Fortier and Villeneuve: Cannibalism and predation by Scomber scombrus larvae 



275 



| Foregut rj Midgut Q~J Reargul/empiy gut 



24 25 265 68 206 104 



B H sli 9 ml V □Parity | | Well digested 



24 25 265 68 206 104 



00 04 08 12 16 20 24 



Hour of day 



Figure 5 



Diel changes in the position (A) and state of digestion (B) 

 of larval fish prey in the gut of larval Atlantic mackerel, 

 Scomber scombrus. Histograms represent the percentage 

 of mackerel larvae with larval fish prey in a given position 

 in gut (A) and the percentage of mackerel larvae with lar- 

 val fish prey in a given state of digestion (B). Numbers 

 above each time class indicate the number of Atlantic mack- 

 erel larvae dissected. 



-r-r-A, 



nJ 



lib 



Atlantic mackerel 

  n= 1.137 

 □ n = 33 



M^ , i 



Yellowtail flounder 

  n = 467 



n "=8 



ittUlw 



*4-*^n 



Silver hake 

 □ n = 7 



ouu^ 



 i — i — i— i — 1 1 1 1 1 1 — i— i 



2 3 4 5 6 7 8 9 10 1) 12 13 



Standard length (mm) 



Figure 6 



Frequency distribution of the standard length of etha- 

 nol-preserved fish larvae in the plankton I black histo- 

 grams) and in the gut of larval Atlantic mackerel. 

 Scomber scombrus (white histograms). Horizontal bars 

 on top of histograms represent the range of hatching 

 length (Fahay, 1983; this study). 



in the gut of Atlantic mackerel larvae is unknown. 

 However, assuming a digestion time of 24 hours, a rela- 

 tive daily predation rate (PR, in %/d) was defined as 

 the proportion of all fish larvae of suitable prey size in 

 a sample that was found in the gut of Atlantic mack- 

 erel larvae: 



PR = (NP a x \00)/iNP L , + DP), 



& a 



where NP 7 = the density offish larvae in mackerel 

 gut per m 3 of water; and 

 DP = the density of larvae of suitable prey 

 size in the plankton per m 3 . 



Predation rate by Atlantic mackerel larvae 3-14 

 mm long ranged from 7% to 57%/d (average of 31%/ 

 d) of the standing stock of the newly hatched fish 



larvae of suitable prey size (or 2% to 27%/d of all fish 

 larvae). Daily predation declined linearly with in- 

 creasing density of copepod nauplii (Fig 8A), the only 

 other prey strongly selected for by Atlantic mackerel 

 larvae (Table 4). Sample size for the regression was 

 low (8) because few RMT collections (for which esti- 

 mates of zooplankton density are available) contained 

 enough Atlantic mackerel larvae to calculate a reli- 

 able index. Daily predation was not significantly cor- 

 related with the density of copepods (P=0.991 ) or the 

 combined density of nauplii and copepods (P=0.658). 

 Daily predation on fish larvae of suitable prey size 

 decreased asymptotically as the average number of 

 alternative copepod prey (nauplii + copepodites + 

 adults) consumed by Atlantic mackerel larvae in- 

 creased (Fig. 8B). The relationship was best described 

 by a reciprocal model; 24% of the variance in daily 



