Using geostatistical analysis for simultaneous estimation of isoscapes and ontogenetic shifts in isotope ratios of highly migratory marine fish

Abstract

Tracking migration of highly migratory marine fish using isotope analysis (iso-logging) has become a promising tool in recent years. However, application of this method is often hampered by the lack of essential information such as spatial variations in isotope ratios across habitats (isoscapes) and ontogenetic shifts of isotope ratios of target animals. Here, we test the utility of geostatistical analysis to generate isoscapes of δ13C and δ15N in the western Pacific and estimate the ontogenetic shifts in δ13C and δ15N values of a target species. We first measured δ13C and δ15N in the white muscle of juvenile (n = 210) and adult (n = 884) skipjack tuna Katsuwonus pelamis sampled across the northwest Pacific. Next we fitted a geostatistical model to account for the observed spatial variations in δ13C and δ15N of skipjack by fork length and other environmental variables with spatial random effects. We then used the best-fit models to predict the isoscapes of δ13C and δ15N in 2021. Furthermore, we measured δ15N of amino acids (δ15NAAs) of skipjack (n = 5) to determine whether the observed spatial variation of isotope ratios resulted from baseline shifts or differences in trophic position. The geostatistical model reasonably estimated both isoscapes and ontogenetic shifts from isotope ratios of skipjack, and the isoscapes showed that δ13C and δ15N can clearly distinguish the latitudinal migration of skipjack in the western Pacific. The δ15NAAs supported the results of the geostatistical model, that is, observed variations in skipjack δ15N were largely derived from a baseline shift rather than regional differences in trophic position. Thus, we showed that geostatistical analysis can provide essential basic information required for iso-logging without compound-specific isotope analysis.

Sharing technical knowledge to understand the distribution patterns and migration history of marine organisms

“…The simulated d 15 N Nitrate values (Figure 3A) are annual mean surface values, whereas the observed values (Figure 3D) are the shallowest data that were measured in a single expedition compiled by and Fripiat et al (2021). The simulated d 15 N Phytplankton values (Figure 3B) are the weighted averages of the euphotic layer throughout the year, whereas the observed values (Figure 3E) are the d 15 N Base values estimated from the d 15 N measurements of bulk nitrogen and amino acids of zooplankton obtained from various depths and seasons (Matsubayashi et al, 2022). The simulated d 15 N PON values (Figure 3C) are the weighted averages of the euphotic layer throughout the year, whereas the observed values (Figure 3F) are the d 15 N Bulk values of surface sediments obtained from various depths and redox conditions compiled by NICOPP (Tesdal et al, 2013), which could be affected by digenesis.…”

“…The isotopic composition of animal tissues and local environments can be used as a natural tag to track an animal's movements through isotopically distinct habitats, called iso-logging (Matsubayashi et al, 2022). Nitrogen isotope (d 15 N) analysis of incremental growth tissues, such as eye lenses and vertebral bones, has proven promising for reconstructing migration routes and feeding environments over the animal's life (Matsubayashi et al, 2017;Harada et al, 2022).…”

Lower thermal tolerance restricts vertical distributions for juvenile albacore tuna (Thunnus alalunga) in the northern limit of their habitats

“…It is theoretically possible that the behavior observed in this individual reflects the ecology of ALB as a species rather than a characteristic unique to the individual. Promising future research direction to overcome the limitation of tagging studies (i.e., data quantity issue) involves using otolith trace element analysis (Macdonald et al, 2013) and Iso-logging (Matsubayashi et al, 2022), to estimate the track of movement simply by collecting substantial biological tissues. Unlike tagging research, these methods are not affected by the recapture rates and therefore could be a useful approach (Macdonald et al, 2013;Matsubayashi et al, 2022) to evaluate the migration in a more quantitative manner.…”

A novel method for fine-scale retrospective isotope analysis in mammals using eye lenses

“…Recent isotopic studies have demonstrated the potential of animal eye lenses for retrospective isotope analyses (Bell-Tilcock et al, 2021; Matsubayashi et al, 2022; Wallace et al, 2014). Eye lenses, primarily composed of structural proteins called crystallins, grow incrementally, and new tissues are sequentially added to the outermost part throughout an individual’s lifetime (Peebles & Hollander, 2020; Wride, 2011).…”

“…Once these tissues are formed, protein synthesis ceases, and they no longer experience metabolic turnover (Peebles & Hollander, 2020). Thus, longitudinal tissue segregation and isotope analysis of eye lenses can enable the reconstruction of individual behavioural histories from isotopic chronology (Bell-Tilcock et al, 2021; Matsubayashi et al, 2022; Peebles & Hollander, 2020). This method has successfully been applied in fish and cephalopods (Hunsicker et al, 2010; Wallace et al, 2014); however, its applicability and appropriate lens segregation methods in mammals have not yet been tested.…”