Session 3: The Ecosystem in Space

Session Chair: Antonio Vázquez

This session consisted of 11 papers, including an invited paper.  The session was focused on the isolation of the fish populations on Flemish Cap or their linkage with the stocks on neighbouring areas, as well the spatial distribution patterns of species. 

It is well known that Flemish Pass is not a barrier for distribution of deep-sea species, such as Greenland halibut and grenadiers, and their population on Flemish Cap were long time ago recognized as belonging to wider distributed stocks. The situation is quite different for the shallowest species, such as cod, American plaice, redfish, and shrimp among commercial species.

Three possible mechanisms to link populations inhabiting the shallowest areas were considered in the invited paper: migration of adult individuals to outside the Flemish Cap, exchange of individuals with neighbouring areas, and larval drift from surrounding areas. For cod, migration of adults to outside the Cap has been proved by tagging experiments, however immigration was never observed. However, a paper presented during this session on mitochondrial DNA analyses concluded that the cod stock on Flemish Cap appears to be a separate stock.

Larval drift from surrounding areas to Flemish Cap was predicted based on oceanographic variables. Before the eastern branch of the Labrador Current moves to Flemish Cap it crosses areas of the Labrador Shelf and Northern Grand Bank where species also inhabiting the Cap are known to spawn. Flemish Cap would be connected in this way more likely with those areas than with central and southern Grand Bank. However, even if larval drift occurs, larval survival is the main factor in determining the resulting recruitment to the Cap. Based on these considerations, larval transport to the Cap from Labrador or Northern Grand Bank is not likely.

A paper on possible mixing of American plaice populations in the area of the Flemish Pass showed that the exchange of American plaice between Flemish Cap and northern Grand Bank is unlikely to occur based on its no occurrence in the deepest strata of Flemish Pass , even though this species reaches deep areas in some seasons.  Furthermore, individuals at both sides of the Flemish Pass were clearly different in mean length at age and in their maturation. Another paper on redfish showed that the three redfish species on Flemish Cap constitute independent stocks according to results of morphometric analyses.

Two papers on northern shrimp on the Flemish Cap detailed the increase in abundance and area of distribution of shrimp in the area.  Differences in year-class strength, between the Flemish Cap and adjacent areas may indicate that shrimp on Flemish Cap are not connected to those on the Newfoundland Shelf.

Papers examining the spatial distribution patterns of several species were presented. The fish fauna in Flemish Cap appears distributed in a persistent structural zonation based on factor analyses of demersal survey trawls during 1995-2000 with redfish being the dominant fish species in the area. Changes in species spatial distributions in the most recent years are related to decreases in the main demersal fish species: cod and American plaice. Declines in the cod and American plaice abundance during 1989-2002 coincided with severe range contraction and a breakdown in the spatial structure of both stocks, which have high degree of spatial overlap.

Results of a longline survey indicated that Greenland halibut and roughhead grenadier (Macrourus berglax) were distributed at depths up to 2 050 m, based on a long-line survey between 700 and 3 000 m depth. Other deep-sea species replaced the above mentioned ones at greater depths. Greenland halibut abundance and biomass appear related to bottom temperature, being the warmer the water, the more abundance of halibut and vice versa.

Discussion of this session brought up information on the witch flounder stock on Flemish Cap.  In this area the species is distributed in the shallowest strata, so depth preferences are quite different from stocks on Labrador and Grand Bank, which are distributed in deep areas. This particular behaviour may point to the isolation of the stock over the Cap.

Some of the changes in depth distribution described in papers in this session may be related to distribution of fish by size.  Large fish tend to occupy deeper waters.  As populations declined and the number of bigger, older fish decreased, an apparent move to shallow water could result.