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

32 CALIFORNIA COOPERATIVE OCEANIC FISHERIES INVESTIGATIONSof temperature than the animal actually experiences.One conspicuous example of how misleading such asimple diagram might be is found in the Mediterranean.Summer surface temperatures are from 22' to28°C and winter values are from about 10°C to17°C. There is no overlap. On the other hand, weknow that these very high surface temperatures insummer are restricted to the upper few meters, andthat by being as little as 15 m beneath the surface thefish might easily find temperatures less than 22°Ceverywhere, even in August. This may apply in manyof the other areas as well in summer, so that theupper limit of surface water temperature may notrepresent the temperature the fishes experience. Likewise,Dr. Hayasi reports, the fish avoid the extremelylow water temperatures of the northern Sea of Japanin winter by staying in the southern area, which ismuch warmer. On the other hand, in some areas thewinter temperatures are inescapable. Fishes in theBlack Sea in winter must undergo temperatures ofabout 8°C: there is no warmer water anywhere inthe Black Sea at that time.On Figures 2 and 3 these open-ocean ranges ofsurface temperature in winter and summer are shownas rectangles with temperature as the vertical scaleand salinity as the horizontal scale. In both cases theentire range of oceanic values of temperature andsalinity is shown in order to emphasize the proportionof these ranges that Engraulis occupies. All exceptone of the species are included on Figure 2 : E, encrasiclzolzis'range is much wider and is shown separatelyon Figure 3.Figure 2 would seen1 to indicate that the oceanichabitat of Engraulis comprises almost the entire rangeof temperature in the ocean but only a small part ofthe salinity range. This would be a very wrong inferenceindeed. Baxter (p. 110) and others have shownthat Engradis retreats to offshore and deeper waterto avoid some extremes of temperatures; the totalrange, which includes extreme summer surface temperaturesthat the fishes may avoid by a slight submergence,and the extreme winter temperatures in thenorthern part of the Sea of Japan that the fish mayavoid by moving a short distance to the south, may bequite misleading. On the other hand, it has also beenshown that the fish do move into the quite brackishwaters of bays and estuaries when the temperature isnot too extreme. Therefore, all of these rectanglesshould extend an uncertain distance to the left on thegraph to include, at least during some part of theyear, waters of very low salinity indeed. This is emphasizedin Figure 3 by the ranges of E. encrasicholus.The open-ocean values from the North Sea southwardpast Spain to western North Africa look very muchlike the ranges on Figure 2 of the other Engraulisspecies. But the ranges in the Baltic, the Mediterranean,the Black Sea and the Sea of Azov emphasize thereal capability of encrasicholns to endure almost-freshas well as extremely saline water and very cold water.It is apparently the seasonal change in temperaturethat causes them to leave the Baltic, North and Azovseas, since the salinity does not change very much withseason.Looking again at Figure 2, some of the open-oceansalinity ranges appear to be very narrow. This signifiesthat the total range of offshore salinity values inthe Engraulis habitat is small, not that Engraulis islimited to a narrow range of salinity, since we knowthat it occurs in brackish waters. It appears thatEngradis can endure the total range of salinity encounteredin the open sea, including the Mediterraneanvalues above 39p/o0, but may be limited toonly part of the temperature range. Various investigatorswho are better informed may correct me, butit seems possible that Engraulis may actually notexperience temperatures much below 7 or 8"C, thoughit may experience these in many areas besides theBlack Sea. Resident populations of E. mordax nearVancouver Island will experience 7 or 8" in winterand if E. anchoitn and E. enchrasicholzis do notretreat completely from the Falkland Current andNorth Sea areas some of the individuals will undergovalues of about 8°C or less.In winter E. encrasicholzcs is found only at temperaturesless than 18" and it may by a slight submergencein summer avoid temperatures more than 22°C.Blackburn (this symposium) has shown that E.australis occurs only between about 10°C and 20"C,and Ahlstrom (1956) has said that most of the eggsand larvae of E. rnordax have been taken in aboutthat range, though some have been taken at more than23°C. The total range of the area occupied by E.ringens is from about 10°C to 23"C, and this includesnorth to south and winter and summer : slightmigrations might reduce this appreciably.E. japonicus capensis simply has no opportunity toendure cold water: about 15°C is the lowest surfacetemperature he may encounter at the tip of Africa,which is only about lat. 35"s. Likewise, a populationof E. encrasicholus that remains in the Mediterraneanmay not find water colder than 10°C at any timeof year.CONDITIONS LIMITING ENGRAULISWe may speculate, then, that Engradis is notlimited by the salinity values encountered in theocean or in estuaries: it not only occurs but spawnsin regions of very low as well as very high salinity.We may speculate also, at least until the other speakerscorrect me, that Engraulis is limited by temperature,since it does not appear in either the coldest orthe warmest oceanic conditions. Perhaps a range from6°C to 22 or 23°C would include all, or nearly all,of its actual occurrences. This would account for itslimited latitudinal range and for its most strikingmigrations from the Baltic, North, and Azov seas,whereas salinity variations would not.This really does not add very much to what most ofyou know already, and it does not by any meansspecify the habitat of Engraulis. By this I mean to saythat there is a vast area of ocean with surface temperaturebetween 6°C and 22°C-about $ of the worldocean, in fact-but that Engraulis inhabits, so far aswe know, less than l/lo of this area. Therefore temperaturealone cannot define the habitat of Engraulis.