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.


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