Good examples are: bigeye tuna in the East Pacific, Indian, and Atlantic; yellowfin tuna in the Atlantic and Pacific; and albacore in the North Pacific. Why? One possible explanation may be due to the positive effects of on-going regulatory measures. However, we cannot explain all of these increases through this reason alone. In many cases, fishing effort has been increasing even under current regulations, while the future projection of the stock level is more optimistic than before: the Bigeye in East Pacific is a good case in point. Generally the increases in the stock biomass seem to be related to the increased recruitment levels (abundance of juvenile small tuna becoming available to fisheries) in recent years.
Estimating the level of recruitment is a very difficult task. Scientists estimate it using the analytical models, based on the catch at age (age structure of catch) and other information. Fisheries target at as large fish as possible within its structural and economic limits. Therefore, their catch is not representative of the population, particularly of the juvenile fish (recruits). Besides, if there is some change in fishing procedures (e.g. adoption of fish aggregating device by purse seiners), the size of fish in the catch changes drastically. Change of size in fish then has an effect on the estimation of recruitments.
With such uncertainties, the alternative and probably more preferable course of action is if we can directly measure the level of the abundance of juvenile fish (i.e. fishery independent parameter). However, none of these methods, including aerial survey, acoustic survey, larvae collections, test fishing for small fish etc. can yet provide with any accuracy, the index of abundance for those juveniles.
There have been some arguments as to whether the recent increase in the recruitment level is real or artifact. For some species, regime shift is recognized. If such a shift is currently occurring, the increase in recruitment level is real induced by the change in environment in favor to the tuna stocks. The favorable oceanographic and/or ecological conditions in the spawning and nursery grounds of tuna are known to reduce the natural mortality at its early life history. Could global warming be contributing to the improvement of tuna stock status?