- Email: [email protected]
Impacts of the Jessica oil spill on sea lion (Zalophus wollebaeki) populations
Marine Pollution Bulletin 47 (2003) 313–318 www.elsevier.com/locate/marpolbul
Impacts of the Jessica oil spill on sea lion (Zalophus wollebaeki) populations Sandie Salazar
*
Charles Darwin Foundation, Puerto Ayora, Gal apagos, Ecuador
Abstract Following the Jessica oil spill, a total of 79 oiled Gal apagos sea lions (Zalophus wollebaeki) were recorded around the islands of San Crist obal, Santa Fe, Isabela and Floreana. Almost half of these animals required washing and other treatment. One sea lion death and a high incidence of conjunctivitis and burns were detected during the period of the oil spill. Sea lion populations exhibited a tendency for decline in the first months following the spill at all three colonies monitored close to the grounding site on San Crist obal. By comparison, declines of similar magnitude occurred at only one of six sea lion colonies monitored on islands more distant from the spill. However, no significant decreases in population numbers were detected for any colony in the year following the spill. Galapagos sea lion populations were partially recovering from the much more catastrophic impact of the 1997/98 El Ni~ no. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gal apagos; Jessica; Impacts; Oil spill; Sea lion; Zalophus wollebaeki
1. Introduction On the early morning of 16 January 2001, the oil tanker Jessica ran aground on a reef off the point of n, at the entrance to Bahıa Naufragio, San El Ca~ no Crist obal. The tanker contained approximately 300 tonnes of bunker fuel and 600 tonnes of diesel. Part of this fuel was rescued from the ship, but over 60% of the total cargo was lost at sea during the following twoweek period. In order to minimize and assess the impact of oil released by the Jessica, the Charles Darwin Research Station and Gal apagos National Park Service formulated a contingency and monitoring plan for affected wildlife. Because sea lions live in large colonies near the grounding site, numerous oiled animals were observed, and studies elsewhere indicated substantive impacts of oil spills on marine mammals (e.g., Hall et al., 1996; Spies et al., 1996; Peterson, 2000). The Gal apagos sea lion (Zalophus wollebaeki) was ranked amongst the highest priority species for remediation and study. The present report describes the principal actions under-
*
Fax: +593-5-527013x3. E-mail address: [email protected] (S. Salazar).
taken and assessment of trends in population numbers following the spill.
2. Methods and results 2.1. Survey of oiled animals During the first two weeks following the spill (17–30 January 2001), counts were made of animals marked by oil along the beaches of the different islands affected. Four colonies of Galapagos sea lions were found to be most affected: La Loberıa and El Malec on on San Crist obal (with 22 animals oiled), Bahıa Santa Fe on Santa Fe (with 41 animals affected) and Playa de los Perros on Floreana (with seven animals oiled) (Fig. 1). The proportion of oiled animals in most of the colonies was less than 10% except in Bahıa Santa Fe, where 19% of individuals exhibited oiling. Other surveys recorded two oiled adults and one oiled pup on Isabela, two (including one dead pup) on Floreana, and two at El Miedo (Santa Fe). In the colony at El Malec on an additional two sea lions were observed with chemical burns, presumably caused by solar heating of oil exposed on skin. Therefore, a total of at least 79 sea lions were considered affected by the oil spill. The two San
0025-326X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-326X(03)00160-7
314
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
Fig. 1. Locations of sea lion colonies affected by the Jessica oil spill.
