Four seasons abundance changes of Zalophus californianus californianus (Lesson 1828), Allen, 1880, in the Gulf of California, Mexico

BIOLOGICAL AND APPLIED SCIENCES

Four seasons abundance changes of Zalophus californianus californianus (Lesson 1828), Allen, 1880, in the Gulf of California, Mexico

M. Octavio Maravilla-Chávez^{I, II}; Alfredo Zavala-González^III; Alfredo Ortega-RubioI, ^*

^ICentro de Investigaciones Biológicas del Noroeste; S. C. La Paz; B. C. S. Mar bermejo 195; Col. Playa Palo de Santa Rita; C. P. 23090; Apdo. Postal 128; [e-mail]; La Paz - Baja California Sur - México

^IIInstituto Nacional de Ecología; Programa de Investigación de los Mamíferos Marinos; Ocampo 1045 Centro La Paz; [e-mail]; B. C. S. 23000; México

^IIIÁrea de Reserva Islas del Golfo de California; Dirección Regional en Baja California Ensenada; B.C. Ave. del Puerto No. 375 Loc. 24; [e-mail]; Ensenada - Baja California - México

ABSTRACT

This paper describes for the first time the relative abundance, population structure and numeric variation of California sea lions in the Gulf of California, Mexico, during four different seasons. The relative abundance estimates varied from the highest 17,162 sea lions in July of 1990 (summer) to a minimum 7,470 in February of 1992 (winter), representing a reduction of 10,000 sea lions. Hypotheses about the migration of part of the population to the Pacific Ocean were compared and discussed against their residency inside the Gulf, and seasonal dispersion of individuals along this inner sea.

Key words:Zalophus californianus californianus, Seasonal variations, relative abundance, migration, Gulf of California, Mexico

RESUMO

O leão marinho da Califórnia (Zalophus californianus californianus) é a única espécie de Otariidae (PINNIPEDIA) que vive em permanência no Golfo da Califórnia, México. Vive em grupos, principalmente em rochas, ilhas ou porções de praias de algumas ilhas. Este artigo descreve pela primeira vez as variações da abundância e estrutura populacional dos leões marinhos desta região, nas quatro estações do ano. Realizamos seis censos, entre Outubro de 1989 e Janeiro de 1992 (3 censos correspondendo ao outono e um a cada uma das outras estações. Os censos foram feitos de barco, em terra ou combinados, com contagem direta e separando os indivíduos em 5 classes de idade e sexo. As estimativas de abundância variaram de 17,162 indivíduos, em Julho de 1990 (na Primavera), ao mínimo de 7,470, no Inverno de 1992, o que representa uma redução de 10,000 leões marinhos. A hipótese da migração de parte da população para o Oceano Pacífico é discutida e comparada com a de residência da população, bem como a nossa proposta de uma dispersão dos indivíduos pelo Golfo do Califórnia.

INTRODUCTION

California sea lions, Zalophus californianus californianus, is one of the four species of Pinnipeds in Mexican waters. It is the most abundant and broadly distributed. Their geographical distribution goes from the border with the United States of America, to the coasts of Nayarit, Mexico, including the Gulf of California (Daugherty, 1965, Zavala-Gonzalez, 1990). Gallo and Ortega (1986) reported two sightings of solitary sea lions south in Acapulco, State of Guerrero, Mexico, while Gallo and Solórzano (1991) extended the area of these sightings near the border of Mexico-Guatemala. Sea lions inhabit mainly beaches, rocks, islets and islands. These groupings are known as "loberas" (rookeries) and can be classified as: breeding (BR), non-breeding (N-BR) and temporary (T). In the BR and N-BR, there are animals all the year round, but only in the BR, there are newborns. The T are for temporary occupation of sea lions, which are generally small rocks without protection against winds and surf. There are 40 recognized rookeries in the Gulf of California, 13 of which are BR, 18 N-BR and at least 9 T (Zavala-Gonzalez, 1990). The last population estimation of the Mexican portion was carried out by Le Boeuf et al., (1983) between 1979 and 1981. They estimated a total of 83,000 animals, of which, 63,000 on the Mexican Pacific Islands and 20,000 in the Gulf of California, with annual productions of 7,000 pups inside the Gulf, and 29,000 on the Mexican Pacific islands. Ten years later, Zavala-Gonzalez (1990) estimated 30,000 California sea lions inside the Gulf of California. One relevant aspect on the behavior of this species is their migratory cycle, described as variations in seasonal numbers of animals (increases-decreases). Description of massive movements of mature males from the coasts of the southern California bight area, U.S.A. towards the north (States of Washington, U.S.A. and British Columbia, Canada) were described by Bartholomew and Boolootian (1960), Hancock (1970), Bigg (1973) and Bonnell et al., (1978). In Mexico, these fluctuations have been reported after the breeding period only for subadult males (Aurioles, 1982, Aurioles et al., 1983; Maravilla, 1986). The aim of this study was to analyze the complete scheme of seasonal fluctuations inside the Gulf, reinforced with recent genetic studies that showed the isolation of sea lions populations from the Gulf regarding those out in the Pacific Ocean (Schramm 2002).

Colonies distribution is shown in Fig. 1.

