DOI: 10.7291/d1469b
title: Recovering populations of the southern sea otters suppress a global marine invader

description: Understanding the role of apex predators on ecosystems is essential for
 designing effective conservation strategies. Supporting recovery of apex
 predators can have many benefits; one that has been rarely examined is
 control of invasive prey. We investigated whether a recovering apex
 predator, the southern sea otter (Enhydra lutris nereis), can exert local
 control over a global marine invader, the green crab (Carcinus maenas). We
 determined that southern sea otters in Elkhorn Slough estuary in
 California can consume large numbers of invasive green crabs and found
 strong negative relationships in space and time between otter and green
 crab abundance. Green crabs persisted at highest abundance in this estuary
 at sites with artificial tidal restriction that were not accessible to
 otters. Green crab abundance remained lower in this estuary than in all
 other estuaries in the region, which lack resident sea otters.
 Conservation organizations and agencies have invested heavily in recovery
 of southern sea otters, increasing their numbers in this estuary.
 Restoration of natural tidal exchange and lost marshes and seagrass beds
 further supports their populations. We have demonstrated that these
 investments in top predator recovery and habitat restoration have reduced
 the impacts of a global invader. Our investigation highlights that
 investment in recovery of top predators can increase beneficial food web
 interactions and resilience of the entire ecosystem.
description: Most of the crab data were collected as part of the long-term
 crab monitoring program at Elkhorn Slough National Estuarine Research
 Reserve. The field protocol we used was the same as the protocol used by
 researchers/collaborators, who collected the data at the other estuaries
 in this study. Briefly, we used standard trapping protocols for crabs,
 soaking baited traps (minnow and Fukui) for 24 hrs, at a tide level of
 zero feet. All crabs were measured, sexed, and identified to species
 level. The majority otter data were collected by USGS,
 during their annual surveys. Protocol publicly available online. The only
 otter data not collected by USGS, were collected by Elkhorn Slough Safari,
 as described in the manuscript. All data were
 QAQC'ed by multiple people, and multiple times. We eliminated crab
 data where a trap was broken or lost during deployment.
description: Microsoft Excel or Apache Open Office Calc (free) 
description: # Recovering populations of the southern sea otters suppress a global
 marine invader Most of the crab data were collected as part of the
 long-term crab monitoring program at Elkhorn Slough National Estuarine
 Research Reserve. The field protocol we used was the same as the protocol
 used by researchers/collaborators, who collected the data at the other
 estuaries in this study. The majority otter data were collected by USGS,
 during their annual surveys. Protocol publicly available online. The only
 otter data not collected by USGS, were collected by Elkhorn Slough Safari,
 as described in the manuscript. All data were QAQC'ed by multiple
 people, and multiple times. We eliminated crab data where a trap was
 broken or lost during deployment. The green crab data are reported in
 catch per unit effort, which was calculated as number of crabs per trap,
 at a site, on a given day. The otter data in Figure 3a, 3b, 4, 5 are
 reported in density, otters/km2. Densities are calculated as described in
 Supporting Information. The otter data in Figure 3c are counts of
 individuals on a given boat tour, and averages calculated per month as
 total numbers for that month divided by number of boat tours. ##
 Description of the data and file structure If a cell has the value
 "null" it means there were no data for that day/year/site etc.
 The same sites were used for the different analyses, but may have
 different groupings. For example VM was a site in the lower estuary,
 without tidal restriction. **File Fig. 3a** *Column names* Site:
 identifier for study site (APM = Azevedo Pond; HI = Hummingbird Island; HL
 = Hudson Landing; KP = Kirby Park; NMF = North Marsh Full; NMM = North
 Marsh Mute; SM = South Marsh, VB = Vierra Beach; WS = Whistlestop Lagoon).
 All sites and abbreviations are also listed in the SI, Table S1 Year: year
 in which data in a given row were collected Sea otter density
 [otters/km^2]: calculated kernel densities of otters within the USGS
 survey cell closest to the site Green crab CPUE [crabs/trap]: average
 number of individual crabs in traps set, at a site, in one day Section of
 estuary: estuary area category (lower, middle, upper) Dist. from mouth
 [km]: distance of "section of estuary" from the mouth of the
 estuary to that section (e.g. lower, middle, upper) Dist. category:
 numerical category of estuary area (0-6 = lower, 6-8 = middle, 8-10=upper
 estuary) **File Fig. 3b** *Column names* Site: identifier for study site
 (APM = Azevedo Pond; HI = Hummingbird Island; HL = Hudson Landing; KP =
 Kirby Park; NMF = North Marsh Full; NMM = North Marsh Mute; SM = South
 Marsh, VB = Vierra Beach; WS = Whistlestop Lagoon). All sites and
 abbreviations are also listed in the SI, Table S1 Year: year in which data
 in a given row were collected Sea otter density [otters/km^2]: calculated
 kernel densities of otters within the USGS survey cell closest to the site
 Green crab CPUE [crabs/trap]: average number of individual crabs in traps
 set, at a site, in one day Hydrology: site category identifying whether
 the site is tidally restricted or not (restriction, no tidal restriction)
 **File Fig. 3c** *Column names* Site: identifier for study site (APM =
 Azevedo Pond; HI = Hummingbird Island; HL = Hudson Landing; KP = Kirby
 Park; NMF = North Marsh Full; NMM = North Marsh Mute; SM = South Marsh, VB
 = Vierra Beach; WS = Whistlestop Lagoon). All sites and abbreviations are
 also listed in the SI, Table S1 Year: year in which data in a given row
 were collected Green crab CPUE [max # crabs/trap/day]: of all the traps
 set at a site, in one day, this is the maximum number of green crabs found
 in one trap, at that site, in one day Month: Month during which the
 Elkhorn Slough Safari counted otters seen during each excursion (Jan-Dec)
 Sea otter abundance [individuals/trip]: average number of individual
 otters per trip, counted during excursions within the month of the prior
 column **File Fig. 5** *Column names* Year: year in which data in a given
 row were collected Estuary: identifier for which west coast estuary in
 which the green crab trapping data came from (Bodega Bay, Drakes Estero,
 Elkhorn Slough, San Francisco Bay, Tomales Bay) Transformed green crab
 CPUE: For cross-estuary comparison we transformed the raw CPUE data using
 the equation: LN(CPUE) + 0.05 ## Sharing/Access information Upon request
 to [rikke@elkhornslough.org](mailto:rikke@elkhornslough.org), we can share
 our data with you directly via Microsoft Excel Crab data were derived from
 the the dissertation of Rikke Jeppesen, and Elkhorn Slough Reserve
 long-term crab monitoring data base, in addition to otter data supplied by
 Tim Tinker, UGSG annual otter surveys. ## Code/Software Code used for the
 population modeling is found the in Supplemental Information of the
 manuscript, in Appendix 1.