Fortier and Villeneuve Cannibalism and predation by Scomber scombrus larvae 



279 



erel larvae increased from to > 10 prey. Given that 

 copepods were not selected for, it appears that the 

 time spent manipulating them or the satiation re- 

 sulting from their ingestion, rather than a switch in 

 prey preference, interfered with the predation by 

 Atlantic mackerel on newly hatched fish larvae. 



The noncompensatory nature of cannibalism 

 and piscivory by Atlantic mackerel larvae 



Cannibalism and interspecific predation have often 

 been proposed as mechanisms for the density-depen- 

 dent regulation of marine fish populations (Ricker, 

 1954; Rothschild, 1986). Because the probability of 

 encounter between Atlantic mackerel larvae should 

 increase with their density. Grave ( 1981) suggested 

 that cannibalism could be a simple mechanism for 

 the density-dependent (i.e. compensatory) regulation 

 of year-class strength in this species. For compensa- 

 tory mortality to occur, an increasing fraction of the 

 prey population must be removed per unit of time as 

 the abundance of prey increases. In the present study, 

 the capacity of mackerel larvae 5-14 mm long to prey 

 on conspecifics and other fish larvae increased ini- 

 tially (but at a decreasing rate) and then saturated 

 quickly as prey density increased (Fig. 7). Our results 

 suggest a type-II functional response (Holling, 1959) 

 in which mortality is actually depensatory, a decreas- 

 ing fraction of the prey population being removed per 

 unit time as prey density increases. This would be the 

 case when mackerel larvae prey on other species offish 

 larvae. In the case of cannibalism, percent mortality is 

 expected to remain constant (i.e. density-independent) 

 with increasing density of Atlantic mackerel larvae 

 because both prey and predators increase in number. 

 As predation on other fish larvae appears depen- 

 satory and cannibalism seems density-independent, 

 we conclude that piscivory by Atlantic mackerel lar- 

 vae 3-14 mm long does not contribute to the den- 

 sity-dependent regulation of population size. In the 

 laboratory, cannibalism by Pacific mackerel ceased 

 when schooling developed at the time of metamorpho- 

 sis ( approximately 15.2 mm SL) (Hunter and Kimbrell, 

 1980). In St. Georges Bay, the longest Atlantic mack- 

 erel cannibal observed was 15.2 mm (Ware and Lam- 

 bert, 1985), but Grave (1981) reported intense canni- 

 balism in early juveniles 13-19 mm SL in the North 

 Sea. Whether piscivory by Atlantic mackerel >14 mm 

 SL is compensatory remains to be determined. 



Potential impact of cannibalism and piscivory 

 by Atlantic mackerel larvae on recruitment 



While predation on conspecifics and other species of 

 fish larvae by Atlantic mackerel larvae <14 mm is 



not compensatory, the resulting mortality may nev- 

 ertheless have some important impact on year-class 

 strength. The latitudinal and vertical distributions 

 of Atlantic mackerel, yellowtail flounder, and silver 

 hake larvae generally coincided on Sable Island 

 Bank, and predation or cannibalism by mackerel lar- 

 vae could be of significance during the early life of 

 these species. As mentioned earlier, the actual mor- 

 tality rate attributable to predation by Atlantic mack- 

 erel larvae cannot be determined without an esti- 

 mate of the transit time offish larvae in the gut. In 

 the laboratory, half the gut content of Pacific mack- 

 erel larvae fed Brachionus plicatilis was evacuated 

 in 2 h (Hunter and Kimbrell, 1980). In the North 

 Sea, digestion time (from food intake to complete 

 defecation of the last food remains) for Atlantic mack- 

 erel larvae 8-14 mm, feeding on copepods, cladocer- 

 ans, and conspecifics was 8-10 h (Grave, 1981). 



In the present study, there were several indica- 

 tions that the digestion of fish larvae by Atlantic 

 mackerel larvae was a relatively slow process. First, 

 a single fish larva was found in the gut of mackerel 

 larvae in 97% of the cases. Second, the frequency of 

 larval fish prey in the gut increased progressively 

 during the day (Fig. 4), an unlikely result if diges- 

 tion and evacuation were rapid. Third, the propor- 

 tion of fish larvae in the fore- and midgut of mack- 

 erel larvae remained high during the day, suggest- 

 ing a slow transit time due to the large size of the 

 prey (Fig. 5). Finally, the high frequency of partly or 

 well-digested fish larvae ( >90% ) in contrast to slightly 

 digested material, also suggested that fish larvae 

 remained for a long time in the gut. All our observa- 

 tions are consistent with a continuous search for fish 

 larvae, saturation of the digestive tractus upon the 

 capture of a larva, slow digestion completed during 

 the night, and evacuation in the early morning. Thus, 

 although the time between capture and complete 

 assimilation of a fish larva could be faster than we 

 estimate, our results suggest a digestion time of 24 h. 



The impact of predation by Atlantic mackerel lar- 

 vae was limited primarily to newly hatched fish lar- 

 vae. Assuming a conservative 24 h for the time be- 

 tween capture and evacuation, the percent mortal- 

 ity rate of fish larvae of suitable prey size attribut- 

 able to predation by Atlantic mackerel larvae ranged 

 from 7% to 57%/d (average of 31%/d). A value of 8 h 

 for digestion time (Grave, 1981) would triple these 

 estimates. Recent studies have often reported high 

 mortality rates for the period just before and after 

 yolk resorption when the swimming abilities of the 

 fish are still poorly developed: 809<7d in jack mack- 

 erel, 7}-achiirus symmetricus (Hewitt et al., 1985), 

 56% and 60%/d in capelin (Fortier and Leggett, 1985; 

 Taggart and Leggett, 1987). For Atlantic mackerel. 



