I POPULATION DYNAMICS of BULL TROUT in LAKE PEND OREILLE

Home , Kokanee salmon, Lake Pend Oreille, Noxon Rapids Dam, Pend Oreille River

POPULATION DYNAMICS OF BULL TROUT IN

LAKE PEND OREILLE, IDAHO

Jonathan L. McCubbins

A Thesis Submitted in Partial Fulfillment of the Requirement of the Degree

MASTER OF SCIENCE

NATURAL RESOURCES (FISHERIES)

College of Natural Resources

UNIVERSITY OF WISCONSIN

Stevens Point, Wisconsin

January, 2013

APPROVED BY THE COMMITTEE OF:

Dr. Michael J. Hansen, Committee Chair Professor of Fisheries College of Natural Resources

Dr. Brian Sloss Assistant Leader, Wisconsin Cooperative Fisheries Research Unit College of Natural Resources

Dr. Tim Ginnett Associate Professor of Wildlife College of Natural Resources

~µ_ DS;)_~ ~ Mr. 'Joseph M. DosSantos Aquatic Program Leader / Fisheries Biologist Avista Utilities, Noxon, Montana

11 ABSTRACT

Introductions of invasive species, habitat loss, and stream fragmentation have caused bull trout Salvelinus confluentus to significantly decline throughout their native range. Consequently, bull trout were listed under the U.S. Endangered Species Act as a threatened species in June,

1998. Bull trout have existed in Lake Pend Oreille and its surrounding tributaries since the last ice age, and the lake was once a world-renowned bull trout fishery. However, non-native species introductions, stream habitat loss and fragmentation in the Lake Pend Oreille system have resulted in a decline of bull trout. To effectively manage bull trout in Lake Pend Oreille, the population dynamics of the species must be understood. Bull trout were captured, marked, released, and recaptured in gill and trap nets during the current lake trout suppression effort.

Abundance was estimated using Chapman’s modification of the Petersen estimator. Bull trout killed during sampling were used to estimate age structure, growth, and maturity. Dead bull trout were dissected to retrieve otoliths for age estimation and to estimate maturity based on development of gonads. Bull trout numbers did not increase as expected in Lake Pend Oreille, but instead abundance remained stable. I found that bull trout residing in Lake Pend Oreille between 2006 and 2008 were between 4 and 14 years of age. I found that growth slowed following maturation of Lake Pend Oreille bull trout and that growth was greatest between age 1 and 2. Males began to mature at a shorter total length than females and most bull trout were mature when they reached 450 mm. My findings improved understanding of bull trout population dynamics in Lake Pend Oreille and suggest that current management techniques are improving the status of bull trout in Lake Pend Oreille.

iii

ACKNOWLEDGMENTS

I would like to thank Avista Corporation for providing the funding for this project and for financial aid throughout my studies at the University of Wisconsin–Stevens Point, including Tim

Swant and Bob Anderson who initiated the funding for this study. I would like to thank the

University of Wisconsin–Stevens Point College of Natural Resources for helping me to advance my education and the staff therein for helping me to achieve my goals. I would like to thank Dr.

Michael J. Hansen for his tremendous support and patience through the development of this thesis and for mentoring me in becoming the best scientist I can be. I would like to thank Dr.

Brian Sloss and Dr. Tim Ginnett for their involvement on my graduate committee and providing their thorough knowledge, intuition and guidance throughout this study. I would like to thank the wonderful Dr. Nancy Nate for her help in the lab while analyzing otoliths and for her inside information on Dr. Hansen. Within Avista Corporation I would like to give a large thanks to Joe

DosSantos for his involvement on my graduate committee and for his guidance as a student and as a professional, Sean Moran for bringing me in to the Lower Clark Fork Valley the right way and for pointers throughout this process, Shana Bernall for her thorough insight and her ability to lead by example and Rob Jakubowski for his assistance while dissecting bull trout mortalities. I would like to thank Ned Horner and Rob Ryan for reviewing previous versions of this thesis and for their knowledgeable pointers in preparation for defending this thesis. I would also like to thank Andy Dux, Jim Fredericks and Chip Corse of Idaho Fish and Game for allowing the use of by-caught bull trout during the lake trout suppression effort and for their patience and support during the analysis of this data and writing of this thesis. I would like to thank Jake Hughes and

Harbor View Fisheries personnel for collecting the majority of the field data for this study.

Thank you to Todd Caspers and Ben Rook for their friendship, for their knowledge when one or

iv all three of us couldn’t seem to wrap our heads around a certain concept and for always lending their ear over empty beer glasses and pizza boxes. I would like to thank my friends in Noxon,

Trout Creek and Columbia Falls, Montana who I consider family for sticking with me through the trying times since my accident and for always offering words of encouragement throughout the process of this project. I would like to thank my mother Patricia and father Charles for their endless love, infinite support and for always encouraging me to do my best in everything I do and my sister Elizabeth and brother Nicholas for their eternal love, support and encouragement during the process of this study and throughout my life. Thank you to my mother and father in- law, Renee and Ovila, for their perpetual love and support throughout graduate school and during the writing of this thesis. I eternally thank my wife Nastastia and my beautiful daughter Kateri for their endless love and support and for always believing in me, even when belief in myself falters at times, I love you both more than life itself. Last but certainly not least, I would like to thank my late grandfather Tom Adkins for inspiring my love of fishing, hunting and the outdoors and for instilling in me a love for fish, especially bull trout.

TABLE OF CONTENTS

ABSTRACT ...... iii

ACKNOWLEDGMENTS ...... iv

TABLE OF CONTENTS ...... vi

LIST OF TABLES ...... vii

LIST OF FIGURES ...... viii

INTRODUCTION ...... 1

METHODS Study Area ...... 6 Fish Sampling ...... 8 Growth ...... 10 Maturity...... 12 Abundance...... 13

