I think Northern Pike grow in length all year around. Something to add to a slippery topic.
Do you have any scientific evidence of growth under the ice of northern pike? Northerns are active under the ice and may even build up their gametes under the ice. Northerns even spawn under the ice at times. However this doesn't necessarily mean they grow under the ice.
Here's something to add to the slippery slope. I've seen it stated in scientific literature several times, that even trout, that are a coldwater fish, cease to grow below 38 F. even if they still feed. According to literature, the feed goes to maintenance of weight and any excess is wasted. I even know one trout farmer in Harrisville, Michigan that was livid a neighbor that was growing out trout for him in his pond, had fed the trout when their was a brief ice out during the middle of winter. He was sure the neighbor had killed some of the trout doing that. He claimed it was really bad to feed them during the winter. He may be wrong of course (wouldn't be the first time a myth has been perpetuated among fish farmers and accepted as fact) but he sure believed it.
In trout, the minimum temperature
for growth is approximately 38°F. At
this temperature and below,
appetites may be suppressed and
their digestive systems operate very
slowly. Trout will require only a
“maintenance” diet (0.5 percent to
1.8 percent body weight/day, depending
upon fish size) at these temperatures;
more than this will result in
poor food conversion and wasted
feed.http://ag.arizona.edu/azaqua/extension/Classroom/pdffiles/223fs.pdf
