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Migratory phenology of the Central American humpback whale (Megaptera novaeangliae) population unit in breeding areas

This study analyzes seven years of photo-identification data to reveal that the Central American humpback whale population follows a highly consistent, asymmetric migratory pattern with a distinct southbound coastal corridor and a more direct northbound return, highlighting both the stability of this endangered population's migration and its potential vulnerability to climate-driven oceanographic shifts.

Original authors: Paula Cabanilles, Raquel Ortells Bañeres, Pamela Martínez-Loustalot, Nicola Ransome, Melvin Castaneda, Katherina Audley, Enrique de Luna, Joëlle De Weerdt, John Calambokidis, Anabel Forte Deltell, Ted

Published 2026-06-24

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CC BY 4.0

Original authors: Paula Cabanilles, Raquel Ortells Bañeres, Pamela Martínez-Loustalot, Nicola Ransome, Melvin Castaneda, Katherina Audley, Enrique de Luna, Joëlle De Weerdt, John Calambokidis, Anabel Forte Deltell, Ted Cheeseman, Marilia Olio, Jorge Urbán R

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the humpback whales of the Central American population as a massive, slow-moving train that travels thousands of miles every year. This "train" doesn't just go from point A to point B; it has a very specific schedule and a unique route for its journey down south versus its trip back north.

This paper is like a detailed timetable and map created by a team of researchers and citizen scientists. They spent seven years (2018–2024) taking photos of the whales' tails to identify them individually, much like recognizing a friend by their unique face. By tracking 672 specific whales across the Pacific coast of Mexico and Central America, they figured out exactly when and where these whales hang out during their breeding season.

Here is the story of their journey, broken down simply:

The "Down South" Trip (The Arrival)

Think of the southbound migration as a leisurely road trip where the whales stop at every scenic viewpoint.

October: The first few whales arrive in Baja California Sur (BCS), the northernmost part of their breeding zone. It's like the train pulling into the first station.
December: The crowd gets bigger in BCS, and the whales start moving south to Nayarit and Jalisco.
January–February: The "train" finally reaches its main destination: Southern Mexico and Central America. This is where the biggest party happens. The whales are most numerous here, and this is where they spend the most time.

The "Back North" Trip (The Departure)

The return journey is completely different. It's less like a road trip with stops and more like a direct flight to save time and energy.

March: Instead of stopping at Nayarit or Jalisco on the way back, the whales seem to skip those middle stops entirely. They reappear directly in Baja California Sur.
The Metaphor: If the southbound trip was a bus making all its stops, the northbound trip is a high-speed express train that bypasses the intermediate stations to get back to the "feeding grounds" (their home base in colder waters) as quickly as possible.

Why This Matters

The researchers found that this pattern is incredibly consistent. Whether the ocean was warm, cold, or in between (due to weather events like El Niño), the whales stuck to the same schedule and route year after year.

The Good News: Because the whales are so predictable, we know exactly where to look for them. This helps protect them. We can tell boat captains, "Slow down here in December," or "Don't fish here in January," because we know the whales will be there.
The Bad News: Because they are so set in their ways, they might struggle if the ocean changes too fast due to climate change. If the "scenic route" they rely on suddenly has no food or becomes dangerous, they might not know how to adapt quickly enough.

The Big Picture

The paper also notes that these whales share the ocean with two other groups of humpback whales. It's like three different bus lines running on the same highway at the same time. By figuring out the specific schedule of this Central American group, scientists can finally tell which whale belongs to which group. This is crucial because this specific group is endangered and growing very slowly. Knowing exactly when and where they are helps ensure that conservation efforts are protecting the right whales at the right time.

In short, this study mapped out the "commute" of a specific group of whales, revealing that they take a scenic, stop-and-go route to their vacation spot, but a direct, no-stops route on the way home.

Technical Summary: Migratory Phenology of the Central American Humpback Whale Population Unit

Problem Statement
Understanding the fine-scale spatiotemporal dynamics of migratory corridors for wide-ranging marine megafauna is critical for effective conservation, yet such data remains incomplete for many populations. Specifically, the migratory phenology of the Central American population unit (PU) of humpback whales (Megaptera novaeangliae) has been poorly resolved. Previous studies were geographically restricted to specific localities or countries, failing to characterize the full breeding corridor from Baja California Sur (BCS) to Central America. Furthermore, the timing of whale passage through northern regions (BCS and Nayarit-Jalisco) and the nature of inter-regional movements (whether these areas serve as secondary breeding grounds or transit corridors) were unclear. This lack of resolution complicates conservation efforts, particularly because the Central American PU is listed as "Endangered" under the U.S. Endangered Species Act, has a slow recovery rate (1.6% annual growth), and overlaps spatially and temporally with two other North Pacific PUs (Coastal México and Offshore México) along the Mexican Pacific. Distinguishing population-specific dynamics is essential to avoid misattributing threats and to refine management strategies.

