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

Transcription

1 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 January 11, 2008 Submitted to: Richard Morat U. S. Fish and Wildlife Service 2800 Cottage Way, Room W-2605 Sacramento, CA and Joelle Buffa U. S. Fish and Wildlife Service San Francisco Bay National Wildlife Refuge Complex 9500 Thornton Ave. Newark, CA Analyses and reporting funded by FWS Coastal Program at San Francisco Bay

2 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. Intraannual 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

3 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 Dropped prey...11 San Francisco Bay fish...12 Colony and Bay comparisons...13 DISCUSSION ACKNOWLEDGMENTS LITERATURE CITED

4 TABLES Table 1. Definitions of breeding stages, Table 2. San Francisco Bay Monitoring Program sampling months, Table 3. Minimum lengths of fish sampled in San Francisco Bay Table 4. Families and species of fish identified in dropped prey, Table 5. Standard lengths and body depths of dropped prey, Table 6. Standard lengths and body depths of dropped prey by breeding stage, Table 7. Two-factor ANOVA results for dropped fish sizes, Table 8. Fish sampled in San Francisco Bay during the Least Tern breeding season (April August), Table 9. Lengths of fish sampled with different gear types in San Francisco Bay, April-August, Table 10. Lengths of suitably-sized fish sampled with different gear types in San Francisco Bay, April-August, Table 11. Lengths of suitably-sized fish sampled with different gear types in San Francisco Bay and in different breeding periods, April-August, Table 12. Two-factor ANOVA results for Bay fish sizes, Table 13. Three-factor ANOVA results comparing dropped fish and Bay fish sizes, Table 14. Spearman correlations by species between species compositions from dropped fish at the colony to fish sampled in San Francisco Bay Table 15. Spearman correlations by year between species compositions from dropped fish at the colony to fish sampled in San Francisco Bay Table 16. Rank preference indices for fish species Table 17. Sizes of northern anchovy and jacksmelt sampled in San Francisco Bay, April-August,

5 FIGURES Figure 1. Fish sampling stations of CDFG s San Francisco Study Figure 2. Dropped prey composition by familiy, Figures 3. Mean midwater trawl CPUE values of suitably-sized fish, April-August, Figure 4. Mean otter trawl CPUE values of suitably-sized fish, April-August,

6 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 success have been collected since the colony s inception in 1976 (Elliott et al. 2007). A four-year study on diet and foraging habits was conducted ( ), which gave some insight into the important prey species of this colony (Elliott 2005, Ehrler et al. 2006). While diet data have been collected and analyzed, comparisons of diet to available prey in San Francisco Bay have not been investigated. San Francisco Bay is the main foraging area for the Alameda Point colony (Bailey 1992, Ehrler et al. 2006). Data on available prey in San Francisco Bay was obtained from the California Department of Fish and Game s (CDFG) San Francisco Bay Study. CDFG has conducted monthly surveys of fish populations at 35 predetermined locations in San Francisco Bay since 1980, and these data provide insight as to the species composition and sizes of fish available to the terns over most of the years the colony has presided in Alameda. The information on Least Tern diet is not as long-standing. While it would be ideal to compare consumed fish to what fish were available in the Bay, there are only three years of diet data determined from fecal sample analysis ( ; Elliott 2005). Therefore, data on dropped prey was chosen for this analysis, as this is the largest diet-related dataset available. Least Terns drop prey items on the ground of the breeding colony mainly as a result of prey being too large for mates or chicks, a lack of hunger by the recipient, or the tern dropping the fish in-transit. There is debate over what dropped prey truly represent. Some argue that many of these fish are suitable and represent a surplus of food (Palmer 1941), while others point out that dropped prey are typically larger than consumed prey, especially with regard to the deep-bodied surfperches (Atwood and Kelly 1984, Robinette et al. 2001). Atwood and Kelly (1984) concluded that the species composition of dropped prey reflected what Least Terns consumed, making these fish decent indicators of diet. Dropped fish are whole prey items that can be identified and measured with relative ease. In addition, collecting dropped prey causes 6

7 relatively less disturbance to the colony than other methods of diet collection, an important factor to consider in investigating diet in an endangered species. METHODS Least Tern data Data on tern arrivals to the colony, departures from the colony, and breeding activity were used to determine the three different phases of breeding in each year: courtship/incubation, chick-rearing, and fledging (Table 1). Dropped prey collection Dropped prey were collected from the ground of the Alameda Least Tern colony by Golden Gate Audubon Society (GGAS) personnel ( ), and PRBO Conservation Science and U.S. Fish and Wildlife (USFWS) personnel ( ). GGAS collected prey on an opportunistic basis, and only numbers of prey species collected in each year are provided (Collins 1995). PRBO and USFWS collected prey during standardized type I nest surveys (i.e., surveys conducted in the breeding area). Type I surveys were generally conducted twice a week throughout most of the breeding season and once a week towards the end of the breeding season. We stored specimens in plastic bags labeled with the collection date. The specimens were later soaked in water, cleaned with a fine artist s paintbrush, and set out to dry. We sequentially numbered the dry fish using a fine tip permanent marker. We measured the total length (from the tip of the snout to the end of the caudal fin), standard length (from the tip of the snout to the end of the hypural bone), and body depth (the widest part of the fish) of each specimen to the nearest millimeter. We recorded the dry weight of each fish to the nearest 0.1 gram. Many specimens in each year of this study had only partial caudal fins, showed signs of tissue loss (due to scavenging or evaporation), or dried in a twisted or bent position. 7

