How much plas­tic pol­lu­tion will kill a tur­tle?

Tehran Times - - SCIENCE -

We know there is a lot of plas­tic in the ocean, and that tur­tles (and other en­dan­gered species) are eat­ing it. It is not un­com­mon to find stranded dead tur­tles with guts full of plas­tic.

But we weren’t re­ally sure whether plas­tic eaten by tur­tles ac­tu­ally kills them, or if they just hap­pen to have plas­tic in­side them when they die. An­other way to look at it would be to ask: how much is too much plas­tic for tur­tles?

This is a re­ally im­por­tant ques­tion. Just be­cause there’s a lot of plas­tic in the ocean, we can’t nec­es­sar­ily pre­sume that an­i­mals are dy­ing from eat­ing it. Even if a few an­i­mals do, that doesn’t mean that ev­ery an­i­mal that eats plas­tic is go­ing to die. If we can es­ti­mate how much plas­tic it takes to kill a tur­tle, we can start to an­swer the ques­tion of ex­actly how tur­tle pop­u­la­tions are af­fected by eat­ing plas­tic de­bris.

In our re­searches, we looked at nearly 1,000 tur­tles that had died and washed up on beaches around Aus­tralia or were found in nets. About 260 of them we ex­am­ined our­selves; the oth­ers were re­ported to the Queens­land Tur­tle Strand­ing Data­base. We care­fully in­ves­ti­gated why the tur­tles died, and for the ones we ex­am­ined, we counted how many pieces of plas­tic they had eaten.

Death from eat­ing plas­tic

Some tur­tles died of causes that were noth­ing to do with plas­tic. They may have been killed by a boat strike, or be­come en­tan­gled in fish­ing lines or derelict nets. Tur­tles have even been known to die af­ter ac­ci­den­tally eat­ing a blue-ringed oc­to­pus. Oth­ers def­i­nitely died from eat­ing plas­tic, with the plas­tic ei­ther punc­tur­ing or block­ing their gut.

Some tur­tles that were killed by things like boat strikes or fish­ing nets nev­er­the­less had large amounts of plas­tic in their guts, de­spite not hav­ing been killed by eat­ing plas­tic. Th­ese tur­tles al­low us to see how much plas­tic an an­i­mal can eat and still be alive and func­tion­ing.

The chart be­low sets out this idea. If an an­i­mal drowned in a fish­ing net, its chance of be­ing killed by plas­tic is zero — and it falls in the lower left of the graph. If a tur­tle’s gut was blocked by a plas­tic bag, its chance of be­ing killed by plas­tic is 100 per­cent, and it’s in the up­per right.

The an­i­mals that were dead with plas­tic in their gut, but had other pos­si­ble causes of death have a chance of death due to plas­tic some­where be­tween 0 and 100 per­cent — we just don’t know, and they can fall any­where in the graph. Once we have all the an­i­mals in the plot, then we can ask whether we see an in­crease in the chance of death due to plas­tic as the amount of plas­tic in an an­i­mal goes up.

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