Trophic Ecology of North Pacific Spiny Dogfish

TROPHIC ECOLOGY OF NORTH PACIFIC SPINY DOGFISH

(SQUALUS SUCKLEYI) OFF CENTRAL CALIFORNIA WATERS

______

A Thesis

Presented to the Faculty of

Moss Landing Marine Laboratories

California State University Monterey Bay

______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Marine Science

______

Jennifer S. Bigman Summer 2013 iii

Jennifer S. Bigman

ABSTRACT

Trophic Ecology of the North Pacific Spiny Dogfish, Squalus suckleyi, off central California by Jennifer S. Bigman Master of Science in Marine Science California State University Monterey Bay, 2013

Studies of predator-prey interactions aid in explaining community linkages, food web dynamics, and energy transfer in marine environments, and must be quantified to construct and implement fisheries management plans. Stomach content analysis (SCA) is the traditional method to determine food habits, which involves explicitly looking at the stomach contents, and results in a brief snapshot of the last meal. Stable isotope analysis (SIA) is an emerging method in marine studies that is based on using ratios of stable elements as tracers in assimilated tissue, and can result in longer-term data on the order of days to the lifetime of an organism. Nitrogen and carbon are the most common elements used for dietary data. Nitrogen is a proxy for trophic position and carbon is a proxy for the source of primary productivity. Because SIA is still in its infancy regarding studies on Chondrichthyans, it is beneficial to validate this method by concurrently studying both SIA and SCA. This study attempts to elucidate both short (SCA) and longer (SIA) term diet as well as factors affecting dietary variation of an abundant and economically important predator, Squalus suckleyi, the North Pacific Spiny Dogfish. Using SCA, three dominant prey types emerged: euphausiids, teleosts, and cephalopods. Euphausiids were found in 41% of stomachs containing food, with 87% of those containing only euphausiids. These crustaceans had the highest Prey-Specific Index of Relative Importance (% PSIRI) of 35.2 %, followed by teleosts (33.1 %) and cephalopods (27.1 %). Out of the factors season, size, geographic space, sex, and depth, only season and space were significant. The results suggested that North Pacific Spiny Dogfish consumed euphausiids during nine months of the year, and during the remaining three months, they fed on fishes and cephalopods, especially anchovies, sardines, and Market squid. The seasonal patterns in diet are though to be a reflection of prey availability. Spatial differences suggested that when offshore, dogfish fed offshore on euphausiids and when inshore, they fed on teleosts and cephalopods. Overall, North Pacific Spiny Dogfish were found to feed on prey with variable trophic positions in both inshore and offshore waters. The resulting isotopic composition of North Pacific Spiny Dogfish was consistent with the expected values of dogfish that consumed known prey species from SCA. This confirms the validity of using SIA to elucidate food habits of this species, and is more beneficial as it requires a drastically smaller sample size than SCA. Females and males did not differ in their trophic position or source of primary productivity. A slight relationship with trophic position and total length was only found in one statistical test, indicating more research to further investigate this relationship is needed. Female tropic position was higher inshore verses offshore, suggesting they feed on fishes and cephalopods inshore and euphausiids offshore. vi

vii

TABLE OF CONTENTS

PAGE

ABSTRACT ...... v LIST OF TABLES ...... viii LIST OF FIGURES ...... ix ACKNOWLEDGEMENTS ...... x

CHAPTER I: Seasonal variation in the diet of the North Pacific Spiny Dogfish, Squalus suckleyi, off central California………………………………………………………………...1 Abstract…………………………………………………………………………………..……2 Introduction………………………………………………………………………………..…..4 Materials and Methods……………………………………………………………………..….9 Results………………………………………………………………………………………..16 Discussion……………………………………………………………………………………20 Literature Cited………………………………………………………………………………32

CHAPTER II: Feeding trends of the North Pacific Spiny Dogfish, Squalus suckleyi as inferred from stable isotope analysis………………………………………………………...55 Abstract………………………………………………………………………………………56 Introduction…………………………………………………………………………………..58 Materials and Methods……………………………………………………………………….61 Results………………………………………………………………………………………..66 Discussion……………………………………………………………………………………69 Literature Cited………………………………………………………………………………78

CONCLUSIONS……………………………………………………………………………93

viii

LIST OF TABLES

Chapter I:

Table 1: Diet composition of North Pacific Spiny Dogfish……...…………………………..43 Table 2: Summary of Results of Individual RDA Models 1-10……………………………..44 Table 3: Summary of Overall RDA (Season + Longitude + Latitude) ……………………...45 Table 4: Summary of Overall RDA Diet Variable Loadings………………………………..45 Table 5: Summary of Overall RDA Factor Loadings………………………………………..45

Chapter II:

Table 1: Sample size, mean and standard deviation of δ15N and δ13C, and source of all specimens used in the food web plot…………………………………………………..…….83

Table 2: Mean and standard deviation of total length, nitrogen, and lipid corrected carbon stable isotope values for all males and females…………………………………………...…83 Appendix 1: Summary table of δ15N, δ13C, and δ13C’, total length (mm), and C:N ratio for all Spiny Dogfish collected in 2004 and 2005 in central California…………………...………..91

LIST OF FIGURES

Chapter I:

Figure 1: Map of Study Site: Monterey Bay, California, U.S.A…………………………….46

Figure 2: Length frequency histogram of female and male Spiny Dogfish specimens collected 2004-2005 from NMFS SCL Survey..…………………...………………………………….47

Figure 3: Cumulative prey curve with associated standard deviation of 11 higher prey categories…………………………...………………………………………………………..48

Figure 4: Graphical representation of %PSIRI and %IRI for the dominant prey categories..49

Figure 5: Graphical representation of the %FO of the dominant prey categories…………...50

Figure 6: Graphical representation %IRI and %PSIRI of the 11 higher prey categories……51

Figure 7: Graphical representation of the %N and %W (Graph A) and %PN and %PW (Graph B) of the 11 higher categories………………………………………………………..52

Figure 8: Graphical representation of the %FO of the 11 higher prey categories…………...53

Figure 9: Ordination biplot of factors and response variables……………………………….54

Chapter II:

Figure 1: Plot of C:N vs. d13C’ for 43 North Pacific Spiny Dogfish specimens………….…84

Figure 2: Collection site of Spiny Dogfish from central California, U.S.A………………....85

Figure 3: Length frequency histogram of females and males of Squalus suckleyi collected in 2004 and 2005 on a NMFS survey…………………………………………………………..86

Figure 4: Dual isotope plot of δ15N and δ13C’ for all individual Spiny Dogfish…………….87

Figure 5: Dual isotope food web of the Spiny Dogfish and its know prey from Chapter 1…88

Figure 6: Linear regression of δ15N and total length (mm)…………………………………89

Figure 7: Linear regression of δ13C’ and total length (mm)…………………….…………..90

ACKNOWLEDGEMENTS

I would like to start off by thanking my committee members, Drs. David Ebert, James Harvey, and Scott Hamilton. Dave, thank you for your enthusiasm, encouragement, support, and knowledge over the last 4 years. I cannot thank you enough for being so supportive as an advisor and advancing my career by introducing me to other researchers at every opportunity. Jim and Scott: thank you for being on my committee and taking the time to contribute to my thesis project. The professors at Moss Landing Marine Laboratories (MLML) are extremely dedicated and passionate about marine science and education and this is why their students become great scientists! I feel very accomplished knowing that I have been able to be a part of MLML. I would also like to thank Bruce Finney, whom I consider an honorary committee member. Thank you for your time, patience, knowledge, and always having your door open so I can bother you with a million questions. Your enthusiasm for stable isotopes and ecology is one of the reasons I want to pursue that field. Many people at MLML were instrumental in this project. I would first like to thank some past PSRC’ers: Joe Bizzarro, Simon Brown, Kelsey James, Jenny Kemper, Megan Winton, and Mariah Boyle. I cannot thank Joe Bizzarro enough for always being there to help me with prey identification, methods, statistics, and edits. Simon helped me from Day 1, and I sincerely appreciate all the time he put into this project, especially the statistics. I am glad we shared the same love for coffee or I am not sure you would have helped me as much. (Just kidding!). Thank you to Jenny Kemper and Mariah Boyle who were a constant source of knowledge for this project. I’d like to thank Kelsey James for her support, advice, and bringing some fun to the lab. Also, thank you Kelsey for filling in as the graduate advisor, and providing support for my defense and graduation. I would also like to thank some present PSRC’ers: James Kunckey and Paul Clerkin for being supportive and bringing some laughter to the lab. I would also like to thank the Geological Oceanography lab, specifically Ivano Aiello, Michelle Drake, and Justin Peglow. Ivano graciously allowed me to have a desk in his lab and provided a job, and I sincerely am thankful. Michelle and Justin: thank you for your

xi support, encouragement, and playful mockery. I would not have had nearly as much fun at MLML if it were not for you two. I would also like to thank the MLML community for being inspiring and supportive. Specifically, I would like to thank the library staff and Joan Parker, Drew Seals, John Machado, and Jocelyn Douglas. Eric Hochberg, of the Santa Barbara Natural History Museum helped with prey identification, and I’d like to thank him for his time and knowledge. Finally, I would like to thank the most important contributors to this project: my family. I cannot begin to express my gratitude to you all. Dad: This project could not have been completed without you. You have given me the best gift any dad could, and that is all the means necessary to achieve my dreams. I know you remember that I said at a very young age that I was going to be a shark biologist, and I thank you for taking me seriously and allowing me to get to this day. Your continued support, both emotionally and financially is sincerely appreciated, and I do not think I can express how thankful I am. I know that without you, I would not be at MLML and would have had a very different and difficult graduate experience. You are the best father anyone could ask for, and I only hope that if I choose to have children, I can provide them with the gift you have given me. Mom: thank you for always being a shoulder to lean on. I am so thankful to have such a wonderful and supportive mother. Your Starbucks giftcards greatly contributed to this project! I am so lucky to have you, and know that you will always be there for me no matter what comes my way. I would also like to thank my siblings: Fran, Abby, Maura, Hillary, Matthew, and Jeffrey. I love you all and thank you for being so wonderful! And finally, I would to thank my husband and best friend, Nick. You have been extremely supportive throughout this endeavor, both emotionally and financially, and I cannot thank you enough. Your undying encouragement, dedication, and love helped me complete this project. This project was funded in part by NOAA Fisheries, the National Shark Research Consortium, the Pacific Shark Research Center (PSRC), and the Council on Ocean Affairs, Science, and Technology at the California State University Monterey Bay. In addition, I received small grants from the following agencies: the Earl and Ethel Myers Oceanographic and Marine Biology Trust, the David and Lucile Packard Foundation, and the Signe Lund Memorial Scholarship. Specimens were collected in collaboration with the PSRC

xii and the NOAA Southwest Fisheries Science Center, Santa Cruz Lab. I would like to express sincere gratitude to those personnel involved in the collection of specimens for this project. This research was conducted under IACUC protocol #801.

Chapter 1: Seasonal variation in the diet of North Pacific Spiny Dogfish,

Squalus suckleyi, off central California

ABSTRACT

The diet of an abundant economically important predator, the North Pacific

Spiny Dogfish, Squalus suckleyi, was examined using traditional stomach content analysis. North Pacific Spiny Dogfish were collected during NOAA Fisheries fishery- independent trawl surveys conducted off central California from January 2004 through January 2005 by the National Marine Fisheries Service. The diet of North

Pacific Spiny Dogfish was dominated by euphausiids with a Prey-specific Index of

Relative Importance (%PSIRI) of 35.2, prey-specific number (%PN 93.6), and prey- specific weight (%PW 92.2). Euphausiids were found in 55 of 134 (41%) stomachs containing food, with 48 of those 55 (87%) containing only euphausiids. These data indicated that not only were euphausiids the most important prey item, but also when eaten, were often the only prey in a shark’s stomach. Teleost fishes were the second most important group with a %PSIRI of 33.1. The most important fish species in the diet included: anchovy, Engraulis mordax (%PSIRI=10.4), sardine,

Sardinops sagax (%PSIRI=9.0), and hake, Merluccius productus (%PSIRI=2.8).

Unidentified fishes (%PSIRI=21.3) also contributed substantially to the diet.

Cephalopods were the third most important prey group (%PSIRI=27.1). Among cephalopods, Market squid, Doryteuthis (=Loligo) opalescens (%PSIRI=6.9), Octopoda

(%PSIRI=5.6), Humboldt squid, Dosidicus gigas (%PSIRI=4.8), and Bobtail squid,

Rossia pacifica (%PSIRI=2.0) were the most important identified taxa; unidentified cephalopods also contributed substantially (%PSIRI=11.7). A redundancy analysis

(RDA) indicated that the diet of North Pacific Spiny Dogfish varied considerably with season, and that they were opportunistic generalists. During upwelling conditions, North Pacific Spiny Dogfish almost exclusively consumed euphausiids, and during all other times of the year, they fed on fishes and cephalopods, especially anchovies, sardines, and Market squid. Spatial differences also explained some dietary variability, suggesting that dogfish are feeding offshore on euphausiids and teleosts and feeding inshore on cephalopods. Size and sex were not significant factors in explaining diet variability.

INTRODUCTION

The U.S. fisheries assessment and management community has shifted its focus to ecosystem-based fisheries management (EBFM) plans, an all-inclusive approach with an overall goal of maintaining the integrity of marine ecosystems and the fisheries and other services they support (Brodziak and Link 2002, Pikitch et al.

2004, Bundy et al. 2011). In the past, fisheries have been managed using regulations of human activity (i.e. total allowable catch, season and area closures), with a focus on target species (Garcia 2003). Since the basis of EBFM plan is managing resources in the framework of their ecosystem, interactions between and among species and factors affecting the ecosystem (i.e. oceanographic processes, human activity) must be considered (Garcia 2003, Bundy et al. 2011). Studies of predator-prey interactions aid in explaining community linkages, food web dynamics, and energy transfer in marine environments, therefore are necessary for constructing an EBFM plan (Pikitch et al. 2004, Cury et al. 2005, Bundy et al. 2011). Understanding the role of upper trophic level predators, such as elasmobranchs, in marine ecosystems is critical because these predators may influence ecosystems in terms of community structure (Heithaus et al. 2010), energy transfer, and food web dynamics

(Wetherbee and Cortés 2004).

Previous researchers have investigated the diet of sharks in the family

Squalidae, because they are abundant globally (Ebert and Compagno 2012). Ebert et al. (1992) examined the feeding ecology of 15 species of sqauloid sharks from

Southern Africa and found that the most important prey items were teleosts and cephalopods. Previous studies on the diet of the North Pacific Spiny Dogfish, S.

5 suckleyi, indicated that its diet was similar to that of other squaloids. Of concern for

EBFM plan is the finding that S. suckleyi preys on commercially important species, such as salmon, herring, hake, and sardines, especially as the sharks increase in size

(Fraser 1946, Jones and Geen 1977a, Brodeur et al. 1987). Other common prey items are cephalopods, namely the commercially fished Doryteuthis genus (Sato

1935, Kaganoskaia 1937, Saunders et al. 1984), and euphausiids (Robinson et al.

1982, Ketchen 1986, Tanasichuk et al. 1991). In sum, previous research indicates that predation by some Squalus spp. is a significant source of mortality for a number of commercially important species (e.g. Bowman et al. 1984, Ketchen 1986).

Because dogfish (Squalus spp.) are highly abundant globally, increased consumption rates of commercially important species must be taken into account when constructing sustainable fisheries management plans. Furthermore, the composition of the diet of S. suckleyi may vary over seasonal and annual scales (Chatwin and

Forrester 1946, Jones and Geen 1977a, Brodeur et al. 1987), yet more work needs to be done to assess the factors that drive dietary variation.

In addition, some Squalus spp. are prey of larger elasmobranchs, e.g. blue sharks, Prionace glauca, (Harvey 1979), six gill sharks, Hexanchus griseus, (Ebert

1994, Ebert 2003), Notorynchus cepedianus, seven gill sharks (Ebert 2003), sand bar sharks, Carcharhinus plumbeus, (Stillwell and Kohler 1993), leopard sharks, Triakis semifasciata, (Ebert 2003), thresher sharks, Alopias vulpinus, (Preti et al. 2012), and white sharks, Carcharadon carcharias, (Ebert 2003). Therefore, any proposed EBFM plan must take into account these predator-prey links.

Because dogfish (Squalus spp.) are ecologically and economically important elasmobranchs that are abundantly distributed throughout the world’s oceans

(Ebert and Compagno, 2012), they have been the focus of numerous ecological interaction studies. Ecological interactions between the closely related Spiny

Dogfish, S. acanthias, and other groundfish species have been extensively studied off

Georges Bank in the Northeast Atlantic. In this system dramatic changes in species diversity and abundance of fish assemblages have occurred in response to fishing- related disturbances dating to the 1960s, which resulted in the decline of targeted benthic-associated fish species (flatfish and gadids) with corresponding increases of small elasmobranchs and pelagic fishes (Fogarty and Murawski 1998, Garrison and

Link 2000b). Because researchers examining trophic ecology found that Spiny

Dogfish exhibit high dietary overlap with the species targeted by the fishery, some researchers proposed a mechanism of competitive release to explain the increase in

Spiny Dogfish populations during the past decades (Fogerty and Murawski 1998,

Garrison and Link 2000 a, b). However it has been argued that no competition for prey resources existed in this large ecosystem because these predators are generalists, and like most fishes, have ontogenetic shifts in diet, which decrease dependence on mutual prey species. (Garrison and Link 2000a). This type of data regarding ecological interactions, elucidated via diet studies, is necessary to understand before developing an EBFM plan.

