Can fisheries be thought of as uncon-trolled experiments in evolution? Inother words, do they produce genetic

change? Scientifically at least, the suggestionshouldn’t cause too much bother. Evolutionby natural selection involves pressures thatkill off individuals with ‘unfavourable’ inher-ited traits, while those with more ‘favourable’features survive and reproduce. Fisherieswould seem to provide such a pressure: mostfisheries target the largest and oldest individ-uals,so fish that are genetically predisposed tomature at larger sizes and older ages are morelikely to be caught before they can reproduce.Such selective harvesting should theoreticallyfavour early- and small-maturing genetictypes — a consequence reminiscent of othercircumstances1,2 in which humans have unin-tentionally selected against that which theydesire most.

Nonetheless,many fisheries scientists andmanagers seem reluctant to acknowledge thepotential for fishing to elicit genetic change3,despite supportive experimental4 and field5,6

data. On page 932 of this issue7, however,Olsen et al. provide compelling evidence ofgenetic change in one of the most notoriouslyoverfished stocks, northern Atlantic cod(Gadus morhua; Fig. 1), arguing that suchchange preceded the collapse of the stock.

The growing number of over-exploitedmarine fishes worldwide has created numer-ous opportunities to examine whether thecollapse of fish populations is associatedwith genetic responses to fishing. Amongthese populations, few have declined morethan the Atlantic cod that range from south-ern Labrador to the northern half of New-foundland’s Grand Bank. By the early 1990s,the numbers of northern cod had declinedby 99.9% relative to their abundance in theearly 1960s8 — a rate of decline almostunmatched among living terrestrial andaquatic species. Concomitant with thisdecrease in population size were significantreductions in age and size at maturity.

Having been documented repeatedly inexploited populations, it is incontestable thatfishing can lead to significant changes in life-history traits, such as age and size at maturity.The question is whether these changes arephenotypic (reflecting non-genetic variationin physical or behavioural characteristics) or genetic. Consider age at maturity, forinstance. As the density of a populationdeclines, relaxed competition for food and

space should lead to individuals growing atan increased rate. Fish generally respond toincreased growth by maturing earlier in life.Thus, fishing could lead to earlier maturitysolely as a consequence of variable pheno-typic responses to growth. Alternatively,however, by selecting against individualswhose genes predispose them to breed atolder ages and larger sizes, fishing mightgenetically alter exploited populations.

To disentangle phenotypic life-historyresponses to fishing from genetic responses,Olsen et al.7 use a new method9 that allowsthem to detect significant changes in age andsize at maturity independently of the effectsthat growth and survival can have on pat-terns of maturation. The method involvesestimating ‘probabilistic reaction norms’(Fig. 2a, overleaf), which describe the likeli-hood that immature individuals of a givenage and size will mature during a specifictime interval, assuming that that size and ageare theoretically enough to allow matura-tion. It is this assumption — that the indivi-duals are already old enough and big enoughto mature — that renders probabilistic reaction norms independent of the effects of growth and survival on maturation.(Although these trajectories are not ‘reactionnorms’ in the strictest sense of the word,as they would then describe how individ-ual genotypes respond to environmentalchange10, Olsen and colleagues’ terminologyis not unprecedented11,and can be justified if

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there is a large environmental component tovariability in individual growth.)

Olsen et al. find that, before the northerncod population collapsed,there was a declinein reaction-norm midpoints (the ages andsizes at which the proportion of mature fishis 50%). In other words, the norms shiftedtowards younger ages and smaller sizes. Theauthors assert that this decline is consistentwith the hypothesis12 that heavy fishing pres-sure selected against genotypes that predis-pose cod to maturing later and larger. Theyalso observe that these life-history changesare not associated with increased growthrate, which could lead to earlier maturity.Together, these findings suggest that geneticchange provides the most parsimoniousexplanation for why age and size at maturitydeclined in northern cod.

