Otter Status Paper - Furbearer Management

Surveys

River Otter Status, Management, and Distribution in theUnited States: Evidence of Large-Scale PopulationIncrease and Range ExpansionNathan M. Roberts,* Matthew J. Lovallo, Shawn M. Crimmins

N.M. RobertsOffice of Applied Science, Wisconsin Department of Natural Resources, 107 Sutliff Avenue, Rhinelander, Wisconsin54501

M.J. LovalloBureau of Wildlife Management, Pennsylvania Game Commission, 2001 Elmerton Avenue, Harrisburg, Pennsylvania16875

S.M. CrimminsCollege of Natural Resources, University of Wisconsin-Stevens Point, 800 Reserve Street, Stevens Point, Wisconsin54481

Present address of S.M. Crimmins: U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, Universityof Alaska Fairbanks, 2140 Koyukuk Drive, Fairbanks, Alaska 99775

Abstract

River otter Lontra canadensis populations in the United States have expanded during the past 50 y as a result ofimprovements in habitat quality and effective management programs implemented by state and federal agencies andnative tribes. Periodic assessments of river otter status, population trends, and geographic distribution are needed todetect changes in populations, assess management approaches, and to identify and prioritize conservation efforts. Wesurveyed state wildlife agency experts to assess the current population and regulatory status of river otters in theirjurisdictions. River otters were legally harvested in 40 states as of 2016. Twenty-two states reported increasingpopulations while 25 reported stable populations. Most states used multiple methods to monitor river otterpopulations including harvest-based surveys, presence–absence surveys, and empirically derived population modelpredictions; harvest-based surveys were the most commonly used monitoring approach. As populations haveexpanded, river otter reintroduction efforts have become less frequent; two additional states had conductedreintroductions since 1998 and only one state had conducted a reintroduction since 2010. We estimated that riverotter distribution increased by 10.2% in the continental United States and by 13.7% in the contiguous United Statesduring an 18-y period. Although populations may continue to increase numerically, river otters may be approachingtheir potential maximum geographic distribution in the United States.

Keywords: distribution; harvest; Lontra canadensis; management; restoration; river otter; United States

Received: October 4, 2018; Accepted: January 3, 2020; Published Online Early: January 2020; Published: June 2020

Citation: Roberts NM, Lovallo MJ, Crimmins SM. 2020. River otter status, management, and distribution in the UnitedStates: evidence of large-scale population increase and range expansion. Journal of Fish and Wildlife Management11(1):279–286; e1944-687X. https://doi.org/10.3996/102018-JFWM-093

Copyright: All material appearing in the Journal of Fish and Wildlife Management is in the public domain and may bereproduced or copied without permission unless specifically noted with the copyright symbol &. Citation of thesource, as given above, is requested.

* Corresponding author: [email protected]

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Introduction

The river otter Lontra canadensis is an elusive speciesthat can serve an important role in aquatic ecosystems asa top predator of fish and crayfish. River otters are alsovalued as a game species and furbearer (Melquist et al.2003). River otters historically inhabited every majorwatershed in the contiguous United States and wereextirpated from much of their historical range as a resultof multiple factors including habitat degradation anddestruction as well as unregulated harvest (Anderson1977). By the early 1900s river otters had beencompletely extirpated from 11 states and had experi-enced severe population declines in 9 other states(Deems and Pursley 1978). The river otter was includedas an Appendix II species under the Convention onInternational Trade in Endangered Species of Wild Faunaand Flora in 1977 as a result of concern that endangeredotter species, such as Pteronura brasiliensis, could bemisidentified as river otter because of a similar superficialappearance, which could lead to unintentional trade ofendangered species; however, it is important to notethat the river otter itself has never been considered aglobally threatened or endangered species (Greenwalt1977). Since inclusion under Convention on InternationalTrade in Endangered Species of Wild Fauna and Flora,the U.S. Fish and Wildlife Service, through the Division ofScientific Authority, housed in the International AffairsProgram, has routinely determined that the export ofriver otter pelts and products from the United States hasnot been detrimental to their viability, thus satisfying theconditions of the Convention on International Trade inEndangered Species of Wild Fauna and Flora agreement(Bowman et al. 2010).