8 Therefore, average weights are likely underestimates, and length measurements are not exact. In addition, while many fish and fish parts were collected and saved, we only analyzed whole fish specimens (i.e., specimens containing a head, body, and caudal peduncle). For the purposes of comparing these fish to the CDFG San Francisco Bay fish data, the standard lengths from dropped fish were used (as this measurement is more accurate than total length), and invertebrates (e.g. Crangon spp.) were dropped from analysis (as these comprised a very small proportion of dropped prey collections). Dropped prey composition has been summarized since collections began in In addition, analysis of variance (ANOVA) with Bonferroni-adjusted t-tests was used to test for inter- and intra-annual differences in size of dropped prey from 2000 to CDFG San Francisco Bay prey data Data on fish in San Francisco Bay have been collected since 1980 by the CDFG s San Francisco Bay Study. For the purposes of this study, we obtained the midwater trawl and otter trawl data for Central and South Bay stations, as these are closest to the Alameda tern colony and likely sample the areas used most frequently by foraging terns (Figure 1). Midwater and otter trawls were conducted most months, particularly during the months Least Terns are present in the Bay (April August); the only exception were the midwater trawls for 1994 (Table 2). Midwater and otter trawls do not sample areas <3-4 m in depth, or rocky areas or areas containing eelgrass, so some species (e.g. topsmelt (Atherinops affinis), jacksmelt (Atherinopsis californiensis), surfperch species (family Embiotocidae), and arrow goby (Clevelandia ios)) are likely underrepresented in the dataset (Kathy Hieb, CDFG, personal communication). However, this trawl data provide the best indices of fish abundances for the time period of interest. The midwater trawls sample the water column, while the otter trawls sample the bottom. The midwater trawl net mouth was 3.7 m x 3.7 m (with a 10.7 m 2 effective mouth opening), with mesh graduated in nine sections from 20.3 cm stretch mesh at the mouth to 1.3 cm (0.5 inch) stretch mesh at the codend. It was towed for 12 minutes obliquely to sample all parts of the 8

9 water column equally. Flow meter counts were recorded and used to calculate the volume of water sampled. The otter trawl had a 4.9 m head rope, a 2.5 cm stretch mesh body, and a 0.6 cm (0.25 inch) delta knotless mesh codend. It was towed on the bottom for five minutes. Distance towed was recorded using a Loran-C or GPS, and was used to calculate the area sampled, assuming a 70% door spread of 3.4 m. For each midwater and otter trawl completed, up to 50 individuals of each species were measured (fork length) and the remainder counted; this subset of length frequency data were used in size comparisons (see Size Comparisons). The length frequency data recorded were expanded by the total catch to estimate the total number of fish for each length; this is called the adjusted frequency. Adjusted frequency numbers for each species were used to calculate the catch per unit effort (CPUE) values for the size range of interest (see Species Compositions). In addition, minimum lengths were established for some species in 1984 (Table 3). Specimens below the designated minimum length were either not counted or recorded, or they were dropped from analyses to be consistent with other years. For more detailed descriptions of the methods for this study may be found at Bay fish data were standardized to a CPUE of fish per 10,000 m 3 for the midwater trawl and fish per 10,000 m 2 for the otter trawl. Each species was evaluated according to its general biology to determine if the midwater or otter trawl was the most appropriate at sampling that species; more specifically, fish that were generally pelagic were evaluated using the midwater trawl data, while demersal fish were evaluated using the otter trawl data. Since Least Terns are small birds and are known to consume small fish (Elliott 2005, Thompson et al. 1997, Atwood and Minsky 1983), CPUE values were calculated by determining the numbers of suitably-sized fish caught in each tow. Based on Elliott (2005), fish less than or equal to 100 mm in length were deemed of suitable size for the terns. This maximum length was applied to all fish except surfperches (family Embiotocidae); these fish are deep-bodied and their body depth cannot exceed the gape width of the tern (Hulsman 1981, Atwood and Kelly 1984, Zuria and Mellink 2005, Thompson et al. 1997). Based on surfperch length and body depth data recorded on dropped fish, we 9

10 determined the maximum length of surfperches to be 50 mm. Inter- and intra-annual sizes in fish were analyzed with ANOVA (and Bonferroni-adjusted t-tests). Least Terns are present in the San Francisco Bay area from April through August (Collins 2000, Elliott and Sydeman 2001, Elliott and Sydeman 2002, Hurt 2003, Hurt 2006); therefore, only tows conducted in these months were used in the analyses. Size comparisons The standard lengths of dropped fish at the colony and the fork lengths of fish sampled in the Bay were compared for While standard length is shorter than fork length, there is typically only a few millimeters difference in these two measurements. Length frequency data for all fish sampled in the Bay were summarized; however, analyses were performed on only the suitably-sized fish. Species compositions I investigated species compositions by comparing proportions of fish species collected from the colony in each year and mean CPUE values for species sampled in the Bay. For Bay fish sampled just prior to the arrival of terns or just after their departure, these data were assigned to the nearest breeding period within that year. Spearman correlations were calculated for species and years to see if colony and Bay fish varied similarly. In addition, rank preference indices were calculated for species found in both the dropped prey and the Bay. In this procedure, ranks for both usage and availability of a resource are calculated, and the difference (i.e., usage-availability) is used in determining whether a resource is preferred (positive value), avoided (negative value), or neutral (zero value; Johnson 1980). 10

