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Exploring the effects of water temperature on egg development in skipjack tuna (Katsuwonus pelamis) has substantial implications for evaluations of wild spawning habitats. In the present study, we examined the hatching success and duration as a function of temperature from 21 to 33°C under captive environments. A high hatching rate of over 50% between 23 and 31°C was observed, with the shortest hatching duration at 31°C. Because the egg period is vulnerable to predators, a shortened hatching duration with warming water would be ecologically advantageous for K. pelamis, as its main spawning grounds are located in tropical areas. [ABSTRACT FROM AUTHOR]

This study compared the microstructures of transverse sections and surface of the otoliths of skipjack tuna. Pairs of otoliths were extracted from the same individuals (fork length [FL]: 6.5–77.5 cm) to examine the difference in the counts of the daily increments between the sections. The increments counted on the transverse section were less than those on the surface section, and the difference between the two sections increased with body size. Even high-resolution scanning electron microscopy did not enhance the difference between the two sections. Increments in the transverse section would not occur daily, which makes otoliths from this section unsuitable for age estimation. Subsequently, we re-evaluated the age estimated using the surface reading with new criteria and compared them with the results of a reference study. Despite successful age estimates using surface otoliths exposed by an etching method, practical difficulties in the method and vague criteria for reading increments have been raised against their reliability for age estimation. By reproducing detailed procedures of the etching method and using reading criteria, we estimated the ages of individuals at various life stages. In applying the criteria, the coefficient of variation (CV) for increment counts among readers decreased. In particular, the criteria reduced the CV for the transition area where the width between increments gradually narrows after the 50th–90th increment. The age estimates (FL < 40 cm) using the criteria are reasonably fit to the growth curve reported in the reference study with validation of daily increment deposition for surface reading. Thus, the new criteria are suitable for estimating age estimation in skipjack tuna up to an FL of 40 cm. • We compared increments between transverse and surface sections of skipjack otoliths. • Low increment counts on the transverse section were not deposited daily. • We reevaluated age estimation based on the otolith surface using new criteria. • The validity and limitation of estimates using surface readings were explored. [ABSTRACT FROM AUTHOR]

Energy intake during the northward migration of tagged juvenile skipjack tuna ( Katsuwonus pelamis ) was estimated using the heat increment of feeding (HIF) determined through peritoneal cavity temperature data acquired with an archival tag. The effectiveness of this method was confirmed by feeding experiments, even in a species without visceral heat exchangers. Applying these experimental results to the data from tagged fish in the wild revealed that the estimated energy intake of skipjack tuna was, on average, 2.2 times greater in temperate waters than in subtropical waters. This difference was likely due to the differences in the food supply between habitats. In contrast, the estimated energy cost of their daily travel distance decreased with northward migration. This result suggests that skipjack spend less energy in acquiring prey in temperate than in subtropical waters since prey are available in higher densities, thereby providing excess energy for growth or lipid reserves. [ABSTRACT FROM AUTHOR]

Formal approaches to physiological energetics, such as Dynamic Energy Budget (DEB) theory, enable interspecies comparisons by uniformly describing how individuals of different species acquire and utilise energy. We used DEB theory to infer the energy budgets of three commercial tuna species (skipjack, Pacific bluefin, and Atlantic bluefin) throughout all stages of ontogenetic development—from an egg to an adult individual and its eggs. Energy budgets were inferred from exhaustive datasets fed into a DEB-based mathematical model tailored to tuna fish, until reaching a high goodness of fit and thus reliable estimates of the model parameters. The life histories of all three species are strongly influenced by morphological and physiological adaptations that accelerate ontogeny during the larval stage, although the effect is more pronounced in bluefin than skipjack tuna. Accelerated ontogeny in energetic terms is a simultaneous improvement of energy acquisition (higher intake) and utilisation (higher expenditure) without changing the capacity of fish to build energy reserve as intake and expenditure increase in unison. High energy expenditure, an even higher intake by necessity, and a limited capacity to build energy reserve, make all three tuna species vulnerable to starvation, thereby theoretically underpinning the description of tunas as "energy speculators". Energy allocation to reproduction maximises fecundity of all three tuna species, thus suggesting that the evolution of tuna favours higher fecundity at the expense of growth. Thinking beyond just physiological energetics (e.g., wild stock projections), DEB-based models are a natural foundation for physiologically-structured population dynamics wherein the environment influences the population growth rate via metabolism. • We inferred energy budgets of skipjack, Pacific bluefin, and Atlantic bluefin tuna. • Morphological and physiological adaptations accelerate ontogeny in the larval stage. • Accelerated ontogeny expands tuna scope for growth, but leaves energy reserve small. • Thermogenesis that starts in early juveniles is a considerable energy sink. • Large body, small reserve, and high energy costs shape tuna ecology and evolution. [ABSTRACT FROM AUTHOR]

Abundance indices derived from fisheries-dependent data (catch-per-unit-effort or CPUE) are known to have potential for bias, in part because of the usual non-random nature of fisheries spatial distributions. However, given the cost and lack of availability of fisheries-independent surveys, fisheries-dependent CPUE remains a common and informative input to fisheries stock assessments. Recent research efforts have focused on the development of spatiotemporal delta-generalized linear mixed models (GLMMs) which simultaneously standardize the CPUE and predict abundance in unfished areas when estimating the abundance index. These models can include local seasonal environmental covariates (e.g. sea surface temperature) and a spatially varying response to regional annual indices (e.g. the El Niño Southern Oscillation) to interpolate into unfished areas. Spatiotemporal delta-GLMMs have been demonstrated in simulation studies to perform better than conventional, non-spatial delta-generalized linear models (GLMs). However, spatiotemporal delta-GLMMs have rarely been evaluated in situations where fisheries spatial sampling patterns change over time (e.g. fisheries expansion or spatial closures). This study develops a simulation framework to evaluate 1) how the nature of fisheries-dependent spatial sampling patterns may bias estimated abundance indices, 2) how shifts in spatial sampling over time impact our ability to estimate temporal changes in catchability, and 3) how including seasonal environmental covariates and/or regional annual indices in spatiotemporal delta-GLMMs can improve the estimation of abundance indices given shifts in spatial sampling. Spatiotemporal delta-GLMMs are then applied to a case study example where the spatial sampling pattern changed dramatically over time (contraction of the Japanese pole-and-line fishery for skipjack tuna Katsuwonus pelamis in the western and central Pacific Ocean). Results from simulations indicate that spatial sampling in proportion to the underlying biomass can produce similar abundance indices to those produced under random sampling. Though estimated abundance indices were not perfect, spatiotemporal GLMMs were generally able to disentangle shifts in spatial sampling from temporal changes in catchability when shifts in spatial sampling were not too extreme. Lastly, the inclusion of seasonal environmental covariates and/or regional oceanographic indices in spatiotemporal GLMMs did not improve abundance index estimation and in some cases resulted in degraded model performance. [ABSTRACT FROM AUTHOR]