Validation of otolith daily increment formation for two temperate syngnathid fishes: the pipefishes Stigmatopora argus and Stigmatopora nigra

Staining protocols affect use of otolith to estimate the demography of the damselfish sergeant major (Abudefduf vaigiensis)

This study assessed the otolith (sagittae, lapilli, and asterisci) increment deposition rate in the range-shifting damselfish, A. vaigiensis, using different concentrations of Alizain Red S and evaluated the impact of staining on increment width. Daily increment deposition was verified in all otolith types and presented clearer fluorescent markings in the lapilli and sagittae than the asterisci, with high stain concentration showing the best clarity. Higher stain concentrations were found to decrease increment width, suggesting care is needed when using stained otoliths as a proxy for growth for this species.

Unmasking pipefish otolith using synchrotron-based scanning X-ray fluorescence

Scientists use otoliths to trace fish life history, especially fish migrations. Otoliths incorporate signatures of individual growth and environmental use. For many species, distinct increment patterns in the otolith are difficult to discern; thus, questions remain about crucial life history information. To unravel the history of such species, we use synchrotron-based scanning X-ray fluorescence. It allows the mapping of elements on the entire otolith at a high spatial resolution. It gives access to precise fish migration history by tagging landmark signature for environmental transition and it also characterises localised growth processes at a mineral level. Freshwater pipefish, which are of conservation concern, have otoliths that are small and fragile. Growth increments are impossible to identify and count; therefore, there is a major lack of knowledge about their life history. We confirm for the first time, by mapping strontium that the two tropical pipefish species studied are diadromous (transition freshwater/marine/freshwater). Mapping of other elements uncovered the existence of different migratory routes during the marine phase. Another major breakthrough is that we can chemically count growth increments solely based on sulphur signal as it is implicated in biomineralization processes. This novel method circumvents reader bias issues and enables age estimation even for otoliths with seemingly untraceable increments. The high spatial resolution elemental mapping methods push back limits of studies on life traits or stock characterisation.

Rapid growth and short life spans characterize pipefish populations in vulnerable seagrass beds

The life-history traits of two species of pipefish (Syngnathidae) from seagrass meadows in New South Wales, Australia, were examined to understand whether they enhance resilience to habitat degradation. The spotted pipefish Stigmatopora argus and wide-bodied pipefish Stigmatopora nigra exhibit some of the shortest life spans known for vertebrates (longevity up to 150 days) and rapid maturity (male S. argus 35 days after hatching (DAH) and male S. nigra at 16-19 DAH), key characteristics of opportunistic species. Growth rates of both species were extremely rapid (up to 2 mm day(-1) ), with seasonal and sex differences in growth rate. It is argued that short life spans and high growth rates may be advantageous for these species, which inhabit one of the most threatened marine ecosystems on earth.

Validation of daily microincrement deposition in otoliths of juvenile and adult Peruvian anchovy Engraulis ringens

Wild adult specimens of the Peruvian anchovy Engraulis ringens were captured and reared to validate the daily periodicity of otolith microincrement formation. The postcapture stress generated spontaneous spawning, making it possible to conduct a rearing trial on larvae first in an artificial nutrient-enriched system (ANES) for 52 days followed by an artificial feeding regime in a culture tank until day 115 post-hatch. Microincrements of the sagittal otoliths of sacrificed juveniles [mean ± S.D. total length (LT ) = 5·13 ± 0·37 cm, range 5-6 cm; c.v. = 7·5%] showed very distinct light and dark zones. The slope of the relationship between the total number of increments after the hatch check and days elapsed after hatching was not significantly different from 1. The transfer from ANES to the artificial feeding regime induced a mark in the sagittal otoliths. The number of microincrements after this induced mark coincided with the number of days elapsed after the transfer date. In parallel experiments, adult E. ringens (mean ± S.D. LT  = 14·92 ± 0·55 cm, range 13-16 cm) were exposed to one of two fluorescent marking immersion treatments with either alizarin red S (ARS; 25 mg l(-1) per 6 h) or oxytetracycline hydrochloride (OTC; 200 mg l(-1) per 10 h). The microincrements between fluorescent bands were distinct, ranging from 0·89 to 2·75 µm (mean ± S.D. =1·43 ± 0·28 µm; c.v. = 32%) and from 0·71 to 2·89 µm (1·53 ± 0·27 µm; c.v. = 35%) for ARS and OTC, respectively. The relationship between the number of microincrements between marks and the number of elapsed days for ARS and OCT treatments indicated that there was a significant correspondence between the number of increases observed and the number of days. Hence, daily microincrements of otoliths of E. ringens are likely to be formed in juveniles and adults under natural conditions.