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Virginia Institute of Marine Science

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The VIMS Juvenile Fish Trawl Survey, which has been in operation since 1955, has undergone considerable changes to the sampling gear, location of sampling sites, and the methodology used to select sampling sites. Recently, a new vessel, the R/V Tidewater, replaced the R/V Fish Hawk, which had been in service for 25 years. In addition to the change in vessel, a new net was used; this net design is more robust to deployment methods and performs more consistently under varying environmental conditions. Therefore, a calibration study was conducted whereby the two research vessels with different nets fished in the same area at the same time. This calibration study provides an estimate of the species-specific factors necessary to ‘convert’ the R/V Tidewater catches to those of the R/V Fish Hawk, taking into account the combination of vessel and net. All other protocols (tow duration, scope, vessel speed, and sample processing) remained unchanged. Comparison sampling with the R/V Tidewater and the R/V Fish Hawk began in April 2014 and concluded in May 2015; additional paired tows were completed in August 2016 to provide sufficient samples for Scup, Black Sea Bass, and adult Summer Flounder. We completed a total of 1,141 paired tows during 97 days-at-sea, capturing a total of 327,526 fishes, crabs, and shrimp aboard the R/V Fish Hawk and 323,580 fishes, crabs, and shrimp aboard the R/V Tidewater. From these data, we developed calibration factors for 41 species groups (species-age or species-size combinations). Calibration factors were estimated from the best-fitting model from among four candidate models that accounted for variability in catches between the two vessels. In addition, we examined species composition of the catches from the paired tows using multivariate analysis and found that catches from the two vessels were similar in all months and strata except for shallow stations in Chesapeake Bay. Our ‘whole survey’ approach allowed us to estimate calibration factors for species in all available habitats that are routinely monitored by the VIMS Juvenile Fish Trawl Survey. Further, our consideration of depth, tidal currents, tow direction, water clarity, tow distance, and salinity in the calibration models ensures that the estimates are applicable across the range of estuarine characteristics that are inhabited by these species. The estimated calibration factors will be applied to catches of the R/V Tidewater at the individual-tow level; relative abundance indices will be estimated using the random-stratified survey design in effect since 1988, thus preserving the integrity of the long-term survey data for estimating relative abundance of juvenile fishes and blue crabs in Chesapeake Bay.


Final Report submitted to National Marine Fisheries Service



trawl survey, fish populations, fish stock assessment, fishery resources

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Creative Commons Attribution 3.0 License
This work is licensed under a Creative Commons Attribution 3.0 License.