254
R.R. Twilley et al.
bina sp., live in the intertidal of mangrove habitats in the Guayas River
estuary. Five species of penaeid shrimp, Penaeus occidentalis (Streets),
P. stylirostris (Stimpson), P. brevirostris (Kingsley), P. vannamei (Boone),
and P. californiensis (Holmes), occur in the Gulf, the last two being the
most abundant (Garda-Saenz et al. 1998). Penaeus californiensis is largely
restricted to shallow marine waters while Penaeus stylirostris and P. vannamei occur near mangrove areas. The densities of post-larvae tend to be
higher during the wet season and, although the abundance of juveniles
lacks seasonal differences, densities appear to be inversely related to the
distance of mangrove tidal channels from the coast (Zimmerman and
Minello 1989). Higher density of post-larvae and juveniles occur in the El
Morro Channel, which is close to spawning grounds (Loesch and Avila
1966; Zimmerman and Minello 1989). Additionally, site-specific DO concentrations correlate with the spatial distribution of shrimp in the Guayas
River estuary. Low DO concentration (0.7-3 ml 1- 1 ) retards growth of P.
vannamei (Seidman and Lawrence 1985) and values of less than 2.1 mll- 1
lead to suspension of feeding (Villalon et al. 1989). The negative effect of
DO may explain lower post-larvae densities in the Salado (2.1 mll- 1 ) than
in the Churute (2.3 ml 1- 1 ), although both subestuaries have similar
distances to spawning grounds. Artisanal fisheries capture post-larvae,
juveniles, and gravid females of Penaeus vannamei and P. stylirostris to
supply the shrimp aquaculture industry (McPadden 1985b), while offshore trawl fisheries capture pre-adults and adults. Trawl by-catch is
composed of the dominant Mysidacea. Decapod crustaceans (family
Caridea) are represented by Paleomonidae (50.6 o/o), Alpheidae (25.3 o/o),
Hippolytidae (21.7 o/o) and Pasiphalidae (2.4 o/o) and blue crabs include
Callinectes sp. (63.6%), C. arcuatus (36.1 o/o), and C. toxotes (0.3%; Garda-Saenz and Pelaez 1998).
17.5 Productivity and Trophic Structure
Variations of net primary productivity (NPP) are more pronounced between inner and outer Gulf regions than between seasons (Stevenson 1981).
While NPP in the outer Gulf is 200 mg C m- 3 day- 1 or less, values in the
inner Gulf are 600-800 mg C m- 3 day- 1 during the wet season and
800-l,OOOmg C m- 3 day- 1 during the dry season, differences being due to
higher turbidity in the wet season. Based on Secchi disk depth, the volume
estimates of NPP on an annual production per unit area range from 200 to
300 mg C m- 2 day- 1 in inner Gulf and from 300 to 1,000 mg C m- 2 day- 1 in
outer Gulf regions. Integration of monthly spatial estimates of daily NPP
R.R. Twilley et al.
bina sp., live in the intertidal of mangrove habitats in the Guayas River
estuary. Five species of penaeid shrimp, Penaeus occidentalis (Streets),
P. stylirostris (Stimpson), P. brevirostris (Kingsley), P. vannamei (Boone),
and P. californiensis (Holmes), occur in the Gulf, the last two being the
most abundant (Garda-Saenz et al. 1998). Penaeus californiensis is largely
restricted to shallow marine waters while Penaeus stylirostris and P. vannamei occur near mangrove areas. The densities of post-larvae tend to be
higher during the wet season and, although the abundance of juveniles
lacks seasonal differences, densities appear to be inversely related to the
distance of mangrove tidal channels from the coast (Zimmerman and
Minello 1989). Higher density of post-larvae and juveniles occur in the El
Morro Channel, which is close to spawning grounds (Loesch and Avila
1966; Zimmerman and Minello 1989). Additionally, site-specific DO concentrations correlate with the spatial distribution of shrimp in the Guayas
River estuary. Low DO concentration (0.7-3 ml 1- 1 ) retards growth of P.
vannamei (Seidman and Lawrence 1985) and values of less than 2.1 mll- 1
lead to suspension of feeding (Villalon et al. 1989). The negative effect of
DO may explain lower post-larvae densities in the Salado (2.1 mll- 1 ) than
in the Churute (2.3 ml 1- 1 ), although both subestuaries have similar
distances to spawning grounds. Artisanal fisheries capture post-larvae,
juveniles, and gravid females of Penaeus vannamei and P. stylirostris to
supply the shrimp aquaculture industry (McPadden 1985b), while offshore trawl fisheries capture pre-adults and adults. Trawl by-catch is
composed of the dominant Mysidacea. Decapod crustaceans (family
Caridea) are represented by Paleomonidae (50.6 o/o), Alpheidae (25.3 o/o),
Hippolytidae (21.7 o/o) and Pasiphalidae (2.4 o/o) and blue crabs include
Callinectes sp. (63.6%), C. arcuatus (36.1 o/o), and C. toxotes (0.3%; Garda-Saenz and Pelaez 1998).
17.5 Productivity and Trophic Structure
Variations of net primary productivity (NPP) are more pronounced between inner and outer Gulf regions than between seasons (Stevenson 1981).
While NPP in the outer Gulf is 200 mg C m- 3 day- 1 or less, values in the
inner Gulf are 600-800 mg C m- 3 day- 1 during the wet season and
800-l,OOOmg C m- 3 day- 1 during the dry season, differences being due to
higher turbidity in the wet season. Based on Secchi disk depth, the volume
estimates of NPP on an annual production per unit area range from 200 to
300 mg C m- 2 day- 1 in inner Gulf and from 300 to 1,000 mg C m- 2 day- 1 in
outer Gulf regions. Integration of monthly spatial estimates of daily NPP