Editing Mälaren (section)

==Ecology==
The most common [[nest]]ing birds on the skerries of Mälaren are also the most common in the Baltic Sea. After a survey in 2005, the ten most common species were found to be [[common tern]], [[European herring gull|herring gull]], [[black-headed gull]], [[common gull]], [[mallard]], [[tufted duck]], [[Canada goose]], [[common goldeneye]], [[lesser black-backed gull]] and [[common sandpiper]]. [[White-tailed eagle]], [[greylag goose]], [[barnacle goose]], [[black-throated diver]], [[red-breasted merganser]] and [[gadwall]] are less common, and some of these latter are endangered in the Mälaren area. Since 1994 a subspecies of [[great cormorant]] ''Phalacrocorax carbo sinensis'', has nested there as well. A 2005 survey tallied 23 breeding colonies with 2178 nests, of which the largest colony had 235 nests. Most experts believe the great cormorant population has peaked and will stabilize at around 2000 nests.<ref name="fåglar">[http://www.skargardsbryggan.com/dokument/malarens_faglar_2005_.pdf Länsstyrelsen i Stockholms län] {{webarchive|url=https://web.archive.org/web/20070928042245/http://www.skargardsbryggan.com/dokument/malarens_faglar_2005_.pdf |date=2007-09-28 }} - Rapport 2006:02: Mälarens Fåglar (pdf, in Swedish)</ref>

One of the characteristic species is the [[osprey]] which has one of its strongest presences in Lake Mälaren. The osprey nests in almost all bays of the lake.<ref name="fåglar"/>

The [[Zebra mussel]] is considered an [[invasive species]] and is causing some problems in Lake Mälaren.

Lake Mälaren has 33 species of naturally occurring fish species, which makes it Sweden’s most diverse lake in regard to fish. Other species have been introduced to the environment, including the common carp and the rainbow trout. The rainbow trout in particular is known to compete with indigenous fish for habitat and food, as they have a faster growth rate and predate on local species. However, they are not considered to have made a significant impact.<ref name=":0">{{Cite journal |last1=Josefsson |first1=M. |last2=Andersson |first2=B. |date=December 2001 |title=The environmental consequences of alien species in the Swedish lakes Mälaren, Hjälmaren, Vänern and Vättern |url=https://pubmed.ncbi.nlm.nih.gov/11878025/ |journal=Ambio |volume=30 |issue=8 |pages=514–521 |doi=10.1579/0044-7447-30.8.514 |issn=0044-7447 |pmid=11878025 |s2cid=2171081 |access-date=2022-10-13 |archive-date=2022-10-13 |archive-url=https://web.archive.org/web/20221013114431/https://pubmed.ncbi.nlm.nih.gov/11878025/ |url-status=live }}</ref>

=== Crayfish ===
Crayfish are of large cultural and economic importance in Sweden, with [[Crayfish party|crayfish parties]] being a longtime annual tradition for many Swedes.

Lake Mälaren was the first lake in Sweden to be affected by the [[crayfish plague]] (''A. astaci'' Schicora'')'' when infected crayfish imported from Finland were introduced to the lake with the intention of human consumption. The plague spread quickly through the lake, exacerbated by boat traffic, which decimated the indigenous [[Astacus astacus|noble crayfish]] (''A. astacus'') population and caused severe economic losses to the local fishing industry. From Mälaren, the plague spread rapidly to all freshwater bodies in Sweden.

After multiple recurrent outbreaks and failed attempts to restore populations of the noble crayfish, Swedish authorities introduced the North American [[signal crayfish]] (''P. leniusculus)'' to L. Mälaren, a species that is resistant to the plague. Since 1969, Mälaren has been continuously stocked with signal crayfish.<ref name=":0" /> However, it was later discovered that signal crayfish were often carriers of crayfish plague. In addition, while populations of noble crayfish and signal crayfish have been known coexist, their larger size, faster growth rate, and aggressiveness often allows them to dominate populations of noble crayfish. This, in addition to habitat degradation,<ref>{{Cite journal |last1=Capinha |first1=César |last2=Larson |first2=Eric R. |last3=Tricarico |first3=Elena |last4=Olden |first4=Julian D. |last5=Gherardi |first5=Francesca |date=August 2013 |title=Effects of climate change, invasive species, and disease on the distribution of native European crayfishes |url=https://pubmed.ncbi.nlm.nih.gov/23531056 |journal=Conservation Biology |volume=27 |issue=4 |pages=731–740 |doi=10.1111/cobi.12043 |issn=1523-1739 |pmid=23531056 |s2cid=25775481 |access-date=2022-10-13 |archive-date=2022-10-13 |archive-url=https://web.archive.org/web/20221013231552/https://pubmed.ncbi.nlm.nih.gov/23531056/ |url-status=live }}</ref> has led to a significant decrease in noble crayfish populations in L. Mälaren.

Signal crayfish have been largely successful in Sweden and rapidly expanded as a replacement population to noble crayfish. Stocking this species is now prohibited within multiple lakes in Sweden in order to protect noble crayfish population. In the Stockholm municipality, which includes L. Mälaren, signal crayfish are now illegal to import, move, or farm.<ref>{{Cite web |title=Kräftfiske - Stockholms stad |url=https://parker.stockholm/fiske/kraftfiske/ |access-date=2022-10-13 |website=parker.stockholm |language=sv |archive-date=2022-10-13 |archive-url=https://web.archive.org/web/20221013114441/https://parker.stockholm/fiske/kraftfiske/ |url-status=dead }}</ref> There have been efforts to encourage farming of noble crayfish populations instead.<ref>{{Cite web |title=Noble Crayfish farming in Sweden |url=https://www.slu.se/en/Collaborative-Centres-and-Projects/slu-aquaculture/education/noble-crayfish-farming-in-sweden/ |access-date=2022-10-13 |website=SLU.SE |language=en |archive-date=2022-10-13 |archive-url=https://web.archive.org/web/20221013114433/https://www.slu.se/en/Collaborative-Centres-and-Projects/slu-aquaculture/education/noble-crayfish-farming-in-sweden/ |url-status=live }}</ref> However, illegal farming of signal crayfish continues to be a problem due to greater public demand for the larger signal crayfish.<ref>{{Cite journal |last1=Bohman |first1=P. |last2=Degerman |first2=E. |last3=Edsman |first3=L. |last4=Sers |first4=B. |date=2011 |title=Exponential increase of signal crayfish in running waters in Sweden – due to illegal introductions? |url=https://www.kmae-journal.org/articles/kmae/abs/2011/02/kmae110017/kmae110017.html |journal=Knowledge and Management of Aquatic Ecosystems |language=en |issue=401 |pages=23 |doi=10.1051/kmae/2011040 |issn=1961-9502 |doi-access=free |access-date=2022-10-13 |archive-date=2022-10-13 |archive-url=https://web.archive.org/web/20221013114431/https://www.kmae-journal.org/articles/kmae/abs/2011/02/kmae110017/kmae110017.html |url-status=live }}</ref>