Deems and Pursley (1978), estimated that river ottersoccupied 71% of their historical range during 1977; by1998, river otter populations had expanded and werethought to occupy 90% of their prior range (Melquist etal. 2003). This continued numeric and geographicexpansion was fostered by conservation managementprograms by state and federal agencies, natural dispersaland expansion of existing populations into increasinglysuitable habitats, and reintroduction efforts by statewildlife agencies and conservation partnerships (Raesly2001). Assessments of river otter populations within thecontiguous United States have occurred periodicallysince their inclusion in Convention on International Tradein Endangered Species of Wild Fauna and Flora (e.g.,Polechla 1990, Raesly 2001, Melquist et al. 2003). Wesurveyed state wildlife agency experts regarding 1) thelegal status of river otters in their state, 2) the statewideriver otter population trend, 3) the results of reintroduc-tion efforts (if applicable), 4) the population status ofriver otters since reintroduction efforts, 5) the method(s)of river otter population monitoring, 6) how frequentlyadjustments are made to harvest regulations (if applica-ble), 7) and the geographic distribution of river otters intheir state. Herein we provide an updated account, as of2016, of river otter population status and distribution inthe United States and discuss conservation priorities.

Methods

We contacted wildlife management agencies from allstates in the contiguous United States during 2016 viaemail (Figure S1, Supplemental Material), and Alaska viaphone, targeting agency personnel that were responsi-ble for river otter management. We asked each agencyexpert to report 1) the legal status of river otters in theirstate, 2) the statewide river otter population trend, 3) theresults of reintroduction efforts (if applicable), 4) thepopulation status of river otters since reintroductionefforts, 5) the method(s) of river otter populationmonitoring, 6) how frequently adjustments are madeto harvest regulations (if applicable), 7) and thegeographic distribution of river otters in their state.Potential responses concerning legal status includedregulated harvest, no harvest, state threatened, or stateendangered. Responses for population trends includedincreasing, stable, or decreasing (although we did notspecify the timeframe). Questions concerning river otterreintroductions assessed the purpose(s) (e.g., reestablishextirpated populations, bolster existing populations, orexpand river otter range beyond current distributions)and outcomes (e.g., expanding, stagnant or geograph-ically restrained, decreasing, or unknown) of reintroduc-tion efforts. Methods of population monitoring includedbridge surveys (presence vs. absence track surveysconducted at bridge crossings), harvest surveys, bow-hunter surveys, camera surveys, winter track surveys,population models from harvest data, harvest effort, andagency field questionnaires (for detailed descriptions ofthese methods, we direct readers to Chilelli et al. [1996],Roberts et al. [2008], and Roberts [2010]). We categorizedthe frequencies of regulatory review within stateagencies into the following: never, every 4–5 y, every2–3 y, annually, and immediately as needed. We talliedresponses to these questions and reported frequency ofresponses. We assessed geographic distribution in eachstate based on a county basis; Alaska does not havecounty jurisdictions, so we gathered information via aphone interview to assess geographic distribution(Alaska Department of Fish and Game, personal com-munication). When river otter presence was unknown ina county, which could occur where there is no harvest ormonitoring effort (i.e., arid or remote regions), werecorded presence in those counties as ‘unknown.’ Whenestimating occupied range area or river otters, we onlysummed the area of the counties with known presenceand excluded those counties that are listed as unknown(Table S1, Supplemental Material). To compare estimatesof occupied area in 1998 with our survey results, wedigitized published maps depicting river otter distribu-tion during 1998 (Melquist et al. 2003) using ArcMap(ESRI 2011).

Results

We received responses from all 48 contiguous statesand interviewed agency personnel in Alaska; not allresponses were complete (Table S1). Based on this 2016survey, 22 states reported increasing populations, 25

River Otter Status in the United States N.M. Roberts et al.


