Using a continuous riverscape survey to examine the effects of the spatial structure of functional habitats on fish distribution

Figures & data

Figure 1. Geographical location of study area. (a) The Seine river basin, France, and the ‘La Bassée’ alluvial floodplain; (b) study segment showing nase abundance for the 266 samples; and (c) a detail at the downstream limits of the segment.

Table 1. Explanatory variables measured or calculated at the sampling unit scale, evaluated for association with the abundance of the species and explanatory variables calculated at the feeding habitat patch scale evaluated for association with the presence/absence of the species.

Variable	Units/category	Scale	Variable type	Data type	Description	Abbreviation
Water depth	cm	Sample (i)	Environmental	Continuous		De
Current velocity	m/s	Sample (i)	Environmental	Continuous		V
Bottom substratum		Sample (i)	Environmental	Categorical	Dominant bottom substratum	Sub
Mud/vegetation	mv
Sand	s
Sand/gravel	sg
Gravel or boulder	g
Distance to a patch of spawning habitat	m	Sample (i)	Spatial	Continuous	The nearest one in upstream or downstream direction	DistS
Distance to a patch of resting habitat	m	Sample (i)	Spatial	Continuous	The nearest one in upstream or downstream direction	DistR
Distance to a patch of feeding habitat	m	Sample (i)	Spatial	Continuous	The nearest one in upstream or downstream direction	DistF
Distance to upstream weir	m	Sample (i)	Spatial	Continuous		Distup
Proportion of resting habitat	%	Sample (i)	Spatial	Continuous	In a 120 m squared window centred on the sampling unit	PrSR
Proportion of feeding habitat	%	Sample (i)	Spatial	Continuous	In a 120 m squared window centred on the sampling unit	PrSF
Proximity index to patches of resting habitat	Dimensionless	Sample (i)	Spatial	Continuous		PxSR*
Proximity index to patches of feeding habitat	Dimensionless	Sample (i)	Spatial	Continuous		PxSF*
Patch area of feeding habitat	m^2	Feeding habitat (Fj)	Spatial	Continuous		AreaF
Proximity index to patches of feeding habitat	Dimensionless	Feeding habitat (Fj)	Spatial	Continuous		PxFF*
Proximity index to patches of resting habitat	Dimensionless	Feeding habitat (Fj)	Spatial	Continuous		PxFR*

*Log-transformed variables; Px: proximity index; i and j: target sample and patch; s: index for other patches; n: number of patches considered in the calculation.

Figure 2. Spatial variables calculated at two scales. (a) At the sample (i) scale: 1– distance to upstream weir (Distup), 2– distance to the nearest patch of spawning/resting/feeding habitat (DistS, DistR, DistF), 3– proportion of resting (or feeding) habitat in a 120 m square window (PrSR, PrSF) around the sample, 4– proximity index to patches of resting (or feeding) habitat (PxSR, PxSF), D_is is the minimal distance between sample (i) and the patch of resting (or feeding) habitat. (b) At the feeding habitat patch (F_j) scale: 5– patch area (AreaF), 6– proximity index to patches of feeding habitat (PxFF, search distance = 60 m), 7– proximity index to patches of resting habitat (PxFR, search distance = 60 m), D_js is the edge-to-edge longitudinal distance between habitat patches. See Table 1 for formula.

Table 2. Results of the AIC-based selection for models of single variables predicting the most probable abundance of C. nasus and B. barbus from the 266 samples collected at dawn and dusk. Models are ranked from the most plausible (low AIC) to least plausible; only those models that ranked higher than the null model are presented.

Species	Model*	AIC	ΔAIC	AIC weight	p-value
C. nasus	DistS	625.31	0	0.25	<0.001
	V	625.73	0.42	0.20	<0.001
	DistR	625.78	0.47	0.20	<0.001
	PrSR	626.39	1.08	0.15	0.001
	De	626.49	1.18	0.14	<0.001
	PrSF	628.17	2.86	0.06	<0.001
	Sub	633.30	7.99	<0.01	0.005 (mv −)
	PxSF	634.52	9.21	<0.01	0.020
	Distup	634.52	13.51	<0.01	0.109
B. barbus	DistS	255.03	0	0.97	0.021
	V	262.69	7.66	0.02	<0.001
	Sub	266.54	11.51	<0.01	<0.001 (s −)
	DistF	272.42	17.39	<0.01	0.032
	PxSR	276.17	21.14	<0.01	0.152
	PrSF	276.94	21.91	<0.01	0.014

Note: Significant bottom substrate, mv: mud and vegetation, s: sand, − : negative effect.

