Research Document - 2000/100

Offshore spawning and year-class strength of northern cod (2J3KL) during the fishing moratorium, 1994-1996.

By J.T. Anderson and G.A. Rose

Abstract

An offshore acoustic survey was conducted during June 1994-1996 in the vicinity of Hawke Channel, traditionally reported as the most important offshore spawning area of northern cod (2J3KL). Estimates of spawner abundance and maturities at age combined with published estimates of fecundity at age were used to calculate the potential number of eggs spawned each year offshore. A pelagic juvenile fish survey was conducted during August-September each year, covering the shelf region offshore from Hamilton Bank to the southern Grand Banks (Divisions 2J3KLNO), including all inshore areas along the northeast coast of Newfoundland. Absolute estimates of abundance for pelagic juvenile cod, typically 60-80 days old, were used to calculate natural mortality between the egg and pelagic juvenile stages. The abundance of cod surveyed offshore in June ranged between 16 - 21 million fish among years. There was a sharp decline in the abundance of spawning cod from 1994 - 1996 contrasted with an increase the abundance of juveniles. Spawning biomass decreased from 17,500 t in 1994 to 8,200 t in 1995 to 5,500 t in 1996. The decline in mature cod resulted from the decline and disappearance of cod from the 1987-89 year-classes. This translated into a decline in the number of eggs spawned from 3.7 x 10¹² eggs in 1994 to 2.7 x 10¹¹ eggs in 1996. These declines were mirrored in the distribution and abundance of pelagic juvenile cod. In 1994, juvenile cod were distributed widely offshore and throughout the inshore area. By 1995, few juveniles were observed offshore but they were distributed abundantly throughout the inshore area. In 1996, no juveniles were sampled offshore and only small numbers of juveniles were encountered inshore. Abundance of pelagic juvenile cod declined from 3.1 x 10⁹ fish in 1994, to 1.5 x 10⁹ fish in 1995 to 1.8 x 10⁸ fish in 1996. Natural mortality was estimated to increase from -0.068 d^-1 in 1994 to -0.084 d^-1 in 1995 to -0.112 d^-1 in 1996 for pelagic cod sampled offshore. The decline in spawning fish offshore occurred two years after the implementation of a fishing moratorium for this stock and, therefore, cannot be linked to fishing. This decline in spawning fish combined with the increased rate of natural mortality between the egg and pelagic juvenile stages among years is consistent with depensation mechanisms for a fish population.

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