Link between striped bass, weather suggests stormy times ahead for fish
Long-term climate patterns may greatly influence how many larvae survive to become adults
More than two months before biologists threw their first net into the water to gauge the success of this year's striped bass reproduction, Ed Martino had the answer, and he never had to leave his desk.
Rockfish reproduction, Martino determined in May, would be "well below average."
The researcher with the National Oceanic and Atmospheric Administration's Cooperative Oxford Laboratory came up with his conclusion by going online and looking at March though May river flows monitored by the U.S. Geological Survey and temperature data from Baltimore-
Washington International Airport for the same period, then plugging the information into a mathematical model.
While Martino crunched numbers in his office, a team of biologists from the Maryland Department of Natural Resources waded into the water at 22 locations once a month from July through September. At each site, they did two sweeps through the water with a 100-foot seine net, then counted everything they caught.
When the work was done, the biologists had averaged 5.6 juvenile striped bass per net haul. That was less than half the long-term average of 11.6. After all of their field work, they had reached the same conclusion as Martino.
His model, which was developed with data from the Maryland DNR, confirms what biologists have thought for years: The weather during any given spring plays a huge role in determining how many larval striped bass survive to be "recruited" into the overall population. But his model puts an exclamation point to just how important weather is: In looking back to 1985, he can account for more than 80 percent of the annual variability in striped bass recruitment in Maryland, where the majority of the East Coast population is spawned.
This year, the model successfully predicted a poor year even though many fishery biologists - including Martino - thought it would be good.
But that predictability may contain a hint of problems on the horizon for striped bass. Although the coastwide population remains above target levels, striped bass recruitment in Maryland has been below average for three consecutive years, largely because the weather hasn't cooperated.
"The Bay is full of spawners, but we are seeing a real reduction in recent years in reproduction," Martino said. "So I think it's pretty obvious that something else is going on in the environment."
That "something else" may be found in work done by Bob Wood, the NOAA scientist in charge of the Oxford Lab. Wood suggests that a broader, long-lasting climate pattern called the Atlantic Multidecadal Oscillation may be affecting striped bass and other fish.
The Atlantic Multidecadal Oscillation is an alternating pattern of warming and cooling over large areas of the Atlantic Ocean, similar to the El Nino, La Nina patterns in the Pacific. The shifts, in turn, affect climate over large regions of North America. Various AMO phases, during which different parts of the Atlantic are warmed or cooled, persist for decades.
During certain AMO phases, which promote wetter winters, cool springs and more frequent nor'easters, the prevailing climate pattern seems to promote improved reproductive success for anadromous fish, such as striped bass, which live most of their lives at sea but return to freshwater to spawn.
During other AMO phases, which promoted drier, warmer springs, the situation is reversed, with fish that spawn on the coastal shelf and whose larvae use estuaries for nurseries, such as menhaden, getting a boost in recruitment. During those times, striped bass reproduction takes a hit.
Wood says those phase shifts are strongly correlated with the rise and fall of striped bass and menhaden stocks in the past.
Striped bass crashed because of overfishing in the 1980s, which was also a time when the AMO was in a phase unfavorable for their recruitment, so fish being caught were not being replaced. The ensuing rebound of striped bass stocks is often touted as a major fishery management success as managers took dramatic actions, including a coastwide moratorium, to protect the spawning stock. And it was. But Wood's work strongly suggests that managers also got lucky - their fishing moratorium coincided with an AMO shift that greatly improved striped bass spawning conditions. "Had the weather not turned, we would have been waiting longer for that recovery," he said.
Meanwhile, as striped bass recruitment bottomed out in the 1970s and 1980s, menhaden recruitment soared, only to fall to persistent low levels in the 1990s and 2000s as striped bass again benefited from the prevailing climate cycles. The same general pattern holds for other anadromous and shelf-spawning fish, Wood said, but the correlation is strongest with the anadromous striped bass and white perch, and the shelf-spawning menhaden and spot.
The exact reason why temperature and the timing of river flows is so important is less certain. Martino and Wood theorize the cool temperatures delay the production of prey until striped bass larvae are most abundant. The high flows may push those plankton and striped bass larvae together so the larvae, which are poor swimmers, have plenty to eat. "The prey almost has to bounce off the heads of the larval striped bass," Martino said.
Conversely, warmer years are beneficial to larval menhaden, which use the same areas for nursery grounds, but arrive earlier and eat different kinds of plankton.
