Proxy measures of fitness suggest coastal fish farms can act as population sources and not ecological traps for wild gadoid fish.
BACKGROUND
Ecological traps form when artificial structures are added to natural habitats and induce mismatches between habitat preferences and fitness consequences. Their existence in terrestrial systems has been documented, yet little evidence suggests they occur in marine environments. Coastal fish farms are widespread artificial structures in coastal ecosystems and are highly attractive to wild fish.


RESULTS
To investigate if coastal salmon farms act as ecological traps for wild Atlantic cod (Gadus morhua) and saithe (Pollachius virens), we compared proxy measures of fitness between farm-associated fish and control fish caught distant from farms in nine locations throughout coastal Norway, the largest coastal fish farming industry in the world. Farms modified wild fish diets in both quality and quantity, thereby providing farm-associated wild fish with a strong trophic subsidy. This translated to greater somatic (saithe: 1.06-1.12 times; cod: 1.06-1.11 times) and liver condition indices (saithe: 1.4-1.8 times; cod: 2.0-2.8 times) than control fish caught distant from farms. Parasite loads of farm-associated wild fish were modified from control fish, with increased external and decreased internal parasites, however the strong effect of the trophic subsidy overrode any effects of altered loads upon condition.


CONCLUSIONS
Proxy measures of fitness provided no evidence that salmon farms function as ecological traps for wild fish. We suggest fish farms may act as population sources for wild fish, provided they are protected from fishing while resident at farms to allow their increased condition to manifest as greater reproductive output.
Department of Zoology, University of Melbourne, Parkville, Victoria, Australia. tim.dempster@sintef.no
Tim
T
Dempster
Pablo
P
Sanchez-Jerez
Damian
D
Fernandez-Jover
Just
J
Bayle-Sempere
Rune
R
Nilsen
Pal-Arne
PA
Bjørn
Ingebrigt
I
Uglem
D000822:Animal Husbandry / Q000379:methods* / Q000592:standards; D000818:Animals; D003247:Conservation of Natural Resources; D017753:Ecosystem; D005399:Fishes / Q000502:physiology*; D006305:Health Status Indicators; D008386:Marine Biology / Q000379:methods; D010271:Parasites; D011157:Population Dynamics
Mar Environ Res. 2007 Feb;63(1):1-18
17095083
Proc Biol Sci. 2005 Apr 7;272(1564):689-96
15870031
Dis Aquat Organ. 2007 Sep 14;77(2):149-58
17972757
J Parasitol. 1993 Dec;79(6):954-8
8277391
Proc Biol Sci. 2009 Oct 7;276(1672):3385-94
19586950
Parasitol Res. 2004 Sep;94(1):1-9
15278439
J Fish Dis. 2008 Mar;31(3):177-86
18261031
Environ Sci Technol. 2010 Nov 15;44(22):8736-43
20977203
Ecology. 2006 May;87(5):1075-85
16761584
Dis Aquat Organ. 2006 Oct 27;72(3):193-9
17190198