We have a couple. One is it is clear we know for sure temperature in the ocean is changing, both at the surface and below the surface; both at depth and in a variety of areas.
We also know that currents and tidal patterns are changing on all three oceans. We are fairly confident that is having an impact on a whole range of biodiversity, and not just Atlantic salmon.
Generally speaking, it's impossible to say it's a linear relationship. For some species it's good, and for some species it's not. So it's not a one-to-one, all good or all bad.
Why is it changing? A big part of that is around climate change. If we look, in particular, at the interaction between the atmosphere and the ocean surface, there we're seeing dramatic change. We can see it in phytoplankton availability, or lack thereof. We can see it in dispersion of nutrients, or lack thereof.
So we know that the system in which fish are swimming is changing significantly. Can we quantify that back yet to individual population level changes? We're struggling with that. A big part of our modelling work on the oceanographic side is trying to understand that a fair bit better. The prevailing theory is that it's ocean conditions that are changing, and when Atlantic salmon, for example, go to sea, that is causing them to return in smaller numbers than before. We don't have anything that suggests there's a massive predation problem or other anthropogenic sources of concern out there.