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Commercial Fisheries News
Volume 34 Number 1
September 2006
Stock assessment: Dogfish ‘not overfished’
Peer review concerns: Fewer females, sex ratio imbalance, low recruitment
CRYSTAL CITY, VA – Mirroring what commercial fishermen say they’ve been seeing for several years now, scientists aboard the Northeast Fisheries Science Center’s 2006 spring survey ran into dogfish – a lot of dogfish.
The numbers were so high that they completely changed the outlook for the stock.
“Here’s the bottom line,” said Paul Rago, chief of the center’s population dynamics branch. “Is the stock overfished? No. Is overfishing occurring? No.”
Rago made those comments on Aug. 15 while providing the Atlantic States Marine Fisheries Commission’s (ASMFC) Spiny Dogfish and Coastal Shark Management Board with an overview of the latest stock assessment.
The new assessment, which was peer reviewed, was released in July as part of the 43rd Stock Assessment Workshop (SAW).
According to the SAW’s summary report, the high spring survey index for dogfish was “not due to one or two exceptionally large tows.”
The report stated, “Five strata had average survey catches that were the highest since 1980. Strata 65 and 66 east of Delmarva had female catch rates that ranked second and first, respectively, over their time series. Stratum 73 off New Jersey also recorded its highest ever female dogfish survey catch. The high average in stratum 73 was attributable to a large catch on the boundary with stratum 74, a much smaller stratum.”
All together, the results indicated “a near two-fold increase in total stock size between 2005 and 2006.” Plus, the stock size of mature females “increased nearly five-fold compared to previous years.”
Rago pointed out that something wasn’t exactly right with the overall picture. Female dogfish only produce four-to-nine pups per litter every two years.
“This is not a biologically plausible rate of change for this population,” he said. “It shows why we never want to put too much emphasis on any single survey estimate.”
Biomass up
Although stock assessment scientists have concerns about the new numbers, the extremely high 2006 survey indices raised the three-year female spawning stock biomass (SSB) estimate to 106,000 metric tons (mt), which is above the 100,000 mt threshold for mature females. Mature females are considered to be those bigger than 80 centimeters, which is roughly 32".
The new total biomass estimate for spiny dogfish, including males and females, exceeded 800,000 mt. Despite this enormous number, managers make many dogfish decisions based on the size of the female spawning population, not the males.
Rago said the sex ratio for mature dogfish – now at seven males to one female – has become extremely skewed. The change in sex ratio from its historical average of about 2-to-1 is coincident with the directed fishery on females. He said scientists worry there might be “reproductive consequences” to this distorted ratio, as evidenced in other shark species.
“The primary concern with the abundance of males is whether this is a source of reduced reproductive success,” said Rago. “We are still not seeing the recruitment we’d expect to see with a population of this size.”
The peer-review committee also noted that the reduced average size of mature females, which produce fewer and smaller pups, may contribute to lower-than-expected recruitment.
In another noteworthy finding, Rago said the overall biomass of dogfish, particularly males, has “shifted inshore,” a phenomenon he called an “enormous change.”
The average distance from shore of male dogfish in the fall of the year used to be 120 kilometers (km) and now it’s 40 km. The average distance from shore for females used to be 60 km and now it’s more like 40-45 km, he said.
Fishing mortality
SAW scientists also updated the overfishing threshold reference point as part of the latest assessment. The old threshold was 0.11 and the new threshold is 0.39. The 2005 estimate of fishing mortality (F) was 0.128.
Since the 0.128 is significantly below 0.39, scientists concluded that overfishing was not occurring. They also said the large increase in the threshold reference point “reflects a shift in size selectivity to larger individuals in the landings and discards.”
For many years now, ASMFC, the New England and Mid-Atlantic Fishery Management Councils, and the National Marine Fisheries Service have been trying to rebuild the dogfish spawning stock biomass. To do so, they’ve prohibited directed fishing on dogfish and tried to keep fishing mortality at 0.03, a level known as F rebuild.
Assessment scientists continued to express concern over recruitment, noting that “recruitment estimates from 1997 to 2003 represent the seven lowest values in the entire time series.”
As a result, the dogfish stock should continue to increase through 2008 and 2009 as fish grow into mature size ranges, but then the population is expected to decline again “due to the low number of recruits that were born during 1997-2003,” scientists said.
Discards
According to Rago, stock assessment scientists “took a much more thorough look at discards” and came up with new estimates of discard mortality for different gear types.
Here are the old discard mortality estimates, which reflect the percentage of dogfish assumed to die when discarded, compared to the new ones:
• Gillnets – old estimate, 75%, new estimate 30%;
• Trawls – old estimate 50%, new estimate still 50%;
• Commercial hook and line – old estimate 25%, new estimate 10%; and
• Recreational fishing – old estimate 100%, new estimate 20%.
Peer review
Three outside scientists affiliated with the Center for Independent Experts at the University of Miami peer reviewed the new stock assessment. Robin Cook of the United Kingdom chaired the panel.
The review was conducted in early June during the 43rd meeting of the Stock Assessment Review Committee (SARC), which presented its findings through the SAW.
Cook expressed numerous concerns about the assessment in a summary of the panel’s review. These included the following statements:
• “The main concerns for this stock are the reduction in female abundance, imbalance in the sex ratio, and the low estimates of recent recruitment. Discards contribute significantly to the fishing-induced mortality and need to be monitored.”
• “The current biomass and fishing mortality estimates are strongly influenced by a very large survey estimate in 2006. This is unlikely to be reliable and probably gives an overoptimistic view of the current state of the stock and an overoptimistic view of future stock development under status quo fishing mortality.”
• “A consistent decline in the average size of females in all surveys supports an impact of the fishery on the females.”
• “The average number of pups and average size of pups has been decreasing over time with the decreasing average size of females.” And
• “It is not clear if the reference points are appropriate to provide scientifically credible advice. … Evaluating the stock status with respect to the reference points is problematic.”
After being presented with results from the latest stock assessment, ASMFC’s dogfish board considered a request from the commonwealth of Massachusetts to immediately reinstate a small-scale directed fishing during 2006.
Janice M. Plante
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