FISHERY BULLETIN: VOL. 80. NO. 2 



coincident in time with the excretion experi- 

 ments showed a mean pelagic biomass over the 

 northwest Africa shelf of 40 to 60 g wet weight/ 

 m 2 (Thorne et al. 1977). Analysis of fish egg and 

 larvae samples later indicated that the relative 

 abundance of sardine to anchovy was about 4:1 

 (Blackburn and Nellen 1976) or mean densities 

 of 32 g wet weight of sardines and 8 g wet weight 

 of anchovy/m 2 . Converting to dry weight the sar- 

 dine and anchovy standing stock would be 8 and 

 2 g/m 2 , respectively. 



Demersal fish stocks on the shelf sampled by 

 bottom trawling gear were more varied in com- 

 position than pelagic stocks. A composite esti- 

 mate of demersal stocks taken in several trawls 

 by three vessels included 2.2 g wet weight/m 2 of 

 fish mainly represented by the families Spari- 

 dae, Sciaenidae, Pomadasyidae, and Congridae 

 (Haedrich et al. 1976). In addition, at depths of 

 about 50 m cephalopods were found in abun- 

 dances of about 1 g wet weight/m 2 , and at 200 m 

 large numbers of the shrimp Plesionika spp. 

 were collected, amounting to 1.44 g wet weight/ 

 m 2 . 



The biomass of pelagic fish in the slope area at 

 depths >200 m was estimated by acoustic mea- 

 surements to be about 80 g wet weight/m 2 and 

 was thought to be composed of jack mackerel, 

 Trach >< rus symmetricus (Thorne et al. 1977). Just 

 offshore of the shelf break concentrations as 

 large as 105 g wet weight/m 2 of fish were occa- 

 sionally observed. Demersal fish biomass was 

 smaller than found on the shallow shelf region 

 and was estimated to be 3.3 g wet weight/m 2 

 from bottom trawls (Haedrich et al. 1976). 



It should be noted that biomass estimates ob- 

 tained from the acoustic survey for the pelagic 

 populations and the trawl sampling for demersal 

 nekton were often highly variable (Thorne et al. 

 1977). Several of the most abundant nekton spe- 

 cies (e.g., Sardinella spp.) were migrating 

 through the study area and a considerable 

 amount of commercial fishing was occurring so 

 the mean biomass values used in regeneration 

 calculation are an attempt to use a reasonable 

 value that was the best estimate of the nekton 

 biomass assessment investigators. 



Regeneration Rates 



Regeneration rates were calculated from nek- 

 ton biomass and fish excretion data for the shelf 

 «200 m) and slope region (>200 m) in the north- 

 west Africa upwelling area. These regions were 



considered separately because of large differ- 

 ences in both the fish and zooplankton popula- 

 tions in these two areas. The sum of ammonium 

 regeneration rates calculated for pelagic fish 

 over the shelf amounts to 2.87 mg-at/m 2 per day 

 (Table 3) while demersal fish regeneration rates 

 were 0.09 mg-at/m 2 per day for a total of 2.96 mg- 

 at/m 2 per day. The anchovy excretion rates were 

 estimated from Engraulis ringens and E. mor- 

 dax values (Whitledge 1978) and Plesionika spp. 

 excretion rates were estimated using values for 

 small sizes of Pleuroncodes planipes, a pelagic 

 crab endemic to the eastern tropical Pacific. 



The ammonium regeneration rates for the 

 slope region were dominated by T. symmetricus 

 (1.80 mg-at/m 2 per day) while demersal fish con- 

 tributed only 0.04 mg-at/m 2 per day. The jack 

 mackerel excretion rate used in the calculation 

 was obtained from specimens examined in the 

 eastern Pacific region (McCarthy and Whitledge 

 1972). 



Table 3.— Regeneration of ammonium by fish over the shelf 

 and slope areas of northwest Africa upwelling ecosystem. 



Estimated from Engraulis ringens and E. mordax rates 

 2 Estimated from Pleuroncodes planipes rates. 



DISCUSSION 



The significance of nutrient regeneration by 

 fish is most apparent when the ammonium re- 

 generation rates are compared with phytoplank- 

 ton uptake rates measured by 15 N-labeled nitrate 

 and ammonium. The mean ammonium and ni- 

 trate uptakes are estimated to be 11 and 10 mg- 

 at/m 2 per day for the shelf region (Maclssac and 

 Dugdale 2 ). The ammonium regeneration rate for 



2 Jane J. Maclsaac and Richard C. Dugdale, University of 

 Southern California, Los Angeles, CA 90007, pers. commun. 

 June 1977. 



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