Those questions are exactly what Justice Cohen investigated. He looked at a range of issues.
We can categorize the types of impacts in different ways. There are impacts on habitat, there are impacts upon fishing pressure, there is climate change and variability. There are a number of environmental and anthropogenic reasons for changes in the returns.
You need to look at these things in their entirety. As he said, the “no smoking gun” point really is valid; you have to look at the whole. You can't say there's one individual feature.