Relationships among vertically structured in situ measures of turbulence, larval fish abundance and feeding success and copepods on Western Bank, Scotian Shelf

Using vertically stratified data of the abundance of silver hake (Merluccius bilinearis ) larvae and concentrations of copepods collected in the field, we examine relationships among the vertical distribution of larval fish, their potential prey, feeding success and water column turbulence. Water column turbulence and associated stratification parameters were estimated from: (i) in situ measures of turbulent kinetic energy dissipation (epsilon) provided by an EPSONDE profiler; (ii) in situ wind speed; (iii) the Richardson number (Ri); and (iv) the buoyancy frequency (N-2). Small (< 5 mm total length) silver hake were more abundant in the least turbulent waters (i.e. at a minimum in the rate of dissipation of turbulent kinetic energy, epsilon < 10(-7) W kg(-1); Ri > 0.25; N-2 > 0.001 (rad s(-1))(2)). Partial correlations amongst epsilon, N-2 and small hake larvae were significant only for N-2. The abundance of larger (> 5 mm total length) hake larvae was positively correlated with depth and was not associated with either epsilon or N-2. Vertical distributions of three potential prey (classified by stage) were variable. Early stage copepodids were positively correlated with N-2 and negatively correlated with epsilon . We found no evidence of diel distribution patterns for small (< 5 mm total length) hake larvae or for any of the developmental stages of the copepods examined. Neither estimate of water column turbulence inferred from wind speed nor from Ri was meaningfully related to in situ estimates of epsilon or to larval fish abundance. Feeding success, measured either as prey items (gut)(-1), average prey length, or total prey volume (gut)(-1), was not related to predicted encounter rates between days. However, the average prey length (gut)(-1) was significantly (P < 0.01) related to water column turbulence. These conflicting results suggest that the relationship between larval feeding and the environment is more complicated than assumed. We conclude that without substantial high resolution in situ examination of the relationship between the vertical distributions of turbulence, larvae and their prey, the growing acceptance in the secondary literature that turbulence has a positive and biologically meaningful effect on trophic interactions between fish and their zooplankton prey (a generalization based largely on modelling and laboratory experiments) is premature.