Marine megavertebrate migrations connect the global ocean

Domain: earth_science
Published: 2025-05-08
Editorial outcome: minor_revision

Abstract
Animal migrations are extensive, ubiquitous, and in decline. To effectively protect migratory species, it is often crucial to identify the interconnected sets of sites they rely upon. Gaps between primary ecological research and synthesised information that is useful to policymakers has limited effective conservation of long-distance migrants, particularly in the marine realm. By synthesising 1304 references to identify 1787 sites and develop model migratory networks for 109 species, we show the minimum extent of marine megafauna connectivity across the global oceans. Our analyses underscore the importance of transboundary cooperation for migratory species conservation at scales larger than current regional structures afford and provide a free online system that will enable policymakers to efficiently summarise how marine migrants use and connect their jurisdictions.

Introduction
From rock art depicting ungulate herds on the move, to Aristophanes suggesting one ‘buy some light clothing’ when the swallows return, humans have long observed the seasonal comings and goings of animals. Modern telemetry devices have now recorded humpback whales (Megaptera novaeangliae) swimming from the coast of Brazil to Antarctica1, leatherback turtles (Dermochelys coriacea) travelling 20,000 km from Indonesia to Oregon2, and Arctic Terns (Sterna paradisaea) undertaking the longest known migration: up to 80,000 km each year from pole to pole3. While these animals demonstrate the phenomenon at its most extreme, perhaps an even more remarkable element of migration is its ubiquity. From Monarch butterflies to blue whales, animals of all sizes, life history strategies, and taxonomic groups take part in migrations4. Unfortunately, the routes and resources critical for numerous migrations are under threat, and many populations of migratory species are in decline4–6. Indeed, nearly half of all marine migratory species are listed as threatened, near threatened, or data deficient7.

Effective conservation of these migratory species requires us to understand not only the scale of their movements, but also their transboundary nature7–10. Migratory movements connect life stages that occur in disparate areas of the planet. As animals move across vast areas, they encounter a diverse suite of threats governed by disjointed sets of policy instruments that impede coordinated protection9,11. Failures of existing mechanisms to comprehensively protect migrants are evident both in management outcomes (migratory fish stocks are overfished at twice the rate of those that remain in a single jurisdiction12) and conservation shortfalls (91% of migratory birds are inadequately protected across their lifecycle13). The unprecedented rate of declines in migratory populations5,7,14, and global attention on the goals of the Kunming-Montreal Global Biodiversity Framework (GBF), have resulted in urgent calls to understand the role of migratory species in ecological communities—before those communities are degraded beyond recognition. However, we can neither understand a species’ ecological roles nor assess any protections provided to a migratory population if we do not understand their migratory cycles.

Animal movement data repositories such as Movebank15, Birdlife International’s Seabird Tracking Database16, and regional or taxonomic data aggregation projects (e.g. the Retrospective Analysis of Antarctic Tracking Data (RAATD)17, Animal Telemetry Network18, ZoaTrack19, OBIS-SEAMAP20) have grown exponentially over the last two decades and are incredibly useful resources for researchers considering migratory connectivity21. While making tracking data available through such repositories is critical, the jump from raw data to usable knowledge still requires synthesis and communication12,22. There are also questions around the effectiveness of data archiving policies to truly ensure raw datasets are accessible21. To address these challenges, a new type of knowledge repository has been developed to fill the gap between raw data repositories and managers, planners, and policymakers. The development of Important Bird and Biodiversity Areas23, Important Marine Mammal Areas24 and Important Shark and Ray Areas25 all signal a shift to aggregating actionable knowledge that can more easily be directly consumed by management and policy processes. These knowledge repositories are complementary to, and frequently reliant upon, raw data repositories. While these taxonomic knowledge repositories have made steady headway in identifying critical areas for megafauna populations, the connectivity generated by movement between the locations has remained in the shadows.

In this work, we aim to summarise not only the known key use areas for a core set of marine migrants, but also the critical links between them. Here we present a platform hosting a freely accessible, cross-taxa, global dataset describing migratory species connectivity in the ocean (MiCO, accessible at: www.mico.eco), which contains networks for over 100 species, representing fishes, marine mammals, sea turtles and seabirds. We present a preliminary estimate of the previously unquantified (minimum) scale and extent of migratory links across the world’s oceans.

Results
This study builds upon a dataset of migration 'sites' and 'routes' identified in Kot et al.26. Defining ‘migration’ itself is an object of extensive discussion27. Here, we use it to mean movements of at least 100 km between key habitat areas, using the limit from the Global Registry of Marine Species28. This captures both seasonal migratory movements between breeding and non-breeding areas29, and within-season foraging movements for some central-place foragers.

Kot et al.26 reviewed literature on ecological connectivity generated by the movements of 173 marine species across 4 taxonomic groups, and we expanded the dataset following that publication. Of the species reviewed, 109 had relevant geospatial information that described connectivity (i.e. at minimum, two identifiable locations/sites linked by the movement of at least one individual), including 40 seabirds (938 sites), 34 marine fish (1198 sites), 28 marine mammals (21 of these being cetaceans) (1430 sites) and 7 sea turtles (1910 sites). After extracting information from these papers on site locations, routes (i.e. connections between sites; n = 5498), and behaviours (e.g. breeding, wintering, foraging, etc.), we synthesised these into summary metasites (n = 1787) and created interactive connectivity networks for each species (e.g. Fig. 1) which were analysed to assess geographic and taxonomic trends in connectivity. Similar numbers of metasites were generated across taxonomic groups (fishes, 521; marine mammals, 401; sea turtles, 513; seabirds, 352 (Fig. 2)). Note that the 'routes', and therefore metaroutes, were recorded using start and end locations of migratory movements, and do not describe a specific path travelled. We found migratory links exist between almost every region in the world (Fig. 3).

