The numbers of larvae larger than 7.75 mm. 

 taken by plankton nets are undoubtedly affected 

 by "net dodging." Most other larval fish taken 

 with jack mackerel in plankton nets show an 

 increase in the ratio of night-caught to day- 

 caught larvae with increasing size of larvae. 

 Farris found that the ratio of night-caught to 

 day-caught larval jack mackerel remained 

 approximately 1:1 for larvae 2 to 12 mm. 

 He concluded that jack mackerel do not evade 

 the net. A second interpretation is that they 

 avoid the net equally well both by day and 

 night. 



The principal rays of the dorsal fin first 

 appear at 5 to 6 mm.and a full complement is 

 attained by 8 mm. The remaining fins begin 

 to form at 6 to 8 mm. The resulting increased 

 motility could decrease the capture of larvae 

 by the net from a probable 100 percent (over 

 the length range 3.50 to 7.75 mm.) of the larvae 

 available to the net to considerably less. 



Survival also appears to increase at this 

 point. If the initial survival rate continued to 

 18.5 mm,, only 1 larva of the original 1 million 

 would survive and the adult stock could not 

 replace itself. The first reduction in mortality 

 appears to take place at about 8 mm. when the 

 larvae become motile. 



The apparent relation between numbers of 

 larvae and length for the range 8.75 to 13.75 

 mm. may be described by log Y = 5.57267 - 

 0.24594X, at this size range survival is 57 

 percent for each millimeter of length increase. 

 This rate of survival would reduce the larval 

 population to 1 fish at 22.5 mm. More prob- 

 ably, the larval mortality is decreasing with 

 increasing size and motility of the fish above 

 8 mm.; the increasing nnotility of the fish also 

 increases its ability to avoid the net, thus 

 causing progressively greater undersampling 

 of fish over 8 mm. There is no indication of 

 a "critical period" in survival, and the only 

 "abrupt" change in the survival curve occurs 

 at the time of fin formation. 



3.23 Adolescent phase 



Little is known about the juvenile jack 

 mackerel. Food studies show that copepods, 

 euphausiids and pteropods constitute most of 

 the food in stomachs, and that copepods are a 

 more important food among juveniles than 

 among adults (Anonymous, 1953). 



3.3 Adult phase 



3.31 Longevity 



Most of the fish taken in the commercial 

 catch are less than 6 years old; the majority 

 are 2, 3, and 4 years old. On rare occasions 

 very large jack mackerel from 10 to over 25 



years old appear in the commercial catch 

 (Anonymous, 1953) and some fish taken in the 

 sport fishery have been reliably aged at over 

 30 years (Fitch, 1956). 



3.32 Hardiness 

 No data. 



3.33 Competitors 



The Pacific mackerel, which is often caught 

 with the jack mackerel and which has many 

 similar habits, is probably the principal com- 

 petitor. 



3.34 Predators 



Predation other than by man has not been 

 studied. 



3.35 Parasites, diseases, injuries, 

 and abnormalities 



No data. 



3.4 Nutrition and growth 



3.41 Feeding 



Feeding takes place at any time of day. 

 One method of catching mackerel at night 

 makes use of chumming under lights, but 

 whether or not they feed in the dark is un- 

 known. Food is taken by selection and pursuit 

 of individual food items. 



3.42 Food 



One study of food habits revealed that 90 

 percent by numbers of identifiable items in 

 jack mackerel stomachs consisted of 

 euphausiids, large copepods, and pteropods. 

 Samples from cannery landings showed that 

 at times jack mackerel feed almost exclusively 

 on juvenile squid and anchovies. Large jack 

 mackerel taken in offshore waters at night 

 contained lantern fish. Both the mackerel and 

 lantern fish were probably attracted to a light 

 suspended over the stern of the vessel. Large 

 jack mackerel taken by the sport fishery are 

 usually caught with large adult anchovies as 

 bait (Fitch, 1956). 



3,42 Growth rate 



Fish 2 years old are about 250 mm. long and 

 3 years, 350 mm. (Fitch, 1956), No other data 

 have been published on growth of juvenile or 

 adult jack mackerel. The following data on 

 preserved specimens are available for condi- 

 tion factor K = ^^^^^t x 10? 



Standard length3). 



11 



