Recent Publication: Cod Body Condition on the Grand Banks

            The collapse of Newfoundland cod in the 1990s is often imagined to have been primarily caused by overfishing. However, the importance of environmental change on the collapse has been heavily debated for the past 30 years due to the collapse occurring at the same time as an abnormally cold period. We know that millions of cod suddenly died in the 90s, but what was the main cause?

Figure 1: The map shows the number of individual cod sampled in each division around Newfoundland and Labrador. The grey lines represent regional bathymetry.

            In our recently published paper titled “Assessing ecosystem-scale synchrony in Atlantic cod body condition” we investigated the evidence for shared changes in cod biology that could be indicative of environmentally driven deaths. We found that cod in multiple populations and ecosystems around Newfoundland and Labrador became much skinnier around the time of collapse, a tell-tale sign that they weren’t getting enough food and that they may have died from starvation.

Figure 2: The predicted weight plot shows the expected weight in kilograms of an individual cod from each NAFO Division over the duration of the study. The red polygon represents the period of collpase and the grey polygons represent the 95% confidence intervals.

We investigated cod weight-at-length, because a larger proportion of heavier fish at a given length is indicative of a well-fed and healthy population. Larger individuals have more energy to help them forage for prey, survive through winter, and invest energy into reproduction. If we found heavier fish around the 90s, then this may have served as an indication that the environment had not played a large role in their collapse. Instead, we found skinnier fish, meaning that the cod collapse was likely at least partially driven or exacerbated by the unusually cold period and reduced food availability in the early 1990s.

            In addition to identifying that cod around Newfoundland and Labrador got skinnier in the early 1990s, our research found that the average weight of cod has continued to fluctuate similarly between populations and ecosystems since the collapse. These synchronous fluctuations indicate that large-scale environmental processes, like changes in the climate, can have similar impacts across cod populations. These large-scale environmental processes may indirectly affect cod by more directly impacting important cod prey, such as capelin (Mallotus villosus). Tracking changes in cod weight can provide an indication of how healthy their populations are, which can help inform the sustainability of fisheries. Hopefully, an improved understanding of the environment’s impact on cod populations will allow us to modify fishing pressure when cod are most vulnerable and prevent future long-term collapses.