Chapter 4: Control of the Spawning Process
The natural spawning by fish is a complex interaction of factors often dependent on a number of environmental factors, some of which we can provide in a hatchery setting, others we cannot. In the proper range and combination, these environmental factors can impact the fish’s physiology, causing a cascade of hormone production resulting in ovulation.
Temperature is perhaps the most important environmental factor affecting fish reproduction, as it impacts growth rate, time when first maturity is reached, and how quickly new eggs develop from one spawning cycle to the next. In a north temperate region, over a species natural range there may be a year or more difference in the times it takes for a fish to first reach maturity. Similarly, the length of the growing season is affected by temperature. A given species may be able have enough nutrient intake in a long growing season to spawn every year, but at the northern part of its range the growing season is shorter, and it may take two seasons to take in enough nutrients to complete a spawning cycle.
Each species has an optimum range of temperature, often no wider than 5°C, where it can successfully reproduce. How long the temperature stays within that temperature range influences the length of the spawning season. In a north temperate climate, the temperature rises in the spring into the appropriate range for spawning. The temperature rise helps to synchronize brood maturity, resulting in the majority of the brooders beginning mature at the start of the spawning season. In a more tropical setting there is less seasonal change in temperature. In such a setting, the same species will have some mature brooders throughout a wider spawning season, but at any one point in time only a portion of the brooders are mature enough for spawning.
In temperate climates not only does the temperature rise in the spring but the hours of daylight increase as well. In a natural setting it is hard to separate the effects of temperature and photoperiod on spawning. Under controlled conditions both temperature and photoperiod can be manipulated to impact spawning. At warmwater hatcheries fish spawning can be delayed in the spring if cool water from a well or other source can be added to the brood holding pond. Similarly, moving the fish into a warm-water setting can advance the spawning season. Under conditions where temperature and photoperiod can be controlled, spawning seasons can be significantly advanced or delayed, or the spawning cycle condensed from a one spawning period/yr. to two or more periods/yr. Lunar phase is another factor influencing natural spawning of a number of marine species and some species of freshwater sunfish. The lunar cycle is regulated under controlled hatchery conditions for a number of marine species, some species spawn more frequently around the dark of the moon and other species around a full moon. The lunar cycle is not commonly regulated in freshwater hatcheries.