Cod (Gadus morhuaL.) migration speeds and transport relative to currents on the north-east Newfoundland Shelf

Abstract

Cod were tracked during shoreward migrations across the north-east Newfoundland shelf for 13, 9, and 36 d in the spring of 1990, 1991, and 1992, respectively, with echo-sounder-equipped vessels. Mean daily ground speeds ranged from 0.08-0.28 m s-1in the three years (max. 0.68 m s-1) over distances up to 59 km (mean daily displacements from 1990-1993 were 10, 24, and 7 km). Mean migration headings were identical in the three years (198°). Cross-track error was smaller and fish were older in 1990 than in 1991 or 1992. Movements along the migration route (198°) coincided with fish movements off the bottom and increasing sea temperatures (325 m station, rises in 2°C isotherm). In 1992, swimming speeds assessed by subtracting currents from ground speeds average 0.10 m s-1(0.23 bl) (maximum 0.28 m s-1; 0.64 bl s-1). After spawning abated, cod exhibited directed movements shoreward. By comparing ground and swimming speeds, we examined hypotheses on passive and active transport. At times, cod actively migrated with advancing flows (ground speeds>swimming speeds), and tended to stem retreating flows (but not always). In contrast, the strong flows across the route (90° to the mean direction) had little effect on cod movements.