Environmental challenges

Aquaculture map

Discharge of nitrogen, phosphorous and organic material

Bellona, 07/01-2009 Fish farming is a significant local source of discharges of nutrient salts. The vast majority of Norwegian aquaculture production takes place in net cages in the sea. Feed spills and excrement are not collected but are released directly into the water (State of the Environment Norway).This produces a fertilising effect in the waters, and if the discharges are too large in proportion to what the waters carrying capasity, water quality will decline. This will have consequences for the surrounding environment and the environmental condition of the fish farm. Discharges of nutrient salts, primarily nitrates and phosphates, and organic materials come from feed spills, fish excrement and dead fish.


Reduced oxygen content
Organic material from fish farms sinks to the bottom and is decomposed by bacteria. This biodegradation process requires oxygen. The discharges thus reduce the oxygen content of the surrounding water, and in extreme cases we may see a total lack of oxygen in the water at the bottom. Discharges of nutrient salts of nitrogen and phosphorus lead to increased algae growth and biomass production in the surrounding water. This increased biomass production leads to a further addition of organic material to the bottom of the fjord, which in turn requires increased amounts of oxygen for bacterial decomposition.


Nitrogen and phosphorus
Two types of nutrient salt, those of nitrogen and phosphorus, represent most of the nutrient salt discharges connected with fish farming (Braaten, 1992). Generally it is salts of phosphorus that are mostly responsible for eutrophication in fresh water, whereas nitrogen salts normally will be more important in salt water (Braaten, 1992). Many of the fish farms are located in fjords and in areas with a large inflow of fresh water, and detailed knowledge is required of the area in question in order to say anything about which nutrient salts are limiting primary production. A fish farm that produces 500 tonnes of salmon, that is, a small farm, releases nutrient salts equivalents to a town of 5,000 to 7,500 inhabitants (State of the Environment Norway). According to the Norwegian Institute for Water Research (NIVA), aquaculture is responsible for 60% and 25%, respectively, of discharges of phosphorus and nitrogen salts along the coast from Lindesnes northward.

Organic material
From fish feed, it is proteins, fats and carbohydrates that constitute nearly all of the organic material (Einen & Mørkøre, 1996).When the organic material hits the bottom, it can be "biodeposited" on the surface of the sediment. Biodeposition means that the sedimentated material is taken up by benthic fauna that are nourished by filtering particles from the water (such as mussels), and the excrement from such fauna is deposited on the seabed (Wasermann, 1994). To prevent a reduction in water quality due to high consumption of oxygen for decomposition we are dependent on adequate currents that can spread the organic material over a larger area. The energy content of the organic material is crucial for the quantity of oxygen that must be added to the water to decompose the organic material (Åsgård & Storebakken, 1993). That is why organic material is converted to energy content in the table below.


Year
Phosphorous
Phosphorous
Nitrogen
Nitrogen
Energy
Energy
 Kg pr. ton fish
Total (ton)
Kg pr. ton fish
Total (ton)
Pr. ton fish (10 MJ)
Total (10MJ)
1985
16,35
566
77,51
2683
0,0227
786
1990
17,56
2630
83,35
12484
0,0227
3400
1991
16,60
2665
78,81
12653
0,0227
3645
1992
17,33
2265
82,27
10754
0,0227
2962
1993
10,60
1737
50,32
8249
0,0271
4444
1994
10,34
2267
49,10
10765
0,0193
4233
1995
10,51
2902
49,89
13780
0,0208
5741
1996
10,41
3338
49,43
15845
0,0218
7000
1997
10,57
3868
50,18
18359
0,0217
7938
1998
10,28
4216
49,32
20237
0,0210
8621
1999
8,87
4202
42,52
20146
0,0203
9630
2000
9,73
4762
46,58
22804
0,0203
9939
2001
8,32
4224
39,79
20206
0,0203
10310
2002
9,81
5355
46,16
25207
0,0203
11085
2003
9,58
5544
45,66
26416
0,0203
11743
2004
8,85
5548
41,86
26253
0,0203
12732
2005
9,57
6176
45,31
29240
0,0203
13101
2006
10,627353
50,23
34789
0,0203
14057
2007
10,62
8640
50,23
40877
0,0203
16517
       

 

Increasing discharges
Since the beginning of the 1990s we have seen a steady increase in discharges of nitrogen and phosphorus salts and organic material from Norwegian aquaculture. Despite some reductions in discharges per tonne of fish produced, the growth in volume leads to a continuous increase in total discharges.

If the assumed increase to 1,050,000 tonnes of salmonids produced in 2010 comes about (Almås, K. et al., 1999), the discharges of nitrogen and phosphorus will climb to 47,355 tonnes and 10,290 tonnes, respectively – an increase of 63% and 62%, respectively. This assumes, however, that fish farming is conducted in the same manner as in 2005

From Lindesnes and northward, aquaculture contributes heavily to the total discharges of nitrogen and phosphorus salts. The discharges of these nutrient salts and organic materials have increased in step with the growth in volume of Norway's farming of salmonids. Even though the industry's measures to improve feed quality and reduce feed spills have reduced the discharges per tonne of fish produced, this reduction has been counteracted by an increased production volume. Continued growth in the farming of salmonids in addition to scaling up the production of other farmed species will lead to increased discharges of nutrient salts and organic material to Norway's coastal and fjord areas. Even though fish farming is responsible for a large part of these discharges, these discharges do not necessarily constitute a large addition of nutrients compared with natural levels, and compared with what is added by ocean currents from foreign discharge sources.

These discharges are harmful only if they exceed the carrying capacity of the area in question. As long as the discharges do not exceed this, they may have a positive impact on the productivity in the area and not inflict any harm on the environment.

The challenge linked to discharges of this type is therefore to calculate the carrying capacity of the location and adjust fish farming activities accordingly. The MOM system (Modelling - Ongrowing Fish Farms - Monitoring) will be an important tool in these efforts.
Copyright © Bellona -- Reprint and copying is recommended if source is stated

Last updated

Support Bellona's work for the environment - Phone 23 23 46 00 | E-MAIL: info@bellona.no