http://www.abc.net.au/news/2014-08-16/health-warning-over-fi...
http://www.fishingworld.com.au/news/toxins-found-at-popular-...
Sydney's ocean beaches are often closed to swimmers because of pollution. Sydney disposes of its sewerage via deep ocean outlets, and this can often make the water quality very bad, depending on ocean currents.
I'm not sure my post warranted a down-vote. Is it because I contradicted you? or do you think I am wrong? I lived on a houseboat on The Georges river/Botany Bay for many years, and have spent some serious time on Sydney's waterways, and offshore. I wouldn't eat anything caught in Sydney's rivers or beaches knowingly.
You can easily observe locals fishing in the river(s) running through Shanghai. The fact that they do it doesn't mean those fish are safe to eat.