The biology of northern shrimp (Pandalus borealis) on the Flemish Cap

Abstract

This study reports on various biological features in the life history of northern shrimp (Pandalus borealis) on Flemish Cap. Northern shrimp on Flemish Cap are males for approximately their first three years. Then usually half change sex at age four years, and the rest at age five years, to become females from then on. Small shrimp are generally found at lesser depth than the larger ones so the size of shrimp increases with depth, especially that of males. Larger females seem to move to shallower waters in March when hatching starts. The size at sex change (L50) and the maximum length (Lmax) are studied within and between seasons. Both L50 and Lmax have decreased in recent years. L50 was 22.5 mm CL on average in the seasons 1996/1997-2003/2004, but has decreased from 23.2 mm CL in 1997/1998 to 22.4 mm CL in 2003/2004 (Icelandic commercial data) and from 24.3 mm CL in 1994 to 20.9 mm CL in 2003 (EU survey data). Also Lmax has decreased from 29.8 mm CL in 1996/1997 to 28.0 nim CL in 2003/2004 (Icelandic commercial data). In spite of this, there is an invariant relationship between L50 and Lmax, where L50 is about 80% of the average Lmax of northern shrimp on Flemish cap. This supports the theory of Charnov and Skúladóttir (2000) on the invariant relationship between L50 and Lmax in sex changing organisms. The same is found in Icelandic offshore waters and the Denmark Strait. The decline in L50 could be related to the observed increase in temperature at 150 m in the last decade from 2.6°C in the years 1992-1996 to 3.5°C in 1997-2003. The sudden decline in the female biomass of shrimp on Flemish Cap between 1992 and 1994 appears to coincide with a drop in the age at sex change (A50) by one year in 1995 and 1996. Moreover, the increase in biomass of shrimp follows the disappearance of cod in the mid-1990s. As female shrimp biomass increased there was a delay in changing sex, showing the versatility of shrimp on Flemish Cap in adjusting to changes in sex-ratio. Growth of shrimp was fast in the earlier years and slower in later years indicating that food may be a limiting factor. The growth during the earlier years is faster than that of northern shrimp in the Barents Sea. Compared to other areas in the 1980s, the growth is slightly faster than that of the warm inshore waters of Iceland, but much slower than the growth of shrimp in the inshore waters of Sweden. The ovigerous period is studied for the first time on Flemish Cap and is estimated to be about eight months.