Environmental Engineering Reference
In-Depth Information
The most comprehensive review of by-catch, based on some 800 studies, showed
its worldwide mean to be about 35%, with specii c rates ranging from less than
10% for cephalopods, more than 60% for redi sh and basses, 75% for l ounders
and soles, more than 80% for eels, and nearly 250% for crabs to more than 500%
for shrimp (Alverson et al. 1994). The great variability of these estimates precludes
coming up with a satisfactory mean discard rate. Discard mortality is also highly
variable, with lows of just a few percent and highs of more than 80% or even 100%
for such species as halibut, king crab, and salmon (Alverson et al. 1994).
Published totals of global discard were estimated to be as high as 27 Mt/year for
the late 1980s (range of about 17-40 Mt) by Alverson et al. (1994) and about 15
(10-20) Mt/year (Alverson 2005) and as little as 7.3 (6.8-8.0) Mt/year (Kelleher
2005) for the beginning of the twenty-i rst century. Zeller and Pauly (2005) pub-
lished a reasoned reconstruction of the past discards, and their best estimates indi-
cate rising totals until the late 1980s (from 4 Mt during the late 1970s to nearly
30 Mt by 1990), followed by declines (around 15 Mt by 1995, and down to 8 Mt
by the year 2000). While this is a welcome news, they also point out that it also
indicates even faster declines in actual landings than was believed previously (by at
least 1.5 Mt and perhaps up to 2.3 Mt/year, compared to the previously estimated
0.39 Mt/year (Watson and Pauly 2001).
While illegal and unaccounted-for catches should obviously be added to reported
landings to get the total of annually harvested marine zoomass, the treatment of
discards is problematic. Those i sh and invertebrates that are dumped back into
the ocean and survive the experience obviously should not be counted as a part of
harvest, and the discarded by-catch that dies serves to support wild marine hetero-
trophs, and it should not be counted as human-appropriated either. At the same
time, there can be no doubt that annual removal followed by the prompt dumping
of millions of tons of live and dead marine organisms amounts to a serious distur-
bance and degradation of aquatic food webs, particularly in the case of i sheries
with very high by-catch rates.
With all of these complications and caveats in mind, I will offer the best estimates
of phytomass needed to support recent levels of i sh catches and aquacultural pro-
duction. Pauly and Christensen (1995) added 27 Mt of discarded by-catch to the
mean 1988-1991 catch (94.3 Mt), assigned fractional trophic levels to 39 species
groups, and used an average interlevel energy transfer efi ciency of 10% to estimate
that about 8% of the world's primary aquatic production was needed to sustain
commercial i shing in the early 1990s, with the shares ranging from about 2% for
the open ocean to 24% for tropical and 35% for nontropical shelf ecosystems. I
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