RESULTS Age ...... 15 Growth ...... 15 Maturity...... 16 Abundance...... 16

DISCUSSION ...... 16

REFERENCES ...... 34

LIST OF TABLES

Table 1. Mean length at age and incremental growth estimates for bull trout in Lake Pend Oreille, Idaho in 2008. Length at age was estimated using the von Bertalanffy length-age model and mean back-calculated length at age. Absolute incremental growth (Ga) columns show growth between adjacent von Bertalanffy and back-calculated lengths at age ...... 21 Table 2. Estimated length and age at maturity for male, female, and all bull trout in Lake Pend Oreille, Idaho in 2008. First Maturity, 50% Mature, and 99.9% Mature indicate the length at which a minimum fraction, 50%, and 99.9% of the population were mature ...... 22 Table 3. Bull trout abundance estimated using Chapman’s estimator for size classes in Lake Pend Oreille, Idaho. M = total number of marked bull trout at large at the end of tagging on 26 May 2008; C = total number of bull trout examined for tags during autumn 2008 (21 August – 7 November); R = total number of tagged fish recaptured during recapture sampling in autumn 2008; N = estimated population abundance; LL(N) = lower 95% confidence limit; and UL(N) = upper 95% confidence limit on N (Ricker 1975) ...... 23 Table 4. Number sampled, estimated abundance, 95% confidence limits, percent of sample that were mature, number of mature bull trout, and 95% confidence limits for mature for bull trout at ages 1–14 in Lake Pend Oreille, Idaho on 26 May 2008 ...... 24

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LIST OF FIGURES

Figure 1. Original distribution of bull trout in North America (McPhail and Baxter 1996) ...... 25 Figure 2. Trap net used for lake trout suppression in Lake Pend Oreille, Idaho (Hughes et al. 2007). Bull trout caught in trap nets and gillnets were used to estimate abundance and biological attributes of the bull trout population in Lake Pend Oreille, Idaho during 2006–2008 ...... 26 Figure 3. Age-bias plots comparing age estimates by two readers (A and B) for the first (upper panel), second (middle panel), and third (lower panel) estimates for 236 bull trout caught in trap nets and gillnets in Lake Pend Oreille, Idaho during 2006–2008 (dark black line = 1:1) ...... 27 Figure 4. Catch at age of 236 bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008 (mean age = 7.6 years) ...... 28 Figure 5. Linear regression of bull trout total length (L) on otolith radius (O) at time of capture in gillnets and trap-nets in Lake Pend Oreille, Idaho during 2006–2008 ...... 29 Figure 6. Standardized residuals from a linear regression (L = –193.57 + 483.97O) of length at capture (L) against otolith radius at capture (O) for bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008 ...... 30 Figure 7. Back-calculated length at age (dots) and von Bertalanffy length-age model fitted to mean back-calculated length at age (curve) for bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008 ...... 31 Figure 8. Maturity at length for all (upper panel), male (middle panel), and female (lower panel) bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008 ...... 32 Figure 9. Average back-calculated length at age for bull trout in Priest and Upper Priest Lakes, Idaho (Bjornn 1961), Flathead Lake, Montana (Fraley and Shepard 1989), and Lake Pend Oreille, Idaho (this study) ...... 33

viii

INTRODUCTION

The family Salmonidae, which includes whitefishes, graylings, trout, salmon and char, is among the oldest families of fish and has been present on earth for millions of years. Based on fossils found in the western United States, major branches of the subfamily Salmoninae were well established by the end of the Miocene epoch, 24 to 5 million years ago. One of these branches was the char (Salvalinus), but other branches included the Eurasian lenok

(Brachymystax), the taimen (Hucho), the brown trout and Atlantic salmon (Salmo) of the

Atlantic Ocean, and the Pacific salmon and trout (Oncorhyncus) of the Pacific Ocean. Modern lines of the sub-family Salmoninae, which include trout, salmon and char, probably separated into their present species 2–5 million years ago, with much of the present diversity taking place within the last 1-million years (Behnke 2002).

The genus Salvelinus can be grouped into three main evolutionary lines, the first two leading to single species, the brook trout (Salvelinus fontinalis) and the lake trout (Salvelinus namaycush), and the third leading to the Arctic char (Salvelinus alpinus), the Dolly Varden

(Salvelinus malma) and the bull trout (Salvelinus confluentus; Behnke 2002). Arctic char, Dolly

Varden, and bull trout are similar in appearance. The bull trout was described as a new species in 1858, but was not recognized as a species until 1978 (Behnke 2002). The bull trout and Dolly

Varden are similar in overall appearance, number of gill rakers, number of vertebrae, and number of pyloric ceaca. Despite these similarities, genetic analysis and chromosome counts indicate that bull trout are a different species from both the Dolly Varden and the Arctic char

(Leary and Allendorf 1997). Arctic char and the northern subspecies of Dolly Varden have 78 chromosomes and the southern subspecies of Dolly Varden has 82 chromosomes. All three species and subspecies have a total chromosomal arm number of 98, which suggests that they are

1 from the same ancestral evolutionary line. In contrast, bull trout have 78 chromosomes with 100 chromosomal arms. Two other char with this karyotype of 100 arms are the white-spotted char

(Salvelinus leucomaenis) of the Far East and the stone char of Kamchatka (scientific name unresolved) (Behnke 2002). Bull trout spawn in natal streams, so they carry genetic similarities to other fish of the same species that rear and spawn in streams. This leads to a pool of different bull trout living within the same system, but carrying genes that are similar to fish that reproduce in the same tributary stream.

The bull trout is native to western North America and is likely named for the stocky, flat head of larger adults and their aggressive feeding habits on other fish (Behnke 2002). The bull trout is a cold water species that is uncommon where water temperatures are warmer than 15°C

(McPhail and Baxter 1996; Selong et al. 2001; Behnke 2002). Bull trout have four life histories, including resident, fluvial, adfluvial, and anadromous (McPhail and Baxter 1996). Anadromous populations are few and are found mainly in the region of Puget Sound and Strait of Georgia in the state of Washington (McPhail and Baxter 1996; Brenkman and Corbett 2005). Adults of the fluvial, adfluvial and anadromous forms are piscivorous and feed mostly on other fish. Juveniles of all forms and adults of the resident form feed mainly on invertebrates (McPhail and Baxter

1996; Behnke 2002; Mogen and Kaeding 2005). Bull trout live up to 24 years but rarely live past the age of 12 years (McPhail and Baxter 1996).

The historical distribution of the bull trout is mostly west of the continental divide, but some populations live east of the divide in the Northwest Territories, British Columbia, Alberta, and the South Saskatchewan River basin in Montana (Mogen and Kaeding 2005; Meeuwig

2008). Bull trout are found from the northern tip of California north to the Yukon Territories and

Alaska (Figure1; McPhail and Baxter 1996; Mogen and Kaeding 2005; Al-Chokhachy et al.