Of potentially greater interest is theimplication that fishing produced a geneticchange in the way in which northern codgenotypes respond to environmentallyinduced variability in growth rate. The ques-tion of whether fishing can mould the shapesof reaction norms (an indicator of this typeof change) was first raised a decade or soago13.The fitness benefits of delayed maturity— more eggs for females, more mates formales — decline as the risk of death fromexploitation increases. So one might predicta ‘flattening’of reaction norms with regard toage at maturity, such that individuals wouldbe favoured if they reproduce as early in life

The cod that got awayJeffrey A. Hutchings

Commercial fishing can reduce the age and size at which fish mature. But it has been unclear whether this reflects changes in genes or in physicalresponses to the environment. A look at Atlantic cod provides an answer.

Figure 1 Atlantic cod: test case for studying human-induced genetic change.

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as possible, and expend higher reproductiveeffort at that age14, irrespective of growth rate(Fig. 2b). With sufficiently high mortality,the potential for phenotypes to changemight decrease, leading to relatively invari-ant phenotypic life-history responses toenvironmental variability — a predictionborne out recently by work on Europeangrayling fish15.

In any case, the potential for fishing togenerate evolutionary change within har-vested populations can no longer be seriouslydiscounted.This may well be the most endur-ing contribution of Olsen and colleagues’research7. If evolutionary change in responseto harvesting proves to be the rule rather than the exception for exploited species, wemust begin to address questions concerningthe magnitude of evolutionary change, thereversibility of such change, and its conse-quences for sustainable harvesting, popula-tion recovery and species persistence.As with

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unintentional selection by humans against,for instance, large animals and antibiotic-susceptible pathogens, the long-term reper-cussions of fishing are almost certainly morecomplicated than previously believed. ■

Jeffrey A. Hutchings is in the Department of Biology,Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada.e-mail: jeff.hutchings@dal.ca

1. Coltman, D. W. et al. Nature 426, 655–658 (2003).

2. Stockwell, C. A., Hendry, A. P. & Kinnison, M. T. Trends Ecol.

Evol. 18, 94–101 (2003).

3 Stokes, K. & Law, R. Mar. Ecol. Prog. Ser. 208, 307–309 (2000).

4. Conover, D. O. & Munch, S. B. Science 297, 94–96 (2002).

5. Handford, P., Bell, G. & Reimchen, T. J. Fish. Res. Bd Can. 34,

954–961 (1977).

Figure 2 Fishing out life histories. a, As Olsen et al.7 describe, the probability of an individualmaturing at a specific age or size can bedetermined from where the negatively slopedprobabilistic reaction norms (solid lines), whichrepresent the age and size at which 50% of apopulation reaches maturity, intersect thepositively sloped growth function (dashed line),which relates length to age. In this hypotheticalexample, an individual growing at the averagerate in 1960 would intercept the 50% maturationprobability contour — the reaction-normmidpoint — at about 3.6 years and 38 cm.Following intensive fishing, an individualgrowing at the same rate in 1990 matures at 2.3 years and 22 cm. b, How traditional reactionnorms might look in an unfished population(solid line), and as fishing increases to low(dotted line) and high (dashed line) levels.(Growth rate is often used as a proxy forenvironmental change in the study of reactionnorms for organisms that continue to grow after maturity.)

Astrophysics

Jump-start for a neutron starDuncan Lorimer

Radio emission from one of the neutron stars in the ‘double-pulsar’system is strangely enhanced in two sections of its orbit — stimulated,perhaps, by radiation from its companion.

One of the most exciting discoveries inastronomy in recent times was thatof the binary system1 J0737�3039,

and its confirmation as a ‘double-pulsar’system earlier this year2. Pulsars are rapidlyspinning neutron stars that form during thesupernova explosions of massive stars;although their masses tend to be slightlylarger than that of our Sun, their radii areonly about 15 km. For the first time, bothneutron stars in this binary have been identi-fied as radio pulsars — one that spins aboutits rotation axis every 22.7 milliseconds(which I shall refer to as ‘A’) and another(‘B’) that spins with a period of 2.77 seconds.

