Population reproductive potential: Evaluation of long-term stock productivity

In order to avoid recruitment overfishing, fish stocks must have sufficient reproductive ability. The spawning stock biomass (SSB), which ignores the value of immature fish, is widely used as an index of stock sustainability. From the perspective of sustainability, immediate reproduction, as well as future spawning, must be considered. We developed an index of long-term stock productivity, called the population reproductive potential (PRP). PRP is defined as the expected total reproductive value of the standing stock. We used PRP to assess the western Atlantic bluefin tuna (WBT) stock. The trends in SSB, numbers (N), biomass and PRP of WBT are inconsistent when compared to each other, due to fluctuation in age composition. We evaluated the long-term productivity of WBT by computer simulation and compared the result with trends in the abundance indices. The result of the computer simulation was highly consistent with the trend in the PRP. Short-term trends in SSB and N often do not reflect long-term stock trends, because they are highly sensitive to age-composition dynamics. The PRP is useful for evaluating stock trends, especially when the age composition is unstable.     

Recovery policy for chub mackerel stock using recruitment-per-spawning

The stock abundance of chub mackerel ( Scomber japonicus ) in the Pacific Ocean off Japan declined in the 1980s and remained at low levels through the 1990s. There were recruitment successes in 1992 and 1996. However, the cohorts born in these years were heavily fished before the age of maturity and chub mackerel has not begun to recover. To investigate the effects of conserving immature fish, we created four recovery policies: (i) policy 0, actual fishing mortality during the 1990s; (ii) policy 1, conserve strong year classes; (iii) policy 2, apply the average fishing mortality in the 1970s–1980s after 1992; and (iv) policy 3, a 55% reduction of the mortality adopted by policy 2. Policy 3 was considered to be the best in terms of final stock abundance and total catch from 1992 to 1999. We also calculate the future projection of stock and catch under these three policies as well as using average fishing mortality from 1993 to 1999. Using average fishing mortality from 1993 to 1999, the stock will not be recovered within the next 20 years. Even under the best policy, the risk that the final stock is not recovered to 3 million tons within the next 10 years is 40%.