Is overfishing a social issue?

Only indirectly. It's mainly an economic issue.

Okay, so the same would go for any environmental issues?

Not necessarily.

what would be an environmental issue that is directly a social issue?

animal conservation?

The first example that comes to mind is land use in some protected areas. Can people with motor vehicles -- especially OTRs -- exist in the same area as environmental preservation?

For more ideas --

http://wps.ablongman.com/ab_socialprob_sprsite_1/11/3023/773955.cw/index.html

http://firstforsustainability.org/risk-management/environmental-and-social-issues.php

so cars polluting the environment would be a social issue?

Also cloning would also be a social issue?

Yes. Cloning is a social issue. Pollution from cars is both social and environmental.