Tuesday, 19 July 2011

Exeter Tuesday: Effects of small-scale fishing efforts on marine turtle populations

(La Islilla Port, Pira, Northern Peru)

Those who eat canned tuna may be familiar with the "dolphin safe label" used to indicate that no dolphins were harmed when the fish were harvested. But dolphins are only one of many marine animals that may be negatively impacted during the fishing process--for example, sharks, sea birds, and turtles are all known to suffer injuries or even death after being caught in nets or on hooks. Although large, industrial fishing outfits are most often blamed for this damage, the combined efforts of smaller parties (known as small-scale fisheries or, sometimes, artisanal fisheries) may also have significant effects on animal populations.

(Leatherback turtle, Dermochelys coriacea, caught in a gillnet boat)

New research published today in the Journal of Applied Ecology describes the effects of small-scale fisheries in Peru on 5 species of marine turtles that forage (and, in rare cases, breed) along the Peruvian coast: green turtles (Chelonia mydas), leatherback turtles (Dermochelys coriacea), olive ridley turtles (Lepidochelys olivacea), hawksbill turtles (Eretomocheyls imbricata), and loggerhead turtles (Caretta caretta). Although outright turtle fishing has been banned in Peru since at least 1995 (green and leatherback turtles have been protected since 1976), these species are often caught accidentally, at which point they are sometimes released, but may also be kept for personal consumption or sale.

(Fishing boats along the Peruvian shoreline.)

The researchers (collaborators from the Marine Turtle Research Group at the Cornwall Campus of the University of Exeter, Pro Delphinus, and NOAA--National Marine Fisheries Service) analyzed 7 years' worth of data collected by observers on fishing boats in each of 4 Peruvian fisheries. Over a total of 264 fishing trips, 807 turtles of 4 species (loggerheads, green, olive ridley, and leatherback turtles) were captured. Using these numbers, the researchers estimated that the annual turtle bycatch across all 4 fisheries is approximately 5900 turtles.

(A bycaught leatherback turtle is released into the wild...after the fishermen are convinced not to keep it.)

"Fishing effort" can be measured as total number of hooks or total length of net deployed, depending on capture technique. The 807 turtles captured here comprise 635 caught with an effort of 900,000 hooks and an additional 172 caught with an effort of 838 km of nets. These represent only a fraction of the estimated 80 million hooks and 100,000 km of nets deployed across Peru each year. Because turtles are not spread evenly across all Peruvian fisheries, it is unlikely that these figures could be used to make a direct extrapolation in order to calculate the total number of turtles captured each year in Peru. All the same, it doesn't take an expert at math to recognize that the fishermen are probably hauling in a lot of turtles along the country's coast.

(A leatherback turtle is brought to shore in northern Peru.)

One additional problem is which turtles are being caught. Adults accounted for approximately 35% of the green turtle bycatch, 71% of leatherbacks, and 67% of olive ridley turtles; the vast majority of loggerheads were juveniles. Because adults are potential breeders, and are therefore responsible for sustaining, or even increasing, the population, it can be particularly damaging if fishermen are catching a disproportionately large number of this age class, as it appears they are.

(Release of the leatherback, "Odiseo" back to the wild.)

There are several potential mitigation techniques available to reduce the impacts of fishing on turtle populations. These include fishery closures, the establishment of marine protection areas, and the use of alternative or additional fishing equipment such as circle hooks, dehookers, net illumination, and nets without flotation devices. However, because fishing is both a livelihood and a direct way of putting food on the table, managers and conservationists will ultimately need to employ techniques that simultaneously save turtles and prevent the locals from going hungry.

To listen to The Journal of Applied Ecology's podcast containing an interview with authors Joanna Alfaro-Shigueto and Brendan Godley, click here. To see the Science Daily write-up of the paper, click here. To see the associated Youtube video, click here.
Alfaro-Shigueto, J., Mangel, J.C., Bernedo, F., Dutton, P.H., Seminoff, J.A., and Godley, B.J. Small-scale fisheries of Peru: a major sink for marine turtles in the Pacific. Journal of Applied Ecology, online advance publication.

All pictures provided by Jeffrey Mangel.

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