SIZE-SPECIFIC VULNERABILITY OF 

 NORTHERN ANCHOVY, ENGRAULIS MORDAX, LARVAE TO 



PREDATION BY FISHES 



Arild Folkvord 1 and John R. Hunter 2 



ABSTRACT 



Vulnerability of larval northern anchovy (6-33 mm SL) to predation by adult northern anchovy and juvenile 

 chub mackerel, Scomber japonicus, was estimated by measuring the response and escape probabilities 

 of larvae. The proportion of larvae responding to the attacks of either predator increased with larval 

 length and differed little between predator species. About 20% of 6 mm larvae responded to attacks 

 of predators while 85-100% of 33 mm larvae responded. The proportion of larvae escaping attacks also 

 increased with larval length, but more larvae of all sizes escaped the attacks of adult northern anchovy 

 than those of juvenile chub mackerel. The rate of consumption of northern anchovy larvae by adult north- 

 ern anchovy was highest when the larvae were 8.5-15 mm long, indicating that greater avoidance suc- 

 cess of larvae in this size range relative to smaller ones may not completely compensate for their greater 

 visibility to predators. 



The events that cause variation in year-class 

 strength in marine fish stocks occur during the first 

 year of life, but no single life stage or period has 

 been identified as being uniquely influential in the 

 establishment of year classes. Mortality rates are 

 size specific over this period with rates being the 

 highest during the egg and yolk-sac stages and 

 declining thereafter (Hunter 1984; Smith 1985). 

 Variation in the relatively low mortality rates of 

 older larval and juvenile stages may be more influ- 

 ential in year-class formation than the variation of 

 the high mortality rates of eggs and first feeding 

 larvae (Smith 1985). Thus all early life stages from 

 egg through juvenile must be considered and knowl- 

 edge of the size- or age-specific vulnerability of lar- 

 vae to predation and starvation is central in any 

 attempt at modeling the recruitment process. 



Starvation is probably a direct source of larval 

 mortality for only a few weeks after the onset of 

 feeding, and most losses in the first year of life may 

 be attributed to predation. Predation is believed to 

 be the major cause of mortality during the egg and 

 yolk-sac stages (Hunter 1984), and incidence of 

 starving jack mackerel, Trachurus symmetricus, 

 and northern anchovy, Engraulis mordax, in the sea 

 indicate that significant starvation mortality is 



^cripps Institution of Oceanography, University of California 

 at San Diego, La Jolla, CA 92093; present address: Austevoll 

 Aquaculture Station, 5392 Storebtf, Norway. 



2 Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La 

 Jolla, CA 92038. 



Manuscript accepted June 1986. 



FISHERY BULLETIN: VOL. 84, NO. 4, 1986. 



restricted to the first 1-2 wk of feeding or about 

 10-20% of the larval period (O'Connell 1980; Hewitt 

 et al. 1985; Theilacker 1986). The vulnerability of 

 larvae to predation has been studied over limited 

 size ranges; laboratory data indicate that yolk-sac 

 larvae seem to be vulnerable to small invertebrate 

 predators (copepods, amphipods, and euphausiids 

 [Hunter 1984]). In addition, some egg and larval 

 predators have been identified in field studies and 

 in several cases loss rates due to predation have been 

 estimated (Moller 1984; Frank and Leggett 1984; 

 Van der Veer 1985; Purcell 1985; older literature 

 summarized by Hunter 1984). 



The objective of this paper was to determine the 

 size-specific vulnerability of northern anchovy lar- 

 vae to predation by pelagic fishes. The size-specific 

 vulnerability of larval Cape anchovy, E. capensis, 

 to cannibalism has been investigated by Brownell 

 (1985) and vulnerability of larval E. mordax to 

 predation by the aquarium fish Amphiprion percula 

 was studied by Webb (1981). The results of the cur- 

 rent study will be compared to these papers in the 

 discussion. 



Our approach was to observe the avoidance 

 behavior of northern anchovy larvae in response to 

 predatory attacks by adult northern anchovy and 

 juvenile chub mackerel, Scomber japonicus. Adult 

 northern anchovy were selected as a predator 

 because it is the most abundant fish stock in the 

 California Current region and because it has a plank- 

 tivorous diet which includes fish eggs and larvae 

 (Baxter 1967; Hunter and Kimbrell 1980). Chub 



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