Crist obal colonies were located immediately adjacent to the Jessica grounding site. Approximately 42 sea lions were cleaned (26 at Santa Fe, nine at San Crist obal and seven at Floreana). All of the treated animals were pups (less than a year old) and small juveniles. The treated animals were cleaned with liquid detergent and excess water, the mucous linings were cleaned with milk, and some sea lions on Santa Fe were hydrated by administration of a subcutaneous physiological solution with vitamin B. Of the 73 sea lions where the degree of oiling was known, 27 were severely oiled (>50% of the body), 12 were moderately oiled (<10%) and 34 lightly oiled. Fortunately, the application of euthanasia was not necessary during this emergency. During monitoring, various pups and juveniles in all the colonies were observed to be affected by an eye infection, the severity of which varied between colonies. The colonies with highest levels of infection (>50% of pups with severe cases) were La Loberıa (San Crist obal), Bahıa Santa Fe (Santa Fe), Caama~ no (Santa Cruz) and Punta Mangle (Fernandina). During the final weeks of January to mid February, specimens of blood and tears were collected for assay from a total of 25 sea lions in the following colonies: La Loberıa, Isla Lobos, Caama~ no and Bahıa Santa Fe. Preliminary diagnosis indicated that the disease was severe conjunctivitis, related to the presence of bacillo-cocci bacteria. A direct relationship between the oil spill and this disease cannot be shown, given that outbreaks of the
disease were seen prior to the spill and historical records exist for this disease in Galapagos. 2.2. Population monitoring Since 1997 the Charles Darwin Research Station has maintained regular population surveys of 12 sea lion colonies in Galapagos (Fig. 2). All of the colonies most affected by the oil spill were included in this population monitoring programme, except Playa de los Perros on the island of Floreana. During November 2001 a general pinniped survey was conducted throughout the archipelago (except Darwin and Wolf islands, where negligible animals live). Total populations of 14,000 Galapagos sea lions and 6000 fur sea lions were estimated in this survey, taking into account that the proportion of the counted animals in a breeding colony was about 50% of the true population. This correction is normally used for estimation of populations of a medium size species, considering the probability of observation (observability) and the census method used (Nichols and Corroy, 1996). Few differences were found between data collected during November 2001 compared with the average number of the animals recorded in surveys earlier in this year at the reproductive colonies of Z. wollebaeki that are monitored regularly. Thus, the census data can be considered reasonably representative of general patterns and not influenced greatly by the particular conditions on the days of sampling.
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
315
Fig. 2. Sea lion colonies monitored in Galapagos since April 1997.
Population trends over the short term (between spill date, or immediately prior to spill, and surveys six months later) were negative for two of the three colonies with oiled animals observed (La Loberıa and El Malec on), and positive for the colony at Bahıa de Santa Fe (Fig. 3). The colony at Playa de los Perros is not a reproductive colony and the number of animals varies greatly between monitoring periods for this site. The downward trend at two oiled colonies was generally unusual for Gal apagos, given that most colonies in Gal apagos showed increasing population numbers as a recovery response to the severe mortality encountered during the 1997/98 El Ni~ no event. Other sites monitored where no animals were observed with effects of oil are useful as reference sites for comparison with the oiled sites. Amongst these sites (Fig. 3), the trend in population numbers over the three months following the spill was generally positive. Only at Isla Lobos (San Crist obal), a site only 8 km from the wreck site but with no animals observed with oiling, and at Plaza Sur off Santa Cruz, was the population trend substantially negative. It is particularly notable that all three colonies on San Crist obal showed population declines, whereas only one of six sites monitored on islands more distant from the spill showed a similar trend. Despite the six-monthly trends, no major changes attributable to oiling were detected during the year following the spill for any of the sites studied. This was indicated by a lack of significant effects shown in t-tests where population numbers of sea lions were compared during the year before and year after the spill for the
various colonies monitored (Table 1). Only one significant effect was found, an increase rather than a decrease in numbers at the oil-affected El Malec on site. While numbers at this colony, located only 2 km from the wreck site, apparently doubled during the year following the spill, the increase occurred between the last survey pre-spill and the first survey post-spill (Fig. 3), and thus probably occurred prior to the spill. During the year following the spill from 14 February to 14 November 2001, population counts at El Malec on in fact declined from 258 to 136 individuals. It is important to note that the sample sizes (dates sampled) and power for t-tests described in Table 1 were low, hence few conclusions can be made because of the likelihood of Type II statistical errors (i.e., a change in population number occurred between years but was not detected because of insufficient sample size). Nevertheless, none of the colonies showed a major decrease in population numbers, and the trend overall was for increase rather than decrease, including the overall trend in population numbers for oil-affected sites combined.
