Every year, shortly after the ice disappears, torpedo-shaped fish slip into the grassy shallows of the lake from this area offshore where the bottom collapses. First comes the female, her green and gold sides, and her ovaries swollen with eggs. The male swims alongside, alert to the opportunity to mate. Over the course of a day or two, they periodically turn their vents towards each other and simultaneously release eggs and milt. In the next moment, they will whip their tails to spread the fertilized eggs over the submerged vegetation. It’s chain pickerel spawning season, and every egg faces an uncertain future.
Fish eggs are gelatinous packets of proteins and fats that bear the imprint of life. In these packages, the poets could see hope; but predators, including other fish attracted to the smell of spawning hormones, see a resistance-free, nutritious meal. Through the evolutionary process of natural selection, egg predation has forced fish and other aquatic animals to develop various traits and behaviors that increase the chances of survival.
Chain Pickerel thwarts the threat of egg predation through a reproductive strategy that is largely volume dependent. Females can lay up to 12,000 eggs for every pound of their mass. Although the eggs are partially hidden among the plants, they receive no protection from either parent. After spawning, both males and females leave them unattended.
Yellow perch also lay and abandon eggs in seagrass beds, but theirs are protected in an outer shell that connects them in a hank. Research by Zoe Almeida and others at Purdue University found that predators can be deterred by harmful chemicals in the skein. Researchers have also observed crayfish attempting to extract yellow perch eggs from their shells, but without success. In aquarium experiments, crayfish and round gobies — a Eurasian fish that invaded the Great Lakes — preferred exposed eggs when presented with the option.
White perch lay eggs differently than yellow perch, as you would expect since white perch is actually a member of the temperate bass family, introduced to inland New England lakes and ponds from its native coastal waters. For about two weeks in late spring, females join males in large spawning groups. This behavior allows fertilization by multiple males, mitigating the risk of combining chromosomes with a “poor quality” male. Females also disperse eggs widely over various substrates, which can help some go undetected. As with chain pickerel and yellow perch, the eggs are left defenseless.
Brown bullheads are more cautious when it comes to choosing mates and raising young. Also known as catfish and retort, members of this species mate in late spring. The female lays her eggs under or near protective cover, normally in a saucer-shaped depression that she digs with her fins. A male will sometimes help prepare the nest, and both sexes provide protection, occasionally aerating and cleaning the eggs while guarding them.
When the eggs hatch after about a week, the young fish are reared by the male or both parents, rarely by the female alone. The adults protect the developing fry as they move through a tight shoal, which resembles a cloud of tadpoles. If you observe the shallow waters of a productive bullhead pond carefully, you may see adults guarding their young. Some observers have even reported seeing parents using their mouths to retrieve stray individuals and bring them back to school.
Fish are not the only group of animals that use a wide range of strategies to ensure their eggs survive in water. Amphibians and insects also exhibit a variety of behaviors. Eastern newts fold the fertilized eggs from the inside, one at a time, into the leaves of submerged vegetation. The common green dragonfly injects its eggs into plant stems just below the water surface. And some species of giant water bugs lay their eggs on the backs of their companions, who ventilate and protect them until they hatch.
It costs energy to produce, hide and defend eggs. Keeping them also involves the threat of predation and a commitment of time that could be spent foraging. On the other hand, leaving the eggs exposed also has its drawbacks. Over millions of years of evolution, aquatic species have evolved many adaptations to balance energy costs and lethal reproductive risks. For underwater eggs, it’s a survival game with countless ways to play.
Dan Lambert is the Associate Director of the Center for Northern Woodlands Education. Illustration by Adelaide Murphy Tyrol. The Outside Story is attributed to and edited by Northern Woodlands magazine (northernwoodlands.org) and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation at nhcf.org.