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states reported stable populations, and 2 states (Arizonaand Washington) reported an uncertain population trend(Table 1). River otters were legally harvested in 40 states,while closed harvest seasons existed in 9 states; Arizona,California, Colorado, Nebraska, New Mexico, RhodeIsland, South Dakota, Utah, and Wyoming (Table 1).

States reported a variety of methods that were used tomonitor river otter populations. Most (n ¼ 31, 63.3%)states used multiple methods to monitor river otterpopulations. Harvest-based surveys were the mostcommonly used monitoring approach, being used by35 states (Table 1). Catch-per-unit effort, populationmodels, and presence vs. absence indices, such asbowhunter surveys and track surveys, were used lessfrequently (Table 1). Only four states—Nevada, RhodeIsland, Utah, and Wyoming—did not report any moni-toring efforts.

As of 2016, 23 states had conducted river otterreintroductions. Ten states indicated that they hadreintroduced river otters to establish populations thathad been extirpated (Table 1). The remaining statesindicated that they reintroduced river otters to expandpopulations in their state beyond their current range,including three states that indicated that reintroductionswere also used to reestablish extirpated populations andseven states that used reintroductions to bolsterpopulations where they already existed (Table 1). Onlyone state, Utah, had conducted river otter reintroduc-tions since 2010. Twenty-one of the 23 states (91.3%)that implemented reintroductions reported that popula-tions were increasing or had reached population goals,and 2 states (Minnesota and North Dakota) indicatedthat populations were stagnant or geographicallyrestrained (Table 1).

Harvest regulations are reviewed frequently by moststates that allow harvest (Table 1). Only three states(Illinois, Kentucky, and North Dakota) did not provide aresponse to our question about regulation reviewfrequency (Table 1). Of the remaining 37 states; 24states reviewed regulations ‘‘promptly’’ or annually and10 states reviewed harvest regulations in cycles of 2–3 y;Michigan reported that regulations are reviewed ‘‘every2 to 3 years’’ and ‘‘promptly as needed’’ (Table 1). Twostates (Connecticut and Tennessee) reported that regu-lations are reviewed ‘‘every 4 to 5 years.’’ South Carolinaand Texas reported that harvest regulations are ‘‘never’’reviewed.

We estimated the county-level geographic distributionoccupied by river otters in the continental United Statesduring 2016 to be approximately 7,422,120 km2—75% ofthe total land area and 94% of the available total waterarea. Within the contiguous United States, we estimatedthat the county-level geographic distribution occupiedby river otters to be approximately 5,686,140 km2—70%of the total land area and 90% of the total water area.(Figure 1). River otters were reported to be ubiquitousthroughout Alaska south of the Brooks Range (AlaskaDepartment of Fish and Game, personal communica-tion). Based on maps published by Melquist et al. (2003),we estimated that during 1998, river otters occupiedapproximately 6,736,558 km2—nearly 65% of the land

area of the contiguous United States—and that thepotential river otter range was 7,485,065 km2. Similarly,we estimated that river otters occupied approximately5,000,550 km2—nearly 63% of the land area of thecontiguous United States—and that the potential riverotter range was approaching 5,556,200 km2. Compari-sons between our survey and published reports from1998 (Melquist et al. 2003) indicated that changes in riverotter distribution occurred primarily in the contiguousUnited States and that river otters had expanded theirrange by approximately 13.7% during an 18-y period(Table 2). We estimated that river otter range in thecontinental United States expanded by 10.2% during thisperiod (Table 3).

Discussion

As per the North American Model of wildlife conser-vation, river otters are held in the public trust by the‘‘government’’ and are managed by state and federalagencies as well as by native tribes (Mahoney and Geist2019). Agency-enforced regulations include restrictionson harvest, harvest season length, harvest methods, andbag limits. Based on our survey, river otters wereharvested in 40 states during 2016. Within states thatharvested otters, regulations concerning river otterharvest seasons were reviewed and adjusted regularly.The continued expansion of river otter populationswhere they are managed by regulated harvest is likelyenhanced by the frequency of these regulatory reviews.

