NSRAA’s Otolith Data Provides Insight On Programs

Posted by on Feb 26, 2016 in front page | 0 comments

NSRAA’s Otolith Data Provides Insight On Programs

NSRAA otolith marks all its chum eggs for identification of adults upon their return, but – until now – the organization was rarely able to study the information in season. This year (2015),  thanks to extra manpower and the need to analyze the cost benefits of the 4.0 group, NSRAA was able to study the data as the season ended, providing timely information as the new year begins.

By examining the otolith markings on harvested fish, NSRAA can gain an understanding of a number of things, including where fish are caught, where they stray and how different rearing techniques might affect marine survival.

Perhaps the most important of these is the ability to compare the survival rates of NSRAA’s 4.0 chum (also known as late-large) with chum raised in its regular program. Typically, fry are released between mid-April and mid-May, at a weight of about 2 grams or a bit under. With the 4.0 program, fish are not released until they double their weight to approximately 4 grams.

“Essentially, to get them to twice the size, you have to feed them twice as much,” explains Chip Blair, NSRAA Data Analyst. Twice the food nearly doubles the cost to raise the fish, of course. NSRAA wants to know whether the program is worth the extra cost. “If we can get one third of 1 percent or more increase in marine survival of the late large over the regulars, it’s worth it.”

NSRAA tried raising chum to 4 grams in the past, but, at best, the survival of these fish equaled that of the regular group. In some cases, it was worse.

“For many of the years we tried it, we experienced unusually cold springs – not the best rearing conditions for any fish,” he says.

Douglas Island Pink and Chum (DIPAC) has experienced ongoing success with its 4.0 program, prompting NSRAA to reconsider and try again.

The idea behind the 4.0 program is that while fry raised and released under the regular program remain in shallow waters near shore before moving out into the ocean, the 4.0 chum readily transition to deeper, off-shore waters immediately upon release. Historically, smaller fry tended to survive better near shore where there was cover from predators, so the fish evolved with these physiological and genetically-driven behaviors. Whale predation, however, seems to be a phenomenon of the last decade.

Humpback whales and other predators have taken to hunting the small fry as they hang out near shore after their release. NSRAA has tried various strategies to counter the problem, including towing the fry away from rearing sites, but there are still many miles of shoreline where predators can hunt the fry as they make their way toward open ocean.

The 4.0 program may be a critical component of NSRAA’s success in the future, especially at Hidden Falls. The otolith data collected this year only begins to paint a picture, but as Chip says, “it’s encouraging.”

Most of the fish from the 4.0 program returned this year as three-year-olds, with a survival ratio as high as 9:1 over those released under the regular program.

“What we found this year, at all the sites we’re doing 4.0s, they’ve been successful by a good margin,” says Chip. “The late-large are surviving better, but there are also more of them coming back at a younger age, which might indicate a smaller ratio of 4.0s to regulars next year, when the four-year-olds return. In any case, the ratios show that the increase in survival more than pays for the extra expense – by a large margin.”

What will NSRAA do if the 4.0 program continues to show significantly better survival rates?

In smaller releases during the startup years at Southeast Cove and Crawfish, NSRAA raised 100 percent of its chum as 4.0s. As release numbers increase for these projects, space constraints will prevent NSRAA from raising 100 percent of chum as 4.0s, but the percentage would be as high as possible.

The information may be particularly helpful at Hidden Falls. Though the 4.0 program is limited by space – in addition to double the food, the fry need double the space for those few weeks as they grow from 2 to 4 grams – there is a potential that NSRAA could increase the number of 4.0 raised at that hatchery.

“If we continue to see they’re doing much better than the regulars, that would show that the predation theory at Hidden Falls is probably true,” Chip says. “So we might raise a higher percentage as 4.0s. Say, instead of doing 33 percent 4.0s, as we do currently, we might do 40-50 percent. It’s a balancing act between rearing space and cost. Another option might be to raise fewer fish with more 4.0s. We’ll probably need a couple more years of analysis before we make the decision to invest that kind of money.”

Chip wasn’t sure what to expect when he analyzed his first round of otolith data this fall.

“These early results are a lot more encouraging than I expected,” he says. “We never would have known that without this information. We’re pretty excited and hoping they’ll continue with the same trend.”

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