CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

30 CALIFOKSI,-\ COOPERATIVE OCEASIC FISHERIES ISVESTIGATIOSSI\ rlling in temperate latitudes and tire richer in nutricntsiind higlier in productivity and zooplanktonT oluiix~ than the central ocean w-atcrs. In temperatelatitudes the depth of tlie mixed layer is notablyshallower in the eastern bountlary currents than furtheroff'sliore.The western boundary current areas in temperatelatitudes are warmer than the central n t er masses.They are not so rich as tlie eastern boundary currentsbut are richer than the central ocean. Zooplanktonvolume is moderate. The mixed layer is much thickerthan in tlie eastern boundary currents.The North Sea is a tolerable environment in spiteof its high latitude only because it receives somewater from the south uia the North Atlantic Currentaiid the Gulf Stream: it is therefore much warmerthan any other water at this latitude. It is shallow,much of it less than 100 m in depth, and fairly wellmixed, so that it does not offer a wide range of properties,though the vertical mixing and advection renderit fairly productive.The Baltic is still shallower and much fresher, thesalinity of the surface water decreasing almost to zeroin the upprr reaches arid even the few deeper basinscontain water of only about 12yL at the bottom.THE RANGE OF TEMPERATURE AND SALINITYIN THE HABITAT OF ENGRAULISThe Mediterranean is a. much more nearly homogeneousbody of water than is the adjacent part of theAtlantic. Salinity is everJ-where quite close to 38.5%0and the temperature, except for the thin suminerwarmedlayer, is near to 13°C. Because of the extremeeraporation 0~7er the area the water is very saliiieand dense, sild ovrrturn to the bottom occurs in partof the Mediterranean. Exchange wit11 the Atlanticoccurs by outflow of saline water to the deeper partof the Atlantic, and inllow of surface water from theAtlantic. For this reason the nutrient concentratioiiin tlie Mediterranean is lo~v and procluctirity is generallylow.3028262422m3v, 20JW* 18mWWIT 160Wn14zw 12[L3t-a IO[LWa2 8Wt-6420-?IiiI.I+austraiis1 :I!I.I !*anchoitai!..[I... -1 ..-.!! -...IIII L-JIIIII>ringensIL rIn o r d a xI!- japonicus0 2 4 6 8 IO 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42ISALINITYIN PARTS PER MILLEFIGURE 2. Temperature and salinity ranges of areas inhabited by Engraulis.