Planktonic hydroids on Georges Bank: ingestion and selection by predatory fishes

Planktonic colonial hydroids (Clytia gracilis) recently have been found to be abundant, but patchily distributed in time and space, on Georges Bank, n orthwest Atlantic Ocean. However, the processes regulating the occurrence o f these hydroid colonies (i.e., seasonality, growth, advection, diffusion, sinking, and predation) are not well understood. The objective of this stud y was to identify and quantify the potential role of predation by fish upon the unattached hydroids. The two components of this study were (i) analyse s of historical fish trawl surveys and stomach contents data collected in t he coastal northwest Atlantic Ocean (including Georges Bank), and (ii) labo ratory experiments testing for the presence of selective feeding by juvenil e cod (Gadus morhua) on hydroids relative to two co-occurring planktonic co pepods (Calanus finmarchicus and Centropages hamatus). We found that 32 and 11 species of fish ingested hydroids in the coastal northwest Atlantic Oce an and Georges Bank, respectively, during 1973-1990. However, hydroids were rarely an important part of the diet of these fishes. The most important p redator of these cnidarians on Georges Bank was winter flounder, with 28.0% of its population having ingested hydroids, with a mean % (by weight) of h ydroids in the diet of 4.1%, during 1973-80. Laboratory experiments indicat ed juvenile cod ingested planktonic hydroids, but overwhelmingly preferred either of the two copepods as prey. While field and laboratory results indi cated that a wide variety of fishes feed on hydroids, we concluded that eme rgences and disappearances of planktonic hydroids on Georges Bank are not g reatly impacted by fish predation. Other factors, in particular physical pr ocesses (i.e., advection, diffusion, and sinking), seasonal cycles of activ ity and inactivity, and predation by invertebrates, should be examined. (C) 2000 Elsevier Science Ltd. All rights reserved.