Unexpectedly high mortality in Pacific herring embryos exposed to the 2007 Cosco Busan oil spill in San Francisco Bay

Abstract

In November 2007, the container ship Cosco Busan released 54,000 gallons of bunker fuel oil into San Francisco Bay. The accident oiled shoreline near spawning habitats for the largest population of Pacific herring on the west coast of the continental United States. We assessed the health and viability of herring embryos from oiled and unoiled locations that were either deposited by natural spawning or incubated in subtidal cages. Three months after the spill, caged embryos at oiled sites showed sublethal cardiac toxicity, as expected from exposure to oil-derived polycyclic aromatic compounds (PACs). By contrast, embryos from the adjacent and shallower intertidal zone showed unexpectedly high rates of tissue necrosis and lethality unrelated to cardiotoxicity. No toxicity was observed in embryos from unoiled sites. Patterns of PACs at oiled sites were consistent with oil exposure against a background of urban sources, although tissue concentrations were lower than expected to cause lethality. Embryos sampled 2 y later from oiled sites showed modest sublethal cardiotoxicity but no elevated necrosis or mortality. Bunker oil contains the chemically uncharacterized remains of crude oil refinement, and one or more of these unidentified chemicals likely interacted with natural sunlight in the intertidal zone to kill herring embryos. This reveals an important discrepancy between the resolving power of current forensic analytical chemistry and biological responses of keystone ecological species in oiled habitats. Nevertheless, we successfully delineated the biological impacts of an oil spill in an urbanized coastal estuary with an overlapping backdrop of atmospheric, vessel, and land-based sources of PAC pollution.

Fig. 1.

Satellite view of study sites in central San Francisco Bay. Landmarks include the Golden Gate Bridge to the west and the Richmond Bridge to the north. Oiled sites include HC, SA, PP, and KC. Nonoiled reference sites include PC, PSQ, and SRB. Arrow indicates location of oil release.

Fig. 2.

Evidence of cardiac dysfunction in embryos and larvae incubated in subtidal cages at oiled sites in 2008. Box-and-whisker plots encompass the distribution of individual data points. (A) Mean heart rate (n = 30) by cage. One cage at SRB was lost at retrieval, one cage at PP was stripped of embryos before retrieval, and video from one cage each at SA and PP were of insufficient quality to provide heart rate data. (B) Mean heart rate by site. Mean heart rates from individual cages were pooled for a site mean. ANOVA indicated 34% of the total variance was caused by the cage, and showed significant effects of site (P < 0.001) and oiled state (P < 0.001). Letters A–C indicate statistically similar datasets identified by post-hoc means comparison with Tukey-Kramer HSD test (α = 0.05). Overall, oiled sites were statistically different from reference sites (Student t test, α = 0.05). (C) Incidence of pericardial edema in larvae hatched in the laboratory after incubation to 7 d postfertilization in subtidal cages. Edema tended to be observed more frequently at oiled sites (Wilcoxon rank-sum test, P = 0.06).

Fig. 3.

Representative images of intact and dechorionated naturally spawned embryos from oiled and nonoiled intertidal sites sampled in 2008. Images were made by using oblique coherent contrast illumination on a Nikon SMZ1000 stereomicroscope, allowing high relief for colorless, transparent specimens. (A) An undissected cluster of embryos in the chorions from reference site SRB with normal translucency. Eyes are darkly pigmented and the trunk and tail are coiled around the brightly transparent yolk. (B) Representative embryo from SRB uncoiled after dechorionation, with anterior to the left and dorsal at top. Longitudinal features evident with oblique coherent contrast illumination from dorsal to ventral are the transparent neural tube, notochord, and gut. (C) An undissected cluster of nonviable embryos from oiled site PP showing pigmented eyes, opacity indicative of deteriorating tissues, and spontaneously ruptured chorions. (D) Representative late-stage embryo from PP with cardiac activity after dechorionation. Arrows indicate ventral pigment cells, arrowheads indicate loss of epidermal integrity.

Fig. 4.

Hatching rates of larvae from natural spawn samples. Values represent mean ± SEM normal hatch rates, calculated as percent morphologically normal larvae hatched from total eggs in eight transect subsamples per site. Grand totals of eggs assessed for each site were 535 (SRB, 2008), 820 (PC, 2009), 683 (PC, 2010), 968 (SA, 2008), 385 (SA, 2010), 549 (PP, 2008), 919 (PP, 2010), 469 (KC, 2008), and 726 (KC, 2010).

Fig. 5.

Comparison of PAC patterns in PEMDs deployed at six sites indicates distinct PAC sources. nMDS was used to represent a large number of PAC compounds in low-dimensional (i.e., 2D) space. nMDS analysis was carried out using Primer version 6.0 as described in SI Appendix. Axes surround a unitless space within which samples are placed according to the degree of similarity in the relative abundance of five influential PAC compounds (preselected from 34 compounds using the Primer BEST routine). Similarity in PAC patterns determines the distance between points in the space: samples with similar patterns are placed close together and dissimilar patterns are further apart. The observed patterns are statistically different from a random configuration of points (stress, 0.05). PSQ and SRB are reference sites; HC, SA, PP, and KC are oiled sites.

Fig. P1.

Defects in embryos incubated at oiled sites were absent at reference sites. (A) Incidence of pericardial edema in larvae hatched in the laboratory after incubation at 7 d postfertilization in subtidal cages. Edema was observed more frequently at oiled sites (Wilcoxon rank-sum test, P = 0.06). (B–E) Representative images of naturally spawned embryos from the intertidal zone at nonoiled (B and C) and oiled (D and E) sites sampled in 2008. Viable embryos are translucent and dead embryos are opaque. Embryos at each type of site developed to a late stage at which eyes were fully pigmented. Clusters of embryos from nonoiled sites were translucent within their eggshells (B), and uncoiled upon eggshell removal (C). Clusters of nonviable embryos from oiled sites showed spontaneously ruptured eggshells and opacity (D), whereas embryos that still had a heart beat failed to uncoil upon eggshell removal (E). Arrows and arrowheads in E indicate ventral pigment cells and loss of epidermal integrity, respectively. (Scale bars: 1 mm.) (F) Hatching rates of larvae from natural spawn samples. Values represent mean ± SEM normal hatch rates, calculated as the percentage of morphologically normal larvae hatched from all eggs in eight transect subsamples per site.