Examination of Sampling Bias for Larval Yellow Perch in Southern Lake Michigan Richard S. Fulford 1,* , James A. Rice 1 , and Fred P. Binkowski 2 1 Zoology Department North Carolina State University Campus Box 7617 Raleigh, North Carolina 27695 2 Great Lakes WATER Institute 600 E. Greenfield Avenue Milwaukee, Wisconsin 53204 ABSTRACT. Evidence suggests larval yellow perch, Perca flavescens, utilize nearshore and offshore habitat during the 30–40 day period between hatch and transition from pelagic to demersal habitat. In a large, open system like Lake Michigan this represents a significant increase in available habitat and it is important to understand how this increase may impact our ability to sample larval yellow perch in an unbiased manner. We measured the vertical distribution of larval yellow perch in southern Lake Michi- gan as a function of size, age, and diel period. Larval yellow perch were collected from two zones (sur- face and epilimnion) from 1 to 32 km from shore in 2001 during the day; on four dates surface samples were also collected at night. Results indicate larval perch are more abundant at the surface at night than during the day. Larval yellow perch < 15 mm total length (TL) and younger than 18 days post hatch were found in both surface and epilimnion habitat during the day, but larvae > 15 mm TL were captured only in the epilimnion and farther than 5 km from shore, which suggests a different spatial distribution for larger, older larvae. Diel differences in larval abundance and size at the surface suggest more and larger larvae will be caught for a similar effort at night as compared to daytime sampling. Observed differences in larval distribution with size and age also suggest that sampling concentrated nearshore and/or at the surface has the potential to under-sample larger/older yellow perch larvae in Lake Michigan. INDEX WORDS: Perca, larvae, sampling, depth, Lake Michigan, pelagic. J. Great Lakes Res. 32:434–441 Internat. Assoc. Great Lakes Res., 2006 INTRODUCTION Pelagic fish larvae are frequently smaller, lack parental protection, and are distributed over a larger area than other types of fish larvae (Sissenwine 1984). It is important to properly characterize the distribution and mortality rate of pelagic larvae to understand how larval dynamics impact annual re- cruitment (Houde 1987). Both the small size and wide dispersal patterns of pelagic larvae present a challenge for conducting field surveys in an unbi- ased manner. In particular, it has been demonstrated that data concerning the horizontal and vertical dis- tribution of pelagic larvae can be critical to the de- * Corresponding author. E-mail: Richard.Fulford@usm.edu Current address: University of Southern Mississippi—Gulf Coast Re- search Lab, P.O. 7000, Ocean Springs, MS 39566-7000. 434 velopment of comprehensive larvae surveys (Thayer et al. 1983) that provide unbiased measures of density, size and age distributions, and mortality (Wang and Eckmann 1994) for each annual cohort. To effectively collect the complete range of larval sizes from hatch to juvenile transition, we must know if vertical or horizontal position is correlated with size, age, or diel sampling period. Yellow perch, Perca flavescens is a popular and commercially important sport fish throughout the Great Lakes region (Francis et al. 1996). Histori- cally, the Lake Michigan yellow perch population has been a productive fishery, but has been in de- cline since the early 1990s. This decline is gener- ally thought to be due to increased mortality during the larval period (Clapp and Makauskas 2002). Yel- low perch larvae occupy the pelagic zone from