d e n captured using the same trawl dur ing juvenile abundance surveys in Bath Creek, a tr ibutary of the Pamlico River, dur ing April through June 1984, was developed (Ahrenholz et al. 1987a).

PATTFRNS OF PHYrOPI,ANKTON BIOMASS IN NORTH C, AROLINIAN ESTUARIES

Informat ion on salinity and chlorophyll a distri- but ions in the Neuse and Pamlico estuaries, origi- nally r epor ted by Hobble and Smith (1975) and Hobbie (1974), is presented to characterize the temporal and spatial distribution of phytoplankton biomass in the study areas. These studies charac-

�9 s, .on / J .

Salinity %,

Menhaden number tow -1 [ ] <500 [ ] 500-750 [ ] 751-1000 �9 > 1000

Chlorophyll a p.g I -~ [ ] <50

[ ] 50-60 [ ] 61-70

�9 >70

Fig. 2. Contours of surface salinity, relative abundance of juvenile menhaden, and chlorophyll a in the Neuse River, North Carolina on May 2, 1984.

Juvenile Menhaden Nurseries 107

%

Salinity %=

Menhaden number tow -1 [ ] <25 [ ] 25-50 [ ] 51-75

�9 >75

Chlorophyll a gg I ~ [ ] <20

[ ] 20-30

[ ] 31-40

[ ] >40

Fig. 3. Contours of surface salinity., relative abundance of juvenile menhaden, and chlorophyll a in the Neuse River, North Carolina on May 15, 1984.

terized the nut r ien t dynamics o f the Pamlico and Neuse estuaries, respectively. These authors developed con tour plots o f the salinity and chlorophyll a concent ra t ion by river k i lometer and t ime for three successive years. We c o m b i n e d their yearly con tour plots into a cont inuous t ime series to provide a descript ion of the seasonal shifts in salinity and chlorophyll a concen t ra t ion in the estuaries. However, the chlorophyll a levels r epo r t ed by Hob- bie (1974) and Hobb ie and Smith (1975) are not directly comparab le to those in this report ; we used a m o r e effective extract ion technique that yielded higher chlorophyll concent ra t ions (D'Elia et al. 1986).

Results

JUVENII,E MENIIADFN DISTRIBUTION AND BIOI,OGICakL AND PHYSICAl. PARAMFTERS

Juvenile m e n h a d e n were cap tu red in all three cruises on the Neuse and Pamlico rivers. On May 2, 1984, juveni le m e n h a d e n were taken at the m o u t h of the Neuse River estuary in 3%0 to 5%0 salinity water, which coinc ided with a band of phy- top lankton biomass that had levels o f chlorophyll a exceeding 70 ~g I -~ (Fig. 2). Catch rates for juvenile m e n h a d e n exceeded 1,000 tish tow l . These rates were the highest obse rved a m o n g the three cruises. However, since the tows on this date werc

done using a pair of small boats, they were ex- pec ted to p roduce h igher catch rates because of the crowding effect o f the tow lines and boats.

On May 15, the Neuse River was resampled with a more extensive survey (Fig. 3). Fish a b u n d a n c e and chlorophyll a levels were lower dur ing this second survey; however, it is impor t an t to r e m e m b e r the trawl was towed differently dur ing the two dates. Two relatively high concent ra t ions of juvenile m e n h a d e n were encoun te red . One g roup o f fish, located in the seaward end of the estuary, f o r m e d a tongue ex tend ing f rom the sou the rn bank in 4%0 to 5%0 salinity water. The densities o f fish exceeded 75 fish tow 1. A second g roup was located near the mid por t ion of the estuary in 2%0 to 3%0 salinity water and were in densities between 25 fish tow 1 and 50 fish tow 1. Both groups were associated with elevated levels o f chlorophyll a in the surface water.

The Pamlico River was surveyed on May 16, 1984 (Fig. 4). T h e highest concent ra t ion of juvenile m e n h a d e n was e n c o u n t e r e d at the head of the estuary in water o f less than 1%o salinity at densities exceeding 300 fish tow -1. T h e fish distribution co- incided with the highest concent ra t ion of chlorophyll a, well in excess of 75 ~g 1-1.

