The Fisheries Act (NI) 1966.
This is the main piece of legislation which underpins DAERA’s overall responsibilities with regard to Inland Fisheries (please note that the Act also describes powers with regard to Sea Fisheries and Aquaculture which also form a separate part of DAERA’s responsibilities):
Fisheries Act (Northern Ireland) 1966 (legislation.gov.uk)
There are also a number of Regulations which further regulate inland fishing activity.
The Fisheries Regulations (N.I.) 2014
These regulate angling and commercial activity and describe licencing requirements, restrictions on methods of fishing and fishing equipment, takeable size of fish and close seasons. (N.B. These Regulations were amended in 2005 and so should always be read in conjunction with any subsequent amending Regulations).
The Eel Fishing Regulations (N.I.) 2010
These describe permitted methods of eel fishing, takeable size of eels and close seasons. (N.B. These Regulations were amended in 2011 and so should always be read in conjunction with any subsequent amending Regulations).
The Public Angling Estate Byelaws (N.I.) 2005
DAERA also manages (directly or in-directly) 90 publicly available fisheries and the permitted methods on such waters are governed through these Byelaws (N.B. These Byelaws were amended in 2006 and so should always be read in conjunction with any subsequent amending Byelaws).
Fisheries (Tagging and Logbook) Byelaws (Northern Ireland) 2001
These describe the requirement for salmon and sea trout logbooks for angling and commercial fishing.
A link to all of these documents can be found here.