Researchers from the Spanish Institute of Oceanography (IEO, CSIC) and the Institute of Marine Research (IIM-CSIC), in collaboration with scientists from the Centre for Fisheries Ecosystems Research in Canada, have published a study analysing the impact of fishing and climate on cod growth in the Flemish Cap area over the last three decades, using biological and oceanographic data obtained in annual research surveys in the area.
Vigo, Wednesday, 1 June 2022. This study, recently published in the journal Frontiers in Marine Science, reports a decline in the growth rate of the cod population over the last 30 years on the Flemish Cap bank, reaching historic lows in 2016, the final year of the study.
The Flemish Cap bank, east of the Grand Banks of Newfoundland, lies outside Canada’s exclusive economic zone (EEZ) and extends from 120 metres deep to beyond 4,000 metres. It is located in a convergence zone between cold, low-salinity waters (the Labrador Current) and warmer, saltier waters (the Gulf Stream), which gives it very distinctive environmental characteristics.
The fisheries operating on Flemish Cap exploit demersal resources, close to the seabed, with cod (Gadus morhua) being one of the iconic species of Galician and Portuguese trawlers. When Canada extended its EEZ to 200 miles in 1977, the efforts of the Spanish and Portuguese trawling fleet shifted from the Grand Banks of Newfoundland to Flemish Cap. This increase in fishing effort, the isolation of this cod population and environmental changes led to the collapse of the population in the mid-1990s. The result was the closure of the fishery in 1999, which did not reopen until ten years later. “However, despite the improvement in the cod population, this work shows that its condition is still not optimal,” explains Rosario Domínguez-Petit, IEO researcher and one of the authors of the study.
The study results show a decline in the growth rate of the Flemish Cap cod population over the last 30 years. This decline is due to several factors, which have also changed over time. “Our results suggest that, although fishing overexploitation in the 1980s and 1990s and rising water temperatures caused the decline in the Flemish Cap cod growth rate before the moratorium, today processes such as food availability and global warming are behind the lack of recovery of this population. The shortage of large females not only means a reduction in catchable biomass, but also lowers the reproductive success of the population, compromising its future,” says Rosario Domínguez-Petit.
“Methodologically, it has been a very demanding piece of work,” comments Raquel Ruiz, lead author of the article and predoctoral researcher at the Centre for Fisheries Ecosystems Research in Canada. Fish growth is recorded in the rings of otoliths, calcareous structures in the head that are equivalent to the rings in tree trunks. “Identifying and measuring these rings requires very meticulous work, both in preparing the sample and in the subsequent image analysis,” the researcher notes.
In addition, “the mathematical modelling was also very complex, because cod change their habits with age, and the fishery also changed its behaviour before and after the collapse of the 1980s and 1990s. Incorporating all this casuistry into a mathematical model was a complex task for which we had advice from Bryan Black, a researcher at the University of Arizona and a pioneer in this type of work, and from Fernando González and Diana González of the Vigo Oceanographic Centre, experts in Flemish Cap fisheries,” says Rosario Domínguez-Petit.
Three years passed from the start of the research in February 2019 to the publication of the work. It was necessary to select the appropriate individuals for this study from the IIM’s historical sample collection, cut, polish and photograph all the samples under a microscope, and carry out all the statistical modelling work.
“This work reaffirms how important it is to maintain data-collection programmes and sample collections. Ecosystem fluctuations can occur over very different time frames. When pollutants are spilled, some responses are immediate and others are detected many years later. In the case of climate change, we move on scales of decades, and it is necessary to analyse long time series of data in order to capture these fluctuations,” explains Rosario Domínguez-Petit.
For this reason, it was essential to have the otoliths that have been collected annually since 1988 to estimate and assess the age of Flemish Cap cod. “If the samples had not been preserved for so many years at the CSIC, we would not have been able to do this work 30 years later,” adds Fran Saborido, IIM researcher and third author of the article.
Cod is an apex predator at the top of the Flemish Cap demersal food web, so any alteration in its population will cause a cascading effect that will ultimately affect the whole ecosystem. This has already been confirmed by previous studies showing that cod abundance is closely related to the abundance of beaked redfish (Sebastes mentella) and shrimp (Pandalus borealis), which are also target species of the trawl fleet.
“Understanding this web of interconnections and the factors that modulate them is key to predicting future trends in abundance and biomass, especially under expected climate-change scenarios, and should be taken into account in stock assessments to establish dynamic reference points that guarantee the sustainability not only of the population itself, but also of the fishery,” says Rosario Domínguez-Petit.