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Fishery Bulletin 104(2) 



al. (2005) noted that in the study region, pelagic fish 

 eggs are generally more abundant near the surface and 

 decrease exponentially in abundance with increasing 

 depth. As a result, if feeding by capelin occurs predomi- 

 nantly in surface waters, the effective density of prey 

 encountered would be greater than the average density 

 over the water column, which would in turn decrease 

 the estimate of the effective volume swept based on 

 our estimates of encounter rates. However, the pat- 

 terns of vertical distribution of both eggs (Pepin et al., 

 2005) and capelin (O'Driscoll et al., 2001) are highly 

 variable among sites and without accurate estimates 

 of these elements from the current study, inferences 

 about the impact on estimates of the effective volume 

 swept would be speculative. In contrast, I implicitly 

 incorporated the probability of attack into the estimate 

 of the effective volume search, essentially giving it a 

 value of 100%. However, Wieland and Koster (1996) 

 found that the visibility of eggs increased with devel- 

 opmental stage and hence may alter the probability 

 of attack by a predator. Lower visibility of early stage 

 eggs could lead to an increase in the estimate of the 

 effective volume swept, but because we could not stage 

 most eggs sampled from the stomachs, the net effect on 

 the estimated volume swept is unclear. 



Planktivorous fish represent important forage for 

 piscivores in several marine ecosystems but they can 

 also play a significant role as predators of early life 

 stages. In the Baltic Sea, spratt iSprattus sprattus L.) 

 feed extensively on cod eggs (Koster and MoUmann, 

 2000) and they may play an important role in regulat- 

 ing the underlying form of the stock-recruitment rela- 

 tionship (Koster et al., 2001). In upwelling systems, 

 anchovies and sardines may also have a significant 

 impact on egg and larval mortality rates (Smith et 

 al., 1989). It has also been suggested that herring 

 (C harengus L.) and mackerel (Scomber scombrus L.) 

 predation on ichtyplankton may play an important 

 role in fish population dynamics in the Gulf of St. 

 Lawrence (Swain and Sinclair, 2000) and on Georges 

 Bank (Garrison et al., 2002). However it is unclear 

 whether capelin play a significant role in the fish 

 community dynamics in the ecosystems they inhabit, 

 despite evidence that they are likely to have a signifi- 

 cant impact on zooplankton abundance (Vesin et al., 

 1981; Akenhead et al., 1982; Skjoldal and Rey, 1989; 

 Hassel et al., 1991). Clupeids, scombrids and engrau- 

 lids can alternate between filter- and particulate-feed- 

 ing modes, which may allow them to more effectively 

 consume relatively small prey (~1 mm), such as fish 

 eggs. There is no information on the feeding modes of 

 capelin, but another osmerid, rainbow smelt (Osmerus 

 mordax, Mitchill 1814), is known to be a primarily 

 particulate feeder (Mills et al., 1995). Particulate feed- 

 ing is generally directed toward larger prey, whereas 

 filter feeding is used when prey are smaller or more 

 abundant. Gill raker structure does not provide signifi- 

 cant insight into the possible feeding modes of these 

 various planktivorous fish. The estimate of reactive 

 distance from the present study (0.024-0.034 m) is 



of the same order as the gape width of capelin (3.6% 

 for TL; Pepin, unpubl. data), making filter feeding a 

 distinctly possible mode of feeding. For known filter 

 feeders, Uotani (1985) reported a maximum gill raker 

 length of 6 mm in Engraiilis japonicus (100 mm TL), 

 Gibson (1988) reported an average gill raker length of 

 4 mm in Clupea harengus (150 mm TL), and Molina et 

 al. (1996) reported gill raker lengths ranging from 5 to 

 9 mm in Scomber japoiucus (150-250 mm SL). On the 

 other hand, Lecomte and Dodson (2004) reported gill 

 raker lengths of 3.3-3.9 mm in Osmerus mordctx (150 

 mm TL). In the case of capelin, I found that gill rak- 

 ers ranged from 3.0-4.5 mm in length (130-170 mm 

 TL) (Pepin, unpubl. data). Inter-raker gaps among 

 all these species ranges from 0.19 mm (£. japonicus) 

 to 0.4 mm (S. Japonicus) and from 0.36 to 0.45 mm 

 in capelin (Pepin, unpubl. data). Thus all these spe- 

 cies, which feed extensively on zooplankton but which 

 use different ingestion modes, are equipped with gill 

 rakers designed to retain small particles. To better 

 understand the role of capelin in Arctic ecosystems, a 

 greater knowledge of feeding will be required. If filter 

 feeding is a dominant feeding mode, a simple filtra- 

 tion or volume-swept model may provide an accurate 

 measure of the potential impact of capelin on fish eggs, 

 whereas greater knowledge of behavior and selectivity 

 will be required if particulate feeding is the dominant 

 feeding mode because selection for or against fish eggs 

 could be influenced by availability of alternate prey 

 (e.g., Kean-Howie et al., 19881. 



In most of the areas where planktivores can have 

 a significant impact on the mortality of fish eggs, the 

 ecosystems can be characterized as being "wasp-waist- 

 ed" (Cury et al., 2000), i.e, as having a relatively high 

 diversity of plankton species and top predators that 

 are all influenced by the abundance and productivity 

 of relatively few and dominant forage fish species that 

 act to transfer energy from secondary producers to up- 

 per trophic levels in a manner similar to that found 

 in many Arctic regions. It is clear that capelin are 

 important prey for a number of top predators in the 

 waters of the Barents Sea (Gjosaster, 1998), Iceland 

 (Vilhjalmsson, 2002), and on the Newfoundland and 

 Labrador coast (Akenhead et al., 1982). and they may 

 also have an important impact on the plankton commu- 

 nity (Skjoldal and Rey, 1989; Hassel et al., 1991). What 

 remains uncertain is whether they play an important 

 regulatory role in the dynamics of the early life stages 

 of fish. Observations from this study revealed that the 

 highest occurrence of fish eggs in the stomachs of cap- 

 elin came from capelin near one of the few remaining 

 major spawning aggregations of the northern cod stock 

 (Rose, 2003). The dynamics of such circular prey-preda- 

 tor interactions need to be studied further in order to 

 understand their potential importance to population 

 regulation (Walters and Kitchell, 2001) but to do so 

 will require consideration of the spatial and temporal 

 patterns of predator-prey interactions if predictive re- 

 lationships are to be derived (Pepin et al., 2002, 2003; 

 Baumann et al., 2003). 



