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Quantifying dispersal between marine protected areas by a highly mobile species, the bottlenose dolphin, Tursiops truncatus - PubMed

  • ️Mon Jan 01 2018

. 2018 Aug 23;8(18):9241-9258.

doi: 10.1002/ece3.4343. eCollection 2018 Sep.

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Quantifying dispersal between marine protected areas by a highly mobile species, the bottlenose dolphin, Tursiops truncatus

Milaja Nykänen et al. Ecol Evol. 2018.

Abstract

The functioning of marine protected areas (MPAs) designated for marine megafauna has been criticized due to the high mobility and dispersal potential of these taxa. However, dispersal within a network of small MPAs can be beneficial as connectivity can result in increased effective population size, maintain genetic diversity, and increase robustness to ecological and environmental changes making populations less susceptible to stochastic genetic and demographic effects (i.e., Allee effect). Here, we use both genetic and photo-identification methods to quantify gene flow and demographic dispersal between MPAs of a highly mobile marine mammal, the bottlenose dolphin Tursiops truncatus. We identify three populations in the waters of western Ireland, two of which have largely nonoverlapping core coastal home ranges and are each strongly spatially associated with specific MPAs. We find high site fidelity of individuals within each of these two coastal populations to their respective MPA. We also find low levels of demographic dispersal between the populations, but it remains unclear whether any new gametes are exchanged between populations through these migrants (genetic dispersal). The population sampled in the Shannon Estuary has a low estimated effective population size and appears to be genetically isolated. The second coastal population, sampled outside of the Shannon, may be demographically and genetically connected to other coastal subpopulations around the coastal waters of the UK. We therefore recommend that the methods applied here should be used on a broader geographically sampled dataset to better assess this connectivity.

Keywords: MPAs; bottlenose dolphins; connectivity; photo‐identification; population structure.

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Figures

Figure 1
Figure 1
GPS

locations of bottlenose dolphin samples collected and used throughout this study and approximate locations of Special Areas of Conservation (

SAC

s) around the British Isles (areas circled). Samples include coastal biopsies of free‐living dolphins (n = 71), samples collected from dead‐stranded animals (n = 25), and one sample from a bycaught animal. Note that some sampling locations indicated by the circles overlap due to the scale of the map

Figure 2
Figure 2
GPS

tracks recorded during boat surveys for bottlenose dolphins on the West coast of Ireland

Figure 3
Figure 3

Examples of bottlenose dolphin fins showing the three grades of mark severity used in photograph analysis. Each dolphin was graded from one to three as follows: (a) grade M1 marks, consisting of significant fin damage or deep scarring that were considered permanent; (b) grade M2 marking that consist of deep tooth rakes and lesions, with only minor cuts present; (c) fin with grade M3 marks, having only superficial rakes and lesions. Grades M1 and M2 are considered to last many years, enabling long‐term identification of these dolphins. In contrast, “superficial” markings (grade M3), such as tooth rakes, may fade and heal within a relatively short period of time and interannual resighting probabilities of these animals are likely to be reduced

Figure 4
Figure 4

(a) Genetic assignment probabilities from

STRUCTURE

(n = 97) with each vertical column corresponding to an individual dolphin and the colors indicating the membership proportions to each of the three clusters. (b)

DAPC

scatterplot clustering the samples (n = 97) according to their first two principal components. The outlier “bnd204” was the only sample assigned differently by

DAPC

and

STRUCTURE

. Red, green, and blue colors represent Coastal Shannon, Coastal Mobile, and Pelagic dolphins, respectively. (c) Map of individual assignment probabilities per population, (I) Coastal Shannon and (

II

) Coastal mobile identified by

TESS

including only coastal biopsies (n = 71). The color scale bar indicates the assignment probabilities. The results are based on analyses run with the complete set of 15 microsatellite loci

Figure 5
Figure 5

Possible migrant dolphin (a male given photo‐identification number #18) has been encountered only within Shannon Estuary

SAC

over 9 years (encounter locations indicated with red dots) but is genetically assigned to coastal mobile population with 79% certainty (green color in assignment probability plot from

STRUCTURE

). Dolphin #1276 (encounter locations indicated with green dots) is a male potentially closely related to #18 (r ≥ 0.45), and he in turn is closely related to #1199 (encounter locations indicated with yellow dots), also a male. Both #1276 and #1199 are strongly assigned to the coastal mobile population

Figure 6
Figure 6

Social network diagram of bottlenose dolphins encountered on at least five occasions during the data collection 1996–2014. Boxes represent a social cluster of individuals encountered in the Shannon Estuary, and circles a cluster of the “mobile” dolphins encountered on the west and north‐west coast of Ireland. The length of the line in the network diagram inversely represents the strength of the association between a dyad calculated as half‐weight index (

HWI

)

Figure 7
Figure 7

Lagged identification rate (

LIR

) for bottlenose dolphins encountered ≥5 times (a) in the Shannon Estuary and (b) outside the Shannon Estuary in the coastal waters of Ireland during the study period 1996–2014. The graph describes the probability that a dolphin photographed at time 0 will be identified again at time X within the area. Data points are represented as green circles (with

SE

), and the best fitting model (see Supporting information Appendix S12) is displayed as the black solid line. Time lag (number of days) is given on logarithmic scale

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