BLACKBURN and NELLEN: EGGS AND LARVAE IN AN UPWELLING AREA 



The Polish data support our conclusion that 

 Trachurus was the principal pelagic fish at the 

 edge of the shelf. Our conclusion that Sardina 

 pilchardus was an important species on the shelf 

 is supported by the results of the Professor Sied- 

 lecki hauls, but not by those from the commercial 

 vessels. Commercial fishing for that species is 

 concentrated farther north, especially between 

 lat. 24° and 26°N (Chabanne and Elwertowski 

 1973; Odra Company results given above). Sardine 

 catches of the Professor Siedlecki were much 

 higher between lat. 22° and 25°N (mean of 62 

 hauls, 2.37 tons/h) than between lat. 20° and 22°N 

 (mean of 15 hauls, 0.17 ton/h). There appears to be 

 no commercial fishing for Engranlis off Spanish 

 Sahara. 



SPATIAL AND TEMPORAL 



DISTRIBUTION OF 



SARDINE AND ANCHOVY EGGS 



In this section we characterize the area in which 

 sardine and anchovy eggs occurred on JOINT-I, 

 and note temporal changes in their abundance. 

 The findings on areal distribution would apply also 

 to adult fish in reproductive condition. We have 

 assembled data on temperature, chlorophyll a, 

 small zooplankton (<500 jum), large zooplankton 

 (>500 jum), sardine eggs, and anchovy eggs for the 

 10 series along lat. 21°40'N. Figures 2 to 4 show the 

 data for three series, including the two series in 

 which sardine eggs were most abundant. Anchovy 

 eggs were most abundant in the 23-24 March 

 series (Figure 2). 



Vertical distributions of temperature and den- 

 sity varied as shown by Barton (1974) and L. A. 

 Codispoti (pers. commun.), and are not discussed in 

 detail. Figure 3 shows typical coastal upwelling 

 and Figure 4 a relaxation of upwelling conditions. 

 Figure 2 shows weak coastal upwelling and up- 

 welling at the shelf edge. Other series showed 

 similar variations. It is doubtful if upwelling ever 

 occurred only at the edge. 



Chlorophyll a in the water column always 

 showed a primary or secondary maximum on the 

 middle or outer part of the shelf, and sometimes 

 another maximum over the slope. The maximum 

 over the slope was found when upwelling occurred 

 at the edge, as in Figure 2, and was probably a 

 result of it. Maxima of small zooplankton were 

 distributed like those of chlorophyll. Both chloro- 

 phyll and small zooplankton were relatively low, 

 close inshore in all series, and also beyond the shelf 



edge in series where second maxima did not occur. 

 Large zooplankton were relatively scarce on the 

 shelf in each series. Their biomass increased 

 sharply at the edge, and generally continued high 

 as far offshore as we sampled. 



Sardine and anchovy eggs were virtually 

 confined to the middle and outer parts of the shelf 

 on all series, regardless of their abundance. Their 

 mean abundance there is given in Table 1, 

 together with means of temperature, chlorophyll, 

 and small zooplankton for the water column in the 

 same area, for each series. Temperature means are 

 approximate. 



DISCUSSION 



Sardine eggs were most abundant on the middle 

 and outer continental shelf during haul series 5 

 and 10, moderately abundant during series 8, and 

 scarce on other series (Table 1). Figures 2 to 4 show 

 the abundance on series 5, 9, and 10. Low numbers 

 of eggs indicate either a small population of adults 

 in the vicinity, or one that is spawning little. Mean 

 biomass of adult fish was estimated acoustically 

 for the same part of the shelf on the same sam- 

 pling line, at various dates commencing 31 March 

 (Thorne et al. in press). This biomass showed an 

 irregular increase with time. It was about 8 g/m^ 

 on 31 March, 40 g/m- on 6 to 9 April and 22 to 26 

 April, and 80 g/m^ on 1 to 6 May. These four 

 periods were close in time to series 6, 7, 9, and 10, 

 respectively. The predominant species was proba- 

 bly sardine as stated earlier. The egg numbers 

 show that adult sardines were probably abundant 

 on series 5 and moderately so on series 8, but we 

 have no acoustic estimates of biomass for those 

 series or for series 1 to 4. 



The low mean egg number on series 6 probably 

 reflected a very small adult population, but it is 

 unlikely that the low numbers on series 7 and 9 did 

 so, in view of the biomass estimates just given. It 

 is more probable that sardine spawning was 

 inhibited during series 7 and 9. The low mean 

 temperatures in the water column during those 

 series, namely, 15.5° and 16.0°C (Table 1), could 

 have been responsible. Furnestin and Furnestin 

 (1959, 1970) stated that spawning of Sardina is 

 absent or feeble below 15.5°C and optimal from 

 16.0° to 18.0°C, especially over 16.5°C, in Moroccan 

 waters. Spawning might, therefore, be low at 15.5° 

 to 16.0°C in waters off Spanish Sahara. The limit- 

 ing effect of temperature appears to be not on the 

 spawned eggs, which can develop at 10°C 



893 



