I do recall reading a report sometime ago that was some sort of assessment of the impact Otters were having/could have on fisheries. A kind of for and against type thing. I might try and dig it out- it's probably easy to locate via a Google search. Also can't remember who commissioned and produced it (which can naturally make a difference in terms of how arguments are nuanced). One thing I do recall, which sort of made a bit of sense, was that otters here generally live shorter lives (something like 8 compared to 15 years sewhere in Europe) and in part (quite a big part I think) this is due to the availability of food. They tend to operate within fairly large territories (as much as 20-25km?) and dwindling food sources are forcing otters to travel more and in doing so they come under threat. Territorial scraps with other otters, attacked by man's best friend and of course a bit of road kill. Although saying that, I have never seen a dead otter on the side of the road in all my travelling years, but the principle makes sense. I think the report also said that cubs are nor produced each year with environmental issues for otters determining whether or not successful breeding takes place. The overall message there simply being that a natural thinning out process is taking place. Which when we think about rivers ravaged by otters resulting in profound reductions in stock levels, has to make sense. Otters would need to move on ultimately. Either that or they make material shifts in the diet and start salivating at the sight of a swan or other wildlife which, you would imagine, get the RSPB hot and bothered. Now, where would that lead?
Clearly if this thinning out process is taking place and that dreaded word "balance" comes into play, then all fine and dandy- apart from the devastation that has taken place in the meantime. And you also get back to, arguably, the more serious issue of recruitment in certain rivers.