VERTICAL PROFILES OF THEORETICAL CONTACT RATES BETWEEN LARVAL COD AND THEIR NAUPLIAR PREY IN LIGHT OF RECENT TURBULENCE MEASUREMENTS U.S. GLOBEC Georges Bank Workshop Woods Hole Oceanographic Institution 16-18 October 1995 Lewis S. Incze Bigelow Laboratory for Ocean Sciences This study focusses on the vertical distribution of copepod nauplii, important prey for early feeding fish larvae, and the distribution of turbulent mixing that might lead to enhanced rates of contact between the predator and its prey. For the workshop I present vertical profiles of nauplii, fluorescence, salinity and temperature at selected stations of the pilot program in May 1992. Nauplii were sampled using a pumping system with intake attached to a CTD. The pump delivered 5.3 m3 min-1 to the deck, where a manifold system was used to continuously subsample at a rate of 15.5 l min-1. The smaller volume was passed through a 40 æm mesh net for one minute to collect copepod nauplii and other small zooplankton. Turbulence in the upper mixed layer was estimated using local winds for six hours prior to sampling; turbulence for the lower, tidally-mixed layer (ò20 m off bottom) and for the pycnocline came from turbulence measurements in June 1995 (Neil Oakey, GLOBEC cruise SJ9508). Higher resolution pump profiling was done as a part of that cruise, but sample analysis is just beginning. Dissipation rates of 10-6 and 10-8 W kg-1 were selected as characteristic of those two domains, respectively, and at that level of averaging are applicable between years. These values agree with recent GLOBEC modeling results (Lynch, Naimie, Werner) and are being used in other models of turbulence and contact rates (Werner et al., this workshop). A complete analysis of the 1992 data set has been submitted to the GLOBEC special volume of Deep Sea Research. I use the term "contact rate" here to refer to the juxtaposition of a predator (anterior end) and its prey within a fixed distance, thus focussing on the physical spatial relationship and ignoring, for the present, the many biological parameters (search behavior, detection, pursuit, capture, handling time) that are poorly known and almost certainly highly variable. I scale the turbulent velocity calculation to a fixed length equal to a hypothetical radius of detection of prey by the predator. I use 0.26 cm, or ca. 1/2 predator body length, a conservative value still within the Kolmogorov length scale for most of the dissipation rates involved in this study -- exception being some low wind conditions in the upper layer. This approach gives rise to substantially lower estimates of turbulent enhancement of contact compared to the method of scaling to mean prey density (represented as N-0.333, where N=No. prey m-3). There is not yet consensus on the proper scale to use, and I forward what might be characterized as "conservative" estimates of turbulent enhancement of contact rates in larval cod. I use a steady predator cruising rate of 0.2 cm s-1, a slow swimming speed but perhaps not so bad for a simplification that ignores the actual pause-travel behavior seen in this larva and the fact that feeding larvae must stop to handle prey. Calculations suggest the following increases due to turbulence in the 1992 data set: from 34-219% in the upper layer, depending on wind strength and depth; 8% in the pycnocline and 110% in the 20-30 m portion of the water column beneath the pycnocline (that is, 20 m or more away from bottom). The hourly contact rates over the entire data set span more than one order of magnitude, from 0.6 to 26.5 prey h-1. At most stratified sites, the calculations predict that the best feeding conditions (contact rates) remain in the pycnocline, where naupliar concentrations were greatest (7 out of 9 stratified stations had maximum nauplii in the pycnocline). At the mixed sites, despite combined effects of wind and tidal turbulence, predicted contact rates between cod larvae and nauplii remained low (ó2 h-1) due to the over-all low abundance of nauplii. Contact rates (C) with and without turbulence are shown in Figure 1 for two stratified stations (11P and 24P) with different vertical structure (physics and biology) and different wind speeds (W), and for an average of two stations in the mixed area. The pycnocline is shaded; swimming speed and contact radius are as described above. Figure 2 shows the sensitivity of the calculations (data are for station 24P) to changes in swimming speed (v) of the predator (top panel) and changes in contact radius (R; lower panel; note that v and R have been reduced in this illustration only for convenience of plotting). Sensitivity to a 0.5 decade increase in Epsilon is intermediate between the responses shown for v and R The sensitivity to R shows how much more we need to know about scaling (R enters into the turbulent velocity calculation) and larval capabilities..... and we have not yet addressed variable behaviors, variable turbulence and the patchy distributions of prey caused by anisotropic turbulence at somewhat larger scales. Advances in theory, sampling and behavioral studies all are needed. Fig. 1 Theoretical encounter rates between cod larvae and copepod nauplii at selected sites with and without turbulence, using conservative parameters (pycnocline is shaded). Fig. 2 Sensitivity of encounter rates to changes in predator speed (v) and en- counter radius (R) using data for station 24P from Fig. 1, with turbulence.