The status of North Pacific Spiny Dogfish stocks is contentious, with differing views on the management and conservation of these species (Fordham et al. 2006,

Bigman et al. in press). The North Pacific Spiny Dogfish has had a long and intense

7 history of exploitation from the 1930s until the 1980s, which is thought to have resulted in a dramatic decline in abundance, yet their status is thought to be different today. The International Union for the Conservation of Nature (IUCN) Red

List assessment lists the Northeast Pacific subpopulation as Vulnerable (Fordham et al. 2006). This assessment occurred before the split of the Spiny Dogfish and the

North Pacific Spiny Dogfish into two distinct species based on genetic, morphological, and meristic differences (Ebert et al. 2010, Verissimo et al. 2010).

The literature on the status of the North Pacific Spiny Dogfish indicates that the current population is abundant, and in a recent assessment, the IUCN Red List

Assessment for North Pacific Spiny Dogfish found that the population is stable, if not increasing (Bigman et al. in press). Because this species is highly abundant, basic life history information, such as food habits, is needed now more than ever to develop an EBFM plan.

To date, the majority of research on North Pacific Spiny Dogfish has been on inshore populations, mainly in British Columbia, with no published diet or life history studies from the southern part of its range. This research has included studies of general life history and/or review papers (Ketchen 1986), diet (Jones and

Geen 1977a, Brodeur et al. 1987, Tanasichuk et al. 1991,), age and growth (Ketchen

1975, Jones and Geen 1977b, Saunders and McFarlane 1993), reproduction

(Ketchen 1972), abundance (Saunders et al. 1984), and management/stock status

(Palsson 2009, Wallace et al. 2009). The furthest south any of these specimens were collected was off Oregon (Brodeur et al. 1987). Therefore, to fully assess the

8 ecological role of Spiny Dogfish along the west coast of the U.S., studies of trophic interactions are needed from the southern portion of its range.

Objectives and Hypothesis:

The main goal of this study is to examine the diet and trophic ecology of

North Pacific Spiny Dogfish in a previously unstudied part of its range, to fill vital data gaps in the trophic ecology of this species, which is necessary for the formulation of effective EBFM plan along the US West Coast. Specific objectives are to: (1) quantify the diet of North Pacific Spiny Dogfish using stomach content analysis and (2) investigate sources of variability in the diet of North Pacific Spiny

Dogfish.

Based upon previous studies of North Pacific Spiny Dogfish off Japan (Sato

1935), Russia (Kaganoskaia 1937), and British Columbia (Fraser 1946, Chatwin and

Forrester 1953, Jones and Geen 1977a), I hypothesized that North Pacific Spiny

Dogfish would be a generalist predator that feeds on fishes, cephalopods, and euphausiids. Differences in diet among seasons were expected, as Monterey Bay has three well-defined seasons (see below) (Skogsberg 1936, Skogsberg and Phelps

1946) that will affect the abundance and distribution of prey, thus ultimately affecting the predator. Geographic spatial variation also was expected to be a significant factor in explaining dietary variation, as prey availability may change spatially and seasonally (Andrews et al. 2009, Bellagia et al. 2012a,b, Brown et al.

2012).

MATERIALS AND METHODS

Study Area

The study area roughly encompasses Monterey Bay, California, ranging from

36˚ 54’ 36”N to 36˚ 39’ 0”N. Monterey Bay is one of the largest embayments on the

West Coast of the U.S., roughly 23 km wide with headlands at its northern and southern points. From the coastal center, Moss Landing, the Bay stretches 11-12 km to the open ocean, and reaches depths greater than 2000 meters. Monterey Bay is characterized by a deep submarine canyon that divides it roughly in half, and other irregularities in the topography of the seafloor. These irregularities and complex bathymetry have a pronounced effect on the seasonal and general distribution, migration, and relative abundance of local organisms (Skogsberg 1936). Three well- defined oceanographic seasons characterize Monterey Bay: Upwelling season (UPS),

Oceanic Current Season (OCS), and Davidson Current Season (DCS) (Skogsberg

1936, Skogsberg and Phelps 1946). From March to July, cold, nutrient-rich waters upwell during UPS. Strong southerly winds combined with the process of Ekman transport move surface waters offshore, resulting in the upwelling of deeper waters which are cold and high in nutrients (Skogsberg 1936, Skogsberg and Phelps 1946).

A weakening of these winds and upwelling characterize the OCS (August to

November), which causes the California Current to move closer to shore. From

December to February, the DCS causes the weakening of the California Current, resulting in the development of an inshore northward moving current.

Sample Collection

North Pacific Spiny Dogfish were collected during approximately monthly trawl surveys conducted during January 2004 to January 2005 along the central

California coast by the National Marine Fisheries Service (NMFS), Santa Cruz Lab.

Haul depths ranged from 93 to 417 meters.

Total length in millimeters and sex was determined for each shark before being frozen on board the vessel. Sharks were then transported to Moss Landing

Marine Laboratories for later processing by Pacific Shark Research Center personnel, in which the stomachs were excised and frozen for later sorting.

Stomach Content Analysis (SCA)

The stomachs were thawed in cold water and contents were sorted into prey categories over a 500-micrometer sieve. The prey items were blotted on paper towels, weighed (resulting in a wet weight) to the nearest 0.001 g, enumerated, and stored in 70% ethanol for later identification to the lowest taxonomic level. Any prey items that weighed less than 0.005 g were given a mass of 0.01 g for use in calculations. Identification of prey to the lowest possible taxonomic level was performed using a dissecting microscope, taxonomic keys, field guides, consultations with experts, and museum collections. For highly digested material, or in such cases where only “hard parts” (e.g., eye lenses, otoliths) remained undigested, the minimum number of individual prey items represented was recorded (Lance et al. 2001). Any highly digested material that could not be placed

11 in a taxonomic category, parasites, or any inorganic material, such as gravel or sand, was noted but excluded from further analyses.

Sample Size Sufficiency

Before performing analyses, cumulative prey curves (Ferry and Cailliet 1996) were used to assess if the number of stomachs sampled was sufficient to describe prey richness. The basis of this method lies in the fact that as sample size increases, the variability in the number of new prey items introduced into the diet should decrease. The estimated number of unique prey categories was plotted against the cumulative number of stomach samples that were examined. Prey curves were generated using the software program R (Version 2.13.1) using the “Vegan

Community Ecology package” (Oksanen et al. 2011).

Because visual examination of the stabilization of prey curves can be quite subjective, a method using linear regression was used for a quantitative assessment

(Bizzarro et al. 2007). The slope (b) of the linear regress ion through the last five sub-samples was used as a test for adequate sample size, and the criteria b ≤ 0.05 was used to signify an acceptable leveling off for the prey curve (Bizzarro et al.

2009).

Diet Description

Relative measures of prey quantity values (following Hyslop 1980,

Amundsen et al. 1996, Brown et al. 2012) were calculated, as they are the standard parameters for SCA and can be used to facilitate comparisons among studies. The

12 prey-specific abundance measures were prey-specific number (%PN) and prey- specific weight (%PW) (Eq. 1, below; Amundsen et al. 1996, Brown et al. 2012). I also calculated the average percent abundance, in terms of percent number (%N) and percent weight (%W), and frequency of occurrence (%FO) (Eq. 2, below; Hyslop

1980). The prey-specific measures (Eq.1) take into account only the stomachs in which the prey item was found. They describe diet on an “inter-individual” level, whereas percent number and weight describe the diet on a population level (Brown et al. 2012).

The following diet indices were examined for spiny dogfish gut contents:

Equation 1. Prey-specific abundance (%PNi, %PWi):

"%Aij j=1 %PAi = ni where PAi is the prey-specific abundance (number or weight), and Aij is the abundance (number or weight) for prey ! i, in an individual stomach sample j, and ni is the number of stomachs containing prey i, with n being the total number of stomachs,

Equation 2. Average percent abundance (%Ni, %Wi):

"%Aij %A = j=1 i n where Ai is the abundance (number or weight), Aij is the abundance (number or ! weight) for prey i, in an individual stomach sample j, and ni is the number of

13 stomachs containing prey i, with n being the total number of stomachs, and

Equation 3. Frequency of Occurrence (FO):

n FO = i i n where ni is the number of stomachs containing prey i, with n being the total number of stomachs !

In addition, I calculated the Prey-Specific Percent Index of Relative

Importance (%PSIRI). The %IRI, or Index of Relative Importance has been traditionally used to provide an unbiased estimate of dietary importance for different prey items, but is inherently biased as it inflates the FO value. Frequency of occurrence is built into the value of both %N and %W, so multiplying by it again introduces the bias (Brown et al. 2012). Therefore, the Index of Relative Importance

(Pinkas et al. 1971) was modified by substituting %N and %W with their corresponding prey-specific abundances, %PN and %PW (Brown et al. 2012):

PSIRIi = FO " (%PNi + %PWi ).

The Prey-Specific IRI sums to 200% and dividing by 2 results in a comparable measure to the standardized %IRI ! (Cortés 1997). One of the most important differences between measures is that the %PSIRI is additive with respect to taxonomic levels. For example, the %PSIRI of all the species in a family combined will be equal to the %PSIRI of the family, which is not the case for %IRI. This additivity allows the %PSIRI to be compared within and among species, and among studies because its values are not dependent on the levels of taxonomic

14 classification used (Brown et al. 2012). Based on the resulting %PSIRI values, all prey were grouped into 11 higher prey categories for use in statistical analyses.

These 11 categories are: anchovy, crab, euphausiids, Humboldt squid, Market squid,

Octopoda, Other Fishes, Other Squid, sardine, unidentified cephalopods, and unidentified fishes.

Statistical Analysis

To elucidate significant environmental and biological sources of dietary variability, a Constrained Redundancy Analysis (RDA) was conducted using the software program R (Version 2.13.1) with the “Vegan Community Ecology package”

(Oksanen et al. 2011. The constraining, or explanatory, variables used in the RDA were space (latitude and longitude), total length, season, depth, and sex. To decide on an overall model, the factors were all tested separately to obtain their individual significance values and total variance explained. This resulted in six single-factor models: (1) total length, (2) sex, (3) season, (4) depth, (5) latitude and longitude, and (6) longitude and latitude. Because “space” is not a one dimensional factor and includes both longitude and latitude, they were both included in the same model.

Interaction factors were tested for all 10 combinations of the factors and no models were significant at 0.05. The order in which factors were entered in this test matters as the first factor entered attempts to explain 50% of the variability and the remaining factors attempt to explain the remaining 50% of the variability.

Significance was tested with 9,999 Monte Carlo permutations. The factors that were not significant (p>0.01) were dropped from the final overall model. The overall

15 model was constructed by adding factors that were significant, and they were entered in the order of most significant to least significant. The RDA was then conducted, and significance again was tested by 9,999 Monte Carlo permutations.

Redundancy Analysis is useful for explaining how one set of variables

(response matrix) may be explained by another set (explanatory matrix) (Borcard et al. 2011). An RDA is a combination of a multivariate multiple linear regression, and principal components analysis (PCA) that can be used to construct a constrained ordination plot (Borcard et al. 2011) for visual examination. The plot has axes computed by the RDA that are linear combinations of explanatory variables

(Borcard et al. 2011). Essentially, they are a series of linear combinations, in successive order, of the explanatory variables that best explain the variation of the response matrix (Borcard et al. 2011).

An inherent problem of diet data is that some samples may share an absence of a prey category. Thus, not transforming the data appropriately can affect the interpretation of results. A Hellinger transformation is useful for community data as it is meaningful for presence-absence data such as that resulting from stomach content analysis (Oksanen et al. 2011). Without using this transformation in a RDA, samples with absent prey categories would be more closely related than that sharing the same prey category (Borcard et al. 2011). The Hellinger transformation produces a Euclidean distance where “abundance values” are first divided by the

“site total abundance” and then the result is square root transformed (Borcard et al.

2011). In this case, the “abundance values” are the %N of a prey category and the

“site total abundance” is the total abundance of all prey in a stomach. For example,

16 the %N of euphausiids in stomach #1 was divided by the total %N of all the prey in that stomach and then square root transformed. The %N was used in statistical calculations as it best describes feeding behavior (Hyslop 1980) as %W can be more biased because of water retention (weight wet) and digestion effects.

RESULTS

Specimen Characteristics

A total of 218 specimens were collected in and around Monterey Bay in 2004 and 2005 (Figure 1). Of these, 123 were males (56.4%) and 95 were females

(43.6%) (Figure 2). Females were 280 mm TL to 1058 mm TL, and males 275 mm

TL to 948 mm TL. There was a lesser frequency of female individuals > 700 mm TL and < 400 mm TL and of male individuals > 900 mm TL and < 500 mm TL. Seventy- three stomachs were empty (33.4%) and these were excluded from further analyses. Eleven specimens had no meta-data associated with them and these also were excluded from further analyses.

Sample Size Sufficiency Results

The cumulative prey curve stabilized as the slope through the last five points

=0.007 (Figure 3). This indicated that the number of samples collected and analyzed was sufficient to characterize the diet of North Pacific Spiny Dogfish at the most specific level of prey identification.

Diet Description

A total of 255 individual prey items were found, representing 37 taxonomic categories including 11 unique species (Table 1). Three prey taxa were dominant in the diet of North Pacific Spiny Dogfish: euphausiids (%PSIRI=35.2%), teleosts

(%PSIRI=33.1%), and cephalopods (%PSIRI =27.1%) (Figure 4). The %FO was fairly similar for all taxa, with teleosts being the most frequent (%FO= 44.8%), followed by euphausiids (%FO=37.9%), and cephalopods (%FO=33.1%) (Figure 5).

North Pacific Spiny Dogfish mainly consumed crustaceans, which were the dominant prey type by %PSIRI, %N, and %W. One taxon of crustaceans

(Euphausiids) comprised the overwhelming majority of all crustaceans consumed.

Euphausiids were the most important prey taxa using %PSIRI and %IRI (Figure 6), and the %N and %W for euphausiids was twice the amount of the next highest prey category, unidentified fishes (Figure 7). When consumed, euphausiids were usually the only prey item found in the stomach: out of 55 stomachs containing euphausiids,

48 contained just euphausiids (%PN=93.6%, %PW=92.2%, %FO=37.9%); (Figures

7, 8) North Pacific Spiny Dogfish commonly consumed only one prey type per meal

(Figure 7). The euphausiids consumed were commonly highly digested thus making any further taxonomic identification unlikely.

Based upon % PSIRI, teleosts were the second most important prey taxa to

North Pacific Spiny Dogfish, but the most frequently consumed (44.8% FO); (Figures

4, 5) the most important teleosts were anchovy, Engaulis mordax, (%PSIRI=10.4%), sardine, Sardinops sagax, (%PSIRI=9.0%), hake, Merluccius productus

(%PSIRI=2.8%), and unidentified fishes (%PSIRI=21.3%) (Figure 6). Although the

18 abundance in number (%N) of anchovies and sardines was similar (6.9 and 6.6%, respectively) anchovies were more common (%FO=10) in the diet of North Pacific

Spiny Dogfish (Figures 7, 8). Hake was less abundant in both number and weight

(2.76% for both) (Figure 7).

Cephalopods (%PSIRI= 27.7) also were of importance in the diet of North

Pacific Spiny Dogfish, but less so than euphausiids and teleosts (Figure 4).

Cephalopods were the least frequent (%FO=33.1) of the three major prey taxa

(teleosts, euphausiids, and cephalopods), but still contributed substantially to the diet in terms of %N and %W (Figure 5). Squid (order Teuthida) was the dominant cephalopod consumed, with a %PSIRI=12.9. Of the squid consumed, Market squid,

Doryteuthis opalescens, and Humboldt squid, Dosidicus gigas, were the major contributors to the diet in terms of abundance in both number and weight; Market squid were most abundant in both number and weight (6.6% and 6.4%, respectively), followed by Humboldt squid (%N=3.3, %W=4.7) (Figure 7). Bobtail squid, Rossia pacifica, were of little importance to the diet (%PSIRI=2). Octopoda

(%PSIRI=4.5) also contributed to the diet of North Pacific Spiny Dogfish, with the presence of two species found in the stomach, Benthoctopus leioderma

(%PSIRI=0.44) and Octopus rubesecens (%PSIRI=0.52). Unidentified cephalopods

(%PSIRI=11.7%) also were important in the diet (Figure 6).

Other, less important contributors to the diet of North Pacific Spiny Dogfish were scavenged food such as fishery offal (pectoral girdle, fish head) and one unidentified crab. Unidentified organic matter (UOM) contributed little to the diet of

North Pacific Spiny Dogfish (%PSIRI=1.66).

