Numbers is one of the iWork apps available thru the Apple Store for your iPad. You can get Numbers (spreadsheet), Pages (word processor) and Keynote (presentations). They roughly equate to Excel, Word and PowerPoint on your average business computer.
I use a Numbers spreadsheet to track my catches… it’s nice to be able to easily do things like calculate average measurements, average weight, date ranges, etc., on the fly.
Anyway, I have my iPad with me pretty much everywhere I go (it’s an extension of my brain these days…) so it’s only natural to use it for this too.