Movement of fish and fishers in the North Atlantic
In a recently published paper Ragnar Edvardsson et al. explore Atlantic cod and cod fishing in historical Iceland using multidisciplinary methods. The take home message is that fluctuating sea temperature in the North Atlantic are likely to have coincided with shifts in Atlantic cod migration routes or, more drastically, the spawning grounds of the migratory cod. This change in fish migrations may have contributed to a temporary collapse of Atlantic cod fishing in Iceland.
Otolith shape as an indicator of life-history
Otoliths, a calcified structures of the inner ear of fish are often used in fisheries science. Fish growth, and thereby otolith growth, is not only reflected in annuli size but also in otolith shape. Otolith shape analysis can therefore be used to retrospectively examine the environmental conditions of the fish and, in turn, assign individuals to stocks or subpopulations. Since stationary and migratory Atlantic cod experience very different environments, including temperature in their lifetime otolith shape variation can be used to distinguish between the two Atlantic cod types.
Atlantic cod migrations react to sea temperature
It is well recorded that Atlantic cod migration patterns around Iceland follow sea temperature, for example, as the cod follow their favored prey the capelin further north. The current results place these migration shifts in a Millennial scale context, showing that Atlantic cod migrations may have fluctuated with temperature for many centuries. Although it makes intuitive sense that sea temperature has always effected fish migrations empirical confirmation is a key to understanding and for framing further research on historical distribution shifts and migration shifts of marine fish.
The rise and fall of cod fishing in Iceland
Moreover, the study claims that these changes in fish migrations are linked to change in the patterns of domestic cod fishing. Icelanders probably fished for subsistence and local trade since the early settlement. The “fish event horizon” commenced in the North Sea in the 11th century with a steady increase in the demand for dried fish. In turn, fish prices increased and the economy of Iceland became more reliant on fishing. The Icelandic commercial cod fisheries thrived in the period between AD 1250 and AD 1550 after which historical references and archaeological evidence suggest stagnation and decline.
The location of the historical fishing stations, and to a lesser extent references to historical fishing seasons, show that the fishers targeted Atlantic cod migrations and very likely also spawning fish. Historical environmental change resulted in fisher movement on varying scales. Coastal erosion destroying boat landings, forced fishermen to relocate to new sites, depletion of in-fjord fishing grounds, forced many fisher-farmers to abandoned their home bases and move to the larger commercial fishing stations. Finally, reoccurring lack of catch over long periods of time, driven by change in migrations, forced fishermen to relocate nationally to fish seasonally and at least from the late 17th century fishermen from the north of Iceland traveled to the commercial fishing stations in the southwest.
The study shows how multidisciplinary collaborations can expose effects easily overlooked by more conventional research. It moreover adds to evidence for successful colonization and exploitation of sub-arctic environments in the Viking Age followed by stagnation or failure to adapt to climate driven changes.