Non-trophic impacts from white sharks complicate population recovery for sea otters.

Jerry H Moxley,Kyle S Van Houtan,Salvador J Jorgensen,Teri E Nicholson

doi:10.1002/ece3.5209

Jerry H Moxley, Kyle S Van Houtan + Show 2 more

Open Access

https://doi.org/10.1002/ece3.5209

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Abstract

Complex interactions between protected populations may challenge the recovery of whole ecosystems. In California, white sharks (Carcharodon carcharias) mistargeting southern sea otters (Enhydra lutris nereis) are an emergent impact to sea otter recovery, inhibiting the broader ecosystem restoration sea otters might provide. Here, we integrate and analyze tracking and stranding data to compare the phenology of interactions between white sharks and their targeted prey (elephant seals, Mirounga angustirostris) with those of mistargeted prey (sea otters, humans). Pronounced seasonal peaks in shark bites to otters and humans overlap in the late boreal summer, immediately before the annual adult white shark migration to elephant seal rookeries. From 1997 to 2017, the seasonal period when sharks bite otters expanded from 2 to 8 months of the year and occurred primarily in regions where kelp cover declined. Immature and male otters, demographics most associated with range expansion, were disproportionately impacted. While sea otters are understood to play a keystone role in kelp forests, recent ecosystem shifts are revealing unprecedented bottom‐up and top‐down interactions. Such shifts challenge ecosystem management programs that rely on static models of species interactions.

Highlights

Recovering carnivores, for exam‐ ple, can increase direct and indirect top‐down effects (Stier et al, 2016) that may inhibit the recovery of protected prey species and compete against larger goals of ecosystem recovery (Marshall et al, 2016; Samhouri et al, 2017)
White sharks are considered threatened by the IUCN Red List, and though they are protected at state, federal, and global scales, their current status in the northeastern Pacific is debated
Though it might not have been significant historically, several factors indicate that white sharks may be currently limiting the recovery of California sea otters

Summary

| INTRODUCTION

Unexpected conservation challenges include conflict between pro‐ tected populations (Marshall, Stier, Samhouri, Kelly, & Ward, 2016; Soulé, Estes, Berger, & Rio, 2003). Though the original causes of the sea otter population decline (hunting, fishery interactions) have been solved, the recent advent of mortalities from white shark bites suggests new threats to recovery (Nicholson et al, 2018; Tinker, Hatfield, Harris, & Ames, 2016). In contrast with otters, the rate of human shark bites over the past 50 years has increased only mar‐ ginally and per‐capita risk has decreased by 90% (Ferretti, Jorgensen, Chapple, Leo, & Micheli, 2015) Though it might not have been significant historically, several factors indicate that white sharks may be currently limiting the recovery of California sea otters. Any appreciable and sustained population growth must come from the range peripher‐ ies It is in these peripheries where a high rate of injurious and fatal interactions with white sharks appears to be stalling popula‐ tion growth (Tinker et al, 2016). We hope the inte‐ grated data and analyses here generate new insights into protected species population recovery and whole ecosystem restoration

| METHODS

Findings

| DISCUSSION