When striped bass rebounded from record low numbers in the late 1980s and early 1990s, fishery managers took the credit, saying their controversial harvest limits had helped to save the stock.
But a recent study suggests they may need to share that credit with someone else: Mother Nature.
Similarly, fishery agencies have been drawing fire in recent years for not cutting commercial menhaden harvests which many anglers blame for reduced levels of this important food source—a favorite of many predators—in the Chesapeake.
But, again, recent research suggests the blame for the menhaden shortage may lie mostly with Mother Nature.
Or, more specifically, in however she determines which weather patterns will dominate the Bay region.
Recent papers by Robert Wood and colleagues show a strong relationship between springtime weather patterns and which fish populations are winners and losers in a given year when it comes to spawning success in the Bay.
“It suggests that a lot of what we see and worry about in terms of fisheries management could be because we don’t account for the natural climate cycles that affect these species,” said Wood, a fisheries scientist and climatologist with the National Oceanic and Atmospheric Administration.
Wood’s work has centered on the impact of two weather patterns, the Azores-Bermuda High, and the Ohio Valley High, on the region.
The Azores-Bermuda High is a high pressure system whose center moves back and forth during the year between the Azores, a chain of islands off Portugal, and the Bermuda Islands in the western Atlantic. It is the system responsible for warm, humid summers in the region, and the closer its center is to Bermuda, the greater its impact on the region’s weather.
But when the high is centered near Bermuda in the late winter and early spring, it triggers an earlier onset of warmer, drier conditions in the mid-Atlantic region.
Competing with that system for influence during the spring is the Ohio Valley High. When that high pressure system is dominant, it causes cool winter and spring conditions, and it is often associated with more rain, snow and runoff. Ohio Valley highs occurred frequently in the late winter and early spring of 2003, contributing to wetter than normal conditions.
Whichever system is dominant in March, according to Wood, brings conditions favorable for the survival of fish larvae for some species, but less favorable conditions for others.
For example, successful spawning for striped bass and white perch, two ocean-living fish that spawn in similar freshwater habitats, is boosted when the Ohio Valley High dominates in March. By contrast, menhaden and spot—ocean-spawning fish whose young move into the Bay during the spring—have their best success when the Azores-Bermuda High dominates that month.
The reason, Wood believes, is that when the Azores-Bermuda high rules the weather early in the spring, it sets up ideal conditions for larval spot and menhaden as they swim up the Bay. Flows down the Bay tend to be weaker, allowing the larvae to more easily swim far up the Chesapeake.
Perhaps more important, the estuarine-loving zooplankton consumed by larval spot and menhaden can flourish early, so they are plentiful in the areas used by young spot and menhaden when they begin showing up in nursery areas during March and April.
By contrast, when the Ohio Valley High rules the March weather, the freshwater spawning areas of striped bass and white perch are expanded downstream, and the supplies of the freshwater zooplankton that their young consume begin multiplying earlier, ensuring they are plentiful by late spring, when recently hatched rockfish and perch need plenty of them to eat.
Basically, that means a cool, late spring benefits striped bass and white perch, while an earlier, warmer spring benefits menhaden and spot.
“Obviously, those things are mutually exclusive if you are looking at the same nursery area, which you really are,” Wood said. “Menhaden need to get up upstream and into the nursery areas as early as March, which is just about a month or so earlier than the striped bass spawn.”
While the strength of each high pressure system fluctuates from year to year, the overall climate pattern seems to be cyclical with conditions tending to favor one group of spawners for more than a decade at a time, Wood said.
That doesn’t mean that a species won’t have successful reproduction during unfavorable conditions. It just means they probably won’t, on average, produce anywhere near as many young as during favorable times.
Normally, fishery managers rely on traditional indicators of a population's health, such as its spawning stock biomass—a measure of the reproductive potential of a population—to predict reproductive success.
Yet the relationship of spawning stock biomass for many species, such as menhaden, to recruitment is weak. That suggests that environmental factors play an important, if not dominant, factor in the reproduction for many species.
In fact, Wood said, almost half of the variation in Atlantic menhaden recruitment since 1966 can be explained by the number of days the Azores-Bermuda High dominates the region during the month of March—a remarkably strong correlation for fishery management.
Wood’s idea is catching on. NOAA’s Chesapeake Bay Office said in a report earlier this year that managers should begin factoring climate variabilities into their fisheries management plans.
A catch threshold that protects stocks during periods of good reproduction may not adequately protect fish during cycles of poor reproduction, said the report, “Fisheries Ecosystem Planning for Chesapeake Bay.”
“Management plans that proved successful 10 years ago can become ineffective if the decadal [climate] regime has shifted,” the report said, which may help to explain why some fishery plans seem adequate for a while and then fail. “Clearly, the impact of the controlling climatic regime should be taken into account in managing fisheries.”
A case in point, Wood’s research suggests, is striped bass and white perch. Both have similar life histories and use similar spawning habitats.
In the late 1970s and much of the 1980s, the region’s climate shifted into a regime unfavorable for spawning success for both species, but striped bass were apparently being fished much harder than white perch.
As a result, the striped bass stock collapsed—even though fishing pressure did not noticeably increase during that period. The white perch population didn’t flourish, but was able to maintain itself during the same time because it was subject to less fishing pressure.
By contrast, sharp catch limits—including total moratoriums in some cases—helped the striped bass spawning stock to rebuild. But the population rebound in the late 1980s and 1990s coincided with a period in which the Ohio Valley High was a dominant feature in the spring, favoring striped bass reproduction.
The 1990s, which had some of the most dominant Ohio Valley Highs in recent decades, included several of the most successful striped bass spawning years on record.
“The recovery didn’t happen until we had some wet, cool years during and just immediately following the moratorium,” Wood said. “What management did was preserve enough spawning stock so when this climate signal turned back around favoring recruitment, we were in good shape to take advantage of that.”
Had people been aware of the influence of climate patterns and acted early to reduce fishing pressure, “managers might have avoided a really big problem,” Wood said
Similarly, the poor reproduction of menhaden in the Chesapeake in recent decades is mirrored by that of spot, another ocean-spawning species whose young move into the Bay and use the same nursery habitats. If the poor reproduction of menhaden was primarily the result of overharvesting by the commercial fishing industry, as some recreational anglers have suggested, the trends between the species should look different, Wood said, but they don’t.
At the same time, Wood said, managers need to be make sure that the population of spawning-age menhaden receives enough protection so that it can take advantage of conditions the next time Mother Nature flicks the climate switch.
“We have evidence to say the menhaden spawning stock is not the problem,” Wood said. “But it is still something we have to be concerned about.”