2008; Meeuwig 2008). Bull trout were once found in the McCloud River in California, the southernmost historical limit of the genus, but this population is now extirpated and the species distribution is shrinking (Behnke 2002). Introductions of invasive species, habitat loss, stream fragmentation, and overexploitation caused bull trout numbers to decline throughout the species native range (Rieman and McIntyre 1996; USFWS 2002; Post et al. 2003; Al-Chokhachy et al.

2008). Consequently, bull trout were listed under the U.S. Endangered Species Act as a threatened species in June 1998 (USFWS 1998).

Bull trout have existed in Lake Pend Oreille, Idaho, and its surrounding tributaries since the last ice age. An introduced population of kokanee salmon (Oncorhynchus nerka) migrated downstream via the Clark Fork River from Flathead Lake, Montana in the 1930s (Hansen et al.

2007). Kokanee became abundant in Lake Pend Oreille by the end of the 1930s (Behnke 2002).

Kokanee were the principal prey for bull trout in Lake Pend Oreille (Vidergar 2000). High abundance of kokanee resulted in increased growth and maximum size of bull trout in Lake Pend

Oreille (Behnke 2002). Subsequently, the lake supported a world-renowned bull trout fishery.

Although unverified weights up to 18 kg have been claimed for bull trout in Kootenay Lake,

British Columbia, where kokanee salmon are native, the verified world record bull trout (14.5 kg) was harvested from Lake Pend Oreille in 1949 (Vidergar 2000; Behnke 2002). Introductions of non-native species to Lake Pend Oreille, principally the lake trout, along with loss of spawning habitat and stream connectivity, have reduced bull trout numbers in Lake Pend Oreille and its surrounding tributaries (Baxter et al. 1999; USFWS 2002).

The lake trout was introduced into Lake Pend Oreille in 1925 (Hughes et al. 2007;

Hansen et. al. 2008). Opossum shrimp Mysis relicta were introduced to Lake Pend Oreille each year between 1966 and 1970 and the population was established by 1972 (Rieman and Falter

1981). Mysis are an important prey for juvenile lake trout, so the introduction of Mysis has been credited for large increases in lake trout abundance in some western lakes (USFWS 2002;

Hughes et al. 2007; Hansen et al. 2008). Although Mysis have been in the lake since about 1970, lake trout recruitment was limited in Lake Pend Oreille until opossum shrimp were abundant enough to provide an abundant source of prey for juveniles (Hansen et. al. 2008). From 1999 to

2006, lake trout abundance increased by 356% in Lake Pend Oreille, so a predator removal program was initiated in spring 2006 (Hansen 2007; Hansen et al. 2010).

To protect spawning adult bull trout, all tributaries to Lake Pend Oreille except the Clark

Fork River have been closed to bull trout harvest since 1964 (USFWS 2002; Downs et al. 2006).

In 1996, two years before bull trout were listed under the U.S. Endangered Species Act, no-kill fishing regulations were established in the entire Lake Pend Oreille watershed of Idaho to eliminate harvest of bull trout (USFWS 2002). Avista Corporation, the United States Fish and

Wildlife Service (USFWS), the Idaho Department of Fish and Game, and the Montana

Department of Fish, Wildlife and Parks are working together to re-establish bull trout populations in the system.

Bull trout in Lake Pend Oreille are adfluvial (Downs et al. 2006; USFWS 2002). They hatch and grow in a third or fourth order tributary and migrate downstream to the lake at ages 1–

5 (Downs et al. 2006). Bull trout then grow and mature in Lake Pend Oreille and return to their natal stream to spawn at ages 6–11 (Downs et al. 2006). Bull trout are more migratory than most other non-adfluvial salmonids. Spawning, foraging, migrating, and overwintering movements can be up to 250 km in large interconnected streams (Al-Chokhachy et al. 2008; Fraley and

Shepard 1989). Migrating spawning adults in Trestle Creek, a third order tributary directly connected to Lake Pend Oreille, mostly spawned in consecutive years (83%) or every other year

(17%; Downs et al. 2006). Most bull trout spawn in consecutive years in Lake Pend Oreille, but estimates vary among spawning streams within the system.

Given the current status of bull trout populations throughout their range, a need to ensure rapid detection of further declines, and a need to evaluate effects of recovery programs, managers must monitor trends in local stocks and regional populations (Maxell 1999). To effectively manage bull trout in Lake Pend Oreille, population dynamics of the species must be understood. Biological attributes, such as age, growth, and maturity, should be assessed and population abundance of bull trout in the lake should be quantified to determine how the population is responding to lake trout suppression and to measure progress toward recovery goals. The draft recovery goal for bull trout in Lake Pend Oreille is to restore a bull trout harvest fishery of at least 200 fish annually, while meeting Federal Recovery Plan draft criteria, which include: (1) a minimum of six local populations of more than 100 adult bull trout; (2) at least

2,500 adult bull trout in the population; and (3) a stable or increasing trend in abundance

(USFWS 2002).

Age, growth, and maturity are important life history attributes for understanding the status of fish populations, and age is the most valuable attribute that can be obtained from sampling (Hilborn and Walters 1992). Age distribution statistics provide the basis for a large proportion of stock assessments in temperate waters (Hilborn and Walters 1992). Modeling of fish populations may provide length-at-age information that is essential in the management of fish populations (DeCicco and Brown 2006). Age is rarely known, so must be estimated to assess age structure, growth, age at maturity, and abundance of fish populations. Otoliths are continuously growing stable bones in the spinal column of most fish species that often accurately record annuli through the entire life of many fish species (DeCicco and Brown 2006; Downs et

5 al. 2006; Zymonas and McMahon 2009). Growth can be used to assess health of a population and to evaluate effects of non-native species interactions because negative interactions with other species may stimulate a compensatory response on a less competitive population by a reduction in growth (Gunckel and Hemmingsen 2002). Maturity status is also important for assessing if a population is compensating for negative interactions with other species by maturing at earlier ages. Age at maturity may vary among multiple spawning populations and life history types in large complex systems. Spawning adults drive abundance within a population, so age and length at maturity are important tools for evaluating fish population status. Abundance estimates are needed to evaluate population status and trends.

My objective was to determine if biological attributes of the bull trout population in Lake

Pend Oreille, Idaho, including age structure, growth, maturity, and abundance, were consistent with a recovering or recovered population. To address my objective, I estimated age, growth, and maturity for the population from samples of bull trout otoliths collected during 2006–2008, and I estimated abundance by mark-recapture for bull trout caught in trap nets and gillnets set in

Lake Pend Oreille during 2007–2008. I expected to find that age, growth, maturity, and abundance of bull trout was consistent with a population that was near carrying capacity.