3. Discussion The effects of oil spills on sea mammals are difficult to predict, but largely depend on the level of skin exposure and ingestion. Pinnipeds generally appear quite resistant to external oiling because the presence of blubber layers minimizes impacts on thermoregulation. By contrast, sea otters can suffer considerably from hypothermia
316
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
Affected: San Cristóbal Island: Punta Suárez 250
300
200
No. individuals
No. individuals
La Lobería 350
250 200 150 100
150 100 50
50 0 0
1997
1998
1999
2000
1997
2001
1998
2000
2001
No reports: Isla Santa Cruz:
El Malecón
Caamaño
350 300 250
No.individuals
No. individuals
1999
200 150 100 50 0
1997
1998
1999
2000
2001
500 450 400 350 300 250 200 150 100 50 0
1997
1998
1999
2000
2001
Plaza Sur
Affected: Santa Fé Island: 900
Santa Fe Bay
800
No. individuals
350
No. individuals
300 250 200 150
700 600 500 400 300 200
100
100
50
0
1997
1998
1999
2000
2001
2000
2001
0
1997
1998
1999
2000
2001
Mosquera 1800
No reports: San Cristóbal Island: No. Individuals
1600
Isla Lobos 400
No. individuals
350 300 250 200
1400 1200 1000 800 600 400 200
150
0
1997
100
1998
1999
50 0
1997
1998
1999
2000
2001
No reports: Isla Española: Gardner Bay 350
No. individuals
300 250 200 150 100 50 0
1997
1998
1999
2000
2001
Fig. 3. Changes over time in maximum number of animals sighted per visit for three colonies affected by the oil spill and six colonies with no reports of oiled animals. A vertical line indicates the date of the oil spill.
when oiled (Geraci and St. Aubin, 1990; Overton et al., 1994; Michel et al., 2000). Following the Exxon Valdez spill, impacts on harbor seals were less severe than on sea otters, although eye illness (Michel et al., 2000) and some decline in population size was observed for seals
(Frost et al., 1999; but see Hoover-Miller et al., 2001). Effects of internal ingestion of oil remain largely unknown (Michel et al., 2000); however, kidney failure, intestinal lining destruction, neural disorders and bioaccumulation are likely in extreme cases (Overton et al.,
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
317
Table 1 Mean densities of sea lions at sites during the 12-month period prior to the Jessica oil spill (19 January 2001), and in 2001 following the spill, with results of t-tests Island
Site
Year pre-spill
Year post-spill
t-value
Reference sites Caama~ no Espa~ nola Espa~ nola Espa~ nola Floreana Mosquera Plaza Sur Seymour Norte San Crist obal
Caama~ no Bahıa Gardner Punta Cevallos Punta Suarez Champion Mosquera Plaza Sur Seymour Norte Isla Lobos
254.5 186.3 34.3 184.7 197.7 576.5 630.8 117.3 231.8
331.7 267.0 64.0 187.5 321.5 538.5 435.7 52.0 230.7
2.23 2.02
Oil-affected sites San Crist obal San Crist obal Santa Fe
La Loberia El Malec on Santa FeBay
145.0 90.3 224.8
110.5 198.7 253.3
*
df
0.01
7 3 2 3 3 4 8 2 5
0.40 3.03 1.06
4 5 5
0.06 1.83 0.74 1.68
p ¼ 0:029.
