Striped bass only taking nibble of crab population
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The Bay’s burgeoning striped bass stock is eating tens of millions of baby blue crabs each year, but probably not enough to take a significant bite out of the total crab population, according to new research.
A study by the Virginia Institute of Marine Science estimated that striped bass ate about 73.5 million small blue crabs that were inhabiting underwater grass beds in the lower and midportions of the Bay last fall.
But that number was dwarfed by the 1.6 billion crabs that the scientists estimated were taking refuge in the same grass beds. “That is an astounding number,” said Jacques van Montfrans, the VIMS scientist who oversaw the study.
The question of whether predation by striped bass — whose numbers in the Bay are at their highest levels in decades — is affecting blue crabs has become increasingly controversial as the crab population has shrunk in recent years.
Not only is the striped bass population high, but the population of menhaden, an important source of food, has been low. This has caused some to hypothesize that predation on blue crabs has increased.
Some watermen have even suggested that fishing pressure on striped bass should be stepped up as a means of helping the blue crab, the Bay’s most valuable commercial fishery.
The VIMS study was the most comprehensive study so far of the striped bass/blue crab relationship in the Bay. It was endorsed by the Bi-State Blue Crab Committee, an advisory panel of state officials, watermen, environmentalists and others who have been working for more than two years to improve Baywide blue crab management.
The VIMS study focused on grass beds in the Lower and Mid Bay because that is where small crabs generally seek refuge from predators; they are far more vulnerable in open bottom areas of the Bay.
Researchers used a special device to “vacuum” young blue crabs from randomly selected grass beds in the study area. The results indicated that the average blue crab density was 13 crabs per square meter of grass bed.
Similarly, the scientists worked with watermen to net all of the fish in a grass bed to determine their numbers as well. They found an average of 8.5 striped bass per acre of grass bed.
A third part of the study examined how long it took striped bass to digest blue crabs; without that, scientists wouldn’t know whether blue crabs found in striped bass stomachs had been eaten within hours — or days. In laboratory work, it turned out that it takes striped bass just about a day — 23 hours — to digest a blue crab.
Then, from September through mid-November, the scientists gathered striped bass from randomly selected grass beds, examining 253 fish in all. During the 2000 study, they found that striped bass had as many as 41 blue crabs in their stomach, but many had none. The average was 5.
When the scientists put all of the numbers together, they estimated that striped bass consumed 73.5 million blue crabs during the fall, but that was only 4.6 percent of the 1.6 billion blue crabs they estimated were in the grass beds. “That’s not a very high percentage,” van Montfrans said.
The study found that blue crabs, by weight, accounted for 52 to 100 percent of the striped bass diet, van Montfrans said. But, he cautioned, that doesn’t mean the Baywide population is heavily consuming blue crabs because the study only looked at striped bass in grass beds, where young crabs are most common. “You can’t extrapolate to the striped bass population as a whole,” he said.
In fact, he said, the study confirms that striped bass were highly opportunistic feeders, eating whatever was most plentiful. The highest rates of blue crab predation took place in beds where crabs were most common; in other areas, striped bass were eating large amounts of fish, worms or shrimp.
Because of a lack of earlier studies, van Montfrans said it is impossible to say whether predation by striped bass on blue crabs has increased in recent years.
But, he said, it is unlikely that the striped bass feeding in seagrass beds are having much of an impact on blue crabs when compared to fishing.
That’s because overall predation rates on juvenile blue crabs are tremendously high. Few of the 73.5 million blue crabs were likely to have survived to become adults. The life cycle of the blue crab is designed to produce huge amounts of young — probably trillions of larvae are produced by the blue crab spawning stock — so they can sustain high levels of predation. If striped bass were not eating the blue crabs, van Montfrans suggested, it’s likely that something else would.
That means, van Montfrans said, that only a “minuscule” number of the crabs eaten by striped bass would have survived to spawn. In contrast, by the time hard shell blue crabs reach almost 3.5 inches, they have almost no natural predators; any adults taken in the fishery reduces the spawning stock.
“These predators are having an impact on a segment of the population that is undergoing high levels of natural mortality,” van Montfrans said, “whereas by the time they get to be commercial size — if left in a natural situation — they would reproduce and contribute to the population reproductively.”
The study focused on the fall because that is when blue crabs are at their smallest and most numerous — and when they are most vulnerable to predation. Further, van Montfrans said, striped bass generally don’t move into grass beds until temperatures begin to drop after summer.
But, he added, watermen indicate that predation can be high in the spring, too. “That may be an important period to assess,” he said.
More work could also be warranted to determine predation rates in unvegetated areas, where small crabs have little cover. “There is a lot we don’t know yet,” he said.
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