Invasive salmonids: niche requirements, interspecific interactions and empty niches Timo Muotka University of Oulu Finnish Environment Institute Kai Korsu University of Oulu Ari Huusko Finnish Game & Fisheries Research Institute
In freshwater ecosystems, invasive species are one of the leading causes of biodiversity loss, second only to habitat destruction
Fish introductions have caused dramatic biotic homogenization of fish faunas Species extirpations have caused a negligible change in % similarity among state fish faunas (B); most of the increase in similarity is due to species introductions (C)
Introductions involve many cosmopolitan species Transfer of rainbow trout (Oncorhynchus mykiss) from its original range to all continents (Petersen et al. 1987)
Invasive salmonids: : a threat to native biodiversity? Brown trout in New Zealand streams Salmo trutta Galaxias Prototroctes oxyrhynchus Brown trout partly caused the extinction of the New Zealand grayling, and continues to threat native galaxiids
#waterfalls Elevation No fish 4.37 339 Trout only 0.42 324 Galaxias only 12.3 567 (Townsend & Crowl 1991)
Trout causes a behavioural trophic cascade in NZ streams McIntosh & Townsend (1996): a cage experiment: : (i) trout present, (ii) (iii) no fish, (ii) galaxias present, fish present Amount of algae decreased in fish presence (esp. trout), Nesameletus mayflies
No change in grazer biomass or abundance but the dominant grazer (Deleatidium sp.) was less active in the presence of trout a behaviourally-induced trophic cascade Trout represents an evolutionarily novel predator archetype against which invertebrate prey have poorly developed antipredator skills
Invasion by alien salmonids: : the North American case Alien species like tend to displace native species which are often forced into upstream refugia Salvelinus confluentus Oncorhynchus clarkii
Exclusion of native brook trout by European brown trout in Valley Creek,, Minnesota: historical evidence Waters (1999) Brook trout biomass decreased sharply from 1973 onwards as brown trout increased; brown trout was first observed in 1969 As an alien species, brook trout is often a strong invader, but within its native range, it gets replaced by brown trout the two species are presumed to be competitors, with brown trout being the superior competitor
EASTERN Headwaters NORTH AMERICA EUROPE Brook trout Brown trout Mainstream Is brook trout a threat to native brown trout in North European streams?
TROUT POPULATIONS IN 142 SITES ACROSS 60 STREAMS IN TWO DRAINAGES A Allopatric brook trout 20% B Allopatric brown trout 38% Sympatric 42% A: Upper River Kemijoki B: River Iijoki (Loukusajoki) BROOK TROUT IS A SUCCESSFUL INVADER!
Brook trout establishment Upper Kemijoki drainage system Brook trout had established populations in the middle reaches of the river by year 1994 10 km
and spread 10 km and spread towards the headwaters (years 1994 vs. 2004)
Effects on native brown trout Brown trout / brook trout ratio, % 100 75 50 25 0 10 km year 1994 2004
Effects on native brown trout Brown trout / brook trout ratio, % 100 75 50 25 0 10 km year 1994 year 2004
OVERALL TROUT DENSITY REMAINED CONSTANT 35 30 25 20 15 10 5 0 1994 2004 allopatric brook trout sympatric allopatric brown trout Total trout density (ind. 100 m -2, +/- 1 s.e.)
Water depth Substrate size t ou th o b sp i ow k r b ijo m ie s c Ke e n ut o tr Stream width 3.3 (1.7 5.0) 7.0 (2.6-16.1) 11.8 (3.5-6.8) 14.8 (3.7-40.0) ph 6.3 6.2 6.8 6.9 brook trout + sympatric brown trout Velocity, width, ph Allopatric brook trout Sympatric Invasion sites Allopatric brown trout Br tr k oo PCA on habitat variables NICHE SEGREGATION Iij o k i (6.1-6.6) (4.9-7.2) (6.6-7.2) (6.3-7.2)
Experimental studies Laboratory experiments Mesocosm experiments Indoor flumes in FGFRI, Paltamo Artificial stream channels in Paltamo presence of brook trout has no effect on the habitat distribution of brown trout (age-0 fish) brook trout decreased its use of riffle habitats in the presence of brown trout No effects on either species: brook trout occurs in pools, brown trout in riffles (age-1 fish)
Brown trout ph Stream size Substrate size Water velocity + Habitat niches Brook trout
Mainstream headwater gradient Only brown trout present Niche space in the absence of the competitor (fundamental niche) Realized vs. fundamental niches
Only brook trout present Both species present; region of potential replacement Mainstream headwater gradient Only brown trout present Niche space in the presence of the competitor (realized niche) Realized vs. fundamental niches
Fisheries management perspective BROOK TROUT COLONIZED EMPTY (OR MARGINALLY USED) NICHE SPACE (HEADWATER STREAMS); IS IT A THREAT TO BROWN TROUT? Distribution of brown trout in Finland Blue: genetically pure stocks, considered threatened in Finland (Red Data Book of Finland, 2000) Practically all threatened populations of the species (particularly in the central parts of the country) are in headwaters!
Fisheries management perspective Headwaters Populations of brown trout are affected by extensive aquaculture releases Mainstream
Fisheries management perspective Populations of brown trout have high evolutionary and adaptive significance Headwaters Mainstream
How should we protect the of brown trout from alien the headwater populations alien species invasions? Management by isolation? Isolation barrier Probably the best option is to prevent stocking of alien species to new river systems stop stocking our inland waters with alien species like brook trout or rainbow trout!