11 RESULTS Dropped prey There were 19 families and 34 species of fish identified in dropped prey samples since 1981 (Table 4). Most of the fish collected were jacksmelt, northern anchovy (Engraulis mordax), and topsmelt (Table 4). When condensing the dropped prey to the family level, there were variations in composition and diversity of prey through the years; families Atherinopsidae, Engraulidae, and Embiotocidae dominated most collections, representing 57.5%, 19.7%, and 11.0%, respectively, of all dropped prey samples (Figure 2). The largest mean length and body depth of dropped fish were observed in 2006 (Table 5). A summary of fish sizes by breeding stage showed that the largest fish were collected in the courtship/incubation period, followed by the chick-rearing period, and the smallest fish collected during the fledging period (Table 6). However, this pattern was not similar in each individual year. The size of fish collected in the courtship/incubation period in 2006 were similar to the sizes of fish collected during chick-rearing (Table 6). Also, fish collected in the fledging stage were larger than the chick-rearing fish in 2000, 2001, and 2003 (Table 6). Significant differences in the lengths of dropped prey were found when considering both year and breeding stage (Table 7). The significant interaction term (year*breeding stage) indicated that sizes of dropped prey did not vary similarly in each breeding stage in each year. An examination of each year revealed that fish collected in the courtship/incubation stage were significantly larger than chick-rearing fish in 2001 (ANOVA: F 2,1768 =19.66, p<0.001; Bonferroniadjusted t-test: p<0.001), 2002 (ANOVA: F 2,1120 =40.32, p<0.001; Bonferroni-adjusted t-test: p<0.001), 2003 (ANOVA: F 2,1727 =26.32, p<0.001; Bonferroni-adjusted t-test: p<0.001), 2004 (ANOVA: F 2,1847 =39.80, p<0.001; Bonferroni-adjusted t-test: p<0.001), and 2005 (ANOVA:F 2,2280 =53.93, p<0.001; Bonferroni-adjusted t-test: p<0.001). Courtship fish were significantly larger than fledging fish in 2002 (Bonferroni-adjusted t-test: p<0.001), 2003 (Bonferroni-adjusted t-test: p<0.001), 2004 (Bonferroni-adjusted t-test: p<0.001),

12 (Bonferroni-adjusted t-test: p<0.001), and 2006 (ANOVA: F 2,846 =23.69, p<0.001; Bonferroniadjusted t-test: p<0.001). Chick-rearing fish were significantly larger than fledging fish in 2002 (Bonferroni-adjusted t-test: p<0.001), 2005 (Bonferroni-adjusted t-test: p<0.001), and 2006 (Bonferroni-adjusted t-test: p<0.001). Fish recovered in the fledging period were significantly larger than chick-rearing fish in 2001 (Bonferroni-adjusted t-test: p<0.001) and 2003 (Bonferroni-adjusted t-test: p=0.039). San Francisco Bay fish Out of the 46 families and 107 species of fish identified in the midwater and otter trawl tows since 1980, there were 40 families and 93 species of fish captured during the Least Tern breeding season (April August; Table 8). Of these, 26 families and 68 species were found to be of suitable-size to Least Terns (Table 8). Trends in species composition differed by species (Figures 3 and 4). Some species displayed an apparent decline (e.g. Pacific herring (Clupea pallasii); Figure 3b), strong abundance peaks (e.g. white croaker (Genyonemus lineatus), Figure 4b; Pacific sand lance (Ammodytes hexapterus), Figure 4d), relatively high abundances compared to the other species (e.g. northern anchovy, Figure 3a; bay goby (Lepidogobius lepidus), Figure 4a), relatively low abundances (e.g. threespine stickleback (Gasterosteus aculeatus), Figure 3d), and some only appeared in recent years (e.g. California grunion (Leuresthes tenuis); Figure 3c). The length of prey varied among years (Table 9). The range in the sizes of fish caught goes well beyond what Least Terns are capable of capturing and eating. When restricting the Bay fish data to only the fish suitably-sized for Least Terns, size differed between years, with the largest fish sampled in 2006 (Table 10). Intra-annual fish sizes showed a general trend of increasing size as each season progressed (Table 11). Statistical results showed significant differences in sizes of fish sampled when considering year and breeding stage (Table 12). Similar to the dropped fish results, the significant interaction term (year*breeding stage) signified varying results, which prompted analyses by year. Fish sampled during the courtship/incubation stage were significantly smaller than chick-rearing fish in 2000 (ANOVA: F 2,8351 =217.06, p<0.001; Bonferroni-adjusted t-test: p=0.040), 2001 (ANOVA: 12

13 F 2,8114 =250.00, p<0.001; Bonferroni-adjusted t-test: p<0.001), 2002 (ANOVA: F 2,11877 =532.17, p<0.001; Bonferroni-adjusted t-test: p<0.001), 2003 (ANOVA: F 2,10158 =371.90, p<0.001; Bonferroni-adjusted t-test: p<0.001), 2004 (ANOVA: F 2,8211 =113.44, p<0.001; Bonferroniadjusted t-test: p<0.001), and 2005 (ANOVA: F 2,6372 =82.75, p<0.001; Bonferroni-adjusted t-test: p<0.001). Bay fish sampled during the chick-rearing period were significantly smaller than fish sampled in the fledging period in all years (ANOVA (2006): F 2,5435 =103.19, p<0.001; Bonferroniadjusted t-test: p<0.001; Bonferroni-adjusted t-tests for : p<0.001; Bonferroniadjusted t-test for 2005: p=0.048). Colony and Bay comparisons The size of suitably-sized fish from the Bay varied significantly from the colony fish when factoring in year and breeding stage (Table 13). Again, the significant interaction terms led to investigating each year individually. Bay fish were significantly larger than colony fish in 2001 (ANOVA: F 1,9886 =17.03, p<0.001), 2003 (ANOVA: F 1,11889 =504.81, p<0.001), 2004 (ANOVA: F 1,10062 =401.66, p<0.001), 2005 (ANOVA: F 1,8656 =631.44, p<0.001), and 2006 (ANOVA: F 1,6285 =75.75, p<0.001). When examining the courtship/incubation period in each year, dropped fish at the colony were significantly larger than Bay fish in 2002 (ANOVA: F 1,5040 =59.16, p<0.001), 2003 (ANOVA: F 1,1540 =23.15, p<0.001), and 2004 (ANOVA: F 1,2603 =7.94, p=0.0049). During the chick-rearing period, fish sampled in the Bay were significantly larger than colony fish in 2001 (ANOVA: F 1,2163 =16.48, p=0.0001), 2003 (ANOVA: F 1,4207 =313.29, p<0.001), 2004 (ANOVA: F 1,2778 =175.14, p<0.001), 2005 (ANOVA: F 1,1548 =83.95, p<0.001), and 2006 (ANOVA: F 1,1643 =5.19, p=0.0228). Similar to the chick-rearing stage, Bay fish were significantly larger than colony fish in the fledging stage of every year (ANOVA results: 2000: F 1,3786 =36.68, p<0.001; 2001: F 1,4827 =217.90, p<0.001; 2002: F 1,5215 =199.17, p<0.001; 2003: F 1,6138 =502.76, p<0.001; 2004: F 1,4677 =628.39, p<0.001; 2005: F 1,4650 =966.90, p<0.001; 2006: F 1,2275 =159.07, p<0.001). When comparing species compositions between the colony and the Bay, 8 species had positive significant correlations and 2 species had near-significant correlations (Table 14). Comparisons by year showed 12 positive significant years and 2 near-significant years of the midwater trawl data, and one positive and one negative near-significant correlations in otter trawls (Table 15). 13

