MUHLING ET AL.: DYNAMIC HABITAT USE OF ALBACORE AND THEIR PRIMARY PREY SPECIES IN THE CALIFORNIA CURRENT SYSTEM CalCOFI Rep., Vol. 60, 2019 DYNAMIC HABITAT USE OF ALBACORE AND THEIR PRIMARY PREY SPECIES IN THE CALIFORNIA CURRENT SYSTEM BARBARA MUHLING, STEPHANIE BRODIE, MICHAEL JACOX OWYN SNODGRASS, DESIREE TOMMASI NOAA Earth System Research Laboratory University of California, Santa Cruz Boulder, CO Institute for Marine Science Santa Cruz, CA CHRISTOPHER A. EDWARDS ph: (858) 546-7197 Ocean Sciences Department [email protected] University of California, Santa Cruz, CA BARBARA MUHLING, OWYN SNODGRASS, YI XU HEIDI DEWAR, DESIREE TOMMASI, JOHN CHILDERS Department of Fisheries and Oceans NOAA Southwest Fisheries Science Center Delta, British Columbia, Canada San Diego, CA STEPHANIE SNYDER STEPHANIE BRODIE, MICHAEL JACOX Thomas More University, NOAA Southwest Fisheries Science Center Crestview Hills, KY Monterey, CA ABSTRACT peiods, krill, and some cephalopods (Smith et al. 2011). Juvenile north Pacific albacore Thunnus( alalunga) for- Many of these forage species are fished commercially, age in the California Current System (CCS), supporting but also support higher-order predators further up the fisheries between Baja California and British Columbia. food chain, such as other exploited species (e.g., tunas, Within the CCS, their distribution, abundance, and for- billfish) and protected resources (e.g., marine mammals aging behaviors are strongly variable interannually. Here, and seabirds) (Pikitch et al. 2004; Link and Browman we use catch logbook data and trawl survey records to 2014). Effectively managing marine ecosystems to pre- investigate how juvenile albacore in the CCS use their serve these trophic linkages, and improve robustness of oceanographic environment, and how their distributions management strategies to environmental variability, thus overlap with the habitats of four key forage species. We requires knowledge of food web structure. show that northern anchovy (Engraulis mordax) and hake Food webs of the California Current System (CCS) (Merluccius productus) habitat is associated with produc- are comparatively well studied (e.g. Field et al. 2006; tive coastal waters found more inshore of core juvenile Kaplan et al. 2013; Rose et al. 2015; Koehn et al. 2016). albacore habitat, whereas Pacific sardine (Sardinops sagax) However, it is not yet clear how the dynamic nature of and boreal clubhook squid (Onychoteuthis borealijaponica) the CCS in space and time impacts trophic structure and habitat overlaps more consistently with that of albacore. predator-prey relationships, which presents a challenge Our results can improve understanding of how albacore for building ecosystem models (Hunsicker et al. 2011). movements relate to foraging strategies, and why prey- The diets of many large pelagic predators may vary both switching behavior occurs. This has relevance for the temporally and spatially, reflecting opportunistic feeding development of ecosystem models for the CCS, and for strategies. Some studies in the CCS have shown a near- the eventual implementation of ecosystem-based fish- exclusive reliance of pelagic predators such as tunas on ery management. one prey species, particularly coastal pelagic fishes such as anchovy (Engraulis mordax), while others show a much INTRODUCTION more diverse diet including crustaceans and cephalopods Ecosystem-based fishery management (EBFM) aims (Pinkas et al. 1971; Bernard et al. 1985; Glaser 2010). to account for environmental and ecosystem factors Existing studies have typically been snapshots, provid- within fisheries assessment and management frameworks ing limited information on how predator-prey interac- (Link 2017). This goal can be achieved through many tions vary at higher temporal and spatial scales. However, possible approaches of varying complexity; including such variability has implications for how energy flows explicit consideration of processes such as climate vari- through the food web, as well as foraging costs and net ability and change, habitat quality, predator-prey rela- energy gain in predators, some of which migrate long tionships in models of species productivity, distribution, distances to reach the CCS (Childers et al. 2011; Fujioka and trophic structure (Pikitch et al. 2004; Link 2017). et al. 2018). Prey-switching behavior in predators may be Optimal management strategies may involve trade-offs, triggered by changes in dominant species in the ambient as managers balance a desire to maximize sustainable prey assemblage, or by active targeting of preferred or catch of target species while preserving ecosystem func- high-energy prey when these are more available (Begoña tion, particularly for major forage species such as clu- Santos et