Environmental Engineering Reference
In-Depth Information
Pauly and Christensen (2002) used the 10% i gure not because they assumed it but
because they found that it comes very close to the mean of more than 100 estimates
they examined for their study. Given the enormous variety of aquatic species and
habitats, it is highly unlikely that we could ever i nd the precise transfer mean,
but we know that actual transfer efi ciencies depart signii cantly from the 10% rate
(Smil 2008). In upwelling areas, which occupy only about 0.2% of the world's
oceans but produce between 15% and 20% of all harvests (Pauly and Christensen
1995), the average efi ciencies may be close, or even above, 15%: Cushing (1969)
found rates between 13.9 and 18.5% in some Pacii c upwelling areas, and a recent
study of marine food webs by Tsagarakis et al. (2010) used mean values of 10%
for the Adriatic but 12.6% for the southern Catalan Sea and 17.4% for the northern
Aegean Sea.
The average trophic level of marine harvests are much easier to address, as several
studies have used fractional data for a large number of species groups in order to
calculate the historical changes in global i shing effort. As a result, we know that
Vitousek et al. (1986) made a serious underestimate when they chose a mean trophic
level of two: since the early 1980s the actual trophic levels of exploited i shes have
been between 3.2 and 3.3 (Zeller et al. 2009). But to which total should be these
multipliers applied? While crop production statistics are among the most accurate
i gures used to assess the human claim on phytomass, and while for many countries
they are available for the entire twentieth century (and even further back), there are
signii cant problems even with the latest i shery statistics, and only a few reliable
national series and a few global approximations exist for the pre-World War II
period. Moiseev (1969) made educated guesses for the global catch in 1800, 1850,
and 1900, with the totals remaining very low, at, respectively, 1 Mt, 1.5 Mt, and
2 Mt.
The FAO began its post-World War II data series in 1947, when it also estimated
the global catch for the last prewar year, putting the 1938 landings at 22 Mt. But
Sarhage and Lundbeck (1992) put the worldwide i sh landings at just 10 Mt during
the late 1930s. If the former total is correct, then the record Japanese prewar marine
harvest would have accounted for only about 20% of the worldwide total; if the
latter is closer to the real value, then Japan's share would have been more than 40%
of global seafood landings. The FAO's data series (in yearbooks since 1947, online
since 1961) shows the global marine catch rising from about 18 Mt in 1950 to just
over 80 Mt in 1988 and 1989, then falling back a bit before getting to about 85
Mt in 1996 and 1997, then slightly declining again, a record that amounts to two
decades of stagnation (FAO 2011a).
