Condition Indices as Surrogates of Energy Density and Lipid Content in Juveniles of Three Fish Species

Figures & data

FIGURE 1. Spatial extent of sampling in Virginia estuaries for Summer Flounder (circles; n = 73), Striped Bass (squares; n = 66), and Atlantic Croakers (triangle; n = 72) used in comparisons of direct and indirect condition indices.

TABLE 1. Descriptive statistics for direct and indirect condition indices (Fulton’s K = Fulton’s condition factor; K_n = relative condition factor; L–M = length–mass; HSI = hepatosomatic index) measured from the same Summer Flounder (n = 73), Striped Bass (n = 66), and Atlantic Croakers (n = 72). The L–M deviations are the deviations from nonlinear regression models for fish length (mm) and mass (g) given by equations (3)–(5). Due to collinearity among predictors, the means of the raw indirect condition data were subtracted from individual observations of condition to estimate relationships between indirect condition indices and energy density (kJ/g of wet mass) or lipid content (g/g of wet mass).

Condition index	Mean	SD	Minimum	Maximum
Summer Flounder
Energy density	4.137	0.538	2.718	5.308
Lipid content	1.861	0.986	0.562	4.715
TL	223.9	34.6	156	289
Fulton’s K	0.900	0.109	0.601	1.103
Kn	0.999	0.129	0.625	1.247
L–M deviation	–2.290	15.039	–66.375	21.226
HSI	0.788	0.421	0.230	2.030
loge(fatmeter)	0.826	1.374	–2.303	2.639
Striped Bass
Energy density	5.032	0.658	3.441	6.869
Lipid content	3.930	1.429	1.262	7.828
FL	143.4	17.6	117	193
Fulton’s K	1.169	0.075	1.025	1.407
Kn	0.977	0.064	0.832	1.157
L–M deviation	–1.030	2.699	–11.801	8.362
HSI	1.876	0.650	0.656	3.516
loge(fatmeter)	3.310	0.249	2.464	3.843
Atlantic Croaker
Energy density	4.700	1.018	2.985	7.530
Lipid content	3.880	2.295	0.811	10.302
TL	160.8	22.7	118	214
Fulton’s K	1.053	0.081	0.895	1.221
Kn	0.970	0.065	0.835	1.177
L–M deviation	–1.715	3.343	–13.203	4.738
HSI	1.256	0.602	0.429	2.770
loge(fatmeter)	3.210	0.435	2.059	3.925

TABLE 2. Values of r² between indirect condition indices (Fulton’s K = Fulton’s condition factor; K_n = relative condition factor; L–M = length–mass; HSI = hepatosomatic index) and log_e transformed energy density (kJ/g of wet mass) and lipid content (g/g of wet mass) for juvenile Summer Flounder, Striped Bass, and Atlantic Croakers. Relative estimates of lipid content measured with the fatmeter were taken from both sides of each individual (Fatmeter_Mean represents the average) and log_e transformed to identify the “best” location for assessing lipid content. An asterisk denotes the fatmeter location used for index comparisons. All values were significant (P < 0.05) unless indicated by footnotes.

Indirect condition index	Lipid	Energy
Summer Flounder
Fulton’s K	0.39	0.46
Kn	0.19	0.32
L–M deviation	0.27	0.38
HSI	0.26	0.45
FatmeterEyed	0.39	0.33
FatmeterNoneyed*	0.40	0.44
FatmeterMean	0.41	0.41
Striped Bass
Fulton’s K	0.30	0.31
Kn	0.25	0.32
L–M deviation	0.23	0.29
HSI	0.21	0.33
FatmeterLeft*	0.22	0.12
FatmeterRight	0.07	0.02^a
FatmeterMean	0.14	0.06^a
Atlantic Croaker
Fulton’s K	0.53	0.48
Kn	0.22	0.23
L–M deviation	0.08	0.08
HSI	0.27	0.31
FatmeterLeft	0.64	0.58
FatmeterRight	0.66	0.61
FatmeterMean*	0.68	0.62

^aP > 0.05.

TABLE 3. General linear models used to predict energy density (kJ/g of wet mass) and lipid content (g/g of wet mass) with indirect condition indices (Fulton’s K = Fulton’s condition factor; K_n = relative condition factor; L–M = length–mass; HSI = hepatosomatic index) and fish length for Summer Flounder (n = 73), Striped Bass (n = 66), and Atlantic Croakers (n = 72). Predictors were centered about their means (Table 1), and Akaike’s information criterion corrected for small sample size (AIC_c) was used for final model selection (ΔAIC_c = difference in AIC_c value between the given model and the best-performing model). Parameter estimates for final models are presented with 95% confidence intervals. The proportion of the total variation in energy density and lipid content explained by the indirect condition indices was determined by using a restricted maximum likelihood (REML)-based r² estimate ().

