Sunday, February 12, 2017
WHAT FISHERMEN DON'T (WANT TO) UNDERSTAND ABOUT SURVEYS
The National Marine Fisheries Service’s decision to reducethis year’s annual catch limit for summer flounder by 30% was met with howls of dismay from elements of both the commercial and recreational fisheries.
Although the decision was justified by six consecutive yearsof below-average recruitment, which has caused the spawning stock biomass todecline to just 58% of the target level, many fishermen’s knee-jerk reaction has been to attack the data underlying NMFS’ decision, rather than in trying to understand how that data was obtained and used.
Some of the most frequently-attacked data has been that developed by NMFS’ annual spring and fall trawl surveys, which survey the abundance of a wide array of species found off New England and the Mid-Atlantic coast.
A letter that the New York Sportfishing Federation sent to the Atlantic States Marine Fisheries Commission is typical. In it, the Federation asks that no reduction occur until a new stock assessment is completed. As part of its argument in favor of status quo regulations, it says
“We strongly believe that the data collected on the trawl surveys that were done by NMFS on their vessel “Bigelow” are grossly inaccurate. The trawl survey vessel is improperly equipped and their gear type and methods for harvesting summer flounder are inadequate, thus rendering this data useless for management purposes. [emphasis added]”
The Federation may well be right in saying that the NMFS vessel Bigelow’s gear is “inadequate” for the task of harvesting summer flounder. However, the Bigelow’s gear is not intended to harvest summer flounder, but to survey the health of the stock, and that makes a big difference when folks talk about the adequacy of the gear used.
While the Federation is a recreational fishing organization, commercial fishermen may have an even harder time in understanding the difference between trying to harvest and trying to survey. A recent and frequently-quoted piece published by the Center for Sustainable Fisheries criticized the NMFS trawl survey with respect to another flatfish, witch flounder, saying
“Fishermen have long maintained that there is a huge disconnect between what they see on the water and the conclusions derived from the NOAA surveys and stock assessments. Their claims have been dismissed as self-serving. Now it seems the fishermen have a strong case. On a recent bottom trawl survey, a typical industry net caught four times as many flatfish as the rig used on the government trawl survey...
“[A] boat was rigged to tow two nets simultaneously, each of the type used on the Bigelow, with one significant difference. One net was fitted with a rockhopper and the other had a chain sweep. Because different fish species behave differently, fishermen use a chain sweep, attached to the leading edge of the net and in contact with the sea floor, when targeting flatfish such as witch flounder. These fish hide in the sediment on soft bottom to evade predators. You will hear fishermen refer to fishing boats with rockhoppers as ‘hard bottom boats’ because those boats typically go after other species such as cod or haddock which tend to be found over rocky or gravel bottom.
“…[The Bigelow’s] exclusive use of a rockhopper has been a point of contention with the fishermen since the Bigelow commenced operations in these waters. Fishermen openly questioned its accuracy in estimating flatfish abundance. The survey work [comparing the two net types] has provided the evidence that their skepticism is well founded.”
That was the fishermen’s view. In reality, the comparison between the two nets provided no such evidence at all.
However, the comments of both the New York Sportfishing Federation and the Center for Sustainable Fisheries does prove that the folks representing those organizations just don't understand the survey process.
When a fisherman pulls a trawl, such trawl is configured to maximize the number of the target fish species caught, minimize time on the water and so reduce expenses, and generally allow the fisherman to operate as efficiently as possible. But when a scientist pulls a trawl, such trawl is configured to catch a representative sample of all the fish, in a consistent and repeatable manner, at each predetermined area where sampling takes place.
The goal of the NMFS survey is not to catch as many of each species as possible; in fact, catching too many fish can slow down and ultimately degrade the sampling process. NMFS spells that out quite clearly, saying
“we want consistently comparable catches, from which we derive a whole series of measurements and samples collected from fish and invertebrates captured on each tow.”
NMFS notes that when too many fish are caught by the trawl
“The time required to sort and process the catch increases, which adds to the time required to conduct the work—either increasing the cost or decreasing the amount of geographical area we can cover. Also, large catches must be subsampled to estimate what has actually been caught during each tow. The combined effects potentially increase error associated with both sampling (reducing the number of stations) and subsampling.”
And that’s what fishermen are seemingly unable to understand. The fact that a net may be configured to catch the most fish doesn’t mean that such net would be the best one to use in a survey. Such a net might very well catch so many fish that the survey quality would decline.
The purpose of the survey is not to catch as many fish as possible, but to determine relative abundance. As NMFS states in the operating protocols for the Northeast Multispecies Bottom Trawl Survey,
“Abundance estimates obtained from this survey are relative abundance indices rather than absolute abundance indices because catch efficiency of the sampling gear is less than 100%. Relative abundance indices are comparable through time because survey catchability is held constant through standardization of gear, vessel, and methodology.”
Thus, even if the NMFS survey gear catches, at best, only 25% of the flatfish that are present, as the Center for Sustainable Fisheries’ comments suggest may be true, so long as it consistently catches 25% of the fish available, the gear used by the Bigelow is proper for its purpose, and not inadequate at all.
The only thing that remains inadequate is many fishermen’s knowledge of how surveys are done, and what information the trawl data is meant to convey.
Such fishermen require a new, improved understanding far more than NMFS requires a new stock assessment of summer flounder.