Convergence of marine megafauna movement patterns in coastal and open oceans

Abstract

The extent of increasing anthropogenic impacts on large marine vertebrates partly depends on the animals' movement patterns. Effective conservation requires identification of the key drivers of movement including intrinsic properties and extrinsic constraints associated with the dynamic nature of the environments the animals inhabit. However, the relative importance of intrinsic versus extrinsic factors remains elusive. We analyze a global dataset of ∼2.8 million locations from >2,600 tracked individuals across 50 marine vertebrates evolutionarily separated by millions of years and using different locomotion modes (fly, swim, walk/paddle). Strikingly, movement patterns show a remarkable convergence, being strongly conserved across species and independent of body length and mass, despite these traits ranging over 10 orders of magnitude among the species studied. This represents a fundamental difference between marine and terrestrial vertebrates not previously identified, likely linked to the reduced costs of locomotion in water. Movement patterns were primarily explained by the interaction between species-specific traits and the habitat(s) they move through, resulting in complex movement patterns when moving close to coasts compared with more predictable patterns when moving in open oceans. This distinct difference may be associated with greater complexity within coastal microhabitats, highlighting a critical role of preferred habitat in shaping marine vertebrate global movements. Efforts to develop understanding of the characteristics of vertebrate movement should consider the habitat(s) through which they move to identify how movement patterns will alter with forecasted severe ocean changes, such as reduced Arctic sea ice cover, sea level rise, and declining oxygen content.

The importance of sample size in marine megafauna tagging studies

“…, Sequeira et al. ). In spite of that, a sample size of one with sufficient track length can still lead to scientific insights.…”

“…Right: high (green) to low (indigo) occupancy of elephant seals on a logarithmic scale (adapted from Rodriguez et al 2017). (J) Global map of tracking data set used to quantify the movement patterns of >50 marine vertebrate species, showing that movement patterns are strongly conserved across species and vary based on the habitats the animals move through; line colors show different tracks for 50 marine megafauna species (adapted from Sequeira et al 2018). a sample size of one is expected to capture a "normal" or common trajectory, the value of such studies actually lies in their ability to show that certain feats are possible. Examples of extraordinary feats detected in single animal studies include the journey of >1,000 km by a leatherback turtle (Dermochelys coriacea) tagged off South Africa (Hughes et al 1998), and the discovery that white sharks (Carcharodon carcharias) can last more than a month on a single large meal (Carey et al 1982, but see Semmens et al 2013).…”

“…The value of different sample sizes is best exemplified in work from individuals who pioneered tagging on the same system or species, necessarily starting with small numbers of tags before attaining larger sample sizes that altered the scope of their work allowing new questions to be addressed. For example, a thread of work tracking leatherback turtles in the Atlantic began with n = 3 (Hays et al 2004a), progressed to n = 21 (Fossette et al 2010), then n = 106 (Fossette et al 2014) to recently become part of a study involving >2,500 tracked marine animals across multiple species (Sequeira et al 2018). At each iteration, the questions that were addressed changed, and this increasing capability is reflected in the synthesis presented here.…”

“…(J) Global map of tracking data set used to quantify the movement patterns of >50 marine vertebrate species, showing that movement patterns are strongly conserved across species and vary based on the habitats the animals move through; line colors show different tracks for 50 marine megafauna species (adapted from Sequeira et al. ).…”

“…) to recently become part of a study involving >2,500 tracked marine animals across multiple species (Sequeira et al. ). At each iteration, the questions that were addressed changed, and this increasing capability is reflected in the synthesis presented here.…”