Dr. Cathal Gallagher:

All the species are different. We had a problem with salmon. In the 1960s and 1970s, for every 100 juveniles that went out to sea after spawning, having spent a couple of years in the river, 20 came back. Now we are down to 5%. There is a way of partitioning. That suggests a big increase in mortality at sea. While we have many theories about what is actually happening, it is very complicated to come to a conclusion because there are individual populations. We believe much of the problem is attributable to climate change, but to measure that you have to ask whether the fish have the same food sources they had in the past. We have some evidence that the food being eaten at sea has less calorific value, for example, and we have evidence that there are more predators in different areas.