Enriqueta Velarde

SEABIRD ECOLOGY, EL NIñO ANOMALIES, AND PREDICTION OF SARDINE FISHERIES IN THE GULF OF CALIFORNIA

Small pelagic fish constitute 25-40% of the fisheries landings in Mexico. More than 70% of these landings, predominantly Pacific sardine (Sardinops caeruleus), are captured in the Gulf of California. Small pelagic fishes are a key component of the Gulfs ecosystem; they are eaten by seabirds, sea mammals, and other fishes. The sardine fishery within the Gulf has been showing signs of overfishing since the early 1990s. To contribute to the sustainable management of this fishery, we developed two statistical models that use oceanographic conditions and seabird breeding and feeding data to predict total fishery catch and catch per unit effort (CPUE) of Pacific sardine in the central Gulf. Total catch was predicted with an accuracy of 54% by a linear model incorporating the Southern Oscillation Index (SOI), the clutch size of Heermanns Gulls ( Larus heermanni), and the proportion of sardine mass in the diet of Elegant Terns (Sterna elegans). CPUE was predicted with an accuracy of 73% by a model based on the proportion of sardines in the diet of Elegant Terns, the reproductive success of Heermanns Gulls, and the springtime sea surface temperature anomaly in the Gulf region. Our results show that the reproductive ecology of seabirds is coupled to the global and local oceanographic conditions and that this infor- mation can be used to predict in advance the outcome of fishing efforts. We propose the use of models of this kind to reduce the effort of the fleet in years when it can be anticipated that CPUE will be low.

Effects of parental age and food availability on the reproductive success of Heermann's Gulls in the Gulf of California

Parental age, body condition, and food availability have been found to influence breeding parameters in seabirds, such as clutch size, number of chicks hatched and fledged, hatching, fledging, and reproductive success. In this paper we analyze the influence of parental age and body condition estimated by body mass, and food availability estimated from catch per unit effort (CPUE) statistics for Pacific sardine (Sardinops caeruleus) + northern anchovy (Engraulis mordax) by the local fishing fleet, on the breeding parameters of the Heermanns Gull (Larus heermanni; a vulnerable species according to Mexican federal law) nesting in Isla Rasa, Gulf of California, Mexico. Results are based on data from 1123 recaptures of known-age individuals, ranging from 4 to 13 years of age, during seven observation years between 1989 and 1997. Ages of mated male and female gulls were positively correlated. Breeding parameters showed their lowest values in 1992, an El Niño year in which the birds also showed significantly lower individual masses for both males and females, and in which the local CPUE of sardine + anchovies was lowest. All breeding parameters increased significantly with parental age and were highest at 10-12 years. No significant statistical interactions were found between food availability and parental age on the breeding parameters. Through a path analysis we found that there is a strong chained relationship between variables: food availability, which is strongly driven by oceanographic conditions, affects both the survival of eggs into hatchlings and the survival of hatchlings into fledglings. This external factor and parental age, a biological factor intrinsic to each nesting couple, explain 41% of the observed between-nest variation in fledgling success.

ENDEMIC AND INTRODUCED VERTEBRATES IN THE DIET OF THE BARN OWL (TYTO ALBA) ON TWO ISLANDS IN THE GULF OF CALIFORNIA, MEXICO

Abstract Through the analysis of 458 barn owl (Tyto alba) pellets collected between 1981 and 1995 on 2 islands in the northern Gulf of California, Mexico, we found significant differences in the main components of the diet of barn owls on these islands, where introduced rodents, bats, and seabirds occur. On Rasa Island, the most important prey was Rattus rattus, followed by Myotis vivesi, Oceanodroma microsoma, Mus musculus, and 4 bird species. On Partida Island, the main prey was M. vivesi, followed by O. microsoma, R. rattus, and 3 bird species. The mean number of prey per pellet was higher on Partida Island (1.08) than on Rasa Island (0.91), but the mean biomass per pellet was higher on Rasa Island (75.8 g) than on Partida Island (25.8 g). The storm-petrels O. microsoma and O. melania were taken by barn owls in proportion to their abundance on both islands. Conversely, barn owls on Rasa Island ate more R. rattus than M. musculus, possibly because of the larger size of the former. Overall, our data suggest that barn owls in the Gulf of California behave as flexible predators that take prey according to availability and profitability.

