FISHERY BULLETIN: VOL. 70, NO. 3 



potential of the larger crustaceans, and even very 

 young fish, is greater than indicated by average 

 area densities. The definite filtering response 

 of the mackerel, which appears to be an adap- 

 tation for capturing the smaller organisms it 

 utilizes at a greater rate than would be possible 

 by biting activity when density is high, implies 

 that high densities are a factor of some conse- 

 quence in the feeding ecology of the species. One 

 possible explanation is that the various kinds of 

 food organisms tend to be distributed conta- 

 giously, with aggregations of considerably high- 

 er density than reflected by area averages. As 

 Ivlev (1961) demonstrated with carp fry, feed- 

 ing rate can be expected to increase with degree 

 of aggregation for a fixed quantity of food or- 

 ganisms. Euphausiids have been observed in 

 schools and breeding swarms (Brinton, 1962). 

 More than likely the filtering response is evoked 

 in mackerel with empty or near-empty stomachs 

 by densities not much above the equivalent of 1 

 or 2 Artemia/Mter, where rate of eff"ective biting 

 must already be on the order of 60 or more per 

 minute. At 4 to 5 Artemia /liter the daily nu- 

 tritional requirement could be obtained in less 

 than 45 min of filter feeding. 



Whereas Artemia adults represent the smal- 

 lest organisms consumed by the mackerel, they 

 represent the largest organisms commonly con- 

 sumed by the anchovy (Loukashkin, 1970); the 

 latter also consumes phytoplankton and crusta- 

 ceans less than 1 mm in length by filter feeding. 

 The wet weight quantities of Artemia adults 

 consumed at different densities (Table 5), based 

 on the present study for the mackerel and on 

 Leong and O'Connell (1969) and O'Connell 

 (1972) for the anchovy, suggest the diff"erences 

 in utilization and nutritional value of large 

 crustaceans for equivalent age groups of the 

 two species. 



The anchovy requires far less food than the 

 mackerel to meet its daily nutritional require- 

 ment, and can obtain the necessary quantity in 

 about 20 min when Artemia adults are at den- 

 sities of 1 or more per liter. In this same period 

 the mackerel consumes more than the anchovy 

 at all densities, but not enough to satisfy the 

 daily requirement. When considered in terms 

 of the weight of the two fish, the greater quan- 



Table 5. — Comparison of Artemia adults consumed (mg 

 wet weight) by the 1 -year-old mackerel and anchovy 

 for different density levels of Artemia in the water.^ 



1 Calculations for the anchovy at age 1-f- and weight 7 g show the 

 nutritional requirement and amounts consumed in 20 min at 1 and 5 

 Artemia/Uter to be 475 mg, but all other values ore the same as for the 

 4-g anchovy. 



titles consumed by the mackerel have a rela- 

 tively lower nutritional value than those con- 

 sumed by the anchovy, except, perhaps, at very 

 high densities. 



The extreme diflference in quantities consumed 

 by the two species at high densities is attribu- 

 table to the filtering capability of the mackerel, 

 and the failure of the anchovy to filter feed on the 

 larger crustaceans, regardless of density. No 

 meaning can be attached to the small diflference 

 at the lowest density level because calculation 

 of the values involved considerable extrapolation. 

 It is nevertheless probable that the anchovy tends 

 to remove large crustaceans at a lower rate than 

 the mackerel at such levels. O'Connell (1972) 

 showed that the feeding activity of the anchovy 

 is likely to be divided between these and the 

 smaller crustaceans captured by filtering if the 

 larger organisms are less than about 5 to 8% 

 of the dry weight of total zooplankton concen- 

 tration. 



From the above considerations it is tentatively 

 hypothesized that the individual mackerel gen- 

 erally utilizes a greater proportion of the large 

 crustaceans in the sea than does the individual 

 anchovy, but that the proportion utilized gen- 

 erally has a relatively low nutritional value for 

 the mackerel. 



980 