14 Rank preference indices showed terns preferred 12 species, avoided 13 species, and neutral towards 2 species (Table 16). DISCUSSION Most of the fish collected from the Least Tern breeding area were Atherinopsids (mainly jacksmelt and topsmelt), northern anchovy, and surfperch species (family Embiotocidae). Jacksmelt, topsmelt and northern anchovy are all pelagic, schooling fishes that can presumably be spotted by Least Terns from the air; these species are also the main fish consumed by Least Terns (Elliott 2005). Northern anchovy is the most abundant fish species in San Francisco Bay (Baxter et al. 1999), and while their abundance in the Bay has varied from year to year (Figure 3a), it is more abundant than the other pelagic, schooling, slender-bodied species. While less abundant than northern anchovy, jacksmelt is a fairly common fish of suitable-size sampled in the Bay, even given that it is likely underrepresented. The Embiotocids are likely overrepresented in the dropped fish samples; they are deep-bodied fish that are often unable to fit through the gape of a tern chick (Hulsman 1981, Atwood and Kelly 1984). For the purposes of this report, a tern gape width was defined as 15 mm (Atwood and Kelly 1984), but Zuria and Mellink (2005) reported mean gape widths of 11.2 mm (adults), 7.2 mm (smaller chicks), and 9.1 mm (larger chicks). The mean body depth of 18 ± 3.6 mm (range=7-27, n=701) in surfperches dropped at the colony is larger than the most conservative gape estimate, and it is more than twice the gape width of small chicks. Therefore, the dropped surfperches are too large for chicks (as well as most adults). The mean length of 63 mm in dropped prey (Table 5) is larger than the mean length of consumed prey (60 mm; Elliott 2005), suggesting that dropped fish represent fish that may be too large to eat. The largest dropped prey were found in 2006, and the largest fish were also sampled in the Bay in This may be driven by the presence of relatively large northern anchovy and jacksmelt sampled during the Least Tern breeding season in that year (Table 17). Dropped prey were significantly smaller than suitably-sized Bay fish in five of the seven years 14

15 analyzed. Some dropped fish (e.g. northern anchovy) are smaller than the minimum lengths used by CDFG, so this may have affected results. Intra-annual patterns in dropped prey size were somewhat unexpected. The literature on different tern species shows that adults switch from bringing larger prey (for courtship and incubating females) to smaller fish (for chicks; Atwood and Kelly 1984, Shealer 1998). While we saw smaller fish in the chick-rearing period compared to the courtship stage in five of the seven years studied, the fledging stage revealed even smaller fish in three of the years studied. I might have expected to see slightly larger fish during the fledging period, as chicks are larger at this time and capable of handling larger fish, and the fish in the Bay are larger at this time as well; yet only two years showed larger fish in the fledging period, and observations of feedings at the Alameda colony confirm that fledglings receive larger fish than smaller chicks (PRBO, unpublished data). However, the breeding stage designations are rough estimates of breeding phenology at the colony and do not necessarily reflect what may be occurring at the colony. For instance, there may be many smaller chicks in the colony during the fledging period (due to re-nesting or late nesting), which may explain why smaller fish are being collected. In addition, fledglings may be receiving more feedings from parents on the surrounding tarmac areas or by the shores of the Bay, so many of the dropped feedings of fledglings may not be collected during the nest surveys in the colony. It is interesting to note that while dropped fish sizes decrease as the season progresses, the opposite is true of Bay fish and the sizes of fish consumed by Least Terns (Elliott 2005). I would expect Bay fish sizes to increase, as this reflects the growth of the age-0 class fish that were spawned in the winter and spring months (Baxter et al. 1999). The intra-annual colony and Bay fish size comparisons illustrate the switch to foraging for appropriately-sized fish: dropped fish were larger than Bay fish during courtship/incubation, and then Bay fish were larger than dropped fish for the remainder of the season (when chicks and fledglings were present). Correlations between species compositions of dropped prey and Bay fish give more insight into the relationship between tern foraging and prey abundance and availability. The slender-bodied, pelagic schooling fish (northern anchovy, Pacific herring, and Pacific sardine (Sardinops sagax)), as well as fish inhabiting the nearshore environment (California grunion, arrow goby, cheekspot 15