On average, countries were connected to 27.9 ± 20.8 (mean ± sd, median = 22) other EEZs or to ABNJ by migratory marine species. The most highly connected countries were the United States and France, with routes linking them to 115 and 114 other national waters, respectively. This is likely due to their high number of overseas territories and large investments in sampling effort, rather than any ecological driver. Ectotherms (fishes, sea turtles) tended to connect tropical regions to each other, with numerous routes recorded, particularly in the equatorial Americas. On the other hand, endotherms (seabirds and marine mammals) had more routes moving from the tropics to the poles, or even pole to pole (Fig. 3). Across all groups, there were extensive inter- and intra-regional movements, emphasizing the importance of both regional and global governance structures in migratory species conservation and management.

The system includes migratory information collected using over 16,000 animal-borne tags (conservative estimates: 6658 deployed on fishes, 2468 on marine mammals, 3367 on sea turtles and 3521 on seabirds). Most animals measured are of unknown sex for all taxa except sea turtles, for which females are most commonly recorded (Fig. 4A). Sea turtles also, along with seabirds, have more data collected from breeding adults than any other life stage, while fish and marine mammals have mostly migratory (or observation) information from animals of unknown life stages (Fig. 4B).

Methods
Data
The MiCO system (https://mico.eco/system) is an online tool that collects, displays, and summarises information about marine migrations to support policymakers. It was developed to describe regional to global-scale connectivity generated by migratory marine megavertebrates, with an initial focus on species utilising areas beyond national jurisdiction. Tutorials on the capabilities of the online system are available at mico.eco/system-help.

The list of species to be evaluated in this study was selected from the initial high-seas species considered by MiCO12 and focused on those listed in CMS appendices or managed by an RFMO, excluding sturgeon, sea ducks and taxonomic families with limited representation. While our approach maximises the utility of the system for specific policy arenas, we acknowledge that species listed under CMS or managed by RFMOs represent a fraction of all migratory marine species. As such, and despite the breadth of work undertaken here, connectivity for more than two-thirds of migratory marine megafauna remains unassessed.

For this study, a total of 5476 sites were identified from the literature across all taxonomic groups: 1910 sea turtle sites, 1430 marine mammal sites, 1198 fish sites, and 938 bird sites. Sites were identified based on the authors’ description of either processed data (26.7% of studies) or tracks (73.3% of studies), and were usually associated with a specific behaviour. If no behaviour was noted in the study, the site was listed as an 'observation'. Reviewers collecting data from the literature identified the centroid of the activity described. While identification of the exact centroid for a given activity was an inherently subjective process, training of reviewers to ensure standardisation, as well as quality control of datasheets by the review manager, increased reproducibility and confidence in the results. A buffer was applied to each centroid to describe the area used. To focus the system on provision of regional to global scale information on connectivity, we used 1° as the smallest buffer for sites, and applied a 5° or 10° buffer to larger sites to allow for differentiation among the size of areas identified in different studies and to increase the efficiency of reviewers assessing the literature. Using such a coarser resolution for sites also decreases the sensitivity of the sites to potential errors in deciding on the exact location of the centroid. These sites made up the base dataset used in this study. The data collected for those sites were subjected to quality control processes to ensure all identified literature was included, reviewer responses were standardised, and georeferencing of sites conformed with site names and species distributions (Supplementary Fig. 1).

Site aggregation
To address duplication of sites identified in different studies of the same species, we developed a site aggregation process (Supplementary Fig. 2). Aggregation of sites into 'metasites' had three objectives: 1) aggregate information across multiple sites that identify a species’ activities in the same location; 2) maintain information on large-scale (>500km) connections generated by animal movements; 3) maintain the resolution of information on critical reproductive life history stages; 4) provide a minimum level of known animals sampled within a site and moving between sites. To address these objectives, we developed a five-step process for site aggregation that was run on the sites and routes for each species in the MiCO System.

Data prep. The site aggregation process iteratively selected a site and identified sites that should be aggregated with it based on a series of rules. Because the process iteratively selected sites to examine, the groupings of sites that were aggregated together were dependent on the order of the sites in the dataset. We use this inherent prioritisation to maintain information at a high resolution for reproductive sites. All reproductive sites were examined first and were ordered from smallest (1°) to largest (10°). All remaining sites were evaluated after the reproductive sites based on size, with the largest sites (10°) considered first. This approach for non-reproductive sites was chosen to limit overlap between non-reproductive sites and generate the most parsimonious network.

Site selection. We iteratively selected the next site (the 'Lookup Site') within the sorted set of sites to evaluate which sites should be aggregated with it. All sites in the sorted site dataset were evaluated to determine if their centroid fell within the radius (buffer) of the lookup site. Those sites that met this criterion ('Nearby Sites') were grouped for further consideration (Supplementary Fig. 3).

The sites identified in Kot et al.26 have very coarse resolutions (1–10° radii). When aggregating sites together with 10° sites, it is possible that sites that were >1000 km away could be selected for inclusion. To ensure that large-scale connectivity between such sites was not lost in the aggregation process, routes and connections were projected to a metre-level Azimuthal Equidistant projection (centred at 0°N, 0°E) and any routes or connections >500 km long were not included in the aggregation process for a given Lookup Site. That is to say, any Nearby Site that included a route or connection to a Lookup Site >500 km away from it was removed from consideration for grouping ('Removed Site') with the Lookup Site. To account for the removal of the site, any sites that were: a) greater than 1° from the Lookup Site centroid, and b) within the buffer of the Removed Site, and c) closer to the Removed Site centroid than the Lookup Site centroid were also removed so that they could later be aggregated with the Removed Site (see Supplementary Figs. 2 and 3 for the decision flowchart).

