SC/51/ Changes in the abundance and distribution of gray whales at Laguna San Ignacio, México during the 1997-98 El Niño and the 1998-99 La Niña. Jorge Urbán R., Alejandro Gómez-Gallardo U., Victor Flores de Sahagún, Miguel Palmeros R. and Stefan Ludwig. Departamento de Biología Marina. Universidad Autónoma de Baja California Sur Ap.Post 19-B. La Paz, B.C.S. 2381 MÉXICO. ABSTRACT The ENSO (El Niño Southern Oscillation) events involve positive sea temperatures anomalies (El Niño) and anoumosly cold waters in central Pacific, regarding in effects on abundance and distribution of various plant and animal species, including several species of pinnipeds and cetaceans. For gray whales in their winter distribution there is no changes related to this events, but in 1984 were low numbers of whales in Laguna San Ignacio reported by Swartz (199). This work present the main changes in the presence of gray whales in Laguna San Ignacio during 1998 and 1999 years from data obtained by 28 weekly surveys (February 8 to March 28) and death monitoring activities in 1996-99. For 1999 we counted the lowest number of adult whales, while 1998 showed an earlier decrement in total numbers. Single whales number were very simmilar among years without any significant changes but for Cow-calf pairs the peak numbers were delated and significantly lower numbers. The number of dead calves was similar to previous studies, but with higher proportions of adult dead whales in 1999 (6%). The minimum calf mortality rate found in 1998 and 1999, were higher than 1987-82 and 1996-97 seasons. During El Niño year we observed a shift in the general distribution pattern towards northern latitudes and opposite during La Niña year. These facts let us hypothesise that this events had effect on the nutritional conditions on the whales, especially mature females. INTRODUCTION The term El Niño referred originally to a relatively weak and warm southward oceanic current that develops almost annually along the coast of southern Ecuador and northern Peru around Christmas (Philander, 199). Among other features of El Niño, there is an atmospheric pressure-field difference named Southern Oscillation (SO) between the eastern and western tropical Pacific (Walker and Bliss, 1932). During the El Niño event there is a positive sea surface temperature anomaly along the equatorial Pacific and the coast of South America. The term ENSO (El Niño / Southern Oscillation) is an event of large scale warm episodes (El Niño phase) in the central Pacific, as one of its phases (Zebiak and Cane, 1987). The other period of this cycle is La Niña, anomously cold water in the central Pacific (Philander, 1985; 199). The El Niño events can cause effects on abundance and distribution of various plant and animal species (Arntz and Fahrbacher, 1991). The strongest El Niño of this century in 1982/83 had strong impact on marine mammal species in Peru, with increased mortality and changes in distribution, for example on the southern sea lion (Otaria byronia) and the South American fur seal (Arctocephalus australis) (Arntz and Fahrbacher, 1991). There are also reports about changes in abundance and distribution for cetaceans during El Niño phases. Tershy et al. (1991), described increase of fin whale (Balaenoptera physalus) and Bryde s whale (Balaenoptera edeni) abundance in the Canal de Ballenas, Gulf of California, during the El Niño period 1982-83. Whitehead et al. (1988), observed no changes in abundance and distribution of sperm whales off the Galapagos Islands during the El Niño year 1987 but shallower dives for food, compared to the cool year 1985.
In relation with the gray whales at Laguna San Ignacio, in 1984, following a major El Niño event in the Pacific, but also with industrial noise playback experiments and significant commercial fishing activity in the lagoon, counts of gray whales decreased significantly by 5% from the 1978-82 levels. This represented a 59% or grater decrease in the number of females with calves using the lagoon (Jones et al. 1988; 1994; Swartz, 199) In this study we present data for abundance, distribution and mortality of gray whales in Laguna San Ignacio, Baja California Sur, México (Fig. 1), during 1996/97 without an El Niño event and during the strong ENSO cycle 1997-1999, with the El Niño event 1997/98 and the following La Niña 1998/99. 18' 14' 113º 1' 58' 54' El Vizcaíno Biosphere Reserve MÉXICO Isla Pelícanos Isla Garza La Laguna 5' La Freidera Punta Piedra 46' Punta Holcomb Isla Abaroa 26º 42 Isla Ana PACIFIC OCEAN Figure 1. Laguna San Ignacio, B.C.S., México. METHODS Twenty eight complete censuses from 1996 to 1999 (one per week from February 8 to March 28), were considered to determine whale abundance and distribution within the lagoon. The surveys followed a basic line transect design and were conducted by boat using the geographical lagoon division and the methodology described by Jones and Swartz (1984) and Urban et al. (1997). During the whale surveys the coastal areas were searched in order to find stranded whales. Once located, they were measured, sexed and classified according to the stranding condition code of Geraci and Lounsbury (1993).