] ' h o u g h not presented , surface water t empera - ture and secchi dep th distributions in both estu-

108 K.D. Friedland et al.

�9 Station

Salinity %0

Menhaden numbertow -1 [--]<100

D100-200

[]201-300

m>3oo

Chlorophyll a lag [3

[ ] <25 [ ] 25-50

[ ] 51-75

[ ] >75

Fig. 4. Contours of surface salinity, relative abundance of juvenile menhaden, and chlorophyll a in the Pamlico River, North Carolina on May 16, 1984.

aries were visually c o m p a r e d to juvenile m e n h a d e n distributions, but no associations were found.

Dur ing the three May surveys, the surface trawl c a p t u r e d p o s t m e t a m o r p h i c j uven i l e m e n h a d e n ranging in size f rom 29 m m (10th percent i le) to 38 m m (90th percent i le) fork length. Size ranges of juveniles were similar in all o f the surveys (Fig. 5) suggesting the fish were f rom the same spawning group. The size distribution of the fish captured on May 2 had a med ian length of 32 m m and an interquart i le range of 30 m m to 34 m m (Fig. 5A). The size distr ibution 13 d later, also in the Neuse River, shifted to a median length of 34 m m and interquart i le range of 32 m m to 35 m m (Fig. 5B). Fish sampled in the Pamlico River on May 16 were only slightly smaller then those found on the Neuse with a median length of 33 m m and an interquart i le range of 31 m m to 35 m m (Fig. 5C).

Fish caught dur ing the May 1984 river cruises were of sizes that also occur red in the creeks of the same rivers. This is i l lustrated by the length fre- quency of catches m a d e in Bath Creek dur ing Apr i l -June surveys (Fig. 5D). The lengths of fish caught in these surveys ranged between approxi- mately 29 m m (10th percent i le) and 50 m m (90th percent i le) .

0 I

15 tB '

:t 0 /

5o . .

Neuse, May 2, 1984 n=496

Neuse, May 15, 1984 n=421

F ~ I T - - I

Parnlico, May 16, 1984 n=492

pnl une 198

LL II1

.>_

ft .

I ' ' I '

30 40 50 60

Fork Length (mm)

Fig. 5. Length frequencies of juvenile menhaden captured during Neuse and Pamlico cruises (A through C) and during juvenile abundance surveys in Bath Creek (D).

PATTERNS OF PHYI'OPLANKTON BIOMASS IN NORTII CAROI.INIAN ESTUARIES

The fresh to salt water gradient and the location of the phy top lank ton biomass m a x i m u m were cor- related with each other, and shifted seasonally in bo th river systems. We c o m p a r e d the posit ion of the chlorophyll m a x i m u m to salinity, bu t we do not suggest that salinity is the p r imary d e t e r m i n a n t of phy top lank ton p roduc t ion in these estuaries.

The location of the estuarine chlorophyll maxi- m u m usually occur red in close proximity to the band of 4%0 to 6%o salinity water in bo th estuarine systems. This band shifted posit ion seasonally. In the Neuse River, it was located at river k i lometer 60 dur ing the spring freshet but occur red fur ther up river (a round ki lometer 30) dur ing late sum- m e r (Fig. 6). Similarly, in the Pamlico River, the 4%0 to 6%o salinity band was found at k i lometer 30 in spring and at k i lometer 10 in s u m m e r (Fig. 7). The location of the chlorophyll a m a x i m u m , as depic ted by the location of the 25 Ixg 1-1 chlorophyll isopleth and secondarily the location of the 10 Ixg 1 -~ isopleth, also shifted seasonally in association with the 4%~ to 6%0 salinity band in bo th


S u r f a c e Salini ty (%~

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I I I I I I I I I I I I I I I I I I I I I I I I I I

A S O N D J F M A M J J A S O N D J F M A M J J A S O N S J F M A M J J A 1 9 7 0 1971 1 9 7 2 1 9 7 3

Y e a r a n d Month Fig. 6. River distance time series plot o f surface salinity and chlorophyll a for the Neuse River, North Carolina. Redrawn from

l tobb ie and Smith (1975).

river systems (Figs. 6 and 7). Dur ing the March to August per iod, when m e n h a d e n juveniles are estuarine residents, salinity typically increased with decreased s u m m e r river flows and the chlorophyll m a x i m u m shifted ups t ream (Figs. 6 and 7).