1994; Seal Conservation Society: http://www.pinnipeds. fsnet.co.uk/species/steller.htm). Chronic pollution of small quantities of oil from shipping and coastal discharges also appears to cause long term impacts in pinnipeds, as is evident with grey seals (Halichoerus grypus) where the skeletal and the reproductive systems can be affected (Jenssen, 1996). Exposure to persistent chemical compounds has also been suggested as the cause of declines in Baltic grey seal populations (Jenssen, 1996). A medium-term impact of oiling was observed for the Australian fur seal (Arctocephalus pusillus doriferus) following the Iron Baron oil spill in Australia (July 1995), with the number of pups born at the Tenth Island (an affected area) reduced during the year following the spill. A negative relationship also was detected between the productivity of fur seal colonies and proximity of island colonies to the spill site (Pemberton, 1998). In Gal apagos, short-term impacts of the Jessica oil spill were apparent in terms of numerous sea lions with oiling observed and treated during the two-week period following the wreck of the Jessica; however, no major long-term negative impacts were detected. Counts of sea lion numbers declined at all three colonies located on San Crist obal near the spill site over the following six months; however, this was more likely caused by temporary emigration or stochastic sampling variation rather than by direct mortality. Amongst the sea lion colonies where oiled individuals were observed, a decline in population numbers over the year following the spill was detected only at La Loberıa, San Crist obal, and the decline at that colony was not statistically significant (Table 1). Population numbers at the site closest to the spill (El Malec on) were considerably higher overall in 2001 than during the previous year, however, a downward trend was experienced through 2001. Part of this decline was presumably caused by the unrelated deaths
of at least nine sea lions through illegal killing for reproductive organs (Salazar and Edgar, 2001). For all sites, any changes in population numbers in 2001 were minor compared to the huge declines in population numbers in 1998 associated with the severe El Ni~ no that concluded in that year. Impacts of the Jessica oil spill were therefore well within the range of natural environmental variability in the region.
Acknowledgements I am grateful to Ian Robinson, who designed the protocol for treatment of oiled sea lions, Godfrey Merlen, Martin Haulena, Alejandro Ulloa, Marilyn Cruz and others who shared their knowledge to help with this unexpected experience. Thanks also to the teams of people from the Charles Darwin Foundation and Galapagos National Park Service who washed the animals and collected data in Santa Fe colony, and to CDF staff in Puerto Baquerizo Moreno for their collaboration. My special thanks to Graham Edgar and Lynn Lougheed for helpful comments and English translation. This study was supported by GEF PNUD, the British Embassy, Ernest Kleinwort Charitable Trust and the Galapagos National Park Service. I also wish to thank the Laboratory of Erasmus University in Holland for conducting the pathological analyses.
References Frost, K.J., Lowry, L.F., VerHoef, J.M., 1999. Monitoring the trend of harbor seals in Prince William Sound, Alaska; after the Exxon Valdez oil spill. Marine Mammal Science 15, 494–506. Geraci, J.C., St. Aubin, D.J., 1990. Sea Mammals & Oil: confronting the risk. Academic Press, Inc, California, USA.
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S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
Hall, A.J., Watkins, J., Hiby, L., 1996. The impact of the 1993 Braer oil spill on grey seals in Shetland. Science of the Total Environment 186, 119–125. Hoover-Miller, A., Parker, K.R., Burns, J.J., 2001. A reassessment of the impact of the, Alaska Exxon Valdez oil spill on harbor seals (Phoca vitulina richardsi) in Prince William Sound. Marine Mammal Science 17, 111–135. Jenssen, B.M., 1996. An overview of exposure to, and effects of, petroleum oil and organochlorine pollution in grey seals (Halichoerus grypus). Science of the Total Environment 186, 109–118. Michel, J., Shigenaka, G., Hoff, R., 2000. Oil Spill Response and Cleanup Techniques. http://response.restoration.noaa.gov/oilaids/ monterey/Chapter5.pdf. Nichols, J.D., Corroy, M.J., 1996. Techniques for estimating abundance and species richness. In: Wilson, D.E., Russell Cole, F., Nichols, J.D., Rudran, R., Foster, M.S. (Eds.), Measuring and Monitoring Biological Diversity, Standard Methods for Mammals, vol. 10. Smithsonian Institution Press, Washington, pp. 177–202.