RESULTS Abundance Gray whale counts from 7 boat weekly transects for each year were used as an index of the abundance of adult whales within the lagoon during four winter seasons (from February 8 to March 28). The maximum combined counts were between second week of February (week I) for 1998 to first of March (week IV) for 1996, where we can observe the higher number in 1997 and the lower in 1999 (Table 1, Fig. 2). During 1998 the peak number of adult whales was not so different of 1997 and even higher than 1996, but the numbers decrease constantly until the last survey. In 1999 we counted the lowest numbers of adult whales but the slope in their abundance is less pronounced than 1998. Table 1. Number of cow-calf pairs (Cc), single whales (Si) and total adults (Adl), counted in 28 complete weekly surveys from February 8 to March 28 of 1996-1999 winter seasons in Laguna San Ignacio, B.C.S., México. Week Cc Si Adl Cc Si Adl Cc Si Adl Cc Si Adl I 62 85 147 87 136 223 52 178 23 13 85 98 II 42 74 116 97 18 25 48 12 168 16 129 145 III 68 14 172 126 127 253 42 57 99 17 144 161 IV 92 115 27 143 65 28 39 38 77 33 91 124 V 83 44 127 133 71 24 62 2 64 45 84 129 VI 5 3 53 19 16 125 38 38 41 49 9 VII 51 51 13 4 134 19 19 39 19 58 3 Number of whales (adults) 25 2 15 1 5 23 253 161 27 I II III IV V VI VII week of observation Figure 2. Number of gray whales (only adults) counted in weekly surveys (February 8 to March 28) in Laguna San Ignacio, B.C.S., México, during 1996 to 1999 winter seasons. For single whales an unsuspected higher number were registered in the second week of February (I) in 1998, with very marked slope until the third week of March (Figure 3). The other years showed very similar patterns and numbers, with peak number between the end of February (III) and beginning of March (IV).
2 178 16 144 number single whales 12 8 4 127 115 I II III IV V VI VII week of observation Figure 3. Number of single whales counted in weekly surveys (February 8 to March 28) in Laguna San Ignacio, B.C.S., México, during 1996 to 1999 winter seasons. The peak numbers for cow-calf pairs were in the first week of March (IV) for 1996 and 1997, while 1998 and 1999 were one week later (V) and with lower numbers than 1996-97 years. 16 14 143 number cow-calf pairs 12 1 8 6 4 92 62 45 2 I II III IV V VI VII week of observation Figure 4. Number of cow-calf pairs counted in weekly surveys (February 8 to March 28) in Laguna San Ignacio, B.C.S., México, during 1996 to 1999 winter seasons. Changes in abundance Considering the total number of adult whales inside Laguna San Ignacio during the four years of study, it does not seem to be any substantial changes in their abundance, except for 1997 (Fig. 5). However, due to the fact that single whales were always the predominant group of both categories and that there is not any significant change in their abundance (H (3, n=28)=2.427, p=.4931) (Fig. 6), the total number of whales throughout the years looks equivalent. Oppositely, the cow-calf pairs display obvious and significant shifts in their numbers (H (3, n=28) =21.3437, p=.1) (Fig. 7)
3 25 2 No. of whales 15 1 5 Min-Max 25%-75% Median value Figure 5. Number of adult whales inside the lagoon during the four years of study. 3 25 2 No. of whales 15 1 5 Min-Max 25%-75% Median value Figure 6. Number of single whales inside the lagoon during the four years of study. 3 25 2 No. of whales 15 1 5 Min-Max 25%-75% Median value Figure 7. Number of cow-calf pairs inside the lagoon during the four years of study.
