Irregularly shaped hotspots on image

[Viktoras Didziulis]

Is there a "standard" method of setting irregularly shaped invisible
hotspots on images displayed on a stack ? 
 
I have a few images with lots of fish on them. Need to highlight each fish
and display its name on mouseEnter. Now I am drawing a polygon around each
fish, setting its tool tip to fish name and putting a script that passes the
long id of the polygon to a handler to change polygon's ink from noop to
blend. And vice versa on mouseLeave. Additionally it displays contents of
polygon's tooltip in a separate field (in case the tooltip does not appear
immediately on mousenter). 
 
The drawback of this approach is that while highlights are irregular,
hotspots are still rectangular and sometimes hotspot of a large fish blocks
all the small fishes around. So I have to adjust ordering of polygons
sending larger to back and bringing the smallest to front to avoid blocking.
 Are there other alternative techniques ? 
 
Best! 
Viktoras