Catching and eating young fish is one way to reduce the levels of
mercury in large fish like marlin and swordfish, but what affect will
this have on the natural population levels over time?
The World Health Organisation has very recently released updated
studies relating to acceptable levels of mercury (re human consumption)
in large and long lived pelagic fish, such as shark, larger tuna
species such as yellowfin/bluefin, marlin and broadbill swordfish. You
can find some very brief lip service to this on the Sydney Fish Market website (if you dig hard enough).

The upshot is the overall levels found in such fish were often
extremely high and recommended to be consumed infrequently, once a
month at most, particularly for pregnant women and kids. (You may
remember the case last year where the wife of ex ironman Craig
Riddington had a miscarriage that was put down to a couple of serves a
week of tuna and swordfish). When the general media got hold of this
story the general consensus was one serve of such fish per week was
maximum to avoid build up of these accumulatory poisons.

Now where this gets more interesting is related to the capture,
marketing, sales and regulatory controls of such fish species. Due to
the overall decimation of the more ‘popular’ eating species by the
commercial fleets (domestic and overseas), some commercial operators
have switched focus to swordfish and now striped marlin in particular,
as more highly rated eating fish become harder and harder to make a
buck out of due to declining numbers.

One of the reasons we have seen such an increase in availability of
these species at fish shops and restaurants is not only the increased
supply, but the much lower relative wholesale cost compared to prime
food fish such as snapper etc. Restaurants will make much higher margin
and this has been one of the reasons behind the ‘groovification’ of
swordfish and marlin as food fish. (Other second and third-rate food or
even bait species such yellowtail, slimy mackerel for example are
increasingly appearing on menus as more desirable species become all
but fished out)

Getting back to the mercury thing, Sydney fish markets has apparently
limited the maximum acceptable size of swordfish to 50 kilos, ie.
juveniles. To put this in context, this fish is long lived and can grow
in excess of 400 kilos, showing that indeed there must be serious (but
veiled perhaps) concerns on mercury build up. The effect of harvesting
numbers of juvenile fish before they become breeders will also
obviously have a deleterious effect on the eventual stock of these fish
in the future, but we can see from examples of decimation of species
such as the bluefin tuna, gemfish, orange roughy, ling, school and
gummy sharks, and not so far off the yellowfin tuna, that the
commercial lobby has more of an influence on the fisheries authorities
bodies than anybody else. Once a specific fishery has been flogged,
they move onto the next one.

Regarding the marlin species, partial addressment of the mercury
problem has been shown by the banning of the sale of blue and black
marlin, however it is now open slather on the smaller striped marlin,
with this species being increasingly targeted and is commonly available
for purchase.  For context, this species grows to around 200
kilos, with 70 to 130 kilos being commonly caught sizes. While this
species is smaller than the blue and black marlin, it is quite long
lived (>10 years) and the results from US and Fijian studies have
shown high levels of mercury to exist.  High levels of heavy
metals such as lead and cadmium, which are also accumulatory and effect
various nervous system functions, have also been found.