Lough Report

### Larval Fish Vertical Distribution, Growth and Condition in Relation to Prey Field and Turbulent Mixing Intensity

    INVESTIGATORS:

    Gregory Lough
    Northeast Fisheries Science Center
    Woods Hole, MA 02543
    (508) 548-5123
    glough@whsun1.wh.whoi.edu

    David Mountain
    Northeast Fisheries Science Center
    Woods Hole, MA 02543
    (508) 548-5123
    dmountai@whsun1.wh.whoi.edu

    Laurence Buckley
    Northeast Fisheries Science Center
    Narragansett, RI 02882
    (401) 792-6671 - URI
    (401) 782-3368 - NEFSC
    lbuckley@gsosun1.gso.uri.edu

    Elaine Caldarone
    Northeast Fisheries Science Center
    Narragansett, RI 02882
    (401) 782-3220
    ecaldaro@whsun1.wh.whoi.edu

    GRANT PERIOD:  October 1, 1991 - September 30, 1996

#### STATEMENT OF OBJECTIVES:

Water column density stratification has been shown to have a significant influence on the feeding and survival of larval fish by concentrating larvae and their prey in the surface waters (Buckley and Lough, 1987). This proposal investigates the feeding and nutritional condition of larval cod and haddock populations as it relates to prey abundance and water column structure on Georges Bank in the spring. A number of hypotheses can be examined which include:

1. A stratified water column favors rapid growth and survival of cod and haddock larvae;

2. Haddock larvae are more dependent than cod larvae upon development of a stratified water column;

3. In stratified waters the depth of capture has an effect on larval growth;

4. In well-mixed waters no difference is observed in growth of larvae captured at discrete depths;

5. In stratified waters larvae captured in the vicinity of the thermocline grow faster than larvae captured at other depths;

6. The prey field has an effect on larval growth rate;

7. Initial larval size decreases as the season progresses;

8. Larvae spawned later in the year grow faster than larvae spawned earlier in the year;

9. Storm events (mixing) temporarily disrupt the larval prey field resulting in lower short-term growth of cod and haddock;

10. Larval cod grow faster than haddock at low prey densities;

11. Larval haddock grow faster than cod at high prey densities.

#### STATEMENT OF WORK:

Research cruises have been conducted on Georges Bank as pilot studies in May 1992 (AL9205), 1993 (AL9306) and 1994 (AL9403II) and as part of the full-scale program in March (SJ9503), April (SL9505) and May (SJ9507) 1995. During the first several days of the process cruise a concentration of cod/haddock eggs/larvae was located by a grid of stations to characterize the larval fish, plankton, and temperature-salinity field in an area sufficiently large to encompass the anticipated advection in which the fine-scale station studies will be carried out. The survey grid was situated so that the stations overlap the shoal front of the well-mixed waters (<60 m) and the shelf-/slope-water front (c. 100 m). Based on real-time sample analyses made during the grid survey, stations were selected across the larval patch for the fine-scale time series observations and a drogue(s) was deployed at the depth corresponding to the weighted center of gravity of the larval vertical distribution. On the drogued station, the sampling scheme used was a combination of fine- to micro-scale observations in order to sample fish larvae and their prey, and environmental parameters. Time series observations were made on station every 1-3 hours, encompassing at least 2 tidal periods. Using this sampling scheme the vertical distribution of larvae and their prey were characterized from the well-mixed shoal waters across the deeper stratified water along the flank of Georges Bank. Over the course of the cruise, detailed snapshots of the seasonal stratification process were developed.

The principal sampling gear used to make vertically stratified hauls for fish eggs, larvae and the larger invertebrate zooplankton was the 1-m2 MOCNESS with 0.333-mm mesh nets (Lough and Potter 1993). The 1/4-m2 MOCNESS with nets of 0.064-mm mesh, was used to sample the smaller prey (e.g., nauplii). Depth strata for both larval fish and zooplankton was nominally at 10-m intervals in the upper 60 m and at 20-m intervals between 60-100 m depth. Extra nets not used in the vertical profile sampling were used to collect specimens for special preservation. Specimens were designated for formalin preservation (larval gut contents), 95% alcohol preservation (larval otoliths), and freezing (biochemical analyses).

All cod/haddock eggs, larvae and juveniles will be removed from the 1-m MOCNESS samples, identified and their standard lengths measured. Larval prey selection and feeding rate (number of prey per larvae) will be determined by larval gut contents identified to the lowest level possible. Zooplankton from the MOCNESS samples will be identified and enumerated and an estimate of biomass determined. For each MOCNESS haul, the fish eggs, larvae, and zooplankton will be standardized to produce a vertical profile, as well as vertical profiles of temperature, salinity, light level, and fluoresence. A complete larval fish prey field will be comprised from the MOCNESS samples. Larval and juvenile fish will be grouped into suitable size classes for comparison with discrete prey fields characterized from the gut contents. These data will be compared with the site-specific time series measurements of water column structure during the stratification process.

The RNA, DNA, and protein content, and lengths of each larvae are being determined from each research cruise. RNA content is correlated with the rate of protein synthesis, while DNA content is an index of cell number. The RNA-DNA ratio, therefore, reflects the protein synthetic capability of larval fish and can be used to estimate in situ protein growth when calibrated with laboratory experiments. The ratio of RNA to DNA is a useful biochemical index of nutritional condition of larvae, the larger the ratio, the greater the potential protein synthetic capacity of the larvae. RNA-DNA ratio is an index of the nutritional condition of the larva over the past 2-3 days.