Metasite centroid definition. To develop a metasite from the Lookup Site and any Nearby Sites that were not removed during site selection, an iterative subprocess was implemented. In this subprocess, the new metasite centroid was calculated as the average latitude and longitude from those remaining sites, and the buffer from the Lookup Site was used as the metasite buffer. Any sites with centroids that did not intersect the metasite buffer were removed, and a new metasite centroid was calculated repeatedly until all sites intersected the metasite buffer. Any sites included in the new metasite were removed from consideration for aggregation into subsequent metasites.

Data duplication check. Information on the sampling technology, temporal scope of site use, and species behaviour within the site was collated from individual sites included within the new metasite. Information on number of individuals, sex, and life history stage was summed. Given that many tracking studies either reuse or build on existing datasets, we implemented a three-step process to identify potentially duplicate data and ensure that our sample size estimates were a minimum estimate of known connectivity in a site, or travelling between sites (for routes and connections).

The first step was to identify duplicate sites. All references used in the Kot et al.26 literature review included a unique reference ID (Zotero ID). If sites had the same Zotero ID and identical latitudes and longitudes, they were considered duplicate sites. In this situation, only the site with the greatest number of individuals was included when summing any count of individuals.

Sites that had a route between them and were aggregated together in a metasite were considered the same site, so as not to double count animals moving within a metasite. Again, only the site with the greatest number of individuals was included when summing any count of individuals.

Finally, if sites included in the same metasite met both of the following conditions, they were assumed to have used the same data and only one site was included:
a. The sites came from references with at least one author with the same surname, and
b. The temporal scope (i.e. months and years) identified for one site was a subset of the other site Outside MiCO Aggregation, https://doi.org/10.48610/aa77f3c.
As above, if these conditions were met, only the site with the greatest number of individuals was included when summing any count of individuals.

Metasite aggregation. Following the data duplication check, the new metasite was added to a database of metasites for the species under consideration. Site identification codes (IDs) used to define connections were then replaced with the corresponding metasite IDs, and all connections that joined the same metasites were aggregated into metaconnections. If metaconnections subsequently connected a metasite to itself, they were removed. If site information was not included when summing numbers of individuals in a metasite (due to data duplication), the individuals on associated routes/connections were not included when calculating the same statistics. Final model outputs (points and polyline feature layers within file geodatabases) were then generated from the metasite information and made available via the MiCO System. All products in the MiCO System are freely distributed under the CC-BY copyright.

Network models. Network models were generated in R using the igraph63 package version 2.1.1, using metasites as nodes and the metaroutes as edges. The radius of a given metasite was used as the radius for the node, and the weight of the edges was based on the number of individuals associated with the metaroute. Nodes were coloured based on the activities occurring in the metasite.

