Acoustic Surveys Indicate Rebuilding Behaviour and Biomass of Northern Cod

The northern cod stock off Newfoundland and Labrador was formerly one of the largest Atlantic cod (Gadus morhua) stocks (estimated 3.5 million t). Acoustic research in the early 1990s documented the concentration, location and behavior of the last observed vertical spawning aggregations in the Bonavista Corridor on the NE continental shelf (450,000 t in 1990) - termed "hyper-aggregation". By mid-1990s the entire stock biomass had declined to <50,000 t.  For over a decade there was zero or negative population growth with poor recruitment and increased natural mortality such that few fish survived beyond age 5 (with virtually no fishing). No aggregations as observed in the early 1990s could be located, as the remaining fish were dispersed and demersally distributed. The sole overwintering-spawning group that was productive was located in a coastal fjord (Smith Sound), and this group continued to exhibit typical vertical behaviors, recruitment and stock growth, with no excessive mortality (despite being fished). Almost 2 decades later, acoustic surveys of overwintering and prespawning cod in the Bonavista Corridor area (2007, 2008, and 2011) have shown a progressive redevelopment of vertical pelagic aggregations and a corresponding increase in both the age structure and biomass from about 10,000 to >100,000 t. This area of the stock was the last to go down and is the first to show rebuilding. Behavioral dynamics appear to signal coming changes in stock status. These dynamics, the changing biomass and age structure of the northern cod, and possible causes related to feeding, predation and climate, will be discussed.