If you spend as much time prowling along the shallow margins of lakes as I do, you come across some very interesting scenes.
Early June is when most aquatic ecosystems really start to kick into high gear. Bugs are emerging and fish are spawning - including many of the bait-fish that larger trout like to feed on. If you have the patience, and move slowly enough, you can catch a glimpse of the spawning behavior one of the more interesting minnows in Alberta, the fathead minnow.
At this time of year, male fathead minnows start excavating nesting sites under objects resting on the bottom in shallow water. This usually includes rotten logs or pieces of wood, but they have also been known to nest (sadly) under bits of trash discarded by uncaring anglers. Once the nest is excavated, the male spends a lot of time defending the spot from other male interlopers intent on taking advantage of all his hard work.
Males are easy to identify at this time of year - they are thicker through the body and more colorful than the females, and they have dark heads with white nuptial tubercles. The tubercles are mostly used to tend and clean the eggs but may also play a role in defense as the male fights off other males or kicks spawned out females out of the nest to make room for the next suitor. Yes, the male allows multiple females to spawn in the nest and then guards all the eggs till they hatch - the better the nesting site, the more females the male gets.
Here is a short video that shows a male on his nesting site, followed by a male and female nesting together before the male evicts the female in anticipation of another hook-up.