Fishermen help scientists uncover the secret life of cod

Fishermen and scientists often butt heads when it comes to cod fisheries. But when the two groups work together, they can reveal important insights into cod biology that make regulations more effective for everyone. Learn how fishermen in the Baltic Sea helped scientists study declining cod populations.


Funk, S., Krumme, U., Temming, A., & Mollmann, C. (2020). Gillnet fishers’ knowledge reveals seasonality in depth and habitat use of cod (Gadus morhua) in the Western Baltic Sea. ICES Journal of Marine Science, 77(5), 1816–1829.


The Rise and Fall of Cod

 Cod is one of the most important fish in the North Atlantic. Demand for cod fishing fueled economies and even ignited an 18 year military dispute between Iceland and the United Kingdom known as “The Cod Wars”. Entire cities were developed to support cod fisheries at a time when the cod were so abundant they were said to practically catch themselves. But centuries of fishing and a changing environment depleted the once plentiful cod populations. The very towns and economies built around the fisheries floundered as governments imposed fishing regulations to try and rescue cod populations. Newfoundland, instituted a complete ban on cod fishing in 1992, putting 38,000 people out of work in a single day.


Despite its diminishing population sizes, cod remains a critical component of commercial fisheries throughout the Atlantic Ocean. This frequently causes tension between fishermen who rely on a profitable cod fishery and scientists who struggle to help cod populations rebound. But effective management for the fishery cannot be achieved by one group alone – a point exemplified by a recent discovery of cod behavior in the Baltic Sea.


Declining cod populations in the Baltic Sea

Atlantic cod, Gadus morhua. Image from Wikimedia commons

Scientists conducted surveys to estimate the size and health of the Baltic sea cod populations, which have been on the decline since the 1990s. Unlike a targeted fishery that can optimize its fishing gear as technology improves and follow migrating schools of fish, scientific surveys rely on consistency: surveying the same area at the same time of year using the same type of fishing method and gear. In this way, scientists can accurately compare the fish populations in an area from year to year. But the very strength of this scientific method quickly becomes a shortcoming when a depleted fish population no longer comes in force to the historically surveyed area. A team of scientists from Germany surveying cod populations in the Western Baltic Sea sensed that their surveys were inadequately capturing the full cod distribution. To solve this problem, they enlisted the help of the cod-catching experts – the fishermen who have spent decades searching for the best cod fishing spots.

When scientists examined  the reported catches from commercial trawlers, which are restricted to deeper waters, they realized that cod inhabited the deep waters of the Western Baltic Sea only for a few months in a year. But where did the fish go for the rest of the year? They must retreat to shallow waters.

The scientists turned to the coastal waters – areas where their own surveys could not cover. To gain insights into cod behaviors, scientists spoke with fishermen who spent their careers catching cod in coastal waters. The scientists interviewed 15 commercial fishermen – most of whom had over 20 years of experience and one had spent over 60 years fishing those waters!


 Finding cod with a little help from fishermen

The scientists asked fishermen for general information on where they fished at different times of the year. How deep did they have to set their nets? What size mesh caught the most fish? How big were the fish? What type of habitat did the fish prefer? The collective experience of career fishermen revealed the hidden patterns of cod behavior in the area. The cod shift their distribution with the seasons – occupying shallower waters in the spring and fall and moving into deeper waters in the summer and winter.

Surveying fishermen revealed that cod depth ranges changed seasonally throughout the year. Figure modified from Funk et al., 2020

What drove this seasonal change in cod distribution? The scientists overlaid catch information with records of ocean temperatures and found that cod change their depth in response to the temperature of the ocean. Cod appear to prefer water temperatures around 53 – 57°F (12 – 14°C). In winter, the cod move to deeper waters that are warmer than surface waters. As surface temperatures increase in the spring, cod move into shallower waters but are driven back into deeper waters as surface temperatures become too warm in the summer. When surface waters begin to cool in the fall, the cod once again move into shallower waters before they must return to depth to avoid cold surface waters in the winter. The scientists speculate that this temperature preference may be driven by food availability or spawning preferences.


Collaboration is the key to successful management

Scientists and fishermen may not see eye-to-eye on every issue, but this study highlights the importance of working together to improve management of fish species. By collaborating with fishermen who have spent their careers chasing fish, scientists can fill gaps in their knowledge and develop more effective management strategies.



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