Gizzard shad population dynamics in eutrophic Missouri reservoirs with emphasis on environmental influences on their growth

Figures & data

Table 1. Summary statistics (grand mean, range given within parentheses) for environmental variables used to assess growth of gizzard shad.

Reservoir	Cumulative temperature (degree days >15°C)	TP (µg/L)	TN (µg/L)	Chla (µg/L)	Turbidity (NTU)	Secchi depth (m)	Zooplankton (µg/L, dry weight)
Long Branch	1438	94	1296	29	18	0.6	184
	(1303–1611)	(70–124)	(1176–1411)	(24–33)	(13–23)	(0.5–0.7)	(160–204)
Mark Twain	1559	107	1570	20	17	0.9	110
	(1423–1777)	(71–139)	(1381–1763)	(18–22)	(9–23)	(0.6–1.1)	(90–143)
Thomas Hill	1968	68	901	12	16	0.6	105
	(1825–2033)	(60–84)	(876–958)	(11–14)	(12–23)	(0.5–0.7)	(93–120)
All	1655	90	1256	20	17	0.7	133
	(1303–2033)	(60–139)	(876–1763)	(11–33)	(9–23)	(0.5–1.1)	(90–204)
Notes: TP, total phosphorus; TN, total nitrogen, and Chla, chlorophyll a.

Figure 1. Length frequencies of gizzard shad collected from the study reservoirs during 2004–2008. Mean (SE) electrofishing (CPUE, number/h) is also shown.

Figure 2. Age composition (%) of gizzard shad in the study reservoirs during 2004–2008.

Figure 3. Von Bertalanffy growth curves for gizzard shad in Long Branch (LOB), Mark Twain (MTL), and Thomas Hill (THL) lakes. Equations are l_t  = 245.533(1–e ^{−0.284( t  + 1.863)}) for LOB (r ²= 0.98; p < 0.0001), l_t  = 233.473(1–e ^{−0.412( t  + 0.830)}) for MTL (r ²= 0.99; p < 0.0001), and l_t  = 244.282(1–e ^{−0.346( t  + 0.755)}) for THL (r ²= 0.99; p < 0.0001). Mean length for age 7 in Thomas Hill Lake was not computed due to very low sample size.

Figure 4. Three-dimensional scatter plot showing the influence of initial total length of gizzard shad and Secchi depth on growth increments of gizzard shad in Long Branch (solid circle), Mark Twain (open circle), and Thomas Hill (solid, upside-down triangle) lakes.

Table 2. Regression models explaining variation in growth increments (mm) of gizzard shad using initial mean total length (ITL) of gizzard shad, cumulative water temperature (Temp), total phosphorus concentration (TP), total nitrogen concentration (TN), chlorophyll concentration (Chla), turbidity (Turb), Secchi depth (Secchi), and zooplankton biomass (Zoop) as explanatory variables (n = 47).

Variables	Values for regression coefficients				r ^2	p	ΔAIC
	α	β 1	β 2	β 3
ITL, Secchi	96.718	−0.384	−17.995		0.778	<0.0001	0
ITL, Secchi, TN	98.570	−0.377	−16.135	−0.003^a	0.782	<0.0001	0.987
ITL, Secchi, Temp	90.053	−0.379	−17.839	0.003^a	0.782	<0.0001	1.083
ITL, Secchi, TP	98.368	−0.380	−16.351	−0.038^a	0.782	<0.0001	1.102
ITL, Secchi, Chla	97.857	−0.381	−17.906	−0.080^a	0.779	<0.0001	1.599
ITL, Secchi, Zoop	97.882	−0.383	−18.431	−0.008^a	0.778	<0.0001	1.896
ITL, Secchi, Turb	96.434	−0.384	−17.964	0.016^a	0.778	<0.0001	1.995
Notes: ^aNot significant at α = 0.05. Models were compared with AIC and ΔAIC indicates the differences in AIC values from the most parsimonious model (for which ΔAIC = 0) for each group of models. Only models within two units of the best model are shown.