Wednesday, November 16, 2011

Give them big Fish a Break!

Likely a big pregnant female - the worst possible loss for the stock!

Very interesting blog post!
It very much echoes a recent conversation I had with Doc where he told me that it is imperative to protect the large mature Sharks whereas in his opinion, a fishery for younger Sharks that would remain within the parameters of their natural mortality was at least conceivable.

Fish species channel their reproductive effort through three main strategies:
opportunistic species have a short life, grow quickly and produce many offspring (e.g., anchovies);
species are long-lived, grow slowly and produce few young (e.g., sharks);
and periodic species are also long-lived and grow slowly, but produce many young (e.g., rock fish) [notice that salmonid and intermediate strategies have also been described].

For equilibrium and periodic species, a long life often implies a large body.
In terms of fitness, as an individual grows and ages, its chances of successful reproduction at least once in a life time increase, as though reproduction resembled a lottery whereby the older managed to bet more frequently than the younger (one side of the gamble). Equilibrium species bet a few descendants at each reproductive bout, whereas periodic species bet from thousands to millions. If reproduction fails one year, the long-lived individual can wait for improved conditions in the next year, e.g., more food, more mates, better environmental conditions.

Many multi-million dollar fisheries target relatively long-lived species, and it will be no surprise to find them on the menu or delicatessen section of our favourite restaurant or shop (e.g., tuna, cod, halibut, caviar [i.e., sturgeon]). That many harvested fish stocks worldwide show a temporal trend of decreasing body size indicates not only the selective removal of the largest fish, but also rapid evolutionary responses to compensate for (to escape from!) big-fish mortality, like retarding body growth and advancing the age at first reproduction.

Whatever the mechanism, if we target big fish then stocks become unpredictable and, what might be even more shocking, if we target only small or only big fish, the result might be population depletion all the same.
Thus, conservation of age structure seems to be the key issue. At an ecosystem level (beyond the coverage of this article), when overharvesting concentrates not only on the largest individuals within a species, but on the species that reach the largest sizes, i.e., apex predator species, we are actually ‘fishing down marine food webs(read it!) – one of the main anthropogenic impacts on global fisheries.

The main take-home message from Berkeley’s and other studies is that the assessment of stock recovery and exploitation must rely not only on biomass, but also on maximum age.

Fish populations and communities lacking their giants risk depletion and extinction.

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