Discussion

Friedland et al. (1989) demons t r a t ed that the distribution pa t te rns of juveni le m e n h a d e n in es- tuar ine creeks were the result of" gradient search behaviors for phy top lank ton food. They were un- certain, however, whether juveni le m e n h a d e n were cont inuously dis tr ibuted t h r o u g h o u t the estuary, or whether juveni les exhibi ted behaviors that re- stricted them to the creeks. T h e main f inding of our study indicates an estuary-wide association between the distr ibution of" phy top lank ton biomass and the relative a b u n d a n c e of juveni le menhaden �9 Since catch-per-unit-effort for juveni le m e n h a d e n in the mains tem of the Neuse and Pamlico rivers was equivalent to that d o c u m e n t e d in creek surveys (Ahrenholz et al. 1989) we con tend that: 1) juveni le m e n h a d e n are dis tr ibuted th roughou t the

estuary; 2) popula t ions in the creeks can be e i ther cont inuous or disjunct with mains tem distributions; and 3) feeding and distributional behaviors observed in creeks also apply to the mains tem estuary. We believe that the mains tem estuary represents a major por t ion of the nursery area for juvenile m e n h a d e n and that the nursery must be viewed in totality as a cont inuous distr ibution of juveni le fish th roughou t the estuary�9 However, we in terpre t our results with some caution due to the l imited t empora l coverage of the river sampling. Until m o r e extensive data can be collected, we must rely on the previous results of Fr iedland et al. (1989) to make inference about the behaviors o f o lder , l a rge r j u v e n i l e s d u r i n g the s u m m e r months .

The Neuse and Pamlico estuaries exhibi t pre- dictable pat terns of phy top lankton distribution, a feature that can be general ized to o the r estuaries providing m e n h a d e n nursery habitat and that in part explains m e n h a d e n estuarine dependence . The factors control l ing phy top lank ton productivity in estuaries are diverse and include light penetra-


Sur face Sal inity (%o) ~ ~ , / ' - ~ ' a k ' ' , f ~ . . ~ , , " ' ' ' . ' " . . L . % . ' " -

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A S O ~ I D J F M h , M 'J ' ' ' J ;~ ~ ~ M n'l ~ ' ; '~A~'J' l~-~ ~'M'S'J L- ;,~l'~'~'J'n~ . ~ . ~ . . ~ .~ . . . , ,.. ~ n . . . ~ , . . , . , . . , . , ~.. 1970 1971 1972 1973

Y e a r and Month Fig. 7. River distance time series plot of surface salinity and chlorophyll a for the Pamlico River, North Carolina. Redrawn from

IIobbie (1974).

tion, water-column stability, and nut r ien t availabil- ity (Anderson ] 986; Pennock and Sharp 1986; Fish- er et al. 1988; Boyer et al. 1993; Harding 1994). What these factors hold in c o m m o n is the relation- ship each has to the mixing and flow dynamics of the estuary. Thus, enhanced phytoplankton product ion zones, similar to those described for the Neuse and Pamlico rivers, exist in o ther estuaries likely to be used by menhaden for nursery areas (Anderson 1986; Pennock and Sharp 1986; Fisher et al. 1988; t Ia rd ing 1994). We suggest that these enhanced phytoplankton zones are critical to the survival of pos tmetamorphic juvenile menhaden , and may have played a role in the evolution of the species estuarine dependence . Estuaries on the east coast of the United States can be viewed as stable productivity zones used by menhaden to facilitate the transition f rom par t iculate-feeding to filter-feeding, and which suppor t the growth of juveniles (Reintjes and Pacheco 1966).

