What an unusual problem – and one I know a good number of reptile enthusiasts (myself included) would love to have! I’m not sure where you live in the UK, but it sounds like snake-central – which is obviously bad news for you, if you don’t welcome their visits.
You’re obviously concerned that the snakes are going to predate your goldfish, but I wonder if you have seen anything that makes you worry that they are eating them? Although grass snakes (Natrix natrix) are known to take the occasional fish, they really don’t often make up much of their diet. Frogs are a much more typical item on their menu, and if your immediate vicinity is supporting the population of these reptiles that the number of incursions into your pond would suggest, I expect there are plenty of frogs around to feed them. As for adders (Vipera berus) although they do swim quite well – and much more readily than most of their relatives – fish-eating just isn’t their thing; a furry mouse on the other hand, or a lizard perhaps, but I can’t see them causing your goldfish any problems.
Frankly, with so much wildlife on your doorstep and a river nearby, if you are losing fish, I’d be more inclined to look to a heron as the culprit.
It’s also a bit unusual for adders and grass snakes to co-exist so closely.
Although adders in the northern parts of their range (which means the UK and much of Scandinavia) are less fussy about it than their southern counterparts, as a general rule, adders tend to be creatures of heaths, moorland and the edges of woodland. Despite the “beware of adders” signs that landowners sometimes use to put off prospective picnickers, adders are seldom found in marshy wetlands – so I’m left intrigued about why they should be visiting your pond. Perhaps, if your garden is a particularly large one, there may be a population of mice that would interest them – rather than the pond itself – and it’s just chance that you’ve spotted them by the water’s edge.
Both species tend to be fairly territorial, with a fairly clearly defined home range, so the logical conclusion would be that your pond forms part of that – but why you have adders sharing what sounds like an ideal grass snake habitat is baffling.
All snakes enjoy a degree of legal protection, principally under the Wildlife and Countryside Act. If you are wanting to relocate animals, you’d probably be well advised to contact your local wildlife trust in the first instance, who should be able to give you some good advice and will most likely come out to have a look at the problem first hand. Their knowledge of the local snake populations and contacts with experts in the area is something that’s certainly worth having. With adders being our only poisonous snake, how you move them – and where too – can be a contentious issue, so for your own sake it’s a good idea to get some professional input to avoid any possible comeback later.
Keeping snakes away from your pond isn’t going to be easy; they are remarkably good at getting under, round or over most barriers. It might be worth making sure that your compost heap is far away from the pond itself, since grass snakes naturally lay their eggs in rotting vegetation, the heat generated acting as an incubator to keep them warm until they hatch. If you are inadvertently providing a grass snake nursery, it might go a long way towards explaining the numbers you’re seeing.
Having said all of that, unless you really are certain that they are eating your fish – or you simply can’t abide snakes – there’s a lot to be said for accepting them as part of the scenery and enjoying their visits. British reptiles are under a great deal of pressure and the wholesale drainage of traditional sites has made life difficult for grass snakes especially; there’s a lot to be said for giving them a home, if you possibly can (but I must admit I’m biased – I really like snakes!).
Hopefully you can find a solution that will work out for you and your slithery visitors.