Sources of Dietary Variability

The constraining factors of season, longitude, and latitude explained a combined ~10.7% of variance in the diet of North Pacific Spiny Dogfish. Out of the six individual RDA models, only model numbers 3, 5, and 6 were significant (p<0.01)

(Table 2). For input into the overall model, model 6 was chosen over model 5

(model 6: p=0.0018, model 5: p=0.0014), as it was slightly more significant. The overall model thus included “season + longitude + latitude” and was statistically significant (F=3.84, p=0.0001), explaining 10.7% of the variation in the diet (Table

3). The first and second RDA axes were statistically significant thus necessitating the examination of these two axes. RDA 1 axis explained 6.7% of the variation in the diet (F=9.63, p=0.0001) and RDA 2 axis explained 2.0% of the variation in the diet

(F=2.94, p=0.0077). The loadings for both the diet variables and the factors for both

RDA 1 and RDA 2 axes are displayed in Tables 4 and 5.

Many explanatory factors and prey items loaded onto the 2 significant RDA axes (Fig. 9), RDA Axis 1 was highly significant (p=0.0001) and RDA 2 axis was marginally significant (p=0.0077), and this can be inferred as more variables loaded along RDA 1. Three factors loaded heavily on RDA 1: Euphausiids, Davidson Current

Season (DCS), and Longitude (all >0.5). Unidentified cephalopods also loaded onto

RDA 1 (0.39) but to a lesser degree. Latitude and longitude loaded heavily onto RDA

2 (both >0.5), and market squid less so (0.40). The results inferred from RDA 1 axis imply that season, specifically DCS, was associated with many prey items both

20 positively and negatively. This means that during DCS, dogfish primarily ate fish, and when it was not this season, they primarily ate euphausiids.

DISCUSSION

North Pacific Spiny Dogfish collected from Monterey Bay, California, were a generalist predator mostly consuming crustaceans, teleost fishes, and cephalopods.

These findings were supported by previous studies of North Pacific Spiny Dogfish diet in Japan (Sato 1935), Russia (Kaganoskaia 1937), and British Columbia

(Chatwin and Forrester 1953, Jones and Geen 1977a, Robinson et al. 1982).

Euphausiids were the dominant crustacean consumed, and were usually the only prey type in an individual stomach, indicating that these dogfish took advantage of an abundance of euphausiids, particularly during the upwelling season. Teleost fishes (mainly sardines and anchovies) were a frequent prey item of North Pacific

Spiny Dogfish. In reference to cephalopods, squid, particularly Market squid and

Humboldt squid, contributed to the diet substantially. Season, longitude, and latitude were the factors that explained the most dietary variation.

North Pacific Spiny Dogfish mainly consumed crustaceans, which were the most important prey type in this study. Only two taxa of crustaceans were found, with euphausiids) accounting for the majority of all crustacean biomass consumed.

Other studies have documented the presence of euphausiids in the diet of North

Pacific Spiny Dogfish (Bonham 1954, Brodeur et al 1987, Tanasichuk et al. 1991), and it has even been cited specifically that North Pacific Spiny Dogfish feed on swarms of euphausiids (Ketchen 1986). Jones and Geen (1977a) analyzed 14,796

North Pacific Spiny Dogfish stomachs from British Columbia, Canada waters, and found euphausiids were one of two dominant prey items. Although euphausiids were not the most frequent prey item (fish were most frequently observed), when eaten, euphausiids were often overwhelmingly dominant in the stomach. This indicated that these dogfish were preying upon euphausiids when they become abundant, presumably with season, supporting the generalist feeding habits of this species.

Teleosts were the second most important prey type to North Pacific Spiny

Dogfish, but the most frequently consumed. In this study, sardines and anchovies, were the most important teleosts found in the diet, with sardines being more important by mass and anchovies more frequently consumed. These prey items are also important contributors to the diet of North Pacific Spiny Dogfish in other regions, as the majority of researchers reported teleosts to be the most important prey group, with herring and sardines the most frequent, and anchovies, herring, hake, smelt, ratfish, cod, pollock, and mackerel of lesser frequency (Sato 1935,

Kaganoskaia 1937, Jones and Geen 1977a). Chatwin and Forrester (1953) found that of 229 stomachs containing food collected off British Columbia in 1953, 100% of them had euchalon (also called smelt; Thaleichthys pacificus) “or at least definite traces of them.” Bonham (1954) examined 1,100 (41% empty) North Pacific Spiny

Dogfish stomachs from 1941 to 1943 collected from Washington waters and found fishes comprised two thirds of the diet, with the main fishes being ratfish,

Hydrolagus colliei and herring, Clupea pallasi. Hake was of lesser importance to the

22 diet, but had a %PN and %PW of 100, indicating that in those stomachs that contained hake, it was the only item consumed at that time.

Cephalopods also were of importance in the diet of North Pacific Spiny

Dogfish, but less so than euphausiids and teleosts. Cephalopods were the least frequent of the three major prey taxa (teleosts, euphausiids, and cephalopods), but still contributed substantially to the diet in terms of %N and %W. Previous researchers also have found cephalopods in the diet of North Pacific Spiny Dogfish, but in no study were they the dominant prey type (Sato 1935, Kaganoskaia 1937

Chatwin and Forrester 1953). Squid (order Teuthida) was the dominant cephalopod consumed by North Pacific Spiny Dogfish in this study; Market squid and Humboldt squid were the major contributors to the diet. Market squid are abundant in

Monterey Bay as Cailliet et al. (1979) found that it dominated catches from commercial anchovy hauls in Monterey Bay sampled from 1975 to 1976. Leos

(1998) examined the effect of a short fishing closure on the biological characteristics of this squid because it is one of the largest and most important fisheries in the

Monterey Bay area. Furthermore, it is a common prey item for other elasmobranchs in this area (Robinson et al. 2007, Rinewalt et al. 2007).

Interestingly, Humboldt squid were present in the diet of North Pacific Spiny

Dogfish. This species is a large, abundant, and aggressive squid of the family

Ommastrephidae (Zeidberg and Robinson 2007). Historically, it can be found from the surface to about 1000 m off the subtropical coasts of North and South America, with a geographical center in the eastern Equatorial Pacific (Field et al. 2007,

Zeidberg and Robison 2007). However, this species was initially sighted in Monterey

Bay in 1997, appearing at the onset of a strong El Niño period followed by a near disappearance until 2001, but returned in abundance in 2002 and has been present in Monterey Bay ever since (Zeidburg and Robinson 2007). This species has greatly expanded its range both northward as far as Canada and the Gulf of Alaska and southward to southern Chile (Field et al. 2007). This expansion appears to coincide with oceanographic conditions linked to climate changes and a reduction in competing top predators (Zeidburg and Robinson 2007). The presence of this squid in the diet may potentially demonstrate a predator altering its diet to consume new prey resources that were made available potentially due to climate changes. This scenario is likely to become common in future years as oceanographic conditions change further allowing for range expansions of both predators and prey.

The condition of squid remains in the stomach shed light on how this prey was consumed, as the squid were commonly connected “chunks”, as if the Dogfish may have taken a bite out of a tentacle and swam away. Scavenging is the most likely reason as to why squid in this condition was found in the stomach contents of

North Pacific Spiny Dogfish (Eric Hochberg, Santa Barbara Natural History Museum, personal communication.). Group foraging could explain the consumption of this large and voracious squid by a smaller North Pacific Spiny Dogfish, but is unlikely as the squid form schools as well (J. Bizzarro, personal communication).

As North Pacific Spiny Dogfish are extremely abundant fish and comprise a large biomass (Ebert 2003, Ebert and Compagno 2012, Bigman et al. in press), they likely have a large predatory impact on their prey species, many of which are commercially valuable and constitute large volume fisheries in the same spatial area

24 in which North Pacific Spiny Dogfish are feeding. Market squid dominated commercial landings of marine species in 2010 in California waters and they made up the largest volume and had the greatest value of any marine fishery with over

13,000 tons landed with an ex-vessel value of approximately $73.8 million dollars

(Zeidburg et al. 2006, CA Dept. Fish and Game 2011). Sardines also support a large fishery from British Columbia, Canada, to Baja California, Mexico, and in 2010, landings were 33,658 tons with an ex-vessel value of $4.3 million (CA Dept. Fish and

Game 2011). The landings have steadily increased since the resurgence of the fishery in 1999 (CA Dept. Fish and Game 2011). The anchovy fishery exists along

Washington, Oregon, and California, and landings average about 8,500 tons per year during the last 10 years with the majority of fish landed in California (CA Dept. Fish and Game 2011). Hake are the largest single groundfish resource (Dark and Wilkins

1994, Buckley and Livingston 1997) comprising the largest fishery by volume off the

Pacific Coast of the U.S. with landings exceeding 267,000 mt in some years (Lomeli and Wakefield 2011). As such, the predatory impact of North Pacific Spiny Dogfish on these commercially important prey species discussed above must be taken into account to ensure the fishery remains viable. In addition, prey species of North

Pacific Spiny Dogfish consume each other, thus increasing the existing competition.

For example Northern anchovies also consume euphausiids (Brodeur et al. 1987), demonstrating that potential competition exists even among species consumed by

North Pacific Spiny Dogfish.

In addition to consuming commercially important species, North Pacific

Spiny Dogfish also consumes similar prey items to economically important fishes

25 such as salmon, sardines, Pacific hake, Pacific herring, and Pacific cod (Gadus macrocephalus). Interactions between Pacific salmon (Oncorhynchus spp.) and North

Pacific Spiny Dogfish have been well studied because it was long believed that they preyed upon salmon smolts. However, the evidence indicates that although North

Pacific Spiny Dogfish consume salmon, predation pressure is relatively minor and focused mostly on juveniles (Beamish et al. 1992, Jones and Geen 1977a). Although

North Pacific Spiny Dogfish do not primarily prey upon salmon, these fishes do consume similar resources, therefore, and have overlapping niches. Euphausiids were the major prey of Pacific salmon species, specifically O. kisutch and O. tschawytscha (Fraser 1946). Pacific hake feed on euphausiids and squid, but most of their diet consists of small-schooling fishes, such as the Northern anchovy and the

Pacific herring and cannibalism on Pacific hake juveniles (Buckley and Livingston

1997). Because of the large biomass of Pacific hake off the U.S. West Coast, they likely have a large predatory impact on some of the same prey species that occur commonly in the diet of North Pacific Spiny Dogfish. Understanding these types of predator-prey interactions are necessary to construct an EBFM plan, as it provides insight to interactions among sympatric species.

Seasonal shifts in oceanographic conditions were an important factor in explaining Spiny Dogfish diet variation. Intra-species seasonal diet differences of elasmobranchs have been well documented (Chatwin and Forrester 1953, Cortés et al. 1996, Rinewalt et al. 2007). These differences mirror seasonal movements of predator and prey species (Wetherbee and Cortés 2004). Laptikhovsky et al. (2001) found seasonal diet shifts in a closely related species, S. acanthius, from the

Falklands shelf. That Squalus species fed primarily on herring in the winter and squid in the summer (Laptikhovsky et al. 2004). Jones and Geen (1977a) reported that North Pacific Spiny Dogfish off British Columbia had a seasonal dietary shift from fishes in winter to invertebrates in summer.

The data presented here demonstrated that S. suckleyi exhibited a major dietary shift from euphausiids to anchovies/sardines with seasonal changes in oceanographic conditions. The ordination plot indicates that when it was DCS

(December to February; weakening of California Current, little to no thermocline, warm upper waters, inshore northward current) North Pacific Spiny Dogfish were not consuming euphausiids but instead consuming anchovies, sardines, and cephalopods. During OCS, North Pacific Spiny Dogfish are also not consuming euphausiids. However, North Pacific Spiny Dogfish are almost exclusively feeding on euphausiids during UPS when strong upwelling brings nutrient rich water to surface those likely benefit populations of this prey.

The abundance of euphausiids varies on long and short-term time scales.

Long-term variations are primarily due to large-scale oceanographic patterns such as El Niño and La Niña events, and short-term time scales are due to localized oceanographic processes (Benson et al. 2002, Marinovic et al. 2002, Croll et al.

2005). For example, Brodeur and Pearcy (1992) reported the prevalence of euphausiids in the diet of 18 different species of fish caught off Oregon and

Washington from 1981 to 1984 varied annually depending upon presence and strength of upwelling. During years with intense upwelling in the California Current

(non El Niño years), there was a greater prevalence of euphausiids in the fishes’

27 stomachs, but during years leading up to and during El Niño events (reduced to absent upwelling), the prevalence of euphausiids in the stomachs decreased

(Brodeur and Pearcy 1992).

Seasonal upwelling in Monterey Bay occurs during spring and early summer

(late March/early April to late October/early November), overlapping with the seasons UPS and OCS used in this study (Marinovic et al. 2002, Croll et al. 2005).

During UPS, euphausiids were the dominant prey item consumed by North Pacific

Spiny Dogfish in this area, coinciding with the period of greatest euphausiid abundance in Monterey Bay (Marinovic et al. 2002, Croll et al. 2005). After the peak of productivity in June, euphausiids begin to decrease into the DCS season, when upwelling decreases (PFEL 2012).

There was a positive relationship between UPS and euphausiid consumption although the relationship was expected to be stronger in the RDA. No euphausiids were consumed during DCS and the RDA displays this in showing a strong negative relationship between the two. Although no relationship existed between euphausiids and OCS (right angles between vectors), they were consumed during this season. An explanation for the weak positive relationship between euphausiids and UPS and no relationship between euphausiids and OCS is the lag of seasonal abundances of zooplankton behind productivity cycles (Marinovic et al. 2002, Croll et al. 2005). Croll et al. (2005) summarized previous research that indicated primary production lags behind the beginning of upwelling by 6-10 days in

Monterey Bay, and the euphausiid abundance lags behind the primary productivity by 3 to 4 months. This lag would explain why the UPS season did not have a direct

28 strong positive relationship with euphausiid consumption but indirectly showed that through the relationship between euphausiids and DCS and OCS, respectively.

Other abundant prey items found in the diet of North Pacific Spiny Dogfish whose abundance may change seasonally included sardines, anchovies, and Market squid. Pacific sardines are the most abundant pelagic fish species in this California

Current ecosystem (Zwolinski et al. 2011), and Chavez et al. (2003) found them to be abundant during warm periods, or times when upwelling was decreased. Their greater abundance and presence during non-upwelling months would explain their presence in the diet of North Pacific Spiny Dogfish in this study during DCS, or when upper waters are warmer and upwelling is weak and/or non-existent. Anchovies are another important prey item of North Pacific Spiny Dogfish in this study, and a possible explanation for this is the large abundance of anchovies in central

California (Ahlstrom 1966). Market squid was present in North Pacific Spiny Dogfish stomachs during OCS, late summer through late fall when the winds and upwelling weaken. Previous researchers have suggested that squid of the genus Doryteuthis are more abundant during warmer months, and that in Monterey Bay, landings of

Market squid decreased with lower temperatures (McInnis and Broenkow 1978,

Robin and Dennis 1999, Zeidberg et al. 2006). This is evident in this study, as the dominant prey type during OCS was market squid.

Spatial differences were significant in explaining dietary variability in this study, and it is well documented that elasmobranch dietary differences can be explained by both large and small-scale geographic differences (Stillwell and Kohler

1982, Bellagia et al. 2012a,b, Brown et al. 2012,). Many researchers found spatial

29 differences were significant in explaining variability in the trophic ecology of some

Squalus spp. (Rae 1967, Jones and Geen 1977a, Alonso et al. 2002). Andrews et al.

(2009) found variability in the trophic ecology with geographic zone, but the study encompassed a much larger study area including samples from Gulf of Alaska to

Washington (Andrews et al. 2009). Considering the relatively small spatial area, the significance of spatial differences in this study is surprising, yet variation in the diet even between locations in close proximity to one another has been documented

(Ajemian et al. 2012, Brown et al. 2012, Drymon et al. 2012). As species abundance often reflects latitudinal and longitudinal gradients (Rinewalt 2007), prey availability may be influenced by latitude and longitude. Since longitude and euphausiids were positively correlated in the RDA, it can be inferred that with increasing longitude (moving offshore) the abundance of euphausiids increase as well. Furthermore, Croll et al. (2005) found that in Monterey Bay, the submarine canyon walls may concentrate prey. Because euphausiids were the most important prey for North Pacific Spiny Dogfish in this study, it can be suggested that North

Pacific Spiny Dogfish may have been moving offshore to feed on euphausiids, but feeding on other prey while inshore. This may explain why spatial variation is significant in the RDA, despite the small study area.

Although the size range of Spiny Dogfish collected in this study was greatly truncated due to the lesser frequency of small and large specimens, nearly the total size distribution of females and males was examined. Size at parturition for both males and females has been reported to be 23-30 cm TL, with an average size of 26-

27 cm TL (Ketchen 1986). Maximum female total length is 130 cm TL (Ketchen

1972, 1975). These truncated distributions could be explained by size and segregation tendencies (see below) of this species. Individuals of intermediate sizes could be more common at the depths sampled in this study. North Pacific Spiny

Dogfish segregate by size and sex, with adults being more demersal and juveniles common in the water column (Ketchen 1986). Further research of smaller and larger females and males would greatly benefit this study because it potentially can determine if size explains a significant percentage of dietary variability.