Additional methods
Two summary exports were made from the file geodatabases: one, a subset of all metaroutes which had source and destination metasites in different EEZs; and two, a list of all metasites and their associated metadata. All subsequent analyses were undertaken in R version 4.3.064, using the packages tidyverse65 version 2.0.0, circlize66 version 0.4.16, jsonlite67 version 1.8.8, and ggExtra68 version 0.10.1. Metasites were allocated to a region of the UN geoscheme based on the latitude and longitude of the metasite centroid. These were geographical locations rather than political, for example, the British Virgin Islands was allocated to the Caribbean rather than to Northern Europe. Twenty-three individual breeding stages (specific to species groups) were summarised into four key life stage categories: ‘Breeding Adult’, ‘Non-breeding Adult’, ‘Juvenile’, and ‘Unknown’. Adjacency matrices were constructed from the data on metaroutes that crossed EEZs, and were used to generate chord diagrams.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The work by Bentley et al. is an excellent step forward in the use of animal movement data to understand how migratory species connect sites across the globe. The work that the authors present is an outstanding task that sets up the scene nicely to better understand how marine ecosystems are inter-connected through large marine vertebrates movement, as well as to better design management strategies across countries and their jurisdictions.\nThe dataset and the web interface available at MiCO have a huge potential for large-scale efforts and collaborations focused on the interconnectivity of regions of the global ocean.\nI commend the authors for their effort and for producing a well-written and robust manuscript. I have added a few comments and questions that I had while reading the paper (see attached PDF), but in general, I think this manuscript should be accepted for publication.\nLooking forward to seeing this published", "reviewer_2": "General comments:\nThe manuscript presents a platform to synthesise information on marine migratory connectivity and inform marine conservation, and reveals some of the first results of the data compilation. It is well written and structured, and the platform if very timely and of high value to help filling the gap between marine science and conservation policy. I have however a few main comments, mostly related to the clarification of some methodological steps and the interpretation of the results.\nRegarding the methods, some key points are presented very vaguely, or not mentioned at all; the methods section is focused on some detailed procedures that could go to supplementary material, while some main points are not explained. First, it should be clarified that the study is describing geographical links, but not quantifying the strength of such links (sensu Webster et al. 2002; TREE; https://doi.org/10.1016/S0169-5347(01)02380-1). The final sentence of the abstract can be misleading, as this study does not quantify the connectivity per se; it provides a first estimate of the scale and scope of the geographical links. I’d suggest changing to “We present a first estimate of the previously unquantified (minimum) scale and extent of migratory connectivity across the world’s oceans.”\nAnother major point is the lack of definition of what a “site” means; it should be clarified how were the “important sites” identified. In Kot et al, a site is defined as “start and end points for routes”, but that is a very vague definition (it would exclude, e.g. the stopover sites used by many species during their migratory journeys). In many cases, the start and end points can be simply the tagging location and the location of the last position due to battery failure, rather sites than are more intensively used by the animals; in such cases, the results would be driven by sampling procedures, rather than the ecology of the species. This can be particularly problematic if the main aim is to inform conservation management. The authors should provide more details in the methods section (or in supplementary material) about the identification of these sites, and show that the sites that were included in the analyses are, in fact, a sample of the important sites for the species – e.g. spawning/ breeding sites, foraging sites, stopover sites (even if currently incomplete and biased towards some life history stages).\nRegarding the results, I think that it should be stated more clearly that what is being presented here is the platform and its (very high) potential to inform marine conservation, but that currently the results are extremely biased towards well-studied regions – and, thus, they should be taken very carefully when considering marine policies; their current value is to inform priorities for future studies, but not necessarily yet priorities for the conservation of most species. On this last point, it would be interesting to estimate the comprehensiveness of the dataset per species/regions.\nMinor comments:\nLines 89-90: If the study includes also within-season movements, that means that it is not only about migratory connectivity, but movement connectivity more broadly. Some central place foragers can travel hundreds of kilometres when commuting between breeding and foraging sites (often larger distances than some migrations); why not removing these movements from the dataset? Or, at least, an estimate of the proportion of within-season connections in the entire dataset should be provided;\nLines 98: It is not clear how the “network models” were built (that is not mentioned in the methods sections); at least a basic explanation should be given, as this is important to understand the tool and the results;\nLines 102-109: This paragraph is repeating information that was provided in the previous one;\nLines 111-116: See my main comment regarding the interpretation of the results; the high number of connections in USA and France are very likely linked to the sampling bias; to be able to interpret the results obtained, it would be needed a sensitivity analysis to estimate how representative is the dataset; reference to Fig 1 in line 116 should be removed, as fig 1 only shows results of ectotherms;\nFigure 2: These figures are a great representation of the results, but it would be important to provide an estimate of the representativeness of the datasets of each group (e.g. using IUCN range maps); note that a label is missing in the figure of the turtle (“Central America”?);\nLine 140 (and also Line 261): List of species – CMS list is not comprehensive regarding marine migrants; if the analyses only include CMS listed species, it should be clarified that the migratory links of many marine species were not considered here;\nLines 140-151: See main comment regarding the interpretation of the results; without providing an estimate of the comprehensiveness of the dataset, it is premature to take any conclusions linked to the ecology of the species, or the threats affecting them; if the global distribution of sea turtles is restricted to tropical and temperate areas, it would be relevant to look to gaps within their range (and there’s no point in comparing it with the seabirds, as we already know that have a much larger distribution); regarding the seabirds, note that temperate/polar species tend to be more migratory than tropical ones (again, it is fundamental to know the representativeness of the dataset to draw any conclusion); reference to the Great Barrier Reef: are