Juvenile fish nursery areas are increasingly being described in the context of optinlal environments (Joseph 1973; Miller and Dunn 1980). In some

cases, the nursery is being strictly def ined by phy- siochemical requi rements of growth (Lankford and Targett 1994). In o ther cases, nurseries are be- lieved to confer protect ion from predat ion or can- nibalism (Kneib 1987). The benefits of the lnen- haden nursery appear to be the optimization of feeding and growth. Juvenile m e n h a d e n are capa- ble of cropping nanophytoplankton , yet incapable of the filtering volumes associated with adult feeding. The phytoplankton species mix and productivity in the estuary favors this c ropping strategy, whereas in adult m e n h a d e n habitats (the lower estuary and coastal ocean) plankton communit ies contain larger plankton at lower biomass concen- trations. In these environments , h igher filtering- volume feeding strategies are required, and thus would not facilitate juvenile growth.

From our work, we propose a hypothesis on the format ion and seasonal evolution of juvenile menhaden nurseries. Prior to nursery formation, larval m e n h a d e n appear to be loosely aggregated and their movements significantly inf luenced by passive mechanisms associated with wind and tide. At this


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Fig. 8. Hypothetical distribution of surface salinity, phytoplankton biomass, and.juvenile menhaden during initiation of juvenile nursery and after seasonal shifts in respective distributions. Panel A represents the spring condition and panel B represents the summer condition. Salinity is depiction of measurement along the mainstem of the estuary. Hatched areas are intended to denote area of higher relative abundance.

stage in their life history, m e n h a d e n larvae are only secondarily associated with pa t te rns of phy- top lankton biomass th rough feeding on zooplankton and large phytoplankton. After m e t a m o r p h o - sis, .juveniles b e c o m e obligate filter-feeding phytoplanktivores, and begin to distribute in g raded pat- t e rns c o r r e l a t e d with the d i s t r i b u t i o n o f phy top lank ton biomass (Friedland et al. 1989). T h e m e n h a d e n n u r s e r y first f o r m s with this change in feeding behavior. Spring condi t ions in the estuary, in f luenced primari ly by flow, control pa t te rns of phy top lank ton p roduc t ion and result in a concent ra t ion of fish in the lower por t ion of the estuary (Fig. 8A).

The nursery evolves with seasonal changes in es- tuar ine flow and the distr ibution of chlorophyll maxima. As water flow abates dur ing the summer,

chlorophyll m a x i m a occur fu r ther ups t ream, and in response, juveni le m e n h a d e n forage in those areas. The river ma ins tem c o m p o n e n t of the nursery moves progressively ups t ream (Fig. 8B). Creeks also have salinity gradients and isolated chlorophyll m a x i m a associated with t hem (Friedland et al. 1989). Creek systems, acting as i n d e p e n d e n t mixing zones, often suppor t subpopula t ions (Fig. 8B).

In our view, the nursery is dynamic in t ime and space and selected by juveni le m e n h a d e n by virtue of changes in the p lank ton community . We define the estuary itself as the nursery grounds, or that region where the nursery is likely to form. In the pures t sense, we consider the nursery to be the spatial domain of a n-dimensional niche for juvenile m e n h a d e n . Food concent ra t ion is the domi-


nan t factor def ining that niche. Menhaden are nursery residents in a variety o f estuarine habitats o f t en hav ing salinity c o n c e n t r a t i o n s e x c e e d i n g those descr ibed in this r epo r t (Pacheco and Gran t 1965; Fr iedland and Haas 1988). T e m p e r a t u r e does not appea r to play a role until fish emigra te to sea dur ing early fall in response to a temperature cue (Friedland and Haas 1988). Al though other factors may play a role in def ining the juveni le niche, none have e m e r g e d tha t provide such a clear cause-and-effect re la t ionship as the distribution of phytoplankton.

Evidence of stratification by size suggests menhaden juveni le nurseries may have some internal organizat ion that we ascribe to feeding. Intra-es- mar ine movemen t s of the nursery popula t ions of gulf m e n h a d e n (Brevoortia patronus) are similar to those observed for Atlantic m e n h a d e n . Deegan (1990) found that the distr ibution of gulf menhaden was related to seasonal succession of phyto- p lank ton productivity within an estuary. However, evidence that the intra-estuarine migrat ion of gulf m e n h a d e n juveniles was s ize-dependent was also found and viewed as a reflect ion of the increased metabol ic demand s of larger fish. O the r evidence of size stratification has also be seen in both Atlan- tic and gulf m e n h a d e n (Ahrenholz et al. 1989). The metabol ic d e m a n d s of m e n h a d e n are m e t mos t directly by feeding, which is a mechanica l process of filtering. I f the distr ibution of schools of a .juvenile cohor t are stratifying by size, we predict that this is re lated to feeding efficiency. We suggest the p lank ton c o m m u n i t y be examined in grea ter detail as it relates to the m o v e m e n t of dif- fe ren t size juveniles within the estuary.

