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Fishery Bulletin 105(2) 



mersal fish, such as plaice, by diurnal vertical migrants 

 (euphausiids, shrimp, etc.) which feed pelagically at 

 night but are found near the seafloor by day (McClel- 

 land, 1990). Larval P. decipiens infections are acquired 

 through consumption of various benthic invertebrates, 

 including mysids, isopods, amphipods, decapods, and 

 polychaetes (McClelland, 2002) The acanthocephalan 

 E. gadi, which was the most significant species in the 

 classification of 4T plaice stocks in this study, reach 

 maturity in the intestines of dozens of species of marine 

 fish in the North Atlantic and use gammaridean and 

 caprellid amphipods and mysids as intermediate hosts 

 (Marcogliese, 1994). 



Among influences on digenean infection parameters 

 in fish are the distributions and abundances of mol- 

 luscan intermediate hosts, where the parasites perform 

 one or more generations of asexual reproduction. Brittle 

 stars, which transmit Fellodistomum metacercariae to 

 plaice, become infected by feeding on cercariae which 

 develop in bivalve moUusks (Koie, 1980). In contrast to 

 the other helminths used as markers here, the digenean 

 S. baccatum, an important component of DFAs involving 

 4Vn plaice, is transmitted actively, through penetration 

 of the skin by cercariae (Wolfgang, 1955). MoUusks 

 that host the asexual reproduction of S. baccatum to 

 the cercarial stage are whelks, especially the common 

 or waved whelk (Buccinum undatum), which is widely 

 distributed and commercially exploited in eastern Ca- 

 nadian waters. 



Infection parameters of larval helminths in 4T-4Vn 

 plaice may also be influenced by temporal and spatial 

 distributions of the final hosts. The digenean S. bac- 

 catum matures and reproduces in the intestines of 

 large piscivorous fish such as sea raven (Hemitripterus 

 americanus) and Atlantic halibut {Hippoglossus hippo- 

 glossus) (Wolfgang, 1955), and the anisakine nematode 

 A. simplex matures and reproduces in the stomachs 

 of cetaceans (Rohde, 2005). Adults of the anisakines 

 Contracaecum osculatum and P. decipiens, and the 

 acanthocephalan C. strumosum occur, respectively, in 

 stomachs and intestines of seals. Surveys of various 

 demersals in waters off Nova Scotia, Canada (Marco- 

 gliese and McClelland, 1992; McClelland et al., 2000; 

 McClelland and Martell, 2001a&b) revealed that in- 

 fection parameters of P. decipiens and Corynosoma 

 wegeneri increased with proximity to Sable Island, site 

 of the largest grey seal (Halichoerus grypus) colony in 

 the Northwest Atlantic. 



Spatial disparities in prevalences and abundances 

 of parasitic helminths in fish may also be traced to 

 variation of physical parameters (temperature, salin- 

 ity, depth, and bottom habitat) that influence distribu- 

 tions of the invertebrate precursor hosts (Williams and 

 Jones, 1994). Our 4T plaice samples were collected at 

 relatively uniform depths (43-88 m), and the substrates 

 in both sampling areas ranged from sandy pelite to 

 sandy gravel and had outcroppings of sandstone bed- 

 rock (see Loring and Nota^). The mean near-bottom 

 temperature for sampling stations in eastern 4T was 

 1.62 (0.49-3. 23)°C (tt = 5), but only 0.21 (-0.01-0.77)°C 



(« = 9) for stations in western 4T. Near-bottom tempera- 

 tures prevalent on the Magdalen Shoals are extremely 

 low throughout the year (Swain et al., 1998), and small 

 variations in temperature may have dramatic effects on 

 the developmental and transmission rates of helminth 

 parasites, as well as on the distributions and devel- 

 opmental rates of their poikilothermic intermediate 

 hosts. Hence, the fact that acanthocephalan infections 

 in southeastern Gulf plaice were much heavier than 

 those found in plaice from the northwestern Magdalen 

 Shoals may, to some extent, reflect the relative warmth 

 of waters occupied in eastern 4T. 



In summary, DFAs of abundances of the acantho- 

 cephalans E. gadi and C. strumosum support the find- 

 ings of earlier mark-recapture studies (Powles, 1965), 

 which indicate the presence of distinct northwestern 

 (Miscou-Magdalen) and southeastern (Cape Breton) 

 plaice stocks in the southern Gulf of St. Lawrence. 

 Moreover, both parasite markers could be employed in 

 future studies of migration and mixing of the two 4T 

 stocks within 4T, and infection parameters of E. gadi 

 alone may prove useful for detecting the presence of 

 southeastern 4T migrants among stocks overwinter- 

 ing in Laurentian Channel waters of the Cabot Strait 

 and 4Vn. The strength of our conclusions may be miti- 

 gated, however, by the fact that 4T and 4Vn plaice were 

 sampled only during September and May, respectively, 

 and samples from the two areas were taken more than 

 a year apart. Hence, the possibility of seasonal or lon- 

 ger term variations in infection parameters of enteric 

 helminths, e.g., Fellodistomum sp. and E. gadi, could 

 not be investigated. Finally, given problems inherent 

 in stepwise procedures (Power et al., 2005), statistical 

 procedures employed in the present study, and in simi- 

 lar studies, could be improved upon. Power et al. (2005), 

 for example, adopted an "all possible subsets" approach 

 to selection of indicator parasites used in linear and 

 quadratic discriminant analyses and in nonparametric 

 classification of bogue landed at Spanish fishing ports. 

 Efforts will be made to address these shortcomings in 

 future surveys. 



Acknowledgments 



The authors are grateful to T. Hurlbut, Fisheries and 

 Oceans Canada, Moncton, for supervising the collection 

 of plaice samples during the 2004 groundfish cruise in 

 the southern Gulf of St. Lawrence. We also thank R. 

 Morin, Fisheries and Oceans Canada, Moncton, and 

 four anonymous referees for their helpful comments and 

 advice on earlier drafts of the manuscript. 



•' Loring, D. H., and J. G. Nota. 1973. Morphology and sedi- 

 ments of the Gulf of St. Lawrence. Fisheries Research Board 

 of Canada Bulletin, 182, 147 p. 116 Lisgart Street, Ottawa, 

 Canada, KIA 0H3. 