The prey items found in these stomachs were mostly highly digested, complicating identification and quantification. This was especially true of small, soft-bodied prey items such as euphausiids, all of which were highly digested and could only be identified to order. The most common identifiable part of euphausiids were the numerous small, black eyes. Thus, eyes and “hard parts” (otoliths, cephalopod beaks, and vertebrae) were the most useful in identifying prey found in the stomachs. Although prey items such as squid and fishes are commonly reported in other studies (Sato 1935, Kaganoskaia 1937, Saunders et al. 1984), “hard parts” were uncommon in the stomachs of the North Pacific Spiny Dogfish we collected in central California. Thus, a majority of the prey had to be identified based upon the highly degraded prey items, which likely affected the taxonomic resolution.

Overall, the North Pacific Spiny Dogfish was a generalist predator that experienced dietary shifts with season and geographic space. They primarily ate euphausiids during two of three seasons, consuming teleosts and cephalopods for the remainder of the year. Important fishes to the diet were sardines and anchovies, and important cephalopods were Market and Humboldt Squid. Spiny Dogfish diet

31 changed seasonally, mirroring the abundance of certain prey in Monterey Bay. Spiny

Dogfish diet also varied with geographic space, indicating that Dogfish consumed euphausiids while offshore and teleosts and cephalopods while inshore.

LITERATURE CITED

Ahlstrom, E. 1966. Co-occurrences of sardine and anchovy larvae in the California Current region off California and Baja, California. California Coop. Oceanic Fisheries Investigation. Vol. 11, July 1965 to July 1966.

Ajemian, M.J., Powers, S.P. 2012 Habitat-specific feeding by cownose rays (Rhinoptera bonasus) of the northern Gulf of Mexico. Envr. Biol Fishes. 95(1):79-97

Ainley, D.G., Spear, L.B., Allen, S.G. 1996. Variation in the diet of Cassin’s auklet reveals spatial, seasonal, and decadel occurrence patterns of euphausiids off California, USA. Mar. Ecol. Prog. Ser. 137:1-10.

Allen, B.R., and Cliff, G. 2000. Sharks caught in the protective gill nets of Kwazulu- Natal, South Africa. 9. The spinner shark (Carcharhinus brevipinna) (Muller and Henle). S. Afr. J. Mar. Sci. 22:199-215.

Alonso, M.K., Crespo, E.A., Garcia, N.A., Pedraza, S.N., Mariotti, P.A., Mora, N.J. 2002. Fishery and ontogenetic driven changes in the diet of the spiny dogfish, S. acanthias, in Patagonian waters, Argentina. Environmental Biology of Fishes 63:193-202.

Amundsen, P. A., H. M. Gabler, and F. J. Staldvik. 1996. A new approach to graphical analysis of feeding strategy from stomach contents data—modification of the Costello (1990) method. Journal of Fish Biology 48:607-614.

Andrews, A.G. 2010. Variation in the trophic position of Spiny Dogfish (Squalus acanthias) in the northeastern Pacific Ocean: an approach using carbon and nitrogen stable isotopes. University of Alaska Fairbanks. Master’s thesis, 106 pgs.

ASMFC (Atlantic States Marine Fisheries Commission). 2008. Spiny Dogfish Technical Committee report. Summary of October 16, 2008 meeting. ASMFC, Alexandria, Virginia.

Baduini, C. L. 1995. Feeding ecology of the basking shark (Cetorhinus maximus) Relative to distribution and abundance of prey. Master’s thesis, Moss Landing Marine Laboratories, San Jose State University.

Beamish, R. J., Thomson, B.L., McFarlane, G.A. 1992. Spiny dogfish predation on Chinook and Coho Salmon and the potential effects on hatchery-produced salmon. Transactions of the American Fisheries Society. 121:444-455.

Beamish, R. J.; McFarlane, G.A.; Sweeting, R. M., Neville, C. M. 2009. Keynote address: the sad history of dogfish management. Pages 1-10 in V.F. Gallucci, G.A. McFarlane, and G.G. Bargmann, editors. Biology and management of dogfish sharks. American Fisheries Society, Bethesda, Maryland.

Bellagia, M., Figueroa, D.E., Sànchez, F., and Bremec, C. 2012a. The feeding ecology of Mustelus schmitti in the southwestern Atlantic: geographic variations and dietary shifts. Envr. Biol. Fish. 95(1):99-114.

Bellagia, M., Figueroa, D.E., Sànchez, F., and Bremec, C. 2012b. Long-term changes in the spiny dogfish (Squalus acanthias) trophic role in the southwestern Atlantic. Hydrobiologia 684:57-67.

Benson, S.R., Croll, D.A. Marinovic, B. B., Chavez, F.P., Harvey, J.T. 2002. Changes in the cetacean assemblage of a coastal upwelling ecosystem during El Nino 1997-98 and La Nina 1999. Prog. Oceanogr. 54:279-291.

Bigelow, H., and Schroeder, W. 1953. Fishes of the Gulf of Maine. Fishery Bulletin of the Fish and Wildlife Service, 74:88-99.

Bigman, J.S., Ebert, D.A., and Goldman, K.J. 2013. Squalus suckleyi. In: IUCN 2013. IUCN redlist of threatened species.

Bizzarro, J.J., H.J. Robinson, C.S. Rinewalt, and D.A. Ebert. 2007. Comparative feeding ecology of four sympatric skate species (Bathyraja and Raja spp.) off central California, U.S.A. Environmental Biology of Fishes 80:197–220.

Bizzarro, J.J., Smith, W.D., Castillo-Geniz, J.L., Ocampo-Torres, A., Marquez-Farias, J.F. Hueter, R.F. 2009. The seasonal importance of smell coastal sharks and rays in the artisanal elasmobranch fishery of Sinaloa, Mexico. PANAMJAS 4(4):513-531.

Bonham, K. 1954. Food of the dogfish, Squalus acanthias. Fisheries Research Paper, Vol. 1. Washington Department of Fisheries, 1954. p. 25-36.

Borcard, D., Gillet, F., and Legendre, P. 2011. Numerical Ecology with R. Use R! series. Springer, NY.

Bowman, R., Eppi, R., Groslein, M. 1984. Diet and consumption of spiny dogfish in the Northwest Atlantic. ICES C.M. 1984.

Brodeur, R.D. and Pearcy, W.G. 1992. Effects of environmental variability on trophic interactions and food web structure in a pelagic upwelling system. Mar Ecol. Prog. Ser 84:101-119.

Brodeur, R.D., H.V. Lorz, and W. G. Pearcy. 1987. Food habits and dietary varability of pelagic nekton off Oregon and Washington, 1979-1984. U.S. Dept. Commer. NOAA Technical Report, NMFS 57, 32 pp.

Brodeur, R.D.; Fleming, I.A.; Bennett, J.M., and Campbell, M.A. 2009. Summer distribution and feeding of spiny dogfish off the Washington and Oregon coasts. Pages 67-76 in Biology and management of dogfish sharks. American Fisheries Society, Bethesda, Maryland

Brodziak, J., and J. Link. 2002. Ecosystem-based fishery management: what it is and how can we do it? Bulletin of Marine Science, 70(2):589-611.

Brown, S. C. 2010. Diet composition of Bathyraja interrupta (Gill and Townsend, 1897) and Bathyraja aleutica (Gilbert, 1896), from the Northern Gulf of Alaska Continental Shelf. Moss Landing Marine Laboratories Master’s Thesis, 53 pp.

Brown, S.C., Bizzarro, J.J., Caillet, G.M., Ebert, D.A. 2012. Breaking with tradition: Redefining measures for diet description with a case study for the Aleutian skate, Bathyraja aleutica (Gilbert 1896). Envr. Biol. Fish 95: 3-20.

Buckley, T.W. and Livingston, P.A. 1997. Geographic variation in the diet of the Pacific hake, with a note on cannibalism. CalCOFI Report Vol. 38: 53-62.

Bundy, A., Link, J.S., Smith, B.E., and Cook, A.M. 2011. You are what you eat, whenever or wherever you eat it: an integrative analysis of fish food habits in Canadian and U.S.A. Journal of Fish Biology. 78(2):514-39.

Cailliet, G.M., Karpov, K.A., and Ambrose, D.A. 1979. Pelagic assemblage as determined from purse seine and large midwater trawl catches in Monterey Bay and their affinities with Loligo Opalescens. Calcofi Rep., Vol 20.

California Department of Fish and Game, 2011. Review of selected California Fisheries for 2010: Coastal pelagic finfish, Market squid, Ocean salmon, groundfish, highly migratory species, Dungeness crab, Spiny lobster, Spot prawn, Kellet’s whelk, and white seabass. CalCOFI Rep. Vol 52. 23pgs.

Chatwin, B.M. and Forrester, C.R. 1953. Feeding habits of dogfish (Squalus suckleyi (Girard)). Fisheries Research Board of Canada, Progress Report #95, pp. 35- 38.

Chavez, F.P., Ryan, J.R., Lluch-Cota S.E., Niquen, M.C. 2003. From anchovies to sardines and back: multidecadal change in the Pacific Ocean. Science Vol 299: 217-221.

Clarke, M.R., Clarke, D.C., Martins, H.R., and da Silva, H.M. 1996. The diet of the blue shark (Prionace glauca L.) in Azorean waters. Arquipelago Life Mar. Sci. (Ponta Delgada) 14A:41-56.

Cliff, G. and Dudley, S.F.J. 1991. Sharks caught in the protective gill nets of Natal, South Africa. 5. The Java shark (Carcharhinus ambionensis) (Muller and Henle) S. Afr. J. Mar. Sci. 11:443-453.

Compagno 1984. FAO species catalogue, vol. 4, pt. I. Sharks of the world. An annotated and illustrated catalogue of shark species known to date, Food and Agriculture Organization of the United Nations, Rome.

Compagno, L., Dando, M. and Fowler, S. (2005). A Field Guide to the Sharks of the World. Harper Collins Publishers Ltd., London.

Cortés, E. 1997. A critical review of methods of studying fish feeding based on analysis of stomach contents: application to elasmobranch fishes. Can. J. Fish. Aquat. Sci. 54: 726–738.

Cortés, E., Manire, C.A., and Hueter, R.E. 1996. Diet, feeding habits, and diel feeding chronology of the bonnethead shark, Sphyrna tiburo, in southwest Florida. Bull. Mar. Sci. 58:353-367.

Cury, P.M., Mullon, C., Garcia, S.M., Shannon, L.J. 2005. Viability theory for an ecosystem approach to fisheries. ICES J. Mar. Sci. 62(3):577-584.

Croll, D.A., Marinovic, B.B., Benson, S.R., Chavez, F.P., Black, N., Ternullo, R., and Tershy, B.R. 2005. From wind to whales: trophic links in a coastal upwelling system. Mar Ecol Prog Ser 289: 117-130.

Dark, T.A. and Wilkins, M.E. 1994. Distribution, abundance, and biological characteristics of groundfish off the coast of Washington, Oregon, and California, 1977-1986. U.S. Dept. Commer., NOAA Tech. Rep. NMFS 177, 73 pp.

Demirhan, S.A. and Seyhan, K. 2007. Life history of spiny dogfish, Squalus acanthias (L. 1758) in the southern Black Sea. Fisheries Research 85:210-216.

Drymon, J.M. Powers, S.P., Carmichael, R.H. 2012. Trophic pplasticity in Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) from north Central Gulf of Mexico. Envr. Biol. Fish, 95(1)21-35.

Ebert, D.A. 1991. Observations on the predatory behaviour of the sevengill shark, Notorynchus cepedianus. S. Afr. J. mar. Sci. 11: 455-465.

Ebert, D.A. 2003. Sharks, Rays, and Chimaeras of California. University of California Press, Berkeley, California, 284 p.

Ebert, D.A., Compagno, L.J.V., and Cowley, P.D. 1992. A preliminary investigation of the feeding ecology of squaloid sharks off the West Coast of Southern Africa. South African Journal of Marine Science, 12:601-609

Ebert, D.A., W.T. White, K.J. Goldman, L.J.V. Compagno, T.S. Daly-Engel, and R.D.Ward. 2010. Resurrection and redescription of Squalus suckleyi (Girarg, 1854) from the North Pacific, with comments on the Squalus acanthias subgroup (Squaliformes: Squalidae). Zootaxa 2612: 22-40.

Field, J.C., Baltz, K., Walker, W.A., Phillips, J.A. 2007. Range expansion and trophic interactions of the Jumbo Squid, Dosidicus gigas, in the California current. CalCOFI Rep., Vol 48: 131-146.

Ferry, L.A., and G.M. Cailliet. 1996. Sample size sufficiency and data analysis: are we characterizing and comparing diet properly? p. 71–80. In: Feeding ecology and nutrition in fish: proceedings of the symposium on the feeding ecology and nutrition in fish. (MacKinlay, D., and Shearer, K., eds). International Congress on the Biology of Fishes, San Francisco, CA, 14–18 July 1996.

Fogarty, M.J. and Murawski, S. A. 1998. Large-sclae disturbance and the structure of marine systems: fishery impacts on Georges Bank. Ecological Applications, 8(1):S6-S22.

Ford, E. 1921. A contribution to our knowledge of the life-histories of the dogfishes l anded at Plymouth. J Marine Biol. Association of the UK. N.S. 12(3):469-505.

Fordham, S., Fowler, S.L., Coelho, R., Goldman, K.J. and Francis, M. 2006. Squalus acanthias (Northeast Pacific subpopulation). In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4.. Downloaded on 27 October 2010.

Fordham, S.V. 2009. Conservation of Atlantic spiny dogfish under U.S. law and CITES. Pages 411-424 in Biology and Management of Dogfish Sharks. American Fisheries Society, Bethesda, Maryland.

Francis, R.C. 1983. Population and trophic dynamics of Pacific hake (Merluccius productus). Can. J. Fish. Aquat. Sci. 40:1925-1943.

Fraser, C. McLean. 1946. Food habits of fishes. Transactions of the Royal Society of Canada. 40: 33-39.

Garcia S. M., Zerbi A., Aliaume C., Do Chi T., Lasserre G. The ecosystem approach to fisheries. 2003. Issues, terminology, principles, institutional foundations, implementation and outlook. FAO Fisheries Technical Paper, 443. 71 pp

Garrison, L.P., Link, J.S. 2000a. Dietary guild structure of the fish community in the Northeast United States continental shelf ecosystem. Marine Ecology Progress Series 202:231-240.

Garrison, L.P., Link, J.S. 2000b. Fishing effects on spatial distribution and trophic guild structure og the fish community in the Georges Bank region. ICES J. Mar. Sci., 57(3):723-730.

Harvey, J.T. 1979. Aspects of the life history of the blue shark, Prionace glauca L., in MontereyBay, California. Moss Landing Marine Laboratory. Master’s thesis, 86 pgs.

Heithaus, M.R., Frid, A., and Vaudo, J.J. 2010. Unraveling the ecological importance of elasmobranchs. Sharks and Their Relatives II: Biodiversity, Adaptive Physiology, and Conservation. Eds., Carrier, J.C., Musick, J.A., and Heithaus, M.R. CRC Press 2010.

Hickling, C.F. 1930. A contribution towards the life-history of the Spurdog. J of Marine Biol. Assoc. U.K. 16:529-576.

Holden, M.J. 1966. The food of the spurdog, Squalus acanthias (L.). J. Const. Int. Explor. Mer. 30:255-266

Hyslop, E.J. 1980. Stomach contents analysis: A review of methods and their application. Journal of Fish Biology 17: 411–429.

Jensen, A.C. 1966. Life history of the spiny dogfish. U.S. Fish and Wildlife Service Fishery Bulletin. 63:527-551.

Jones, B.C. and Geen, G.H. 1976. Taxonomic reevaluation of the spiny dogfish (Squalus acanthias L.) in the northeastern Pacific ocean. J. Fish. Res. Board Can., 33(11): 2500–2506.

Jones, B.C. and Geen, G.H. 1977a. Food and feeding of spiny dogfish (Squalus acanthias) in British Columbia waters. Journal of Fisheries Research Board Canada. 34: 2067-2078.

Jones, B.C. and Geen, G.H. 1977b. Age determination of an elasmobranch (Squalus acanthias) by x-ray spectrometry. J. Fish. Res. Board Can., 34(1): 44–48.

Kaganovskaia, S.M. 1937 Materials on the fisheries biology of spiny dogfish (Squalus acanthias L.). Izv. TINRO 10:105-115.

Ketchen, K.S. 1972. Size at maturity, fecundity, and embryonic growth of the spiny dogfish (Squalus acanthias) in British Columbia waters. Journal of Fisheries Research Board of Canada. 29:1717-1723.

Ketchen, K.S. 1975. Age and growth of dogfish, Squalus acanthias, in British Columbia waters. Journal of the Fisheries Research Board of Canada 32:43- 59.

Ketchen, K.S. 1986. The Spiny Dogfish (Squalus acanthias) in the Northeast Pacific and a History of its Utilization. Canadian Special Publication of Fisheries and Aquatic Sciences 88. 78pgs.

Kohler, N.E. 1987. Aspects of the feeding ecology of the blue shark, Prionace glauca in the Western North Atlantic, doctoral dissertation, University of Rhode Island, Kingston.

Lance, M.M., A.J. Orr, S.D. Riemer, M.J. Weise, and J.L. Laake. 2001. Pinniped Food Habits and Prey Identification Techniques Protocol. AFSC Processed Report 2001-2004. NMFS Alaska Fisheries Science Center.