16 goby (Ilypnus gilberti), and yellowfin goby (Acanthogobius flavimanus)), appear to be the significant positive findings, indicating that terns may be choosing these species in accordance with their availability. White croaker, bay goby, and brown rockfish (Sebastes auriculatus) are generally demersal or benthopelagic species that are not necessarily found in nearshore habitats; these species may have been made locally available in nearshore environments (e.g., during ebb tide). Not surprisingly, other demersal fish (e.g. English sole (Parophrys vetulus), Pacific staghorn sculpin (Leptocottus armatus)) and surfperch species showed no significant results with colony fish. However, jacksmelt the most abundant species collected at the colony had a nonsignificant negative relationship, as well as topsmelt (another important dropped fish); this may be due to underrepresentation in the Bay fish trawls due to sampling methods. The correlations by year also illustrate how midwater trawls sample the fish utilized by terns. Also, when examining only the midwater trawls, Bay fish species compositions and dropped fish compositions varied in similar ways in most years. This lends more evidence that the dropped fish indicate changes in availability and abundance of species in San Francisco Bay. The rank preference indices tell a somewhat different story. There appear to be preferences toward the same slender-bodied pelagic species (northern anchovy, Pacific herring, and Pacific sardine) and some benthic, nearshore species (cheekspot goby, bay goby, white croaker). However, these values indicate that terns avoid jacksmelt and topsmelt, meaning their relative abundance in the environment is large compared to their relative importance in the diet (or in this case, in the dropped prey collections). If jacksmelt and topsmelt are truly underrepresented in Bay trawl surveys, then their true availability in the environment would be even larger than what is reflected here, and their true rank preference indices would show an even stronger avoidance. It is possible that jacksmelt and topsmelt are underrepresented in the dropped fish samples; they may be eaten more often and/or be less likely to be too large for consumption. In addition, our classification of suitably-sized prey may actually include fish that are too large for terns, making the CPUE calculations erroneously high and creating significant findings where there may not be any (or vice versa). This may be the case for fish other than surfperches that may have body depth limitations to consider (e.g., flatfishes), but data to calculate minimum sizes were not available for this investigation. Therefore, these correlation and rank preference indices should be viewed with caution, as they may not reflect selectivity accurately. 16

17 Despite the shortcomings of the selectivity analyses, northern anchovy, Pacific herring, and, to a lesser extent, Pacific sardine are consistently shown to be important prey items to the Least Terns. These are high-caloric species, making good meals for growing tern chicks, and positive relationships between anchovy abundance in the dropped prey and reproductive success of the Alameda Least Tern colony have already been shown (Elliott et al. 2007). Elegant Tern (Sterna elegans) chicks fed anchovies (the high energy content food) showed a faster gain of body mass and wing length, and had higher lipid reserves than chicks fed topsmelt (the low energy content food; Dahdul and Horn 2003). The authors deduced that a tern colony that had to switch from a high-energy food item (e.g. anchovy) to a low-energy food item (i.e. topsmelt) may produce underweight fledglings with less fat reserves, and parents may have to increase provisioning to chicks, thereby leading to decreased survival and reproductive success. Terns are opportunistic foragers and generally stay within just a few miles of the breeding colony to catch their prey (Atwood and Minsky 1983), so terns may be taking jacksmelt in large numbers as they are more readily available and found closer to the colony than the preferred northern anchovy or Pacific herring. It is difficult to say what we can or cannot conclude about the dropped fish. Northern anchovy are important, yet they are the second most common fish in the dropped fish collections. The results on jacksmelt are less clear; this is the most abundant fish dropped at the colony and their abundances at the colony do not match the trawl abundances. This could be due to underrpresentation in the trawl data, or it could be a less desirable fish that is more abundant near the colony and makes an easy fish to catch and bring back to the chick. ACKNOWLEDGMENTS I would like to thank Kathy Hieb (CDFG) for providing the San Francisco Bay fish data and giving suggestions on the analyses; these data were collected by the Interagency Program for the San Francisco Estuary and the CDFG s San Francisco Bay study. Rachel Hurt, Susan Euing and Joelle Buffa (USFWS) provided breeding data on the Alameda Point Least Tern colony; Rachel, Susan, other Refuge staff and interns collected dropped fish during in-colony reproductive surveys in I also thank Richard Morat and the FWS Coastal Program 17

18 at San Francisco Bay for contributing funding to conduct these analyses. Jennifer Roth (PRBO) provided additional input into this report. In , dropped fish were collected during the Least Tern foraging study for the Oakland Harbor deepening project (Ehrler et al. 2006) under contract/subcontract with Tetra Tech, Inc.; data were collected under prime contract DACW07-02-F-0028 ( ), subcontract /GS-10F-0268K (2004), and subcontract /GS-10F-0268K (2005). This is PRBO Contribution Number

19 LITERATURE CITED Atwood, J. L. and P. R. Kelly Fish dropped on breeding colonies as indicators of Least Tern food habits. Wilson Bulletin 96: Atwood, J. L. and D. E. Minsky Least Tern foraging ecology at three major California breeding colonies. Western Birds 14: Bailey, S. F California Least Tern foraging and other off-colony activities around Alameda Naval Air Station during Unpublished Report, Dept. of Ornithology and Mammalogy, California Academy of Sciences. Baxter, R., K. Hieb, S. DeLeón, K. Fleming and J. Orsi Report on the fish, shrimp, and crab sampling in the San Francisco estuary, California. Prepared for the Interagency Ecological Program for the Sacramento-San Joaquin Estuary, California Department of Fish and Game, Technical Report 63. Collins, L. D California Least Tern nesting season at the Alameda Naval Air Station, Unpublished report prepared for Western Division, Naval Facilities Engineering Command. Collins, L. D California Least Tern monitoring report for 1999 at the Naval Air Station, Alameda. Unpublished report prepared for Engineering Field Activity West, Naval Facilities Engineering Command. Dahdul, W. M. and M. H. Horn Energy allocation and postnatal growth in captive Elegant Tern (Sterna elegans) chicks: responses to high- versus low-energy diets. Auk 120(4): Ehrler, C. P., M. L. Elliott, J. E. Roth, J. R. Steinbeck, A. K. Miller, W. J. Sydeman, and A. M. Zoidis Oakland Harbor Deepening Project (-50 ): Least Tern, Fish, and Plume Monitoring. Project Year 2005 and Four-Year Final Monitoring Report. Tetra Tech, Inc., San Francisco, California. July Elliott, M.L. and W.J. Sydeman, Breeding Status of the California Least Tern at Alameda Point (former Naval Air Station, Alameda), Alameda, California, Unpublished Report, Point Reyes Bird Observatory, Stinson Beach, California. Elliott, M.L. and W.J. Sydeman, Breeding Status of the California Least Tern at Alameda Point (former Naval Air Station, Alameda), Alameda, California, Unpublished Report, Point Reyes Bird Observatory, Stinson Beach, California. Elliott, M. L Diet, prey, and foraging habits of the California Least Tern (Sterna antillarum browni). Unpublished Master s thesis, San Francisco State University. 19

