Cod Gadus morhua L. populations as behavioural units: inference from time series on juvenile abundance in the eastern Skagerrak

Abstract

Abundance and distribution of cod Gadus morhua in various size intervals and age groups between 2000 and 2005 were followed in coastal trawl surveys. In spite of a reduction in fishing pressure in recent years and high cod recruitment in the Skagerrak region in 2001 and 2003, no recovery could be evidenced. The survey data clearly showed that low cod density areas were not recolonized, even though abundance of juvenile cod remained high for about a year after the recruitment episodes. Increased abundance of fish >400 mm total length was only discernible at some scattered locations where other studies also have suggested local populations still to be present. The intermittent high recruitment has been linked to an inflow of egg and larvae from the North Sea, a theory which also has gained support from genetic studies. It was thus argued that the disappearance of the juvenile cod from the inshore is an effect of a migratory behaviour; the fish of offshore origin eventually leave the coast for the open Skagerrak or the North Sea. These findings support a view on cod populations as essentially behavioural entities, whereas dispersal of early life stages may be less important as a structuring mechanism.

Migratory behaviour of Atlantic cod Gadus morhua: natal homing is the prime stock-separating mechanism

“…Knutsen et al 2003, Svedäng & Svenson 2006. We think these directional movements represent migratory behaviour based on individual decision making of the fish, suggesting a philopatric tendency.…”

“…Circumstantial evidence for natal philopatry has been shown for cod in the eastern part of the Skagerrak (Svedäng & Svenson 2006). An almost total eradication of locally spawning cod aggregations along the Swedish Skagerrak coast has, in some years, resulted in the apparent anomaly of low adult cod abundance coinciding with high juvenile abundance (Svedäng 2003, Svedäng & Svenson 2006.…”

“…An almost total eradication of locally spawning cod aggregations along the Swedish Skagerrak coast has, in some years, resulted in the apparent anomaly of low adult cod abundance coinciding with high juvenile abundance (Svedäng 2003, Svedäng & Svenson 2006. Svedäng (2003) and Cardinale & Svedäng (2004) hypothesised that most juvenile cod along the eastern Skagerrak coast had been passively transported from offshore spawning areas (Munk et al 1999), and the following (unexpected) low abundance of adult cod was due to return migration of juvenile and/or maturing fish which had reached a certain size or age (Pihl & Ulmestrand 1993).…”

“…Hunter et al 2003), the migratory movements of individual fish can be mapped from retrieved tags. The purpose of our study was to examine -using observational data on migratory behaviour -the following 2 hypotheses: (1) that the cod population in the eastern North Sea region comprises a mixture of resident and migratory stocks (Munk et al 1999, Knutsen et al 2004, Svedäng & Svenson 2006; (2) that if natal homing is a part of the behavioural repertoire of cod, recorded movements should be directional and related to spawning locations.…”

Decline and Recovery of Atlantic Cod (Gadus morhua) Stocks throughout the North Atlantic

“…Much of the increase in the cod at West Greenland during the 1920s and 1930s was due to eggs and larvae imported by currents from Iceland (Buch et al 1994), but it remains uncertain whether the fish from Iceland established self-supporting spawning populations at West Greenland, as stated by Anisimov et al (2007), or depended on the inflow of new recruits from Iceland. Along the eastern Skagerrak coast, where local spawning aggregations have been depleted, juvenile cod may still occur in high numbers in some years, but most are recruited from offshore spawning areas, mainly in the North Sea, and they return to offshore areas instead of spawning locally along the coast (Svedäng and Svenson 2006). For the S. Labrador-E. Newfoundland stock, there is concern that fishing on inshore populations will lessen the likelihood that those populations will expand to offshore waters and establish spawning groups there .…”