Laptikhovsky, V.V., Arkhipkin, A.I., and Henderson, A.C. 2001. Feeding habits and dietary overlap in spiny dogfish Squalus acanthias (Squalidae) and narrowmouth catshark Schroederichthys bivius (Scylorhinidae). J. Mar. Biol. Ass. UK. 81: 1015-1018.

Leos, R.R. 1998. The biological characteristics of the Monterey Bay squid catch and the effect of a two day per week fishing closure. Calcofi Rep. Vol 39:204-211.

Link, J.S.; Garrison, L.P.; Almeida, F.P. 2002. Ecological interactions between elasmobranchs and groundfish species on the northeastern U.S. continental shelf I. Evaluation of predation. North American Journal of Fisheries Management 22:550-562.

Livingston, P.A. and Bailey, K.M. 1985. Trophic role of the Pacific whiting, Merluccius productus. Mar. Fish. Rev. 47(2):16-22.

Lomeli, M.J.M., and Wakefield, W.W. 2012. Efforts to reduce Chinook (Oncorhynchus Tshawytscha) and rockfish (Sebastes spp.) bycatch in the U.S. west coast Pacific hake (Merluccius productus) fishery. Fisheries Research 119-120: 128-132.

Lyle, J.M. 1983. Food and feeding habits of the lesser spotted dogfish, Scyliorhinus canicula, (L.), in the Isle of Man waters. J. Fish. Biol. 23:725-737.

Marinovic, B.B., Croll, D.A., Gong, N., Benson, S.R., Chavez, F.P. 2002. Effects of the

1997-199 El Nino and La Nina events on zooplankton abundance and euphausiid community composition within the Monterey Bay coastal upwelling. Prog Oceanogr 54:265-277.

McFarlane GA, Beamish RJ. 1987. Validation of the dorsal spine method of age determination for spiny dogﬁsh. In: Summerfelt RC, Hall GE (eds) Age and growth of ﬁsh. Iowa State University Press, Ames, Iowa, pp 287–300.

McInnis, R.R. and Broenkow W.M. 1978. Correlations between squid catches and oceanography conditions in Monterey Bay, California. Fish and Fame Fish Bulletin 169:1616-170.

Oksanen, J., Blanchet, G.F., Kindt,R., Legendre, P., and O’Hara, R.B. 2011. Vegan: Community Ecology Package. R package version 117-8. http://CRANR- Projector/package=vegan.

Olsen, A.M. 1984. Synopsis of biological data on the school shark Galeorhinus australis, FAO Fisheries Synopsis 139. Fisheries and Aquaculture Department, Food and Agriculutural Organization, Rome.

Pacific Coast Groundfish Fishery Management Plan for the California, Oregon, and Washington Groundfish Fishery, as Amended through Amendment 19. 2008. Pacific Fishery Management Council. 167 pgs.

Pacific Fisheries Information Network PacFIN report #310 dated, 22 Nov. 2010. Pacific States Marine Fisheries Commission, Portland Oregon, www.pfmc.org

Palsson, W.A. 2009. The Status of Spiny Dogfsh in Puget Sound. Pages 53-65 in Biology and Management of Dogfish Sharks. American Fisheries Society, Bethesda, Maryland

Pikitch, E.K., Santora, C., Babcock, E.A., Bakun, A., Bonfil, R., Conover, D.O., Dayton, P., Doukakis, P., Fluharty, D., Heneman, B., Houde, E.D., Link, J., Livingston, P.A., Mangel, M., McAllister, M.K., Pope, J., Sainsbury, K.J. 2004. Ecosystem-based fishery management. Science, 305:346-347.

Pinkas, L.M., Oliphant, S., Iverson, I.L.K. 1971. Food habits of albacore, bluefin tuna, and Bonito in California waters. California Dept. of Fish and Game 152:1-105.

Platell, M.E., Potter, I.C., Clarke, K.R. 1998. Resource partitioning by four species of elasmobranchs (Batoidea: Urolophidae) in coastal waters of temperate Australia. Mar. Biol. 131:719-734.

Preti, A. Soykan, C., Dewar, H., Wells, R.J.D., Spear, N., Kohin, S. 2012. Comparative feeding ecology of the shortfin mako, blue, and thresher sharks in the California Current. Envr. Biol. Fish. 95(1)127-146.

Rae, B.B. 1967. The food of the dogfish, Squalus acanthias L.. Marine Research, Edinburgh, H.MS.O. Edinburg, Scotland. 19 pgs.

Rago, P.J. and Sosebee, K. 2009. The agony of recovery: scientific challenges of spiny dogfish recovery programs. Pages 343-372 in Biology and management of dogfish sharks. American Fisheries Society, Bethesda, Maryland.

Rinewalt, C.S., Ebert, D.A., and Cailliet, G.M. 2007. Food habits of the sandpaper skate, Bathyraja kincaidii (Garman 1908) off central California: seasonal variation in diet linked to oceanographic conditions. Environmental Biology of Fishes, Special Issue: Skates. 80(2-3):147-163.

Robin, J.P. and Dennis, V. 1999. Squid stock fluctuations and water temperature temporal analysis of English Channel Loliginidae. J. Appl. Ecol. 36:101-110.

Robinson, C.K., Lapi, L.A., Carter, E.W. 1982. Stomach contents of spiny dogfish (Squalus acanthias) caught near the Qualicum and Fraser Rivers, April-May 1980-81. Canadian Manuscript Report of Fisheries and Aquatic Sciences 1656. iii+21pp.

Robinson, H.J., Cailliet, G.M., and Ebert, D.A. 2007. Food habits of the longnose skate, Raja rhina (Jordan and Gilbert 1880), in central California waters. Environmental Biology of Fishes, Special Issue: Skates. 80(2-3):165-179.

Sato, S 1935. A note on the Pacific dogfish, Squalus suckleyi, (Girard), in the coastal waters of Hokkaido, Japan. Jour. Fac. Sci. ser. Zool. 4:127-141. Sapporo.

Saunders, M.W. and McFarlane, G.A. 1993. Age and length at maturity of the female spiny dogfish, Squalus acanthias, in the Strait of Georgia, B.C., Canada. Environmental Biology of Fishes 38:49-57.

Saunders, M.W., McFarlane, G.A., and Smith, M.S. 1984. Abundance, distribution, and biology of spiny dogfish (Squalus acanthias) in Hecate Strait during Sept. 7- Oct. 1, 1982. Canadian Manuscript of Fisheries and Aquatic Sciences, No. 1754. 161 pp.

Skogsberg, T. (1936) Hydrography of Monterey Bay, California. Thermal Conditions, 1929-1933. Transactions of the American Philosophical Society held at Philadelphia for Promoting Useful Knowledge, 29, 1-152.

Skogsberg, T. and A. Phelps (1946) Hydrography of the Monterey Bay, California Thermal Conditions, Part 2 (1934-1937).Proceedings of the American Philosophical Society, 90, 350-386.

Stillwell, C. E. and N. E. Kohler. 1982. Food, feeding habits, and estimates of daily ration of the shortfinmako (Isurus oxyrinchus) in the Northwest Atlantic. Can.

J. Fish. Aquat. Sci. 39:407–414.

Stillwell, C.E. and Kohler, N.E. 1993. Food habits of the sandbar shark Carcharhinus plumbeus off the United Stated northeast coast, with estimates of daily ration. Fishery Bulletin 91(1):138-150.

Talent, L.G. 1976. Food habits of the leopard shark, Triakis semifasicata, in Elkhorn Slough, Monterey Bay, California. Calif. Fish and Game 68:224-234.

Tanasichuk, R.W. 1998a. Interannual variation in the population biology and productivity of Euphausia pacifica in Barkley Sound, Canada, with special reference to the 1992 and 1993 warm ocean years. Mar Ecol. Prog. Ser. 173:163-180.

Tanasichuk, R.W. 1998b. Interannual variations in the population biology and productivity of Thyanoessa spinifera in Barkely Sound, Canada, with special reference to the 1992 and 1993 warm ocean years. Mar Ecol. Prog Ser. 173: 181-195.

Tanasichuk, R.W., D.M. Ware, W. Shaw and G. A. McFarlane. 1991. Variations in diet, daily ration and feeding periodicity of Pacific hake (Merluccius productus) and spiny dogfish (Squalus acanthias) off the lower west coast of Vancouver Island. Can. J. Fish. Aquat. Sci. 48:2118-2128.

Taylor LR, Compagno LJV, Struhsaker PJ. 1983. Megamouth—A new species, genus, and family of lamnoid shark (Megachasma pelagios, family Megachasmidae) from the Hawaiian Islands. Proc Calif Acad Sci 43:87–110.

Templeman, W. 1944. The life history of the spiny dogfish (Squalus acanthias) and the vitamin values of dogfish liver oil. Dept. Nat. Res. Bull. (Fish) 15. 102 pgs.

Tricas, T.C. 1979. Relationships of the blue shark, Prionace glauca, and its prey species near Santa Catalina Island, California. Fish Bull. 77:175-182.

Vega, N.M., Gallucci, V.F., Hauser, L., and Franks, J. 2009. Differences in growth in the spiny dogfish over a latitudinal gradient in the eastern North Pacific. Pages 169-179 in Biology and management of dogfish sharks. American Fisheries Society, Bethesda, Maryland.

Wallace, S.S.; McFarlane, G.A.; Campana, S.E.; King, J.R. 2009. Status of spiny dogfish in Atlantic and Pacific Canada. Pages 313-334 in Biology and management of dogfish sharks. American Fisheries Society, Bethesda, Maryland.

Webber, J. D. and J. J. Cech, Jr. 1998. Nondestructive diet analysis of the leopard shark from two sites inTomales Bay, California. Calif. Fish. Game 84:18–24.

Wetherbee, B.M. and Cortés, E. (2004). Food consumption and feeding habits. In Carrier, J.C., Musick, J.A., and Heithaus, M.R. (Eds.), Biology of Sharks and their Relatives(pp. 225-246). Boca Raton, Florida: CRC Press.

Wetherbee, B.M., Cortés, E., and Bizzarro, J.J. 2012. Food consumption and feeding habits. In: Carrier, J.C., Musick, J.A., and Heithaus, M.R. (eds) Biology of Sharks and their Relatives, Edition 2. CRC Press, Boca Raton, Florida: 453-486.

Zeidberg, L., and Robinson, B.H. 2007. Invasive range expansion by Humboldt squid, Dosidicus gigas, in the eastern North Pacific. Proc. Natl. Acad. Sci, USA. 104:12948-12950.

Zeidberg, L., Hamner, W., Nezlin, N., Henry, A. 2006. The fishery for California market squid (Loligo opalescens) (Cephalopoda: Myopsioda) from 1981 through 2003. Fish Bulletin 104:46-59.

Zwolinski, J.P., Demer, D.A., Byers, K.A., Cutter, G.R., Renfree, J.S., Sessions, T.S., Macewicz, B.J. 2011. Distributions and abundances of Pacific sardine (Sardinops sagax) and other pelagic fishes in the California Current Ecosystem during spring 2006, 2008, and 2010, estimated from acoustic- trawl surveys.

TABLES

Table 1: Diet composition of North Pacific Spiny Dogfish, as expressed by the following mectrics: percentage number (%N), percentage weight (%W), percentage prey-specific number (%PN), percentage prey-specific weight (%PW), percentage Index of Relative Importance (%IRI), and percentage prey-specific Index of Relative Importance (%PSIRI)

Lowest %N %W %FO %PN %PW %IRI %PSIRI Identification ACTINOPTERYGII 34.66 31.57 0.45 77.31 70.43 29.69 33.11 Clupeiformes 14.89 12.55 0.19 77.11 64.97 5.30 13.72 Clupeidae 7.99 6.99 0.12 68.14 59.65 1.76 7.49 Sardinops sagax 6.55 6.21 0.09 73.08 69.30 1.14 6.38 Clupea pallasi 1.44 0.78 0.03 41.67 22.66 0.08 1.11 Engraulidae Engraulis mordax 6.90 5.55 0.10 66.71 53.66 1.29 6.23 Sardinops sagax 6.55 6.21 0.09 73.08 69.30 1.14 6.38 Merluccius productus 2.76 2.76 0.03 100.00 100.00 0.15 2.76 Embiotocidae Embiotoca jacksoni 0.34 0.04 0.01 50.00 5.54 0.00 0.19 Pleuronectiformes 1.55 1.68 0.02 75.00 81.11 0.07 1.61 Sebastidae 1.03 0.99 0.01 75.00 72.09 0.03 1.01

Unidentified Fish 14.08 13.56 0.21 65.84 63.40 5.91 13.82

CEPHALOPODA 25.69 29.68 0.33 77.60 89.67 18.33 27.69 Octopoda 3.79 5.13 0.06 68.75 93.01 0.49 4.46 Octopus rubescens 0.34 0.69 0.01 50.00 99.42 0.01 0.52 Benthoctopus leioderma 0.34 0.54 0.01 50.00 78.13 0.01 0.44 Sepiolida Rossia pacifica 1.21 1.22 0.02 58.33 59.06 0.05 1.21 Teuthida Doryteuthis opalescens 6.55 6.36 0.07 95.00 92.19 0.89 6.45 Dosidicus gigas 3.33 4.65 0.05 69.05 96.25 0.39 3.99

Unidentified Cephalopoda 8.39 9.86 0.12 71.57 84.10 2.14 9.13

MALACOSTRACA 35.86 35.08 0.39 92.86 90.84 27.40 35.47 Decapoda Cancer 0.34 0.13 0.01 50.00 18.30 0.00 0.24 Euphausiacea 35.52 34.96 0.38 93.64 92.15 26.73 35.24

Scavenge 2.07 2.07 0.02 100.00 100.00 0.09 2.07

Unidentified Organic Matter 1.72 1.60 0.02 83.33 77.19 0.07 1.66

Table 2: Summary of Results of Individual RDA Models 1-10 Model Factor Variance F-value P-value Explained (%) 1 Total length 0.97 1.28 0.2414 2 Sex 0.46 0.60 0.7484 3 Season 7.1 5.00 0.0001* 4 Depth 1.1 1.41 0.1927 5 Space (Latitude + Longitude) 4.2 2.88 0.0018* 6 Space (Longitude + Latitude) 4.2 2.88 0.0014* 7 Interaction of Total length 2.5 1.39 0.1171 and Depth 8 Interaction of Depth and 2.5 1.39 0.1214 Total Length 9 Interaction of Total length 3.3 1.9 0.0176 and Sex 10 Interaction of Sex and Total 3.3 1.9 0.0171 Length * denotes significant p-value

Table 3: Summary of Overall RDA (Season + Longitude + Latitude) VARIANCE F-VALUE P-VALUE EXPLAINED (%) Overall RDA 10.7 3.84 0.0001* RDA 1 6.7 9.63 0.0001* RDA 2 2.0 2.94 0.0077* *denotes significant p-value

Table 4: Summary of Overall RDA Diet Variable Loadings DIET VARIABLES RDA 1 LOADING RDA 2 LOADING Anchovy 0.179268 -0.007683 Euphausiid -0.653848 -0.113885 Humboldt Squid -0.052240 -0.055537 Market Squid -0.119972 0.402251 Octopoda -0.009847 -0.029543 Other Fishes 0.040286 -0.155895 Other Squid -0.083860 -0.015820 Sardines 0.211174 -0.063233 Unidentified Cephalopods 0.388240 -0.008993 Unidentified Fishes 0.105353 -0.059047

Table 5: Summary of Overall RDA Factor Loadings FACTOR RDA 1 RDA 2 SEASON-OCS 0.007379 0.29334 SEASON-UPS -0.138645 -0.12804 SEASON-DCS 0.578250 0.03638 LATITUDE -0.04400 0.8971 LONGITUDE -0.54805 0.5129

FIGURES

Figure 1: Study Site in Monterey Bay, California, U.S.A. Arrows indicate GPS haul locations. All specimens caught in the 20 hauls were used for SCA (Chapter 1). White arrows indicate hauls in which subsamples were selected for use in Chapter 2.

0 Female Male 10

40 200 300 400 500 600 700 800 900 1000

Figure 2: Length frequency histogram of female (red) and male (blue) North Pacific Spiny Dogfish specimens collected 2004-2005 from NMFS SCL Survey

Figure 3: Cumulative prey curve with associated standard deviation of 11 higher prey categories. Slope through the last 5 points, b=0.007.

40 %PSIRI

35 %IRI

0 Euphausiid Teleost Cephalopod

Figure 4: Graphical representation of the Prey-specific Index of Relative Importance (%PSIRI) and Index of Relative Importance (%IRI) for the 3 dominant prey categories.

Figure 5: Graphical representation of the percentage Frequency of Occurrence (%FO) of the 3 dominant prey categories

Figure 6: Graphical representation of the Index of Relative Importance (%IRI) and Prey-Specific Index of Relative Importance (%PSIRI) of the 11 higher prey categories. UNID ceph= unidentified cephalopods, UNID Fish= unidentified fishes

Figure 7: Graphical representation of the percent number (%N) and percent weight (%W) (Graph A) prey-specific number (%PN) and prey-specific weight (%PW) (Graph B) of the 11 higher categories. UNID Fish= unidentified fishes, and UNID ceph= unidentified cephalopods.