20 Elliott, M. L., R. Hurt, and W. J. Sydeman Breeding biology and status of the California Least Tern Sterna antillarum browni at Alameda Point, San Francisco Bay, California. Waterbirds 30(3): Hulsman, K Width of gape as a determinant of size of prey eaten by terns. Emu 81: Hurt, Rachel Breeding Status of the California Least Tern at Alameda Point, Alameda, California, Unpublished Report, U.S. Fish and Wildlife Service, Fremont, California. Hurt, Rachel Breeding Status of the California Least Tern at Alameda Point, Alameda, California, Unpublished Report Prepared for the U.S. Navy, U.S. Fish and Wildlife Service, Fremont, California. Johnson, D. H The comparison of usage and availability measurements for evaluating resource preference. Ecology 61(1): Palmer, R. S A behavior study of the Common Tern (Sterna hirundo hirundo L.). Proceedings of the Boston Society of Natural History 42: Robinette, D., N. Collier, A. Brown, and W. J. Sydeman Monitoring and management of the California Least Tern colony at Purisma Point, Vandenberg Air Force Base, Unpublished Report, Point Reyes Bird Observatory, Stinson Beach, CA. Shealer, D. A Size-selective predation by a specialist forager, the Roseate Tern. Auk 115(2): Thompson, B. C., J. A. Jackson, J. Burger, L. A. Hill, E. M. Kirsch and J. A. Atwood Least Tern (Sterna antillarum) in Birds of North America, No. 290 (A. Poole and F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists Union, Washington, D.C. Zuria, I. and E. Mellink Fish abundance and the 1995 nesting season of the Least Tern at Bahía de San Jorge, Northern Gulf of California, México. Waterbirds 28(2):

21 Table 1. Definitions of breeding stages, Breeding stage Year Courtship / Incubation Chick-rearing Fledging 2000 April 28 - June 3 June 4 - July 1 July 2 - Aug April 24 - June 1 June 2 - June 30 July 1 - August April 22 - June 1 June 2 - June 30 July 1 - July April 19 - May 30 May 31 - June 28 June 29 - August April 20 - May 28 May 29 - June 26 June 27 - August April 18 - May 29 May 30 - June 27 June 28 - August April 13 - May 30 June 1 - June 28 June 29 - August 14 21

22 Table 2. San Francisco Bay Monitoring Program sampling months, Year Midwater trawl Otter trawl 1980 January-November January-November 1981 January-December January-December 1982 January-December January-December 1983 January-December January-December 1984 January-December January-December 1985 January-December January-December 1986 January-December January-December 1987 January-December January-December 1988 January-December January-December 1989 January-August January-August 1990 February-October February-October 1991 February-October February-October 1992 February-October February-October 1993 February-October February-October 1994 February-April February-October 1995 April-December 1 January-December April-December January-December 1997 January-December 2 January-December 1998 January-December January-December 1999 January-October January-October 2000 January-December January-December 2001 January-December 3 January-December February-December February-December 2003 January-December 4 January-December January-December January-December 2005 January-December January-December 2006 January-December 5 January-December 5 1 Did not sample August 1995 due to mechanical problems. 2 MWT: In 1997 sampled only San Pablo Bay January to March, all stations April to December. 3 Both nets, did not sample March MWT and OT: survey 5 was conducted in early June (6/2/03-6/4/03); last survey in survey 6 conducted in early July (7/1/03). 5 MWT and OT: part of survey 4 conducted in early May (5/1/06 and 5/3/06). 22

23 Table 3. Minimum lengths of fish sampled in San Francisco Bay. Species Minimum length (mm) American shad 20 bay goby 20 California grunion 20 California halibut 20 California tonguefish 20 chameleon goby 20 cheekspot goby 20 Chinook salmon 20 delta smelt 30 diamond turbot 20 English sole 20 longfin smelt 40 northern anchovy 40 Pacific herring 30 Pacific staghorn sculpin 20 plainfin midshipman 20 shimofuri goby 20 Shokihaze goby 20 speckled sanddab 20 starry flounder 20 striped bass 25 white croaker 20 yellowfin goby 20 23

24 Table 4. Families and species of fish identified in dropped prey, Family Species Scientific name Number Percentage of total collection Clupeidae Pacific herring Clupea pallasii % Pacific sardine Sardinops sagax % Clupeidae spp % Engraulidae northern anchovy Engraulis mordax % Salmonidae Osmeridae Chinook salmon Oncorhynchus tshawytscha % coho salmon Oncorhynchus kisutch % Salmonidae spp % delta smelt Hypomesus pretiosus % longfin smelt Spirinchus thaleichthys % surf smelt Hypomesus pretiosus % Osmeridae spp % Batrachoididae plainfin midshipman Porichthys notatus % Scomberesocidae Pacific saury Cololabis saira % Fundulidae rainwater killifish Lucania parva % Cyprinidae Cyprinidae spp % Atherinopsidae California grunion Leuresthes tenuis % jacksmelt Atherinopsis californiensis % topsmelt Atherinops affinis % Mississippi silverside Menidia beryllina % Atherinopsidae spp % Gasterosteidae threespine stickleback Gasterosteus aculeatus % Scorpaenidae brown rockfish Sebastes auriculatus % rockfish spp. Sebastes spp % Hexagrammidae unidentified greenling Hexagrammos spp % Cottidae Pacific staghorn sculpin Leptocottus armatus % Cottidae spp % Moronidae striped bass Morone saxatilis % Sciaenidae white croaker Genyonemus lineatus % Sciaenidae spp % Embiotocidae black perch Embiotoca jacksoni % walleye surfperch Hyperprosopon argenteum % silver surfperch Hyperprosopon ellipticum % 24

