George Novak

Signs of Life

Like a planet in space, a rainbow trout egg sparks and crackles as biological processes begin a miraculous transformation, the same that progresses silently in the inscrutable waters of New Zealand’s wild rivers every day. But even in clean rivers, the odds are stacked against this small vessel of life—only one in a thousand eggs will hatch and survive until adulthood.

Written by       Photographed by George Novak

Even in the first days of life, the eggs are vulnerable. Those that are invalid at laying can become infested with Saprolegnia, a cotton wool-like fungus that can spread to healthy eggs also, sometimes resulting in the death of the whole clutch.
Even in the first days of life, the eggs are vulnerable. Those that are invalid at laying can become infested with Saprolegnia, a cotton wool-like fungus that can spread to healthy eggs also, sometimes resulting in the death of the whole clutch.
At 10-times magnification, the camera’s lens takes us inside the egg, into a rich and sheltered environment. The developing jelly-like embryo is connected to a yolk sac, its primary source of nutrition, but also pillowed by bubbles of oil that fuel the rapid cell division taking place and make up deficiencies in the diet once the alevin begins feeding. Although sheltered by the shell of the egg, the embryo is still susceptible to pollution in the environment, especially in New Zealand rivers, which are high in nitrogen from farm run-off, effluent and fertilisers. While the nitrogen itself doesn’t seem to hamper development of the embryo, it is a stimulant for algae and fungi, often leading to large mat-like growths that smother aquatic life. After just one week, this embryo has developed an unpigmented eye, mouth and the yolk sac critical for its nourishment over the next 40 days. At this stage, the embryo is so delicate that even small movements of the egg can halt development forever. Egg incubation is measured in ‘degree days’—about 280 for rainbow trout (28 days at 10ºC), 340 for brown trout and about 500 for Quinnat salmon. (Some commercial salmon farms adjust the temperature of incubation to hatch eggs at a specific time.) Ten days before the alevin hatches, its eyes become visible through the shell of the egg (below), which is now quite robust and can be handled with little risk to the developing embryos.
At 10-times magnification, the camera’s lens takes us inside the egg, into a rich and sheltered environment. The developing jelly-like embryo is connected to a yolk sac, its primary source of nutrition, but also pillowed by bubbles of oil that fuel the rapid cell division taking place and make up deficiencies in the diet once the alevin begins feeding.
Although sheltered by the shell of the egg, the embryo is still susceptible to pollution in the environment, especially in New Zealand rivers, which are high in nitrogen from farm run-off, effluent and fertilisers. While the nitrogen itself doesn’t seem to hamper development of the embryo, it is a stimulant for algae and fungi, often leading to large mat-like growths that smother aquatic life.
After just one week, this embryo has developed an unpigmented eye, mouth and the yolk sac critical for its nourishment over the next 40 days. At this stage, the embryo is so delicate that even small movements of the egg can halt development forever.
Egg incubation is measured in ‘degree days’—about 280 for rainbow trout (28 days at 10ºC), 340 for brown trout and about 500 for Quinnat salmon. (Some commercial salmon farms adjust the temperature of incubation to hatch eggs at a specific time.)
Ten days before the alevin hatches, its eyes become visible through the shell of the egg (below), which is now quite robust and can be handled with little risk to the developing embryos.