Overton, E.B., Sharp, W.D., Roberts, P., 1994. Toxicity of petroleum. In: Cockerham, L.G., Shane, B.S. (Eds.), Basic Environmental Toxicology. CRC Press, Inc, USA, pp. 133–156. Available from. Pemberton, D., 1998. Fur seal monitoring report. In: Iron Baron Oil Spill July 1995, Long Term Environmental Impact and Recovery, Department of Primary Industries, Water & Environment, Parks and Wildlife Service, Hobart, Tasmania. Peterson, C.H., 2000. The ‘‘Exxon Valdez’’ oil spill in Alaska: acute, indirect and chronic effects on the ecosystem. Advances in Marine Biology 39, 3–84. Salazar, S., Edgar, G., 2001. Galapagos Sea lion Massacre. Marine Pollution Bulletin 42, 705. Spies, R.B., Rice, S.D., Wolfe, D.A., Wright, B.A., 1996. The effects of the Exxon Valdez oil spill on the Alaskan coastal environment. In: Rice, S.D., Spies, R.B., Wolfe, D.A., Wright, B. (Eds.), Proceedings of the Exxon Valdez Oil Spill Symposium, vol. 18. American Fisheries Society, Bethesda, MD, pp. 1–16.
Impacts of the Jessica oil spill on sea lion (Zalophus wollebaeki) populations Sandie Salazar
*
Charles Darwin Foundation, Puerto Ayora, Gal apagos, Ecuador
Abstract Following the Jessica oil spill, a total of 79 oiled Gal apagos sea lions (Zalophus wollebaeki) were recorded around the islands of San Crist obal, Santa Fe, Isabela and Floreana. Almost half of these animals required washing and other treatment. One sea lion death and a high incidence of conjunctivitis and burns were detected during the period of the oil spill. Sea lion populations exhibited a tendency for decline in the first months following the spill at all three colonies monitored close to the grounding site on San Crist obal. By comparison, declines of similar magnitude occurred at only one of six sea lion colonies monitored on islands more distant from the spill. However, no significant decreases in population numbers were detected for any colony in the year following the spill. Galapagos sea lion populations were partially recovering from the much more catastrophic impact of the 1997/98 El Ni~ no. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gal apagos; Jessica; Impacts; Oil spill; Sea lion; Zalophus wollebaeki
1. Introduction On the early morning of 16 January 2001, the oil tanker Jessica ran aground on a reef off the point of n, at the entrance to Bahıa Naufragio, San El Ca~ no Crist obal. The tanker contained approximately 300 tonnes of bunker fuel and 600 tonnes of diesel. Part of this fuel was rescued from the ship, but over 60% of the total cargo was lost at sea during the following twoweek period. In order to minimize and assess the impact of oil released by the Jessica, the Charles Darwin Research Station and Gal apagos National Park Service formulated a contingency and monitoring plan for affected wildlife. Because sea lions live in large colonies near the grounding site, numerous oiled animals were observed, and studies elsewhere indicated substantive impacts of oil spills on marine mammals (e.g., Hall et al., 1996; Spies et al., 1996; Peterson, 2000). The Gal apagos sea lion (Zalophus wollebaeki) was ranked amongst the highest priority species for remediation and study. The present report describes the principal actions under-
*
Fax: +593-5-527013x3. E-mail address: [email protected] (S. Salazar).
taken and assessment of trends in population numbers following the spill.