River otter reintroductions were widespread duringthe late 20th century (Raesly 2001). As of 2016, .4,000river otters have been translocated to 23 states in one ofthe most ambitious and extensive carnivore restorationefforts in history (Erb et al. 2018). Our survey indicatedthat ,50% of the river otter reintroductions implement-ed in the United States were in areas where otters hadbeen extirpated. Other restoration efforts were conduct-ed to bolster existing river otter populations and toexpand otter distributions and provide connectivitywithin states that had existing, but not ubiquitous, riverotter populations (Table 1). The majority of theseprojects were implemented in the mid-1980s and werecompleted by the late 1990s; only two states (New Yorkand Pennsylvania) were actively engaged in reintroduc-tions during 1998 (Raesly 2001). As of 2016, only twoadditional states had conducted reintroductions (NewMexico and North Dakota) since Raesly’s (2001) previousassessment and Utah was the only state that hadreintroduced river otters since 2010. The cessation ofwide-scale reintroduction activities may suggest that themajority of suitable habitats are currently occupied byestablished river otter populations. Raesly (2001) deter-mined that, although it is preferable to acquire riverotters from proximal populations to maximize geneticsimilarity and retain subspecies integrity, most jurisdic-tions used the most easily obtained river otters, oftenfrom Louisiana, during reintroduction projects. Theoverall impacts, or extent, of genetic infusion fromdistant populations that were sourced for these reintro-



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toth

ree

year

s

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wM

exic

oIn

cre

asin

gN

otr

app

ing

seas

on

Cam

era

surv

eys

Re

est

ablis

han

ext

irp

ate

d

po

pu

lati

on

Po

pu

lati

on

incr

eas

ing

No

tap

plic

able

Ne

wY

ork

Incr

eas

ing

Re

gu

late

dtr

app

ing

seas

on

Har

vest

surv

eys

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arve

ste

ffo

rt,

po

pu

lati

on

mo

de

ls,

bri

dg

esu

rve

ys,

win

ter

trac

ksu

rve

ys

Esta

blis

hin

un

occ

up

ied

po

rtio

no

fst

ate

and

bo

lste

r

exi

stin

gp

op

ula

tio

ns

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pu

lati

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incr

eas

ing

Ad

just

me

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rom

ptl

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ed

ed

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lish

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pie

d

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ate

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r

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stin

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ula

tio

ns

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et

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just

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rom

ptl

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ter

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mo

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ling

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est

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han

ext

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ate

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po

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on

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ge

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ally

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stra

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d

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se

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ter

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ys

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est

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han

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d

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on

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et

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nu

ally

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ma

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ing

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able

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ing

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ied

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nu

ally

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od

eIs

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dSt

able

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pin

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aso

nN

on

ere

po

rte

dN

ot

app

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le

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leR

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ula

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g

seas

on

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vest

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eys

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plic

able

No

tap

plic

able

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ver

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thD

ako

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cre

asin

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otr

app

ing

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on

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de

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arve

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dro

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ill

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eys

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est

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ext

irp

ate

d

po

pu

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on

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pu

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on

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eas

ing

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tap

plic

able

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nn

esse

eIn

cre

asin

gR

eg

ula

ted

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pin

g

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on

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vest

surv

eys

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blis

hin

un

occ

up

ied

po

rtio

no

fst

ate

Po

pu

lati

on

incr

eas

ing

Fou

rto

five

year

s



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duction efforts are unknown, but our results suggest thatrange expansion has occurred nevertheless.