Figure 8: Graphical representation of the Frequency of Occurrence (%FO) of the 11 higher prey categories

Figure 9: Ordination biplot of factors and response variables (including space [latitude+longitude] and season [UPS, OCS, DCS]). ANCH= anchovy, DCS= Davidson Current season, Euph= euphausiid, HS= Humboldt squid, Lat= latitude, Long= longitude, MS= Market squid, OCS= Oceanic Current season, OF= other fishes, SAR= sardine, UC= unidentified cephalopods, UF= unidentified fishes, UPS= Upwelling season

Ch. 2: Feeding trends of the North Pacific Spiny Dogfish, Squalus suckleyi as inferred from stable isotope analysis

ABSTRACT

Data on food habits of apex predators is required to develop appropriate and effective ecosystem-based management plans. Feeding habits of an abundant and economically important predator, the North Pacific Spiny Dogfish, Squalus suckleyi, was elucidated using stable isotope analysis of white muscle tissue. North Pacific Spiny

Dogfish were collected from NOAA Fisheries fishery-independent trawl surveys conducted off central California from January 2004 through January 2005. Prey taxa commonly found in the diet of Spiny Dogfish (see stomach content analysis in Chapter 1) were analyzed to assess trophic position and overall contribution to the diet. The mean and standard deviation of δ15N for males was 14.8‰ ± 0.5 and for females it was 14.5 ‰

± 0.5. The mean and standard deviation of δ13C’ (lipid-extracted and lipid-normalized) for males was -15.0 ‰ ± 0.4 and for females it was -15.2 ‰ ± 0.3. In general, the resulting isotopic composition of North Pacific Spiny Dogfish was consistent with the expected isotopic values of dogfish that consumed the known prey species from stomach content analysis, confirming the validity of using stable isotope analysis to assess trophic position of this species. No significant difference was found for δ15N or δ13C’ values between females and males (δ15N: t=-1.535, p=0.132; δ13C’: t=-1.688, p=0.099). A marginally significant relationship was found between total length and δ15N (r2=0.097, p=0.039), but no significant relationship was observed between total length and δ13C’

(r2=0.051, p=0.142). δ15N of females increased with increasing δ13C’ (r2=0.52), indicating that females feeding more inshore had greater trophic positions than those that fed offshore, although a redundancy analysis determined that there was no relationship

58 between δ15N or δ13C’ for size (F=1.19, p=0.31) or sex (F=0.99, p=0.18). The resulting

δ15N or δ13C’ values were similar to other values found for the same species, but from different regions. Based on stable isotope analysis it was concluded that North Pacific

Spiny Dogfish from central California waters had carbon and nitrogen stable isotope values indicating that they feed on prey with variable trophic positions in inshore and offshore waters.

INTRODUCTION

It is crucial to quantify trophic interactions of apex predators, such as elasmobranchs, for the development and implementation of successful marine ecosystem- based fisheries management plans (EBFM) (Brodziak and Link 2002, Pikitch et al. 2004,

Bundy et al. 2011). Quantifying trophic interactions requires conducting diet studies on organisms, and the traditional method is stomach content analysis (SCA) (see Chapter 1).

SCA is a proven method to obtain data on a species’ diet composition and to establish predator-prey linkages, but it is not without limitations (Hyslop 1980, Assis 1996, Cortés

1997, 1999). While SCA provides taxonomic resolution that cannot be rivaled by other methods, it is resource intensive and time consuming. Many predatory fish have a high percentage of empty stomachs, thus requiring even more specimens to be sampled (see

Wetherbee and Cortes 2004). In addition, SCA only provides insight into the last meal ingested by the organism, precluding the understanding of any longer term patterns. This may lead to the over- or under-estimation of the importance of some prey items contributing to the diet, thus biasing data for the construction of an effective EBFM plan.

Stable isotope analysis (SIA) is another method that has been used in marine studies to determine long-term diet composition (Peterson and Fry 1987). SIA has been routinely used to elucidate the diet of other taxa including birds, mammals, and bony fishes (Hobson and Clark 1992a,b, Lesage et al. 2001), and an increasing number of studies over the past decade have used SIA to examine trophic ecology of elasmobranchs

(i.e. Fisk et al. 2002, Estrada et al. 2003, MacNeil et al. 2006). This method is based upon the fact that isotopic compositions change in predictable ways as tissues are produced and then consumed, assimilated, and transferred through different levels of the food web.

Tissue turnover times are often on the order of months to years, so isotopic composition of tissues reflects an integration of the diet through time (Logan and Lutcavage 2010).

Numerous studies have validated that the stable isotope ratios of elements such as carbon (13C/12C) and nitrogen (15N/14N) in organismal tissue can be used as tracers to identify predator-prey relationships as well as the trophic position occupied by particular species and individuals (Peterson and Fry 1987, Post 2002). Because the carbon isotope ratio (δ13C) changes little in moving up trophic positions (typically from 0-1‰ per trophic position), they reflect the source of primary production at the base of the food chain (France 1995, Rau et al 1983, Peterson and Fry 1987, Post 2002). In marine systems, tissues depleted in 13C are often indicative of pelagic or “offshore” sources of prey at the base of the food chain whereas more enriched 13C values reflect the contribution of benthic, littoral, or “inshore” sources of carbon to the diet (France 1995,

Hobson 1999, Cherel and Hobson 2007, Carlisle et al. 2012). In contrast, δ15N correlates positively with trophic positions occupied by particular species. Due to isotopic fractionation, 15N is concentrated in tissues relative to 14N, and δ15N increases with trophic positions (often ranging from 3-5‰ per trophic position increase) (Minagawa and

Wada 1984, Peterson and Fry 1987). A recent study based upon a controlled 1000+ day feeding experiment on leopard sharks (Triakis semifasciata) determined elasmobranch- specific discrimination factors to be 1.7 ‰ for δ13C and 3.7‰ for δ15N (Kim et al. 2010).

Due to elasmobranch physiology and biology, potential biases can influence the results of stable isotope analysis. First, elasmobranchs retain trimethylamine oxide

(TMAO), a major osmolyte that functions in homeostasis, but is depleted (contains lighter isotopes) in nitrogen. Second, urea, which is depleted in 15N and enriched in 13C,

61 is also retained in elasmobranchs for osmoregulation (Kim and Koch 2012). Third, lipids are stored in the liver to maintain neutral buoyancy while swimming and lipids are depleted in 13C relative to other somatic tissues (Kim and Koch 2012). The C:N ratio is often used as a proxy for lipid content, and is useful in assessing the accuracy of lipid extraction techniques (Post et al. 2007, Drymon et al. 2012, B. Finney, pers. comm.).

Thus, dietary studies of elasmobranchs that use stable isotope analysis need to be aware of and account for potential biases in the preparation and analysis phase.

Many researchers have used SCA and SIA to assess trophic ecology, but few have concurrently examined stomach content and stable isotope analysis together to confirm the validity of solely using SIA (Estrada et al. 2003, Boyle 2010). These types of studies are crucial in determining if SIA is valid without the taxonomic resolution of SCA, and further research needs to be conducted to confirm that SIA may be used to estimate trophic ecology of elasmobranchs.

Objectives and Hypothesis:

The primary goal of this study was to investigate long-term trophic ecology of

North Pacific Spiny Dogfish (Squalus suckleyi) (referred to as Spiny Dogfish for the remainder of this chapter) from central California waters using stable isotope analysis.

My specific objectives were to (1) use SIA to determine sources of primary productivity

(δ13C) and trophic position (δ15N), (2) construct a food web plot with Spiny Dogfish and known prey species (Chapter 1), (3) investigate the role of sex and size in explaning patterns of variation of δ13C and δ15N, and (4) compare resulting isotope data to that of

62 published studies on the same species in other regions (Andrews 2010, Reum &

Essington 2013).

I expected Spiny Dogfish from central California waters to have a large variance in δ13C values as they were found to feed on both inshore and offshore species, and a

δ15N in between that of fishes/cephalopods and euphausiids as both are important contributors to the diet (Chapter 1). Size was expected to be a significant factor in explaining variability in the isotope ratios, as previous researchers of stable isotopes of elasmobranchs reported significant positive relationships with δ15N and size (Hussey et al. 2011, Shiffman 2011, Courtney & Foy 2012). One study found such shifts for Spiny

Dogfish specifically, although farther north in Alaska and British Columbia (Andrews

2010). Sex was not expected to have a significant effect in explaining variability in the nitrogen or carbon stable isotopes (Andrews 2010, Reum & Essington 2013). Although dietary variation was explained by season using SCA (see Ch.1), complete 13C and 15N turnover in white muscle tissue would require >500 and 300 days respectively (Logan &

Lutcavage 2010), thus not allowing for detection of seasonal changes. Other researchers also have confirmed that white muscle tissue has too long of a turnover period to detect seasonal changes (MacNeil et al. 2006, Malpica-Cruz et al. 2012). Thus, season was not tested.

MATERIALS AND METHODS

Sample Collection

The National Marine Fisheries Service (NMFS) Santa Cruz Lab (SCL) collected

Spiny Dogfish tissue samples during monthly trawl surveys along the central California

63 coast from January 2004 to January 2005. Specimens from these surveys were collected from 20 hauls with depths ranging from 93 to 417 meters (Figure 1). Four euphausiid samples also were collected opportunistically from the same region in 2012. Stable isotope values for other prey (known from Ch.1) were taken from the literature (Table 1).

A subset of samples used for stomach content analysis (Ch.1) was selected for stable isotope analysis to ensure the results would not be biased towards diet type. A total of 43 samples were analyzed in this study, with 23 females and 20 males, across a range of sizes for each sex. This sample selection allowed for testing of ontogenetic shifts in diet/trophic position and for differences between sexes.

Sample Preparation

Based on Kim and Koch (2012), the following methods were used for stable isotopic analysis. A small section of clean white muscle was excised and placed into a clean glass scintillation vial. Scalpels and forceps were rinsed with deionized water (DI) and dried with Kim Wipes between each sample to avoid contamination. Traditionally ethanol or methanol would be used for this step but this may affect carbon isotope results so DI water was chosen for this step.

Lipids were removed from all tissue samples. For this process, 10mL of petroleum ether was pipetted into each glass vial and they were then placed in an ultrasonic water bath for 15 minutes. After that period of time, the petroleum ether was decanted and the process repeated once more.

Following the petroleum ether rinse, the samples were rinsed with DI water to remove the urea from the tissue, as it can bias the N data (Kim and Koch 2012). For this

step, 10mL of water was pipetted into the vials, and then vials were put into the water

bath for 15 minutes. The water was then decanted and this process was repeated one more

time.

The tissue was dried in an oven at 55˚C overnight. A mortar and pestle was used

to break down each sample into powder, and both were cleaned in between each sample.

To analyze the tissue, the samples were weighed into tin capsules (Costech, 3.5 x 5m).

Samples were analyzed using an ECS 4010 (Elemental Combustion System 4010)

interfaced with a Delta V Advantage mass spectrometer through the ConFlo IV system at

Idaho State University. Three in-house standards (ISU Peptone, Costech Acetanilide, and

DORM-3) were used to directly calibrate against international standards.

Delta Notation

The results from SIA will be notated in the standard δ notation using the

following equation:

$ ' Rsample "x = & ( ) #1) *1000%o , %& Rstd ()

15 13 where "x is equal to either δ N or δ C, Rsample is directly measured, and is equal to the

13 12 ! 15 14 isotopic ratio ( C/ C or N/ N) of the sample, and Rstd is equal to the isotopic ratio of

! the standard (PeeDee Belemnite for carbon and atmospheric nitrogen for nitrogen).

δ13C Correction

The percentage weight of C and N in each sample was used to calculate a C:N

weight ratio which is commonly used as a proxy for lipid content. In tissues with over 5%

65 lipid, or a C:N > 3.5, biases in δ13C may occur if tissue is not lipid extracted using chemical techniques or normalized using mathematical techniques (Post et al. 2007).

Resulting samples had variable and often increased C:N ratios ranging as high as 6.98

(Figure 2, Appendix 1) even after lipid extraction. Some previous researchers suggested greater and variable C:N ratios for dogfish and other squaloids (Somniosus pacificus) in the eastern North Pacific (Andrews 2010, Courtney and Foy 2012), but further research is needed to fully understand the lipid content in this family of fishes. Due to this increased and variable C:N ratio, the δ13C values were then lipid-normalized based on a lipid-free correction equation (Reum and Essington 2013). Many lipid-normalization equations are available in the literature (McConnaughey and McRoy 1979, Logan et al. 2008, Reum

2011), but one was chosen specifically for Spiny Dogfish (Reum and Essington 2013).

This equation had a residual error with a mean and standard deviation of 0.38 ± 0.30, and was used as it best provides the most accurate δ13C’ values as it is species-specific. The resulting value is notated as δ13C’: and calculated as:

13 22.73 13 ! C' = 8.39 ! +! Cbulk C : Nbulk

13 13 where δ C’ are the resulting lipid-normalized δ C values, C:Nbulk is the C:N ratio of the

13 sample, and δ Cbulk is the uncorrected carbon stable isotope ratio of the sample.

Spiny Dogfish Food Web

δ15N and δ13C for Spiny Dogfish and known prey from Chapter 1 will be plotted to visualize a food web. The most common prey items found in the SCA portion of this study (Chapter 1) were selected and are as follows: euphausiids (Euphausiidae), Pacific sardine (Sardinops sagax), Northern Anchovy (Engraulis mordax), Market Squid

(Doryteuthis opalescens), and Humboldt Squid (Dosidicus gigas). Euphausiid samples were collected opportunistically from the same area as the Spiny Dogfish but in 2012. All other carbon and nitrogen stable isotope values were obtained from the literature from studies in Monterey Bay as close to 2004 and 2005 as possible. Pacific Sardine stable isotope values were from Sydeman et al. (1997), Northern Anchovy and Market Squid stable isotope values were from Becker et al. (2007), and large and small Humboldt

Squid stable isotope values were from Ruiz-Cooley et al. (2004). All prey isotope values obtained from the literature were lipid extracted. Table 1 displays the prey species, stable isotope values, and source of data. The trophic discrimination factors of 1.7 ‰ for δ13C and 3.7‰ for δ15N will be used to interpret the plot (Kim et al. 2010).

Investigating the Role of Size and Sex in Variability of δ15N and δ13C

Both simple and rigorous statistics were used to investigate the relationship of

δ15N and δ13C’ with respect to size and sex. A two sample independent t-test was used to test differences in δ15N and δ13C’ between males and females. A linear regression was used to examine the relationship between total length and δ15N and between total length and δ13C’.

A constrained redundancy analysis (RDA) was used to investigate the variation of

δ15N and δ13C’ with size and sex. The RDA was conducted using the software program R

(Version 2.13.1) with the “Vegan Community Ecology package” loaded (Oksanen et al.

2011). The constraining, or explanatory, variables used were size (total length) and sex

(male vs. female). Factors were first tested separately to obtain individual significance

67 values and then entered into the overall model in order of most significant to least significant. Significance was tested using 9,999 Monte Carlo permutations.

Redundancy Analysis is useful for explaining how one set of variables (response matrix) may be explained by another set (explanatory matrix) (Borcard et al. 2011). An

RDA is a combination of multivariate multiple linear regression and principal components analysis (PCA) that can be used to construct a constrained ordination plot for visual examination. The plot has axes computed by the RDA that are linear combinations of explanatory variables. Essentially, they are a series of linear combinations, in successive order, of the explanatory variables that best explain the variation of the response matrix (Borcard et al. 2011).

RESULTS

Collection

A total of 43 Spiny Dogfish (23 female, 20 male) and four euphausiids were analyzed for SIA (Figure 3). Females were 28.0 to 75.5 cm TL and males were 30.9 to

89.7 cm TL (Figure 3).

Stable Isotope Values

Spiny Dogfish stable isotope values varied among specimens; δ15N were 13.4 to

15.6 ‰ and δ13C’ were -15.8 to -14.4 ‰ (Figure 4, Table 2, Appendix 1). There was no separation of males and females into distinct groups or any other kind of sorting. There was a positive relationship among female δ15N and δ13C’ (r2=0.52, p=0.00). In contrast, no relationship (r2=0.02, p=0.861)) was found between male δ15N and δ13C’.

Spiny Dogfish Food Web

The dual isotope plot (Figure 5) indicated female and male lipid-normalized stable isotope values of Spiny Dogfish fit well into the span of carbon and nitrogen stable isotope values of their most common prey deduced from Chapter 1. The spread of data points appears to follow a linear trend, showing the stepwise enrichment from one trophic level to the next. After accounting for the trophic discrimination factors, male and female

Spiny Dogfish plotted in between the euphausiid samples and the cluster of fish and squid samples. Based on the sorting of prey on this plot, the prey species may be separated into three distinct groups based on their δ15N and δ13C: low, intermediary, and high trophic positions. The low trophic position group consisted of euphausiids as they plotted at the lowest trophic position across all carbon isotope values. The intermediary group included the Northern Anchovy, Pacific Sardine, Market Squid, and small Humboldt Squid. This group plots in a cluster towards the middle of the plot. The higher trophic position group consisted of large Humboldt Squid, which occupied the highest trophic position across all carbon isotope values.

