Can I reduce a prawn/shrimp broth I have a freshly made shrimp broth. 
 Onions, parsnip, tomato paste and white pepper corns along with shrimp shells.

https://www.msc.org/where-to-buy/product-finder/products/cfpsproduct-19BA7EBA-B7E7-40E4-8C33-8566D3AF25A0  is similar to the shrimp I have used. 

It was simmering for about 15 minutes, then I strained it. 

It is my experience that more simmering makes it turn bitter or too compact  (I'm lacking a better word). 

Can I simmer/reduce the liquid to about half without imparting any off flavours?
Remove anything you suspect might affect flavour negatively (or, more simply, strain the whole caboodle), then reduce to concentrate flavour.

For fish broth, you shouldn't simmer the fish bones / heads for more than half an hour and I assume a similar rule applies for shrimp.