快猫短视频

STRICTER quotas on catches of North Sea cod and other fish recommended this
month may not be enough to stop them dying out. The quotas are based on models
that may overestimate how fast populations recover, because they don鈥檛 take the
effect of environmental changes into account.

The European Commission has recommended that fishing quotas for North Sea cod
should be slashed to protect dwindling stocks. European Union ministers are
expected to accept these proposals this week, when they set quotas for European
waters.

But Michael Lesser and his colleagues at the University of New Hampshire in
Durham have found that in some areas increased UV radiation resulting from the
thinning of atmospheric ozone could be helping to kill cod larvae. Unless this
stock loss is built into models 鈥渨e might get ourselves into even more trouble鈥�,
says Lesser.

Atlantic cod spawn in deep water, but the developing embryos float upwards to
finish their development within 25 metres of the surface. At these depths they
can be exposed to the increased levels of UV light.

In lab experiments, Lesser found that 90 per cent of Atlantic cod larvae died
within 10 days of being exposed to the level of UV light known to penetrate 8
metres down in the Gulf of Maine. When the larvae were shielded with Plexiglas,
which screens out UV, only one in three of them died.

What鈥檚 more, the few that survived exposure to UV were worse off for the
experience. They had higher levels of DNA damage and were slightly shorter than
the shielded larvae. Lesser suggests that this is the result of delayed
development, which in turn raises the risk of them being eaten because they
spend more time as plankton.

鈥淭his type of data has not been incorporated into any type of fishery model
used to make population estimates,鈥� says Lesser. Around 99 per cent of juvenile
cod are thought to be eaten by predators before they can reproduce. But Lesser
argues that the actual survival rate could be lower if rising UV levels are
included in the calculations.

More field measurements are needed to find out whether UV is making a
difference to cod or other species in the wild, says Kent Smedbol of Dalhousie
University in Nova Scotia, who has studied cod spawning. Changes in water
density due to salinity and temperature can affect the depth to which embryos
float, and particulates in the water can act like a sunscreen. 鈥淎 few things
have to come together for this to matter,鈥� he says.

Lesser agrees. The actual exposure of real fish populations to UV radiation
must be measured, he says, particularly because it will not be the same
everywhere. 鈥淭he water quality in the Gulf of St Lawrence is very different than
the water quality in the Gulf of Maine.鈥�

Other environmental factors altered by human activity may also be important.
Earlier this year, researchers at Britain鈥檚 Centre for Environment, Fisheries
and Aquaculture Science in Lowestoft found a link between rising surface
temperatures in the North Sea and reduced survival rates of young cod (
Nature, vol 404, p 142).