Site Fidelity and Movement of Atlantic Cod (Gadus morhua) on Piled Boulder Reefs as Determined by Acoustic Telemetry

Principal Investigator:

James Lindholm, Ph.D., Pfleger Institute of Environmental Research

Co-Investigator:

Peter Auster, Ph.D., National Undersea Research Center at the University of Connecticut

Key Project Personnel:

Stephen Kibner, USCG Retired

Timeline:

Initiated in June 2002, on-going

Atlantic cod (Gadus morhua), a species with ecological, economic and cultural significance for New England, is a demersal, omnivorous fish found throughout the north Atlantic, from the surface to a depth of 600 meters. Cod populations in the Gulf of Maine have been exploited commercially for several hundred years, and continue to be heavily exploited today. The fact that cod swim in the ocean is well understood. Precisely how cod and other fish species move, relative to different features of the undersea landscape, is much less understood. However, the successful conservation and management of cod in the Gulf of Maine, and at the scale of the Stellwagen Bank National Marine Sanctuary (SBNMS) in particular, is highly dependent on this information.

This project is using acoustic telemetry technology to quantify cod movement over different features of the landscape to inform management of the SBNMS and the greater Gulf of Maine region.

The piled boulder reefs and isolated boulders that occur on the tops of many topographic rises within the SBNMS are glacial in origin. The boulder reefs are often linear features formed by submerged and eroded glacial eskers (i.e., linear mounds of gravel-boulders formed along the lines of advance of a glacier). Each reef was characterized by multiple piled boulders providing meters of relief above the seafloor, and multiple interstitial spaces between boulders. Representative, sessile invertebrate fauna included the stalked tunicate (Boltenia ovifera), massive sponges of the genus Polymastia, and the anemone (Bolocera tuidae). The water depth was 65 m at each site. The maximum distance between individual piled boulder reefs was 24 km (the distance from the northernmost reef to the southernmost reef), while the minimum distance between reefs was 4 km (between the two northernmost reefs). The boulder reefs were selected inside the Western Gulf of Maine Closed Area (WGoMCA), which overlaps the SBNMS boundary, and has excluded all commercial fishing gear targeting demersal fishes since May 1998.

Tagging in this region reduces the probability of fish being caught during the course of the study, though recreational fishing continues within the closed area.

Combined sun-illuminated topography and acoustic backscatter map of VR2 receiver sites at piled boulder reefs in the SBNMS.

Each receiver is shown with an estimated radius of detection of 400m. Cod are caught and tagged externally with V8SC coded-acoustic transmitters (each of which emits a unique ID code). They are then released on the seafloor using a custom-designed fish release device, within the overlap of the SBNMS and the Western Gulf of Maine Closed Area (WGoMCA).

Atlantic cod with an acoustic transmitter attached externally at the base of the first dorsal fin.The tagging gun used for insertion of the T-bar anchor is visible in the lower-center of the image.

Tagged cod about to be returned to the seafloor in a PVC fish release device.

Movements of tagged cod are recorded by an array of four VR2 acoustic receivers deployed on the seafloor. Data are collected at the scale of minutes for several months at a time. Preliminary tracking occurred in the gravel habitat of northeastern Stellwagen Bank in 2001 (Lindholm and Auster, 2003). From May 2002 through October 2002 and from September 2004 through March 2005, cod movement was investigated at four piled boulder reef sites. The same piled boulder reefs were used in both periods in order to quantify any influence of seasonality on cod movement behavior. To-date, a total of 120 cod have been acoustically-tagged ranging in size from 39 ñ 60 cm total length.

A large cod (60 cm) prior to release.

To-date, three broad categories of movement behavior were identified at each of the four piled boulder reefs, across years and across seasons. Multiple cod showed high site fidelity (present > 90% of the study) to the boulder reef where they were caught and released. A second set of cod were recorded moving among the four piled boulder reefs included in the study (including movements as far as 24 km). A third group of cod were recorded only briefly at the boulder reef where they were caught and released before leaving the study area. In each case, neither cod size nor season (Summer v. Winter) appear to be factors in the recorded movement behavior.

Time at-liberty for each of the 38 Atlantic cod released in 2002 at each of the four piled boulder reefs.

The high site fidelity of many cod to individual piled boulder reefs suggests that habitat-specific management measures, such as marine reserves, may offer significant protection to cod within the Sanctuary.

Time at-liberty for each of the 37 Atlantic cod released in 2004 at each of the four piled boulder reefs. The purple circles indicate cod tagged at Reef 3, where the receiver has not yet been recovered.

PIER gratefully acknowledges the support of NOAA’s Stellwagen Bank National Marine Sanctuary