Good news from the oceans or just fishy tales? PDF Print E-mail
In brief
Written by Albert Norström   
Friday, 19 August 2011 20:08

 

Emmet Duffy over at SeaMonster has a lovely post on the recent great news that cod stocks around the Scotian shelf in the north Atlantic seem to be recovering, and dragging along the whole ecosystems into a new, more desirable regime again. With the collapse of cod stocks, the entire of area around the Scotian shelf was pushed over into a new regime, dominated by pelagic forage fish such as herring and big blooms of algae. Now that cod seems to be coming back, the algae seem to be on the decrease aswell, due to a series of trophic cascade interactions. Regime shifts in marine systems are certainly not irreversible, as this example shows. But recovery can take a very long time, during which livelihoods of fishermen can be wrecked and whole coastal towns that were dependent on cod fishing turned into ghost towns. The question is whether recovery of the ecological domain, can fuel a larger-scale transformation into a more sustainable trajectory for the whole related social-ecological system. The authors of the cod recovery paper, Frank et al, also raise other complications that can arise in the ecological domain:

“Recovery in other over-exploited ecosystems such as the Black Sea, Northern Benguela, the Sea of Japan, and elsewhere has been delayed by jellyfish blooms, the presence of invasive species and by eutrophication, all of which are possible in the system we describe. The widespread body size reductions of benthic fishes documented for other exploited northwest Atlantic systems, if not reversed, could also slow the recovery of the benthic fish complex and adversely affect food web structure. The evolving global climate could alter the ecosystem positively or negatively. These uncertainties notwithstanding, the answer to the critical question of whether or not such profound changes in the dynamics of large marine ecosystems are reversible seems to be ‘yes’. This bodes well for other perturbed, formerly cod-dominated systems at latitudes to the north of the eastern Scotian Shelf that have yet to recover. Indeed, subtle signs of cod recovery have been appearing in other sub-arctic northwest Atlantic ecosystems during the past few years.”

Over in Europe, where the state of European fisheries has long been described as one of collapsed stocks due to decades of overexploitation and gross mismanagement, there are also good news. In a recent letter (note that letters are not peer-reviewed) to the journal Nature, Massimiliano Cardinale and others argue that European fisheries and fish stocks are not in the bad condition often ascribed to them. Any good news from European waters are welcome, but there are good reasons to remain only (very) carefully optimistic and not begin premature celebrations. Cardinale et al have looked at data for the 40 European fish stocks for which maximum sustainable yields (MSYs) are known, and claim that 43% (of those 40 fish stocks) are fished at or below the MSY. Indeed this is an improvement from the levels we had in 2000 (12.5% of fish stocks in Europe at or below MSY levels). In addition to this improvement, Cardinale et al report trends of falling fishing mortalities for these stocks and increases in average spawning biomasses. The reasons to remain only carefully optimistic, and slightly wary of these good news, is that we still have 57% of those fish stocks being fished at levels above the MSY. In addition, most ecologists, unlike fisheries biologists, are extremely skeptical of MSYs. That is because MSYs usually land at approximately half of the original fish population densities. In other words, when a fish stock is fished at MSY then it already has been reduced to almost half of its original size. This can have huge impacts on the functioning and resilience of the ecosystems; one can try and visualize what would happen to say any prey population if you reduced their predator densities by 50%. In fact, a great recent paper by Llope et al in Global Change Biology shows through sophisticated modeling and simulations how removal of top predators severely eroded the resilience of the Black Sea ecosystem.

All in all, however, even if these two separate stories are two small specks of light in the darkness, they are nonetheless, signs of light. And we welcome them.

 

 

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