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Reproduction dependent on habitat quality
Pike-perch generally reach sexual maturity around 2 to 3 years for males and 3 to 4 years for females. The corresponding size is often between 40 and 50 cm, depending on the environment. As with pike, few individuals reach this stage. Each breeding adult is already the product of an extremely severe natural selection process.
For reproduction, pike-perch need clean substrates, moderately turbid water, stable spring temperatures and relatively stable water levels. The artificialization of banks, the disappearance of drowned wood, the tidal range of hydroelectric reservoirs or sudden variations in water level can compromise the success of egg-laying. Rapid tidal changes can dry out or silt up the nest.
Spring warming triggers reproduction
Pike-perch spawn later than pike. It generally begins when the water reaches 12 to 15°C, usually between April and May, depending on the region and altitude.
Unlike pike, zander do not lay their eggs on flooded vegetation. The male, now adorned with a "charcoal" coat due to the increase in reproductive hormones, selects a hard substrate: gravel, roots, drowned wood, compact sandy areas. It carefully cleans the surface with its caudal fin and body movements. This creates a circular bowl 30 to 80 cm in diameter. Spawning can then take place.
High fertility... but targeted
The female deposits her eggs in the nest prepared by the male. The eggs are slightly adhesive and attach themselves to the substrate. Fecundity is high: between 150,000 and 200,000 eggs per kg of female. A 4 kg female can therefore produce over 600,000 eggs, but unlike the pike, success does not depend on numbers alone. It depends on parental care.
After laying, the female leaves the area. The male stays behind. For 8 to 15 days, he ventilates the eggs by creating a water current, eliminating any fine particles that may have settled on the eggs and attacking any intruders with great virulence. Ventilation is vital, as pike-perch eggs are susceptible to asphyxiation and fungal growth. Without water circulation, mortality rises rapidly.
As with most fish, pike-perch embryonic development depends on cumulative heat. It is estimated that it takes between 110 and 130 degree-days to reach hatching. Schematically, at 13°C, hatching takes place in 9 to 10 days, while at 15°C, it can take 7 to 8 days. If the temperature drops abruptly, or if the male abandons the nest, incubation may be compromised.
Misleading survival ratios
While 600,000 eggs may seem to guarantee abundant offspring, in reality, natural selection is severe. In good conditions, 50-80% of eggs can hatch if the nest is properly tended; without parental care, mortality can exceed 90% within a few hours.
There is also strong predation on the little treats formed by fresh eggs on a nest. The main predators of eggs are breams, perch, gobies, crayfish, sun perch, catfish and a few opportunistic cyprinids. If the male is removed from the nest, even briefly, it can be emptied within minutes.
Once hatched, the larvae remain briefly clustered above the nest, then disperse into the water column. At this stage, they become planktonophagous and extremely vulnerable. In natural environments, less than 5% of larvae reach the juvenile stage (a few centimetres long), less than 1% reach the age of one year, and only between 0.2 and 0.5% reach sexual maturity.
In other words, out of 600,000 eggs laid, only a few hundred will produce yearling pike-perch, and only a handful will reproduce. What's more, pike-perch are piscivorous very early on, as soon as they reach a few centimetres in length. A growth differential is enough to trigger opportunistic cannibalism. In closed or densely populated environments, this phenomenon can account for a significant proportion of summer mortality. As with pike, this is a natural regulatory mechanism, but it becomes more pronounced when habitats are simplified or alternative food resources are lacking.
A species particularly sensitive to nest fishing pressure
The male's territorial behavior is the pike-perch's main vulnerability. During the breeding season, it attacks any intruder, actively defends its nest, and therefore becomes very easy to capture. And therein lies the problem. Unlike other species, catching a male pike-perch is not a trivial matter. Even if he is quickly released, he may lose the precise location of the nest, abandon custody or return too late. And in his absence, the eggs are consumed almost immediately. A single capture can destroy several hundred thousand eggs.
Why closure makes sense
The pike-perch breeding season lasts just a few weeks. Yet it is the key to several years of recruitment. During this thermal window of around 120 degree-days, everything depends on the continued presence of the male at the nest. Targeted fishing pressure on territorial individuals can have an impact disproportionate to the number of catches made. Closure therefore protects a specific biological behavior, not just a population.
Behind a clutch of several hundred thousand eggs, the reality is simple: only a few individuals will reach adulthood.The pike-perch is not an inexhaustible resource. Its reproductive strategy, effective in a stable environment, becomes fragile in the face of human disturbance. Respecting the spawning period means giving the male time to ventilate the eggs, to reach the thermal accumulation necessary for hatching, and to give a new generation a chance to exist.
