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Commercial Fisheries News
Volume 35 Number 12
August 2008
Scientists study herring predation impacts
PORTLAND, ME – A couple of years ago, scientists determined that four major predator groups – marine mammals, medium demersal fish, large pelagic fish, and seabirds – ate three times more Atlantic herring than commercial fishermen catch.
Now, scientists are hoping this predation information can be used in the next herring stock assessment to give everyone a better understanding of how the resource is utilized – both in nature and by industry.
“We’ve taken a first stab at incorporating ecosystem principles into the assessment,” said Bill Overholtz of the National Marine Fisheries Service’s Northeast Fisheries Science Center during the New England Fishery Management Council’s June 3-5 meeting in Portland.
“I think we’re getting some interesting results,” he said. “Hopefully we can bring some of these new methods and analyses forward in the assessment arena.”
The next herring stock assessment, which is conducted jointly with Canada through the Transboundary Resource Assessment Committee (TRAC), isn’t scheduled to take place until December 2009. However, industry members already are keeping an eye out for new information that might play a role in it.
Overholtz and fellow researchers Jason Link and Larry Jacobson carried out the center’s work on herring predation, and Overholtz presented a detailed summary of the findings at a special all-day herring session on March 1, 2007 at the Maine Fishermen’s Forum (see CFN April 2007 for details).
During this recent June council meeting, he reviewed that same information and provided a preview of the center’s preliminary calculations for how total herring predation – if incorporated into the next assessment by the TRAC – might impact existing biological reference points for herring.
“Predation mortality should be included in stock assessments of prey fish,” said Overholtz. “If we can do it, it only adds to our knowledge base.”
What eats herring?
Overholtz first provided the council with a recap of the four major herring predator groups. These are:
Marine mammals – Over time, this has become the biggest predator category. In 2002, marine mammals ate an estimated 150,000 metric tons (mt) of herring, with the primary consumers being fin, humpback, minke, and pilot whales, along with harbor porpoise, white-sided dolphins, harbor seals, and gray seals.
Medium demersal fish – This category, which used to dominate the herring predator field, has dipped to number-two. Still, medium demersal fish ate on the order of 130,000 mt of herring in 2002, with the key consumers being spiny dogfish and silver hake, followed by, to a much lesser extent, cod, white hake, monkfish, winter skates, thorny skates, sea ravens, red hake, bluefish, summer flounder, and pollock.
Large pelagic fish – The three most important herring consumers in this category were bluefin tuna, blue sharks, and mako sharks, with herring consumption estimated at 25,000 mt in 2002; and
Seabirds – Herring consumption by seabirds in 2002 was estimated at roughly 9,000 mt, making it the smallest predator group of the four. The most significant consumers were northern gannets, shearwaters, blacklegged kittiwakes, blackback gulls, herring gulls, and northern fulmars.
All together, herring predators ate roughly 314,000 mt of herring in 2002.
In comparison, commercial fishermen landed 93,157 mt in 2002. Landings from 1995 to 2005 averaged 107,018 mt and, in 2007, industry landings were down to 78,172 mt, according to the latest update of the Atlantic Herring Stock Assessment and Fishery Evaluation Report released by the New England council in May.
The science center’s research team expected predation to increase to 350,000 mt in the future if fish stocks rebound and marine mammal populations continue to grow.
Shift in consumption
Overholtz said the science center’s herring predation work covered the Gulf of Maine and Georges Bank but did not include the Scotian shelf.
He also noted that, over the years, predation patterns have changed significantly. In 1991, the medium demersal fish category as a whole consumed over 200,000 mt of herring, with dogfish alone accounting for 98,000 mt of that amount. However, the 2002 estimate of herring consumption by the medium demersal fish category dropped to roughly 130,000 mt.
Marine mammals, on the other hand, have steadily increased their herring consumption and now top the predator field.
Here’s how the four predator categories shifted in just over a decade’s time:
Herring consumption by predators in 1991 – medium demersal fish, 70%; marine mammals, 24%; large pelagic fish, 5%; and seabirds, 1%.
Herring consumption by predators in 2002 – marine mammals, 48%; medium demersal fish, 41%; large pelagic fish, 8%; and seabirds, 3%.
Objectives
According to Overholtz, the research team had four major objectives while investigating this predation situation. These were to:
Estimate total consumption of herring by predator groups;
Estimate predation mortality on herring;
Determine the impact of predation on herring recovery and dynamics; and
Quantify the effects of predation on long-term yield potential.
The team conducted much of this work in the early 2000s, which is when the herring plan contained a “proxy” for maximum sustainable yield (MSY) of 220,000 mt.
That number has since been replaced by 194,000 mt in the council’s herring plan as a result of the 2006 TRAC assessment, but for their initial ecosystem analysis, center scientists used the former number – 220,000 mt – because that’s what was available at the time.
New model
Using a new model that incorporated not only predation but also other types of natural mortality and fishing mortality, the team calculated that long-term yield at MSY for the herring resource would be 528,000 mt instead of 220,000 mt. The team thinks this much larger number – or something close to it based on more recent data – could be the new MSY reference point if the TRAC takes an ecosystem approach in the 2009 assessment.
In one scenario, the team calculated that if MSY was 528,000 mt and predators consumed 350,000 mt, the remaining amount available for a commercial fishery would be 178,000 mt, which is significantly more than the fishery has been harvesting in recent years.
“There appears to be a fair amount of fishery yield that can be taken even though there is a fairly large predator base out there,” said Overholtz.
As predator consumption increases, the amount of yield available for the commercial fishery would decrease, and if predator consumption decreased, the fishery yield could increase, assuming the stock overall remained healthy, he explained.
“You have to look at the trade offs between predation and the fishery,” Overholtz said.
Janice M. Plante
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