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Least Tern Dropped Prey Analyses at Alameda Point, San Francisco Bay, California

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Least Tern Dropped Prey Analyses at Alameda Point, San Francisco Bay, California Least Tern Dropped Prey Analyses at Alameda Point, San Francisco Bay, California. by Meredith L. Elliott Marine Ecology Division PRBO Conservation Science 3820 Cypress Drive, #11 Petaluma, CA 94954 January 11, 2008 Submitted to: Richard Morat U. S. Fish and Wildlife Service 2800 Cottage Way, Room W-2605 Sacramento, CA 95825-1846 and Joelle Buffa U. S. Fish and Wildlife Service San Francisco Bay National Wildlife Refuge Complex 9500 Thornton Ave. Newark, CA 94560 Analyses and reporting funded by FWS Coastal Program at San Francisco Bay EXECUTIVE SUMMARY Prey dropped on seabird colonies may be used as an indicator of foraging and diet. Dropped fish have been collected from the Alameda Point Least Tern colony in most years since 1981, and findings were compared to California Department of Fish and Game (CDFG) fish trawl data of Central and South San Francisco Bay; sizes of dropped fish have been collected since 2000, and these results were compared to sizes of fish found in the Bay. Most of the dropped prey were jacksmelt (Atherinopsis californiensis), topsmelt (Atherinops affinis), and northern anchovy (Engraulis mordax). Surfperch species (family Embiotocidae) were also present in the dropped fish and likely represent fish too large to fit through the gape of a tern chick. Northern anchovy and jacksmelt were the most common prey species sampled in San Francisco Bay. Fish dropped at the colony were generally smaller than Bay fish, although terns chose larger fish than expected during the early part of the breeding season (during courtship and incubation). The largest dropped prey were found in 2006; this was also found in the Bay fish results and was probably driven by significantly larger jacksmelt and northern anchovy in the Bay in that same year. Dropped prey appeared to get smaller as the season progressed, indicating that adult terns switch from feeding larger fish to their mates to feeding smaller fish to chicks. Intra- annual Bay fish size results were the opposite, reflecting the growth of the age-0 class fish that had spawned in the winter and spring months. Selectivity analyses illustrate the importance of small, pelagic, schooling fishes to Least Terns, particularly northern anchovy, Pacific herring (Clupea pallasii), and Pacific sardine (Sardinops sagax). Terns also forage on benthic fishes found in the nearshore environment, such as arrow goby (Clevelandia ios) and cheekspot goby (Ilypnus gilberti). Despite the large numbers of jacksmelt and topsmelt collected from the Least Tern colony, results suggest that terns are not selecting these species over northern anchovy. Northern anchovy, Pacific herring, and Pacific sardine are all relatively high in fat and may be the preferred prey to terns. However, if these species are not locally available, terns may switch to catching jacksmelt and topsmelt, as these species may be more abundant near the colony during the breeding season. 2 TABLE OF CONTENTS EXECUTIVE SUMMARY.................................................................................................................................2 TABLE OF CONTENTS .................................................................................................................................3 TABLES ...............................................................................................................................................................4 FIGURES .............................................................................................................................................................5 INTRODUCTION ...........................................................................................................................................6 METHODS .........................................................................................................................................................7 Least Tern data.............................................................................................................................................................7 Dropped prey collection..............................................................................................................................................7 CDFG San Francisco Bay prey data.........................................................................................................................8 Size comparisons.......................................................................................................................................................10 Species compositions ................................................................................................................................................10 RESULTS.......................................................................................................................................................... 11 Dropped prey..............................................................................................................................................................11 San Francisco Bay fish..............................................................................................................................................12 Colony and Bay comparisons..................................................................................................................................13 DISCUSSION.................................................................................................................................................. 14 ACKNOWLEDGMENTS............................................................................................................................. 17 LITERATURE CITED.................................................................................................................................... 19 3 TABLES Table 1. Definitions of breeding stages, 2000-2006............................................................................... 21 Table 2. San Francisco Bay Monitoring Program sampling months, 1980-2006.............................. 22 Table 3. Minimum lengths of fish sampled in San Francisco Bay......................................................... 23 Table 4. Families and species of fish identified in dropped prey, 1981-2006. .................................. 24 Table 5. Standard lengths and body depths of dropped prey, 2000-2006........................................ 26 Table 6. Standard lengths and body depths of dropped prey by breeding stage, 2000-2006....... 27 Table 7. Two-factor ANOVA results for dropped fish sizes, 2000-2006......................................... 28 Table 8. Fish sampled in San Francisco Bay during the Least Tern breeding season (April – August), 1980-2006....................................................................................................................................... 29 Table 9. Lengths of fish sampled with different gear types in San Francisco Bay, April-August, 2000-2006........................................................................................................................................................ 33 Table 10. Lengths of suitably-sized fish sampled with different gear types in San Francisco Bay, April-August, 2000-2006.............................................................................................................................. 34 Table 11. Lengths of suitably-sized fish sampled with different gear types in San Francisco Bay and in different breeding periods, April-August, 2000-2006................................................................ 35 Table 12. Two-factor ANOVA results for Bay fish sizes, 2000-2006................................................ 36 Table 13. Three-factor ANOVA results comparing dropped fish and Bay fish sizes, 2000-2006. ........................................................................................................................................................................... 37 Table 14. Spearman correlations by species between species compositions from dropped fish at the colony to fish sampled in San Francisco Bay. ................................................................................... 38 Table 15. Spearman correlations by year between species compositions from dropped fish at the colony to fish sampled in San Francisco Bay. ................................................................................... 39 Table 16. Rank preference indices for fish species. ............................................................................... 40 Table 17. Sizes of northern anchovy and jacksmelt sampled in San Francisco Bay, April-August, 2000-2006........................................................................................................................................................ 41 4 FIGURES Figure 1. Fish sampling stations of CDFG’s San Francisco Study........................................................ 42 Figure 2. Dropped prey composition by familiy, 1981-2006................................................................ 43 Figures 3. Mean midwater trawl CPUE values of suitably-sized fish, April-August, 1980-2006... 44 Figure 4. Mean otter trawl CPUE values of suitably-sized fish, April-August, 1980-2006............. 45 5 INTRODUCTION The Least Tern (Sterna antillarum browni) is a federal and state endangered species (Thompson et al. 1997). The Alameda Point Least Tern colony is the largest colony in California north of San Luis Obsipo County, and data on population size and breeding
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