VISUAL THRESHOLD FOR SCHOOLING IN NORTHERN ANCHOVY 



ENGRAULIS MORDAX 



John Hunter and Ragan Nicholl' 



ABSTRACT 



The visual threshold fur schooling was determined for two groups of 50 adult northern anchovy in the 

 laboratory. The index of dispersion and the mean distance to the nearest neighbor were used to measure 

 changes in schooling as a function of light intensity. The threshold light intensity for schooling, (i x 10" " 

 Wcm''-(2.6 X 10"'' mc), was estimated to occur at a depth of 30 m on a starlit night and at 38 mduringa 

 full moon, when the chlorophyll concentration is 0.2 mg Chi a ni"'^. At 2.0 mg Chi a ni"-^ the threshold oc- 

 curs at a depth of 8 m on a starlit night and at 20 m under full moon light. Sufficient light appears to exist at 

 night within the upper 10 m for schooling to occur in most of the habiUit of the anchovy. The vertical 

 distribution of newly spawned anchovy eggs indicated that the maximum depth of spawning may be similar 

 to the maximum depth of schooling and that the visual threshold for schooling could be used to forecast max- 

 imum spawning depth in the sea. 



Vision plays a primary role in the maintenance of 

 most fish schools in the sea. Other sense organs, par- 

 ticularly the lateral line, are important in coor- 

 dinating movements and spacing of fish within the 

 school (Pitcher et al. 1976), but it is unlikely that 

 lateral line sense alone is sufficient for maintaining 

 the integrity of schools at night in the sea. In fact, a 

 large number of laboratory studies indicate that if 

 light is sufficiently reduced, fish no longer maintain 

 schools (Whitney 1969; Blaxter 1970). Thus the 

 visual threshold for schooling and the depth of pene- 

 tration of light probably determine the maximum 

 depth at which pelagic fishes are able to school at 

 night in the sea. 



Our objective was to determine the visual thresh- 

 old for schooling in adult northern anchovy, 

 Engraulis mordax, and to use this information to 

 forecast the maximum schooling depth for anchovy 

 at night in the sea. This calculation is of ecological in- 

 terest because the maximum depth for schooling is 

 probably also the maximum depth for spawning and 

 for nocturnal feeding. Anchovy spawn only at night 

 and visual recognition of other fish is probably as 

 essential for spawning as it is for schooling. During 

 what we believe was spawning behavior, several 

 males rapidly pursued a female over an irregular 

 path, a tactic probably not possible using senses 

 other than vision. This is a casual laboratory observa- 

 tion and requires further documentation, however. 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La .Jolla, CA 

 92038. 



Northern anchovy feed at night as well as in the day 

 (Loukashkin 1970; Hunter and Kimbrell 1980). Light 

 may not be necessary for filter feeding, but it is 

 essential for particulate feeding, although somewhat 

 higher light levels may be required for feeding than 

 for schooling (Hunter 1968). Anchovy migrate 

 toward the surface at sunset (Mais 1974), and schools 

 are less frequently detected at night using sonar and 

 not detected with echo sounders (Smith 1970). 

 Nevertheless, no doubt exists that schooling con- 

 tinues at night because the anchovy fisher^' is typi- 

 cally a night fishery and because profiles of schools 

 are detectable at night owing to the bioluminescent 

 disturbances their movements produce (Squire 

 1978). 



METHODS 



Apparatus and Laboratory Procedures 



Two groups of 50 northern anchovy (group 1, 

 mean length = 10.5 cm SL; group 2, = 9.8 cm SL) 

 were maintained in a 4.6 m diameter tank supplied 

 with running seawater (group 1 , mean temperature 

 = 16.9°C; group 2, 21.0°C). To simplify photo- 

 graphic analysis the school of 50 fish was constrained 

 to a somewhat two dimensional form by maintaining 

 them in water 45 cm deep. The fish were fed adult 

 Artemia at the daily time of 1000, and the tank was 

 cleaned 1 h after feeding. 



The tank, which was constructed of blue vinyl, was 

 located in a light tight rectangular enclosure in which 

 the walls and ceiling were covered with white vinyl 



Manuscript accepted August 1984. 



FISHERY BULLETIN: VOL. 83, NO. 3, 1985. 



235 



