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

Improve Your Coarse Fishing (UK) - - Contents -

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”

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