Fine-scale temporal and spatial distributions of Atlantic cod (Gadus morhua) on a western Gulf of Maine spawning ground

We utilized a combination of acoustic telemetry and archival data storage tags (DSTs) to study the fine-scale temporal and spatial distributions of spawning Atlantic cod in Ipswich Bay, located in the western Gulf of Maine. Pre-spawning cod were tagged (200 DSTs, 26 acoustic tags) as they neared a known spawning ground. Fish equipped with acoustic tags were relocated by a receiver array and a boat-mounted directional receiver over the following eight weeks. Group activity was analyzed by minimum convex polygon (MCP), and individual kernel distribution estimates were created for 13 fish. Individual cod utilized spawning areas <60 km2 in size (mean 41 km2) with a high degree of spatial overlap. Activity was distributed along a roughly V-shaped “corridor” bounded by elevated bottom features. Cod aggregated around specific humps and ridges, and there were two separate weeks during the two-month study when the majority of cod converged on a single aggregation “hot-spot.”The last detection date for each cod was used as a proxy for departure date. Seven cod were tracked making rapid departures across the spawning ground in <24 h. Residence on the spawning ground ranged from 8 to 53 days (mean 30 days). Residence time was confirmed by DST data. Of the 26 recaptured cod whose DSTs were analyzed, 14 cod were observed to make clear deep-water (>100 m) shifts occurring 23–50 days after tagging. Data from three cod equipped with both an acoustic transmitter and a DST showed concurrence between last detections and deep-water shifts.Our findings demonstrate that coastal cod in the western Gulf of Maine aggregate around fine-scale bathymetric features on the spawning ground, utilize relatively small areas during spawning, are highly mobile within those areas, and tend to move as a group.

► We studied the fine-scale distribution of spawning Atlantic cod.
► Individual utilized spawning areas <60 km2, and spawning areas overlapped extensively.
► Spawning occurred near elevated bathymetric features.
► During the study, the majority of cod converged on a single aggregation ‘hot-spot.’
► Residence on the spawning ground ranged from 8 to 53 days (mean 30 days).