THE IMPACT OF THE DRIFTNET BY-CATCH
ON THE OCEANIC ECOSYSTEM
Introduction
The sudden increase of driftnet vessel effort in the 1980s was seen as a
threat to the long-term sustainability of some of the important fishing
resources - in particular albacore - as well as representing a serious environmental
threat with respect to the by-catch of marine mammals and other living resources
taken by the gear (UN Secretary General 1990). Although the impact of large-scale
driftnetting on the range of affected species or the total biomass was not
yet fully understood, scientists had become increasingly concerned about
the potential effects of large-scale removals of target and non-target species
on the North Pacific ecosystem as a whole (U.S. Summary Report 1991). For
example, blue sharks, a top predator, "are part of a very intricate
assembly of animals that co-exist according to a very long, evolved interactive
equilibrium" (Bardach pers. comm. 1991). It was conservatively estimated
that in 1990 alone some 2.4 million blue shark were taken out of the North
Pacific by the combined efforts of five driftnet fisheries. Removing that
many top predators could change the equilibrium in a large-scale ecosystem,
possibly for a long time, and there was a high likelihood that the populations
of target species, as well as other species, would be detrimentally affected
as a result.
Fisheries, Biological, and Ecological Data
Requirements
Driftnet data collection programs collect only certain kinds of data: a
sample of the number, species, and sometimes the size of the marine creatures
caught and hauled on board. The number, species, and size composition of
the target species and by-catch species that fall out of the nets are either
not recorded or insufficiently recorded, making reliable stock estimates
impossible. Data that are not recorded by the driftnet fleets include spawning
locations and seasons, migration and distribution patterns, and feeding
and spawning behavior. It is not possible to acquire an understanding of
the pelagic ecosystem and reliable estimates of natural mortality, stock
construction and recruitment from sporadic and minimal observer programs
and unreliable commercial driftnet catch/effort data. The cry that more
data was needed was only a stalling tactic by the driftnet fishing nations
so they could continue to deplete the high seas stocks for their own benefit
in the interim.
By the end of the 1980s, ecologists were only beginning to understand the
ramifications of the previous two decades of large-scale driftnet fishing
on the pelagic ocean ecosystem as a whole. In addition, it was no longer
possible to collect the critical baseline data needed to assess the impact
of driftnet fishing on the oceanic ecosystem, including species distribution
patterns and the factors that limit their dispersal, pre driftnet population
sizes, growth parameters, age structures, mortality estimates from all sources,
survival rates, reproduction and life history parameters, feeding and schooling
behavior, and species interactions, including dependance, competition, and
predator/prey relationships. Very little of this data can be gleaned from
driftnet catch records. Most importantly, driftnet fleets were taking so
many species in such large quantities and in such a short period of time,
the pelagic ecosystem was drastically changing before its normal state could
even be understood.
Both the short and long term impact of driftnet fishing on the by-catch
species populations and their interactions within the oceanic ecosystem
is potentially devastating. The life history of every species is unique.
Some reproduce slowly, some are distributed in patches across the ocean,
some migrate long distances from breeding sites. The level of effort required
to take the MSY of the target species may be more effort than many of the
by-catch species can stand. As noted by the FAO (1990) "the rate of
harvest must be restricted to that rate which allows populations of the
slowest growing and reproducing species to survive." That level may
make driftnet fishing uneconomical. Under customary international environmental
law, the precautionary principle, elucidated in the UN driftnet resolutions,
places the burden on the driftnet fishing nations to prove that driftnetting
is not harmful to pelagic resources before it can be permitted to take place.
The driftnet fishing nations did not meet this burden despite two decades
of opportunity to do so.
Return to Table of Contents
Return to Earthtrust's DriftNetwork Page
