Condition Indices of larval Pacific Herring (Clupea pallasi ) in the San Francisco Estuary.  

Abstract

The short-term objective of this research is to determine the "nutritional" condition of larval Pacific herring (Clupea pallasi) in San Francisco Bay using various morphometric characters.  Our longer-term objective is to relate variation in larval herring condition to variation in other physical (e.g., temperature, salinity) and biological (e.g., copepod prey) variables, and to evaluate the larvae’s utility as ecological indicators of ecosystem condition or status. Two sites in San Francisco Bay (San Pablo Bay and Central Bay) were sampled each month from November 1999 through May 2000.  Duplicate or triplicate nighttime, water column total hauls were made at each station with a 1m2/500 µm mesh Tucker trawl.  A total of 562 herring larvae were sorted, identified and measured.  For each larva, six morphometric characters were measured; three (anal body depth, pectoral body depth, and dry weight) that are sensitive to growth (food) and three (eye diameter, standard length, and head width) that are relatively insensitive to growth (food).  From these six measurements we derived nine potential indices of condition by dividing each "food sensitive" measurement (e.g., body weight) by each "food insensitive" measurement (e.g., eye diameter). All morphometric measurements increased between November and May, indicating larval growth and were significantly different between locations (p<0.001).  All condition indices also changed with season, with a larger proportion of fish having higher values later in the season (e.g., late March), suggesting "better" larval condition at this time. Chlorophyll biomass and copepod egg production showed similar patterns of variation as the larval herring condition indices, suggesting the importance of trophic linkages.  Our future research will focus on better understanding the causal mechanisms underlying larval herring condition, as well as evaluating the utility of these indices as indicators of ecosystem condition or status.  

Introduction

 Pacific herring (Clupea pallasi) are one of the most abundant and important commercial fisheries in SanFrancisco Bay. Adult herring migrate from the coastal ocean into the bay during early/mid winter and spawn on solid substrate, with larvae and early juveniles present in the water column throughout late winter and  spring.  Poorly-fed and well-fed larval fish can be distinguished using morphological indices (Ferron and Leggett 1994).  This is achieved by using characters that are sensitive (e.g., body weight, body height) and less sensitive (e.g., eye diameter, body length) to growth. The results are then presented as a ratio or an index of recent growth.  These factors can be thought of as representing "apparent condition" which may be used to determine relationships between a condition index and environmental characteristics (McGurk 1985).

Primary Objectives

• To determine the "nutritional" condition of larval Pacific herring usingmorphometric characters, some of which are sensitive to growth (food),e.g. body weight, and some of which are relatively insensitive to growth (food), e.g. eye diameter.

• To relate variation in larval herring condition to variation in other physical(e.g. temperature, salinity) and biological (e.g.copepod prey) variables, and evaluate their utility as ecological indicators.

Materials and Methods

Field:  

Triplicate tows were double stepped oblique from the surface to 3-5 m above the bottom using a 500 micron mesh net attached to a 1 m2  Tucker trawl.     Hydrographic data were collected using a Seabird 19 CTD. Two casts were made at each station, one before the tows  and one after the tows.   We sampled once monthly at night between November 1999 and May 2000.  We sampled USGS stations 13 (San Pablo Bay) and 17(Central Bay) (Figure 1).

Figure 1. Map of sampling stations

Rinsing the 1 m2 Tucker trawl

Laboratory:  

Herring larvae were sorted, identified, and counted. Fifty individuals from each sample were removed for image analysis. Six morphometric characters were measured: Anal Body Depth Pectoral Body Depth Eye diameter Standard Length Head Width Dry Weight

Resulting Indices of Condition:  

Preliminary Results

Measurement Results Summary

• We sorted, identified, and measured 562 larvae from two locations.

• All of the body measurements were significantly different between locations. Larger mean values were observed in San Pablo Bay.

• Length frequency distributions were significantly different between locations with a greater number of larger fish occuring in San Pablo Bay and a greater number of smaller fish occuring in Central Bay.

• All of the body measurements increased over time, indicating growth.

Condition Indices Results Summary

• A non-parametric ANOVA indicated that  mean values for each index were significantly different between months (p<0.05).

•  A non-parametric ANOVA showed that indices 3 (BW/HW),4(ABD/ED), and 7(PBD/ED) were significantly different between locations.

• Two of the nine indices of condition showed an increase in value over time (Index 8 (PBD/SL) and Index 9 (PBD/HW)).Morphometric indices appear to be useful tools for evaluating"nutritional condition" in larval herring in San Francisco Bay.

Conclusions

• Changes in condition indices with season, with a larger proportion of fish having higher values later in the season (late March), suggest "better" larval condition at this time.

• More large fish occurred in San Pablo Bay, suggesting that this location may offer more resources to the larvae.  In contrast, a larger number of small fish were observed in Central Bay.  The effects of location will be more closely examined in the future.

• Other data on chlorophyll and copepod egg production show similar patterns, indicating the importance of trophic linkages.

• These indices may prove useful as indicators of ecosystem condition or health.

• Future research will focus on better understanding the causal mechanisms underlying larval herring condition.

References

Ferron, A. and W.C.Leggett. 1994. An appraisal of condition measures for marine fish larvae. Pages 217-304 in J.H.S Blaxter and A.J. Southward, eds, Advances in Marine Biology (30). Academic Press, San Diego, CA.

McGurk, M.D. 1985. Multivariate analysis of morphometry and dry weight of Pacific herring larvae. Marine Biology 86:1-11.  

Acknowledgements

The authors wish to thank EPA-STAR for funding for this project. Special thanks to Jay Tustin and Dave Morgan for trips on the RV Questuary and to Darren Gewant for help in the field.