METHODS

Study Area.—Lake Pend Oreille is a natural, temperate, oligotrophic lake that lies in the glacially formed Purcell Trench (USFWS 2002; Downs et al. 2006; Hansen et al. 2007).

Summer water temperatures (May–October) average 9°C in the upper 45 m of the water column, but surface temperatures may reach 24°C in hot summers (Hansen et al. 2007; Hughes et al.

2007). Lake Pend Oreille is the largest natural lake in Idaho and the fifth deepest lake in the nation, with a mean depth of 164 m, a maximum depth of 351 m and a surface area of 38,300 ha

(Vidergar 2000; USFWS 2002; Downs et al. 2006; Hansen et al. 2007; Hughes et al. 2007).

Stratification occurs from late June to September and the thermocline usually lies at 10–24 m

(Hansen et al. 2007; Hughes et al. 2007). Lake Pend Oreille is fed by streams originating in the

Selkirk Mountains to the Northwest, the Cabinet Mountains to the Northeast, and the Coeur d’Alene Mountains to the East (Vidergar 2000). These mountain ranges make up most of the largely undeveloped, steep, rocky terrain of the shoreline and littoral zone (Hansen et al. 2007;

Hughes et al. 2007). The rest of the littoral zone (the northern end and bays) consists of gradual or moderately sloping bottom (Hansen et al. 2007; Hughes et al. 2007). Most fish habitat occurs in the pelagic area of the lake (Hansen et al. 2007; Hughes et al. 2007).

Lake Pend Oreille is enclosed by a dam on its inlet and outlet. The Clark Fork River, the largest tributary to Lake Pend Oreille, is blocked by Cabinet Gorge Hydroelectric Development, which was completed in 1952 (USFWS 2002; Hansen et al. 2007). Thompson Falls

Hydroelectric Development was completed in 1913 and lies 113 km upstream of Lake Pend

Oreille on the Clark Fork River (USFWS 2002). Noxon Rapids Dam was constructed in 1959 and lies between the other two major dams, thereby creating a series of three impoundments of the lower Clark Fork River (USFWS 2002). All three dams presently block upstream migration of fishes (USFWS 2002; Hansen et al. 2007; Hughes et al. 2007). The Pend Oreille River, the lake’s outlet, was impounded by Albeni Falls Hydroelectric Development, which was completed in 1955 (Hansen et al. 2007). This dam regulates the top 3.5 m of the lake (Hansen et al. 2007;

Hughes et al. 2007). From July through September, pool elevation is regulated at 628.7 m, whereas winter pool elevation ranges from 625.1 to 626.4 m (Hansen et al. 2007).

The Lake Pend Oreille native fish assemblage consists of bull trout, westslope cutthroat trout Oncorhynchus clarki lewisi, mountain whitefish Prosopium williamsoni, pygmy whitefish

Prosopium coulterii, slimy sculpin Cottus cognates, five cyprinids and two catostomids. Non- native sport fish species include kokanee salmon Oncorhynchus nerka, Gerrard-strain rainbow trout Oncorhynchus mykiss, lake trout Salvelinus namaycush, lake whitefish Coregonus clupeaformis, smallmouth bass Micropterus dolomieu and several other species that are present in low numbers including northern pike Esox lucius, brown trout Salmo trutta, largemouth bass

Micropterus salmoides, perch Perca flavescens and walleye Sander vitreus.

Field Sampling.—Trap netting and gillnetting were used to suppress lake trout in Lake

Pend Oreille to restore the kokanee salmon population to harvestable numbers and to reduce competition between bull trout and other predators (lake trout and rainbow trout). Data collected came primarily from bull trout by-catch during lake trout suppression netting from autumn 2006 through autumn 2008. Gillnets were randomly distributed throughout the lake to reduce bias in bull trout capture-based estimates, whereas trap nets were set in locations where shoreline slope permitted setting trap nets. Three boats were used for setting trap nets and two boats (one 47- foot boat and one 35-foot boat) were used for lifting trap nets and fishing gill nets (Peterson and

Maiolie 2005; Hughes et al. 2007).

Large trap nets used for lake trout suppression in Lake Pend Oreille were constructed of marlex twine, lead-core polypropylene line, and in-line plastic floats (Hughes et al. 2007). Trap nets were set on the bottom of the lake to lead fish into a boxlike receptacle (the pot) at the deep end of the lead or fence (Figure 2; Hughes et al. 2007). Fish follow the lead net into the heart portion of the trap and then move into the pot (Hughes et al. 2007). Trap nets varied in size: two nets were 15-m deep with 6-m pots, two nets were 12-m deep with 6-m pots; nine nets were 9-m deep with 6-m pots; and one net was 6-m deep with a 6-m pot. Trap nets were fished from 21

March through 1 June 2007, 4 September through 30 October 2007, 8 March through 26 May

2008, and 25 August through 6 November 2008.

Gill nets used for lake trout suppression were 2.4-m tall and mesh sizes ranged 51–127 mm stretch measure (Hughes et al. 2007). Three nets 91-m in length were tied together to construct one 274-m gang (Hughes et al. 2007). When deployed, several gangs were often tied together and deployed in a zigzag layout to increase catch efficiency. Contract fishermen refer to this as catching the fish in the "bite”. Gill nets were either set from morning until evening for day sets or from evening until morning for night sets (Hughes et al. 2007). Gillnets were fished from 27 March through 4 June 2007, 11 September through 1 November 2007, 3 March through

26 May 2008, and 21 August through 7 November 2008.

Data for age, growth, and maturity was obtained through collection of otoliths from bull trout that did not survive capture during lake trout suppression efforts. Incidental mortalities in

2008 were added to the total number of otoliths gathered from bull trout killed during netting since fall 2006. These deceased bull trout provided samples for use in estimating population attributes, including age structure, growth, and maturity. Such biological attributes provide information about population status and life history that is crucial to effectively manage the bull trout population in Lake Pend Oreille. This objective required estimation of age and growth history from otolith samples retrieved from all dead bull trout collected. Otoliths were analyzed to back-calculate growth history and age structure.

