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, northwest Atlantic Ocean. However, the processes regulating the occurrence of these hydroid colonies (i.e., seasonality, growth, advection, diffusion, sinking, and predation) are not well understood. The objective of this study was to identify and quantify the potential role of predation by fish upon the unattached hydroids. The two components of this study were (i) analyses of historical fish trawl surveys and stomach contents data collected in the coastal northwest Atlantic Ocean (including Georges Bank), and (ii) laboratory experiments testing for the presence of selective feeding by juvenile cod (Gadus morhua) on hydroids relative to two co-occurring planktonic copepods (Calanus finmarchicus and Centropages hamatus). We found that 32 and 11 species of fish ingested hydroids in the coastal northwest Atlantic Ocean and Georges Bank, respectively, during 1973–1990. However, hydroids were rarely an important part of the diet of these fishes. The most important predator of these cnidarians on Georges Bank was winter flounder, with 28.0% of its population having ingested hydroids, with a mean % (by weight) of hydroids in the diet of 4.1%, during 1973–80. Laboratory experiments indicated juvenile cod ingested planktonic hydroids, but overwhelmingly preferred either of the two copepods as prey. While field and laboratory results indicated that a wide variety of fishes feed on hydroids, we concluded that emergences and disappearances of planktonic hydroids on Georges Bank are not greatly impacted by fish predation. Other factors, in particular physical processes (i.e., advection, diffusion, and sinking), seasonal cycles of activity and inactivity, and predation by invertebrates, should be examined.