The t rophodynamics of estuaries and mar ine ecosystems remains an enigmat ic p rob lem. Do preda tors define the size f requency and species compos i t ion of the phy top lank ton communi ty in estuaries, or are observed p lank ton communi t i e s the result o f physical factors and nut r ien t avail- ability? Considerable progress has been made in character iz ing the relative role o f top-down versus bo t tom-up mechan i sms in freshwater food webs (McQueen et al. 1986; Moss et al. 1994; Ross et al. 1994). However, progress in mar ine and estuarine ecosystem has been e i ther l imited to very simple food chains (Kobler et al. 1994) or to only a few trophic connec t ions (Roman et al. 1988; Kioboe and Nielsen 1994; Nielsen and Kioboe 1994; Pur- cell et al. 1994; Soli~ and Krstulovi~ 1994). Men- haden have been identif ied as impor t an t grazers, and thus an impor t an t t rophic link, in most of the ecosystems they occur (McHugh 1967; Oviatt et al. 1971; Lewis and Peters 1994). In the context of estuaries, juveni le m e n h a d e n , by virtue of their filter ing capabilities and their a b u n d a n c e (Friedland

et al. 1984), may have a significant top-down influence on the p lankton community . The predict- ability of m e n h a d e n distr ibution versus chlorophyll distr ibution presents a methodologica l solution to addressing this p rob lem. Following the basic ap- p roach out l ined in this study, a synoptic survey of the estuary could be achieved that would allow stratified estimates of m e n h a d e n abundance , thus permi t t ing the grazing impac t to be computed . The same app roach is be ing used with allied species using informat ion on the feeding and distri- but ional habits of their juveniles (MacNeill and Brandt 1990; Luo and Brandt 1993).

The re is evidence that fishing pressure has had a d i spropor t iona te impac t on es tua r ine -dependen t species, therefore , the informat ional needs of fisheries m a n a g e m e n t may be most critical for these resources (Houde and Ru the r fo rd 1993). O u r findings have implicat ions for the design of resource surveys for .juvenile m e n h a d e n and o the r es tuar ine-dependen t species. Juvenile a b u n d a n c e surveys for m e n h a d e n were designed a round the concep t that juveni le catch rates in index creeks would provide an index of r ec ru i tmen t to the adul t popula t ion (Ahrenholz et al. 1989). The creek habitats were chosen because of sampl ing conve- n ience and because the creek popula t ions appea red to be more stable over time, thus more re- liably providing catches of juveni le m e n h a d e n (Pa- checo and Grant 1965; Wilkens and Lewis 1971). After careful evaluation, the creek index catch rates failed to reflect known changes in recruit- m e n t and were eventually a b a n d o n e d (Ahrenholz et al. 1989). The i r failure to predic t r ec ru i tmen t of the adult popula t ion supports the conten t ion that the creeks were isolated segments of the nursery popula t ion , and probably not indicative of the popula t ion as a whole. However, the p reced ing discussion of a census of the grazing c o m m u n i t y for t rophodynamics research is equally useful in the context of recru i tment . A stratified survey that pro- vides an est imate of the grazing impac t also pro- vides an est imate of recru i tment . To be of general interest in the m a n a g e m e n t of the species, the survey would have to cover the ent i re nursery range.

ACKNOWLEDGMENTS

We thank S. Rodgers, E Serchuk, R. Roundtree, A. Powell, and .J. Smith for productive reviews of the manuscript, and J. Hobbie and N. Smith for use of data on the Neuse and Pamlico rivers.

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Received for consideration, February 6, 1995 Accepted far pubhcatwn, May 23, 1995

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Formation and Seasonal Evolution of Atlantic Menhaden Juvenile Nurseries in Coastal Estuaries