NOTE Bertram Size-dependent predation risk in larval fishes 



373 



significant positive relationship between the minimum 

 mass of prey consumed and prey length when those 

 five trials were excluded from the analysis (F=43.9, 

 df=l,16, P«0.001). Thus, both estimates of prey con- 

 sumption indicate a positive relationship between prey 

 length and total mass of prey consumed. 



My analysis is consistent with the interpretation 

 that predators actively select the largest, most prof- 

 itable prey to maximize both the mass of prey con- 

 sumed and the caloric gain from an attack, as pre- 

 dicted by foraging theory. Because of this pattern of 

 prey selection, predators consumed significantly 

 more biomass as prey size increased, between trials, 

 despite a decline in average predation mortality. 

 More generally, the result confirms that larger lar- 

 vae may be subject to higher rates of predation not- 

 withstanding that their overall vulnerability to pre- 

 dation is inversely related to length. A future chal- 

 lenge will be to incorporate this result from several 

 levels of analysis into longitudinal studies that ex- 

 amine the effects of mortality caused by predation 

 on the size structure of cohorts of growing prey. 



The above arguments on prey profitability are con- 

 sistent with the interpretation of Pepin et al. (1992) 

 that predators actively select larger prey because of 

 greater encounter or attack probabilities that result 

 from greater activity or a longer perception distance 

 (i.e. the distance that a larva is visible to a predator) 

 relative to smaller prey. Indeed predators may have 

 selected larger prey because of the combined effects 

 of higher profitabilities and encounter rates for larger 

 prey. It is noteworthy, however, that prey encounter 

 rates and profitability may not always increase in 

 unison with increasing prey size because encounter 

 rates will be dependent on prey behavior and the 

 perception capabilities and size of the predator. Sepa- 

 rating the general effects of prey encounter rate and 

 profitability on predator size-selectivity (see also 

 Juanes and Conover, 1994) will require mechanistic 

 studies and modelling efforts that address prey be- 

 havior, profitability of the prey to a predator, as well 

 as encounter, attack, and capture rates, all of which 

 will require corresponding information on predator for- 

 aging technique and perception capabilities over a broad 

 range of prey sizes and prey-to-predator size ratios. 



Acknowledgments 



I thank D. A. Roff for the discussion which prompted 

 the analysis presented herein. P. Pepin graciously 



provided unpublished data. T J. Miller, W. C. Leggett, 

 P. Pepin, and three anonymous reviewers helped to 

 improve the paper. Support was provided by a Natu- 

 ral Sciences and Engineering Research Council of 

 Canada post doctoral fellowship. 



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