Yeah...I just saw that too...
The difference here is that although it is a public park, the park is managed by City of Brampton...when then is a different kettle lake of fish...
Looking at this issue deeper...
From the Fish and Wildlife Conservation Act...
PART II
HUNTING, TRAPPING, FISHING AND RELATED ACTIVITIES
General Restrictions
Trespassing
10. (1) A person shall not,
(a) enter premises in contravention of the Trespass to Property Act for the purpose of hunting or fishing;
(b) enter premises in contravention of the Trespass to Property Act in possession of a firearm, fishing rod or other hunting or fishing device;
© engage in hunting or fishing in contravention of the Trespass to Property Act;
(d) fail to leave premises in contravention of the Trespass to Property Act, if the person was on the premises for the purpose of hunting or fishing; or
(e) fail to leave premises in contravention of the Trespass to Property Act, if the person was on the premises in possession of a firearm, fishing rod or other hunting or fishing device.
Notice under Trespass to Property Act
(2) The Minister may authorize a person to give notice for the purposes of the Trespass to Property Act with respect to hunting or fishing on Crown land.
Interference with notice
(3) A person shall not interfere with any signs or markings that, under the Trespass to Property Act, give notice that,
(a) hunting or fishing is prohibited; or
(b) entry is prohibited for the purpose of hunting or fishing.
PART V
LAWS OF OTHER JURISDICTIONS
Laws of other jurisdictions
58. (1) A person shall not possess wildlife, invertebrates or fish that,
(a) were killed, captured, taken, possessed, transported, bought or sold contrary to the laws of another jurisdiction; or
(b) were removed from another jurisdiction, contrary to the laws of that jurisdiction.
Sorry...that's just how the chip falls...I guess.
But as a resident of Brampton, you can certainly challenge those By-law as OT pointed out...with the help of your local representative. That's what we have done in Toronto by challenging Toronto Park and Rec and actually had the No Fishing signs revoked.