Is it possible to identify a particular gene sequence from next gen sequencing techniques (especially RNA-seq )?
Okay, so you have RNASeq, but no reference genome for your fish?  Just transcript sequence from zebrafish?  Are you just trying to see if this gene is expressed, or you need to a more global view to compare it to?

If you just want to see if the gene is expressed, easy thing to do is to make a reference just made up of zebrafish transcripts, align to that.  If there are too many differences between your fish and zebrafish, that won't work.

In which case, the next thing you could try is de novo assembly of your RNAseq data, then use blast to compare the zebrafish trasncripts to your assembled transcripts.  Blast will make alignments where a short read aligner might not.