Sunday, January 11, 2015
Climate change has been in the news quite a bit lately.
Globally, 2014 was the warmest year ever recorded, and there’s no reason to believe that 2015 won’t be pretty warm as well.
The ocean is warming as well as the air, and that can’t help but have an effect on fish.
One group of researchers has determined that warming waters will have a significant impact on Atlantic cod off the northeastern coast, as they lose thermal habitat—that is, waters become too warm—on parts of Georges Bank and much of the Mid-Atlantic Bight. In addition, increased water temperatures in the Gulf of Maine are likely to lead to lower cod recruitment and make the fish more vulnerable to high fishing mortality rates.
Rhode Island Sea Grant has issued a report which says that
“There has been mounting evidence that over extended periods of time, even small increases in water temperature can significantly affect species composition, distribution, and abundance of fish communities…It has been projected that with warming, the geographical distribution of cold-water species will shift toward the poles and to deeper waters where temperatures are cold, causing a general reduction of cold-water species while expanding the ranges of warm-water species.”
Rutgers University has gone a step farther. Its School of Environmental and Biological Sciences has created the OceanAdapt website, which allows people to see the changes in the centers of abundance for various species, in terms of both depth and latitude, as the shifts described by Rhode Island Sea Grant take place.
There’s no question that a warming ocean is creating new challenges for fisheries managers.
Unfortunately, one of the biggest challenges has nothing to do with the warming ocean itself. Instead, it is taking the form of fishermen who fight against needed regulations, and now have added climate change to a list of excuses--which already includes predation, pollution and habitat change--used to try to evade responsibility for declining fish stocks.
While there is no question that warming waters are bringing change to fish distribution, those changes are not consistent across all species. And rather than absolving fishermen from responsibility for declining stocks, they actually increase the need for more intense, and more restrictive, regulation of existing fisheries.
The National Marine Fisheries Service recently described a study that investigated how four popular fish stocks have responded to a warming ocean. The stocks involved were black sea bass, scup, summer flounder and the sourthern New England/Mid-Atlantic Bight stock of winter flounder. The study incorporated data collected between 1972 and 2008 by NMFS’ northeastern trawl surveys.
It turns out that the fish don’t all react in the same way.
Winter flounder exhibited no northward movement at all.
The other three stocks did show northward movements, but the reasons for those movements were not all the same.
The researchers found that, in the case of two of the stocks,
“Northerly shifts in scup and black sea bass are linked to increases in temperature and are more tied to climate than fishing.”
However, in the case of summer flounder
“The fish were not shifting northward with warmer conditions, but simply re-colonizing their former habitat areas.”
That was happening because of
“…a decrease in fishing pressure and an expansion of the age structure…
“Fishing typically removes the larger fish from a population. Large, older summer flounder are typically found further north, and as exploitation reduced the number of summer flounder in the 1980s and 1990s, larger fish were preferentially harvested by the fishery. The remaining summer flounder population, dominated by smaller fish, subsequently became centered further south. The northward shift of the stock in recent decades was linked to an increase in the number of larger, older fish as the population has rebuilt.”
Thus, as the study advises,
“multiple factors specific to individual species need to be considered when developing management regulations for living marine resources.”
NMFS, in announcing the study, noted that
“climate change can have major impacts on the distribution of fish, but the effects of fishing can be just as important and occur on a more immediate time scale.”
If we extend that thought to the cod discussed earlier in this essay, we can look at a study entitled “Reconciling overfishing and climate change with stock dynamics of Atlantic cod (Gadus morhua) over 500 years”, which appeared in the Canadian Journal of Fisheries and Aquatic Sciences in 2011.
In that article, the researcher reported that a population model that only considered fishing impacts on cod population trends between 1505 and 2004 failed to accurately predict historical fluctuations in the stock. A model that relied only on climate failed almost as badly.
However, a model that considered both factors tracked historical changes in the cod population with reasonable accuracy.
When the model is applied to fairly recent times—roughly the last 200 years—the finding was that
“This model suggests temporal differentiation between fishing and climate effects, including (i) declines during the Little Ice Age (18001880) caused by lower productivity, (ii) collapses in the 1960s caused by overfishing, (iii) collapses in the late 1980s caused by both, and (iv) rebuilding now hindered by depensatory effects of low numbers.”
Based on that finding, it is folly to attempt to separate climate change from overfishing when we look at stock declines. While both are detrimental to healthy stocks, together they create a sort of synergy that makes it even more difficult to conserve and rebuild fish populations.
Although conducted on the other side of the Atlantic, the conclusions drawn in the paper “Fishing, climate change and north-east Atlantic cod stocks,” written by three members of the Marine Biological Association of the UK in 2007, probably provide good guidance for the maintenance of our western Atlantic cod stocks as well. The authors found that
“Cod stocks in the north-east Atlantic, especially the North Sea, have been impacted upon heavily owing to sustained overfishing by modern fleets. It seems unlikely that global climate change warming is the primary factor causing the decline in fish numbers, but evidence suggests that this phenomenon may be exerting additional pressures on already overexploited stocks by further reducing recruitment rates that are already low due to reduced [spawning stock biomass]. Indirect effects of climate change on the availability of prey for juvenile fish are predicted to be detrimental to the recovery of fish stocks, and current climate conditions are likely to drive this decline in suitable prey supply for the foreseeable future. Temperature effects on cod growth, reproduction and recruitment will cause significant effects that require consideration when making stock assessments and catch forecasts…
“There is a need to take precautionary measures to give stocks a chance to rebuild. Although climate change appears to be one of the factors responsible for non-recovery of cod in the North Sea, fisheries are no longer sustainable at the present level of fishing effort. Future policy must take both factors into account when deciding upon sustainable catch quotas… [emphasis added]”
Thus, there is no scientific justification for using climate change as an excuse to allow overfishing. To the contrary, climate change probably amplifies the problems that overfishing causes, and makes the adoption of conservative annual catch limits even more important than it might have been before.
That’s not what a lot of the folks up in New England want to hear, but it is true nonetheless.