Most, if not all, North American freshwater fish species reproduce only once a year. Some, such as lake trout and brook trout, spawn in the autumn. Most others spawn in spring and summer. But how do they know when conditions are right? How do they all coordinate to ensure spawning successfully produces baby fish each year?
Without a calendar to consult, fish have to rely on their senses to tell them when environmental conditions are right for spawning. For many temperate lake species, like sunfishes, the most reliable cue to predict when to spawn is water temperature. Water temperatures in lakes follow a predictable pattern each year: warming in late spring and summer as day lengths get longer and cooling by late summer and fall as day lengths shorten. Over millennia, the pumpkinseed sunfish have settled on about 65-70 degrees Fahrenheit as their trigger for the initiation of spawning.
But what if water temperatures are constant, say the 68 degrees that our pumpkinseed sunfish experience in our Upper River tank in the Land of Opportunity at ECHO? Do they spawn constantly, or not at all?
As it happens, pumpkinseed sunfish also are sensing the length of each day. Day lengths are even more predictable than water temperature and not dependent upon weather patterns which can change rapidly within the Basin. A pumpkinseed sunfish's hormone levels are primed by longer days so that it is ready to spawn when temperatures hit the mark. If water temperatures are always above the mark, like here at ECHO, then the length of the day tells our sunfish when the time is right. In the past month, the pumpkinseed sunfish in our Upper River tank have initiated their spawning behavior; the same behaviors will likely not be observed in Lake Champlain for several more week.
So what happens? The males in the population stake out territories in shallow water by creating saucer-shaped nests, fanning silt away from the bottom and not letting other fishes near their nesting site:
When a female arrives, the male engages her in a dance-like spawning ritual. They swim in circles together, and the female dips to the side as they spawn:
Bluegill sunfish also engage in very similar behavior:
Once the male has spawned, often with multiple females, he continues to guard his nest from intruders until the eggs hatch and his offspring swim away together in a cluster.
While the behavior observed here at ECHO is interesting, there are larger concerns when we create changes in water temperatures in the wild. Two areas of concern are industrial cooling water releases and water releases from dams. Water is often a cheap and available resource for industrial operations for cooling power plants and other applications. Heat is essentially released as a by-product and is often regulated as a pollutant because of its effects on aquatic ecosystems, including alteration of animal life-cycles. When water is released from large reservoirs for energy production or flood control, temperatures vary from warmer surface waters to cooler (and sometimes oxygen-deprived) deeper waters. Again, there are regulations that require dam operators consider the type of water that they release. As with any human activity, there is always more to understand about our effects on the ecosystems that support us, and more creative solutions to the problems we cause that are yet to be discovered.