25 Table 4 (continued). Family Species Scientific name Number Percentage of total collection Embiotocidae shiner perch Cymatogaster aggregata % striped seaperch Embiotoca lateralis % dwarf perch Micrometrus minimus 05 pile perch Rhacochilus vacca 03 white seaperch Phanerodon furcatus 36 Embiotocidae spp Ammodytidae Pacific sand lance Ammodytes hexapterus 03 Gobiidae 7 0. % 4 0. % % 8 0. % 5 0. % % % % % % bay goby Lepidogobius lepidus 15 yellowfin goby Acanthogobius flavimanus 60 cheekspot goby Ilypnus gilberti 13 arrow goby Clevelandia ios 24 Gobiidae spp. sole Pleuronectidae Eng lish Parophrys vetulus % unidentifiab le % TOTAL % 25

26 Table 5. Standard lengths and body depths of dropped prey, Standard length (mm) Body depth (mm) Year mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 63 ± ±

27 Table 6. Standard lengths and body depths of dropped prey by breeding stage, a) Courtship/incubation Standard length (mm) Body depth (mm) Year mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 71 ± ± b) Chick-rearing Standard length (mm) Body depth (mm) Year mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 63 ± ± c) Fledging Standard length (mm) Body depth (mm) Year mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 62 ± ±

28 Table 7. Two-factor ANOVA results for dropped fish sizes, Source Partial SS df MS F Prob>F Model year breeding stage year * breeding stage Residual Total Number of observations = Root MSE = R 2 = Adjusted R 2 =

29 Table 8. Fish sampled in San Francisco Bay during the Least Tern breeding season (April August), Family Species Scientific Name Gear type used in analysis Number of suitablysized specimens Percentage of all suitablysized specimens Petromyzontidae Pacific lamprey Lampetra tridentata otter trawl river lamprey Lampetra ayresii otter trawl Hexanchidae broadnose sevengill shark Notorynchus cepedianus otter trawl Squalidae spiny dogfish Squalus acanthias otter trawl Alopiidae thresher shark Alopias vulpinus midwater trawl Triakidae leopard shark Triakis semifasciata otter trawl brown smoothhound Mustelus henlei otter trawl Torpedinidae Pacific electric ray Torpedo californica otter trawl Platyrhynidae thornback Platyrhinoidis triseriata otter trawl Rhinobatidae shovelnose guitarfish Rhinobatos productus otter trawl Rajidae big skate Raja binoculata otter trawl Myliobatidae bat ray Myliobatis californica otter trawl Acipenseridae green sturgeon Acipenser medirostris otter trawl white sturgeon Acipenser transmontanus otter trawl Clupeidae threadfin shad Dorosoma petenense midwater trawl Pacific herring Clupea pallasii midwater trawl Pacific sardine Sardinops sagax midwater trawl American shad Alosa sapidissima midwater trawl Engraulidae northern anchovy Engraulis mordax midwater trawl Salmonidae steelhead trout Oncorhynchus mykiss midwater trawl chinook salmon Oncorhynchus tshawytscha midwater trawl Osmeridae surf smelt Hypomesus pretiosus otter trawl eulachon Thaleichthys pacificus midwater trawl whitebait smelt Allosmerus elongatus otter trawl night smelt Spirinchus starksi otter trawl longfin smelt Spirinchus thaleichthys otter trawl Synodontidae California lizardfish Synodus lucioceps otter trawl Batrachoididae plainfin midshipman Porichthys notatus otter trawl Ophidiidae spotted cusk-eel Chilara taylori otter trawl Gadidae Pacific tomcod Microgadus proximus otter trawl Fundulidae rainwater killifish Lucania parva midwater trawl

30 Table 8 (continued). Family Species Scientific Name Gear type used in analysis Number of suitablysized specimens Percentage of all suitablysized specimens Atherinopsidae California grunion Leuresthes tenuis midwater trawl jacksmelt Atherinopsis californiensis midwater trawl topsmelt Atherinops affinis midwater trawl Gasterosteidae threespine stickleback Gasterosteus aculeatus midwater trawl Syngnathidae bay pipefish Syngnathus leptorhynchus otter trawl Scorpaenidae brown rockfish Sebastes auriculatus otter trawl black rockfish Sebastes melanops midwater trawl blue rockfish Sebastes mystinus otter trawl yellowtail rockfish Sebastes flavidus otter trawl vermilion rockfish Sebastes miniatus otter trawl unidentified rockfish Sebastes spp. otter trawl Hexagrammidae lingcod Ophiodon elongatus otter trawl kelp greenling Hexagrammos decagrammus otter trawl Cottidae cabezon Scorpaenichthys marmoratus otter trawl brown Irish lord Hemilepidotus spinosus otter trawl Pacific staghorn sculpin Leptocottus armatus otter trawl buffalo sculpin Enophrys bison otter trawl padded sculpin Artedius fenestralis otter trawl bonyhead sculpin Artedius notospilotus otter trawl scalyhead sculpin Artedius harringtoni otter trawl prickly sculpin Cottus asper otter trawl Agonidae pygmy poacher Odontopyxis trispinosa otter trawl Liparidae showy snailfish Liparis pulchellus otter trawl slipskin snailfish Liparis fucensis otter trawl Moronidae striped bass Morone saxatilis midwater trawl Carangidae jack mackerel Trachurus symmetricus midwater trawl Sciaenidae queenfish Seriphus politus midwater trawl white croaker Genyonemus lineatus otter trawl

