Some animals — elephants, for instance — are easy to study because researchers can camp out and observe their habits. Fish, in contrast, can be somewhat of a mystery. Gaze out over the water's surface, and the secret lives of fish remain hidden to the casual viewer.
"It's always intrigued me that we have to understand the lives of fishes indirectly — we can't often observe what fish are doing, but have to connect the dots by using a battery of different studies," explains fisheries ecologist and AAAS member David Secor at the University of Maryland Center for Environmental Science's Chesapeake Biological Laboratory in Solomons, Maryland.
"After 20 years of this, we've made so many great discoveries about the lives of fish with electric tags and otoliths — it's been a great study of ecology," says Secor.
Another fisheries scientist and AAAS member at the Chesapeake Biological Laboratory, Edward Houde, has studied fisheries for some 50 years and was introduced at a January presentation as the 'godfather of ecosystem-based management in the Chesapeake Bay management community.' He earned that unofficial honor for his role in bringing ecosystem-based management to the Chesapeake region back in 2001 by co-writing a 400-page strategic plan that detailed ecosystem-based fisheries planning especially for the Chesapeake Bay. Houde calls it common sense: "Do no harm to the ecosystem that produces your fish."
Both AAAS researchers investigate fish populations in and out of the Chesapeake Bay, the nation's largest estuary.
People have always depended on fishing and harvesting in the Chesapeake region, and no less now. Fisheries — from striped bass to oysters — support the regional food supply and economy. To figure out how to better manage our natural resources, Secor and Houde are among the scientists plunging in to discover clues from underwater. Each recently presented a study funded in 2011 by the NOAA Chesapeake Bay Office's Fisheries Science Program. Their findings in these two (currently under way) studies will help direct better fisheries management along the Atlantic coast and in a Bay riddled with serious water quality issues.
One fish, two fish
Silvery-colored with a black spot just behind the gill, menhaden are the second most-harvested fish in the U.S., where the species is primarily used for oil and omega-3 fatty acids. It's also considered a keystone species and among the most abundant and important finfish species in the Chesapeake Bay and coastal Atlantic waters.
"They really haven't gotten the research they've deserved over the years," Secor says. So Secor and Houde launched a study to look at how to improve estimates of juvenile menhaden's abundance, examine historical trends, and compare growth rates across nursery habitats in the Bay. Menhaden feed on plankton — low on the food web — so they can exert a strong influence up the food chain. Research started last year — focusing on larvae — and will continue this year with a focus on juveniles.
"The Chesapeake Bay is historically the biggest nursery area for juveniles on the Atlantic coast," says Secor. Menhaden are pelagic school fish, and over a decade, their abundance may rise and fall as much as twenty- to thirtyfold, explains Secor, though scientists aren't sure what drives those fluctuations in the Chesapeake.
During the late 1970s and '80s, menhaden numbers soared. But just a decade earlier in the '60s — and again in the '90s — abundance was low. Biologists have their hypotheses about why menhaden populations spiked: It could have been changes in the oceanography, since they reproduce over shelf habitats most months of the year. Other hypotheses focus on changes in the Bay or a climate regime shift, explains Houde.
They found that most menhaden larvae enter the Chesapeake during the months of October, November, and December, but fewer survive. Scientists aren't sure why, but suspect that the Bay is too cold, or there's not enough food. Larvae that arrive in January, February and March tend to be fewer in number but have a higher survival rates.
Secor and Houde's study will also look at the fish study methods of seining — the traditional method, in which a wide net stretched between two posts is drawn through shallow waters — and mid-water trawling.
"Anyone who's waded out in the midst of a menhaden school knows that they're very good at evasion," Secor says of seining. "Its efficiency depends on the weather, depends on the tide, and depends on experience [of the surveyors]." Often, frustrated researchers will see pods just beyond reach of the seine.
"So this caused some questions about 'what is the overall average of seine catches telling us in terms of actual juvenile abundance in the Chesapeake Bay,'" Secor says.
There's reason to believe that the mid-water trawl may be better suited for the lifestyle of young menhaden, which tend to swim towards the surface in deeper waters. The 18-foot-wide, cone-shaped trawl is drawn by a research vessel, which can be more expensive. But the trawl can sample the whole water column, and is effective at trapping schools of menhaden herded in front of the net.
In 2010 they found that sites near the mouth of a river yielded zero seine catches but 259 trawl catches. However, at the most-upstream site, the trawl couldn't physically get to the site, but the seine pulled in 52 fish.
They'll replicate what Maryland Department of Natural Resources has done for decades by sampling at the same sites throughout the Bay and its tributaries. In addition, they'll audit Maryland DNR's historical seine surveys — dating back to 1959 — to figure out how to compare them to newer data and study menhaden's decadal variations.
To study juvenile growth rates, Secor and Houde will sample the earstone, called the otolith. A slice through the calcium carbonate otolith — about one to three millimeters long —reveals rings like the growth rings of a tree. One ring equals one day's growth, and the first ring — signifying the birth — is only about 20 microns across. Daily rings are only one to five microns wide, and a juvenile typically has over 100 of these rings that researchers must painstakingly count.
Growth rates offer clues about the local habitat conditions, and different sub-estuaries may provide different growth conditions, depending on temperature, salinity, and more. Menhaden growth rates can vary as much as 50 percent across the tributaries of the Chesapeake Bay.
Finally, they'll examine ecosystem-based management: what constitutes an important nursery.
Fisheries scientists have noticed that fish abundances bounce up and down from year to year, but they don't know if there's a pattern. That's the mystery driving another study by Houde, who with colleague William Connelly, has set out to study the Chesapeake's juvenile anadromous fish — which are born in fresh water, but spend most of their life in the open ocean. These fish, such as striped bass, herrings, and white perch, return to Bay tributaries to spawn.
Fish stocks rise and fall even if a species isn't fished, according to Houde, depending on the success of reproduction in any year. In the Chesapeake, some of the anadromous fish populations can fluctuate by ten- to fiftyfold in the abundance of young that are produced.
For many anadromous fishes, such as striped bass, the strength of their 'year class' (fish hatched in a given year) is determined in the early life stages, sometimes in the first month of life.
Houde and Connelly will investigate population patterns for these juvenile anadromous fish, examining differences in reproductive success among species and among Bay tributaries. Since these less-than-year-old fish are not harvested on a large scale, the explanation has to be an environmental impact on reproductive success. Their answers, they hope, will help inform managers on the status of tributary systems.
To dig deeper, they'll analyze Chesapeake Bay Program monitoring surveys and historic Maryland DNR data sets, searching for patterns as fish abundances rise and fall.
"We proposed to undertake a relatively comprehensive study looking back at the 60-year data set to understand what the causes were, and understand how things like temperature affect reproductive success," says Houde, who'll investigate temperature, salinity patterns, and freshwater flow. In the spring, high freshwater flow often means higher zooplankton and plankton production, which supports fish production.
"You have to have the right combination of flow, temperature, and food at the right times," says Houde, whose team will see if those criteria are similar for every species. Eventually, they would like to develop forecasting models to look at flow, temperature, and plankton production and be able to predict the abundance for species such as juvenile striped bass.
To complete their research, Secor and Houde must overcome bumps in the road. Last year, the Fisheries Science Program awarded funding to both studies, but there wasn't a call for studies to fund this year. Both say they need to find another funding source to be able to wrap up their studies.
In addition, fisheries scientists everywhere will be looking ahead to how fish populations will be affected as the environment changes — which seem to be imminent. "The fisheries challenge will be to understand how these populations of fish are resilient to absorbing future changes and shock, like climate change," Secor says.