Sisal is often untreated too - though if you plan on crocheting a net, sisal is a beast to hook.
You can get legit fishnet off some etsy sites and maybe Richard's Shells (if I remember correctly.)
I wish I hadn't lost the organic jute brand link I found.
After emailing a few manufacturers I was told, by the only two who responded to me, that their jute twine could very well be unfit for pet use/consumption (due to oils used in the manufacturing process, or protective chemicals utilized for storage or shipping).
Just thought I'd fan the flames of paranoia.... :roll:
That said an awful lot of crabbers use nothin'-special run-of-the-mill hardware store jute, with no ill effects that we can register.
I encourage you to research jute production processes & deside for your self. Start with 'jute' in wikipedia, then investigate different brands available in stores you have access to. Email customer service.... though often they won't respond: it's a technical question that service reps don't seem equipped to answer.
Good luck!