Mortality During the study, we found three calves dead in 1996, six calves and one adult in 1997, three calves in 1998 and, two calves and three adults in 1999 (Table II). Based on Rice and Wolman (1971), and Jones y Swartz (1984); animals between 3 and 8.99 m were considered calves of the year, between 9 to 1.99 m immatures, and whales more larger to 11 m adults. Table II. Summary of length and sex composition of dead gray whales found in San Ignacio Lagoon, B.C.S. (January-March 1996; February-March 1997; January-April 1998; January-April 1999). Date Age class Sex Length (m) 1996 January 31 calf - - February 1 calf - - March 15 calf - - 1997 February 13 calf male 4.5 February 16 calf female 5. February 17 calf female 4.6 February 21 calf male 4.7 February 23 adult male 12.1 February 27 calf male 5.7 March 15 calf male 5. 1998 January 21 calf male 4.1 January 27 calf female 4. February 14 calf female 4. 1999 February 6 calf female 4.2 February 18 adult? 12.5 March 4 adult female 12. March 15 adult female 14.1 March 28 calf female 8. In relation with the age class, it is important to note the absence of inmatures dead whales during 1996 to 1999. The number of dead calves was similar to previous studies. The most important change observed correspond to the adult dead whales found in 1999 (6%), two females and one of unknown sex, and the presence of only two calves, a new born (4.2 m) and a estimated 3-4 months old calf (Fig. 8). 1% 4 7 5 6 6 4 3 3 Percentage of dead whales 8% 6% 4% 2% 7 5 % 77 78 79 8 81 82 96 97 98 99 Calf Adult Immature Figure 8. Percentage of dead whales found in San Ignacio Lagoon between 1977-1982 (Jones and Swarts, 1984) and 1996 to 1999 (numbers inside the column is the total dead whales).
For the comparison with previous studies, we estimated the minimum calf mortality rate for each season. This was estimated by dividing the number of dead calves discovered per season by the estimated gross calf production per season (number of living calves at the maximum combined count + number of dead calves per season). The minimum calf mortality rate found in 1998 and 1999, were higher than 1987-82 and 1996-97 seasons (Figure 9). In other words, the minimum calf mortality was higher during the El Niño and La Niña events. 12 1 Minimum calf mortality rate 8 6 4 2 1978-82 95 % C.I. Mean Figure 9. Minimum calf mortality rate estimated in 1996, 97, 98 and 1999; and comparison with 1978-82 reported by Jones y Swarts (1984). DISCUSION The changes in the abundance of gray whales in Laguna San Ignacio are related with the general distribution of these whales in the Mexican Pacific. Considering the distribution pattern during 1996 and 1997 as -normal-, during the breeding season of 1998, El Niño year, we observed a shift in the general distribution pattern towards northern latitudes, with a very few whales in their southern range, including Bahía Magdalena, and unusual high numbers, including calves, in southern California and the northern Baja California Peninsula. The opposite happened during the breeding season of 1999, La Niña year, when we observe the presence of gray whales in places and latitudes where usually they are not present as the Northern Gulf of California and Bahía de Banderas, Jalisco, in mainland. These changes in the distribution pattern are related with higher temperatures in 1998 and lower temperatures in 1999. The age/class group of gray whales more affected were the cow-calf pairs. They are the ones that appear to prefer a certain water temperature, in which they give birth and rear their calves (Figure 1). In relation with the mortality, it is important to underline the presence of three dead adult whales, at least two of them females, during 1999. This is consistent with the mortality of gray whales in all the coast of Baja California Península during the same breeding season when were observed more than 7 dead adult whales of which more than 75% were females (Pérez-Cortés et al., 1999).
