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 in Marine Science _______________ by Jennifer S. Bigman Summer 2013 iii Copyright © 2013 by Jennifer S. Bigman All Rights Reserved iv v 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 ix 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 x 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