Statistical Analysis

A Kolmorgorov-Smirnov test for normality confirmed that the stable isotope data followed a normal distribution (δ15N: p=0.95 and δ13C’: p=0.32). No significant difference was found for δ15N or δ13C’ values between females and males (δ15N: t=-

1.535, p=0.132; δ13C’: t=-1.688, p=0.099). There was a marginally significant positive relationship between total length and δ15N (r2=0.097, p=0.039; Figure 6), but no

69 significant relationship between total length and δ13C’ (r2=0.051, p=0.142; Figure 7). The

RDA was not significant for either size (F=1.19, p=0.31) or sex (F=0.99, p=0.18), so no overall model was conducted.

DISCUSSION

Overall, muscle tissue of Spiny Dogfish from central California waters had carbon and nitrogen stable isotope values indicating Spiny Dogfish fed on inshore and offshore prey with variable trophic positions. Two other recently published studies on this same species, but from other regions, found similar results with the current study (Andrews

2010, Reum and Essington 2013). Other fishes also take advantage of both inshore and offshore habitats as determined from their carbon and nitrogen isotope values (McMeans et al. 2010, Hussey et al. 2011, Carlisle et al. 2012). McMeans et al. (2010) found that

Greenland Sharks (Somniosus microcephalus) stomachs contained benthic (inshore) and pelagic (offshore) teleosts and their δ13C values supported this conclusion. Carlisle et al.

(2012) examined 53 white sharks from the Northeastern Pacific Ocean and found that they foraged in offshore and coastal (inshore) habitats. Although there was a significant linear relationship between the nitrogen stable isotope and total length, the RDA did not detect a significant relationship between the two. More research needs to be conducted in order to fully understand this trend. Neither linear regressions nor the RDA detected any differences between either isotope or sex of the fish.

The Spiny Dogfish food web, as visualized by the dual isotope plot, indicated that

Spiny Dogfish fit into the prey space of its known prey as determined in Chapter 1 of this study. This indicates that the stable isotope signature of carbon and nitrogen are accurate

70 and appropriate measures of food habits of this species. This also has been confirmed for other elasmobranch species (Estrada et al. 2003, Boyle et al. 2012). Estrada et al. 2003 used stable isotopes of nitrogen and carbon of Blue, (Prionace glauca), Shortfin Mako

(Isurus oxyrinchus), Thresher (Alopias vulpinnis), and Basking Sharks (Cetorhinus maximus) to calculate trophic positions and compared them with published trophic positions calculated from stomach content data, finding trophic position calculated using either technique were not significantly different.

The Spiny Dogfish food web plot also shows that different prey groups sort along the different axes based upon their isotopic signatures, and these groups may potentially be related to habitat type. The spread of data in Figure 5 appears to fall on a positive linear trend, confirming that the inshore sources are more enriched in 13C than the offshore sources, as expected. The low trophic position of euphausiids sorted as expected based on their known lesser trophic position and largely pelagic, offshore habitat (Dilling et al. 1998). The clustering of the intermediary group including Northern Anchovy,

Pacific Sardine, Market Squid, and small Humboldt Squid, indicated that they all had similar trophic positions and occupied similar inshore habitats. The greater trophic position of Humboldt Squid was expected, due to their largely piscivorous diet

(Nigmatullin et al. 2001).

The significant linear regression of total length and the nitrogen stable isotope value indicated that there may be a shift from lesser trophic position prey to greater trophic position prey as Spiny Dogfish increase in size, thus progress in development.

The marginally significant relationship of the nitrogen stable isotope and total length indicated that directly, and the positive relationship between female δ15N and δ13C’

71 indicated that indirectly. Because size was not significant in the RDA, more work needs to be done to fully understand this trend. Many researchers have documented an ontogenetic habitat shift in Spiny Dogfish with smaller, immature individuals having a more offshore, pelagic habitat and larger, mature individuals using a more benthic, demersal or inshore habitat (Jensen 1966, Ketchen 1986, Tribuzio et al. 2009). The positive relationship between female δ15N and δ13C’ in my study supported this previous research, and indicated that those Spiny Dogfish that fed inshore (presumably on higher trophic position species such as fishes and cephalopods) may have greater trophic positions than those that fed offshore (presumably on krill). Andrews (2010) found direct evidence of an ontogenetic shift in the diet of Spiny Dogfish using stable isotope analysis at five of seven sites. Other researchers also have reported such a shift for Spiny dogfish and other Squalus spp. using stomach content analysis (Jones and Geen 1977, Bowman et al. 1984, Laptikhovsky et al. 2001, Alonso et al. 2002). Alonso et al. (2002) examined

132 Squalus acanthias specimens collected from Patagonian waters from 1996 to 1998 and found differences in diet between sexes and age classes. Alonso et al. (200) found immature sharks’ diet consisted of mostly of pelagic-type prey such as fish and ctenophores and with the onset of maturity, the diet shifted to more demersal-benthic and demersal-pelagic types such as mollusks and benthic fish such as flounder, hake, skates, and midshipman (Alonso et al. 2002).

As both statistical tests indicated, sex-specific differences in diet were not significant, therefore, males and females were not feeding on different prey types.

Andrews (2010) and Reum and Essington (2013) came to similar conclusions when testing the relationship of sex and δ13C and/or δ15N. This is supported by two other

72 studies on Spiny Dogfish, Jones and Geen (1977) and Beamish and Sweeting (2009), which found minimal differences in diet between male and females. This is most likely due to the aggregations of this species and their ontogenetic shift in habitat (discussed above). It has been suggested that Spiny Dogfish may be remaining in these aggregations for extended periods of time thus sharing feeding histories (Reum and Essington 2013). If that hypothesis were true, it would explain the presence of size and ontogenetic differences in diet without sex-specific differences because as these dogfish increase in size, they shift from a pelagic to benthic habitat with different prey availabilities, but both male and female counterparts are in the same respective habitat.

Overall, negligible variation was explained by sex or size related differences in the diet of Spiny Dogfish from central California. Other predictors such as intrinsic variability, individual specialization in resource use, age, reproductive status, body condition, could explain the remaining variability but such factors were not tested.

Previous researchers have found inter-individual differences in physiology to be substantial in accounting for variability in δ13C and δ15N (Barnes et al. 2008, Kim et al.

2012). Barnes et al. (2008) found that intrinsic variability in isotopes of European Sea

Bass, Dicentrarchus labrax, raised in laboratory settings and fed identical diets was 1.8

‰ δ13C and 1.4 ‰ δ15N. Kim et al. (2012) reported inter-individual values of Leopard

Shark, Triakis semifasciata, muscle δ13C and δ15N to be 1.6 ‰ and 0.6 ‰. Intrinsic variability in δ13C and δ15N is a possible explanation for variation in δ13C and δ15N in my study, as the difference between the greatest and least δ13C and δ15N values were 0.9

‰ and 1 ‰, respectively.

Comparisons with other studies

Two studies have recently been conducted on stable isotopes of nitrogen and carbon on Spiny Dogfish that had similar results to my study. Andrews (2010) examined geographical, ontogenetic, and spatial variation of Squalus acanthias (=Squalus suckleyi) carbon and nitrogen stable isotope ratios, with the majority of the specimens collected from various regions in Alaska, with two sites further south: one in British

Columbia and the other in Puget Sound, Washington. Geographical variation was found in isotope values for dogfish collected from Puget Sound, Washington; Howe Sounds,

British Columbia; Dixon Entrance, Alaska; Yakutat Bay, Alaska; and central Gulf of

Alaska. Reum and Essington (2013) investigated the variability of carbon and nitrogen stable isotope values of Spiny Dogfish with respect to season and space.

Overall, my study, Andrews (2010), and Reum and Essington (2013) found similar stable isotope results except for a few differences (Table 3). The size range of my study encompassed smaller individuals than Andrews (2010) and Reum and Essington

(2013), but the latter two studies included larger males and females. My study and

Andrews (2010) found an ontogenetic shift in δ15N, but Reum and Essington found only that 3% of the variability in δ15N and δ13C was explained by size of the fish. More work needs to be done to fully determine if there is an ontogenetic shift in diet, and there may be underlying relationships in certain areas due to stronger offshore or latitudinal gradients. The range of δ15N was similar across all studies except for the Spiny Dogfish collected from Alaska (Andrews 2010), which were 2.5 ‰ lower on average than regions further south. The δ13C varied considerably across studies but the different lipid correction and normalization techniques potentially complicate the comparisons. My

74 study and Reum and Essington (2013) used the same lipid normalization technique and analyzed Spiny Dogfish from similar habitats—embayments. The δ13C values in my study overlapped with the values in Reum and Essington (2013), but the δ13C from Puget

Sound, WA had a larger range, encompassing more values that were depleted in δ13C by

~2‰. This may be due to the existence of variability in isotopic baselines from different areas in the North Pacific (Graham et al. 2009, B. Finney, pers. comm.). The majority of the data from all three studies shows that sex does not have a significant impact on δ13C or δ15N, indicating that female and male Spiny Dogfish consume similar prey types across varying latitudes.

The lipid extraction process is somewhat controversial, and this study and the stable isotope papers cited here used a variety of methods in which to lipid-normalize and/or extract their data. Many researchers decide not to lipid extract thus not correcting for the bias lipids have on δ13C (Andrews 2010, Vaudo and Heithaus 2011, Pethybridge et al. 2012). Lipids are traditionally extracted using the Bligh and Dyer (1959) method, which uses a chloroform methanol mixture to dissolve lipids (see Borrell et al. 2011,

Hussey et al. 2011, Kinney et al. 2011). Other researchers have developed ways to extract lipids using less harmful chemicals such as a petroleum ether and deionized water rinse

Kim and Koch (2012). Yet other researchers rely on mathematical calculations to lipid- normalize data such as McConnaughey and McRoy (1979), Logan et al. (2008), Reum

(2011), Reum and Essington (2013), among others. This study choose to lipid extract based on Kim et al. (2012) using a petroleum ether and deionized water rinse. The resulting C:N ratios were too high and thus a lipid-normalization model was selected following Reum and Essington’s (2013) equation for the same species. Although the

75 lipid-normalization in this study was deemed necessary, other studies have found unusually high C:N ratios for Spiny Dogfish (Andrews 2010) and other cold-water elasmobranchs (Courtney and Foy 2012). Nonetheless, every species is different, and researches should choose an appropriate lipid extraction or lipid normalization equation on a species-specific basis.

In conclusion, Spiny Dogfish collected in Monterey Bay, CA, had carbon and nitrogen stable isotope values consistent with consuming variable trophic position prey from inshore and offshore habitats. When plotted with stable isotope values of known prey, Spiny Dogfish fit into “prey space” and a linear relationship existed among

A significant linear relationship between the nitrogen stable isotope and total length existed, but as an RDA did not detect this relationship, more research needs to be conducted in order to fully understand this trend. Sex was not found to be a significant factor in explaining variability in either isotope, indicating that males and females had shared feeding histories. The results presented in my study are similar to two other studies on the same species collected further north, and this conclusion has important implications for an ecosystem-based management plan.

LITERATURE CITED

Alonso, KA; Crespo, EA; Garcia, NA; pedraza, SN; Mariotti, PA; and Mora, NJ. 2002. Fishery and ontogenetic driven changes in the diet of the spiny dogfish, Squalus acanthias, in Patagonian waters, Argentina. Environmental Biology of Fishes, 63: 193-202.

Andrews, AG III (2010). Variation in the Trophic Position of Spiny Dogﬁsh (Squalus acanthias) in the Northeastern Paciﬁc Ocean: An Approach Using Carbon and NitrogenStable Isotopes. MS Thesis, University of Alaska Fairbanks, AK.

Assis, CA. 1996. A generalised index for stomach contents analysis in fish. Scientia Marina 60(2-3):385-389.

Barnes, C; Jennings, S; Polunin, NVC; Lancaster, JE (2008). The importance of quantifying inherent variability when interpreting stable isotope field data. Oceologia 155:227-235.

Beamish, RJ; and Sweeting, RM. 2009. Spiny dogfish in the pelagic waters of the Strait of Georgia and Puget Sound. In: Gallucci VF, McFarlane GA, Bargmann G (eds) Biology and Management of dogfish sharks. American Fishery Society, pp 101-118.

Becker, BH; Peery, ZM; and Beissinger, SR. 2007. Ocean climate and prey availability affect the trophic level and reproductive success of the marbeled murrelet, an endangered seabird. Marine Ecology Progress Series. 329: 267-279

Bligh, EG and Dyer, WJ. 1959. A rapid method of total lipid extraction and purification. Canadian J. of Physiology and Pharmacology 37, 911-917.

Borcard, D, Gillet, F, and Legendre, P. 2011. Numerical Ecology with R. Use R! series. Springer, NY.

Borrell, A; Aguilar, A; Gazo, M; Kumarran, RP; and Cardona L. 2011. Stable isotope profiles in Whale Shark (Rhincodon typus) suggest segregation and dissimilarities in the diet depending on sex and size. Environ. Biol. Fish. 92: 559- 567

Bowman, R., Eppi, R., Groslein, M. 1984. Diet and consumption of spiny dogfish in the Northwest Atlantic. ICES C.M. 1984.

Boyle, MD. 2010. Trophic interactions of Bathyraja trachura and sympatric fishes. CSUMB Thesis, 87 pgs.

Boyle, MD; Ebert, DA, Cailliet, GM. 2012. Stable-isotope analysis of a deep-sea benthic fish assemblage: evidence of an enriched benthic food web. Environmental Journal of Fish Biology. 80, 1485-1507.

Brodziak, J, and J Link. 2002. Ecosystem-based fishery management: what it is and how can we do it? Bulletin of Marine Science, 70(2):589-611.

Bundy, A, Link, JS, Smith, BE, and Cook, AM 2011. You are what you eat, whenever or wherever you eat it: an integrative analysis of fish food habits in Canadian and U.S.A. Journal of Fish Biology. 78(2):514-39.

Carlisle, AB; Kim, SL; Semmens, BX’ Madigan, DJ; Jorgensen, SJ; Perle, CR; Anderson, SD; Chapple, TK; Kanive, PE; and Block, BA. 2012. Using stable isotope analysis to understand the migration and trophic ecology of Northeastern Pacific White Sharks (Carcharadon carcharias). PLoS ONE 7(2): e30492. doi:10.1371/journal.pone.0030492

Cherel, Y. and Hobson, KA. 2007. Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean/ Marine Ecology Progress Series, 329: 281-287.

Cortés, E 1997. A critical review of methods of studying fish feeding based on analysis of stomach contents: application to elasmobranch fishes. Can. J. Fish. Aquat. Sci. 54: 726–738.

Cortés, E 1999 Standardized diet compositions and trophic levels of sharks. ICES J. Mar. Sci. 56:707-717.

Courtney, DL and R Foy. 2012. Pacific Sleeper Shark Somniosus pacificus trophic ecology in the eastern North Pacific Ocean inferred from carbon and nitrogen stable-isotope ratios and diet. Journal of Fish Biology, 80:1508-1545.

Drymon, JM; Powers, SP; Carmichael, RH. 2012. Trophic plasticity of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) from the north central Gulf of Mexico. Environ Biol Fish. 95:21-35.

Estrada, JA, AN Rice, ME Lutcavage, and G.M. Skomal. 2003. Predicting trophic position in sharks of the north–west Atlantic Ocean using stable isotope analysis. Journal of the Marine Biological Association of the United Kingdom 83 (6): 1347–1350.

Fisk, AT, et al. (2002). Using anthropogenic contaminants and stable isotopes to assess the feeding ecology of Greenland sharks. Ecology 83(8): 2162-2172.

France, RL. 1995. Differentiation between littoral and pelagic food webs in lakes using stable carbon isotopes. Limnol. Oceanogr., 40(7), 1310-1313.

Hobson, KA. 1999. Tracing origins and migration of wildlife using stable isotope analysis: a review. Oceologia, 120:314-326.

Hobson, KA, and Clark, RG. 1992a. Assessing avian diets using stable isotopes I: turnover of 13C in tissues. The Condor, 94(1)181-188.

Hobson, KA and Clark, RG. 1992b. Assessing avian diets using stable isotopes II: factors influencing diet-tissue fractionation. The Condor. 94(1)189-197.

Hussey, NE; Dudley, SFJ; McCarthy ID; Cliff, G; and Fisk, A. 2011. Stable isotope profiles of large marine predators: viable indicators of trophic position, diet, and movement in sharks? Can. J. of Fish. Aquat. Sci. 68: 2029-2045.

Hyslop, EJ 1980. Stomach contents analysis: A review of methods and their application. Journal of Fish Biology 17: 411–429.

Jensen, AC. 1965. Life history of the spiny dogfish. Fish. Bull 65.3: 527-554.

Ketchen, K. 1986. The spiny dogﬁsh (Squalus acanthias) in the northeast Paciﬁc and a history of its utilization. Canadian Special Publication of Fisheries and AquaticSciences 88.

Kim, SL and Koch, PL 2012. Methods to collect, preserve, and prepare elasmobranch tissues for stable isotope analysis. Environmental Biology and Fishes. 95: 53-63.

Kim, SL, Casper, DR, Galvan-Magana, F, Ochoa-Diaz, R, Hernandez-Aguilar, SB, Koch, PL 2012. Carbon and nitrogen discrimination factors for elasmobranch soft tissues based on a long-term controlled feeding study. Environmental Biology of Fishes. 95: 37-52.

