Opie's observation regarding the increased numbers of herons in fall and winter was probably a major contributor to the declining overwinter survival of fall stocked fingerlings during the years that the PFBC stocked them there. Fall and winter flows are often low, which benefits avian predators and when most other waters are frozen, there is very little if any ice on the Tully, making it and local spring-fed limestoners a magnet for herons in the region. Additionally, juvenile heron behavior is such that they migrate north in late summer before they head south, so as a result SE Pa sees an inflow of herons from the Chesapeake and Delaware Bay nesting regions in Aug and Sept.
The population of herons going back to Blue Marsh's construction (first impounded in 1979) has been aided by the increase in "heron feeding habitat" and the increased abundance of fish caused by creation of the lake. That has undoubtedly spilled over to enhance heron abundance on the creek. Additionally, while there may be a debate as to whether there was once one heron rookery in the area or none prior to Blue Marsh's construction, there is no debate that there are at least three now. My observation has been that the numbers of herons present along the stream in spring and early summer is substantial as well.
Great Blue Herons are quite capable of consuming one to three trout per day and there may be a "night crew" that is separate from the "day crew" that most anglers see, meaning that predation is taking a bigger toll than most anglers know. Yes, herons feed at night. One only needs to do a lot of night electrofishing or fishing along lakes and rivers, drive along the causeway at Pymatuning at night, or have an office that looks out onto a lake, as I do, to see the evidence.
