Behaviour of foraging marine turtles 311
TABLE 2 Individual behavioural parameters of movement bouts before and after the closest significant change point to the beginning of the scallop harvest season (25 June 2016).
Turtle ID
Green turtles Cm_1 Cm_2
Kemp’s ridley turtle Lk_1
Loggerhead turtle Cc_1
Date of first fix after sig- nificant change point
25 June 27 June
25 June 2 July
Travel speed (km/h)
Before
0.163 0.060
0.153 0.109
which can also have implications on the species’ foraging behaviour and energy budgets (Jones & Seminoff, 2013). In terms of behaviour, the presence of vessels and associated noise can alter regular behavioural patterns, by decreasing surfacing time and increasing flight responses to escape or avoid a perceived threat (Hazel et al., 2007; Wirsing et al., 2008). Such movements correspond with the increase in speed and directionality identified for the turtles tracked, and could explain the lower rate and quality of satellite- derived locations received during the scallop harvest season. Frequent flight movements will probably increase the metabolic rate of individuals, and energy expenditure, which in turn decreases the internal energy available for storage and somatic growth (Hamann et al., 2002; Jones & Seminoff, 2013). Further investigation of how the observed distributional and behavioural changes relate to the foraging ecology and overall energy expenditure of turtles in the region is warranted. This would provide insights into the direct impact of recreational fisheries on turtles, and the overall impact from the high density of boats and swimmers. Indirect effects of recreational fisheries to marine turtles
could occur through impacts on seagrass beds (Hallac et al., 2012), which are important habitats for several species of marine turtles (Bjorndal et al., 1997; Schmid et al., 2003; Bjorndal & Bolten, 2010). A major impact of boating activ- ities in coastal habitats is the fragmentation and degradation of seagrass beds as a result of extensive scarring and anchor- ing. The fragmentation of seagrass beds negatively affects the growth and regeneration rates of seagrass species, dis- rupts associated invertebrate communities, increases ero- sion and suspended sediments (thus decreasing visibility), and has been linked to decreasing biodiversity in coastal ha- bitats (Hallac et al., 2012; Nordlund & Gullstrom, 2013). Problems derived from seagrass scarring have been high- lighted during multiple and extensive seagrass monitoring programmes along the Eastern Gulf of Mexico (FWC, 2013), and can indirectly affect availability and quality of food for turtles in the region. It is possible that damage to marine turtle foraging habitat is influencing the changes
After
0.629 0.047
0.248 0.163
Persistence velocity ± SD Before
After
0.005 ± 0.034 0.009 ± 0.015
0.000 ± 0.031 0.011 ± 0.024
0.055 ± 0.150 0.003 ± 0.010
0.013 ± 0.056 0.003 ± 0.041
Tortuosity Before
0.270 0.058
0.001 0.000
After
0.007 0.056
0.001 0.027
in distribution of turtles during and after the scallop season. Marine turtle hotspots outside the scallop harvest season appear to overlap with areas highly used by harvesters dur- ing the scallop harvest season, whereas marine turtle hot- spots during the scallop harvest season appear to shift and not overlap as much with areas of high vessel density. Nevertheless, these results should be interpreted with caution, given that turtle sightings were recorded opportunistically. However, it is likely that the number of turtles that actually overlap with the scallop fishing area is higher than we re- corded, given that limited accessibility to high density vessel areas hinders the detection of turtles. Changes in the relative distribution of individuals between seasons can alter the dynamics of themarine turtle populations, increasing dens- ity of turtles in patches avoided by harvesters, which are probably the less damaged habitats. Marine turtle distributions are influenced by multiple
factors including, but not limited to, local and oceanic cur- rents (e.g. Wildermann et al., 2017), abundance and access to prey (Witt et al., 2007), shelter (Christiansen et al., 2017), presence of predators (Heithaus, 2013), and seasonal shifts in water parameters (Schmid & Witzell, 2006; Shimada et al., 2016). Moreover, intra- and inter-specific differences in behaviour are also common among marine turtles. For example, distributional dichotomy has been shown in green turtles, which have distinctive resting (nocturnal) and foraging (diurnal) areas within their home range (Makowski et al., 2006; Christiansen et al., 2017). Such be- haviour could explain the use of multiple patches by turtle Cm_1 (Fig. 2). Temperature-dependent changes in behav- iour are also typical between seasons (summer/winter) (Schmid & Witzell, 2006). Home range size is expected to increase in the summer when turtles are more active (Shimada et al., 2016) but our results show an apparent de- crease in home range size following the start of the scallop harvest season. Given that the tracking of turtles took place during summermonths (June–August) when average water temperatures are fairly constant (28.8–30 °C; NOAA, 2017), it is unlikely that temperature had a substantial influence in the observed changes. Nevertheless, further integrative
Oryx, 2020, 54(3), 307–314 © 2018 Fauna & Flora International. This is a work of the U.S. Government and is not subject to copyright protection in the United States. doi:10.1017/S0030605318000182
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