Scientists Study Drop in Blue King Crab Populations
KODIAK, Alaska A groundbreaking experiment to learn more about blue king crab larvae has produced surprising results for research fishery biologist Brad Stevens and his team.
The experiment examined conditions for cultivation of larval crabs.
"We've probably learned more about blue king crabs in the last year than anything that has been published about these animals," Stevens said.
He said a few publications describe the crabs taken from the ocean, but no one has worked with them in a laboratory before, cultivated the larvae, or looked at hatching and embryonic development. "It is new ground we're breaking," Stevens said.
He was approached by a representative of the Pribilof Islands group, Central Bering Sea Fishermen's Association, which asked whether there was something he could do to help the group's members understand the blue king crab population fluctuations and decline.
The blue king crab fishery in the Pribilofs was closed in 1999 because of a low population and has remained closed.
Stevens received about $85,000 of grant funding from the North Pacific Research Board in 2003 for the experiment, which took place last year.
"My interest is looking at habitats. We could either go out there and do it, which is a very expensive operation requiring a lot of ship time and technology. We looked at that closely and decided it wasn't feasible," he said. "The other approach was to look at the larvae in a laboratory. That was the approach I decided to start with."
The first step was to produce enough small crabs for the experiment. Part of the funding was used to develop a cultivation procedure.
"We had been doing this with red king crab for four years with variable success. Sometimes we'd get 10 percent survival of the larvae, sometimes we'd get up to 50 percent but never exceeded that," Stevens said.
"We were looking for really good improvement," he said. "To do the kind of research we want to do on settlement behavior, we needed about 1,000 settling crabs. It takes space to cultivate those, so we need to do it efficiently."
The experiment targeted three things: diet, temperature and density of cultivation, or the number of larvae per liter.
The larvae were primarily fed with brine shrimp, also known as artemia, or "sea monkeys."
Stevens and his team set up a "sea monkey" farm in the lab where the eggs hatched overnight.
They fed the crab larvae with four different combinations of brine shrimp, including some fattened with different types of diatoms, or microscopic algae.
The diet with the highest survival rate -- 92 percent -- was the one in which diatoms were added to the water with the brine shrimp to feed the crab larvae.
Nutrition from brine shrimp alone gave them about 50 percent survival rate, compared with about a 1 percent rate in the wild.
"That was much higher than we expected to get," Stevens said. "We were very happy with that result."
The experiment also found that the crabs' best survival occurred in water at nearly 43 degrees Fahrenheit.
Another part of the experiment was to find out when crabs hatch.
"That question can be fine-tuned," Stevens said. "For instance, we can ask what month they hatch. Within that period of time, how long does it take an individual crab to hatch and what occurs on a daily basis?
"We have done this with red king crab and with Tanner crab, and were checking off species as we get a chance to do this because timing tells you about the reproductive strategy of these animals."
King crabs have the longest hatching period of any crab species in the world, taking 30 days to release all their larvae.
"They do it in little spurts every day," Stevens said. "We put these crabs in individual tubs, let them hatch and collect the larvae every day. We can determine the hatch time.
"Every evening, as soon as we go home and turn off the lights, in the next hour, they start releasing larvae and they do it for about two hours. Every day they do that for a month."
He said that was unexpected. Now Stevens is preparing another grant proposal to the North Pacific Research Board to request two years of funding to look at some yet unanswered questions.
One question is about the crabs' settlement behavior, and other questions are about the hatching process, such as why it takes 30 days to release their eggs.
"We want to look at the effects of temperature on development," he said.
"For instance, if you put this animal in cold water, does it just slow down the whole process of development or does it slow down at a specific stage?" he said. "Do they develop all the way up to the point where they are going to hatch and then they just stop until they get some hatching cue, or does it slow down the whole process?"
His goal is to find out why things happen the way they do, but the information does not necessarily translate into ways to increase crab populations.
"At this point, I don't foresee being able to control any of that," he said. "We are not going to be able to go out and change the conditions under which these animals are living out in the ocean." The research aims to explain the reasons for crab population fluctuations.
Stevens began with the hypothesis that the decrease of the blue king crab population occurred somewhere between hatching and settling.
"Either the larvae are not surviving to the point where they become first-year crabs or they're not ending up in the right place where they can settle," he said.
He noted that one possibility is that currents carry them away from preferred habitats.
Data gained through the experiment are directly applicable to aquaculture.
"People can see that it's going downhill, and they want to do something about it," Stevens said. "They are interested in the idea of aquaculture.
"I do not think it's feasible or economical to farm king crabs because it takes them 10 years to get to legal size for the fishery. They take up a lot of space, they are highly cannibalistic and they are a top-level predator so they need to be fed a lot."
Source: Knight Ridder/Tribune Business News