2. Methods and results 2.1. Survey of oiled animals During the first two weeks following the spill (17–30 January 2001), counts were made of animals marked by oil along the beaches of the different islands affected. Four colonies of Galapagos sea lions were found to be most affected: La Loberıa and El Malec on on San Crist obal (with 22 animals oiled), Bahıa Santa Fe on Santa Fe (with 41 animals affected) and Playa de los Perros on Floreana (with seven animals oiled) (Fig. 1). The proportion of oiled animals in most of the colonies was less than 10% except in Bahıa Santa Fe, where 19% of individuals exhibited oiling. Other surveys recorded two oiled adults and one oiled pup on Isabela, two (including one dead pup) on Floreana, and two at El Miedo (Santa Fe). In the colony at El Malec on an additional two sea lions were observed with chemical burns, presumably caused by solar heating of oil exposed on skin. Therefore, a total of at least 79 sea lions were considered affected by the oil spill. The two San
0025-326X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-326X(03)00160-7
314
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
Fig. 1. Locations of sea lion colonies affected by the Jessica oil spill.
Crist obal colonies were located immediately adjacent to the Jessica grounding site. Approximately 42 sea lions were cleaned (26 at Santa Fe, nine at San Crist obal and seven at Floreana). All of the treated animals were pups (less than a year old) and small juveniles. The treated animals were cleaned with liquid detergent and excess water, the mucous linings were cleaned with milk, and some sea lions on Santa Fe were hydrated by administration of a subcutaneous physiological solution with vitamin B. Of the 73 sea lions where the degree of oiling was known, 27 were severely oiled (>50% of the body), 12 were moderately oiled (<10%) and 34 lightly oiled. Fortunately, the application of euthanasia was not necessary during this emergency. During monitoring, various pups and juveniles in all the colonies were observed to be affected by an eye infection, the severity of which varied between colonies. The colonies with highest levels of infection (>50% of pups with severe cases) were La Loberıa (San Crist obal), Bahıa Santa Fe (Santa Fe), Caama~ no (Santa Cruz) and Punta Mangle (Fernandina). During the final weeks of January to mid February, specimens of blood and tears were collected for assay from a total of 25 sea lions in the following colonies: La Loberıa, Isla Lobos, Caama~ no and Bahıa Santa Fe. Preliminary diagnosis indicated that the disease was severe conjunctivitis, related to the presence of bacillo-cocci bacteria. A direct relationship between the oil spill and this disease cannot be shown, given that outbreaks of the
disease were seen prior to the spill and historical records exist for this disease in Galapagos. 2.2. Population monitoring Since 1997 the Charles Darwin Research Station has maintained regular population surveys of 12 sea lion colonies in Galapagos (Fig. 2). All of the colonies most affected by the oil spill were included in this population monitoring programme, except Playa de los Perros on the island of Floreana. During November 2001 a general pinniped survey was conducted throughout the archipelago (except Darwin and Wolf islands, where negligible animals live). Total populations of 14,000 Galapagos sea lions and 6000 fur sea lions were estimated in this survey, taking into account that the proportion of the counted animals in a breeding colony was about 50% of the true population. This correction is normally used for estimation of populations of a medium size species, considering the probability of observation (observability) and the census method used (Nichols and Corroy, 1996). Few differences were found between data collected during November 2001 compared with the average number of the animals recorded in surveys earlier in this year at the reproductive colonies of Z. wollebaeki that are monitored regularly. Thus, the census data can be considered reasonably representative of general patterns and not influenced greatly by the particular conditions on the days of sampling.
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
315
Fig. 2. Sea lion colonies monitored in Galapagos since April 1997.
