This is a very challenging time to be a cod fisherman in the Gulf of Maine, and presumably, it's also a tough time to be a cod. Fisheries managers have drastically reduced the amount of cod fishermen will be allowed to catch next year, and this follows a large reduction last year. Like many of us working at the intersection of oceanography and fisheries, I'm interested in what's up with cod? There are a lot of hypotheses being kicked around, but as you might imagine, I'm interested in whether changes in temperature or other environmental factors are playing a role. Here's a quick and dirty take on whether recent warming could be impacting cod.
First, temperature. I began with the AVHRR OI data set that I've been using to characterize the 2012 story. I grabbed the pixel near NERACOOS Buoy E off of the central Maine coast. Here is the temperature pattern at that location:
The thin blue line is the daily anomalies (smoothed over 15 day window). The gray circles are the mean for each year. The black line is the overall trend. The trend is significant, but there is considerable year-to-year variability. A trend of 0.026° per year is pretty consistent with the general global warming trend. This would project to a 1° increase by 2050 and 2.3° by 2100. The really striking pattern in the figure are the last three, extremely warm years. If you fit a line to the 2004-2012 anomalies, you get a trend that is 10 times faster than the long term trend. I'm getting on statistically dubious grounds with this calculation, but it does suggest that we may have entered a new temperature regime.
Most of the published relationships between temperature and cod use some form of bottom temperature. I used the 2002-2012 temperature data from buoy E to develop a simple statistical model for the temperature at 50m as a function of the surface temperature and the day of the year. The long-term mean temperature at this location was 6.89°C, which is a bit lower than the "standard" 8° value that most people report. So, I added 1.11° to the annual bottom temperatures to get the mean close to right:
Why the big deal about 8°? Well, Ken Drinkwater's 2005 paper on cod and climate change suggests that for regions above 8°, additional warming tends to reduce the fitness of cod. For regions below 8°, warming increases fitness. Using this benchmark, we had a period from 1999-2002 when cod reproduction may have been reduced by temperature. The 10°C value for 2012 is getting scarily close to the 12°C point where Drinkwater says "cod stocks are not observed much."
The red line in the figure is the September-October mean (not adjusted). I then applied a relationship from Fogarty et al. (2008) relating the probability of catching a cod in the NMFS bottom trawl survey with the September-October bottom temperature.
Not surprising, this has many of the same patterns as the temperature values. There is a gradual long-term trend (probability declines by 0.004 per year) and a much faster trend over the last 8 years (0.02 per year). While the trends are interesting, I'm interested in what appears to be a shift in the mean in 1999. Before 1999, the values are centered around 0.42. After 1999, the mean probability appears near 0.37. By this metric, 2012 looks very, very scary. The probability of finding a cod this year was about half what it was in the early 1980s