Thank you.
The current challenge is not estimating how much fish seals consume. For example, a grey seal eats about a tonne and a half of fish per year. We even have a pretty good idea of the percentage each species' diet represents. We have a problem with the samples because it all depends on where we go. Sable Island seals do not have the same diet as Magdalen Islands seals. So there is a sampling problem. However, we can measure things to a certain extent and figure out average consumption. We do that quite well.
That said, we have a hard time measuring the impact of consumption on the target species. Are seals responsible for 30%, 50% or 90% of the natural mortality for cod? That is where we run into problems because we do not have good ways to measure cod mortality and to determine causes of mortality in the species.
The second problem is determining how killing all the seals would benefit the cod. Is there a direct connection between seals and cod? Would other predators fill the niche created by killing the seals? The ecological link is a problem.