RIASSUNTO
1. INTRODUCTION.
In recent years, interest and concern for marine mammals (seals, dolphins and whales) particular on the Dutch continental shelf has greatly increased. These concerns have led the Dutch Government to consider banning seismic research in the Dutch Continental Shelf during the period April to October, with effect from 1991.
Seismic research is intensive during the summer season because current methods depend on good weather conditions. The decline in larger species in the Atlantic Ocean is the result of whaling activities.
The importance of the environment, as well as the interest of the E&P industrie in the Dutch Continental Shelf, demand careful consideration of this problem.
Four aspects need consideration:
the presence or absence of marine mammals,
seismic research techniques,
the interaction between both of the above,
the effectiveness of protective measures.
In the following sections, the occurrence of marine mammals in the Dutch Continental Shelf, the ecology of the various species, the development and current status of seismic research and the effect of such seismic investigations upon marine mammals are discussed. Conclusions are drawn in Section 5 concerning the necessity and effectiveness of the proposed ban. Likewise, recommendations have been made in Section 6 on a possible course of action.
2. MARINE MAMMALS ON THE DUTCH CONTINENTAL SHELF.
2.1 Occurrence and the threat to survival.
The marine mammals of the Dutch coastal waters have come under great pressure in recent decades as a result of human activity. Indeed, the mammal fauna of the whole North Sea has been greatly reduced compared with the situation a few decades ago. The great whales, with the exception of the lesser rorqual, have almost disappeared from the North Sea. In the southern North Sea, whales have probably never been part of the permanent fauna, even though they were were more frequently seen earlier this century (pers. comm., C. Smeenk).
Dolphins and porpoises on the other hand, have reduced significantly(1). Healthy populations of marine mammals are important for a variety of reasons(4), as outlined below.
As top predators, they are good indicators of the quality of the marine environment.
As possible indicators of threat to human health because of the morphological and physiological compatability of marine mammals and humans.
The decline in larger species in the Atlantic Ocean is the result of whaling activities.
A definite explanation for the decline of the smaller species is difficult for the following reasons(1,2,3):
A clear picture of the numbers, habits, population dynamics and spread of smaller marine mammals is not available.
No single factor has ever been proven, suggesting a combination of factors to be responsible.
Insufficient information is available concerning the qualitative and quantitative effects of potential factors on the ecology of these species.
Some suggestions can be made on the reasons for reduced numbers of marine mammals(1,33).
Direct influence may arise from the following:
Habitat loss (the disappearance of suitable surroundings) especially affects coastal species, e.g. the damming of the Zuiderzee and the Delta estuaries.
Hunting for dolphins and porpoises: (this is not currently of importance in the North Sea)
Dolphins and porpoises may be caught in fishing nets. Vertical nets used by the Danish fishing industry are estimated to cause up to 4,000 casualties per year.
Disturbance, e.g. tourism, military activities, acoustic signals from shipping, platforms and seismic research may also be factors.
2.1 Occurrence and the threat to survival.
The marine mammals of the Dutch coastal waters have come under great pressure in recent decades as a result of human activity. Indeed, the mammal fauna of the whole North Sea has been greatly reduced compared with the situation a few decades ago. The great whales, with the exception of the lesser rorqual, have almost disappeared from the North Sea. In the southern North Sea, whales have probably never been part of the permanent fauna, even though they were were more frequently seen earlier this century (pers. comm., C. Smeenk).
Dolphins and porpoises on the other hand, have reduced significantly(1). Healthy populations of marine mammals are important for a variety of reasons(4), as outlined below.
As top predators, they are good indicators of the quality of the marine environment.
As possible indicators of threat to human health because of the morphological and physiological compatability of marine mammals and humans.
The decline in larger species in the Atlantic Ocean is the result of whaling activities.
A definite explanation for the decline of the smaller species is difficult for the following reasons(1,2,3):
A clear picture of the numbers, habits, population dynamics and spread of smaller marine mammals is not available.
No single factor has ever been proven, suggesting a combination of factors to be responsible.
Insufficient information is available concerning the qualitative and quantitative effects of potential factors on the ecology of these species.
Some suggestions can be made on the reasons for reduced numbers of marine mammals(1,33).
Direct influence may arise from the following:
Habitat loss (the disappearance of suitable surroundings) especially affects coastal species, e.g. the damming of the Zuiderzee and the Delta estuaries.
Hunting for dolphins and porpoises: (this is not currently of importance in the North Sea)
Dolphins and porpoises may be caught in fishing nets. Vertical nets used by the Danish fishing industry are estimated to cause up to 4,000 casualties per year.
Disturbance, e.g. tourism, military activities, acoustic signals from shipping, platforms and seismic research may also be factors.