most of the seabird species breeding there migratory?;\nLine 256: The MICO system (…) summarizes INFORMATION ABOUT marine migration (does not “summarizes marine migration”); in the following sentence, it should be “connectivity generated by…”;\nLines 265-270: See main comments regarding the definition of a site; it should be clarified here how these sites were identified at the first place form the literature review, as defining the geographic location/boundaries of sites is not a trivial task. The definition of sites can have a major impact on the results (and consequently on all the conclusions);\nLines 292-315: I found this explanation a bit confusing; what does “the largest sites” mean (lines 287, 288)? What’s the size of the buffer used to define metasites (line 294)? Were the data projected to a metric system? (distance metrics are given sometimes in km, others in degrees; it should be standardized or explained more clearly in the procedures);\nLine 221: Fisheries are also a primary source of mortality for the other groups;\nLine 216: The tool was not built to identify stakeholders, so I don’t think that is possible to “quickly identify” stakeholders with it; it can eventually facilitate the search;"}, "author_response": "Response to Reviewers: NCOMMS-24-30449A, Bentley et al.\nWe thank the two anonymous reviewers for their attention to our manuscript, and below\noutline the changes we have made in response to their helpful comments.\nReviewer comments are in plain text, while author responses are noted in italics.\nReviewer #1 (Remarks to the Author):\nThe work by Bentley et al. is an excellent step forward in the use of animal movement data\nto understand how migratory species connect sites across the globe. The work that the\nauthors present is an outstanding task that sets up the scene nicely to better understand\nhow marine ecosystems are inter-connected through large marine vertebrates movement, as\nwell as to better design management strategies across countries and their jurisdictions.\nThe dataset and the web interface available at MiCO have a huge potential for large-scale\nefforts and collaborations focused on the interconnectivity of regions of the global ocean.\nI commend the authors for their effort and for producing a well-written and robust\nmanuscript. I have added a few comments and questions that I had while reading the paper\n(see attached PDF), but in general, I think this manuscript should be accepted for\npublication.\nLooking forward to seeing this published\nResponse: We are glad the reviewer found the manuscript so useful, and we are grateful for\ntheir thoughtful comments.\nMinor comments extracted from PDF:\nL 70: This transition is rather abrupt, as it is missing a statement of what the overarching\ngoal of the study is. Whilst is implicit in the preceding text, I think it would be worth explicitly\nstating it,\nResponse: Thank you for pointing this out. To smooth the transition between the two\nexisting sentences (here for context) we have included additional text (here in bold) on new\nL 70: “While these taxonomic knowledge repositories have made steady headway in\nidentifying critical areas for megafauna populations, the connectivity generated by movement\nbetween the locations has remained in the shadows. We aim to summarise not only the\nknown key use areas for a core set of marine migrants, but also the critical links\nbetween them. Here we present the first freely accessible, cross-taxa, global dataset\ndescribing migratory connectivity in the ocean (MiCO, accessible at: www.mico.eco),...”\nFig 1: Whilst this is a great illustration for the MiCO interphase, the figure is a bit too crowded\nand could be a bit confusing. Are there ways to make it a bit cleaner/easier-to-read?\nResponse: We have simplified the figure to show one species only, rather than two species.\nWe agree the image is a bit busy and do therefore direct the reader to the dynamic versions\navailable online in the figure 1 caption: “View the dynamic versions of this and over 100\nother network models at mico.eco/system/mapper.”\nL 143: I wonder if there's also a bias associated to the different tagging/research effort\nbetween sites in the global north (and their territories) and the global south that might be\nworth to explore and it'd bound to help understand the under-representation in tropical areas\nResponse: We agree with the reviewer, and include text on new L 160 which discusses this\npoint: “The existence of biases in biological datasets is well-known27,35, and in our system\nthese are observed in both geography and taxonomy. Specifically, geographic biases\nmimicking the wealth and distribution of university researchers was evident (Fig. 3). Data on\nmigratory connectivity has been collected from locations (mostly the USA, UK, and Europe)\nthat broadly reflect the distribution of ecological research published worldwide36.”\nFig 4: I find it hard to visualize a reason why tagged marine mammals are so biased to\nunknown, unless the majority of the data included comes from cetaceans (as in the case of\npinnipeds this information is easily accesible). Or could it be that the information has been\ncollected but has not been made available by the researchers?\nResponse: The reviewer is entirely correct that the observed bias towards animals of\nunknown sex in the “marine mammal” section because almost all the marine mammal data\nare from cetaceans, we have added text on L 96-97 noting the study contains, “28 marine\nmammals (21 of these cetaceans)”. The full list of species is viewable on mico.eco/explore.\nL 267: Is there a reason for this? Is this due to data resolution? Animal behaviour?\nResponse: We have included the following explanation for sizes of radii on new L 274-279:\n“A buffer was applied to each centroid to describe the area used. To focus the system on\nprovision of regional to global scale information on connectivity, we used 1° as the smallest\nbuffer for sites, and applied a 5° or 10° buffer to larger sites us to allow for differentiation\namong the size of areas identified in different studies and to increase the efficiency of\nreviewers assessing the literature. Using such a coarse resolution for sites also decreased\nthe sensitivity of the sites to potential errors in deciding on the exact location of the centroid.”\nL 277: Can species richness be considered in the identification of metasites?\nResponse: As metasites are calculated only within individual species, in this work we do not\nconsider species richness when identifying metasites. However, we agree with the reviewer\nthat this is an interesting angle and have a PhD candidate within our group currently\nconsidering how best to synthesise multi-species sites, potentially including species richness\nas a metric.\nReviewer #2 (Remarks to the Author):\nGeneral comments:\nThe manuscript presents a platform to synthesise information on marine migratory\nconnectivity and inform marine conservation, and reveals some of the first results of the data\ncompilation. It is well written and structured, and the platform if very timely and of high value\nto help filling the gap between marine science and conservation policy. I have however a few\nmain comments, mostly related to the clarification of some methodological steps and the\ninterpretation of the results.\nResponse: We’re gratified to hear that the reviewer found value in the study and the\nplatform, and we thank them for their constructive comments.