Ray Hilborn: Analysis Shows California Sardine Decline Not Caused by Too High Harvest Rate
Posted with permission from SEAFOODNEWS — Please do not repost without permission.
SEAFOODNEWS.COM [SeafoodNews] (Commentary) by Ray Hilborn April 22, 2015
Two items in the last weeks fisheries news have again caused a lot of media and NGO interest forage fish. First was publication in the Proceedings of the National Academy of Sciences of a paper entitled “Fishing amplifies forage fish population collapses” and the second was the closure of the fishery for California sardine. Oceana in particular argued that overfishing is part of the cause of the sardine decline and the take home message from the PNAS paper seems to support this because it showed that in the years preceding a “collapse” fishing pressure was unusually high.
However what the PNAS paper failed to highlight was the real cause of forage fish declines. Forage fish abundance is driven primarily by the birth and survival of juvenile fish producing what is called “recruitment”. Forage fish declines are almost always caused by declines in recruitment, declines that often happen when stocks are large and fishing pressure low. The typical scenario for a stock collapse is (1) recruitment declines at a time of high abundance, (2) abundance then begins to decline as fewer young fish enters the population, (3) the catch declines more slowly than abundance so the harvest rate increases, and then (4) the population reaches a critical level that was called “collapsed” in the PNAS paper.
Looking back at the years preceding collapse it appears that the collapse was caused by high fishing pressure, when in reality it was caused by a natural decline in recruitment that occurred several years earlier and was not caused by fishing.
The decline of California sardines did not follow this pattern, because the harvest control rule has reduced harvest as the stock declined, and as fisheries management practices have improved this is now standard practice. The average harvest rate for California sardines has only been 10% per year for the last 10 years, compared to a natural mortality rate of over 30% per year. Even if there had been no fishing the decline in California sardine would have been almost exactly the same.
In many historical forage fish declines fishing pressure was much higher, often well over 50% of the population was taken each year and as the PNAS paper highlighted, this kind of fishing pressure does amplify the decline. However many fisheries agencies have learned from this experience and not only keep fishing pressure much lower than in the past, but reduce it more rapidly when recruitment declines.
So the lesson from the most recent decline of California sardine is we have to adapt to the natural fluctuations that nature provides. Yes, sea lions and birds will suffer when their food declines, but this has been happening for thousands of years long before industrial fishing. With good fisheries management as is now practiced in the U.S. and elsewhere forage fish declines will not be caused by fishing.
Ray Hilborn is a Professor in the School of Aquatic and Fishery Sciences, University of Washington specializing in natural resource management and conservation. He is one of the most respected experts on marine fishery population dynamics in the world.
Subscribe to SEAFOODNEWS.COM to read the original post.