There have been instances in the past where "suspicious" shortages have been created to drive up the prices of products. It has happened with many products such as: milk, chicken, beef, etc. It there truly is an overfishing concern, then the production of this product should be cut back without raising the price.
Most likely the over fishing concerns of bluefin tuna are legitimate. We do consume a lot of tuna. Yes, sometimes environmentalist can use scare tactics, but this isn't the case all the time. The amount of tuna consumption has been increasing as more people are getting away from eating red meat. Therefore, over fishing of bluefin tuna is a real concern.
No, overfishing concerns regarding bluefin tuna are not being over-inflated in order to get restaurant operators to pay more for them. These fish are indeed being harvested at rates that are unsustainable. If they continue to be overfished, it will soon be the case that there will be no more bluefin tuna to enjoy.
Saying that there are overfishing concerns wouldn't do anything to the market price. In the end, the amount of fish available is what it is. The amount available and the popularity of something determins what a person is willing to pay for the item, or in this case the restaurant. Over fishing doesn't affect price.