trout_halfhatched

A newly-hatched alevin nudges a sibling that is yet to hatch. Clearly visible are the blood vessels in the yolk sac and the brain cap over the eyes where the top of the skull will form. The eyes are the most distinct feature, but the developing dorsal fin on the back and the blood-rich gills behind the eyes are also obvious. Rainbow trout choose to breed in winter in rivers with a moderate flow and gravel bottom. Hard stones such as granite or greywacke provide gaps for the eggs to fall into and a good surface for algae growth and insect production to provide food for the emerging fry. Although pumice is the predominant geology in the Volcanic Plateau —which includes Taupo and the Rotorua lakes—it is more mobile and has fine particles that tend to seal the gaps around the developing eggs, depriving them of oxygen. Egg survival in pumice is very low, a reason for the man-made trout hatchery in Rotorua, which produces up to 100,000 one-year-old fish for the local lakes each year.
A newly-hatched alevin nudges a sibling that is yet to hatch. Clearly visible are the blood vessels in the yolk sac and the brain cap over the eyes where the top of the skull will form. The eyes are the most distinct feature, but the developing dorsal fin on the back and the blood-rich gills behind the eyes are also obvious.
Rainbow trout choose to breed in winter in rivers with a moderate flow and gravel bottom. Hard stones such as granite or greywacke provide gaps for the eggs to fall into and a good surface for algae growth and insect production to provide food for the emerging fry. Although pumice is the predominant geology in the Volcanic Plateau —which includes Taupo and the Rotorua lakes—it is more mobile and has fine particles that tend to seal the gaps around the developing eggs, depriving them of oxygen. Egg survival in pumice is very low, a reason for the man-made trout hatchery in Rotorua, which produces up to 100,000 one-year-old fish for the local lakes each year.
At extreme magnification, the formation of the upper and lower jaw of the alevin is obvious, and painterly scarlet lines trace the course of major blood vessels. Fine dark lines above the eye are the first signs of the coloration that will protect the fish from predators. The eye, which will be the primary mechanism for hunting, is already well developed. Fish have no eyelids, no iris and a much larger pupil to allow as much light to enter as possible. This results in relatively low visual acuity (or ‘resolution’) but phenomenal light sensitivity—a trout can find and catch a black fishing fly on a dark night. Rather than an iris, trout use pigments to filter bright light, which can take several minutes to take effect. The alevins become fry when the yolk sac has been used up, and grow into parr, which show vertical bars on their flanks as they develop the coloration associated with camouflage. Over their lifetime, the young rainbow trout will double their weight as many as 14 times as they grow from a 300-milligram egg into an adult weighing more than five kilograms.
At extreme magnification, the formation of the upper and lower jaw of the alevin is obvious, and painterly scarlet lines trace the course of major blood vessels. Fine dark lines above the eye are the first signs of the coloration that will protect the fish from predators.
The eye, which will be the primary mechanism for hunting, is already well developed. Fish have no eyelids, no iris and a much larger pupil to allow as much light to enter as possible. This results in relatively low visual acuity (or ‘resolution’) but phenomenal light sensitivity—a trout can find and catch a black fishing fly on a dark night.
Rather than an iris, trout use pigments to filter bright light, which can take several minutes to take effect.
The alevins become fry when the yolk sac has been used up, and grow into parr, which show vertical bars on their flanks as they develop the coloration associated with camouflage. Over their lifetime, the young rainbow trout will double their weight as many as 14 times as they grow from a 300-milligram egg into an adult weighing more than five kilograms.
After a month, the alevins have consumed the contents of their yolk sacs and emerged from the gravel as fry. The two sides of their abdomens—where the yolk sac once was—have fused, a process known as buttoning up. They are now well developed, with strong fins, and colours and stripes to avoid detection by predators. But their fight for survival is only just beginning—the average survival rate from egg to adult is as little as one in a thousand.
After a month, the alevins have consumed the contents of their yolk sacs and emerged from the gravel as fry. The two sides of their abdomens—where the yolk sac once was—have fused, a process known as buttoning up. They are now well developed, with strong fins, and colours and stripes to avoid detection by predators. But their fight for survival is only just beginning—the average survival rate from egg to adult is as little as one in a thousand.

Growth Rate

Rainbow trout eggs are approximately five millimetres in diameter and weigh about one third of a gram when they are laid into a depression in the gravel river bed, or spawning redd, by the female trout. On hatching the fry unfurl to about 10 mm long and remain hidden in the gravel feeding off the yolk sac for three weeks. They emerge into a world where everything wants to eat them and have to find shelter from both the fierce flow of the river and their predators. The survivors quickly find shelter in shallow backwaters and start looking for food. From here on their lives are dominated by the requirement for shelter, and their growth rate determined by food availability and water temperature. The faster they grow, the safer they are, and because they are cold-blooded and a cold water fish, they grow best in the chill waters of spring. As water temperature increases the fish use more of the energy derived from food for the metabolic needs of their body and have less left over for growth. Eventually, when the temperature gets too high, growth stops altogether and they struggle to survive unless they can seek out cool water in deep lakes or spring-fed inflows.

+ Read sidebar

More by

More by George Novak