Larval otolith analysis will be performed on the alcohol preserved specimens to accurately age the larvae and derive growth curves using the methods described in Bolz and Lough (1988). At the individual level, the outer-most otolith increments will be examined in terms of width to correlate with larval morphological parameters, condition (RNA/DNA ratios), and prey field. In this way we may be able to index daily growth with measures of the available prey in different water column conditions. With such an index it may be possible to use otolith analysis to assess which fraction of the larval population that survives, and to indicate conditions that survivors encountered at various times in their early life.

The evolution of vertical stratification along the southern flank and associated meterological sequence of events needs to be summarized for the experiment relative to the biological time series. Essential physical parameters for correlation with biological vertical profiles will be taken from the moored current meter arrays and the shipboard CTD time-series profiles. Empirical measures of vertical current shear in the water column will be used to parameterize estimates of small-scale turbulent mixing intensity, which are important for comparing predator-prey contact rates. The contribution of wind and tidal stress in shoal areas to turbulent mixing intensity will be evaluated.

Implicit within the dynamics of well-mixed versus stratified waters is the theory of turbulence related predator-prey contact rates (Rothschild and Osborn, 1988). An initial descriptive approach to comparing different turbulent environments will be to use a three-dimension, multivariate response-surface analysis of feeding rates of size specific larvae versus prey density and turbulence level. Ultimately, we are working towards a coupled physical-biological model to evaluate the potential effects of climate change on larval survival through changes in food availability due to changes in the stratification process on Georges Bank.

#### SUMMARY OF KEY FINDINGS:

All fish eggs and larvae have been removed from the 1992 and 1993 MOCNESS samples, identified, measured and standardized vertical profiles made. Processing of the 1994 and 1995 samples is in progress; selected profiles representative of each site can be found in Cruise Reports. Cruise Reports are available from the principle investigators or the U.S. GLOBEC Georges Bank Program Service and Data Management Administrator, Robert Groman, WHOI, Swift House, MS #38, Woods Hole, MA 02543, (508) 457-2000 x2409, rgroman@whoi.edu, FAX:(508) 457-2169.

Biochemical analysis of the larval fish collected during the May 1992 cruise has been completed and a manuscript prepared. Insufficient numbers of larvae were collected at stratified areas for comparison with the well-mixed sites. Based on their RNA-DNA ratio, cod larvae sampled from the well-mixed sampling sites appeared to be in good condition and growing rapidly. No significant differences were observed in RNA-DNA ratios or RNA concentration of cod larvae collected during day tows compared to night tows. The RNA-DNA ratio of cod and haddock differed with depth of capture at the well-mixed sites sampled, although the sample sizes were small. Ten percent of the haddock sampled at the mixed sites were in poor condition and tended to be found in the surface 10 m. Further interpretation of the values is hampered by the lack of corresponding prey field data.

All of the larvae collected during the 1993 and 1994 field seasons have been analyzed for their RNA, DNA, and protein content. Statistical analysis of the results is in progress. Biochemical analysis of the 1995 field caught larvae has begun. Weather conditions during the 1993-95 research cruises allowed for larvae and their prey field to be sampled from both well- mixed and stratified areas of Georges Bank. These samples will provide a good data base for the development of models of the stratification process and its effect on larval growth and survival. The larger number of larvae sampled at discrete depths during these cruises will allow us to investigate the possible segregation of larvae at different depths, as suggested by the 1992 field season data.

A laboratory study relating temperature, larval cod RNA-DNA ratio and growth rate has been completed. A series of cod larvae reared at three temperatures and three food levels were sampled. The larvae are being analyzed for their RNA, DNA, and protein content and measured for length. A general model relating temperature, larval RNA/DNA and growth rate is being constructed.

Preliminary analysis of larval cod and haddock otoliths collected during the May 1993 cruise has begun. The samples being examined are from fish that have been analyzed for RNA/DNA content; otoliths are extracted from the resultant residue. Because of the minute size of the otoliths the recovery rate is only 35%, but the large number of samples processed offsets this low rate. Examination of the recovered otoliths shows them to be in excellent condition. For each otolith, daily growth increments are counted, and measurements are made to determine the width and length of the sagittae, the diameter of the nuclear check and the yolksac check, and the width of the last three increments. RNA/DNA values will be compared to these measurements in an attempt to correlate the growth patterns evident in the otolith microstructure with the larval condition index biochemically.

The Georges Bank Information System for the NW Atlantic Study is based on the Joint Global Ocean Flux Study (JGOFS) data management software system. This software is a distributed, object-based data system, with the primary data sets residing with the responsible scientists when possible. It is built upon a standard protocol (National Center for Supercomputing Application's HyperText Transmission Protocol) and uses NCSA's HTTP Daemon (HTTPD) on machines serving data. Electronic access and viewing of both data and information is done via World Wide Web brower software such as Mosaic and Netscape. Anyone with a computer connected to the network and the browser software can access the Program's information, documentation and data with the uniform resource locator (URL): http://globec.whoi.edu.

#### REFERENCES:

Bolz, G. R., and R. G. Lough. 1988. Growth through the first six months of Atlantic cod, Gadus morhua, and haddock, Melanogrammus aeglefinus, based on daily otolith increments. Fish. Bull., U.S. 86: 223-323.

Buckley, L. J., and R. G. Lough. 1987. Recent growth, biochemical composition, and prey field of larval haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua) on Georges Bank. Can. J. Fish. Aquat. Sci. 44: 14-25.

Lough, R. G., and D. C. Potter. 1993. Vertical distribution patterns and diel migrations of larval and juvenile haddock, Melanogrammus aeglefinus, and Atlantic cod, Gadus morhua, on Georges Bank. Fish. Bull., U.S. 91:281-303.

Rothschild, B. J., and T. R. Osborn. 1988. Small-scale turbulence and plankton contact rates. J. Plankton Res. 10: 465-474.