This fact, let us to hypothesise that El Niño and La Niña events had effect on the nutritional conditions on the whales, especially mature females, perhaps due to reduction on the availability of food resources provoked by oceanographic changes in their feeding areas. Although dead calves numbers in 1998 and 1999 were not different to previous years, if we take in account the lower numbers of cow-calf pairs that visited those years the lagoon, found that the minimum calf mortality rate was significantly higher in 1999 and in less degree in 1998. The reasons of this higher mortality could be due an poor healthy conditions of the mothers and/or because the oceanographic conditions in the lagoon were not the optimal for their first days of life. REFERENCES Arntz, W.E. and E. Fahrbacher. 1991. El Niño-Klimaexperiment der Natur, Ed. Birkhauser, Basel, 264 pp. Geraci, J.R. and Lounsbury, V.J. 1993. Marine Mammals Ashore, a field guide for strandings. Texas A&M Sea Grant Publication, Texas, USA. 35pp. Jones, M.L. and Swartz, S.L. 1984. Demography and phenology of gray whales and evaluation of whale-watching activities in Laguna San Ignacio, Baja California Sur, Mexico. In: Jones, M.L., Swartz, S.L. and Leatherwood, S. (eds.) The gray whale, Eschrichtius robustus. Academic Press, Inc., Orlando, Florida, pp. 39-374 Jones, M.L., S.L. Swartz, and M.E. Dahlheim. 1988. Census of gray whales in the San Ignacio Lagoon in 1985. A follow-up study in response to low whale counts in 1984. Final Report to U.S. Marine Mammal. Commn., 35 pp. Jones, M.L., Swartz, S.L. and Dahlheim, M.E. 1994. Census of gray whale abundance in San Ignacio Lagoon: a follow-up study in response to low whale counts recorded during an acoustic playback study of noise-effects on gray whales. U.S. Department of Commerce N.T.I.S. Final Report. Publication PB 94-19562. 32pp. Pérez-Cortés, H., J. Urbán R., F. Ollervides, V. Sánchez S., J. Pettis, P. Loreto y M.A. Palmeros. 1999. Varamiento de ballenas grises en B.C.S. durante el invierno 98/99 Situación anormal? paper presented at the XXIV Reunión Internacional para el Estudio de los Mamíferos Marinos. Mazatlán, México, April 18-22, 1999. Philander, S.G.H. 1985. El Niño and La Niña. J. Atmos. Sci., 42, 2652-2662. Philander, S.G.H. 199. El Niño, La Niña and the Southern Oscillation. Academic Press, 293 pp. Rice, D.W. y Wolman, A.A. 1971. The life history and ecology of the gray whale (Eschrichtius robustus). Spec. Publ. Am. Soc. Mammal. 3, 1-142 Swartz, S.L. 199. Comments on the carryng capacity of the gray whale breeding lagoons in Baja California and alternative breeding sites. Appendix 4. Annex I (Grey Whales). Report of the Scientific Committee. Fortieth Report of the International Whaling Commission. pp. 162-163 Tershy, B.R., D. Breese and S. Alvarez-Borrego. 1991. Increase in cetacean and seabird abundance in the Canal de Ballenas during an El Niño Southern Oscillation event. Mar.-Ecol.- Prog.-Ser., 69 (3), 299-32. Urbán R., J., A. Gómez-Gallardo U., Flores de Sahagún, V., Cifuentes L., J., S. Ludwig, and Palmeros R., M. 1997. Gray whale studies at Laguna San Ignacio, B.C.S., México, Winter 1996. SC/4/AS19. Rep. int. Whal. Commn 47:625-633. Walker, G.T. and E.W: Bliss. 1932. World Weather V. Mem. Roy Meteor. Soc., 4, 53-84. Whitehead, H., V. Papastavrou and S. Smith. 1988: Sperm whales and El Niño off the Galapagos Islands. Paper SC/4/Sp4 presented to the IWC Scientific Committee, (unpublished). Zebiak, S.E. and M.A. Cane. 1987. A model of El Niño/Southern Oscillation. Mon. Wea. Rev., 115, 2262-2278.