Otoliths were removed and stored in a paper envelope for later analysis. Otoliths were mounted in an epoxy mixture of Araldite GY 502 and Aradur 956 and a thin section of the otolith-epoxy matrix was removed using a Buehler Isomet 1000 Precision Saw. Cross sections were mounted on a slide with super glue. To estimate age, multiple readings by two different

9 readers were used to resolve discrepancies between each reader. Annuli on each otolith were counted five independent times (twice by one reader and three times by the other reader). One reader was inexperienced in counting annuli on fish parts, so precision increased dramatically after each reading. Between each reading, readers consulted to resolve differences between their counts of annuli. Precision was quantified by age-bias plots and the coefficient of variation (CV) between each pair of readings.

Maturity status was assessed by analysis of gonads of deceased bull trout. Total length, size and shape of gonads, presence of residual eggs, and secondary sex characteristics (e.g., pronounced kipe and spawning coloration) were evaluated to determine gender and maturity status. For example, a bull trout that was longer than 300 mm and displayed a pronounced kipe, but had underdeveloped gonads, was marked as “mature, possible alternate year spawner”. If a bull trout was captured in late fall, was of an adult size, displayed spawning coloration, and retained residual eggs or milt, the fish was recorded as “mature, spawned this year”. Date of capture and secondary sex characteristics were evaluated when eggs or milt were not present.

Growth.—Fish length at annulus formation was back calculated for each fish from otolith cross sections to increase the total amount of length-age data for growth analysis and to provide length-age data for lengths of fish too small to be sampled. To evaluate linearity of the relationship between growth in fish length and otolith radius, I regressed fish length against otolith radius (Francis 1990):

Lc   Oc c

In the equation, Lc = length of fish at time of capture; Oc = otolith radius at time of capture; α and

β = parameters estimated using linear regression; and εc = additive process error. I tested

10 linearity by fitting a second order polynomial to the relationship between standardized residuals and predicted length. The regression coefficient for the squared term differed significantly from zero, so I found the relationship to be linear.

I used the biological intercept back-calculation model to back calculate length at age for each fish in the sample (Campana 1990):

Oa  Oc  La  Lc Lc  L0  Oc  O0 

In the model, La and Oa = back-calculated fish length and otolith radius at each annulus; Lc and

Oc = fish length and otolith radius at time of capture; and L0 and O0 = fish length and otolith radius at the initiation of proportionality between fish and otolith growth (biological intercept).

The biological intercept model can virtually eliminate Lee’s phenomenon and back-calculation bias inherent in the Fraser-Lee model (Campana 1990). I used a biological intercept developed for lake trout (L0 = 21.7 mm; O0 = 0.137 mm; Hansen et al. 2012) because of the similar early life history between lake trout and bull trout (egg size = 5–6 mm, incubation period = 223 days, size at hatching = 15–17 mm; and size at emergence = 22–28 mm; McPhail and Baxter 1996).

The von Bertalanffy length-age model was fitted to mean back-calculated length at age of bull trout (Ricker 1975; Hilborn and Walters 1992):

Ktto  Lt  L 1 e   i

In the length-age model, Lt = mean back-calculated length at age t; L∞ = the asymptotic length to which an average bull trout would grow if allowed to grow indefinitely; K = the instantaneous rate at which an average bull trout grew toward its asymptotic length; to= the hypothetical age at which bull trout length was zero; and εi = additive process error. Parameters were estimated

11 using nonlinear least-squares regression for male, female, and all bull trout. Likelihood-ratio tests were then used to compare growth between male and female bull trout in Lake Pend Oreille.

Parameters of the von Bertalanffy model were estimated separately for each gender (full model) and for both genders combined (reduced model). I then compared residual sums-of-squares for the full and reduced models in a likelihood-ratio test. The full model was accepted if the residual sum-of- squares was significantly lower for the full model than for the reduced model (i.e. separate growth curves for male and female bull trout). The reduced model was accepted if the residual sum-of- squares was not significantly lower for the full model than for the reduced model (i.e. a single growth curve for both genders of bull trout combined).

Absolute incremental growth was calculated for age 1 and older bull trout, for which back-calculated length at age data were available. Absolute incremental growth rate Ga was calculated as the difference in length l between adjacent ages 1 and 2:

Ga  l2  l1

In the model, l1 and l2 = mean back-calculated lengths and mean lengths at age of capture for successive ages. Mean incremental growth rates for the bull trout population in Lake Pend

Oreille were computed as arithmetic means of individual growth rates for age increments.

Maturity.—Dead bull trout caught during netting operations were frozen after capture for estimation of maturity. Frozen bull trout specimens were thawed for analysis of gonads and secondary sex characteristics and classified as: (1) ripe = will spawn in autumn; or (2) no gonad development = will not spawn in autumn. Maturity at length was modeled using a logistic nonlinear regression model (Quinn and Deriso 1999):

1 M x  rX X    i 1 e m

In the model, Mx = proportion of mature fish at each length x; r = instantaneous rate at which the

proportion of mature fish reaches 1.0; Xm = length at which 50% of the fish were mature; and εi = additive process error. Parameters were estimated using nonlinear least-squares regression.

Abundance.—Abundance of bull trout was estimated by mark-recapture during ongoing trap-netting and gillnetting by using mark-recapture protocols established for estimating abundance of lake trout in Lake Pend Oreille (Hansen et al. 2007). Bull trout captured alive and in good condition during lake trout suppression efforts were marked between autumn 2007 (4

September – 1 November) and spring 2008 (3 March – 26 May). T-bar anchor tags were used for marking because of their ease of application, high retention in other salmonids, and high visibility when recaptured (McAllister et al. 1992; Parsons and Reed 2005; Hartman and Janney

2006). Fish were marked with two uniquely numbered tags to estimate tag loss. Tags were inserted into the muscle tissue that lies below the anterior-central region of the dorsal fin, and the tagging gun was rotated 90° during withdrawal to secure the tag between the bones supporting the fin rays. Bull trout that were sickly were not tagged and released. Two contract fishing boats operated simultaneously, but only one technician was available to mark fish, so only 50% of all bull trout were marked and released. Total length was recorded for all captured bull trout and more than 10% of all bull trout captured were marked. In autumn 2008 (21 August – 7

November), bull trout captured during lake trout suppression efforts were checked for tags and fin clips. Live fish were then released back into the lake and dead fish were collected for otolith analysis and assessment of age, growth, and maturity.