\nRegarding the methods, some key points are presented very vaguely, or not mentioned at\nall; the methods section is focused on some detailed procedures that could go to\nsupplementary material, while some main points are not explained.\nResponse: As recommended by the editor we have retained a reasonable amount of detail\nwithin the manuscript text, as there is no limit to word count for methods. In addition, we\nhave endeavoured to clarify the specific methodological points as noted below in the\nreviewer’s comments. We hope that this improves the clarity of the work.\nFirst, it should be clarified that the study is describing geographical links, but not quantifying\nthe strength of such links (sensu Webster et al. 2002; TREE; https://doi.org/10.1016/S0169-\n5347(01)02380-1). The final sentence of the abstract can be misleading, as this study does\nnot quantify the connectivity per se; it provides a first estimate of the scale and scope of the\ngeographical links. I’d suggest changing to “We present a first estimate of the previously\nunquantified (minimum) scale and extent of migratory connectivity across the world’s\noceans.”\nResponse: We agree with the reviewer’s point that we do not quantify the strength of\nconnectivity in the manner of Webster et al. We have changed the final sentence of the\nIntroduction (new L 74-75) (as we believe this is the sentence to which the above comment\nis addressed) to read: “We present a first estimate of the previously unquantified (minimum)\nscale and extent of migratory links across the world’s oceans.\nAnother major point is the lack of definition of what a “site” means; it should be clarified how\nwere the “important sites” identified. In Kot et al, a site is defined as “start and end points for\nroutes”, but that is a very vague definition (it would exclude, e.g. the stopover sites used by\nmany species during their migratory journeys). In many cases, the start and end points can\nbe simply the tagging location and the location of the last position due to battery failure,\nrather sites than are more intensively used by the animals; in such cases, the results would\nbe driven by sampling procedures, rather than the ecology of the species. This can be\nparticularly problematic if the main aim is to inform conservation management. The authors\nshould provide more details in the methods section (or in supplementary material) about the\nidentification of these sites, and show that the sites that were included in the analyses are,\nin fact, a sample of the important sites for the species – e.g. spawning/ breeding sites,\nforaging sites, stopover sites (even if currently incomplete and biased towards some life history\nstages).\nResponse: We agree that more clarity was needed in the description of how sites were\ndefined. We have added the following text to new L 267-279: “Sites were identified based\non the authors description of either processed data (26.7% of studies) or tracks (73.3% of\nstudies), and were usually associated with a specific behaviour. If no behaviour was noted in\nthe study, the site was listed as an “observation”. Reviewers collecting data from the\nliterature identified the centroid of the activity described. While identification of the exact\ncentroid for a given activity was an inherently subjective process, training of reviewers to\nensure standardisation a, as well as quality control of datasheets by the review manager,\nincrease reproducibility and confidence in the results. A buffer was applied to each centroid\nto describe the area used. To focus the system on provision of regional to global scale\ninformation on connectivity, we used 1° as the smallest buffer for sites, and applied a 5° or\n10° buffer to larger sites us to allow for differentiation among the size of areas identified in\ndifferent studies and to increase the efficiency of reviewers assessing the literature. Using\nsuch a coarse resolution for sites also decreased the sensitivity of the sites to potential\nerrors in deciding on the exact location of the centroid.”\nRegarding the results, I think that it should be stated more clearly that what is being\npresented here is the platform and its (very high) potential to inform marine conservation, but\nthat currently the results are extremely biased towards well-studied regions – and, thus, they\nshould be taken very carefully when considering marine policies; their current value is to\ninform priorities for future studies, but not necessarily yet priorities for the conservation of\nmost species. On this last point, it would be interesting to estimate the comprehensiveness\nof the dataset per species/regions.\nResponse: We thank the reviewer for this comment and have text on L 71-72 to emphasise\nthat this is a presentation of the platform itself: “Here we present a platform hosting the first\nfreely accessible, cross-taxa, global dataset describing migratory species connectivity in the\nocean”. We agree that estimating the comprehensiveness of the data per species would be\nan interesting study – we have PhD candidates who are taxa experts engaging with this\nquestion in e.g. marine turtles, but an assessment of the representativity of the entire dataset\nis outside of the scope of this work, which as the reviewer notes presented the system itself.\nDue to the challengers in producing comparable estimates that account for methodological\ndifferences in undertaking this assessment between e.g. colonial breeders (seabirds,\npinnipeds) vs. cetaceans, this will be undertaken as part of follow-on projects. The platform\ndoes provide an indication of the range coverage of connectivity data, by allowing users to\nvisualise migratory links alongside the best available underlying range maps for the species,\nand we have added text to indicate this on L 155-156: “The system includes the functionality\nto visualise migratory links overlaid upon the best available range maps for each species, as\ncoarse indicator of these potential gaps.”\nMinor comments:\nLines 89-90: If the study includes also within-season movements, that means that it is not\nonly about migratory connectivity, but movement connectivity more broadly. Some central\nplace foragers can travel hundreds of kilometres when commuting between breeding and\nforaging sites (often larger distances than some migrations); why not removing these\nmovements from the dataset? Or, at least, an estimate of the proportion of within-season\nconnections in the entire dataset should be provided;\nResponse: We agree that the study expands the strictest definition of migratory connectivity\n(sensu Webster et al.) and considers connectivity caused by multiple kinds of large-scale\nmovements. This text on new L 88 which we hope more clearly articulates the scope of the\nstudy: “Defining ‘migration’ itself is an object of extensive discussion28. Here, we use it to\nmean movements of at least 100 km between key habitat areas, using the limit from the\nGlobal Registry of Marine Species29. This captures both seasonal migratory movements\nbetween breeding and non- breeding areas30, and within-season foraging movements for\nsome central-place foragers.” We chose not to remove movements that are within-season,\nbecause they still represent key links between important areas for highly-mobile species. We\nagree with the reviewer that the foraging movements of many central-place foragers exceed\nthe distances travelled for other species on their seasonal migrations. For the purposes of\nthis study, we have included all movements, as they represent critical connections between\ndisparate areas.