Quantifying habitat selection and variability in habitat suitability for juvenile white sharks

Abstract

While adult white sharks (Carcharodon carcharias) are apex predators with a circumglobal distribution, juvenile white sharks (JWS) feed primarily on bottom dwelling fishes and tend to be coastally associated. Despite the assumedly easier access to juveniles compared to large, migratory adults, limited information is available on the movements, environments, and distributions of individuals during this life stage. To quantify movement and understand their distribution in the southern California Bight, JWS were captured and fitted with dorsal fin-mounted satellite transmitters (SPOT tags; n = 18). Nine individuals crossed the U.S. border into Baja California, Mexico. Individuals used shallow habitats (134.96 ± 191.1 m) close to shore (7.16 ± 5.65 km). A generalized linear model with a binomial distribution was used to predict the presence of individuals based on several environmental predictors from these areas. Juveniles were found to select shallow habitats (< 1000 m deep) close to land (< 30 km of the shoreline) in waters ranging from 14 to 24°C. Southern California was found to be suitable eight months of the year, while coastal habitats in Baja California were suitable year-round. The model predicted seasonal movement with sharks moving from southern California to Baja California during winter. Additionally, habitat distribution changed inter-annually with sharks having a more northerly distribution during years with a higher Pacific Decadal Oscillation index, suggesting sharks may forego their annual fall migrations to Baja California, Mexico, during El Niño years. Model predictions aligned with fishery-dependent catch data, with a greater number of sharks being captured during periods and/or areas of increased habitat suitability. Thus, habitat models could be useful for predicting the presence of JWS in other areas, and can be used as a tool for potentially reducing fishery interactions during seasons and locations where there is increased susceptibility of incidental catch.

Fig 1. Habitat suitability by environmental parameters.

Presences (standardized daily positions) are shown as black circles with a value of 1, while available points are shown as a having a value of 0. Values are plotted against distance to land (A), temperature (B) and depth (C). Conditional predicted responses for each variable are displayed as red lines by fixing the other two parameters.

Fig 2. Map of habitat suitability in the Northeast Pacific Ocean.

Map of the Northeast Pacific Ocean, with colors representing the monthly overlap in the 25% core area.

Fig 3. Monthly habitat suitability in the Northeast Pacific Ocean.

Areas of suitable habitat for JWS in the Northeast Pacific by each month of the year, with red being areas that are highly suitable while blue is unsuitable. Monthly averages are based on environmental variables throughout the study period (2002–2015).

Fig 4. Median suitable latitude over time.

The 10-day running mean of the median latitude of the 25% core area used each day. There were significant monthly and yearly differences in the median position. In the summer of 2014 and 2015 there were northerly expanses in median core latitude.

Fig 5. JWS captures by predicted suitability.

The number of JWS captured (black circles) in each block-month (left y- axis) against the predicted habitat suitability. Habitat suitability predicted the mean number of JWS caught (red line). Sum total effort (right y-axis, Hours/fathom length) across habitat suitability (purple line) is overlaid, and decreases with increasing habitat suitability.

Fig 6. Habitat suitability hot spots for the Northwest Atlantic.

Overlap of the monthly 25% core areas for JWS in the Northwest Atlantic. Colors represent the number of months that area represents core suitable habitat for JWSs. There are hot spots off Long Island Sound and off the Outer Banks of North Carolina (insets).