Kim, SL; Martinez del Rio, C; Casper, D; Koch, PL. 2012. Isotopic incorporation rates for shark tisues from a long-term captive feeding study. J. Exp. Biol. 215: 2495- 2500.

Kinney, MJ; Hussey, NE; Fisk, AT; Tobin, AJ; and Simpfendorfer, CA. 2011. Communal or competitive? Stable isotope analysis portioning within a communal shark nursery. Marine Ecology Progress Series. 439: 263-276.

Lesage, V; Hammill, MO; Kovacs, KM 2000. Marine mammals and the community structure of the estuary and Gulf of St. Lawrence, Canada: evidence from stable isotope analysis. Marine Ecology Progress Series, 210:203-221.

Logan, JM and Lutcavage, ME. 2010. Stable isotope dynamics in elasmobranch fishes. Hydrobiologia 644:231-244.

Logan, JM; Jardine, TD; Miller, TJ; Bunn, SE; Cunjak, RA; Lutcavage, ME. 2008. Lipid corrections in carbon and nitrogen stable isotope analyses: comparison of chemical extraction and modeling methods. J. Animal Ecology. 77: 838-846.

MacNeil, MA, KG Drouillard & AT Fisk, 2006. Variable uptake and elimination of stable nitrogen isotopes between tissues in fish. Canadian Journal of Fisheries and Aquatic Sciences 63: 345–353.

Malpica-Cruz, L; Herzka, SZ; Sosa-Nishizaki, O; and Lazo, JP. 2012. Tissue-specific isotope trophic discrimination factors and turnover rates in a marine elasmobranch: empirical and modeling results. Can. J. Fish. Aquat. Sci. 69: 551-564.

McConnaughey, T and McRoy, CP. 1979. Food-web structure and the fraction of carbon isotopes in the Bering Sea. Marine Biology. 53: 257-262.

Minagawa, M and Wada, E. 1984. Stepwise enrichment of δ15N along food chains: further evidence and the relation between δ15N and animal age. Geochimica et Cosmochimica Acta 48: 1135-1140.

Nigmatullin, CH; Nesis, KN; and Arkhipkin, AI. 2001. A review of the biology of the jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae). Fisheries Research. 54: 9-19.

Oksanen, J, Blanchet, GF, Kindt, R, Legendre, P, and O’Hara, RB. 2011. Vegan: Community Ecology Package. R package version 117-8. http://CRANR- Projector/package=vegan.

Peterson, BJ and B Fry (1987). Stable isotopes in ecosystem studies. Annual Review Ecology and Systematics 18: 293-320.

Pethybridge, H; Butler, ECV; Cossa, D; Daley, R; Boudou, A. 2012. Trophic structure and biomagnification of mercury in an assemblage of deepwater chondrichthyans from southeastern Austrailia. Marine Ecology Progress Series. 451: 161-174

Pikitch, EK, Santora, C, Babcock, EA, Bakun, A, Bonfil, R, Conover, DO, Dayton, P, Doukakis, P, Fluharty, D, Heneman, B, Houde, ED, Link, J, Livingston, PA, Mangel, M, McAllister, MK, Pope, J, Sainsbury, KJ. 2004. Ecosystem-based fishery management. Science, 305:346-347.

Post, DM. 2002. Using stable isotopes to estimate trophic position: models, methods, and assumptions. Ecology 83 (3): 703–718.

Post, DM; Layman, CA; Arrington, DA; Takimoto, G; Quattrochi, J; and Montana, CG. 2007. Getting to the fat of the matter: models, methods and assumptions for dealing with lipids in stable isotope analyses. Oceologia, 152, 179-189.

Rau, GH; Means, AJ; Young, DR; Olson, RJ; Shafer HA, and Kaplan, IR. 1983. Animal 13C/12C correlated with trophic level in pelagic food webs. Ecology, 64: 1314- 1318.

Reum CP. 2011. Lipid correction model of carbon stable isotopes for a cosmopolitan predator, spiny dogfish, Squalus acanthias. J. Fish. Biol. 79: 2060-2066.

Reum, CP and Essington, TE. 2013. Spatial and seasonal variation in δ15N and δ13C values in a mesopredator shark, Squalus suckleyi, revealed through multitissue analysis. Marine Biology. 160:399-411.

Ruiz-Cooley, RI; Gendron, D; Aguiniga, S; Mesnick, S; and Camquiry, JD. 2004. Trophic relationships between sperm whales and jumbo squid using stable isotopes of carbon and nitrogen. Marine Ecology Progress Series. 277: 275- 283.

Shiffman, D. 2011. Feeding ecology of the sanbar shark (Carcharhinus plumbeus) in South Carolina estuaries using δ13C and δ15N stable isotope analysis. COC. Master’s thesis.

Sydeman, WJ; Hobson, KA; Pyle, P’ and McLaren, EB. 1997. Trophic relationships among seabirds in central California: combined stable isotope and conventional dietary approach. Condor, 99: 327-336.

Tribuzio, C., Gallucci, V. F. & Bargmann, G. G. (2009). Reproductive biology and management implications for spiny dogﬁsh in Puget Sound, Washington. In Biology and Management of Dogﬁsh Sharks (Gallucci, V. F., McFarlane, G. A. & Bargmann, G. G.,eds), pp. 181–194. Bethesda, MD: American Fisheries Society.

Vaudo, JJ and Heithaus, MR. 2011. Dietary niche overlap in a nearshore elasmobranch mesopredator community. Marine Ecology Progress Series. 425: 247-260.

Wetherbee, BM and Cortés, E. (2004). Food consumption and feeding habits. In Carrier, JC, Musick, JA, and Heithaus, MR. (Eds.), Biology of Sharks and their Relatives(pp. 225-246). Boca Raton, Florida: CRC Press.

TABLES

Table 1: Sample size, mean and standard deviation of δ15N and δ13C, and source of all specimens used in the food web plot. Species Sample Mean δ15N ± Mean δ13C± Source Size (n) SD SD Female Dogfish 23 14.5 ± 0.5 -18.3 ± 0.9 This study Male Dogfish 20 14.8 ± 0.5 -18.2 ± 1.0 This Study Female Dogfish, Corrected 23 See above -15.2 ± 0.3 This Study Male Dogfish, corrected 20 See above -15.0 ± 0.4 This Study Euphausiids 4 8.1 ± 1.3 -22.9 ± 0.4 This Study Pacific Sardine 3 12.9 ± 0.1 -17.0 ± 0.3 Sydeman et al. 1997 Northern Anchovy 27 13.6 ± 0.5 -17.2 ± 1.0 Becker et al. 2007 Market Squid 21 13.7 ± 0.6 -15.6 ± 0.7 Becker et al. 2007 Humboldt Squid, large 5 16.8 ± 0.4 -14.8 ± 0.5 Ruiz-Cooley et al. 2004 Humboldt Squid, small 5 14.5 ± 0.5 -16.2 ± 0.3 Ruiz-Cooley et al. 2004

Table 2: Mean and standard deviation of total length, nitrogen, and lipid corrected carbon stable isotope values for all males and females. SEX TL (mm) ± SD δ15N ± SD δ13C ± SD Male 614 ± 169.6 14.76 ± 0.53 -15.03 ± 0.38 Female 553 ± 126.5 14.50 ± 0.47 -15.24 ± 0.47

Table 3: Comparisons for stable isotope analysis across three studies: my study, Andrews (2010), and Reum and Essington (2013). This Study Andrews 2010 Andrews 2010 Reum and Ch. 1 Ch.2 Essington 2013 Location Monterey Bay, Gulf of Alaska GOA to British Puget Sound, CA (GOA) to Puget Columbia WA Sound, WA

Sample Size (n) 43;♂=20 60;♂=10 491;♂=186 & 97;♂ & ♀=47 &♀=23 &♀=50 ♀=305

Total Length ♂: 30.9-89.7 ♂: 72-81 ♂: 61-96 ♂: 43-87 Range (cm) ♀: 28.0-75.5 ♀: 72-81 ♀: 52-113 ♀: 35-119

15 13.4-15.6 12.0-13.4 10.8-15.6 12.4-15.8 Range δ N (‰) 13 -15.6 to -14.4 -21.3 to -17.9 -21.2 to -16.8 -17.8 - to 14.2 Range δ C (‰)

Lipid Yes, Reum and No Yes, Yes, Reum and Correction? Essington 2013 McConnaughy Essington 2013 and McRoy(1979)

Size 15 Did not look 15 minor, T-test: δ N Yes for δ N Differences? b/c size range explained 3% too small variation

Sex No No Tested each site minor, Differences? separately; 3 explained 4% out of 6 variance

FIGURES

Figure 1: Collection sites of North Pacific Spiny Dogfish in central California, U.S.A. Arrows indicated GPS haul location. All specimens caught in the 20 hauls were used for SCA (Chapter 1). White arrows indicate hauls in which subsamples selected for use in SIA (Chapter 2).

Figure 2: C:N vs. δ13C’ values for 43 North Pacific Spiny Dogfish specimens.

Figure 3: Length frequency histogram of females (red) and males (blue) of Squalus suckleyi collected in 2004 and 2005 on a NMFS survey.

Figure 4: δ15N and δ13C’ values for all individual Spiny Dogfish. Closed circles represent females and open circles represent males. In this plot, nitrogen (δ15N) is a proxy for trophic position, with larger δ15N values indicating a greater trophic position. Carbon (δ13C’) is a proxy for source of primary production, either inshore vs. offshore or benthic vs. pelagic. Less negative δ13C’ indicate a benthic or inshore source, and more negative δ13C’ values indicate an offshore or pelagic source.

Figure 5: Dual isotope values of the Spiny Dogfish and its know prey from Chapter 1. The open black square represents the lipid-extracted and normalized carbon isotope signal (δ13C’) and the solid black square represents the trophic discrimination factor adjusted δ13C’ for North Pacific Spiny Dogfish. Error bars represent the first standard deviation around the mean. All prey values with the exception of euphausiids were taken from literature (see Table 1). Trophic discrimination factors of 3.7 for nitrogen and 1.7 for carbon were used to correct the stable isotope values (Kim et al. 2012).

Figure 6: Linear regression of δ15N and total length (mm). The trend line indicates the significant relationship.

Figure 7: Linear regression of δ13C’ and total length (mm). No significant relationship exists between total length and carbon isotope value.

APPENDIX

Appendix 1: Summary table of δ15N, δ13C, and δ13C’, total length (mm), and C:N ratio for all Spiny Dogfish collected in 2004 and 2005 in central California

Fish Total Length Sample ID Sex (mm) δ15N δ13C δ13C' C:N 30205104 F 451 14.61 -17.33 -15.12 4.11 30205107 M 493 15.01 -17.01 -14.91 4.12 30205108 F 526 14.93 -17.61 -14.91 3.73 30205109 F 516 14.14 -17.59 -15.35 4.15 30205110 M 508 14.78 -17.63 -14.66 3.50 30205112 M 476 15.13 -16.88 -15.15 4.44 30205183 M 897 15.04 -19.60 -15.81 3.20 30205302 M 839 15.50 -19.80 -14.66 2.10 30205305 F 588 14.32 -18.48 -15.05 3.28 30205306 M 825 15.56 -17.50 -14.80 3.71 30205311 M 454 14.04 -19.47 -15.30 2.84 30205403 F 632 14.53 -19.38 -15.31 2.91 30205405 F 671 14.82 -17.28 -14.98 4.02 30205408 F 718 14.17 -19.38 -15.57 3.13 72604110 M 711 14.53 -19.67 -15.18 2.61 72604205 F 755 14.63 -18.83 -15.24 3.22 72604228 M 690 15.21 -18.04 -15.18 3.69 72604327 M 721 13.79 -17.50 -14.54 3.47 82504105 F 485 14.68 -16.96 -14.85 4.11 82504110 F 539 14.72 -17.63 -15.06 3.85 82504112 F 431 14.65 -17.65 -15.54 4.30 82504113 M 435 14.83 -17.04 -15.09 4.27 82504114 M 602 14.99 -17.61 -15.35 4.14 82504313 M 657 14.27 -19.23 -15.56 3.23

82504316 M 581 13.87 -17.95 -15.32 3.88 82504318 F 600 13.93 -18.56 -15.35 3.50 82504319 F 658 14.22 -19.05 -15.41 3.23 82504321 F 686 13.97 -19.24 -15.43 3.11 82504322 F 678 14.65 -19.60 -15.54 2.96 82504335 F 280 13.44 -18.43 -15.57 3.79 82504337 M 315 14.12 -17.04 -14.42 3.66 82504338 M 309 14.55 -17.22 -14.35 3.48 302050102 F 502 15.44 -17.02 -14.88 4.09 302050308 F 470 15.08 -17.25 -14.89 3.95 302050403 F 632 15.30 -18.93 -15.00 2.94 302050404 F 558 14.50 -18.80 -15.46 3.43 302050406 M 562 14.66 -18.44 -15.25 3.49 726040101 M 756 15.52 -18.16 -15.02 3.47 726040202 M 788 15.01 -19.89 -15.10 2.39 825040101 F 647 14.07 -19.71 -15.46 2.82 825040324 M 671 14.81 -18.24 -14.96 3.36 82504336a F 325 14.66 -18.61 -15.22 3.35 82504336b F 372 14.06 -17.87 -15.36 3.97

CONCLUSION

North Pacific Spiny Dogfish collected from central California were opportunistic predators, consuming prey occupying variable trophic positions in inshore and offshore habitats. Three dominant prey types emerged from stomach content analysis and these were reflected in the stable isotope values of Spiny Dogfish, confirming the validity of using SIA to elucidate diet for this species. This is the first detailed, quantitative diet study of this species off central California, as previous research has only been conducted further north. In order of importance, the dominant prey taxa were euphausiids, teleosts, and cephalopods. Seasonal and spatial differences were significant in explaining dietary variation for stomach content analysis. There was no difference in diet between females and males for either method, and stable isotope analysis yielded a slight ontogenetic shift in trophic position.

Euphausiids dominated the diet of North Pacific Spiny Dogfish by number and mass, but teleosts were more frequent. Euphausiids that were found in the stomachs of

North Pacific Spiny Dogfish were highly digested, making lower taxonomic identification difficult. The most important teleosts to the diet were sardines followed by anchovies. Sardines dominated by mass, but anchovies dominated by number and were more commonly encountered. Unidentified fishes also were important in the diet, but the highly digested state of the majority of prey items made further identification of many specimens difficult. The most important cephalopods in the diet of Spiny Dogfish were

Market squid, Octopoda, and Humboldt squid. The consumption of Humboldt squid by

North Pacific Spiny Dogfish was presumed to be by scavenging.

There was a significant shift in consumption of prey types with the season, which reflects movements and migration of prey species. The variation in the diet among the seasons is likely due to the seasonal abundance of euphausiids, which were the most important prey type consumed. When present, euphausiids were frequently the only prey item in a stomach. When euphausiids were not abundant during the other two seasons, teleosts, mainly sardines and anchovies, and cephalopods, mainly market and Humboldt squid, were important prey. Further research with greater sample sizes for each season is needed to adequately address the seasonal variation in the diet of the North Pacific Spiny

Dogfish, and the effect of euphausiid availability on North Pacific Spiny Dogfish foraging. The three largest species of shark (whale shark, Rhincodon typus; basking shark, Cetorhinus maximus; and megamouth shark, Megochasma pelagios) feed on euphausiids and zooplankton (Taylor et al. 1983, Baduini 1995) providing evidence that euphausiids can provide adequate energy for metabolism and other processes. Also, previous researchers of Spiny Dogfish have noted the presence of euphausiids in the diet

(Robinson et al. 1982, Ketchen 1986).

As expected, space was a significant factor in explaining dietary variability.

Species abundance often reflects latitudinal and longitudinal gradients, thus, prey availability may be influenced by latitude and longitude. I found that with increasing longitude (i.e. moving offshore) the abundance of euphausiids increased, indicating that

Spiny Dogfish may have been moving offshore to feed on euphausiids, and feeding on other prey while inshore.

The stable isotope values confirmed that Spiny Dogfish fed on euphausiids offshore and fishes and cephalopods while inshore. SIA is an appropriate and effective

94 method to elucidate the diet of this species as evident from North Pacific Spiny Dogfish fitting well into the span of prey space. Spiny Dogfish may have a slight increase in trophic position ontogenetically which is supported by previous studies. Males and females did not differ significantly in their diets, and this is most likely due to the schooling nature of this species. Two recently published studies on the same species but in higher latitude waters had results that were similar to this study, suggesting that the

North Pacific Spiny Dogfish feeds on similar prey items in similar habitats along the entire western side of its range. These data can be used to inform management plans because Spiny Dogfish on the U.S. West Coast can be treated as one management unit rather than breaking this species into groups.

This study has elucidated the diet of an abundant economically important predator in the eastern North Pacific. A sustainable fishery for Spiny Dogfish began in British

Columbia in September 2011, and has potential to expand to other parts of this species’ range. This increase in demand for this species may soon necessitate its own management plan as well as stress the need for continued monitoring of the trophic ecology and predator-prey interactions of this species. This study determined food habits on short and longer time scales and can contribute crucial data needed to develop an effective and appropriate EBFM plan.