\nLines 98: It is not clear how the “network models” were built (that is not mentioned in the\nmethods sections); at least a basic explanation should be given, as this is important to\nunderstand the tool and the results;\nResponse: We agree that a clearer explanation should have been provided. We have added\nthe following text to the Methods section in new L 371-375: “Network models: Network\nmodels were generated in R using the igraph package, using metasites as nodes and the\nmetaroutes as edges. The radius of a given metasite was used as the radius for the node,\nand the weight of the edges was based on the number of individuals associated with the\nmetaroute. Nodes were coloured based on the activities/behaviours occurring in the\nmetasite.”\nLines 102-109: This paragraph is repeating information that was provided in the previous\none;\nResponse: We have adjusted the text to avoid this repetition, and incorporated these lines\ninto the paragraph above (beginning L 92) which now reads: “Kot et al. 2023 reviewed\nliterature on ecological connectivity generated by the movements of 173 marine species\nacross 4 taxonomic groups, and we expanded the dataset following that publication. Of the\nspecies reviewed, 109 had relevant geospatial information that described connectivity (i.e.,\nat minimum, two identifiable locations/sites linked by the movement of at least one\nindividual), including 40 seabirds (938 sites), 34 marine fish (1,198 sites), 28 marine\nmammals (21 of these being cetaceans) (1,430 sites) and 7 sea turtles (1,910 sites). After\nextracting information from these papers on site locations, routes (i.e., connections between\nsites; n = 5,498), and behaviours (e.g. breeding, wintering, foraging etc.), we synthesized\nthese into summary metasites (n = 1,787) and created interactive connectivity networks for\neach species (e.g. Fig. 1) which were analysed to assess geographic and taxonomic trends\nin\nin connectivity. Similar numbers of metasites were generated across taxonomic groups\n(fishes, 521; marine mammals, 401; sea turtles, 513; seabirds, 352 (Fig. 3)). Note that the\n“routes”, and therefore metaroutes, were recorded using start and end locations of migratory\nmovements, and do not describe a specific path travelled. We found migratory links exist\nbetween almost every region in the world (Fig. 2).”\nLines 111-116: See my main comment regarding the interpretation of the results; the high\nnumber of connections in USA and France are very likely linked to the sampling bias; to be\nable to interpret the results obtained, it would be needed a sensitivity analysis to estimate\nhow representative is the dataset; reference to Fig 1 in line 116 should be removed, as fig 1\nonly shows results of ectotherms;\nResponse: We completely agree and have added the following text to emphasise this is\nlikely driven by sampling on L 109-110: “This is likely due to their high number of overseas\nterritories and large investments in sampling effort, rather than any ecological driver.” We\nhave also removed the erroneous reference to Fig. 1.\nFigure 2: These figures are a great representation of the results, but it would be important to\nprovide an estimate of the representativeness of the datasets of each group (e.g. using\nIUCN range maps); note that a label is missing in the figure of the turtle (“Central America”?);\nResponse: We completely agree that IUCN range maps are a good estimate for\nrepresentativeness: the system has the capacity to visualise these along with the migratory\nlinks, new text on L 155 indicates this: “The system includes the functionality to visualise\nmigratory links overlaid upon the best available range maps for each species, as coarse\nindicator of these potential gaps.” We have also added this range maps to the visualisation\nof new Figure 1 to illustrate the point and note in the figure caption that “The crimson hash\nindicates the IUCN range map for this species, highlighting range areas which lack\nconnectivity information.” A single quantitative measure of representativeness is challenging\nacross all taxonomic groups; however this is part of our next steps for work using this\nsystem.\nLine 140 (and also Line 261): List of species – CMS list is not comprehensive regarding\nmarine migrants; if the analyses only include CMS listed species, it should be clarified that\nthe migratory links of many marine species were not considered here;\nResponse: We agree that CMS-listed and RFMO-managed species are a limited number of\nthe total number of marine species that may be considered to migrate or undertake\nextensive movements. We have added text at the end of the Introduction (L 70) to affirm this\nstudy is for “a core set of marine migrants”, and also have added a sentence and amended\nanother in the Methods (new L 261-265) to more fully explain this: “While our approach\nmaximises the utility of the system for specific policy arenas, we acknowledge that species\nlisted under CMS or managed by RFMOs represent a fraction of all migratory marine\nspecies. As such, and despite the breadth of work undertaken here, connectivity for more\nthan two thirds of migratory marine megafauna remains unassessed.”\nLines 140-151: See main comment regarding the interpretation of the results; without\nproviding an estimate of the comprehensiveness of the dataset, it is premature to take any\nconclusions linked to the ecology of the species, or the threats affecting them; if the global\ndistribution of sea turtles is restricted to tropical and temperate areas, it would be relevant to\nlook to gaps within their range (and there’s no point in comparing it with the seabirds, as we\nalready know that have a much larger distribution); regarding the seabirds, note that\ntemperate/polar species tend to be more migratory than tropical ones (again, it is\nfundamental to know the representativeness of the dataset to draw any conclusion);\nreference to the Great Barrier Reef: are most of the seabird species breeding there\nmigratory?;\nResponse: We agree with the reviewer that because this dataset is unlikely to be\nrepresentative, ecological conclusions should not be drawn from it. In this section we aimed\nto illustrate ways in which our data align (or do not) with well-established understanding of\nspecies’ ecologies, propose potential reasons for this, and highlight where known gaps exist.\nWe have rewritten the section to make this clearer on L 139-142: “For example, the clumped\ndistribution of sea turtle research near the equator is expected, given the largely tropical\ndistribution of this taxon31. On the other hand, our dataset displays a clear bias in seabird\ndata collected in the Southern Ocean, despite seabirds being widely distributed, including in\nthe tropics.” The seabirds of the Great Barrier Reef are extremely understudied, though they\ndo arrive and depart key breeding colonies for distant sites throughout the year they do not\nmigrate north-south in the typical sense – to avoid confusion we have remove the reference\nto this system.\nLine 256: The MICO system (…) summarizes INFORMATION ABOUT marine migration\n(does not “summarizes marine migration”); in the following sentence, it should be\n“connectivity generated by…”;\nResponse: Yes, we agree completely and have rephrased the text on L 254-255 to read as\nsuggested: “The MiCO system (https://mico.eco/system) is an online tool that collects,\ndisplays, and summarizes information about marine migrations to support policymakers.” We\nhave also fixed the typo in the following line, from “generate” to “generated” as noted.