Ecology of humpback whales (Megaptera novaengliae) off the pacific coast of Nicaragua: Population characteristics, migratory patterns and feeding behaviors

Humpback whales (Megaptera novaeangliae) are known for their extensive annual migrations between high-latitude feeding grounds and tropical or subtropical breeding and calving areas. Effective conservation of these threatened populations requires a thorough understanding of their ecology in these regions. This study focuses on the distribution patterns, population characteristics, migratory connections, and the influence of oceanic features on humpback whales within the Central American breeding area, specifically the Central American humpback whale population (CAM). Despite known threats such as vessel collisions and fishing gear entanglement, little is understood about this population's ecology, with prior research mostly limited to southern Costa Rica and Guatemala. Nicaragua, centrally located in Central America, had no existing data on the endangered CAM population.

Our research was conducted at two locations on Nicaragua’s Pacific coast: northern Nicaragua (Padre Ramos) and southern Nicaragua (San Juan del Sur). Data were collected during the dry seasons from 2004 to 2008 and 2016 to 2018 in northern Nicaragua, and from 2016 to 2023 (excluding 2019) in southern Nicaragua. Methods included photo-identification of whale tails, song recordings, behavioral assessments, sea surface temperature measurements, and GPS tracking. Chapters 2 and Chapter 3 of our study aimed to clarify the population characteristics of the CAM population and site connectivity by addressing questions such as the number of whales migrating to Nicaragua, the frequency of sightings at both sites, the presence of breeding groups including mother-calf pairs, competitive groups, and singers, the environmental factors that influence their distribution patterns, and the acoustic patterns between both sites. We observed, using the Chapman’s modified Petersen two-sample estimator, that a relatively small number of whales (ranging from 109 to 517) from the CAM population use Nicaragua as a breeding area annually. Social groups, including mother-calf pairs, singers, and competitive groups, exhibited different habitat use patterns depending on the study site, suggesting that social groups may seek specific areas within the Central American breeding grounds based on their needs, such as resting or feeding. A limited number of individuals (7 %; 12 out of 176) were recaptured in both northern and southern Nicaragua based on photo-identification. An overall unstructured song pattern was observed on both northern and southern Nicaragua, with common themes and units.

To further understand the migratory patterns, timing, and site affinity of CAM humpback whales with other breeding and feeding areas, Chapter 4 analyzed photo-identification data collected in Nicaragua by Cascadia Research Collective (2004–2008) and Association ELI-S (2016–2021). These images were compared with an international research and citizen-science database using automated image recognition. Photographic recaptures of 176 individuals were documented over a 36-year period in Central America and Mexico's breeding areas, and in the north American West Coast feeding areas, including British Columbia, Oregon, Washington, and California. Given the minimal photographic recaptures and the different habitat use patterns found between northern and southern Nicaragua (see Chapter 2), we explored whether differences in migratory destinations could be identified between both sites. Our research revealed that individuals from northern and southern Nicaragua displayed distinct site affinities throughout their migration, leading to spatial structuring of migratory routes. We found that photo-identified whales were recaptured in breeding areas between October and April, and in feeding areas along the U.S. West Coast year-round, with peak recaptures between June and October. This study provided the first evidence of fine-scale site affinity of individual humpback whales within Nicaraguan waters and to other breeding and feeding areas.

Through our citizen science program initiated in 2015, we discovered in Chapter 5 that humpback whale sightings were reported between July and October (wet season), which is outside the known temporal breeding range for the CAM population in Nicaragua. Humpback whales migrating from Chile/Antarctica feeding areas have previously been reported as far north as Costa Rica during the wet season. Therefore, determining the migratory connections of these individuals was essential to understand their population origins. Through international collaboration and the use of artificial intelligence on the Happywhale online matching platform, we confirmed the presence of a second humpback whale population, Breeding Stock G (BSG), based on a single photographic recapture linked to the Antarctica feeding area.

During the dry season (November to April), oceanographic conditions shift due to the annual cycle of the Costa Rica Dome (CRD), an open-ocean upwelling system. The CRD, a nutrient-rich shallow thermocline reaching coastal Central America, affects marine productivity and was observed to influence the presence of CAM humpback whales in Nicaraguan waters. In Chapter 6, we observed unusual feeding behaviors in Nicaragua, despite humpback whales typically fasting during migrations. These whales were observed feeding alongside other species, such as seabirds and dolphins. To assess the CRD's impact on humpback whale sightings and supplemental feeding behaviors, Chapter 7 employed linear mixed models to evaluate the influence of time-lagged chlorophyll-a on CAM abundance, and finding the four-month delayed chlorophyll-a to be the best predictor.

In conclusion, this research enhances the understanding of humpback whale distribution, migration patterns, and their adaptation to local marine ecosystem productivity in the Eastern Pacific. Future research should explore the residency times and site fidelity of the CAM and BSG populations, understand the population characteristics and habitat use of the BSG population, assess the importance of supplemental feeding for reproductive success and survival, investigate mother-calf behaviors, and evaluate population connectivity between CAM and BSG using acoustical measurements. Conservation efforts should focus on establishing Marine Protected Areas, developing specific legislation on ship speed reduction and ship strikes in critical nursing and breeding areas, implementing temporal fishing restrictions, and using baseline data to assess entanglement risks. These measures aim to reduce anthropogenic pressures on humpback whales, particularly for the most vulnerable mother-calf groups. This research has already contributed to the development of whale-watching guidelines in Nicaragua, the regional and international assessment of habitat use patterns, and the management of the species across the North Pacific.