DFO MS Thesis Defense presented by Sean Boisvert
Department of Fisheries Oceanography
"A Numerical Investigation of Size-Selectivity in a Modified Scallop Dredge"
By: Sean Boisvert
Advisor: Geoffrey Cowles (University of Massachusetts Dartmouth)
Committee Members:
Pingguo He (University of Massachusetts Dartmouth), and Douglas Zemeckis (Rutgers University)
Thursday May 8th, 2025
2pm
SMAST East 101-103
836 S. Rodney French Blvd, New Bedford
and via Zoom
Abstract:
The US Atlantic sea scallop (Placopecten magellanicus) fishery, one of the most economically significant in the country, faces challenges related to unintentional capture of undersized scallops and other non-target species. Minimizing retention of bycatch and small scallops can result in economic gain and healthier populations. To address these issues, a modified scallop dredge was developed by Atlantic Cape Fisheries, LLC and features a modified cutting bar with a foreface angle that can be adjusted to a range of angles relative to the seabed to improve the size-selective sorting process. This study complements field research conducted using paired trawls of the modified dredge and standard turtle deflector dredge (TDD) by investigating the underlying hydrodynamic effects of different cutting bar angles and tow speeds on scallop escapement. A coupled computational fluid dynamics (CFD) and particle tracking model approach is used to analyze these effects. The unsteady viscous flowfield is computed with the FUN3D CFD flow solver using an unstructured body-fitted mesh to resolve the boundary layer on the dredge frame. The resulting time-dependent velocity field is used to drive simulations of the trajectories of scallops using a dynamical particle tracking scheme implemented in MATLAB. This model quantifies escape probabilities and other metrics such as shedding frequency, vertical particle velocities, average time to reach the twine top, average height achieved, and average particle trajectories. Simulations were conducted for multiple cutting bar angles, tow speeds, and scallop sizes to assess the modified dredge’s effectiveness compared to the standard TDD. We examined scallop trajectories across 26 size bins (30-160 mm shell height) for nine cutting bar angles (15°-75°) and the standard TDD at a nominal tow speed of 2.5 ms-1 (~5 knots), with additional experiments at 2 ms-1 (~4 knots) and 3 ms-1 (~6 knots) for three angles (30°, 45°, 60°). In total, 468 numerical experiments tracked 15 million particles, requiring approximately 50 hours of computational walltime. Results show that shedding frequency decreased as cutting bar angle increased. The 15° and 30° angle cutting bar angle yielded the highest escape probabilities, particularly for smaller scallops, exceeding the TDD by up to 40%. Higher angles (60°-75°) produced intermittent high vertical velocities but led to lower escape probabilities, attributed to less frequent eddy formation. Scallops released near the cutting bar (0.1 m above seabed, 0.4 m downstream) had the highest escape probabilities. Tow speed had a positive effect on escape probability, especially for the TDD configuration. This study bridges the gap between field trials and the fundamental understanding of scallop sorting mechanisms in the dredge’s wake. The findings of this research have important implications for the design and testing of modified fishing gear. The use of CFD modeling, as evidenced in this study, presents a valid, cost-effective alternative to traditional at-sea gear testing, allowing for extensive design exploration and optimization in a controlled and computationally efficient manner.
Join Zoom Meeting
https://umassd.zoom.us/j/94519380509
Note: Meeting passcode required, email contact below to receive
To request the Zoom passcode, or for any other questions, please email Callie Rumbut at c.rumbut@umassd.edu
SMAST East 101-103 classrooms and zoom
: 836 S. Rodney French Blvd., New Bedford MA 02744
Callie Rumbut
c.rumbut@umassd.edu
https://umassd.zoom.us/j/94519380509