Clare Leschin-Hoar of NPR writes this month about the recent Proceedings of the National Academy of Sciences paper (PNAS) which detail fish recruitment challenges related to climate change effects. Gregory L. Britten and his researchers claim to have established a link between declines in the amount of phytoplankton and declines in various types of juvenile fish. The worst of these declines are said to be happening in North Atlantic groundfish populations.
The paper analyzed a variety of regions globally and found some irregularities – in the Gulf of Alaska for example there were no such declines in juvenile fish populations. However in Australia and South America the declines in phytoplankton and juvenile fish were present similarly to those seen in the North Atlantic.
Leschin-Hoar relates Britten’s findings to those of Rebecca Asch who published another PNAS paper this year which found a number of fish have changed spawning seasons due to altered food availability. “It’s no longer just pull back on fishing and watch the stock rebound. It’s also a question of monitoring and understanding the ability of stocks to rebound, and that’s what we demonstrated in this study. The rebound potential is affected as well,” says Britten.
Comment by Ray Hilborn, University of Washington, @hilbornr
Fish Stocks Are Declining Worldwide, And Climate Change Is On The Hook
This is the title of a recent NPR posting — again perpetuating a myth that most fish stocks are declining.
Let’s look at the basic question: are fish stocks declining? We know a lot about the status of fish stocks in some parts of the world, and very little about the trends in others. We have good data for most developed countries and the major high seas tuna fisheries. These data are assembled and compiled in the RAM Legacy Stock Assessment database, available to the public at ramlegacy.org. This database contains trends in abundance for fish stocks comprising about 40% of the global fish catch and includes the majority of stocks from Europe, North America, Japan, Russia, Peru, Chile, Argentina, Australia, New Zealand and South Africa. Major fisheries of the world that are not in the data base are primarily in S. and SE Asia.
The figure below shows the trend in abundance of fish stocks in these different regions and the solid green line in each case represents the abundance that would produce maximum sustainable yield, the traditional target for fisheries management. In general countries and international agencies try to maintain the stock biomass around the green line, where long term sustainable yield is maximized. The declines of concern are those declines that continue below the green line, when you want to be rebuilding stocks, not seeing them decline. Finally a further caution, what are plotted here are averages, and in almost all regions some stocks are in poor condition (that is below the green line) and some are well above.
Clearly not all fish stocks are declining; in recent years they are increasing in the Atlantic Ocean Tuna, European fisheries, both EU and non-EU (Iceland and Norway), US East Coast, Southeast and Gulf, and US West coast.
Fish stocks in recent years are stable in Australia, Canadian East Coast, South Africa, Russia and Japan and Alaska.
We do see long term declines in Canada’s West Coast, the Indian Ocean Tuna, New Zealand, Pacific Ocean Tuna and South America. A characteristic of each of these regions is that they are late developing fisheries, the Pacific and Indian Oceans didn’t see wide scale industrial fishing until much later than the Atlantic Ocean and the decline seen is part of the process of developing new fisheries fishing down to the traditional target of MSY and is planned. The unknown at the moment is whether the Pacific and Indian Ocean Tuna fisheries will be managed to stabilize around the target level, or they will continue to decline as seen in South America.
For the places we don’t have good data (Africa and Asia), what we do know suggests those areas are seeing significant declines in abundance.
So clearly not all fish stocks are in decline—the pattern depends on the region. We can see from this graph that with good fisheries management stocks can recover. The NPR story got the big picture wrong, it isn’t climate change that is on the hook, it is the presence of effective fisheries management that determines the trend in abundance of fish stocks.
The scientific paper on which the NPR story was produced was much more subtle and did not say that fish stocks were decline – that was invented by the authors of the NPR story. The paper estimated that the recruitment potential of the fisheries was declining, specifically that the number of 1 year old fish per adult fish showed a decline in many regions of the world. Interestingly, the paper identified the N. Atlantic as the region of most concern, but when we look at abundance data, the N. Atlantic is the place we see the most stock rebuilding.
The number of 1 year old produced is known as recruitment, and the original paper used the data in the RAM Legacy Stock Assessment database to estimate these trends. The statistics used in the original paper are complex, but we can look quite simply at the trends in recruitment – not the recruitment per spawning adult as done in the paper.
This graph shows the recruitment trend for all stocks in the RAM Legacy Stock Assessment database, with blue the trend if all stocks given the same weight, and red with large stocks giving much more weight. The size of the dots or squares shows the relative number of stocks for which we have data in each year. We do see a clear trend in recruitment decline, with perhaps 10 or 15% decline over the 40 years of available data.
Is this decline in recruitment due to climate change? That is one possibility, but it is also possibly due to stocks being fished to lower abundance over that time as seen in the first graph. However, regardless of the reason, this decline is small and fish stocks can easily rebuild if good fisheries management is put in place.