The near-collapse of the Faroe Plateau cod (Gadus morhua L.) stock in the 1990s: The effect of food availability on spatial distribution, recruitment, natural production and fishery

Abstract

1. This thesis aims to explain why there was a near-collapse of the Faroe Plateau cod stock in the 1990s and why it recovered so quickly. Contrary to what local people on the Faroes believe, these fluctuations were not explained by large-scale migrations between, e.g., Icelandic and Faroese fishing grounds, but rather by local processes on the Faroe Plateau. When cod were in poor condition they moved towards land and hampered the survival of 1-year-old cod, probably by displacing 1-year-old cod to normal cod-depths where they were cannibalised. The survival of 2- year-old cod was, however, positively correlated with the total contemporary biomass of age 3+ cod, probably caused by reduced predation mortality or enhanced foraging, up to an abundance of around 100 thousand tonnes of age 3+ cod, above which there was a sharp decline.

2. There was no negative correlation between the stock size of cod and the individual growth rate. Cod seemed to be reluctant to enter the food-rich deep waters (> 150 m) indicating that the shallower areas had to be ‘filled up’ first before cod moved to deep waters. Cod in shallow waters fed on sandeels, juvenile Norway pout and benthic crustaceans, whereas cod in deep waters mostly fed on adult Norway pout and Blue whiting. Cod in good condition tended to have a larger stomach content than slim cod, and, in deep waters, there was a positive correlation between feeding success and local cod density.

3. Cod tended to be caught on longlines when in poor condition, coinciding with a low feeding on natural prey organisms and a more pronounced feeding migration. Hence, the total fishing mortality, which was overall very high, tended to vary negatively with the individual growth rate.

4. The information in 1-3 indicated that a low ecosystem productivity during the late 1980s combined with a high fishing mortality were the main reasons why the cod fishery nearly collapsed in the 1990s. The recovery was likely caused by favorable environmental conditions and a low fishing mortality. A simulation study strongly suggested that the cod fishery on the Faroe Plateau is not sustainable and that the stock will probably collapse in the future. An immediate reduction of the fishing mortality of 40-60% could reverse the negative trend and result in an optimal harvesting. However, the common belief among Faroese fishermen / local people in the existence of large-scale migrations of fish, and a reluctance to accept short-term reductions in catch rates, may represent major obstacles in rebuilding the Faroe Plateau cod stock.