How do we know that 100,000 sharks and about 1,400 sea turtles are caught every year? Or that up to two sharks die for every swordfish caught?
All of the numbers come from Canadian government and ICCAT (the international regulator) science documents. They base their numbers on observer data from the fishery and tagging research.
For loggerheads, the DFO Socio-Economic Analysis states:
“Bycatch in the Canadian Tuna and Swordfish fisheries was estimated to be 1,200 Loggerhead Sea Turtles (with a 95% confidence range of 700-1,800) annually from 2002 to 2008. Assuming that post-hooking mortality is in the range of 20-45%, this would result in the deaths of roughly 200-500 oceanic/neretic juvenile Loggerhead Sea Turtles annually from the Canadian Tuna and Swordfish longline fishery.”
For leatherbacks, the DFO 2004 report on leatherback turtles after they were listed under Canada’s Species at Risk Act states:
“Data from [the pelagic longline] fleet indicate that about 170 incidental captures occur per year.”
We have taken these two numbers estimations 170 + 1200, to come up with roughly 1400 turtles.
For the shark numbers we have looked at the ICCAT and DFO status reports of blue shark, short-fin mako and porbeagle sharks, as well as DFO science research released in 2009 (Campana, 2009) and 2008 (Brazner and McMillan, 2008).
The 100 000 sharks number is based on blue shark catch. The ICCAT science and Campana (2009) estimate a catch of 2450mt, based on the average shark weight of 23.4 kg you get 100 000 sharks. This number can also be worked out in a second way. Using the observer data of an avg of 98 sharks/set, combined with the avg number of hooks per set and you also get approximately 100 000 blue sharks.
Add to this the reported landings of short fin mako shark (around 60-70 tonnes per year) and porbeagle (30-40 tonnes per year). Keep in mind that the numbers for these two sharks does not include an estimation of discards – the one that are cut off the lines.
Current post-release mortality estimates – the number of sharks that die after being caught – are 35% for blue shark (Campana et al, 2009) and between 40% and 50% for porbeagle and shortfin mako, meaning that more than 35,000 sharks are killed every year for 20,000 swordfish
Whether you interpret this as 5 sharks CAUGHT or 2 sharks KILLED for each swordfish (1.75 rounded up to account for other species), this is a problem.