I guess it depends on the type of fishing.
If you are fly fishing, having waded out in the river and fighting the current, or big game fishing where you really have to fight the fish to land it, or perhaps commercial fishing where bringing in nets or the like is genuine physical work, then potentially you could burn plenty of calories.
But if you are just sitting next to a rod, well, all you are doing is ... sitting.
Also, Spark's activity tracker assumes continuous activity. Depending on how far you went, etc, a 3 hour fishing expedition may only involve 90 minutes active fishing, with the rest of the time just sitting as you transit to and from the fishing spot, etc.