al. 2013). The importance of these behaviors to 79 MUHLING ET AL.: DYNAMIC HABITAT USE OF ALBACORE AND THEIR PRIMARY PREY SPECIES IN THE CALIFORNIA CURRENT SYSTEM CalCOFI Rep., Vol. 60, 2019 Figure 1. Map of the study area and ROMS domain. The total number of sets in the albacore surface fishery 2002–15 (left), and hauls in the NOAA SWFSC trawl surveys 2003–16 (right) are also shown. Locations with < 3 vessels reporting data for the troll fishery are not shown. pelagic predators in the CCS is not yet well known. A tance of subsurface water column structure in species first step to understanding foraging ecology is therefore distributions. We compared results between three con- to define the spatial and environmental niches occupied trasting years with different environmental conditions: a by interacting predator and prey species, and to assess weak El Niño year (2004), a cool La Niña year (2012), how the degree of overlap between these varies in space and a very warm El Niño/marine heat wave year (2015). and time. Overall, we aimed to provide a better understanding of Foraging behaviors in commercially important preda- how these species overlap in space and time, and how tors have implications for the fisheries that target them. environmental variability impacts their distributions. Switching between shallow-living prey species and those that live deeper in the water column, or undertake diel METHODS vertical migrations, may impact the availability of pred- ators to fishing gear. For example, commercial and rec- Biological data sources reational tuna fisheries in the CCS mostly use surface Albacore catch per unit effort (CPUE) was defined gear, which is largely deployed during daylight hours as the number of fish recorded per vessel-day in the (Teo 2017; Runcie et al. 2018). Consequently, shifts in US pole-and-line and troll fisheries. These were the vertical distribution of tuna in response to forage will obtained from logbooks from US vessels submitted to impact gear vulnerability and catch. Improving under- the National Oceanic and Atmospheric Administration standing of spatiotemporal predator-prey relationships (NOAA) National Marine Fisheries Service (NMFS). between commercially important species thus has the Data were available throughout the central and east- potential to benefit fishers, future management strate- ern North Pacific Ocean from 1961 through the end gies in the CCS, and to contribute to the implementa- of 2015. However, we limited records used for SDM tion of EBFM. training to those contained within the ROMS domain In this study, we used statistical species distribution (fig. 1, and see further description below), and within the models (SDMs) to predict the distribution of a top pred- time period covered by the MODIS Aqua and VIIRS ator (albacore: Thunnus alalunga) and five key prey spe- ocean color missions (mid-2002 onwards). This ensured cies (northern anchovy: Engraulis mordax; hake: Merluccius that at least two of the three ocean color satellites (Sea- productus; boreal clubhook squid: Onychoteuthis borealija- WiFS, MODIS Aqua, VIIRS) were available to source ponica; Pacific sardine: Sardinops sagax; and Pacific saury: surface chlorophyll for most of the sets, limiting obser- (Cololabis saira) in the CCS. Environmental predictors vations lost to clouds. To account for varying degrees of were sourced from a high-resolution, data-assimilative accuracy in fishing locations reporting in the logbooks, CCS configuration of the Regional Ocean Modeling we removed records where fishing latitude and longi- System (ROMS), allowing examination of the impor- tude were both reported in whole degrees (n = 2,937), 80 MUHLING ET AL.: DYNAMIC HABITAT USE OF ALBACORE AND THEIR PRIMARY PREY SPECIES IN THE CALIFORNIA CURRENT SYSTEM CalCOFI Rep., Vol. 60, 2019 assuming that these were approximate locations. Loca- fore proceeded with anchovy, saury, hake, sardine, and tions of all remaining records were then coarsened to clubhook squid as the five prey species for which to show catch per vessel/day at 0.25˚ resolution, to align construct SDMs. with expected accuracy in location reporting, and vessel Occurrence records for prey species were obtained movements while fishing (see Nieto et al. 2017). These from trawl surveys conducted by the NOAA Southwest filtering criteria resulted in 129,693 spatially explicit, Fisheries Science Center (SWFSC), with 1,486 hauls daily data points for environmental data extraction. completed between 2003 and 2016. Sampling effort was A fishery-independent data set was also available to primarily concentrated in spring (April: 460 hauls) and validate albacore habitat predictions. Since 2001, NMFS summer (July–August: 691 hauls), but some samples were and the American Fishermens