Cod tagging project being done at the University of New Hampshire

The goal of the project is to study the activity and fine scale distribution of spawning cod in the western Gulf of Maine (Ipswich Bay) using a combination of data storage tags (DSTs) and acoustic telemetry.

Between April 21st and May 17th, 2006, two hundred cod were captured on the spawning grounds and externally tagged with Star Oddi milli DSTs.  Thirty of the same fish also had an acoustic transmitter surgically implanted.  The DSTs were programmed to record both water temperature and depth of each fish at 12-minute intervals for approximately six months, and each was imprinted with information on how to report the recapture of a DST-tagged fish. Fishermen have returned twenty-one DSTs so far (10.5% recapture rate). The time at liberty for recaptured tags ranges from 20 days (2,361 measurements for both depth and temperature/tag) to 169 days (20,358 measurements of depth and temperature/tag).  Recapture locations have ranged from Platts Bank (90 km to the northeast of the tagging location) to Stellwagen Bank (80km to the southeast).  Data analyses are just beginning, but preliminary results suggests that spawning cod in Ipswich Bay occupy relatively uniform and consistent depths in May and early June, sometimes punctuated by brief ascents to much shallower depths, which may reflect spawning events.  There is typically a dramatic shift to deeper depths in mid-June, showing that individuals have moved out of the study area.  This depth change coincides with the initiation of a wide range of vertical movement in the water column, indicating a change in behavior as well as location.

The cod tagged with an acoustic transmitter were tracked daily, from May 6th through June 30th, with both stationary and mobile hydrophones, to determine their movement during their residence in Ipswich Bay. Tracking data are being used to create a positional history of each fish during the spawning period.  All relocated fish were found to aggregate around a small number of elevated, hard bottom features of Ipswich Bay, and frequently moved back and forth between these landmarks during the tracking period. 

A gradual eastward movement was observed for several cod in June until they appeared to leave the area entirely.  For some fish, acoustic tracking showed abrupt and rapid eastward movement out of Ipswich Bay, indicating the exact date and time that a fish left the spawning area and moved offshore, and the approximate swimming speed and heading.  By the last week of June, only six acoustic transmitters could be relocated.  The disappearance of tracked fish in mid-June was concurrent with the shift in vertical activity and depth seen in most recaptured DSTs, and supports the idea that most tagged fish moved out of the spawning area by mid-June.  Out of the 21 recaptured DSTs, 4 DSTs came from cod that contained acoustic transmitters (13% recapture rate for the 30 acoustically tracked fish).  The positional tracking data of these individual cod will be combined with the vertical profiles of their DSTs to create a three-dimensional history of their behavior and location during the spawning period.