Population trends over the short term (between spill date, or immediately prior to spill, and surveys six months later) were negative for two of the three colonies with oiled animals observed (La Loberıa and El Malec on), and positive for the colony at Bahıa de Santa Fe (Fig. 3). The colony at Playa de los Perros is not a reproductive colony and the number of animals varies greatly between monitoring periods for this site. The downward trend at two oiled colonies was generally unusual for Gal apagos, given that most colonies in Gal apagos showed increasing population numbers as a recovery response to the severe mortality encountered during the 1997/98 El Ni~ no event. Other sites monitored where no animals were observed with effects of oil are useful as reference sites for comparison with the oiled sites. Amongst these sites (Fig. 3), the trend in population numbers over the three months following the spill was generally positive. Only at Isla Lobos (San Crist obal), a site only 8 km from the wreck site but with no animals observed with oiling, and at Plaza Sur off Santa Cruz, was the population trend substantially negative. It is particularly notable that all three colonies on San Crist obal showed population declines, whereas only one of six sites monitored on islands more distant from the spill showed a similar trend. Despite the six-monthly trends, no major changes attributable to oiling were detected during the year following the spill for any of the sites studied. This was indicated by a lack of significant effects shown in t-tests where population numbers of sea lions were compared during the year before and year after the spill for the
various colonies monitored (Table 1). Only one significant effect was found, an increase rather than a decrease in numbers at the oil-affected El Malec on site. While numbers at this colony, located only 2 km from the wreck site, apparently doubled during the year following the spill, the increase occurred between the last survey pre-spill and the first survey post-spill (Fig. 3), and thus probably occurred prior to the spill. During the year following the spill from 14 February to 14 November 2001, population counts at El Malec on in fact declined from 258 to 136 individuals. It is important to note that the sample sizes (dates sampled) and power for t-tests described in Table 1 were low, hence few conclusions can be made because of the likelihood of Type II statistical errors (i.e., a change in population number occurred between years but was not detected because of insufficient sample size). Nevertheless, none of the colonies showed a major decrease in population numbers, and the trend overall was for increase rather than decrease, including the overall trend in population numbers for oil-affected sites combined.
3. Discussion The effects of oil spills on sea mammals are difficult to predict, but largely depend on the level of skin exposure and ingestion. Pinnipeds generally appear quite resistant to external oiling because the presence of blubber layers minimizes impacts on thermoregulation. By contrast, sea otters can suffer considerably from hypothermia
316
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
Affected: San Cristóbal Island: Punta Suárez 250
300
200
No. individuals
No. individuals
La Lobería 350
250 200 150 100
150 100 50
50 0 0
1997
1998
1999
2000
1997
2001
1998
2000
2001
No reports: Isla Santa Cruz:
El Malecón
Caamaño
350 300 250
No.individuals
No. individuals
1999
200 150 100 50 0
1997
1998
1999
2000
2001
500 450 400 350 300 250 200 150 100 50 0
1997
1998
1999
2000
2001
Plaza Sur
Affected: Santa Fé Island: 900
Santa Fe Bay
800
No. individuals
350
No. individuals
300 250 200 150
700 600 500 400 300 200
100
100
50
0
1997
1998
1999
2000
2001
2000
2001
0
1997
1998
1999
2000
2001
Mosquera 1800
No reports: San Cristóbal Island: No. Individuals
1600
Isla Lobos 400
No. individuals
350 300 250 200
1400 1200 1000 800 600 400 200
150
0
1997
100
1998
1999
50 0
1997
1998
1999
2000
2001
No reports: Isla Española: Gardner Bay 350
No. individuals
300 250 200 150 100 50 0
1997
1998
1999
2000
2001
Fig. 3. Changes over time in maximum number of animals sighted per visit for three colonies affected by the oil spill and six colonies with no reports of oiled animals. A vertical line indicates the date of the oil spill.
