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the coupling of AMO and NAO phases were found to be 
important drivers of long-term flows in the Mississippi 
River and Atchafalaya River and in the Pearl River and 
Pascagoula River, respectively (Sanchez-Rubio et ah, 
2011a). The effect of ENSO on river flow is more evi- 
dent in smaller river basins closely associated with the 
Gulf Coast environment (Sanchez-Rubio et al., 2011a). 
Mississippi River flow is tightly linked to physical 
and biological processes associated with the develop- 
ment and survival of early stages of Gulf menhaden 
on the continental shelf and in estuaries of the north- 
central GOM (Grimes and Finucane, 1991; Govoni and 
Grimes, 1992). From fall through early spring, biologi- 
cal productivity is enhanced by offshore transport and 
upwelling induced by northerly (north-south wind di- 
rection) and easterly (east-west wind direction) winds. 
Vector winds (Hitchcock et al., 1997) spread and proj- 
ect the Mississippi-Atchafalaya River plume over shelf 
waters as far as 100 km (Riley, 1937), resulting in a 
buoyant, nutrient-rich, freshwater mass with a well- 
defined frontal zone that veers westward in spring (Go- 
voni et al., 1989; Govoni and Grimes, 1992; Hitchcock 
et al., 1997). Gulf menhaden larvae aggregate in the 
river plume front on the continental shelf where the 
Coriolis force, wind speed, and wind direction shape 
the trajectory and properties of the plume that trans- 
ports larvae (Govoni et al., 1989; Govoni and Grimes, 
1992; Hitchcock et al., 1997) as far west as Texas (Din- 
nel and Wiseman, 1986). 
Mississippi-Atchafalaya river flow is positively re- 
lated to the seaward projection and areal coverage of 
the plume (Wright and Coleman, 1971; Walker and 
Rouse 2 ) and to the influx of nutrient-rich river water 
(Bratkovich et al., 1994). Seaward projection and areal 
expansion through high river discharge may prolong 
shoreward transport of larvae and increase vulnerabil- 
ity to predation (Govoni, 1997). Nutrient enhancement 
(Riley, 1937; Lohrenz et al., 1990; Dagg and Whitledge, 
1991) during high river discharge increases food avail- 
ability and growth, thereby decreasing predation and 
increasing survival of Gulf menhaden larvae (Govoni, 
1997). The river plume eventually enters the coastal 
current (Cochrane and Kelly, 1986; Wiseman and Kelly, 
1994), exchanging water (Dinnel and Wiseman, 1986; 
Wiseman and Garvine, 1995) and transporting Gulf 
menhaden larvae (Govoni, 1993) between the current 
and the coastal boundary zone. Larvae, once they are 
within the coastal boundary zone, enter estuaries with 
the vernal rise in sea level (Shaw et al., 1985). 
Juvenile development takes place in the estuary. 
During this period, interannual, strong northerly winds 
(coming from the north) promote coastal upwelling, in- 
crease nearshore productivity, and produce extremely 
low tide levels along the Gulf Coast with a resultant 
2 Walker, N. D., and L. J. Rouse Jr. 1993. Satellite assess- 
ment of Mississippi River discharge plume variability. OCS 
Study MMS 93-0044. U.S. Dep. Interior, Minerals Man- 
age. Serv., Gulf of Mexico OCS Region, New Orleans, 50 p. 
[Available at website.] 
die off of marsh vegetation (Kirby, 1972; Day et al., 
1973). This vegetation, accumulated during the winter, 
is later washed out by spring flood tides, stimulating 
spring plankton production (Guillory et al., 1983). The 
availability of food is further increased by high river 
flow that introduces new organic material to inshore 
waters and re-suspends existing detritus (Deegan and 
Thompson, 1985; Madden et al., 1988), fostering faster 
growth and increased survival of young Gulf menhaden 
(Deegan, 1990). 
For early life history stages, food availability is a 
critical aspect of habitat suitability (Christmas et al., 
1982; Deegan, 1986), and climate plays a crucial role 
in the structuring of habitats. Despite the economic 
and ecological importance of Gulf menhaden, studies 
of climate-related hydrological regimes on population 
abundances have been limited primarily to the study 
of annual and interannual factors thought to influence 
survival of larvae and juveniles. In this study, the ef- 
fects of decadal AMO and NAO phases as well as in- 
terannual ENSO events on northern GOM hydrological 
conditions were examined and related to abundances 
of menhaden documented in fishery-independent and 
fishery-dependent data. 
Two other species of menhaden have overlapping 
distribution ranges with the Gulf menhaden (Tolan 
and Newstead, 2004). In the western GOM, juveniles 
of the finescale menhaden ( Brevoortia gunteri) may co- 
occur with Gulf menhaden, and juvenile populations in 
that area are therefore referred to in this manuscript 
as menhaden species (Tolan and Newstead, 2004; An- 
derson and Karel, 2014). The range of yellowfin men- 
haden (B. smithi) overlaps that of Gulf menhaden in 
the eastern portion of the northern GOM; however, es- 
tuarine collections of Brevoortia in Alabama bays have 
yielded only Gulf menhaden (Boschung et al., 2004). 
Menhaden in fishery-independent studies in the cen- 
tral GOM (Louisiana, Mississippi, and Alabama) are 
almost exclusively Gulf menhaden (SEDAR 3 ). A host 
of studies have examined stock structure in the north- 
ern GOM, and all reveal that the fishery is composed 
of Gulf menhaden and other menhaden species, the 
latter of which represent less than 1% of the harvest 
(VanderKooy and Smith 4 ). 
We examined fishery-independent data and fishery- 
dependent data to identify the responses of menha- 
den populations to local meteorological and hydrologi- 
cal conditions imposed by the coupling of AMO and 
NAO phases and by ENSO events. Fishery-indepen- 
dent data included data 1) on Gulf menhaden from 
surveys conducted with beam plankton trawls IBPLs] 
in the central region, 2) on Gulf menhaden from 
3 SEDAR (Southeast Data, Assessment, and Review). 2013. 
SEDAR 32A — Gulf of Mexico menhaden assessment report, 
422 p. SEDAR, North Charleston, NC. [Available at web- 
site.] 
4 VanderKooy, S. J, and J. W. Smith (eds.). 2015. The menha- 
den fishery of the Gulf of Mexico, United States: a regional 
management plan, 181 p. Gulf States Mar. Fish. Comm. 240 
[Available at website.] 