\nThanks!\nLines 265-270: See main comments regarding the definition of a site; it should be clarified\nhere how these sites were identified at the first place form the literature review, as defining\nthe geographic location/boundaries of sites is not a trivial task. The definition of sites can\nhave a major impact on the results (and consequently on all the conclusions);\nResponse: As above, we agree that more clarity was needed in the description of how sites\nwere defined. We have added more information in the Methods (new L 267-279): “Sites were\nidentified based on the authors’ description of either processed data (26.7% of studies) or\ntracks (73.3% of studies), and were usually associated with a specific behaviour. If no\nbehaviour was noted in the study, the site was listed as an “observation”. Reviewers\ncollecting data from the literature identified the centroid of the activity described. While\nidentification of the exact centroid for a given activity was an inherently subjective process,\ntraining of reviewers to ensure standardisation, as well as quality control of datasheets by\nthe review manager, increased reproducibility, and confidence in the results. A buffer was\napplied to each centroid to describe the area used. To focus the system on provision of\nregional to global scale information on connectivity, we used 1° as the smallest buffer for\nsites, and applied a 5° or 10° buffer to larger sites us to allow for differentiation among the\nsize of areas identified in different studies and to increase the efficiency of reviewers\nassessing the literature. Using such a coarse resolution for sites also decreased the\nsensitivity of the sites to potential errors in deciding on the exact location of the centroid.”\nLines 292-315: I found this explanation a bit confusing; what does “the largest sites” mean\n(lines 287, 288)? What’s the size of the buffer used to define metasites (line 294)? Were the\ndata projected to a metric system? (distance metrics are given sometimes in km, others in\ndegrees; it should be standardized or explained more clearly in the procedures);\nResponse: We apologise for the confusing description. It is fundamentally difficult to\ndescribe an iterative process with words and they are usually more easily explained through\nprocess diagrams (e.g., Fig S3). We have amended and added to the text on L 311-321 in\nhopes of providing more clarity. It now reads: “When aggregating sites together with 10°\nsites, it is possible that sites that were >1000 km away could be selected for inclusion. To\nensure that large-scale connectivity between such sites was not lost in the aggregation\nprocess, routes and connections were projected to a meter-level Azimuthal Equidistant\nprojection (centred at 0° N, 0° E) and any routes or connections >500km long were not\nincluded in the aggregation process for a given Lookup Site. That is to say, any Nearby Site\nthat included a route or connection to a Lookup Site >500km away from it was removed from\nconsideration for grouping (“Removed Site”) with the Lookup Site. To account for the\nremoval of the site, any sites that were: a) greater than 1° from the Lookup Site centroid,\nand\nb) within the buffer of the Removed Site, and c) closer to the Removed Site centroid than the\nLookup Site centroid were also removed so that they could later be aggregated with the\nRemoved Site (see Fig S3 for the decision flowchart).” We have also added specific values\nto any mention of large or small sites.\nLine 221: Fisheries are also a primary source of mortality for the other groups;\nResponse: We agree with the reviewer’s comment, and have rewritten the sentence as\nbelow, with additional citations: “(a primary source of mortality for turtles50, sharks51,\nseabirds52, and marine mammals53)” (New L 210)\nLine 216: The tool was not built to identify stakeholders, so I don’t think that is possible to\n“quickly identify” stakeholders with it; it can eventually facilitate the search;\nResponse: We have rewritten this sentence to clarify the system outputs can facilitate\ngovernment engagement with e.g. appropriate bodies governing potential stressors on new\nL 213-214, “Our networks can support interested parties to better understand with which\nRegional Fisheries Management Organisations (RFMOs) species from their jurisdiction are\ninteracting, thus opening the door for engagement from non-fishing nations. ”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Dear authors,\nI have read the new version of the manuscript and the responses to the first round of comments and I am happy with the responses and changes of this newer version. I think this manuscript should be accepted for publication\n\nWhile I didn,t run the examples, I checked the GitHub site and check that the code and data were available", "reviewer_2": "The manuscript is now greatly improved, and I do not have any major comment to make. I suggest a few minor changes (line numbers correspond to the version without tracked changes):\nLine 35: add the word “about”, or “up to”, before “80,000 km”, as the migratory distance of the Arctic terns is not exactly 80,000 km\nLine 56: reference number 17, you may want to cite instead the recently published article describing the Seabird Tracking database (Carneiro et al. 2024: https://doi.org/10.1016/j.biocon.2024.110813)\nLines 106-107; I suggest changing “other EEZs (including ABNJ)” to “other EEZs or to ABNJ”.\nLines 288 and 315 and 316: Add a space between “500” and “km”\nFigure 2: “Central America” label is still missing from the figure of the sea turtles"}, "author_response": "NCOMMS-24-30449A Response to Reviewers. Author responses noted in italics.\nReviewer #1 (Remarks to the Author):\nDear authors,\nI have read the new version of the manuscript and the responses to the first round of\ncomments and I am happy with the responses and changes of this newer version. I\nthink this manuscript should be accepted for publication\nWe thank the reviewer for their thoughtful participation throughout the review\nprocess and for feedback which improved the manuscript.\nReviewer #1 (Remarks on code availability):\nWhile I didn,t run the examples, I checked the GitHub site and check that the code\nand data were available\nCode and data are indeed available and additional information has been added as\nrequested by the Author checklist.\nReviewer #2 (Remarks to the Author):\nThe manuscript is now greatly improved, and I do not have any major comment to\nmake. I suggest a few minor changes (line numbers correspond to the version\nwithout tracked changes):\nWe are grateful for the reviewer’s time and attention to detail as well as their\ncomments which improved the manuscript.\nLine 35: add the word “about”, or “up to”, before “80,000 km”, as the migratory\ndistance of the Arctic terns is not exactly 80,000 km\nAmended as suggested.\nLine 56: reference number 17, you may want to cite instead the recently published\narticle describing the Seabird Tracking database (Carneiro et al.\n2024: https://doi.org/10.1016/j.biocon.2024.110813)\nAmended citation as suggested.\nLines 106-107; I suggest changing “other EEZs (including ABNJ)” to “other EEZs or\nto ABNJ”.\nAmended as suggested.\nLines 288 and 315 and 316: Add a space between “500” and “km”\nAmended as suggested.\nFigure 2: “Central America” label is still missing from the figure of the sea turtles\nAmended in vectorised version, thanks for catching this."}]

Decision letter