when oiled (Geraci and St. Aubin, 1990; Overton et al., 1994; Michel et al., 2000). Following the Exxon Valdez spill, impacts on harbor seals were less severe than on sea otters, although eye illness (Michel et al., 2000) and some decline in population size was observed for seals
(Frost et al., 1999; but see Hoover-Miller et al., 2001). Effects of internal ingestion of oil remain largely unknown (Michel et al., 2000); however, kidney failure, intestinal lining destruction, neural disorders and bioaccumulation are likely in extreme cases (Overton et al.,
S. Salazar / Marine Pollution Bulletin 47 (2003) 313–318
317
Table 1 Mean densities of sea lions at sites during the 12-month period prior to the Jessica oil spill (19 January 2001), and in 2001 following the spill, with results of t-tests Island
Site
Year pre-spill
Year post-spill
t-value
Reference sites Caama~ no Espa~ nola Espa~ nola Espa~ nola Floreana Mosquera Plaza Sur Seymour Norte San Crist obal
Caama~ no Bahıa Gardner Punta Cevallos Punta Suarez Champion Mosquera Plaza Sur Seymour Norte Isla Lobos
254.5 186.3 34.3 184.7 197.7 576.5 630.8 117.3 231.8
331.7 267.0 64.0 187.5 321.5 538.5 435.7 52.0 230.7
2.23 2.02
Oil-affected sites San Crist obal San Crist obal Santa Fe
La Loberia El Malec on Santa FeBay
145.0 90.3 224.8
110.5 198.7 253.3
*
df
0.01
7 3 2 3 3 4 8 2 5
0.40 3.03 1.06
4 5 5
0.06 1.83 0.74 1.68
p ¼ 0:029.
1994; Seal Conservation Society: http://www.pinnipeds. fsnet.co.uk/species/steller.htm). Chronic pollution of small quantities of oil from shipping and coastal discharges also appears to cause long term impacts in pinnipeds, as is evident with grey seals (Halichoerus grypus) where the skeletal and the reproductive systems can be affected (Jenssen, 1996). Exposure to persistent chemical compounds has also been suggested as the cause of declines in Baltic grey seal populations (Jenssen, 1996). A medium-term impact of oiling was observed for the Australian fur seal (Arctocephalus pusillus doriferus) following the Iron Baron oil spill in Australia (July 1995), with the number of pups born at the Tenth Island (an affected area) reduced during the year following the spill. A negative relationship also was detected between the productivity of fur seal colonies and proximity of island colonies to the spill site (Pemberton, 1998). In Gal apagos, short-term impacts of the Jessica oil spill were apparent in terms of numerous sea lions with oiling observed and treated during the two-week period following the wreck of the Jessica; however, no major long-term negative impacts were detected. Counts of sea lion numbers declined at all three colonies located on San Crist obal near the spill site over the following six months; however, this was more likely caused by temporary emigration or stochastic sampling variation rather than by direct mortality. Amongst the sea lion colonies where oiled individuals were observed, a decline in population numbers over the year following the spill was detected only at La Loberıa, San Crist obal, and the decline at that colony was not statistically significant (Table 1). Population numbers at the site closest to the spill (El Malec on) were considerably higher overall in 2001 than during the previous year, however, a downward trend was experienced through 2001. Part of this decline was presumably caused by the unrelated deaths
of at least nine sea lions through illegal killing for reproductive organs (Salazar and Edgar, 2001). For all sites, any changes in population numbers in 2001 were minor compared to the huge declines in population numbers in 1998 associated with the severe El Ni~ no that concluded in that year. Impacts of the Jessica oil spill were therefore well within the range of natural environmental variability in the region.
Acknowledgements I am grateful to Ian Robinson, who designed the protocol for treatment of oiled sea lions, Godfrey Merlen, Martin Haulena, Alejandro Ulloa, Marilyn Cruz and others who shared their knowledge to help with this unexpected experience. Thanks also to the teams of people from the Charles Darwin Foundation and Galapagos National Park Service who washed the animals and collected data in Santa Fe colony, and to CDF staff in Puerto Baquerizo Moreno for their collaboration. My special thanks to Graham Edgar and Lynn Lougheed for helpful comments and English translation. This study was supported by GEF PNUD, the British Embassy, Ernest Kleinwort Charitable Trust and the Galapagos National Park Service. I also wish to thank the Laboratory of Erasmus University in Holland for conducting the pathological analyses.
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