5 Dynamics of fish larvae, zooplankton, and hydrographical characteristics in the West Greenland large marine ecosystem 1950–1984

Abstract

Abstract A relatively long series of plankton and hydrographic samples for the West Greenland ecosystem allows us to explore links between climate, physical oceanography, and abundance of major zooplankton and ichthyoplankton species. These linkages, manifest in food chains, fish stocks dynamics, and regime shifts in large marine ecosystems are important for fisheries management. Patterns of change in larval fish abundances and hydrographic characteristics were analyzed with data from three transects off southwest Greenland sampled between 1925 and 1984. Factor analysis and orthogonal varimax rotation on 8 physical oceanographic variables highlighted gradients in salinity, temperature, and temperature range (stratification). The period in the 1950s tended to be warmer than the 1970s, and a period of low salinity was present in the late 1960s and early 1970s. Non-parametric density estimation methods identified associations between the hydrographic characteristics and the distributions of fish and shrimp larvae: Atlantic cod ( Gadus morhua ), Redfish ( Sebastes marinus and S. mentella ), Greenland halibut ( Reinhardtius hippoglossoides ), Long rough dab ( Hippoglossoides platessoides ), Wolffish ( Anarhichas sp.), Sandeels ( Ammodytes sp.) and northern shrimp ( Pandalus borealis ). The fish larvae were generally distributed non-randomly to at least one of the hydrographic characteristics and to the indices of plankton and copepod abundance. The fish and shrimp larvae showed generally stronger associations with the indices of plankton and copepod abundance than with the hydrographic characteristics. The patterns suggest a suite of fish species whose larvae are weakly adapted to different oceanographic conditions and different ecological niches. For all but sandeels, June and July abundances are likely to be higher when water masses are relatively saline and well stratified, and when plankton, especially copepods, are abundant. These relationships were then carried further to determine synchronous temporal patterns among oceanographic characteristics, abundance indices of zooplankton, fish larvae, and year-class strengths. There was a decreasing trend in the plankton abundance indices from the late 1950s and early 1960s to the 1970s as well as a weak significant positive association to sea temperature. This trend culminated in the year 1969 and 1970 the period of the “Great Salinity Anomaly”. The variability in time series of zooplankton, fish and shrimp larvae could not be related to an impact of the GSA. There were weak significant associations between Atlantic cod year-class strength (given number at age 3) and both temperature and cod larvae abundance. The patterns explain reasonably well the decline in cod, redfish and long rough dab as consequences of changes in the relative influences of the East Greenland Polar Current and the Irminger Current, but do not explain the major increases in shrimp and Greenland halibut in the 1980s and 1990s. At least the former may be a consequence of reduced predation mortality. The associations are also generally consistent with a survey series of adult groundfish started in the 1980s in East and West Greenland waters.