River otters are challenging to monitor because oftheir secretive nature and the fact that they naturallyoccur at relatively low densities on the landscape.Although state wildlife-management agencies use avariety of techniques to monitor changes in river otterpopulation size and distribution, regulated harvest-management programs provide the foundation for themuch of the population monitoring that occurs. Of the40 states that incorporate harvest management pro-grams, the majority used data from regulated harvestseasons (e.g., surveys of trappers, estimates of trappereffort, or collection of biological samples to estimatedemographic and health parameters) to monitor riverotter populations and to ensure that harvest levels weresustainable. For many furbearers, including river otter,harvest-based data are the only information availablewith sufficient sample sizes large enough for robustanalyses (White et al. 2015). Chilelli et al. (1996)encouraged jurisdictions to standardize monitoringmethodology. Two decades later, there is still consider-able variation in monitoring methods and effort acrossjurisdictions. Some variation is to be expected becauseeach jurisdiction has different resources and priorities forwildlife programs. As populations grow and expand, it isreasonable to assume that some jurisdictions may shiftmonitoring resources toward less secure species. Juris-dictions that have harvest are afforded more monitoringopportunities through harvest-dependent data sources,such as catch-per-unit effort, distribution of harvests, andsex and age-structure of harvests. We reiterate Chilelli etal. (1996) and suggest that jurisdictions that allowharvest should collect information on the distributionof harvest, some measure of participation or effort, and,ideally, some estimate of age-structure of the harvest.These harvest-dependent data can be synthesized inappropriate population models to yield a wealth ofinformation on population status and trends (Roberts2010). In the absence of harvest-dependent data,presence–absence data (such as track surveys), canindicate population distribution and trajectories (Robertset al. 2008).

Ta

ble

1.

Co

nti

nu

ed

.

Sta

te

Po

pu

lati

on

tre

nd

Ha

rve

stst

atu

s

Po

pu

lati

on

mo

nit

ori

ng

me

tho

d(s

)u

sed

Inte

nt

of

rein

tro

du

ctio

n

Su

cce

sso

f

rein

tro

du

ctio

n

Fre

qu

en

cyo

fh

arv

est

reg

ula

tio

ns

rev

iew

Te

xas

Stab

leR

eg

ula

ted

trap

pin

g

seas

on

Har

vest

surv

eys

No

tap

plic

able

No

tap

plic

able

Ne

ver

Uta

hSt

able

No

trap

pin

gse

aso

nN

ocu

rre

nt

mo

nit

ori

ng

Esta

blis

hin

un

occ

up

ied

po

rtio

no

fst

ate

and

bo

lste

r

exi

stin

gp

op

ula

tio

ns

Po

pu

lati

on

incr

eas

ing

No

tap

plic

able

Ve

rmo

nt

Incr

eas

ing

Re

gu

late

dtr

app

ing

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on

Har

vest

surv

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arve

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ffo

rt,

po

pu

lati

on

mo

del

s

No

tap

plic

able

No

tap

plic

able

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just

me

nts

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ep

rom

ptl

yas

ne

ed

ed

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gin

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pin

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on

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vest

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lish

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pie

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no

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ate

and

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lste

r

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stin

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ns

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pu

lati

on

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ing

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oto

thre

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ars

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hin

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nce

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inia

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op

ula

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n

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de

ls,

bri

dg

esu

rve

ys

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est

ablis

han

ext

irp

ate

d

po

pu

lati

on

Go

alfu

llym

et

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nu

ally

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con

sin

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eas

ing

Re

gu

late

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app

ing

seas

on

Har

vest

surv

eys

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arve

ste

ffo

rt,

po

pu

lati

on

mo

del

s,b

ow

hu

nte

r

surv

eys

,b

rid

ge

surv

eys

,ca

me

ra

surv

eys

,w

inte

rtr

ack

surv

eys

,fi

eld

-

staf

fq

ue

stio

nn

aire

No

tap

plic

able

No

tap

plic

able

An

nu

ally

Wyo

min

gIn

cre

asin

gN

otr

app

ing

seas

on

No

syst

em

atic

mo

nit

ori

ng

eff

ort

No

tap

plic

able

No

tap

plic

able

No

tap

plic

able

Table 2. Estimated range of river otters Lontra canadensis in theUnited States during 2016 compared with 1998 (estimatedfrom Melquist et al. 2003), excluding Alaska.

