FISHERY BULLETIN: VOL. 74, NO. 4 



(Larrafieta 1960), but on the adults, as to whether 

 or not they release eggs. The adults occur in most 

 parts of the water column (Furnestin and Fur- 

 nestin 1970; Thorne et al. in press), which is the 

 reason for considering mean water temperature 

 here. Furnestin and Furnestin (1970) make it clear 

 that spawning depends on the temperatures over 

 most of the water column, not necessarily on those 

 in the upper 25 m where most eggs are found. 

 Thick layers of water below 15.5°C make an area 

 unsuitable for sardine spawning even if there is 

 warm water at the surface, according to those 

 authors. Figure 3 shows such a situation for series 

 9. From the criteria of Furnestin and Furnestin 

 and the vertical distributions of temperature in 

 our 10 series (examples given in Figures 2-4), it 

 can be said that temperature conditions on series 

 3, 4, 7, and 9 were unsuitable for sardine spawning 

 on the middle and outer shelf. Conditions on the 

 other series were relatively suitable with mean 

 temperatures for the water column at 16.5° or 

 17.0°C. It can then be deduced that adult sardines 

 were scarce on series 1 and 2, because few eggs 

 were found. We have no information about rela- 

 tive abundance of adults on series 3 and 4; they 

 could have been present but not spawning. Rela- 

 tive abundance of adult sardines on the other 

 series is given as low, medium, or high in Table 1, 

 according to indications discussed above. 



This succession of changes in abundance of 

 adults is too irregular to be attributed to growth of 

 individuals in a stationary population. It must be 

 due largely to movements into and out of the small 

 area studied. In the last major change, the biomass 

 approximately doubled in about 2 wk between 

 series 9 and 10. No pelagic fish species has such a 

 high growth rate for adult individuals. It was 

 noted during April and May that fish on the 

 continental shelf were more abundant north of the 

 sampling line (as far as lat. 22°20'N, which was the 

 limit of the acoustic surveys) than along the 

 sampling line (Thorne et al. in press). The fishing 

 results of the Professor Siedlecki also indicated 

 that sardines were more abundant to the north of 

 our area than within it. It is therefore very 

 probable that the biomass increase between series 

 9 and 10 represented a movement of sardines into 

 the study area from the north. 



It is of interest to consider possible causes of the 

 sardine movements. A population of sardines 

 living off the southern part of the coast of Spanish 

 Sahara would be likely to move into a particular 

 area, like our study area, when conditions were 



894 



suitable to them and move out of the area when 

 conditions became unsuitable. The principal de- 

 terminants of distribution of pelagic fish are 

 believed to be temperature and food supply. Tem- 

 perature conditions in the study area were suit- 

 able for adult sardines during the whole period of 

 JOINT-I, since they occur in waters from 14° to 

 18°C off Morocco (Furnestin and Furnestin 1970). 

 Changes in abundance of food might however 

 have caused movements of sardines into and out of 

 the study area. No studies of the diet of Sardina 

 pilchardus have been made off Spanish Sahara 

 except for two fish mentioned later. Elsewhere in 

 its range, including waters off Morocco, it feeds on 

 phytoplankton and small zooplankton (Larrafieta 

 1960; Furnestin and Furnestin 1970). The dis- 

 tribution of sardines along the sampling line was 

 like that of phytoplankton and small zooplankton 

 as shown earlier: all three having maxima on the 

 middle and outer parts of the continental shelf. 

 This suggests that relative abundance of one or 

 both of those kinds of food determines sardine 

 distribution in a spatial sense and might do so in a 

 temporal sense. 



Comparison of means of zooplankton concen- 

 tration with data on sardine abundance (Table 1) 

 shows no relation between them. If means of 

 chlorophyll concentration are used, there is the 

 following relation: sardine abundance is low when 

 chlorophyll values are 115 mg/m- or less, and 

 medium or high when chlorophyll values are 147 

 mg/m- or more. This suggests that sardines en- 

 tered the study area in order to feed on phyto- 

 plankton when it was relatively abundant and left 

 the area when phytoplankton was relatively 

 scarce. 



No adult sardines were obtained during 

 JOINT-I. On cruise AUFTRIEB 1975 we caught 

 two sardines in the same area in February. M. 

 Elbrachter kindly identified the contents of their 

 stomachs: one contained no organisms except 

 foraminifera, and the other contained phyto- 

 plankton in good condition, including 15 species of 

 diatoms, and 2 species of dinoflagellates, and 2 

 copepods. Thus 5. pilchardus feeds on phyto- 

 plankton and zooplankton off Spanish Sahara, as it 

 does off Morocco and in other parts of its range. 

 Phytoplankton might be an important part of the 

 diet of the Sahara sardine, suflRciently to cause the 

 sardine to move in relation to changes in phyto- 

 plankton abundance as suggested by our data, but 

 we cannot be certain. More work on the diet of the 

 sardine off Spanish Sahara is needed. Mauri tanian 



