European otters and American mink competition

[MrsNibblet]

Hello all,

I'm interested to know about current research on the European otter and American mink, particularly in relation to competition/predation. What is the mechanism that is resulted in a rapid decline in mink following the re-colonisation of the otter?

Hope you can help

[eeyore]

see the thread entitled mink vs otter for masses of posts on this

[MrsNibblet]

Thanks. I've looked but I'm interested in the evidence on the mechanism for decline rather than the evidence for a decline, e.g. Harrington et al (2009) explored the possibility that mink could be avoiding the scent of otter?

[camo]

Hi Mrs Niblett

The thread eeyore refers to started off well but has degenerated into a silly piece of nonsense however there was some good stuff which was far more theoretical than my anecodatal thoughts. However i would be interested in your own position, thoughts and evidence to date. clearly Scent must be important as Otters cover such a large territory that they may not visit some parts of their territory for days if not weeks. Otter spraint is very well located to maximise its effect, use of Otter made mounds, and us eof stones ellivate the spraint and increase teh effect. Then their deposits under bridges must distribute their scent as the bridge acts like a wind tunnel and increases the spread of scent.

Roy

[MrsNibblet]

Undoubtedly otters are impacting on mink.....see Strachan and Jefferies (1996) and Laura's Bonesi's research. I suspect it is because the otter is much larger than the mink, and perhaps better adapted to an aquatic lifestyle. Maybe it is exploitation competition ie. when one species utilises a resource so that it is unavailable to another. Otters are specialists...they mainly consume aquatic prey, while mink are generalist, they'll eat fish, mammals etc. The diets of the species are known to overlap e.g. Wise et al found 40% overlap in a eutrophic lake, while a study conducted in Sweden found 50%-70% overlap depending on season. If otter is better adapted to an aquatic lifestyle then it will perhaps force mink to consume other prey. A few studies have found that mink diet shifts to a more terrestrial based diet particularly in winter when resources are limiting.

It could also be interference compeition when one species displays antagonistic behaviour to another and therefore precludes it from accessing resources....I think a few studies have found the remains of mink in the spraints of otter.

I think there is so much research still needed in this area

[camo]

Some random thoughts:

As the decline of the Water Vole is directly related to the increase in Mink, riparian groups are encouraged by the increasing Otter population and subsequent reduction in Mink. This supports the view of a changing population however you are correct to suggest the methodology is less certain.

My own anecotal evidence suggests Otters will kill and eat anything that offers a meal however their size and their clumsiness on land makes them more efficent in water. The Mink is able to access smaller hinding paces and therefore whilst the W Vole may have been safe from the Otter is at risk from the Mink. The Mink is also much more fleet of foot on land and therefore i would suggest the evidence of different diets may be due to opportunity rather than an Otter's fishy pallete.

Clearly at double the size an Otter must impact on Mink comfort by sheer size, couple this with a general territorial nature the Otter will not encourage a Mink. The Otter is a bully and will maintain a larger territory than it porbably needs because it has very little competition. I.e. it does because it knows it can!

I have never witnessed Mink predation by Otters but i accept they are opportunists and would eat a Mink. I know of a nature reserve where there is no control of Mink and the Otters there definetly co-exist. Where Mink rafts are used in strong Otter populations the greatest majority of Mink caught have tended to be young animals. This may be down to older wiser mink avoiding traps but it may also suggest prime territories for Mink are outside of Otters territories. It may be the young Mink are just passing through.

Roy

[MrsNibblet]

Regarding water vole being safeguarded by the comeback of otters....possibly true to some extend in that if mink are reducing the number of mink, which are the main perceived predator of the water vole, then this is good news for the wv. However, many forget that at the start of the last century, when water vole and otters were common on the Cambridgeshire fens and dykes, water vole were a staple part of otter diet. Also stephens (1957ish) looked at the stomach contents of otters and found the remains of water voles. In addition, otters may be forcing mink into sub-optimal habitat e.g. still water fisheries...and then disaster!

You are right that there are many instances when mink and otters co-exist...perhaps this occurs when resources are not limited so there is no need to compete, or when the species are in low abundance....again, so not worth competitng?

[nigel_b]

Originally Posted by camo

Then their deposits under bridges must distribute their scent as the bridge acts like a wind tunnel and increases the spread of scent.

Roy

I believe the bridge acts as a physical bottle neck which has to be passed through thus guaranteeing the intruder passes close to the spraint.

[camo]

i agree it shows intelligent selection of a site, probably through a history of effectiveness

[The Woodman]

Originally Posted by MrsNibblet

However, many forget that at the start of the last century, when water vole and otters were common on the Cambridgeshire fens and dykes, water vole were a staple part of otter diet. Also Stephens (1957ish) looked at the stomach contents of otters and found the remains of water voles. In addition, otters may be forcing mink into sub-optimal habitat e.g. still water fisheries...and then disaster!

In Stephens report where she summarises the diet in Table 3, Main foods &c..Arvicola is given as 1.4%. Given the abundance of Arvicola at that time (of the report), I think the number is very low. Other prey items Salmonidae and Anguillidae are listed as 21.1% and 18.5% respectively which may put predation into perspective. It is a small sample and therefore some latitude should be given.

From my experience, I'm not convinced that otter may be forcing mink into sub optimal habitat. Mink will locate a fishery that provides them with an optimal food source and from what I've seen in the north west, their preference is smaller silver fish. Where mink are left alone in these circumstances, they will not travel far from their captive food source.

It's not mink that's the problem here, otter is rapidly discovering those stillwaters with easily caught prey e.g. carp. The otter population is probably nearing the pre-crash levels (EA 5th Otter Survey) and utilising new sources of food that were not available in the 60's and early 70's. Stocking densities are much higher as are the higher weight and therefore more valuable fish.

I was called to a small fishery last Friday where the owner wanted to know what had eaten 27 large brown trout that had died in anoxic conditions under thick ice and floated to the bankside on the thaw. All the signs were there and the otters work has a raised profile in the conciousness of the owner. He will undoubtedly be speaking to other fishery owners on the subject and I fear for a backlash. This has ben discussed in other threads so I'll shut up now.