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Commercial Fisheries News
Volume 37 Number 10
June 2010
Scientists warn: Don’t directly compare Bigelow, Albatross IV survey catches
WOODS HOLE, MA – No doubt about it, the Northeast region’s new survey vessel, the Henry B. Bigelow, catches more fish than her predecessor, the Albatross IV.
But scientists are going to great lengths to caution fishermen that catches between the two vessels are not directly comparable. They say practically everything about the whole trawl survey process – from the gear to the towing speed – is now different.
Russell Brown, chief of the Northeast Fisheries Science Center’s Ecosystems Surveys Branch, outlined these changes in a message that accompanied a report containing results from last spring’s bottom trawl survey, which was conducted Feb. 27-May 9, 2009 and ranged from Cape Hatteras to the Gulf of Maine.
At first blush, the report’s charts seem to indicate the presence of large quantities of fish for several species – an observation that industry members were quick to pick up on.
But Brown urged fishermen against jumping to conclusions about what the results might mean for stock status.
“Many significant changes in survey methodology were implemented … that have significant implications for the use of these data,” he said.
What’s different
According to Brown, changes in survey gear and protocols include the following:
Autotrawl system – The Bigelow is equipped with an autotrawl system that balances warp tensions throughout the duration of survey tows, a feature that was not part of the Albatross IV surveys.
Bottom trawl system – The Albatross used a Yankee 36 bottom trawl and 450-kilogram (kg) Euronet polyvalent trawl doors, while the Bigelow uses a 400x12 four-seam bottom trawl “designed and extensively tested with the fishing industry, fishery management, and academic stakeholders in conjunction with science center scientists through the Mid-Atlantic and New England Trawl Survey Advisory Panel.” The new bottom trawl is fished with 550-kg, 2.2-meter Polyice oval trawl doors.
Towing speed – Survey towing speed has been decreased from 3.8 knots to 3 knots.
Tow duration – Previous surveys ran 30-minute tows, timed from when the winches were locked until they were re-engaged. On the Bigelow, tows involve 20 minutes of actual bottom time as determined by net monitoring systems.
Tow distance – Changes in towing speed and tow duration have resulted in a decrease in average tow distance from 1.9 nautical miles (nm) to 1 nm.
Brown said, “The shorter tow distance allows us to conduct additional tows in areas that are constrained by fixed fishing gear, untrawlable bottom, and steep contours along the edge of the continental shelf.”
He also added, “While some commercial fishery stakeholders are likely to express concern about the reduction in tow duration, a preliminary analysis of the length frequency data from paired Albatross IV and Bigelow tows shows few differences in the largest sized fish of each species caught by the vessels.”
Sampling stations – The number of days for the survey has been increased from 48 to 60. Old sampling stations have been reallocated and station density has increased significantly for offshore strata “that have historically demonstrated higher densities of fish, particularly in the Mid-Atlantic and Southern New England regions.”
Because the Bigelow cannot sample inshore areas effectively, inshore stations were reduced, but inshore areas in the Mid-Atlantic will continue to be sampled through the Northeast Area Monitoring and Assessment Program – or NEAMAP – bottom trawl survey, which is a cooperative effort with industry.
“Catchability”
Brown said the science center conducted an extensive comparison of the “catchability” of the Albatross IV’s Yankee 36 net under the old protocols vs. the Bigelow’s new net with revised protocols.
He called the resulting database “one of the most comprehensive ever produced to study the catchability characteristics of a fisheries bottom trawl survey.”
As fishermen predicted, preliminary results from this work showed that the Bigelow had “significantly higher catch rates for nearly all species except those with very small total body size,” such as anchovy-like fish.
The results of the study were peer reviewed in August of 2009 and its analytical approaches will now be used to compare pre-2009 surveys from the Albatross IV and, occasionally, the Delaware II, with results from new Bigelow surveys.
Brown concluded his message with one final warning against directly comparing old Albatross survey catches with those from the Bigelow.
“Given the changes in vessel, trawling gear, tow speed, tow duration, sampling allocation, and towing procedures, straight-forward comparisons of catches in this report with spring bottom trawl survey catches in previous Resource Survey Reports are not appropriate without employing statistical approaches that are reviewed and endorsed for stock assessment applications through peer review processes,” he said.
For more information about stock assessment surveys, visit the science center’s web site at <www.nefsc.noaa.gov/femad/ecosurvey/mainpage> and look under “Resource Survey Reports.” /cfn/
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