Insights into the horizontal movements, migration patterns, and stock affiliation of California swordfish

Abstract

This study reports on the horizontal movements of swordfish (Xiphias gladius L.) tagged during deep‐set fishery trials off the California coastline. Position estimates from several electronic tag types were used to better understand swordfish stock structure and regional affiliation with current boundary hypotheses used to manage swordfish in the eastern north Pacific. Swordfish were outfitted with (a) satellite‐linked mark–recapture tags (n = 66), (b) electronic data storage tags that were recaptured (n = 16), (c) fin‐mounted Argos transmitters (n = 6), and (d) satellite‐linked archival tags (n = 4). Twenty‐six percent of tagged swordfish reported close to (<225 km) their deployment location within the southern California Bight (SCB). Of the 50 swordfish that moved outside the SCB, 76% exhibited affiliation to the Eastern Pacific Ocean (EPO) management unit, 20% moved into the Western and Central North Pacific (WCNP) and 4% spent time within both the EPO and WCNP boundaries. Mean displacement between deployment and reporting locations was 1,250 ± 1,375 km, with daily rates of movement up to 55 km/day. Seasonal migrations ranged from the equator (0.8°N.132.4°W) to the Hawaiian Islands (17.0°N/154.2°W), with multiple individuals returning to the initial tagging locations the subsequent season. Seasonal site fidelity exhibited by several individuals highlights the importance of the SCB foraging grounds. While no evidence of trans‐equatorial or trans‐Pacific crossing was documented, extensive movements validate the highly migratory nature of California swordfish and support the need for future inclusion of spatial distribution data in management. Findings suggest that SCB swordfish may exhibit a higher level of EPO connectivity than previously proposed.

Exploring timescales of predictability in species distributions

“…By partitioning temporal scales of environmental variability, our results contribute to understanding the mechanisms that structure species responses to the environment at multiple scales. Here, the monthly climatology of environmental covariates was likely a good predictor of swordfish distributions because swordfish exhibit seasonality and some degree of site fidelity in their movements (Sepulveda et al 2020). The incremental improvement in model predictive performance with the addition of the low‐frequency component reflects the high mobility of swordfish, which allows them to move in response to local and interannual variability.…”

“…Swordfish exhibit extreme diel vertical migration, and in the CCS forage at depths up to 500 m during the day and in surface waters at night (Sepulveda et al 2010, 2018, Dewar et al 2011). The CCS is an important foraging ground for north Pacific swordfish, with some adults undertaking spawning migrations to tropical waters near Hawaii during May–August (Grall et al 1983) and other adults residing in the CCS year‐round (Abecassis et al 2012, Sepulveda et al 2020). Within the CCS, swordfish are generally distributed at higher latitudes in summer and lower latitudes in winter (Hanan et al 1993, Sepulveda et al 2018), with some individuals showing high site fidelity following extensive seasonal movements (Sepulveda et al 2020).…”

“…The CCS is an important foraging ground for north Pacific swordfish, with some adults undertaking spawning migrations to tropical waters near Hawaii during May–August (Grall et al 1983) and other adults residing in the CCS year‐round (Abecassis et al 2012, Sepulveda et al 2020). Within the CCS, swordfish are generally distributed at higher latitudes in summer and lower latitudes in winter (Hanan et al 1993, Sepulveda et al 2018), with some individuals showing high site fidelity following extensive seasonal movements (Sepulveda et al 2020). The presence and distribution of swordfish in the CCS thus depend on a complex combination of movements across multiple scales, including large‐scale spawning migrations, non‐spawning seasonal latitudinal movements, fine‐scale foraging, diel vertical migration and site fidelity.…”

The transboundary nature of the world’s exploited marine species

“…However, it is important to acknowledge that there is considerable uncertainty associated in determining levels of connectivity across time and space for marine populations at different life stages, from larvae 22 to adults 23 . This is of special concern, but-not-limited-to 23 , highly migratory species whose ranges span multiple jurisdictions and the high seas, such as tunas 71 , billfishes 72 and sharks 73 challenging management decisions based on meta-populations 71 . Here, we limited the definition of “transboundary” to include species spanning only adjacent countries (e.g., the USA and Canada), excluding countries that were separated by another nation (e.g., Canada and Mexico) and/or the high seas (e.g., Canada and Spain).…”

Flexible use of a dynamic energy landscape buffers a marine predator against extreme climate variability

“…[3,4]). Foraging migrations can consist of relatively rigid movement behaviours such as foraging site fidelity that exploit predictable productivity hotspots [5,6]), or migration routes that are optimized to match average patterns of resource availability [7]. Alternatively, animals may exhibit flexible movement strategies, using proximate cues to track resources as they become seasonally available across spatial gradients [8,9].…”