Harvest management - Ecological Applications: Toward a Sustainable World - page 197

the Peruvian anchovy fi shery in 1972-3, the population had already suffered a small
dip in the upward rise in catches in the mid-1960s resulting from an 'El Niño event'
(Figure 7.6). Such an event involves the incursion of warm tropical water from the
north, reducing the upwelling of nutrient-rich water that normally fuels the
anchovy's food chain. Subsequent increases in fi shing effort meant that the effects
of later El Niño events, particularly in 1973 but also in 1983, were much more severe.
One positive aspect of a crash associated with large-scale climate events, is that the
population is more likely to recover when favorable conditions have returned than
would be the case if the crash were solely the result of overfi shing.

7. 2 . 6 Species that
are especially
vulnerable when rare

A further shortcoming of the yield curve concept needs to be highlighted. Even in
the absence of climatic variation, some species may not exhibit hump-shaped curves
like those in Figure 7.3a and b. This can happen for two reasons. First, recruitment
rate may be particularly low at the smallest population sizes: for instance, recruit-
ment of young salmon may be perilously low at low densities because of intense
predation by larger fi sh, while recruitment of baby whales may be low at low densi-
ties simply because males and females may never meet. The same outcome - higher
mortality than expected at low density - occurs if harvesting effi ciency is higher
when the population is small. For instance, many sardines, anchovies and herrings
are especially prone to capture at low densities because they form a small number
of large schools that follow predictable migratory paths and can be intercepted by
fi shing boats. In both cases, the species are particularly prone to overexploita-
tion because they lack the ability to compensate by rapid population growth
when rare.
Dulvy et al. (2003) list 32 species of marine fi sh, mammals and birds that, as a
result of overexploitation, have been driven locally extinct (e.g. common skate Dip-
turus batis and sea otter Enhydra lutris ) or globally extinct (e.g. the fl ightless great
auk Alca impennis and Steller's sea cow Hydrodamalis gigas ). The sy ndrome associ-
ated with extinction risk is ease of capture, large size and slow reproductive rate -
essentially the same pattern as for the terrestrial megafauna (e.g. mammoths and
giant sloths) that went extinct 11,000 years ago as a result of human hunting in
North America. I have already noted in Section 3.4 how large body size tends to
correlate closely with extinction risk.
However, despite the theoretical risk of extinction by overfi shing, there is sub-
stance to the claim that economic extinction will usually occur before biological
extinction. In other words, the fi shery goes extinct before the fi sh. Paradoxically,
however, nontarget species (by-catch), taken incidentally in a fi shery, are more at
risk of extinction. Some of the larger species that went extinct some time ago were
the target of unmanaged hunting. Nowadays, it is still large species that are most
vulnerable but these are predominantly by-catch species, including turtles (e.g.
Caretta caretta ), por poises and dolphins (e.g. Stenella coeruleoabla ), and albatrosses
(e.g. Diomedia exulans ). As by-catch, these large animals are vulnerable to extinction
on a second count too. Although target species may be subject to a sustainable har-
vesting regime, as long as their harvest continues the by-catch species will also be
taken, and quite possibly at unsustainable levels. Monitoring of by-catch requires
additional management expenditure together with the costs of developing innova-
tive gear - such as turtle excluder devices in trawl nets, bird-scaring lines to keep
seabirds from baited hooks and acoustic pingers to alert marine mammals to the