This duo promise to surpass even theoriginal Nobel-prizewinning pulsar in abinary system3 as a testing ground for relativ-ity,but they are also a fantastic laboratory forstudying pulsar emission. The intense mag-netic fields of pulsars accelerate charged par-ticles around them, causing the emission ofbeams of radiation that sweep the sky like therotating beams of a lighthouse.Already thereare intriguing observations2 of the emissionfrom the double-pulsar system — in particu-lar that pulsar B seems to emit most stronglyin two separate parts of its orbit.On page 919of this issue, Jenet and Ransom4 offer anexplanation for this strange effect, in a modelthat will have important implications for ourunderstanding of this binary system.

The rotation periods of pulsars increaseover time, reflecting the loss of rotationalkinetic energy of the spinning neutron star asit emits a ‘wind’of electromagnetic radiationalong its emission beams. The difference inspin properties of the neutron stars in thedouble-pulsar binary means that their windscarry away energy at significantly different

rates: the rate of loss of energy from A is some3,000 times greater than that from B. This,and the compactness of the pulsars’ orbit,implies that the energy carried in the respec-tive winds from A and B is actually balancedinside the emission region of B (ref. 2). As aresult, the energetics of A can be expected todominate the system.

The panels of Fig. 1 show the geometry of the double-pulsar system, as seen fromabove the orbital plane. The two stars hurtlearound their common centre of mass every2.4 hours, at 0.1% of the speed of light. Thetwo regions of strongest radio emission frompulsar B are indicated in orange in Fig. 1d.Because observers on the Earth are looking atthe system nearly edge on, essentially in thesame plane as the orbit, it is not surprisingthat the emission from B is strongest when itis closest to the Earth and A is furthest away.But it is not immediately obvious why thereis a clear break in the emission between thetwo parts of the orbit.

Jenet and Ransom4 postulate that theemission from B is somehow stimulated —jump-started into action — when the light-house beam of A sweeps through B’s emissionregion. The authors make the reasonableassumption that A’s beam is a wide, hollowcone1 whose size and opening angle can bedetermined directly. It is then a relativelystraightforward geometrical exercise to showthat pulsar B intercepts A’s beam at preciselythe points of the orbit where increased emission is observed2. From current obser-vations, the various angles in the system areconstrained such that they fit two slightlydifferent solutions of Jenet and Ransom’smodel, both of which produce the effectshown in Fig.1.

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6. Sinclair, A. F., Swain, D. P. & Hanson, J. M. Can. J. Fish. Aquat.

Sci. 59, 361–371 (2002).

7. Olsen, E. M. et al. Nature 428, 932–935 (2004).

8. COSEWIC (Committee on the Status of Endangered Wildlife in

Canada). COSEWIC Assessment and Update Status Report on the

Atlantic Cod, Gadus morhua (COSEWIC, Ottawa, 2003).

Available at www.sararegistry.gc.ca

9. Heino, M., Dieckmann, U. & Godø, O. R. Evolution 56, 669–678

(2002).

10.Schlichting, C. D. & Pigliucci, M. Phenotypic Evolution: A

Reaction Norm Perspective (Sinauer, Sunderland, 1998).

11.Stearns, S. C. & Koella, J. C. Evolution 40, 893–913 (1986).

12.Hutchings, J. A. Can. J. Fish. Aquat. Sci. 56, 1612–1623 (1999).

13.Stokes, T. K., McGlade, J. M. & Law, R. (eds) The Exploitation of

Evolving Resources (Springer, Berlin, 1993).

14.Hutchings, J. A. in Handbook of Fish Biology and Fisheries Vol. 1.

Fish Biology (eds Hart, P. J. B. & Reynolds, J. D.) 149–174

(Blackwell, Oxford, 2002).

15.Haugen, T. O. & Vøllestad, L. A. J. Evol. Biol. 13, 897–905 (2000).

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