Predation of Nesting Larids by Peregrine Falcons at Rasa Island, Gulf of California, Mexico

Peregrine Falcon (Falco peregrinus) diets vary widely. On the Pacific Coast of North America, they feed on petrels and alcids (Oceanodroma, Synthliboramphus, Ptychoramphus) (Green 1916, Brooks 1926, Bent 1938, Beebe 1960), while in the Gulf of California they take Eared Grebe (Podiceps nigricollis), Black Storm-Petrel (Oceanodroma melania), Least Storm-Petrel (O. microsoma), Red-necked Phalarope (Phalaropus lobatus), Bonapartes Gull (Larus philadelphia), Heermanns Gull (L. heermanni) and Craveris Murrelet (Synthliboramphus craveri) (Porter and Jenkins 1988). Due to seabird coloniality, close to exclusive dependence of individual Peregrine Falcons on particular prey species may put their survival in an inverse relation to the degree of pollution of marine areas where their prey feed. In the case of Rasa Island, 95% of the population of the two main prey species nest synchronously, providing close to 300,000 prey available for three months.

Warm oceanographic anomalies and fishing pressure drive seabird nesting north

Since 2000, Elegant Terns have begun migrating northward in the face of warm oceanographic anomalies. Parallel studies of nesting colonies in Mexico and the United States show that Elegant Terns (Thalasseus elegans) have expanded from the Gulf of California Midriff Island Region into Southern California, but the expansion fluctuates from year to year. A strong inverse relationship between nesting pairs in three Southern California nesting areas [San Diego saltworks, Bolsa Chica Ecological Reserve, and Los Angeles Harbor (1991 to 2014)] and Isla Rasa in the Midriff (1980 to 2014) shows that terns migrate northward when confronting warm oceanographic anomalies (>1.0°C), which may decrease fish availability and hamper nesting success. Migration pulses are triggered by sea surface temperature anomalies localized in the Midriff and, secondarily, by reductions in the sardine population as a result of intensive fishing. This behavior is new; before year 2000, the terns stayed in the Midriff even when oceanographic conditions were adverse. Our results show that terns are responding dynamically to rapidly changing oceanographic conditions and fish availability by migrating 600 km northwest in search of more productive waters.

FEDERICO CRAVERI AND CHANGES IN NESTING SEABIRDS ON ISLA RASA, GULF OF CALIFORNIA, SINCE 1856

Abstract Isla Rasa in the Gulf of California has been recognized as an important seabird breeding island for more than 150 y. Yet during most of this period, the island suffered human disturbances that severely reduced seabird nesting success. Today, nearly two decades after disturbances ended, Isla Rasa supports about 90% of the global populations of Heermanns gulls (Larus heermanni) and elegant terns (Thalasseus elegans), plus smaller populations of other species. However, the recently available 1856 journals of Italian naturalist Federico Craveri, and the oral traditions of the Comcaac (Seri people), suggest that the array of nesting species has changed. Apparently, California brown pelicans (Pelecanus occidentalis californicus) abandoned Isla Rasa as a breeding site at the onset of disturbances, and terns first colonized it around 1920.

Are seabirds' life history traits maladaptive under present oceanographic variability? The case of Heermann's Gull (Larus heermanni)

ABSTRACT Ocean warming events are becoming more frequent, with potential consequences for birds that use marine resources. We analyzed the annual population growth of Heermanns Gulls (Larus heermanni) through matrix-based demographic models under normal and high sea surface temperature (SST) conditions. Under stable, normal conditions, predicted population growth was ∼4%, with survival of fledglings and juveniles contributing to population growth. Under high SST conditions, predicted population growth dropped to −15% and adult survival became the key factor determining species fitness. Simulations showed a declining growth rate as the frequency of warm SST anomalies increased. Heermanns Gulls can resist oceanographic anomalies of up to 1 event every 5 yr without seriously compromising their population growth rate or their individual fitness, but populations may decline rapidly if the frequency of warm-phase anomalies continues to increase, as it has done during the last decade.