The pis­ca­tory arms race

The bat­tle between preda­tory fish and their prey is a never-end­ing war of at­tri­tion across gen­er­a­tions with con­tin­u­ous evo­lu­tion

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FISH have evolved many strate­gies to avoid be­ing eaten. Some, such as shoal­ing, are more ob­vi­ous than oth­ers, but over mil­lions of years each be­hav­iour and trait has given them a slightly bet­ter chance of sur­vival, lead­ing to its grad­ual spread through­out the pop­u­la­tion. Those fish with a handy trait will have a slightly bet­ter chance of sur­vival. Mul­ti­plied over many gen­er­a­tion this will see their prog­eny be­com­ing more suc­cess­ful. This is pure Dar­winian evo­lu­tion – small changes, mag­ni­fied by time, giv­ing im­proved sur­vival. Of course, it is not just the prey fish that are evolv­ing. The preda­tors will be adapt­ing just the same, giv­ing rise to an evo­lu­tion­ary arms race as preda­tor and prey evolve in uni­son. One of the most com­mon strate­gies that is thought to have evolved in re­sponse to pre­da­tion pres­sure is shoal­ing. Just as in herd­ing an­i­mals on land, many in­di­vid­u­als have a much bet­ter chance of spot­ting a po­ten­tial dan­ger sooner than an in­di­vid­ual. This means that the risk of be­ing eaten for each fish in a shoal is less than if they were on their own. Shoal mates can also spend more time feed­ing, which helps to off­set the ex­tra com­pe­ti­tion they face by liv­ing in such close prox­im­ity to oth­ers. Shoal­ing is only ef­fec­tive when fish can quickly com­mu­ni­cate the dan­ger to oth­ers. This is some­thing that they can do in lit­er­ally a blink of an eye as their body lan­guage trans­mits the alert sig­nal to their shoal mates. A stiff­en­ing of the fins, jerky move­ments and a bend­ing of the back are all sig­nals of ap­proach­ing dan­ger. Of course, preda­tory fish such as pike, have learned to com­bat the many eyes of a shoal of fish. The bro­ken cam­ou­flage pat­tern pro­duced by the ver­ti­cal stripes on a pike help to dis­guise it. The slow, tiny move­ments of a pike also do not draw at­ten­tion to it, en­abling it to creep up close to prey fish. Per­haps the big­gest adap­ta­tion that a pike uses is to hunt mainly at dawn and dusk when light lev­els are low. With their large eyes pike are able to op­er­ate well in the dull gloom of early

morn­ing, while the less acute eyes of roach are not able to op­er­ate as ef­fec­tively, giv­ing the pike a big ad­van­tage. Good eye­sight, es­pe­cially in low light, is a use­ful trait for a preda­tor. Big cats and owls share the same abil­ity as many preda­tory fish. Zan­der share the same adap­ta­tion as lions, hav­ing a layer of re­flec­tive cells in the back of the eye, called the tape­tum, which re­flect light back on to the cone and rod cells in the eye­ball. This gives them greatly im­proved eye­sight in low light, en­abling them to hunt ef­fec­tively even un­der the re­stricted light of a new moon. Spines are an­other use­ful weapon against pre­da­tion, and perch use these to good ef­fect. The suc­cess of pike hunt­ing perch is sig­nif­i­cantly worse than hunt­ing roach of a sim­i­lar size. This can be put down to the spines, which in­crease the time it takes for a pike to swal­low a perch and make its ini­tial catch less suc­cess­ful. Preda­tors can evolve to deal with a wide range of prey and, gen­er­ally speak­ing, the most com­mon prey fish in a body of wa­ter will make up the bulk of a pike’s diet. This has been seen most clearly in lakes where new, highly suc­cess­ful, species have been in­tro­duced. Here, the pike switch to feed­ing on the most abun­dant prey and adapt their be­hav­iour ac­cord­ingly. Not all preda­tors are able to con­sume all prey, and many are lim­ited to feed­ing on a nar­row range of prey sizes. Pike, for ex­am­ple, can con­sume prey at least as large as their mouth gape, but are much more ef­fec­tive when han­dling prey that are no more than 60% as long as their mouth gape. For a prey fish it is im­por­tant to grow fast and min­imise the time that they are a handy snack size for a preda­tor. The same ap­plies for prey fish of all sizes, from be­ing tiny fry, where most of their preda­tors will be in­ver­te­brates, such as wa­ter scor­pi­ons and dragon­fly lar­vae, to young adults which may be sub­jected to cor­morant pre­da­tion. Prey fish pop­u­la­tions can also, over a few gen­er­a­tions, al­ter their body shape to re­duce their risk of be­ing pre­dated. In lakes with pike, cru­cian carp tend to have a rounder body and deeper pro­file than in lakes with no pike. The deep body makes the cru­cians more dif­fi­cult to eat, so their sur­vival is slightly bet­ter. When pike are in­tro­duced to lakes where they were pre­vi­ously ab­sent the cru­cian pop­u­la­tion switches to deeper bod­ied fish in just a few years, hint­ing at the ben­e­fits of chang­ing shape. To en­sure that they are big­ger than their prey, preda­tory fish will nor­mally lay their eggs a few weeks be­fore their prey. Pike, for ex­am­ple, spawn in late March, whereas their main prey in many wa­ters – roach – spawn in mid-May. By the time the roach eggs hatch the pike are al­ready big enough to start hunt­ing and eat­ing them, en­sur­ing that there is no easy es­cape for the poor roach. Many fish use cam­ou­flage to help re­duce their risk of be­ing eaten. If you look at most coarse fish they have a dark back and a lighter coloured belly. When viewed from above the dark back blends in with the bot­tom,

“Pike are more ef­fec­tive when han­dling prey that are no more than 60% as long as their mouth gape”

mak­ing them more dif­fi­cult for preda­tory birds to spot. From a pike’s view­point look­ing up­wards, the re­flec­tive belly blends in with the bright sur­face, again af­ford­ing them a de­gree of cam­ou­flage. Pre­da­tion is cer­tainly one of the strong­est pres­sures driv­ing evo­lu­tion. Although it nor­mally hap­pens at a glacial speed, of­ten tak­ing hun­dreds of gen­er­a­tions and thou­sands of years to give even small changes, evo­lu­tion has given us the in­ter­est­ing preda­tor-prey dy­nam­ics that we see to­day.

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