FISHERY BULLETIN: VOL. 80, NO. 2 



Table 1.— Excretion rate measurements of specimens from northwest Africa. 



day and 0.31 to 0.76 fig urea-N/mg dry weight 

 per day. Specimens starved 24 h excreted 0.90 

 and 0.26 fig NH 4 -N and urea-N/mg dry weight 

 per day. A 2-d starvation lowered ammonium 

 excretion to 0.64 while urea was 0.35 fig urea- 

 N/mg dry weight per day. These rates are some- 

 what higher than the means of 0.17 and 0.05 fig 

 N/mg dry weight per day for ammonium and 

 urea measured at 12°C for Pacific staghorn scul- 

 pin, Leptocottus armatus; starry flounder, 

 Platichthys stellatus; and blue sea perch, Taenio- 

 toca lateralis (Wood 1958). After temperature 

 corrections are made using a Qio of 2.0 these rates 

 are still larger than those measured by Wood 

 (1958). Although linear temperature adjust- 

 ments can be estimated, the food conversion effi- 

 ciency can be variable and cannot be estimated 

 by simple increases in rates as temperature in- 

 creases (Pandian 1970); therefore, excretion 

 rates can be expected to be somewhat nonlinear 

 also. Feeding studies on the Atlantic menhaden 

 showed that ingestion rates are equivalent to 

 0.30-1.2 fig N/mg dry weight per day for filter 

 feeding on Thalassiosira rotula (Durbin and 

 Durbin 1975). Using an assimilation efficiency of 

 80% and growth rate of 5% assimilation, the ni- 

 trogen excretion rates should be about 0.23 fig 

 N/mg dry weight per day. This rate is about 20 to 

 33% of the rates calculated for the smaller speci- 

 mens in this study. 



The other demersal species, P. coupei and C. 

 cantharus, excreted 0.91 fig NH 4 -N/mg dry 

 weight per day when specimens were fresh and 

 0.64 ng NH 4 -N/mg dry weight per day when 

 starved 48 h. The pelagic Sardinella spp. showed 

 an ammonium excretion rate of 4.61 fig NH 4 /mg 

 dry weight per day. Ammonium excretion by the 

 blue shark was 0.1 1 fig N/mg dry weight per day 

 while urea excretion rates were slightly higher 

 (0.13 fig N/mg dry weight per day) than values 

 for the demersal fish. 



Measured quantities of orthophosphate in ex- 

 cretion samples were smaller than ammonium 

 but displayed an approximate linear increase 

 with time. Dissolved organic phosphorus (DOP) 

 was often excreted in quantities similar to ortho- 

 phosphate. The sum of orthophosphate and DOP, 

 representing total phosphorus excretion, is a 

 linear function of time over the relatively short 

 experimental period (Fig. 3). Phosphorus excre- 

 tion rates (Table 1) were smaller after the fish 

 had been starved 1 d. However, specimens 

 starved 2 d showed high phosphorus excretion 

 rates. Ammonium was decreased after 2 d of 

 starvation, indicating elemental excretion ratios 

 may change as starvation proceeds. Nitrate, 

 nitrite, and silicate were excreted in insignifi- 

 cant or zero concentrations in all experiments. 

 Control chambers starting with the same initial 

 water as the experimental tanks showed no con- 



0.6 



50 100 



TIME (min ) 



150 



Figure 3.— Tank concentration of orthophosphate and dis- 

 solved organic phosphate during an excretion experiment. 



330 