Direct	Indirect	Model	AICc	ΔAICc
Summer Flounder
Energy	Fulton’s K	loge(Energy) = (1.411 ± 0.023) + (0.830 ± 0.211)·K	–121.6	0.0	0.46
	Kn	loge(Energy) = (1.411 ± 0.023) + (0.733 ± 0.195)·Kn + (0.002 ± 0.001)·TL	–106.6	15.0	0.44
	L–M deviation	loge(Energy) = (1.411 ± 0.025) + (0.006 ± 0.002)·Deviation	–102.0	19.6	0.38
	HSI	loge(Energy) = (1.411 ± 0.023) + (0.213 ± 0.056)·HSI	–117.1	4.5	0.44
	Fatmeter	loge(Energy) = (1.411 ± 0.024) + (0.064 ± 0.017)·loge(Fatmeter)	–113.3	8.3	0.43
Lipid	Fulton’s K	loge(Lipid) = (0.475 ± 0.104) + (3.194 ± 0.953)·K	92.3	0.0	0.38
	Kn	loge(Lipid) = (0.475 ± 0.101) + (2.723 ± 0.848)·Kn + (0.008 ± 0.003)·TL	99.4	7.1	0.41
	L–M deviation	loge(Lipid) = (0.475 ± 0.099) + (0.024 ± 0.007)·Deviation + (0.007 ± 0.003)·TL	105.2	12.9	0.44
	HSI	loge(Lipid) = (0.475 ± 0.114) + (0.683 ± 0.272)·HSI	108.2	15.9	0.25
	Fatmeter	loge(Lipid) = (0.475 ± 0.103) + (0.258 ± 0.075)·loge(Fatmeter)	95.9	3.6	0.39
Striped Bass
Energy	Fulton’s K	loge(Energy) = (1.606 ± 0.028) + (0.997 ± 0.369)·K	–93.0	0.7	0.30
	Kn	loge(Energy) = (1.606 ± 0.028) + (1.180 ± 0.432)·Kn	–93.7	0.0	0.31
	L–M deviation	loge(Energy) = (1.606 ± 0.028) + (0.027 ± 0.011)·Deviation	–83.4	10.3	0.28
	HSI	loge(Energy) = (1.606 ± 0.027) + (0.119 ± 0.042)·HSI	–90.1	3.6	0.32
	Fatmeter	loge(Energy) = (1.606 ± 0.031) + (0.185 ± 0.063)·loge(Fatmeter)	–74.5	19.2	0.10
Lipid	Fulton’s K	loge(Lipid) = (1.297 ± 0.082) + (2.891 ± 1.101)·K	47.0	0.0	0.29
	Kn	loge(Lipid) = (1.297 ± 0.086) + (3.073 ± 1.343)·Kn	51.5	4.5	0.23
	L–M deviation	loge(Lipid) = (1.297 ± 0.087) + (0.070 ± 0.032)·Deviation	60.6	13.6	0.22
	HSI	loge(Lipid) = (1.297 ± 0.088) + (0.282 ± 0.136)·HSI	58.9	11.9	0.20
	Fatmeter	loge(Lipid) = (1.297 ± 0.087) + (0.752 ± 0.352)·loge(Fatmeter)	56.3	9.3	0.21
Atlantic Croaker
Energy	Fulton’s K	loge(Energy) = (1.525 ± 0.036) + (1.811 ± 0.449)·K	–58.4	26.6	0.47
	Kn	loge(Energy) = (1.525 ± 0.036) + (1.858 ± 0.558)·Kn + (0.005 ± 0.002)·TL	–49.2	35.8	0.49
	L–M deviation	loge(Energy) = (1.525 ± 0.039) + (0.033 ± 0.013)·Deviation + (0.006 ± 0.002)·TL	–30.5	54.5	0.40
	HSI	loge(Energy) = (1.525 ± 0.041) + (0.196 ± 0.069)·HSI	–34.7	50.3	0.30
	Fatmeter	loge(Energy) = (1.477 ± 0.029) + (0.490 ± 0.076)·loge(Fatmeter) – (0.001 ± 0.001)·TL + (0.010 ± 0.003)·[loge(Fatmeter) × TL]	–85.0	0.0	0.75
Lipid	Fulton’s K	loge(Lipid) = (1.172 ± 0.103) + (5.728 ± 1.288)·K	89.2	22.6	0.52
	Kn	loge(Lipid) = (1.172 ± 0.102) + (5.630 ± 1.601)·Kn + (0.016 ± 0.005)·TL	96.3	29.7	0.53
	L–M deviation	loge(Lipid) = (1.099 ± 0.107) + (0.139 ± 0.037)·Deviation + (0.019 ± 0.005)·TL – (0.003 ± 0.001)·(Deviation × TL)	113.2	46.6	0.55
	HSI	loge(Lipid) = (1.172 ± 0.119) + (0.440 ± 0.210)·HSI + (0.010 ± 0.006)·TL	122.3	55.7	0.36
	Fatmeter	loge(Lipid) = (1.172 ± 0.086) + (1.204 ± 0.199)·loge(Fatmeter)	66.6	0.0	0.67

FIGURE 2. (A) Linear regressions (solid lines) of energy density and selected indirect condition indices for Summer Flounder and Striped Bass; (B) regressions of lipid content and selected indirect indices for all three species; and (C) partial regression plot used to assess the relationship between relative lipid estimates measured with the fatmeter and energy density for Atlantic Croakers. The indices shown are those selected from the general linear models based on the lowest Akaike’s information criterion corrected for small sample size (Table 3; Fulton’s K = Fulton’s condition factor). The solid symbols indicate laboratory-held individuals; the dashed lines denote 95% confidence intervals around the equations used to predict log_e transformed energy and lipid content (see Table 3).