274 



Fishery Bulletin 94(2), 1996 



Table 3 



Percent composition of diet and feeding statistics by length classes for larval Atlantic mackerel, Scomber scombrus, sampled 

 southwest of Sable Island (Scotian Shelf) from 18 to 23 July 1991. 



Copepodites and adult copepods. 



Include euphausiids, chaetognaths, gastropods, phytoplankton cell 



decreasing frequency. 



Nauplu. copepodites, and adult copepods. 



cladocerans, amphipods. euphausnd nauplii, and crab larvae in order of 



and the density of these prey in the environment. 

 The feeding ratio for copepod nauplii (mean number 

 of nauplii per gut) was not correlated with the den- 

 sity of nauplii in situ for any length class of Atlantic 

 mackerel larvae (P>0.33). The feeding ratio for cope- 

 pods (copepodites and adults) was negatively corre- 

 lated with the density of this prey for Atlantic mack- 

 erel larvae 6 to <7 mm (P=0.03, n=l) but not signifi- 

 cantly correlated with prey density for other length 

 classes (5 to <6 and >7 mm, P>0.67). 



In RMT collections, the relationship between the 

 incidence of piscivory (proportion of feeding Atlantic 

 mackerel with at least one fish larva in the gut) and 

 the density offish larvae of suitable prey size (sum 

 of Atlantic mackerels and redfishes <5.5 mm, yel- 

 lowtail flounders <6.5 mm, and silver hakes <2.5 mm ) 

 was adequately described by an Ivlev equation (Fig. 

 7, A-C). Incidence of piscivory was limited at densi- 

 ties of larval fish prey <0.1 larvae/m ! and quickly 

 reached a plateau as availability increased. The 

 height of the plateau (maximum incidence of 

 piscivory) increased with predator length. The pa- 

 rameters of the Ivlev equation varied little when the 

 data from the multi-net collections were added to the 

 RMT data set (Fig. 7, D-F). 



Effect of alternative invertebrate prey on 

 piscivory 



In the present study, the absolute predation rate could 

 not be assessed because the transit time for fish larvae 



O 03 



5 c 



Hi 



\\ 



Hour of day 

 Figure 4 



[)ail\ feeding act i vi t\ "I larval Atlantic mackerel, Scorn bci 

 scombrus. (A) Mean number (and standard deviation) of 

 invertebrate prey per dissected gut. (B) Percentage of dis- 

 sected mackerel larvae with at least one fish larva in the 

 gut. Black histograms represent nighttime, gray histo- 

 grams twilight, and white histograms daytime. Number 

 above time class indicates the number of dissected mack- 

 erel larvae. 



