Deree: Age and growth, dietary habits, and parasitism of Enchelyopus ambrius 



49 



.S 



o 



— X- back calculated 

 — ♦— observed 



X >-« w X X x-x  » 



/ i M M M I M I M M M I! I I X I I I I M M I M I M I M i V -X-X-X-X-X 

 95 115 135 155 175 195 215 235 255 275 295 315 



Total length (mm) 



Figure 7 



Length-frequency distribution of back-calculated lengths at age vs. observed lengths at age of £. 

 cimbrius collected during the GM 93-20 survey between 2 August and 13 August 1993 plotted in 

 groups of 5. 



The dietary habits of .E. cimbrius may contribute 

 to its constant absolute growth. Fourbeard rocklings 

 generally forage on small animals in large numbers 

 instead of large animals in small numbers. Fishes 

 that swim actively and forage for small organisms 

 (i.e. "grazers") generally have a higher unit of en- 

 ergy expenditure per unit of energy input than do 

 fishes that are lie-and-wait predators on large or- 

 ganisms. In addition, preference for cold tempera- 

 tures may explain further why E. cimbrius exhibits 

 constant linear growth. I speculate that£. cimbrius 

 uses its energy to forage; energy that would other- 

 wise have contributed to absolute growth. 



Table 6 



Prevalence (P), abundance (A), standard error (SE). and 

 intensity (I) of Raphidascaridinae sp., Grillotia erinaceus. 

 Genolmea laticauda, and Gonocerca phycidis in E. cimbrius 

 collected during the GM 93-20 and GM 94-12 surveys. 



Parasite 



P(%) 



A + SE I range 



Raphidascaridinae sp. 

 Grillotia erinaceus 

 Genolinea laticauda 

 Gonocerca phycidis 



Dietary habits 



Prey items ofE. cimbrius surveyed in this study were 

 predominately bivalves, particularly Yoldia sp., in both 

 number and frequency. Calanoid copepods, although 

 present in great numbers, were consumed by few indi- 

 viduals. The importance of copepods is replaced by 

 bivalves, Yoldia sp., and polychaetes in the diet of larger 

 fish ( > 1 19 mm TL ). After their first year, fish shift from 

 a diet dominated by planktonic copepods to a diet com- 

 prising infauna (bivalves and polychaetes). 



Calanoid copepods composed the majority (71.2%) 

 of the prey taken, with bivalves (although consumed 

 in relatively high frequency (80.0%)) second in im- 



portance by number ( 19.2%) in 1-year-old fish. Juve- 

 niles in the present study exhibited feeding prefer- 

 ences for various calanoid copepods similar to those 

 of pelagic larval gadids off the Isle of Man (Nagab- 

 hushanam, 1965.) This preference for planktonic or- 

 ganisms suggests that juveniles lead a less seden- 

 tary existence and feed more frequently in the water 

 column than do adults. In addition, the selection for 

 smaller prey is directly related to mouth size (Moyle 

 and Cech, 1988). 



On Georges Bank, fourbeard rocklings >100 mm 

 TL consumed motile epifauna, particularly Crangon 

 septemspinosa (46.3%), in the highest percentage by 



