A B S T R A C TMarine mammal depredation can have significant negative effects on the performance of fixed fishing gears. Gray seals (Halichoerus grypus grypus) have been observed feeding on fish weir catches in Nantucket Sound, and remnants of longfin inshore squid (Doryteuthis pealeii ) and finfish in the nets are attributable to seal depredation. A dual-frequency identification sonar (DIDSON) deployed in a weir to monitor squid and fish occurrence captured observations of seal presence and behavior. Continuous recordings of 24+ h were subsampled at multiple intervals to optimize the likelihood of seal detection while minimizing processing time. Targets detected and identified to species included squid, bluefish (Pomatomus saltatrix), and gray seals. Seal occurrence in the weir was observed primarily at night. The presence and behavior of seals may negatively affect the efficiency of the weir due to disruption of the passage of schooling squid and finfish into the catch chamber. Behavioral observations collected using DIDSON can aid in the design of fishing gear modifications to reduce seal depredation.

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