Protect large spawners and properly assess stocks

In order to reduce over­fish­ing, the sta­tus and pro­duc­tiv­i­ty of many fish stocks are assessed by means of stock analy­ses. The fecun­di­ty of female fish is an impor­tant para­me­ter for this. How­ev­er, most cal­cu­la­tions con­tain a sys­tem­at­ic error: the num­ber of eggs of small­er spawn­ing fish is over­es­ti­mat­ed, while that of larg­er fish is under­es­ti­mat­ed — and it is pre­cise­ly the “super spawn­ers” that are tar­get­ed by fish­eries. A recent study with the par­tic­i­pa­tion of the Leib­niz Insti­tute of Fresh­wa­ter Ecol­o­gy and Inland Fish­eries (IGB) and the Hum­boldt Uni­ver­si­ty of Berlin shows that the recov­ery poten­tial of many fish stocks is thus over­es­ti­mat­ed and the risk of over­fish­ing can increase.

Incorrect basic biological assumptions

Almost all cal­cu­la­tions used to esti­mate the sta­tus of exploit­ed fish stocks are based on the assump­tion that the egg count of a female fish increas­es in direct pro­por­tion to her weight (isom­e­try). How­ev­er, recent stud­ies show that in most fish species, large, heav­ier females lay more eggs per body mass than younger, lighter females. Thus, egg num­bers increase dis­pro­por­tion­ate­ly with weight rather than uni­form­ly (hyper­al­lom­e­try). If old, large females are among the spawn­ing fish of a stock, more eggs will be pro­duced than if the same total bio­mass con­sists pre­dom­i­nant­ly of young, small fish.

Researchers from Aus­tralia, the U.S. and Ger­many used mod­el cal­cu­la­tions for 32 marine fish species to inves­ti­gate what hap­pens when two of the most impor­tant ref­er­ence points for fish­eries man­age­ment are adjust­ed accord­ing to the new find­ings on the rela­tion­ship between egg num­ber and fish weight: the spawn­ing poten­tial of a fished stock rel­a­tive to the spawn­ing poten­tial of an unfished stock, and the max­i­mum sus­tain­able yield of a fish stock. Max­i­mum sus­tain­able yield quan­ti­fies the amount of fish that can be tak­en from a stock over the long term. If more fish are caught, the stock is said to be over­fished. Both spawn­ing poten­tial and sus­tain­able yield are used world­wide to set fish­ing quo­tas and imple­ment con­ser­va­tion programs.

Spawning potential is often overestimated in calculations for catch quotas

The results of the study show: On aver­age, the spawn­ing or repro­duc­tive poten­tial of the 32 fish species ana­lyzed is over­es­ti­mat­ed by 22 per­cent. How­ev­er, the val­ues vary from species to species between 3 and 78 per­cent. This is because the hyper­al­lom­e­try of fecun­di­ty varies among species. For exam­ple, the over­es­ti­ma­tion of spawn­ing poten­tial is par­tic­u­lar­ly high for Pacif­ic sar­dine at 78 per­cent, com­pared to cod at 18 per­cent and her­ring at 11 percent.

Systematic overfishing of large fish:

If the wrong bio­log­i­cal rela­tion­ships to egg num­bers per fish weight are assumed when cal­cu­lat­ing the repro­duc­tive poten­tial of an exploit­ed fish stock, the allow­able catch rates are on aver­age 2.7 times too high. Of course, not every fish lar­va sur­vives in nature. This reg­u­la­to­ry capac­i­ty of fish stocks is tak­en into account in the analy­sis of max­i­mum sus­tain­able yield. But even when cal­cu­lat­ing max­i­mum sus­tain­able yield, assum­ing the wrong bio­log­i­cal rela­tion­ships leads to the deriva­tion of max­i­mum catch rates that are 1.2 times high­er than would be appro­pri­ate for sustainability.

“This means that tar­get­ing large spawn­ing fish — as is com­mon in fish­eries — reduces the repro­duc­tive poten­tial of a stock and its yield­ing capac­i­ty. This can sys­tem­at­i­cal­ly fuel over­fish­ing or slow or even pre­vent stock recov­ery,” explains fish­eries pro­fes­sor Robert Arling­haus of IGB and HU Berlin, co-author of the study.

Previous textbook opinion on fisheries management: catch the big ones!

Almost all fish stocks in the oceans, lakes and rivers are man­aged so that the larg­er fish are selec­tive­ly caught and the small­er ones sur­vive to spawn at least once in their life­time. That’s the think­ing behind the wide­spread min­i­mum size require­ment. This catch reg­u­la­tion is intend­ed to pre­vent fish stocks from col­laps­ing despite inten­sive exploita­tion. This approach is based on the assump­tion that the old­er and larg­er ani­mals do lit­tle to renew the stock, and even cost yield poten­tial because they no longer grow as fast as small­er and younger fish.

“This pre­vi­ous­ly blan­ket man­age­ment prac­tice and the basic bio­log­i­cal assump­tions behind it regard­ing the sup­pos­ed­ly lim­it­ed pro­duc­tiv­i­ty of large fish are out­dat­ed in light of our results. On the con­trary, bio­log­i­cal pro­duc­tiv­i­ty, which includes egg pro­duc­tion, increas­es rather than decreas­es with fish size; accord­ing­ly, selec­tive har­vest of the very large fish can also weak­en stocks.”

- Robert Arlinghaus

More sustainable fisheries by protecting the big ones

Mea­sures that help pro­tect large fish can boost fish­ing yields and help pro­tect fish stocks, accord­ing to the researchers — for exam­ple, more selec­tive fish­ing meth­ods that spare large fish as well as young ones. In recre­ation­al fish­ing, catch win­dows could replace tra­di­tion­al min­i­mum sizes. But pro­tec­tion zones or closed sea­sons can also be use­ful. In detail, the best mea­sures depend on the fish species and fish­ing meth­ods and can­not be generalized.

The authors rec­om­mend that future stock assess­ments be recal­i­brat­ed to account for the greater repro­duc­tive capac­i­ty of larg­er fish. “Both con­ser­va­tion and fish­eries and angling can ben­e­fit from more accu­rate stock analy­sis,” Robert Arling­haus concludes.