Methodology
This study provides the first population-specific analysis of migratory phenology across the entire breeding corridor of the Central American PU, spanning seven consecutive breeding seasons (2018–2024).

Data Sources: The analysis integrated 2,696 photo-identification (photo-ID) sightings of 672 unique individuals. Data were compiled from three core research catalogues (PRIMMA/UABCS, La Orca de Sayulita, and Proyecto Megaptera El Salvador) and supplemented by citizen science contributions via the Happywhale platform.
Study Area: The corridor was divided into three regions: Baja California Sur (BCS), Nayarit-Jalisco (NY-JA), and Southern México-Central America (SMex-CAm).
Individual Classification: Individuals were classified as belonging to the Central American PU if they were sighted within the breeding range (Guerrero to Panamá) and either lacked feeding ground data or were matched to feeding areas between California and Washington (U.S.), excluding those matched to feeding grounds between British Columbia and Russia.
Statistical Analysis:
- Spatiotemporal Distribution: A generalized linear model (GLM) with a quasibinomial error distribution modeled the monthly proportion of unique individuals per region. A Type II ANOVA tested for interactions between region, month, and breeding season.
- Inter-regional Movements: Recaptures (individuals sighted in multiple regions within a season) were analyzed to define directional movements (six possible directions). A GLM tested for differences in recapture proportions among movement directions and across breeding seasons.
- Temporal Dynamics: Intra-seasonal movements were examined across three discrete periods: early (up to Dec 31), mid (Jan 1–Feb 15), and late (Feb 16 onward).

Key Results

Asymmetric Migratory Pattern: The study revealed a highly consistent and asymmetric migratory structure between southbound and northbound movements.
- Southbound Migration: Whales appeared in BCS as early as October, peaking in December. They moved through NY-JA primarily in December–January and reached SMex-CAm, where presence peaked in January–February.
- Northbound Migration: The return migration showed significantly weaker connectivity between SMex-CAm and NY-JA. Most individuals reappeared directly in BCS in March, bypassing intermediate coastal regions.
Movement Transitions: Of 317 documented cross-regional recaptures, the most common southbound transition was NY-JA to SMex-CAm ( $n=121$ ), while the most common northbound transition was SMex-CAm to BCS ( $n=50$ ).
Temporal Stability: Statistical analysis confirmed no significant effect of breeding season on distribution or movement patterns ( $p > 0.95$ ), indicating that the migratory timing and routes remained stable across the seven years, despite interannual environmental variability (including El Niño and La Niña events).
Seasonal Shifts: Early and mid-season movements were predominantly southbound. Late-season recaptures were predominantly northbound, with a distinct shift in route preference away from the NY-JA corridor toward a more direct path to BCS.

Significance and Claims
The paper claims to provide the first broad-scale, population-specific characterization of the Central American PU's migratory phenology across its entire breeding range.

Conservation Utility: The persistence of this spatiotemporal pattern suggests a highly stable migratory corridor. While this stability facilitates temporally and spatially targeted conservation measures (e.g., seasonal vessel-speed regulations, whale-watching management), it may also confer vulnerability if climate-driven shifts in oceanographic conditions alter resource availability faster than the population can adapt its fixed migratory behavior.
Population Distinction: The results offer essential insights for distinguishing the Central American PU from overlapping North Pacific PUs. By defining specific arrival and departure windows for each region, the study enables more accurate threat attribution (e.g., distinguishing which population is at risk from specific fishing or shipping interactions in specific regions at specific times).
Ecological Insight: The observed directional asymmetry suggests that the PU utilizes different routes depending on travel direction: a coastal pathway for the southbound migration (potentially for opportunistic feeding or shelter) and a more direct, offshore route for the northbound return (prioritizing speed to reach feeding grounds after energy depletion).

The authors note limitations, including potential sampling biases inherent to photo-ID (favoring males over lactating females) and the uneven effort of citizen science data. However, the dataset represents approximately 45% of the estimated population size, supporting the reliability of the inferred patterns. The study concludes that while the migratory corridor is robust, its rigidity under changing climate conditions warrants careful management to ensure the long-term viability of this endangered population.

The "Down South" Trip (The Arrival)

The "Back North" Trip (The Departure)

Why This Matters

The Big Picture

Technical Summary: Migratory Phenology of the Central American Humpback Whale Population Unit

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