Below the Surface: Science behind spawning

It’s a miracle that any vulnerable eggs and fry reach adulthood. Here’s what happens after coarse fish spawn

IF YOU look in the margins of rivers and pools over the next few weeks you might be lucky enough to see one of nature’s great miracles. While actually seeing coarse fish spawn is a rare, the tiny off-white eggs can often be found in vast quantities in the margins. Look for these pin-prick balls of jelly on reeds, submerged plants and willow tree roots and water moss. A few days later you may be able to spot the tiny embryonic fish, no more than a centimetre long in the shallow margins. Over millions of years, each spring has seen millions of coarse fish across Europe engage in the annual spawning ritual. The number of eggs laid in the short spawning window for each species is incalculable, but must be a truly astronomical number. Yet, from these potential fish, only a tiny fraction will survive for even a few days. Which begs the questions why should spawning be so wasteful and why has a less wasteful system not evolved? Each egg represents a tiny life-support system that must protect and feed the tiny embryo. Inside the jelly-like protein ‘shell’ of the egg is the off-white yolk. Rich in protein and fat, it is the ideal first food for the developing fish. From a tiny black speck to a wriggling embryo, it only takes around a week for the eggs to hatch. At this stage the tiny fish are nothing like the adults. Lacking fins and gills, they still possess a large yolk sack that will sustain them for several more days until they have developed sufficiently to start feeding. Fish eggs and embryos are totally reliant upon the environment in which they have been deposited by the adults. With no way of moving, if the conditions become too hot, or they are left high and dry, or perhaps silt builds up on them, then they will die. Almost all coarse fish are disperse spawners. This means that they lay their eggs over a random area of the bottom or on plants that have the right criteria for spawning. Once the eggs are released they have to fend for themselves with the parents having no further contact. Compared with fish that do look after their eggs and young, coarse fish have a very low survival rate. Often, only one in a hundred eggs will survive the few short days leading up to them hatching. If the parents lay the eggs on the wrong substrate then egg survival can be zero. Although wasteful, we have to remember that a single coarse fish will lay many thousands of eggs. A 1lb roach, for example, can lay around 70,000 eggs each year! What is lost in low survival is compensated by the sheer number of eggs laid.

In the week or two that it takes the eggs to hatch they face many dangers. Predation from invertebrates, such as dragonfly larvae, can have a surprisingly large impact, with studies showing that a high percentage of eggs can be eaten, especially among plant spawning species. Other fish will also make a feast of fish eggs. Shoals of roach will often gather when carp are spawning to feast on the freshly-laid eggs, and few coarse fish will pass up this protein-rich free meal. Eels are particularly drawn to spawning activity, and can often be found in large numbers just after other coarse fish have spawned. The colder the weather, the longer it takes the tiny fish to develop so hatching will be delayed. In years when the weather turns cold after the fish have spawned, the numbers of larvae surviving can be low. The health and size of the parents can have a large bearing on the chances of survival of eggs and young fish. Bigger lay more eggs and they will also be considerably larger than the eggs of smaller individuals. Larger eggs have a higher chance of survival and the young hatchlings may be larger too, an advantage. Coarse fish may live long lives, maybe 2050 years. Apart from their first few years, these fish will spawn every year but, to guarantee their genes are passed on to future generations, there only needs to be one egg that survives until adulthood for each parent. In bumper years when the conditions are more suitable and egg survival is higher, then many more fish may survive and this can lead to a rapid increase in the number reaching adulthood. If the eggs were much larger, so less of them were laid, or the parents invested much more effort in looking after them, then in the good times they would be less able to make the best of the favourable conditions. Laying lots of tiny eggs and abandoning them to their fate is a bit like an insurance policy. In the bad years there may be no return on the energy invested in the eggs, but in the good years the population can flourish. So laying lots of tiny eggs is a gamble but, in an unpredictable environment it is a calculated risk, one that has seen coarse fish survive for aeons.

“What is lost in low survival is compensated by the number of eggs laid”