North Atlantic climate variability: consequences for pelagic ecosystems

When considering policies and management plans for the conservation of endangered marine animals there is often a focus on anthropogenic activities that may be harmful for their safety and therefore increase the mortality rate. As a consequence environmental factors such as variability in climate that affect birth rates are overlooked, yet climate variability has considerable effects on the conservation prospects of endangered animals. The changes in zooplankton composition (the abundance of the copepod Ca/anus finmarchicus decreases) on the feeding grounds of the right whale (Euba/aena glacialis) following years with a negative NAO (North Atlantic Oscillation) Index have detrimental effects on the calving rates of this mammal. There is a tight coupling between the NAO Index and the coupled slope water system (CSWS) affecting the Regional Slope Water Temperature in the Northwest Atlantic. This in turn has a tight coupling with the abundance of C. finmarchicus, probably caused by the changes in advective transport. Because of the reproductive physiology of right whales, years with drops in abundance of prey result in years with almost no births. When prey returns the opposite happens: birth peaks. The past decade there has been a predominantly low abundance of C. finmarchicus, resulting in bad conditions for calving. When C finmarcbiws returned many female right whales were ready to become pregnant, because of the preceding years without pregnancy. Knowing the results of certain climate conditions and the stage female whales are in, it might become possible to predict calving rates with reasonable certainty. Periods with very high or very low calving rates can be explained using this knowledge. Also commercial fish stocks are influenced by climate variability through the changes in plankton composition. The example of cod in the North Sea shows the importance of water temperature. During the relatively warm past decade cod larvae were not able to grow up because of low prey abundances. In 1996 the recruitment was higher because of lower temperatures, but the following years low recruitments returned. The 1996 yearclass lacks the ability to become mature, as most cod are fished before their fourth year. During the past decade the amount of cod diminished because of a combination of low recruitment and fisheries that kept on their high fishing pressure. As the environmental conditions are thought to remain, the only way to give the North Sea cod population a chance for recovery is by prohibiting cod fisheries for indefinite time. The examples used in this report show the importance of long-term monitoring of the marine environment. Even though nothing seems to happen at first sight, over longer periods fluctuations, changes, and shifts can become visible, which could be useful in explaining unexpected events, like rapid diminishing or fast growth of populations. Using this knowledge can result in more suitable management measures and conservation policies, because of having the ability to utilize the obtained knowledge directly as a certain event occurs or tends to occur.