A simple fish stock assessment model applied to over 500 years of catch data demonstrated that if Canadian authorities had allowed for the rebuilding of the stock of northern Atlantic cod off Newfoundland and Labrador in the 1980s, annual catches of about 200,000 tonnes could have been sustained.
A new study by researchers from the Sea Around Us initiative at the University of British Columbia, the GEOMAR Helmholtz Centre for Ocean Research and Dalhousie University modelled the cod population trajectory for the entire period from 1508 to 2019. While the earlier fishery, which used lines and later traps, was sustainable and generated catches of 100,000 to 200,000 tonnes per year for 400 years, the unleashing, in the 1960s, of bottom trawlers onto Northern cod reduced their biomass to levels that could not sustain high catches.
“Our assessment suggests that the biomass —the weight of the population in the water— of northern cod is currently around 2% of what it was earlier,” said Rebecca Schijns, lead author of the study and a researcher with the Sea Around Us at UBC’s Institute for the Oceans and Fisheries.
“The interesting thing is that we got to these results by applying a computer intensive but very simple stock assessment method known as CMSY to catch data for five centuries. Different from previous assessments that required large amounts of information, this method basically requires only a time series of annual catches,” Schijns said. “The other information that is required is available from the scientific literature, and from people with knowledge of the fishery.”
Working with such a long time series allowed the researchers to reliably estimate maximum sustainable yield – the highest catch that a fish stock can support in the long-term, given that environmental conditions remain more or less constant– for northern cod at 380,000 tonnes per year.
But such high catches are now only a dream. Following a massive 810,000-tonne catch taken mostly by foreign vessels in 1968, catches started declining. Moreover, once Canada declared a fishery exclusion zone in 1977, fishing was not halted to allow the stock to rebuild. Rather, a new, heavily subsidized local trawler fleet was let loose on the depleted stock. This led to a final collapse of the fishery, which still remained open to small-scale fishers even during the moratorium that was imposed in 1992. Moreover, in recent years, every time Northern cod appear to increase, the fishing quota is raised.
“As a student, I was on board of a German trawler of fishing off Newfoundland and Labrador in 1973 and I have vivid memories of this cod rush,” said Daniel Pauly, co-author of the study and the Sea Around Us principal investigator. “If artisanal fishers in the outports had been listened to when they warned about running out of cod to catch, things would be different now. The scientists then monitoring the cod stock ignored small-scale fishers and relied only on the data from trawlers which, however, did not reflect the cod stock’s decline because the trawlers could follow the cod further out than the small-scale fishers.”
Paying attention to what local and/or Indigenous fishers have to say and integrating centuries-old catch data into stock assessments help better understand the total impact of fisheries on marine ecosystems and provide clues on how to effectively manage marine populations for the long term.
“The CMSY method proved to be useful to assess the data-rich cod stock, but it also works with stocks for which we have only a catch data. This method is able to provide more reliable estimates of stock status by incorporating past data-limited periods,” Pauly said.
The CMSY method, thus, offers researchers, fisheries managers and policymakers the possibility of taking a comprehensive look into the status of the world’s most important fish stocks.
“Ancient catch data exist for several stocks, such as Bluefin tuna in the Mediterranean, which started being commercialized around the 8th century; Atlantic herring in the Baltic Sea, whose fishery started in the 13th century, and Atlantic salmon in the Celtic Sea, whose fishery started in the 14th century,” Jeffrey Hutchings, co-author of the study and a researcher at Dalhousie University, said. “There is a real opportunity to use these data to design policies that prevent collapses similar to that of the cod stock.”
The paper “Five centuries of cod catches in Eastern Canada” was published in the ICES Journal of Marine Science, DOI: https://doi.org/10.1093/icesjms/fsab153