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An Upwelling System

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An Upwelling System Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 450. 1982. Workshop 3 Resources and ecology of fish (including ichthyoplankton) Summary by O. J. Østvedt On several occasions during the past years I have heard improve our knowledge of the relationship between the complaints that the studies made within the framework different trophic levels. One important fact is that the of CINECA have almost neglected studies of the living demersal fish stocks in the CINECA region are much resources in the region. However, the invited lecturers more abundant than in other upwelling areas. It is of and the contributed papers on fish resources, ecology course somewhat difficult to distinguish between of fish, and ichthyoplankton have clearly demonstrated demersal and pelagic fish. Horse mackerel for example that our knowledge of the fish resources has greatly are sometimes referred to as demersal and at other improved also as a result of the CINECA programme. times as pelagic. In addition to the three lectures dealing with fisheries In the CINECA region, large, unutilized resources and resources, eight contributed papers and five post­ of mesopelagic fish have been detected. The assess­ ers were discussed during Workshop 3. ment of mesopelagic fish in the CINECA region based Several papers referred to acoustic surveys on de­ on acoustic surveys has led to extensive discussion, and mersal, pelagic, or mesopelagic fish. These investiga­ the estimated biomass has been found too large relative tions and other data (particularly statistics on commer­ to the area and productivity of the region. Another cial fisheries) have demonstrated a large seasonal dis­ important point was revealed by the data on the feed­ placement of the major fish stocks. Comparing this sea­ ing of myctophids, which show that these small pelagic sonal migration of fish stocks with the changes in the fish feed mainly on carnivorous copepods, which physical and biological environment clearly demon­ means they are high in the food chain. It is quite clear, strated the effect of upwelling on migration. More however, that the mesopelagic resources are very sub­ detailed discussions on the interaction between fish and stantial and deserve further investigation. the environment took place, however, during Work­ An important result of CINECA is the extensive shop 4. work on fish larvae and the feeding of the different fish One important observation referred to the south­ species, as revealed by several contributions and post­ ward displacement of Sardina pilchardus during the last ers. The distribution of fish larvae in relation to the decade. While in earlier years 5. pilchardus was only environment is particularly important. Too little is rarely found south of Cape Blanc, there are today good known about the spawning season and area of spawn­ fisheries even in Senegal waters. A possible explana­ ing for even the most important commercial species. tion discussed during the Workshop was a general cool­ Data indicate that in the case of many species they may ing down of the environment, but the effect of heavy not have well-defined spawning grounds, but may shift fishery on other pelagic species, particularly Sardinella, according to the environmental conditions. It would be could just as well have given the sardine better oppor­ very useful to compile all the data in the CINECA tunities for survival. Thus interaction between species programme and to compare them with other informa­ could also be the explanation. Reference to the influ­ tion available on the biology and maturity stages of the ence of the environment on Sardina pilchardus was also different species. Few of the scientists working on ich­ made with respect to other areas, notably in Portu­ thyoplankton are able to cover a full season because so guese waters. many interests have to be taken into account when As to the evaluation of the different fish stocks, the planning larger expeditions. Therefore a compilation symposium has shown that work is progressing well and and full consideration of all the data on larvae would that further work is already planned, led by the certainly provide a great deal of information on the Fisheries Project under CECAF in Dakar. However, recruitment of the various fish stocks and on the sea­ the discussion revealed that any attempt to assess the sonal cycles in their reproduction. Such a study would resources in the different regions must take into also furnish information on whether discrete stocks account the influence of upwelling on the fish distribu­ exist in different areas. Since several fish species, both tion - and not only seasonal variations but long-term pelagic and demersal, apparently migrate up and down fluctuations as well. the coast, they might be considered as single stock units With all the data presented to the symposium on for management purposes. plankton and production it should also be possible to 450.
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