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. 2013 Jul 17;280(1766):20130836.
doi: 10.1098/rspb.2013.0836. Print 2013 Sep 7.

Travelling light: white sharks (Carcharodon carcharias) rely on body lipid stores to power ocean-basin scale migration

Gen Del Raye  1 , Salvador J Jorgensen, Kira Krumhansl, Juan M Ezcurra, Barbara A Block
Affiliations

Travelling light: white sharks (Carcharodon carcharias) rely on body lipid stores to power ocean-basin scale migration

Gen Del Raye et al. Proc Biol Sci. .

Abstract

Many species undertake long-distance annual migrations between foraging and reproductive areas. Such migrants depend on the efficient packaging, storage and utilization of energy to succeed. A diverse assemblage of organisms accomplishes this through the use of lipid reserves; yet, it remains unclear whether the migrations of elasmobranchs, which include the largest gill breathers on Earth, depend on such a mechanism. We examine depth records from pop-up satellite archival tags to discern changes in buoyancy as a proxy for energy storage in Eastern Pacific white sharks, and assess whether lipid depletion fuels long-distance (approx. 4000 km) migrations. We develop new algorithms to assess body condition, buoyancy and drift rate during drift dives and validate the techniques using a captive white shark. In the wild, we document a consistent increase in drift rate over the course of all migrations, indicating a decrease in buoyancy caused by the depletion of lipid reserves. These results comprise, to our knowledge, the first assessment of energy storage and budgeting in migrating sharks. The methods provide a basis for further insights into using electronic tags to reveal the energetic strategies of a wide range of elasmobranchs.

Keywords: bioenergetics; buoyancy; migration; white shark.

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Figures

Figure 1.
Figure 1.
Conceptual model illustrating how body condition (lipid reserve) is expected to determine drift rate. (a) An increased liver (blue) mass (i.e. high-lipid reserves) results in a relative decrease in overall body density. In this scenario, lower drag forces are sufficient to reach terminal velocity during drift dives, yielding a slower drift rate (red line). (b) As lipid stores become depleted, the smaller liver mass results in an increase in overall body density leading to a faster drift rate.
Figure 2.
Figure 2.
Captive shark experiment. Two fixed cameras outside the tank were used to track the movement of the shark across orthogonal planes. Pitch and the length-to-girth ratio were measured when the shark's body was parallel to the side camera. The occurrence of tail beats was detectable from the top camera. (Online version in colour.)
Figure 3.
Figure 3.
Drift rate versus body condition under captive conditions. Change in drift rate over 47 days for a captive juvenile white shark (red triangles) and change in girth-to-length ratio (blue circles) during the same period. Linear regressions are significant to p < 0.001.
Figure 4.
Figure 4.
White shark migration trajectories. Median daily position estimates (points) from light/temperature geolocation fitted to a state-space movement model. Grey shading represents posterior distribution confidence limits (lower = 0.025, upper = 0.975, n = 2000 iterations). Highly linear movement patterns during migration suggest a predominance of directed travel over ARS behaviour during these periods.
Figure 5.
Figure 5.
Representative track for a shark migrating between the California coastal zone and the Hawaiian Islands. (a) Drift rate over time compared with (b) longitude (as an indicator of migration). Longitudes 122°–124° W correspond to the California Coast and longitudes 155°–157° W to the Hawaiian Islands. Transiting migrations are indicated by grey shading. Error bars represent standard deviation. (c) Time series of depth change for dives used to estimate drift rate, with dives at the beginning of the migration in red, those at the end of the migration in blue and all remaining dives shown in grey. (d) A selection of 1000 dives selected at random from the records, showing a high variability in within-dive rate of depth change (nonlinear) compared with those selected for the calculation of drift rates. (e) Squared drift rate plotted against time. Points represent mean drift rates for 10 day intervals (x-axis value±5 days). Dotted lines represent 95% CIs of the least-squares fit.
Figure 6.
Figure 6.
Drift rate profiles of migrant versus coastal sharks. Squared drift rate over time for five migrations (colour) versus five coastal records (black). Migrant sharks show a consistent rise in drift rate over time, implying a steady loss of lipid reserves, while coastal records show a buoyancy steady state. The single example of an eastward migration (green) shows a slope consistent with the other westward migrations. Black dotted lines show the average slope for each of the two groups. A two-tailed t-test revealed that the two samples were significantly different with a p-value < 0.005.
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