Chapman’s modification of the Peterson estimator was used to estimate bull trout abundance in Lake Pend Oreille (Ricker 1975):

( )( ) ̂ ( )

 In the model, N = estimated population abundance on 26 May 2008 (end of marking period); M

= total number of marked fish at large at the end of the tagging period (26 May 2008); C = total number of fish examined for marks during recapture sampling in autumn 2008 (21 August – 7

November); and R = total number of tagged fish recaptured during recapture sampling in autumn

2008 (21 August – 7 November). I estimated 95% confidence limits for bull trout abundance using 95% binomial confidence limits for the R/C ratio (Seber 1982):

( )( ) ( ̂) ( )

For these equations, L1 and L2 are exact 95% confidence limits for the binomial proportion R/C

(equations 24.28–24.29 in Zar 1999) and other terms are as defined for the Chapman estimator.

Chapman’s estimator assumes the population is closed (constant N assumption), that all marked and unmarked individuals are equally vulnerable to sampling (constant catchability assumption), and that no tags are lost or go unnoticed (no tag loss assumption; Pollock et al.

1990). First, to address the constant N assumption, I assumed that recruitment and immigration were negligible between marking and recapture because bull trout mostly migrate out of their natal streams during spring and autumn, rather than the period between marking and recapture sampling (Downs et al. 2006). Further, I assumed that survival and emigration were similar

14 between marked and unmarked bull trout because anchor tags have negligible effects on survival and behavior of other salmonids (Rikardsen 2000; Walsh and Winkelman 2004). Next, to address the constant catchability assumption, I assumed that randomized gillnetting subjected marked and unmarked bull trout to an equal likelihood of capture during recapture sampling.

Last, to address the no tag loss assumption, I double-tagged bull trout to estimate tag loss and assumed that anchor tags were all visible and recognized during recapture sampling.

RESULTS

Age.—Otoliths were collected from 236 bull trout ranging from 285 to 771 mm in total length and 4 to 14 years in age. Precision of estimated age improved with successive readings by two readers (Figure 3). By the third reading, estimated ages differed by no more than 1 year for all otoliths. Nearly half of all bull trout sampled were ages 7 or 8 years, 85% of the sample was ages 6–9 years, and mean age was 7.6 years (Figure 4).

Growth.—Length was linearly related to and described 62% of the variance in otolith radius for bull trout in Lake Pend Oreille (Figure 5):

L  193.57  483.97O

Residuals were randomly related to otolith radius (t = -1.596; df = 231; P = 0.112), so error variance was independent of length and the underlying relationship was linear (Figures 6).

Therefore, the biological-intercept, direct-proportion model was appropriate for back-calculating growth histories of bull trout in Lake Pend Oreille.

Variation in back-calculated length increased with bull trout age in Lake Pend Oreille

(Figure 7). Growth did not differ significantly between males and females (F = 0.306; df = 3, 24;

P = 0.82), so males and females were combined (Figure 7). Average back-calculated length at

15 age for bull trout in Lake Pend Oreille increased from 107 mm at age 1 to 636 mm at age 14

(Table 1). An average bull trout grew from length zero at t0 = 0.00657 years at an average instantaneous rate of K = 0.166/year toward an average asymptotic length of L∞ = 705 mm.

Incremental growth averaged 45 mm per year and declined from 91 mm/year at age 1 to 12 mm/year at age 13 (Table 1).

Maturity.—Male bull trout matured at a smaller size and younger age than female bull trout in Lake Pend Oreille. Bull trout first started to mature at 400 mm and age 5.1, 50% were mature at 425 mm and age 5.6, and 99.9% of bull trout were mature at 450 mm and age 6.1

(Figure 8; Table 2). Males started to mature at 350 mm and age 4.1, 50% were mature at 400 mm and age 5.1, and 99.9% were mature at 425 mm and age 5.6 (Figure 8; Table 2). Females started to mature at 425 mm and age 5.6, 50% were mature at 450 mm and age 6.1, and 99.9% were mature at 475 mm and age 6.8 (Figure 8; Table 2).

Abundance.—In autumn 2007 (4 September – 1 November) and spring 2008 (3 March –

26 May), 247 bull trout were tagged and released while gillnetting and trap netting in Lake Pend

Oreille. In autumn 2008 (25 August – 6 November), 12 tagged bull trout were recaptured out of

655 total bull trout examined for tags (Table 3). Bull trout abundance was 12,513 fish (95% confidence interval = 7,456–22,521 fish) and density was 0.33 bull trout/ ha (0.19–0.59 bull trout/ha) on 26 May 2008. Of 12,513 total bull trout present in Lake Pend Oreille, 9,914 (5,914

– 17,833) were 350 mm or longer, 8,004 (4,540 – 13,564) were 400 mm or longer, 6,913 (3,857

– 13,463) were 425 mm or longer, and 5,721 (5,721 – 11,138) were 450 mm or longer in Lake

Pend Oreille on May 28, 2008 (Table 3). Based on a length of 425 mm at 50% maturity, 6,913

(3,857 – 13,463) mature bull trout were present in Lake Pend Oreille on 26 May 2008 (Table 4).

DISCUSSION

Bull trout in Lake Pend Oreille live to older ages than was found during an earlier study of bull trout in Lake Pend Oreille (Vidergar 2000). In 1999, bull trout ranged from age 6 to age

12 in Lake Pend Oreille, perhaps because the population had been recently fished (Vidergar

2000). In contrast, my estimate from 2008 represented a bull trout population that had not been fished in more than a decade, so the population ranged from age 4 to age 14. Average length of bull trout I sampled in 2008 (524.5 mm) was similar to that in 1999 (524 mm; Vidergar 2000), so vulnerability to sampling was likely similar in the two studies. Bull trout migrate from spawning streams to Lake Pend Oreille between age 1 and age 5, with a peak at age 3 (Downs et al. 2006), so were likely not fully vulnerable to capture during sampling until age 3. Absence of bull trout of ages 1–3 in my study and the study by Vidergar (2000) was likely caused by size selectivity of sampling, through either mesh selectivity or behavior of young bull trout that prevented them from encountering trap nets or gillnets.

