Can the river location within a fjord explain the density of Atlantic salmon and sea trout?

Figures & data

Figure 1. Map of the Hardangerfjord and all anadromous rivers in the study. Note the arrow from which the distance from the fjord outlet to the river was measured.

Table I. Categorization of rivers in the Hardangerfjord. Shortened code is abbreviation of name used in graph. Area is the area of river polygons from digital maps using ArcGis^©. Length is distance from anadromous obstacle to river outlet following a transect in the middle of the river. Distance is distance from river outlet to outlet of the Hardangerfjord (Bømlafjord). Glacier is presence of glacier in the catchment. Physical is a categorization of the human physical alteration of the river. Regulation is the presence of river regulation in the river. Discharge is the categorization of whether the discharge has been severely reduced.

River name	Code	Area (m^2)	Length (km)	Distance	Glacier	Physical	Regulation	Discharge
Opo*	OP	72,000	1.5	180	Yes	Low	No	Not reduced
Sima	SI	63,000	4.3	169	Yes	High	Yes	Reduced
Eidfjord^1	EI	249,000	5.6	165	Yes	Medium	Yes	Reduced
Osa^2	OS	37,600	4.0	160	Yes	High	Yes	Reduced
Granvin	GR	122,000	7.5	148	No	Medium	No	Not reduced
Kinso	KI	128,000	4.2	147	No	Low	Yes	Not reduced
Steinsdalselva	STE	99,000	5.0	129	No	Medium	No	Not reduced
Øysteseelva*	ØYS	18,000	0.9	128	No	High	No	Not reduced
Jondalselva*	JO	25,000	0.9	105	No	Medium	Yes	Reduced
Strandadalselva	STR	33,800	2.0	97	No	Low	No	Not reduced
Øyreselva*	ØYR	28,000	1.2	94	Yes	Low	Yes	Reduced
Austrepollelven*	AU	22,300	1.9	92	Yes	High	Yes	Reduced
Bondhuselva	BO	45,000	2.5	89	Yes	Low	Yes	Reduced
Æneselva	ÆN	128,500	5.7	80.8	Yes	Low	No	Not reduced
Hattebergselva	HA	36,000	2.0	64	Yes	Medium	No	Not reduced
Omvikselva	OM	47,000	4.4	61	No	Medium	No	Not reduced
Uskedalselva	US	135,000	10.3	56	No	Medium	No	Not reduced
Etne	ET	288,900	12.2	53	No	Medium	Yes	Not reduced

¹ Eidfjord is a combination of Bjoreio and Eio; ² Osa is the combination of Nordøla and Austdøla. * Rivers not used in the analysis of salmon.

Figure 2. Pre-fisheries abundance (upper panel) and fish per hectare (lower panel) of wild Atlantic salmon observed in the rivers of the Hardangerfjord in the period 2004–2011. For rivers codes, see Table I. Grey symbols indicate values from 2011.

Figure 3. Pre-fisheries abundance (upper panel) and fish per hectare (lower panel) of sea trout observed in the rivers of the Hardangerfjord in the period 2004–2011. For rivers codes, see Table I. Grey symbols indicate values from 2011.

Figure 4. Salmon per hectare versus distance to coast where each plot represents a year from 2004 to 2011. Grey names denote rivers where a significant part of the water discharge has been reduced. For rivers codes, see Table I. Etneelva is in some cases out of scale and the values are denoted in the upper right corner. Both the linear mixed effect model with (dashed line) and without (solid line) data from Etneelva is plotted in each graph.

Table II. Results of the linear mixed-effects model of density of fish (DF) without river Etne. River is defined as a random effect. The standard deviation (s.d.) of the random effect and the residual SD was 0.49 and 0.38, respectively; numDF and denDF represent the numerator and denominator degrees of freedom in the test of the respective factors.

Fixed effects	numDF	denDF	F-value	p-value
(Intercept)	1	54	219.58	<0.0001
Distance	1	10	15.39	0.0318
Year	7	54	8.09	<0.0001

Figure 5. Sea trout per hectare versus distance to coast where each plot represents a year from 2004 to 2011. Grey names denote rivers where a significant part of the water discharge has been reduced. For rivers codes, see Table I.

Figure 6. Total number of observed farmed salmon (upper panel) and estimated percent farmed salmon in spawning populations (lower panel). For rivers codes, see Table I. Grey symbols indicate values from 2011.