698 



Assessment of sampling methods to estimate 

 horseshoe crab (JLimu/us polyphemus L.) 

 egg density in Delaware Bay 



Penelope S. Pooler 



David R. Smith 



us. Geological Survey 



Leetown Science Center 



11700 Leetown Road 



Kearneysville, West Virginia 25430 



E-mail address (for D. R Smith, contact auttior): david_r_smitti@usgs gov 



Robert E. Loveland 



Department of Ecology and Evolution 



Cook College 



Rutgers University 



New Brunswick, New Jersey 08901 



Understanding the reliability of egg 

 density estimates at multiple scales 

 will help develop effective monitoring 

 programs. 



We addressed all three questions 

 with respect to eggs found in both 

 shallow (0-5 cm) and deep (0-20 cm) 

 sediments. Horseshoe crabs are gener- 

 ally thought to lay most of their eggs at 

 a depth of 15-20 cm (Brockmann, 1990; 

 Botton et al., 1994). Through processes 

 of bioturbation and wave-generated 

 sediment activation, horseshoe crab 

 eggs are brought onto the beach and 

 made available to foraging shorebirds 

 (Botton et al., 1994; Kraeuter and Feg- 

 ley, 1994; Jackson et al., 2002). 



Materials and methods 



Mark L. Botton 



Fordtiam University 



113 West 60"' Street 



New York, New York 10023 



Stewart F. Michels 



Delaware Division of Fisfi and Wildlife 



RO. Box 330 



Little Creek, Delaware 19961 



Each spring horseshoe crabs {Limtilus 

 polyphemus L.) emerge from Delaware 

 Bay to spawn and deposit their eggs on 

 the foreshore of sandy beaches (Shuster 

 and Botton, 1985; Smith et al., 2002a). 

 From mid-May to early June, migra- 

 tory shorebirds stopover in Delaware 

 Bay and forage heavily on horseshoe 

 crab eggs that have been transported 

 up onto the beach (Botton et al., 1994; 

 Burger et al., 1997; Tsipoura and 

 Burger, 1999). Thus, estimating the 

 quantity of horseshoe crab eggs in 

 Delaware Bay beaches can be useful 

 for monitoring spawning activity and 

 assessing the amount of forage avail- 

 able to migratory shorebirds. 



We evaluated procedures to estimate 

 horseshoe crab egg density by asking 

 three questions that address sampling 

 at a different spatial scale. 1) How 

 many samples of sediment are needed 



for precise estimation of egg density 

 within a segment of beach? 2) Does egg 

 density within a segment of beach ad- 

 equately represent egg density across 

 a larger stretch of beach? 3) How many 

 beach segments should be sampled 

 to monitor bay-wide egg density? We 

 chose these three questions because 

 the objective of egg studies might focus 

 on any of these scales. We ask the first 

 question to determine the sampling 

 effort necessary to detect changes in 

 egg density over time within a specific 

 beach segment. The second question 

 allows us to examine the reliability of 

 using egg density in a beach segment to 

 infer egg density over a larger stretch 

 of beach. The third question deals with 

 the level of precision in estimates of 

 bay-wide egg density and how many 

 beaches must be sampled to detect 

 bay-wide declines in density over time. 



During May and June 1999, we col- 

 lected sediment on 16 beaches in 

 Delaware Bay (Fig. 1), eight along the 

 eastern shore (New Jersey) and eight 

 along the western shore (Delaware), to 

 estimate egg density. Methods used to 

 collect sediment and extract horseshoe 

 crab eggs are summarized in the pres- 

 ent study, but are presented in detail 

 in Smith et al. (2002b). Beach sediment 

 was collected in cores (5 cm diameter) 

 within a 3-m wide strip along a 100-m 

 segment of beach. Each 3-m wide strip 

 was centered on the mid-beach eleva- 

 tion where a majority of horseshoe 

 crab nests occur (Botton et al., 1988). 

 The mid-beach elevation is halfway 

 between the spring high water level 

 and the beach break at the low tide ter- 

 race. Within each egg-sampling strip, 

 40 locations were selected randomly for 

 sediment collection. At each location, a 

 pair of core samples was taken: one to a 

 depth of 5 cm and the other to a depth 

 of 20 cm. We sampled eggs on 25-26 

 May and 14-15 June 1999, which fol- 

 lowed the heaviest spawning activity 

 in Delaware Bay that year (Smith et 

 al., 2002a). We mixed the entire core 

 contents thoroughly and then removed 

 80-mL aliquots. We ran the aliquots 



Manuscript approved for publication 

 12 February 2003 by Scientific Editor. 



Manuscript received 4 April 2003 at NMFS 

 Scientific Publications Office. 



Fish Bull. 101:698-703 (2003). 



