Then what is the reference level (known as biological reference point)? Most of the international tuna management Conventions define their objective as to maintain stock size at a level which produces the maximum yield and yet sustainable (MSY). Many people think that the MSY level is one value specific to a stock. This is a misconception of the term MSY’. The value of MSY changes as the fishing pattern changes. For example, as purse seine fishery started in most of the Oceans, catching smaller-sized fish schooling around floating objects, in the 1990s, MSY (of yellowfin and bigeye, exploited by the purse seine fishery) becomes less than a half of the level estimated for the earlier period when longline alone catches large fish.
In reality, it is not that simple to maintain tuna stocks at MSY level. Firstly, fish stock is not stable, as environmental conditions vary from year to year. Secondly, fishery is also not stable but variable, depending on socio-economic factors, as shown in the previous paragraph. As a stock abundance fluctuates, the same amount of catch and/or effort can overexploit or underexploit the stock. Can we account for all these variables in managing tuna stocks at a sustainable level for a long period?
In order to overcome these difficulties, in recent years, other less sensitive (or robust） reference points have been considered for the management purpose. Unlike MSY which is a target level, a limit reference point is easier to apply for the management. It is a precautional level below which the stock would have a greater risk of deterioration. Therefore, the management is designed not to allow the stock declines below that level, which is easier than to maintain a stock size at an unstable target reference point. The limit reference point considered for north Pacific albacore is the mean of the lowest 10 estimates of stock biomass in the past.
Therefore, to claim that a tuna stock is overfished on the basis of catch rates or catches is a very simplistic and unscientific view of the complexities involved in analysis and management.