I found that asymptotic length was shorter and incremental growth was lower for bull trout in Lake Pend Oreille than studies of nearby populations (Figure 9; Bjornn 1961; Fraley and

Shepard 1989). Bull trout growth decreases in the presence of sympatric char species (Gunckel and Hemmingsen 2002). In particular, bull trout are at a disadvantage in the presence of lake trout because they are preyed upon and out-competed by lake trout when the two species are sympatric (Martinez et al. 2009). A significant decline in incremental growth rates after age 6 may indicate that bull trout are having difficulty recovering weight after spawning. In 1999, bull trout lived up to age 13 and average incremental growth was 68 mm/year in Lake Pend Oreille

(Vidergar 2000). In contrast, I found that the oldest bull trout was age 14 and incremental growth averaged 45 mm/year. Estimated growth rates from other studies on bull trout in nearby systems were also higher (78–89 mm/year) than I found for bull trout in Lake Pend Oreille

(Bjornn 1961; Fraley and Shepard 1989; Vidergar 2000). Slower growth of bull trout in Lake

Pend Oreille may be attributable to limited availability of preferred prey such as kokanee salmon. If non-native species suppression diminishes competitor species populations, inter- specific density-dependent effects may decrease on bull trout, thereby leaving more preferred prey (kokanee salmon) in the lake for bull trout consumption. However, in Trestle Creek, a bull trout spawning tributary directly connected to Lake Pend Oreille, average incremental growth of adult bull trout was 28 mm/year from 2000 to 2001 and 20 mm/year from 2001 to 2002 (Downs and Jakubowski 2005). I estimated average growth of adult bull trout (≥ 400 mm) in Lake Pend

Oreille to be 38 mm/year in 2007 and 2008, an increase in average growth of 15 mm/year for adult bull trout in the lake since 2001 and 2002. This may suggest that competitor species suppression is decreasing inter-specific density dependence on adult bull trout in Lake Pend

Oreille. An increase in growth of adult bull trout in the lake indicates overall improvement in population health. Results from my study and the Trestle Creek study from 2000 to 2002 may not be directly comparable because average growth for my study was estimated using average back-calculated length at age and the Trestle Creek study used length at age of capture. Further, my study used random catches from the entire lake whereas the Trestle Creek study sampled fish captured in Trestle Creek. Although such differences in methods reduce comparability of the two studies, adult bull trout sampled for both studies forage and grow within the lake and spend only a short time in spawning tributaries, so the two studies may still be comparable.

Bull trout matured between age 4 and age 7 in Lake Pend Oreille, like in other systems

(Fraley and Shepard 1989; Pratt and Huston 1993; Rieman and McIntyre 1995). In my study, males matured earlier than females and age at maturity for all bull trout was between age 4 and 7 in Lake Pend Oreille. This may suggest that the bull trout population in Lake Pend Oreille is

18 compensating for competitor species by reducing reproductive age to adjust for fewer reproductive adults. However, this could also be caused by variability within and between different systems containing bull trout. Increased availability of preferred food items such as kokanee salmon may also have contributed to this difference in age at maturity. Competitor species suppression may be allowing bull trout to consume kokanee more readily, thereby increasing growth and allowing earlier maturation and higher fecundity rates of mature bull trout.

Abundance of bull trout estimated to be present in Lake Pend Oreille was similar in 2008

(12,513; 7,456 – 22,521; my study) and 1999 (12,134; 8,252 – 22,915; Vidergar 2000). Lake trout suppression is potentially reducing non-native species and having minimal impacts on the bull trout population in Lake Pend Oreille. Because my estimate of bull trout abundance in Lake

Pend Oreille was similar to the estimate in 1999, the population may be nearing carrying capacity (Vidergar 2000). Stable to increasing abundance is one of three draft criteria in the

Federal Recovery Plan for the Lake Pend Oreille watershed (USFWS 2002). Redd-count surveys on all known tributaries to Lake Pend Oreille are increasing (Downs and Jakubowski

2005). If redd counts are accurate, my abundance estimate should have been higher than in

1999, because total redd counts were higher in 1999 (705 redds) than in 2007 (654 redds) or

2008 (584 redds). This discrepancy between redd counts and abundance in the lake is likely because abundance in the lake includes sub-adult year classes that are not present during redd counts. Stable to increasing abundance suggests that management has restored the population to near carrying capacity.

My estimates of bull trout age structure, growth, maturity, and population density are consistent with a population that is either at or near carrying capacity or one that is limited by some other factor. Average growth slowed since 1999, possibly because of different assessment

19 methods, but more likely because of limited food resources. Kokanee salmon were on the verge of collapse by the early 2000s so bull trout needed to compete with lake trout for small numbers of kokanee. Since 2002, growth of adult bull trout may have increased in response to lake trout suppression that reduced lake trout population density and thereby increased available kokanee for bull trout. Age at maturity was between age 4 and 7, which could indicate compensatory effects of remaining numbers of competitor species, a result of increased growth in adult bull trout, or a larger sample size than during 1999. Bull trout abundance increased significantly with removal of nonnative competitor species (Renner 2005), and current suppression efforts are significantly reducing the lake trout population in Lake Pend Oreille (Hansen 2007; Hansen et al.

2010). Incidental by-catch mortality on bull trout was 1.9% in Lake Pend Oreille in 2007 and

2008, so suppression of lake trout has not negatively affected the bull trout population. Benefits of lake trout suppression are far greater than negative effects of by-catch losses of bull trout.

My estimates of biological attributes and abundance show the population to be passing all population dynamics parameters established in the draft Recovery Plan except for maintaining a harvest fishery for bull trout in the lake, which is essentially being met by incidental by-catch mortality with no ill effects on the population (USFWS 2002). Adult bull trout abundance in

Lake Pend Oreille exceeds 2,500 and population abundance is at least stable. If these patterns continue, a small harvest fishery for bull trout in Lake Pend Oreille could be established in the near future. Competitor species suppression has been necessary for maintaining bull trout in

Lake Pend Oreille and should be continued as long as it does not negatively affect bull trout. In addition, continuation of competitor species suppression will reduce predation on bull trout and increase availability of kokanee salmon for bull trout consumption.

The Clark Fork River did not historically pose a migration barrier and therefore was thought to be the largest producer of bull trout to the Lake Pend Oreille system. Since hydroelectric dam development (Cabinet Gorge Dam, Noxon Rapids Dam and Thompson Falls

Dam), the Clark Fork River has not been a migratory corridor for upstream migrating adult bull trout. I believe this to be the most significant reason that bull trout have not been able to reach historic numbers in recent years in Lake Pend Oreille. Streams directly connected to Lake Pend

Oreille have compensated for the loss of production from the lower Clark Fork River to their full capacity but have not produced the numbers historically produced by the Clark Fork River and its tributaries. Full upstream passage at Cabinet Gorge, Noxon Rapids and Thompson Falls

Dams could bolster the numbers of bull trout in Lake Pend Oreille and potentially bring adult bull trout numbers back to historic numbers. Compensation of the bull trout population found by this study is thought to be from lack of bull trout production due to the blockage of upstream migration of adult bull trout in the Clark Fork River.

