Introduction to Pond Fish
The breeding triggers for coldwater fish are the rise in water temperature and increasing day length that occur in spring. Spring also sees an increase in insect life and aquatic crustaceans, providing food to bring adult fish into breeding condition and to sustain the fry. The breeding season is thus more prescribed than in many tropical species, for whom rainfall rather than temperature is the most significant factor.
You can estimate when your fish are likely to breed by taking regular measurements of the water temperature in the pond, since spawning typically occurs at around 68°F (20°C). This temperature ensures that the eggs develop at the correct rate: at more than 9°F (5°C) above or below this figure, there is an increased likelihood that the fry will hatch with deformities, because they will develop either too quickly or too slowly.
Physical changes in the fish will also indicate that they are coming into breeding condition. For example, males of the Cyprinidae family display white swellings called tubercles. In fish such as orfe, rudd, goldfish, and koi, the tubercles appear on the gill plates and along the pectoral fins, while in Red Shiners they run along the top of the head. In some groups, such as sticklebacks, the males become more brightly colored to attract mates and deter rivals. In all coldwater species, the females become fatter-bodied than their male counterparts as they swell with eggs prior to spawning. There is also a significant increase in activity in the pond at this time, as the males chase the females relentlessly, often butting or nuzzling against them.
- Tubercles are white, pimplelike lumps that male cyprinids, such as Shubunkins, develop when they are in spawning condition. The tubercles may help to arouse the female when they are rubbed against her body during courtship.
- A gravid female goldfish has a more rounded body profile, but she rapidly regains her normal shape after spawning. She may spawn several times during spring and summer, producing several thousand eggs in total.
Coldwater pond fish show no long-term pair-bonding; any pairings that do occur are purely temporary. Fertilization is external, with eggs and sperm being released into the water simultaneously. Most pond species, including goldfish and koi, are egg-scatterers. They randomly discharge their sticky eggs, which either sink to the substrate or attach to the stems and leaves of aquatic plants. Only a small proportion of eggs will be fertilized, so the fish compensate by producing large numbers of them—up to 400,000 per spawning in the case of koi.
After spawning, egg-scatterers have no further involvement with either their eggs or offspring, but some pond species take more care to ensure that the maximum number of young will survive. The Fathead Minnow (see p.360), for example, lays its eggs in caves or under rocky overhangs in order to hide them from predators, while male sticklebacks keep a protective watch over both their eggs and the newly hatched fry.
Early life of fry
The reason that coldwater fish spawn in the spring is that this is the time of year when conditions are most favorable for the survival of the young. The algal bloom that grows in spring, and which is often cursed by fishkeepers, is actually crucial to the survival of the fry, since it provides them with their first food. The young fish eat not only the algae but also the microscopic creatures called infusoria (see pp.67–68) that live among them.
Dragonfly larvae, water boatmen, and many other pond invertebrates—not to mention fish (including the fry’s own parents)—will readily prey on the young fish. As a result, the fry spend most of their early weeks hiding among aquatic vegetation, rarely straying far from plant cover. It can take between one and seven years for the fish to reach sexual maturity, depending on the species and the temperature of the water in the pond.
FROGS VERSUS GOLDFISH
In spring, frogs often visit garden ponds to spawn at the same time that goldfish are breeding. On rare occasions, this results in frogs accidentally killing goldfish. If a goldfish swims past a male frog, he may grab the passing fish in the mistaken belief that it is a female of his own species. If the frog grips the fish by its head, closing off its gill covers, the goldfish will suffocate, since the frog’s mating embrace lasts a long time, often for hours. There are casualties on both sides, however, since goldfish will sometimes prey on frog tadpoles in their pond. (They avoid toads, which are toxic to them.)
GOLDFISH EGGS AND FRY
The incubation period of goldfish eggs and the growth rate of the fry are both temperature-dependent: generally, the warmer the water, the more rapidly the young develop. The fry, which measure less than 1 ⁄4 in (0.5 mm) long on hatching, are nourished at first by their egg sacs. After a few days, they are free-swimming and actively seeking food.
- Goldfish embryos can be seen curled up inside their eggs in the closeup view above. The eggs usually develop on oxygenating plants (left), held in position by their sticky coating.
- Newly hatched goldfish fry hide among vegetation for about two months. By the time they emerge, they have dark-colored bodies to camouflage them in their murky surroundings.