Mountain, David
| PowerPoint slides | |
HTML slides |
Estimates of the number of cod and haddock eggs hatching each year were made during the U.S. GLOBEC (1995-1999) program and during the NMFS MARMAP (1977-1987) program. Comparing the egg hatching abundance each year with the estimated resulting recruitment provides an indication of the larval-juvenile survivorship each year for both species. Egg hatching - recruitment plots suggest that the survivorship for haddock was about three times greater during U.S. GLOBEC than during MARMAP, and the reverse was true for cod. There appears to have been a major shift in survivorship between the species in the two time periods. A major difference oceanographically between the two periods was the low salinity event during the U.S. GLOBEC years. A recent analysis of the zooplankton community structure on Georges Bank (J. Kane, NMFS) has shown two major clusters - one encompassing the MARMAP years and the other in the 1990's including the U.S. GLOBEC years. The two clusters correspond to 'average' and 'low' salinity periods. The 1990's cluster was characterized by more, smaller copepods, relative to the MARMAP period. The salinity, zooplankton community structure and the cod/haddock survivorship show major differences between the same time periods, suggesting a possible connection between the changes in water properties, zooplankton and gadid survival patterns.
The low salinity event has been shown by oxygen isotope analysis to have a high latitude origin. Comparison of satellite-derived estimates of the Labrador Current transport near Flemish Pass show variability similar to the salinity variability, with the current transport leading by one year. The salinity variability also shows similarity to model estimates of export of water from the Arctic, with a two year lag. The implication is that the salinity variability on Georges Bank during U.S. GLOBEC, which may have links to variability in the zooplankton and early life history of the fish stocks on the bank, may be related to changes in the Arctic. A theme session will be proposed for the Ocean Sciences Meeting in March 2008 on the influence of changes in the Arctic on sub-Arctic and mid-latitude ecosystems.