Table 2. Estimated length and age at maturity for male, female, and all bull trout in Lake Pend Oreille, Idaho in 2008. First Maturity, 50% Mature, and 99.9% Mature indicate the length at which a minimum fraction, 50%, and 99.9% of the population were mature. First Maturity 50% Mature 99.9% Mature Male 350 mm 400 mm 425 mm 4.1 years 5.1 years 5.6 years

Female 425 mm 450 mm 475 mm 5.6 years 6.1 years 6.8 years

All 400 mm 425 mm 450 mm 5.1 years 5.6 years 6.1 years

Table 3. Bull trout abundance estimated using Chapman’s estimator for size classes in Lake Pend Oreille, Idaho. M = total number of marked bull trout at large at the end of tagging on 26 May 2008; C = total number of bull trout examined for tags during autumn 2008 (21 August – 7 November); R = total number of tagged fish recaptured during recapture sampling in autumn 2008; N = estimated population abundance; LL(N) = lower 95% confidence limit; and UL(N) = upper 95% confidence limit on N (Ricker 1975).

M C R N LL(N) UL(N) All LPO BLT 247 655 12 12,513 7,456 22,521 ≥350 mm 230 557 12 9,914 5,914 17,833 ≥400 mm 190 460 10 8,004 4,580 15,135 ≥425 mm 164 418 9 6,913 3,857 13,463 ≥450 mm 152 373 9 5,721 3,196 11,138 ≥500 mm 96 280 6 3,893 1,961 8,482 ≥550 mm 51 183 5 1,594 767 3,631 ≥600 mm 25 95 3 623 262 1,537 ≥650 mm 12 52 3 171 73 422 ≥700 mm 6 28 1 101 32 197

Table 4. Number sampled, estimated abundance, 95% confidence limits, percent of sample that were mature, number of mature bull trout, and 95% confidence limits for mature for bull trout at ages 1–14 in Lake Pend Oreille, Idaho on 26 May 2008. Count Chapman LL(N) UL(N) Age All N LL(N) UL(N) % Mature N Mature Mature Mature 1 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 4 3 160 95 288 0 0 0 0 5 16 852 508 1,533 0.39 328 196 591 6 40 2,130 1,269 3,833 0.5 1,065 635 1,917 7 55 2,929 1,745 5,271 0.67 1,968 1,172 3,542 8 55 2,929 1,745 5,271 0.86 2,516 1,499 4,528 9 49 2,609 1,555 4,696 0.95 2,485 1,481 4,473 10 14 745 444 1,342 0.98 732 436 1,318 11 1 53 32 96 0.99 53 32 95 12 1 53 32 96 1 53 32 96 14 1 53 32 96 1 53 32 96 Total: 235 12,513 7,456 22,521 - 9,254 5,514 16,655

Figure 1. Original distribution of bull trout in North America (McPhail and Baxter 1996).

Deep Shore

Pot Tunnel Lead Net

21 to 52 meters 52 21to Hear

274 m Wing Net

Figure 2. Trap net used for lake trout suppression in Lake Pend Oreille, Idaho (Hughes et al. 2007). Bull trout caught in trap nets and gillnets were used to estimate abundance and biological attributes of the bull trout population in Lake Pend Oreille, Idaho during 2006–2008.

14 0 0 12 0 10 0 8 0 000000 0 000000 6 o o>o o o o o o 0 4 0 2 Age Estimate Reader A Reader Estimate Age 0 0 2 4 6 8 10 12 14 16

14 12 10 8 6 4 2 Age Estimate Reader A Reader Estimate Age 0 0 2 4 6 8 10 12 14 16

14 12 10 8 6 4 2 Age Estmate Reader A Reader Estmate Age 0 0 2 4 6 8 10 12 14 16 Age Estimate Reader B Figure 3. Age-bias plots comparing age estimates by two readers (A and B) for the first (upper panel), second (middle panel), and third (lower panel) estimates for 236 bull trout caught in trap nets and gillnets in Lake Pend Oreille, Idaho during 2006–2008 (dark black line = 1:1).

30 Number 20

0 4 5 6 7 8 9 10 11 12 13 14 Age (years)

Figure 4. Catch at age of 236 bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008 (mean age = 7.6 years).

800 L = 483.97O − 193.57 0 0 0 R² = 0.61 0 0 0 0 700 g O 61>0 o~ogo 0

0 600 too o <2> 0- 0 0 0 500 8 0 0 0 fl> oo oo oo> 400 0 Oo 00 0 Fish Length (mm) Fish 300

200 1.0 1.2 1.4 1.6 1.8 2.0 Otolith Radius (mm)

Figure 5. Linear regression of bull trout total length (L) on otolith radius (O) at time of capture in gillnets and trap-nets in Lake Pend Oreille, Idaho during 2006–2008.

3 0 0

2 0

0 c9 1 0

0 0 0 g O O O § c9 80 0 0 fff° O 0 0 c9 o o a.. oo o c9cP

-3 1.0 1.2 1.4 1.6 1.8 2.0 Otolith Radius (mm)

Figure 6. Standardized residuals from a linear regression (L = –193.57 + 483.97O) of length at capture (L) against otolith radius at capture (O) for bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008.

800 0 0 0 0 0 0 0 0

600

400 Length (mm) Length 200

0 0 2 4 6 8 10 12 14 Age (years)

Figure 7. Back-calculated length at age (dots) and von Bertalanffy length-age model fitted to mean back-calculated length at age (curve) for bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008.

100% 80% 60% 40% 20% 0% 0 100 200 300 400 500 600 700 800

100%

80% 60% 40%

Percent Mature Percent 20% 0% 0 100 200 300 400 500 600 700 800

100% 80% 60% 40% 20% 0% 0 100 200 300 400 500 600 700 800

Length (mm) Figure 8. Maturity at length for all (upper panel), male (middle panel), and female (lower panel) bull trout caught in gillnets and trap nets in Lake Pend Oreille, Idaho during 2006–2008.

800 Priest Lake Upper Priest Lake 700 Flathead Lake .....········ Lake Pend Oreille 600 , ,,. .. ·· / , , ..· 500

400

300 Length (mm) Length 200

100

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Age (years) Figure 9. Average back-calculated length at age for bull trout in Priest and Upper Priest Lakes, Idaho (Bjornn 1961), Flathead Lake, Montana (Fraley and Shepard 1989), and Lake Pend Oreille, Idaho (this study).

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