MATERIALS AND METHODS

Abundance and Distribution. To determine the seasonal distribution of this population, the study area was chosen all along the Gulf of California including 10 different colonies including 9 of the 13 breeding rookeries and one temporary (Table 1). The study period was divided as: spring = March 21 - June 21; summer = June 22 - Sept 20; autumn = Sept. 21 - Dec. 20 and winter = Dec 21 - March 20.

For the determination of relative abundance, each locality from land and/or by sea, was censused counting every animal on the beaches, rocks or entire islands, separating and classifying them into five different sex/age groups recognized for this species on the field (Le Boeuf, et al., 1983, Aurioles, 1982; Aurioles, et al., 1983; Maravilla, 1986, and Zavala-Gonzalez 1990). (Table 2). These were Mature Males (M), Mature Females (F), Subadult Males (SM), Juveniles (J), Pups (P) and Miscellaneous (m) which included those not positively identified or in the water during the censuses.

For island estimations, raw data from census without any correction factor were used.

RESULTS AND DISCUSSION

Population censuses

Tables 3 (a and ^b) show the numbers for population estimations obtained at each rookery by direct counts for the six field trips.

Every rookery was visited during each field trip. During the first trip (autumn of 1989) a total of 11,562 individuals were counted. For the autumn of 1990, 11,585 animals, and for third autumn census at the end of this season (November 1991), census produced an estimation of 10,867 sea lions.

^{Table 3b} showed that numbers during spring and summer corresponded to the breeding and post breeding periods, when the population numbers reached the highest values for this region.

The summer census (July 1990) correspond to the end of the breeding season with 17,162 sea lions being highest recorded number. By the end of spring 1991, 15,043 individuals were counted. Finally, winter estimation in February 1992 produced a total of 7,470 individuals in these 10 islands. Looking at data for the autumn periods, relatively most important rookeries in numbers of individuals were San Jorge, San Esteban and San Pedro Martir, which accounted between the 67 to 72 % of the total. Autumn of 1991 was considered more representative instead of the mean of three periods considering the continuity with previous period (Spring 91) (Table 4).

Adult Females were the most abundant group regardless of the season. They accounted from 41 to 53% of population (Table 5). According to the season, Pups represented 2^nd or 3^rd place. In summer, they contributed with 32%. These results agreed with those obtained by Aurioles (1982), Le Boeuf et al., (1983), and Zavala-Gonzalez (1990) with the maximum abundance and pup production during summer (breeding season). During the autumn, after the breeding season, juveniles surpassed the number of pups. Also some females and males moved out of the breeding rookeries in search for food. More than 50% of females are absent during the winter (Table 5). Also, the total number of sea lions registered a reduction of more than 50% from summer to winter. Zavala-Gonzalez (1990) proposed two peaks of abundance: during late spring and late summer. The first, produced by breeding adults (males and females) arriving to the breeding colonies and the annual production of pups, and second produced by arrival of SM and J. Similar results were obtained in this work for spring census corresponding to the middle of the breeding period (May to June - July). For summer census, also there were coincidences with Zavala's proposal. Aurioles et al., (1983), and Maravilla (1986) proposed a migration pattern for subadult males, going out of the Gulf of California. Zavala-Gonzalez (1990) observed same behavior adding the impact of juveniles. Present result showed the low impact produced by subadult males over the total population numbers. In some rookeries (e.g. Islotes), they impacted because of the small population number on the island (In the autumn 1989 there were 81 SM, almost 50 more than mean = 30). Other categories varied, for pups, which saw reduction at the end of the first year of life about 50% is due to mortality (Aurioles and Sinsel, 1988). Juveniles, non-lactating females and males tended to disperse (Zavala-Gonzalez, 1990).

Le Boeuf et al., (1983) observed some sea lions at sea, few of them close to the islands. During our navigation along the Gulf with sunlight, we registered important numbers of sea lions at sea. Qualitatively, we encountered more sea lions at sea during our winter cruises. The occurrence of enough food resources in the middle-northern Gulf, where about 81% of the sea lions population was concentrated (Aurioles and Zavala, 1994), along with recent genetic findings supported our proposal of the residence of the "population" of the Gulf of California. Schramm (2002) found genetic isolation between the sea lions from the Pacific Ocean, and those living inside the Gulf of California. These two populations seemed to be in progressive isolation. All this supported our proposal for a new explanation of seasonal changes in numbers among the different rookeries being the dispersal of animals along the entire Gulf looking for food resources, more than migration and exchanges with sea lions from outside the Gulf.

ACKNOWLEDGEMENTS

We would like to thank the Mexican Navy, officers and crews for providing us with the best platform to cruise along the Gulf islands and to all who helped during the censuses. We thank to the Instituto Nacional de la Pesca, who supported the first author during the research. We also thank to Norma Susana Ortega Rubio, Gabriela Romero Pulido, Drs. Manuel E. dos Santos and Carolina Alvite fot their helping reviewing the Resumo in Portuguese.

Received: April 05, 2004;

Revised: December 23, 2004;

Accepted: June 06, 2005.

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*

Author for correspondence

Publication Dates

• Publication in this collection
  10 Apr 2006
• Date of issue
  Jan 2006

History

• Accepted
  06 June 2005
• Reviewed
  23 Dec 2004
• Received
  05 Apr 2004