Year Estimated range

1998 5,000,550 km2

2016 5,686,140 km2

Percent change 13.7

Table 3. Estimated range of river otters Lontra canadensis in theUnited States during 2016 compared with 1998 (estimatedfrom Melquist et al. 2003).

Year Estimated range

1998 6,736,558 km2

2016 7,422,120 km2

Percent change 10.2



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Results from this 2016 assessment provide the mostrecent account of river otter status and distribution inUnited States to our knowledge. We found that riverotters have expanded their range significantly in thecontiguous United States since the last assessment in1998 (Melquist et al. 2003). Although methodologies andsampling scales differed from the approach used in ourassessment, comparisons of land areas occupied duringeach period provided a coarse-scale basis for estimatesof population expansion or retraction during an 18-yperiod. Given the large extent of the species’ range, wewere limited to county-level inference. Although isunlikely that river otters inhabit the entire county, webelieve this to be a sufficient and consistent with otherspecies assessments. It is also possible that we underes-timated the geographic extent of the range given thatpresence is unknown in some counties, particularly inarid regions, and these areas were excluded from ourestimate. Similarly, it is possible that false-negativesoccurred, particularly at lower population densities.Nevertheless, these results provide an empirical, albeitcoarse, contemporary estimate of river otter range in theUnited States. Melquist et al. (2003) suggested that in1998, as a result of wetland conservation and restorationefforts, river otters had expanded and inhabited approx-imately 90% of their former range. Our area estimates ofriver otter distribution in the contiguous United Statesduring 2016 (e.g., 5,686,140 km2) were strikingly similar(i.e., only 2.3% greater) to what Melquist et al. (2003)suggested as ‘potential’ range in their 1998 assessment

of 5,556,200 km2. This expansion was facilitated byrestoration programs as well as natural dispersal andexpansion of established populations.

River otters have a high dispersal capacity and arecapable of extensive long-distance movements overland, including mountainous terrain and across water-sheds (Magoun and Valkenburg 1977, Melquist andHornocker 1983). Although populations may continue toincrease numerically, river otters may be approachingtheir potential geographic distribution in the UnitedStates. Continued monitoring of the population statusand distribution at state and national levels would likelydetect any significant changes should they occur.

Supplemental Material

Please note: The Journal of Fish and Wildlife Managementis not responsible for the content or functionality of anysupplemental material. Queries should be directed to thecorresponding author for the article.

Figure S1. Example questionnaire distributed to statewildlife agencies in 2006.

Found at DOI: https://doi.org/10.3996/102018-JFWM-093.S1 (599 KB PDF).

Table S1. River otter Lontra canadensis status andmanagement questionnaire responses by state agencies

Figure 1. County-level distribution of river otter Lontra canadensis in the United States estimated by state wildlife agencies in in2016 (Alaska does not have county-level data and there is no occurrence in Hawaii).



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https://doi.org/10.3996/102018-JFWM-093.S1


in the United States during 2016 (Hawaii excluded).Description of metadata is included.

Found at DOI: https://doi.org/10.3996/102018-JFWM-093.S2 (38 KB XLSX).

Acknowledgments

We thank the many state natural resource agencies fortheir responses and cooperation. We also thank theAssociation of Fish and Wildlife Agencies for logisticalassistance. T. Peltier, N. Forman, and W. Dodge alsoassisted with several aspects of this effort. We would alsolike to thank the Associate Editor and anonymousreviewers for constructive and value suggestions andcomments that improved the quality of this manuscript.

Any use of trade, product, website, or firm names inthis publication is for descriptive purposes only and doesnot imply endorsement by the U.S. Government.

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Bowman M, Davies P, Redgwell C. 2010. Lyster’sinternational wildlife law. England: Cambridge Univer-sity Press.

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Documents

Otter Status Paper - Furbearer Management