31 Table 8 (continued). Family Species Scientific Name Gear type used in analysis Number of suitablysized specimens Percentage of all suitablysized specimens Embiotocidae rubberlip seaperch Rhacochilus toxotes otter trawl black perch Embiotoca jacksoni otter trawl barred surfperch Amphistichus argenteus otter trawl calico surfperch Amphistichus koelzi otter trawl redtail surfperch Amphistichus rhodoterus otter trawl spotfin surfperch Hyperprosopon anale otter trawl walleye surfperch Hyperprosopon argenteum midwater trawl silver surfperch Hyperprosopon ellipticum otter trawl shiner perch Cymatogaster aggregata otter trawl striped seaperch Embiotoca lateralis otter trawl dwarf perch Micrometrus minimus otter trawl pile perch Rhacochilus vacca otter trawl white seaperch Phanerodon furcatus otter trawl unidentified surfperch Embiotocidae spp. otter trawl Chaenopsidae onespot fringehead Neoclinus uninotatus otter trawl Pholidae saddleback gunnel Pholis ornata otter trawl Ammodytidae Pacific sand lance Ammodytes hexapterus otter trawl Gobiidae chameleon goby Tridentiger trigonocephalus otter trawl bay goby Lepidogobius lepidus otter trawl yellowfin goby Acanthogobius flavimanus otter trawl cheekspot goby Ilypnus gilberti otter trawl arrow goby Clevelandia ios otter trawl shimofuri goby Tridentiger bifasciatus otter trawl Shokihaze goby Tridentiger barbatus otter trawl Stromateidae Pacific pompano Peprilus simillimus midwater trawl Cynoglossidae California tonguefish Symphurus atricaudus otter trawl Paralichthyidae California halibut Paralichthys californicus otter trawl Pacific sanddab Citharichthys sordidus otter trawl speckled sanddab Citharichthys stigmaeus otter trawl

32 Table 8 (continued). Family Species Scientific Name Gear type used in analysis Number of suitablysized specimens Percentage of all suitablysized specimens Pleuronectidae curlfin sole Pleuronichthys decurrens otter trawl hornyhead turbot Pleuronichthys verticalis otter trawl sand sole Psettichthys melanostictus otter trawl diamond turbot Pleuronichthys guttulatus otter trawl English sole Parophrys vetulus otter trawl starry flounder Platichthys stellatus otter trawl Dover sole Microstomus pacificus otter trawl TOTAL

33 Table 9. Lengths of fish sampled with different gear types in San Francisco Bay, April-August, midwater trawl otter trawl both nets Year mean ± s.d. range n mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Total 80 ± ± ±

34 Table 10. Lengths of suitably-sized fish sampled with different gear types in San Francisco Bay, April-August, midwater trawl otter trawl both nets Year mean ± s.d. range n mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Total 73 ± ± ±

35 Table 11. Lengths of suitably-sized fish sampled with different gear types in San Francisco Bay and in different breeding periods, April- August, a) Courtship/incubation midwater trawl otter trawl both nets Year mean ± s.d. range n mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 69 ± ± ± b) Chick-rearing midwater trawl otter trawl both nets Year mean ± s.d. range n mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 74 ± ± ± c) Fledging midwater trawl otter trawl both nets Year mean ± s.d. range n mean ± s.d. range n mean ± s.d. range n ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± TOTAL 76 ± ± ±

36 Table 12. Two-factor ANOVA results for Bay fish sizes, Source Partial SS df MS F Prob>F Model year breeding stage year*breeding stage Residual Total Number of observations = Root MSE = R 2 = Adjusted R 2 =

37 Table 13. Three-factor ANOVA results comparing dropped fish and Bay fish sizes, Source Partiall SS df MS F Prob>F Model collection (Bay or colony) year breeding collection*year collection*breeding Residual Total Number of observations = Root MSE = R 2 = Adjusted R 2 =

38 Table 14. Spearman correlations by species between species compositions from dropped fish at the colony to fish sampled in San Francisco Bay. Gear type Species ρ n p Midwater trawl Chinook salmon northern anchovy Pacific herring Pacific sardine <0.001 topsmelt jacksmelt California grunion <0.001 striped bass threespine stickleback walleye surfperch Otter trawl surf smelt longfin smelt white croaker yellowfin goby arrow goby bay goby cheekspot goby Pacific sand lance Pacific staghorn sculpin brown rockfish unidentified rockfish plainfin midshipman dwarf perch shiner perch white seaperch black perch English sole Note: Yellow highlights designate significant results (p<0.05), and pink highlights indicate near-significant results (0.1>p>0.05). 38

39 Table 15. Spearman correlations by year between species compositions from dropped fish at the colony to fish sampled in San Francisco Bay. Gear type Year ρ n p Midwater trawl Otter trawl Note: Yellow highlights designate significant results (p<0.05), and pink highlights indicate near-significant results (0.1>p>0.05). 39

40 Table 16. Rank preference indices for fish species. Gear type Species (Rank colony) (Rank Bay) Selectivity Midwater trawl Pacific herring 2 preferred Pacific sardine 4 strongly preferred northern anchovy 1 mildly preferred Chinook salmon 0 neutral California grunion 0 neutral jacksmelt -2 avoided topsmelt -3 strongly avoided threespine stickleback 1 mildly preferred striped bass -1 mildly avoided walleye surfperch -2 avoided Otter trawl longfin smelt 10 strongly preferred surf smelt -13 strongly avoided plainfin midshipman 8 preferred brown rockfish 4 preferred unidentified rockfish spp. -3 avoided Pacific staghorn sculpin 4 preferred white croaker 5 preferred black perch -5 avoided shiner perch -7 avoided dwarf surfperch -4 avoided white seaperch -12 strongly avoided Pacific sand lance -2 avoided bay goby 5 preferred yellowfin goby -7 avoided cheekspot goby 1 mildly preferred arrow goby -6 avoided English sole 12 strongly preferred 40

41 Table 17. Sizes of northern anchovy and jacksmelt sampled in San Francisco Bay, April- August, northern anchovy jacksmelt Year mean ± s.d. range n ± ± ± ± ± ± ± Total 79 ± ± ± ± ± ± ± ± Total 75 ±

42 Figure 1. Fish sampling stations of CDFG s San Francisco Study. Note: Data from stations in the South Bay ( ) and Central Bay ( ) were used in this report. Map obtained from the IEP/CDFG website: 42

43 Figure 2. Dropped prey composition by familiy, (Note: No prey were collected in years 1983 and ). 43