"Long before it's in the papers"
January 27, 2015


Overfishing has profoundly changed the fish, study finds

June 23, 2011
Courtesy of the Wildlife Conservation Society
and World Science staff

Fish in our cen­tu­ry live fast and die young. So con­cludes a new study by sci­en­tists who com­pared fish caught re­cently off Ken­ya with the bones of fish from me­di­e­val trash.

Mod­ern fish of course aren’t vic­tims of their own reck­less ways: the cul­prit rath­er is over­fish­ing, say the re­search­ers, from the New York-based Wild­life Con­serva­t­ion So­ci­e­ty. That, they claim, has caused an ec­o­sys­tem-wide tran­si­tion that may not be easily re­vers­i­ble.

Re­search­ers com­pared the re­mains of fish from an­cient Swa­hi­li ref­use heaps in Ken­ya with da­ta from re­cent­ly caught fish to un­der­stand how fish com­mu­ni­ties there have changed over time. (Pho­to cred­it: T. Mc­Clana­han/WCS)

Over the cen­turies, they ex­plain, fish­ing has dec­i­mat­ed larg­er and longer-lived spe­cies com­monly caught in the Mid­dle Ages. Re­main­ing fish com­mun­i­ties con­tain more spe­cies with shorter life spans, faster growth rates, smaller av­er­age sizes, and few­er “top preda­tors.”

The study used more than 5,000 sam­ples of an­cient fish re­mains dat­ing be­tween 1250 and 600 years ago, and ap­pears in the cur­rent on­line edi­tion of the jour­nal Con­serva­t­ion Bi­ol­o­gy.

Tim Mc­Clana­han and John­stone Omukoto of the so­ci­e­ty stud­ied the life his­to­ries of mod­ern fish com­mun­i­ties gath­ered from fish caught in both heavily-fished and pro­tected sites along the Ken­yan coast. They com­pared this with da­ta gath­ered from fish re­mains ex­ca­vat­ed from an an­cient Swa­hi­li set­tle­ment in Shanga, Ken­ya.

“An­cient Swa­hi­li mid­dens [trash dumps] rep­re­sent a time cap­sule of da­ta, con­taining in­forma­t­ion on the com­po­si­tion of the re­gion’s fish as­sem­blages and how hu­man com­mun­i­ties in­flu­enced the ma­rine en­vi­ron­ment,” Mc­Clana­han said. “The his­tor­i­cal da­ta sug­gest that fish­ing re­moves the slow­er-grow­ing, longer-lived spe­cies over time and that ma­rine pro­tected ar­eas are only par­tially suc­cess­ful in reco­vering the fish com­mun­i­ties of the past.”

The pair found that where­as an­cient fish com­mun­i­ties had a high per­cent­age of top predators—spe­cies that prey on fish and large in­ver­te­brates such as snails, sea urchins, and clams—mod­ern fish com­mun­i­ties con­tain more spe­cies that feed on plants, small in­ver­te­brates like sea lice, and gen­er­ally smaller spe­cies that feed low­er on the food chain. Mod­ern fish as­sem­blages, they added, al­so con­tain more spe­cies that are smaller with high­er growth and mor­tal­ity rates.

The re­search­ers al­so found that the num­ber of fish bones in the mid­dens peak­ed be­tween 1000 and 1100 A.D. be­fore de­clin­ing. The bones of sheep and goats be­come more prev­a­lent in high­er lev­els of the ground as­so­ci­at­ed with lat­er dates, they said, sug­gesting a shift in hu­man di­et to do­mes­ti­cat­ed an­i­mals.

“The ar­che­o­logical ev­i­dence demon­strates the in­cred­i­ble longe­vity of hu­man­ity’s util­iz­a­tion of coast­al fish­er­ies, while em­pha­siz­ing the crit­i­cal need to ac­tively man­age slow­er grow­ing, longer-lived spe­cies,” said Ca­leb Mc­Clen­nen, di­rec­tor of the so­ci­e­ty’s Ma­rine Pro­gram. “The ev­i­dence from Ken­ya aligns with find­ings from around the world that for mil­len­nia hu­man­ity has re­lied on the world’s oceans for our bas­ic need­s—but has more re­cently failed to do so in a man­ner that al­so will suf­fi­ciently sus­tain that re­source.”

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Fish in our century live fast and die young. So concludes a new study by scientists who compared recent fish caught off Kenya with the bones of fish from medieval trash. Modern fish of course aren’t victims of their own reckless ways: the culprit rather is overfishing, say the researchers, from the New York-based Wildlife Conservation Society. That, they claim, has caused an ecosystem-wide transition that may not be easily reversible. Over the centuries, they explain, fishing has decimated the larger and longer-lived species commonly caught in the Middle Ages. Remaining fish communities, they say, contain more species with shorter life spans, faster growth rates, smaller average sizes, and fewer “top predators.” The study used more than 5,000 samples of ancient fish remains dating between 1250 and 600 years ago, and appears in the current online edition of the journal Conservation Biology. Tim McClanahan and Johnstone Omukoto of the society studied the life histories of modern fish communities gathered from fish caught in both heavily-fished and protected sites on the Kenyan coast. They compared this with data gathered from fish remains excavated from an ancient Swahili settlement in Shanga, Kenya. “Ancient Swahili middens [trash dumps] represent a time capsule of data, containing information on the composition of the region’s fish assemblages and how human communities influenced the marine environment,” McClanahan said. “The historical data suggest that fishing removes the slower-growing, longer-lived species over time and that marine protected areas are only partially successful in recovering the fish communities of the past.” The pair found that whereas ancient fish communities had a high percentage of top predators—species that prey on fish and large invertebrates such as snails, sea urchins, and clams—modern fish communities contain more species that feed on plants, small invertebrates like sea lice, and generally smaller species that feed lower on the food chain. Modern fish assemblages, they added, also contain more species that are smaller with higher growth and mortality rates. The researchers also found that the number of fish bones in the middens peaked between 1000 and 1100 A.D. before declining. The bones of sheep and goats become more prevalent in higher levels of the ground associated with later dates, they said, suggesting a shift in human diet to domesticated animals. “The archeological evidence demonstrates the incredible longevity of humanity’s utilization of coastal fisheries, while emphasizing the critical need to actively manage slower growing, longer-lived species within an ecosystem approach,” said Caleb McClennen, director of the society’s Marine Program. “The evidence from Kenya aligns with findings from around the world that for millennia humanity has relied on the world’s oceans for our basic needs—but has more recently failed to do so in a manner that also will sufficiently sustain that resource.”