The hypothesis is not new; Wood first published it years ago, but he has since strengthened the climate - recruitment connection, adding more data and looking at actual striped bass and menhaden harvests dating back to the 1880s. A new publication is in the works, which he hopes will prod more discussion among managers.
A better understanding of these long-term patterns can be a huge aid for fishery managers. Had they understood they were in the midst of a down-cycle for striped bass recruitment in the 1980s, for instance, managers might have acted sooner to curb fishing pressure, Wood said.
A 2004 report, Fisheries Ecosystem Planning for Chesapeake Bay
, noted that an understanding of climate cycles has been important for improved management of some species in other places, such as salmon in the Pacific Northwest. It said Bay species are likely influenced by long-term climate patterns as well, and understanding those patterns is critical to maintaining healthy populations.
"Management plans that proved successful 10 years ago can become ineffective if the decadal regime has shifted," the report said. "Alternating strategies, linked to the corresponding climate regime, appear warranted."
There are problems in using that information in management, though. The understanding of regional climate patterns is far from complete, and it is much easier to observe what happened in the past than to predict what will happen in the future. As a result, it's hard to say with certainty whether the last three years of poor reproduction stemmed from a change in the AMO and will persist into the future - or their correlation is just coincidence.
"It is quite possible that he (Wood) is correct. The problem that we have, and it is a typical problem with this type of analysis, is that they are always based on an analysis of the past," said Alexi Sharov, a stock assessment scientist with the Maryland DNR. "In many fisheries around the world, strong correlations were found between recruitment and certain environmental factors based on historical data, but the attempts to use these correlations to predict recruitment were not very successful, indicating that we are dealing with a very complex, intricate process."
Also, while striped bass recruitment has been poor the last three years, there's been no boom in menhaden recruitment. The menhaden recruitment index remains below average, although 2008 - 10 were the best three consecutive years for menhaden recruitment measured in Maryland since 1990 - 92.
But managers have become increasingly concerned about the status of the striped bass stock. The total number of fish estimated to be in the coastal population has declined by a quarter in recent years, although it still remains at what fisheries scientists consider safe levels.
The recruitment needed to replenish those stocks has been poor in many places where it's monitored, especially in Maryland, where striped bass reproduction has the strongest connection to future populations. Recruitment has been below average for the last three years, and four of the last five, in the state.
Because of concerns about the population, the Atlantic States Marine Fisheries Commission, a multi-state agency that regulates catches of fish that migrate along the coast, recently decided to assess the striped bass stock next year, rather than 2012 as previously scheduled.
Doug Vaughan, a NOAA stock assessment scientist, called Wood's climate work a "good hypothesis" that offers an explanation why fisheries scientists often are not able to make tight relationships between the size of a fish species' population, and its reproductive success.
"If you are trying to base your projections on some spawner-recruit relationship, it is not likely to do very well," he said. As a result, efforts to make population projections are likely to move toward incorporating variables such as climate patterns although "it is difficult," Vaughan acknowledged.
If the region has indeed entered a climate phase that favors poor striped bass recruitment, it doesn't necessarily mean every year will be poor. Other, more local, weather patterns can overpower the prevailing regional conditions caused by the AMO. "It doesn't control everything, but it controls the probability of having a cold, wet year," Wood said.
Nor does it mean biologists should stop measuring the actual number of fish.
"Rather than guessing whether the prediction worked or not, we actually would still prefer to measure the actual success by doing the juvenile survey and knowing for sure that we have either a poor year class, an average year class or a strong year class," Sharov said. "A prediction is still a prediction."
Indeed, Martino's model had predicted good reproduction in 2008. It turned out to be the worst striped bass recruitment in recent years. He thinks a sudden, severe cold snap in late spring was the culprit.
But the tight relationship between weather, climate and fish production may require adjustments to traditional fishery management theory that aims to maintain high abundances of a species at all times. Instead, managers may need to adopt more precautionary catch levels to protect smaller stocks when they are faced with persistent unfavorable climate conditions.
"No one should think that fishing doesn't have an effect," Wood said. "At the end of the day, this is telling us there is not a static carrying capacity."
Read the entire story in the Chesapeake Bay Journal....
"Wood suggests that a broader, long-lasting climate pattern called the Atlantic Multidecadal Oscillation may be affecting striped bass and other fish."