Research Document - 2008/081

Results of the Atlantic mackerel (Scomber scombrus L.) egg survey conducted in the southern Gulf of St. Lawrence in 2007

By F. Grégoire, C. Lévesque, J-L. Beaulieu and
M-H. Gendron

Abstract

An egg survey for assessing the spawning biomass of Atlantic mackerel (Scomber scombrus L.) was conducted in the southern Gulf of St. Lawrence between 21 and 29 June 2007. Some very low egg abundance, less than 10 eggs/m², was recorded at nearly 50% of the sampled stations. The highest abundance, of more than 400 eggs/m², was recorded at the stations located to the northwest of the Magdalen Islands and in Chaleur Bay. These stations were associated to water temperatures (0-10 m layer) varying between 11.2 °C and 12.2 °C. The mean egg production for the entire sampled area was calculated at 49.7 eggs/m², and total production at 3.45 x 10¹² eggs. A spawning biomass of 76,532 t was associated with this egg production, which represented the lowest recorded value since 1979. An important reduction in the estimated mackerel spawning stock biomass has been observed in the southern Gulf of St. Lawrence since mid-1990s. Commercial landings alone cannot explain this reduction of biomass that occurred at the same time as the waters of the Gulf of St. Lawrence, i.e. the cold intermediate layer or CIL, were cooling. These changes in water temperature may have affected the timing of migration and spawning. They may be responsible for the decrease of the landings in Nova Scotia and southern Gulf and their marked increase on the east and west coasts of Newfoundland. The present coverage of the egg survey in the southern Gulf may no longer represent the whole spawning area and period in eastern Canada. Information from other areas suggests the spawning area may be much larger. To determine the full extent of the mackerel spawning area, the egg survey should be expanded outside the southern Gulf of St. Lawrence. Such a broad scale egg survey, that would include US waters, has never been done before in the Northwest Atlantic.

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