By Hilary Feldman
Sometimes it can be useful to take a step back to get a little perspective. In the case of Atlantic cod, that step is giant – going back 4,500 years. A new study published in Proceedings of the Royal Society compares the size of fish in the past to those caught today.
The Baltic cod is one of the Eastern Atlantic stocks – each forming a genetically distinct population of the Atlantic cod, Gadus morhua. Cod live throughout the North Atlantic, including coastal areas and the open ocean. Adults reach 1.5 to 2 metres in length and weigh about 40 kilograms. Colour patterns vary with habitat. Eggs are released in spawning grounds near the coast, and remain in surface waters for 2-3 weeks. The larval cod hatch and live in the open ocean and feed on plankton, migrating to feed off the coast in the summer. Adults tend to live in colder water near the ocean floor. Seasonal migration is common in many cod populations, related to food availability, water temperature, and spawning grounds.
Looking at otoliths and vertebrae – bones known to reflect fish size and age – allows comparisons. The cod caught in the past, from a Neolithic settlement in modern Sweden, were significantly larger and older. Neolithic fishermen would have kept to shallow coastal areas and had little access to deeper water. The oldest and largest fish swim in deep water. By contrast, modern trawlers catch fish in deeper offshore areas. But overall, these fish are still smaller and younger compared to the distant past. The oldest fish caught – both in the past and recently – were 10 years old, although maximum lifespan is recorded as 20 years.
The implications are wide-ranging. Fish species that are overexploited by humans may evolve in response. Possible adaptations include juvenescence – where fish become reproductively mature at a younger age and smaller size. The result has economic effects, as more fish need to be landed to maintain catch size and individual fish reproduction may be decreased. Modern methods allow targeting of bigger fish, which tend to be the oldest females. Older females are much more efficient egg-producers and also use better spawning areas.
Even within the past century, reproductive maturity has shifted. In 1959, fish reached breeding age at 5-6 years. In 1979, more fish matured at 3 years. And in 2004, the median age was around 2 years.
Cod populations have been declining for decades due to fishing pressure. Archeological information may offer some insights about changes to fish species. There are some speculative elements to such studies; for example, fishing techniques have changed and could bias the findings. However, the overall trend is striking. For all the modern equipment innovations and fish-tracking devices, today’s cod catches are smaller and younger individuals.