Rockhopper Catch Efficiency Study
Two similar nets with different types of groundgear were towed at the same time to estimate just how efficiently the survey net captures flatfish.
Scientists and fishermen want to understand the "efficiency" of our bottom trawl survey gear: what fraction of the fish in the path of the net do we catch? This work looked at how flatfish catch compared when our survey net was rigged with two different sweeps—one intended to maximize bottom contact, and one intended to roll over the ocean bottom.
- What is the capture efficiency of the Northeast Fisheries Science Center (NEFSC) standard rockhopper sweep?
- Is capture efficiency different during the day and night?
- Does capture efficiency vary with species or size?
The project goal is to estimate the relative catch efficiency of the NEFSC standard rockhopper sweep for several flatfish species.
These gear efficiency studies used a commercial vessel to tow two trawls, side-by-side, in a twin-trawl rig. Two NEFSC survey trawl nets were rigged with different sweeps—the part of the gear that contacts the bottom while fishing, also known as groundgear. One net was rigged with the NEFSC standard rockhopper sweep, and the other with a chain sweep. Rockhoppers are used to allow the net to roll over the bottom and do not get snagged as often. Chain sweeps are designed to maximize bottom contact and capture of bottom-dwelling species. Flotation and headrope slack were removed from the trawl rigged with the chain sweep to maximize the gear’s efficiency in capturing bottom-dwelling species.
In 2015, a 12-day trip was conducted on Eastern Georges Bank and off Southern New England, targeting yellowtail flounder. In 2016 a 12-day trip was conducted on the Northern Edge of George Bank and on Stellwagen Bank, targeting witch flounder and American plaice. In 2017, a 10-day trip was conducted targeting summer flounder off Southern New England from Montauk, Long Island to Nantucket; and red hake in the western Gulf of Maine off Cape Ann.
The field experiments were performed on Salt Pond Fisheries twin trawl rig F/V Karen Elizabeth with two nets towed side-by-side simultaneously. The net on one side of the twin trawl was a standard NEFSC bottom trawl fitted with the rockhopper sweep. The net on the other side of the twin rig was fitted with a sweep specifically designed by industry to maximize flatfish capture. Restrictor cables were used to ensure consistent trawl geometry, and a trawl monitoring system was employed to monitor wingspread using spread sensors sewn into the outer portion of the wings. The study targeted areas where flatfish abundance was likely to be high. In 2017, the target species were expanded to include Red Hake. Tows were made in a range of sediment types that included soft mud, sand, coarse gravel and shell hash.
Some very small fish, particularly yellowtail flounder, windowpane flounder, and red hake, were caught much more often in the chain sweep than in the rockhopper. During the day, the efficiency of the rockhopper sweep increased with the size of the fish for several species. For some tows at night, the rockhopper sweep was about 50 percent efficient and did not vary with the size of the animals. This finding may be linked to animals coming up off the bottom at night to feed. There was a decline in rockhopper efficiency for some fish larger than 20 inches.
Methods were developed to estimate relative catch efficiency, and its uncertainty, for rockhopper and chainsweep gears for the NEFSC bottom trawl survey and to generate calibrated swept-area numbers at length and biomass estimates (see Miller 2013).
We conducted an independent peer review of the field studies to evaluate whether they were carried out with acceptable scientific methods and standards, and whether enough data were collected to reliably estimate relative efficiency by species. The review panel concluded that the sampling design and the resulting number of stations with positive catches in the twin trawl studies can be used to estimate maximum efficiency of the NEFSC bottom trawl survey for most of the flatfishes.
Using Experimental Results
This experimental work translated into data and information that could be used to determine how many flatfish and red hake in the path of the standard survey net are not captured, and how that should be accounted for in stock assessments. The results from these studies have been used to update NEFSC survey catchability estimates for some stocks starting in 2019. The specific application of the research findings is different in each assessment, varying by species, fish sizes, and areas sampled.
Understanding stock assessment model accuracy
In 2017, assessments for 19 Northeast groundfish stocks were updated. Results of the Sweep Efficiency Research can be used as a diagnostic tool to check the accuracy of the bottom trawl survey biomass estimates for flatfish species.
Setting catch advice when models are not enough
Sometimes modeling does not provide a clear enough picture of stock status to develop catch advice so alternative methods must be agreed. The results of the Sweep Efficiency Research have been used in several of these situations when the only measure available is an index of Northeast Fisheries Science Center bottom trawl survey catch per trawl haul over time.
Direct input to an assessment model
For some flatfish species the results from the sweep efficiency research can be used within the assessment to convert data from the survey into a population estimate.
- Northeast Trawl Advisory Panel
The panel includes New England and Mid-Atlantic Fishery Management Council members, commercial trawl fishermen, academic and industry gear experts, leaders of other regional surveys, state scientists, and the Center staff assigned to work with this important panel.
- Salt Pond Fisheries - Christopher Roebuck (owner)
- Terry Alexander, NEFMC member
- Vincent Balzano, NEFMC member
- Tony DiLernia, MAFMC member
- Wendy Gabriel, NEFSC scientist
- James Gartland, MAFMC scientist
- William Gerencer, NEFMC stakeholder
- Vito Giacalone, NEFMC stakeholder
- David Goethel, NEFMC stakeholder
- Dustin Gregg, MAFMC scientist
- Pingguo He, NEFMC scientist
- Michael Luisi, MAFMC member
- Anna Mercer, NEFSC scientist
- Timothy Miller, NEFSC scientist
- Frank Mirarchi, NEFMC stakeholder
- Christopher Parkins, ASMFC representative
- Michael Pol, NEFMC scientist
- Philip Politis, NEFSC scientist
- Christopher Roebuck, MAFMC stakeholder
- Robert Ruhle, ASMFC representative
- Michael Sissenwine, NEFMC scientist
- Northeast Trawl Advisory Panel
- Captain Chris Roebuck and crew
- Field Staff
Miller TJ, Martin M, Politis P, Legault CM, Blaylock J. 2017. Some statistical approaches to combine paired observations of chain sweep and rockhopper gear and catches from NEFSC and DFO trawl surveys in estimating Georges Bank yellowtail flounder biomass. TRAC Working Paper 2017/XX. 36 p.
Miller TJ, Richardson DE, Politis P, Blaylock J. 2017. NEFSC bottom trawl catch efficiency and biomass estimates for 2009-2017 for 8 flatfish stocks included in the 2017 Northeast Groundfish Operational Assessments. 2017 Groundfish Operational Assessment working paper. Northeast Fisheries Science Center.
Operational Assessment of 14 Northeast Groundfish Stocks, Updated through 2018
Miller TJ, Richardson DE, Politis P, Roebuck CD, Manderson JP, Martin MH, Jones AW. 2023. Estimation of survey efficiency and biomass for commercially important species from industry-based paired gear experiments. Fisheries Research. Vol. 259.
- Contact Phil Politis