OCONNELL and ZWEIFEL: FEEDING OF PACIFIC MACKEREL 



averag-e capacity might well be less than 32% 

 of weight for the asymptotic number specified. 

 The asymptotic number, which was most 

 strongly influenced by the trials at 22 Artemia/ 

 liter, may slightly underestimate the total num- 

 ber that can be ingested because of losses by 

 defecation. 



DISCUSSION 



It is evident that Artemia adults approximate 

 the smaller crustacean sizes utilized by the mack- 

 erel and that the fish resorts to filter feeding 

 to increase the rate of consumption as density 

 exceeds some level where particulate feeding 

 becomes relatively inefficient. The ecological 

 meaning of this feeding pattern is indicated by 

 considering the resulting relation between food 

 density and rate of food accumulation in the 

 digestive tracts in respect to 1 ) the daily nutri- 

 tional requirement of the species, 2) the density 

 levels of crustaceans in the sea, and 3) the feed- 

 ing rates of the northern anchovy (Engraulis 

 mordax) . 



Hatanaka et al. (1957) showed that S. japon- 

 icus in coastal regions of Japan tend to utilize 

 small fish in late summer and autumn but to rely 

 largely on euphausiids and other crustaceans of 

 similar size in other seasons. They concluded 

 that mackerel a little over 1-year-old and aver- 

 aging 149 g in weight required 8% of their body 

 weight per day in crustaceans to sustain the 

 growth rate observed in nature, which was esti- 

 mated as 0.42% of body weight per day. The 

 feeding functions derived in the present study 

 indicate that the times required for the mack- 

 erel to obtain this daily requirement at the A?'- 

 temia densities tested, or their equivalent for 

 euphausiids, would be 



The equivalent densities for euphausiids are 

 given on the assumption that the mackerel feed- 

 ing rate is keyed to concentration in terms of 

 biomass rather than to numbers per unit volume 

 as such. Euphausiids near the surface at night 

 in the eastern Pacific, largely Euphausia pacifica, 



average 6 mm in body length (O'Connell, 1971), 

 and individuals of this length are 3 mg wet 

 weight (Lasker, 1966), or 50% more than the 

 A7'temia. 



The feeding times given above are relatively 

 short, but the Artemia densities are much higher 

 than those reported for comparable organisms 

 in the sea. Brinton (1962) showed average den- 

 sities of E. pacifica, largely juveniles, to be about 

 0.02/liter near the surface at night oflf southern 

 California. O'Connell (1971) showed an aver- 

 age of 0.03/liter over much the same region, 

 with perhaps 5 to 10% of the area having den- 

 sities approaching 0.1/liter at any one time. 

 These estimates would be elevated, perhaps 

 doubled, if other large crustaceans were added on 

 a biomass equivalent basis. The highest of these 

 area densities would enable the mackerel to ob- 

 tain its daily nutritional requirement in about 

 half a day of particulate feeding, but the more 

 commonly prevailing level would not permit the 

 mackerel to obtain its requirement within the 

 space of a day. 



In all probability the mackerel obtains much 

 of its needs from euphausiids and other crusta- 

 ceans of similar size, but there is good evidence 

 that it depends to some extent on larger organ- 

 isms, such as fish up to one-third of its own body 

 length (Hatanaka and Takahashi, 1960), to se- 

 cure the daily requirement over a reasonable 

 length of time. Though stomachs tended to be 

 fuller and growth better during the season when 

 the mackerel feeds primarily on fish (Hatanaka 

 et al., 1957) , it must be remembered that the 

 8% daily feeding requirement used here pertains 

 to maintenance and growth when euphausiids 

 were the primary food. If the feeding rates in- 

 dicated for higher densities of Artemia apply for 

 biomass equivalents of much larger organisms, 

 the large capacity of the mackerel suggests that 

 relatively infrequent encounters with such or- 

 ganisms would sustain the daily requirement in 

 an average sense. The present study and that 

 of Kariya and Takahashi (1969) indicate that 

 feeding can be expected to continue towards full 

 capacity regardless of state of fullness when food 

 becomes available. 



Though mackerel may have to depend in part 

 on larger organisms, it is possible that the food 



979 