*For definition of variables, see Table 1. Dispersion parameter for the negative binomial (θ): C. nasus θ ∼ 0.14–0.17; B. barbus θ ∼ 0.10–0.17.

Table 3. Results of the AIC-based selection for models that are a combination of the ‘best’ single-variable models predicting the most probable abundance of C. nasus and B. barbus from the 266 samples collected at dawn and dusk. Only models containing non-correlated explanatory variables (Kendall correlation < |0.3|) are proposed.

Species	Model	AIC	ΔAIC	AIC weight
C. nasus	V, DistS, De, DistR	600.36	0	0.86
	De, DistR, DistS	604.06	3.7	0.14
	V, DistR, DistS	613.52	13.16	<0.01
	DistR, DistS	616.38	16.02	<0.01
	V, DistS	616.40	16.04	<0.01
B. barbus	V, DistS	246.99	0	0.43
	V, DistS, PxSR	247.74	0.75	0.30
	V, DistS, Sub, PrSF	249.11	2.12	0.15
	V, DistS, Sub	249.50	2.51	0.12
	DistS, PrSF, PxSR	256.38	9.39	<0.01

Note: For definition of variables, see Table 1. C. nasus θ ∼ 0.24–0.25; B. barbus θ ∼ 0.21–0.24.

Table 4. Parameter estimates and significance levels for the best models predicting the most probable abundance of C. nasus and B. barbus from the 266 samples collected at dawn and dusk.

	C. nasus			B. barbus
Parameter	Estimate	SE	p-value	Estimate	SE	p-value
Intercept	1.3907	0.5965	0.020	−2.1991	0.4761	<0.001
De	−0.0205	0.0052	<0.001
V	1.1404	0.5468	0.037	2.1915	0.7136	0.002
DistS	−0.0058	0.0022	0.008	−0.0674	0.0386	0.080
DistR	0.0102	0.0041	0.014

Figure 3. (a) Proximity index to patches of feeding habitat (PxFF) and resting habitat (PxFR), plotted versus their minimal longitudinal distance to upstream weir. (b) Longitudinal distribution of the mean densities (±SD) of >0+ barbel and nase in the 48 patches of feeding habitat, plotted versus their minimal longitudinal distance to upstream weir. White dots indicate patches with null density.

Table 5. Results of the AIC-based selection for models predicting the presence at dawn and dusk of C. nasus and B. barbus within feeding habitat patches (48 patches). Models are ranked from the most plausible (low AIC) to least plausible; only those models that ranked higher than the null model are presented.

Species	Model	AICc	ΔAICc	AICc weight
	AreaF, PxFF	60.33	0.0	0.50
C. nasus	AreaF	60.75	0.4	0.41
	PxFF	63.88	3.6	0.08
	AreaF, PxFF	50.63	0.0	0.63
B. barbus	AreaF	51.78	1.2	0.36
	PxFF	59.61	9.0	<0.01

Note: For definition of variables, see Table 1.

Table 6. Parameter estimates and significance levels for the best models predicting the presence at dawn and dusk of C. nasus and B. barbus within feeding habitat patches (48 patches).

	C. nasus			B. barbus
Parameter	Estimate	SE	p-value	Estimate	SE	p-value
Intercept	−1.1560	0.4821	0.016	−2.0024	0.5856	<0.001
AreaF	0.0002	0.0001	0.049	0.0003	0.0001	0.008
PxFF	0.7103	0.5472	0.194	0.8989	0.5713	0.115

Figure 4. Response curves for the probability of presence within feeding habitat patches using GLM models in Table 6. Predicted values were computed for the observed ranges of AreaF and PxFF (min/max value) in the ‘La Bassée’ segment.

Figure 5. Conceptual model describing the role of local environment and the spatial habitat relationships in explaining the probability of presence of studied species.