Pons et al. Authors’ Note
In our paper that came out last week in Fish and Fisheries we examine the current status of large commercial tuna and billfish stocks. For tunas, 8 of 22 stocks have biomass (B) below the biomass that produces the maximum sustainable yield (BMSY) and three (southern bluefin, Western Atlantic bluefin and Pacific bluefin) are overfished according to the definition used commonly in the US (B < 0.5BMSY). Most tunas have sustainable biomass and fishing mortality rates (left panel, Fig. 1), with median B/BMSY of 1.12 and with fishing rates lower than those that would lead to MSY (median F/FMSY of 0.81). Billfishes are in slightly worse shape than tunas (right panel, Fig. 1), with a median B/BMSY of 0.85 and F/FMSY of 1.01. Nine out of 18 billfish stocks have biomass below BMSY and four (Eastern Atlantic sailfish, Western Atlantic sailfish, white marlin and Western Pacific striped marlin) have biomass below 0.5BMSY. k
In general, fishing mortality has declined for most of tuna and billfish stocks in the last decade (Fig. 2) due to an improvement in management by the tuna’s Regional Fishery Management Organizations (tRFMOs). In our paper, we showed that the implementation of total allowable catches (TACs) to limit catches is the management measure that leads to the fastest rebuilding of overfished tuna and billfish stocks. In addition, limits in fishing capacity, seasonal closures and minimum size regulations were also important in reducing fishing pressure but not as important as TACs in increasing stock biomass. Although TACs have been implemented by the International Commission for the Conservation of Atlantic Tunas and the Commission for the Conservation of Southern Bluefin Tuna, only recently has a TAC been set for Pacific bluefin tuna to control the intensive fishing pressure that this stock is experiencing. We hope to see signs of rebuilding for this stock in the near future.
The main conclusion of our paper is that the current status of tunas and billfishes is influenced by many factors, but with strong management and enforcement, overfished stocks have the capacity to rebuild and many of them are already rebuilt (e.g. North Atlantic swordfish, Eastern Atlantic bluefin tuna and Atlantic bigeye tuna).
One Response
My question is about the reference points employed for these stocks. In the U.S., MSY is written in to the Magnuson-Stevens Act, which governs fishery management. However, NMFS does not follow this dictate. Instead, NMFS does not follow this dictate. Instead, NMFS assessments substitute “proxies” for MSY, usually Spawning Potential Ratio reference points. The problem is that the SPR approach will normally produce more conservative RPs than true MSY RPs. The SPR approach, when employed without stock recruit modeling, ignores compensation, although compensation is a central finding of population ecology. Due to compensation, stocks can be fished harder, with juvenile survival increases often compensating for the fishery take to a great extent.
Incorrect specification of M is another issue that can occur with SPR. Because SPR as I have seen it applied, requires the specification of M, incorrect values of M can cause the RP to be too conservative, or possibly too liberal. By employing the SPR approach, an assessment can cause a fishery to lose fish that a true MSY approach would allow. Granted in some situations, estimation of MSY can be difficult. However, the effort is no longer made in US assessments, as a rule.
Were the reference points illustrated here proxies or actual MSY reference points?